US20110236226A1

US20110236226A1 - Method of operating a compressor and an apparatus therefor

Info

Publication number: US20110236226A1
Application number: US13/133,167
Authority: US
Inventors: Carolus Antonius Cornelis Van De Lisdonk; Joannes Alphonsus Maria Wiegerinck
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2008-12-09
Filing date: 2009-12-07
Publication date: 2011-09-29
Also published as: CN102365518A; CY1123073T1; EP2356389A2; AU2009326302A1; AP2011005718A0; WO2010066662A3; KR101703502B1; AP2997A; KR20110098790A; JP2012514707A; EP2356389B1; WO2010066662A2; CN102365518B; AU2009326302B2

Abstract

One or more compressor feed streams are passed to one or more inlets of a first compressor. Each compressor feed stream passes through a compressor feed valve. The one or more compressor feed streams are in the first compressor to provide a compressed discharge stream at the outlet of the first compressor, while monitoring for a first indicator of a blocked outlet event of the first compressor and instructing the closure of the compressor feed valves (12, 22, 32, 42) when the first indicator of the first compressor (100) blocked outlet event is detected. This provides a method of operating the compressor for the reduction of relief lead during a blocked outlet event, and an apparatus therefor.

Description

The present invention provides a method of operating a compressor, for the reduction of relief load during a blocked outlet event, and an apparatus for the reduction of relief load during a blocked outlet event of a compressor.
A common application for a compressor is as a refrigerant compressor, whereby it is used to compress a refrigerant stream for instance in the liquefaction of natural gas (NG). The refrigerant stream is used to extract heat from the natural gas under liquefaction, for instance by means of indirect heat exchanging the refrigerant against the natural gas in one or more heat exchangers.
Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure.
Usually, natural gas enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.
The cooling stages for the liquefaction of natural gas utilise one or more refrigerants being circulated in one or more refrigerant circuits. A refrigerant circuit can comprise one or more refrigerant compressors, one or more heat exchangers and one of more cooling devices. In such circuits, an at least partly evaporated refrigerant stream can be compressed in a refrigerant compressor to provide a compressed refrigerant discharge stream. The compressed refrigerant stream can then be cooled in a cooling device, such as an air or water cooler, or in a heat exchanger against another refrigerant stream, to provide a cooled refrigerant stream. The cooled refrigerant stream can then be optionally expanded and then passed to a heat exchanger where it can cool the natural gas.
If a blockage or restriction occurs downstream of the compressor discharge (a so-called “blocked outlet event”), and normal operation continues, the pressure in the compressed refrigerant discharge stream will increase, potentially above the design pressure and mechanical failure and loss of containment may occur. Consequently, the compressed refrigerant discharge stream is normally fitted with a pressure relief valve which passes the compressed refrigerant discharge stream to a flare system before mechanical failure can occur. The flare system must be sized to handle the anticipated relief load for such an event. The piping and flare system may account for considerable capital expenditure. It is therefore desirable to minimise the size of the piping and flare system required to handle such flows.
US 2005/0022552 discloses a gas compression apparatus comprising a gas compressor and a driver, the driver slowing down when the power requirement of the gas compressor exceeds the maximum power of the driver, and a recycle line having a pressure relief device in fluid communication with an outlet of the gas compressor. The pressure relief device is in addition to any anti-surge valves in the recycle line. The recycle line receives a stream of compressed gas from upstream of the compressor outlet. The pressure relief device opens when the discharge pressure from the outlet reaches a designated pressure, supplying the compressed gas to an inlet of the gas compressor, thereby increasing the mass flow rate through the compressor and regulating the driver.
The apparatus disclosed in US 2005/0022552 operates to open recycle lines which increase the mass flow through the compressor. This will require more compression power and as a consequence the maximum driver power will be reached at some point, resulting in a reduction of speed. This could reduce the discharge pressure before it reaches the design pressure of the system. This depends, however, on the normal operating pressure, the design pressure and the maximum power of the driver. If there is, for example, more driver power available this could lead to overpressure and subsequently damage to the compressor or associated piping systems and loss of containment.
In a first aspect, the present invention provides a method of operating a compressor, for the reduction of relief load during a blocked outlet event, and an apparatus therefore, the method comprising at least the steps of:

(a) passing one or more compressor feed streams to one or more inlets of a first compressor, each compressor feed stream passing through a compressor feed valve;
(b) compressing the one or more compressor feed streams in the first compressor to provide a compressed discharge stream at the outlet of the first compressor;
(c) monitoring for a first indicator of a blocked outlet event of the first compressor; and
(d) instructing the closure of the compressor feed valves upon detection of the first indicator of the first compressor blocked outlet event.

In a second aspect, the present invention provides an apparatus for the reduction of relief load during a blocked outlet event of a compressor, comprising at least:
a first compressor having an outlet for a compressed discharge stream and one or more inlets for one or more compressor feed streams, each compressor feed stream passing through a compressor feed valve; and
a first controller, the first controller in communication with a first device to provide a first indicator of a blocked outlet event of the first compressor, said first controller in further communication with each compressor feed valve, such that the first controller transmits a first signal to close each compressor feed valves upon detection of the first indicator of the blocked outlet event of the first compressor.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying non-limited drawings in which:

FIG. 1 is a diagrammatic scheme of a method of and apparatus for operating a compressor according to one embodiment; and

FIG. 2 is a diagrammatic scheme of a method of and apparatus for operating a refrigerant compressor according to a second embodiment.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.
The present method and apparatus are intended to reduce the relief flow associated with a blocked outlet event of a compressor. Upon detection of a first indicator of a blocked outlet event, normal operation of the compressor cycle is discontinued and the closure of the compressor feed valves is instructed.
By instructing the closure of the compressor feed valves when the first indicator of the first compressor blocked outlet event is detected, the mass flow through the compressor is sought to be reduced. If one of the compressor feed valves closes, the relief flow is reduced.
In the context of the present disclosure, “closure” of and “to close” a compressor feed valve preferably means “the shutting” respectively “to shut” the compressor feed valve concerned, such as to essentially fully block the compressor feed stream that passes through the compressor feed valve concerned. Preferably, any compressor feed valve concerned of which closure may be instructed is provided in the form of a trip valve under snap-action control (sometimes also referred to as two-position control), as opposed to a control valve wherein throttling takes place.
Referring to the drawings, FIG. 1 shows a method of and apparatus 1 for operating a compressor 100 having a compressor first feed stream 10 and a compressor second feed stream 20 according to a first embodiment.
However, the method disclosed herein can be used in any system having a compressor with two or more compressor feed streams, such as n feed streams, where n is an integer greater than or equal to 2, more preferably in the range of from 2 to 6.
Each compressor feed stream 10, 20 passes through a compressor feed valve 12, 22. Two feed valves are shown in FIG. 1. However, if there are n compressor feed streams, then n compressor feed valves can be provided, with one for each feed stream. FIG. 1 shows compressor first feed stream 10 passing through compressor first feed valve 12 and compressor second feed stream 20 passing through compressor second feed valve 22.
The first compressor 100 can be a single stage or a multistage compressor. Examples of multistage compressors include those in which two or more compression stages are housed in a single casing, or those in which multiple compressors are arranged in series on a common mechanical drive shaft to sequentially compress a compressor feed stream. The first compressor 100 may be any compressor known in the art, such as a centrifugal, diagonal, axial-flow, reciprocating, rotary screw, rotary vane, scroll or diaphragm compressor. In the embodiment of FIG. 1 two compression stages are shown.
The compressor first feed stream 10 enters the first compressor 100 at a first inlet 18. The compressor second feed stream 20, which is at a higher pressure than the compressor first feed stream, enters the first compressor 100 at a second inlet 28. The compressor first and second feed streams 10, 20 are compressed to provide a compressed discharge stream 150 at an outlet 105 of the first compressor 100. The first compressor 100 can be driven by a first driver 200 which is discussed in more detail below.
The compressed discharge stream 150 can be provided with a recycle stream 110, 120 for each compressor feed stream 10, 20. Thus, for n compressor feed streams, n recycle streams may be provided.
Each recycle stream 10, 20 connects the compressor discharge stream 150 to a compressor feed stream 10, 20, between the respective inlet 18, 28 and respective compressor feed valve 12, 22. Each recycle stream 10, 20 is provided with a recycle expansion device 112, 122, such as a Joule-Thomson valve. FIG. 1 shows a first recycle stream 110 from the compressed discharge stream 150 passing through a first recycle expansion device 112 before joining the compressor first feed stream 10 between the compressor first feed valve 12 and first inlet 18. Similarly, FIG. 1 shows a second recycle stream 120 from the compressed discharge stream 150 passing through a second recycle expansion device 122 before joining the compressor second feed stream 20 between the compressor second feed valve 22 and second inlet 28.
The recycle streams 110, 120 can protect the first compressor from damage due to surge, an event which can occur when the flow of the compressor feed streams is too low, leading to rapid pulsations in flow. This event can be mitigated by opening the recycle valves 112, 122 to return at least a portion of the compressed discharge stream 150 to the inlets 18, 28 of the first compressor 150, thereby increasing the mass flow into the first compressor 150.
The portion of the compressed discharge stream 150 which is not recycled in recycle streams 110, 120 is passed downstream as continuing compressed stream 160. Continuing compressed stream 160 therefore represents the total flow of the compressed discharge stream 150 minus any recycle flow returned to the suction side of the first compressor 100 via recycle streams 110, 120.
A blockage or restriction downstream of the discharge of the first compressor 100 may increase the pressure of the compressed discharge stream 150 and any stream downstream of this between the first compressor 100 and the blockage or restriction. So-called compressor blocked outlet events can lead to an overpressure at the outlet of the first compressor, which may exceed design specifications, causing mechanical failure, if steps are not taken to prevent the pressure increase.
A blocked outlet event is therefore defined herein as a blockage or flow restriction downstream of the discharge of the first compressor which reduces the flow of the compressed discharge stream below that necessary for normal operation, thus leading to an increase in pressure at the outlet of the first compressor.
Blocked outlet events may arise for a number of reasons, which will be dependent upon the nature of the first compressor 100 and the circuit within which the compressor is operating. Examples of compressor blocked outlet events include the failure of a valve which is in fluid communication with the compressed discharge stream 150, for instance the failure of discharge isolation valve 350 in the closed position, the blockage of a unit, such as a condenser which is in fluid communication with the compressed discharge stream 150, or the loss of condensing capability of any condenser which is in fluid communication with the compressed discharge stream 150.
In known systems which do not take steps to address a compressor blocked outlet event, the compressor cycle continues to operate, such that the compressor feed streams will continue to flow into the inlets of the compressor. In addition, any anti-surge control system present will open any recycle valves. This will cause an increase in discharge pressure, and also an increase in suction pressure if any recycle valves are opened. If the compressor can be supplied with sufficient power, the discharge pressure will continue to rise and may exceed the design pressure of the system unless a relief valve is provided.
Conventionally, such a relief valve is connected to a flare system which is sized to handle the entire relief flow from a blocked outlet event i.e. the mass flow which is generated from the compression of all of the compressor feed streams and must be relieved in order to prevent a further pressure increase at the outlet of the compressor.
FIG. 1 discloses a preferred embodiment in which an optional pressure relief valve 250 is provided in fluid communication with the compressed discharge stream 150. In the embodiment shown, the pressure relief valve 250 is connected to the continuing compressed stream 160, which is provided by that portion of the compressed discharge stream 150 which is not recycled to the inlet(s) of the first compressor 100. The pressure relief valve 250 may be connected to a flare system 300. A flare system 300 may comprise a flare stack and associated piping (not shown).
The method and apparatus disclosed herein provide at least one, and more preferably two barrier systems for the protection of the apparatus 1, including the first compressor 100 and compressed discharge stream 150.
The first barrier system addresses the increase in pressure of the compressed discharge stream 150 after a blocked outlet event by instructing the closure of the compressor feed valves 12, 22. This is intended to prevent a further increase in the pressure at the outlet 105 of the first compressor 100 by preventing admission of additional mass into the first compressor 150 via the compressor feed streams 10, 20
The first barrier system monitors for a first indicator of a first compressor blocked outlet event. The first indicator can be one or more of the group comprising: a selected discharge pressure of the compressed discharge stream 150, a selected discharge temperature of the compressed discharge stream 150, a selected power of the first compressor 100, a selected suction pressure of the one of more compressor feed streams 10, 20 and a selected suction temperature of the one or more compressor feed streams 10, 20.
The first indicator should preferably be selective for a blocked outlet event, and dis-selective for other events that occur under normal operating conditions when no blocked outlet event is present. For example, if a process measurable (e.g. discharge pressure or other measurables listed in the group above) is used, then the selected value of that measurable should be high enough or low enough that normal variations of the process measurable that may occur under normal operating conditions, without a blocked outlet, do not reach and/or cross the selected value and causing detection of a first indicator of a blocked outlet event. The selected value should preferably be high enough or low enough that the process measurable in question reaches or crosses the selected value only only in case of a blocked outlet event.
Preferably, the first indicator of a first compressor blocked outlet event comprises, or more preferably consists of, a selected discharge pressure of the compressed discharge stream 150. This discharge pressure is considered to be one of the most direct indicators of a compressor blocked outlet event and also relatively easily measurable. Thus, for example, the first indicator may be a selected pressure in the compressed discharge stream 150. When this selected pressure is reached, the closure of the compressor feed valves 12,22 is instructed. A number of the compressor feed valves 12, 22, from zero to all of the compressor feed valves 12, 22 may close as instructed. Zero, some or all of the compressor feed valves 12, 22 may fail to close.
The method disclosed herein can thus comprise at least the steps of:

(a) passing a compressor first feed stream 10 having a compressor first feed valve 12 to a first inlet 18 of the first compressor 100; and passing a compressor second feed stream 20 having a compressor second feed valve 22 to a second inlet 28 of the first compressor 100;
(b) compressing the first and second compressor feed streams 10, 20 in the first compressor 100 to provide a compressed discharge stream 150 at the outlet 105 of the first compressor 100;
(c) monitoring for a first indicator of a blocked outlet event of the first compressor 100; and
(d) instructing the closure of both of the compressor first and second feed valves 12, 22 if the first indicator of the blocked outlet event of the first compressor 100 is detected.

A number of different scenarios, each having their own probability, exist when instructing the closure of the compressor feed valves 12, 22 in the first barrier system.
For instance, in the embodiment of FIG. 1, both the compressor first and second feed valves 12, 22 may close, one of the two compressor first and second feed valves 12, 22 may close, or neither of the compressor first and second feed valves 12, 22 may close in response to being instructed to do so. For example, if the feed valves 12, 22 are electronically actuated valves, one or more of the feed valves may fail to close due to valve freezing, or the failure of the electronic instructing signal to reach the valve.
The probability of each scenario occurring can be calculated by carrying out an appropriate risk assessment. If the probability of the successful closure of all of the compressor feed valves 12, 22 is acceptably high and the probability of one or more of the compressor feed valves 12, 22 failing to close is tolerably low, then the pressure relief valve 250 and the flare system 300 would not be required.
If the probability of at least one of the compressor feed valves 12, 22 failing to close is unacceptably high, then there is a high probability that at least one of the compressor feed streams 10, 20 would continue to supply the first compressor 100. During a blocked outlet event, this would lead to an increase in the pressure at the outlet 105 of the first compressor 100 and thus in the compressed discharge stream 150 and compressed continuing stream 160. In order to relieve this pressure, a pressure relief valve 250 in fluid connection with the compressed discharge stream 150 can be provided. The pressure relief valve 250 is configured to open in response to the increase in discharge pressure in as a result of a blocked outlet event, providing an outlet for the continuing compressed stream 160. The pressure relief valve 250 can be connected to a flare system 300 to process the portion of the compressed continuing stream 160 passing through the pressure relief valve 250.
If the probability of the successful closure of one or more of the compressor feed valves 12, 22 is acceptably high and the probability of none of the compressor feed valves closing is tolerably low, then the size of one or both of the flare system 300 and the relief valve 250 can be reduced compared to one which is designed to accommodate the output of all of the compressor feed streams 10, 20.
For example, if the probabilities of each of the feed valves 12, 22 failing to close are the same, the probability of increasing numbers of the feed valves failing to close decreases. In the case of FIG. 1 where there are two feed valves, if each feed valve has a probability of failing to close of 0.1, the probability of both feed valves failing to close is 0.01, the probability of one failing to close is 0.18, the probability of one or both failing to close is 0.19 and the probability of neither failing to close is 0.81. Thus, if the probability of 0.19 represents a tolerably low risk, then the apparatus can be provided without a pressure relief valve 250 and flare system 300.
However, if the probability of 0.19 of one or both of the feed valves failing was decided to represent an unacceptably high risk, while the probability of 0.01 of both feed valves 12, 22 failing to close is held to represent a tolerably low risk, then a pressure relief valve 250 can be provided in fluid communication with the compressed discharge stream 150. In this case the pressure relief valve 250 can be connected to a flare system 300. One or both of the pressure relief valve 250 and the flare system 300 may be sized to accommodate at least the relief flow from first compressor. In the embodiment of FIG. 1 the relief flow will correspond to the continuing compressed stream 160 generated by the compressor feed stream with the largest flow rate.
If the probability of 0.01 of both of the feed valves failing to close is held to represent an intolerably high risk, then one or both of the pressure relief valve 250 and the flare system 300 is sized to accommodate at least the flow from the continuing compressed stream 160 generated by both compressor feed streams 10, 20.
In one embodiment, the first barrier system can be provided with a first safety criterion, which defines a number of the compressor feed valves 10, 20 of which failure to close can be tolerated.
For instance, the first safety criterion may be set as the highest number of compressor feed valves which can be allowed to fail to close upon instructing them to close, without causing an intolerable risk such as e.g. causing loss of containment. This safety criterion can thus be used to calculate the minimum capacity of one or both of any pressure relief valve 250 and any flare system 300.
When the first safety criterion is set at the failure of zero feed stream valves, no pressure relief valve 250 or flare system 300 is required. This means that the failure to close of one or more of the feed valves has been calculated to be tolerably low by a risk assessment.
When present, one or both of the pressure relief valve 250 and the flare system 300 can be designed to accommodate at least the relief flow resulting from the compression of the compressor feed streams 10, 20 continuing to flow into the first compressor 100 during the failure to close of the number of feed valves 12, 22 defined in the first safety criterion.
Thus, in general, the term “relief flow” is defined as the flow which must be relieved after a blocked outlet event in order to prevent a further pressure increase between the outlet 105 of the first compressor 100 and the blockage or flow restriction. In the embodiment of FIG. 1 the relief flow is equivalent to the flow rate of the continuing compressed stream 160. This can be contrasted with the flow of the compressed discharge stream 150, which would also include the mass flow provided to the first compressor 100 by any recycle streams 110, 120.
For instance, if there are n compressor feed valves 12, 22, where n is an integer greater than or equal to 1, the first safety criterion can be defined as the failure of a number m of the compressor feed valves 12, 22 to close, where m is an integer in the range of from 0 to n, more preferably m is an integer in the range of from 0 to (n−1) or of from 1 to n, even more preferably m is an integer in the range of from 1 to (n−1).
Thus, in general, the relief flow for the first safety criterion can be defined as the flow resulting from the compression in the first compressor 100 of the m compressor feed streams 10, 20 in which the feed valves 12, 22 fail to close.
Thus, in the case of a first safety criterion in which m=n, the risk assessment has revealed that the probability of even one of the feed valves failing to close is intolerably high, such that one or both of the pressure relief valve 250 and the flare system 300 is sized to accommodate at least the relief flow generated by the compression of all of the compressor feed streams 10, 20.
In the case of a first safety criterion in which m<n, the failure of m+1 or more of the compressor feed valves 12, 22 to close has been calculated in a risk assessment to be tolerably low. The pressure relief valve 250 and/or flare system 300 can thus be sized to accommodate at least the relief flow produced by m compressor feed streams 10, 20.
The first safety criterion will be specific to a particular apparatus and set up. The value m will be statistically determined in a risk assessment such that the failure of m+1 or more valves to close represents a tolerable risk, having an acceptably low probability while the failure of m valves to close is of higher probability and represents an intolerable risk.
In one embodiment, the first safety criterion, m, may be determined by a stochastic analysis for n valves, for example by:

(i) calculating the probability for each of the n+1 possible outcomes of the number of valves closing or remaining open, which represent from 0 up to n of the feed valves failing to close e.g. 0, 1, 2, 3 etc. of the feed valves failing to close;
(ii) calculating the total probabilities for the scenarios in which “0”, “1 or more” to “n−1 or more” and “n” of the feed valves fail to close e.g. “0”, “1 or more”, “2 or more”, “3 or more” etc. of the feed valves failing to close;
(iii) selecting the scenario with the highest probability calculated in step (ii) which represents a tolerable risk, if one exists and;
(iv) setting the first safety criterion at the failure of one fewer valves than the lowest number of valves in the range selected in step (iii), unless a) all of the total probabilities calculated in step (ii) represent a tolerable risk, in which case the safety criterion is set at 0 valves, or b) none of the total probabilities calculated in step (ii) represent a tolerable risk, in which case the safety criterion is set at n valves.

The selection of the first safety criterion will now be explained by way of example for a system having four feed valves and using entirely hypothetical valve failure and tolerable risk probabilities. A system with four feed valves, in which each has a hypothetical and independent probability of failing to close of 0.1, the probabilities of the scenarios in which 0, 1, 2, 3 and 4 feed valves fail to close can be calculated as 0.656, 0.292, 0.049, 0.004 and 0.0001 respectively (step (i)). The total probabilities of 0, 1 or more, 2 or more, 3 or more and 4 feed valves failing to close can then be calculated as 0.6561, 0.3429, 0.0523, 0.0037 and 0.0001 respectively (step (ii)). If a tolerable risk is set at a scenario probability of 0.01 or less, then 3 or more feed valves failing to close is calculated to occur at the highest probability which is determined to represent a tolerable risk (step (iii)). The safety criterion can be set at 2 feed valves failing to close (step (iv)).
Returning to FIG. 1, in the situation in which the first safety criterion is defined as the failure of m=0 of the compressor feed stream valves 12, 22 to close i.e. all the compressor feed stream valves 12, 22 will close as instructed, an apparatus is provided in which it has been determined that there is a tolerably low risk of 1 or more valves failing to close. This means that the pressure relief valve 250 and flare system 300 is unnecessary and need not be present, or if a communal flare system 300 is present which is shared by multiple process units e.g. two or more compressors, such a communal flare system need not be sized to accommodate the relief flow produced by any of the compressor feed streams 10, 20.
In the embodiment of FIG. 1, there are two compressor feed streams 10, 20 and two compressor feed valves 12, 22. The safety criterion of the failure of one of the compressor feed valves 12, 22 to close could be selected if the probability of this event occurring was unacceptably high, while the probability of both of the compressor feed valves 12, 22 failing to close was tolerably low.
One or both of the pressure relief valve 250 and flare system 300 can be sized to accommodate at least the relief flow generated by the first safety criterion i.e. the failure to close of 1 of the 2 feed valves 12, 22. Thus, the flare system is sized to process a relief flow produced by a single compressor feed stream i.e. one or other of the compressor first and second feed streams 10, 20.
In the situation where the compressor feed streams 10, 20 have different mass flows, the flare system should be sized to process a relief load generated by the compressor feed stream 10, 20 with the larger mass flow. In general, if the safety criterion is set at a number m feed valves 12, 22 failing to open, then the flare system 300 should be sized to process the relief load generated by the m streams with the largest mass flow.
In a preferred embodiment, at least the pressure relief valve 250 is sized to accommodate at least the relief flow generated by the first safety criterion.
In a further embodiment, the pressure relief valve 250 is sized to at least accommodate the relief flow generated by the first safety criterion, while the size of the flare system 300 may be sufficient to accommodate flows greater than the relief flow generated by the first safety criterion. Such a situation can occur where the flare system 300 is shared with one or more other processing units which provide the limiting case on the capacity of the flare system 300 i.e. the other processing units require a greater flare capacity than the flow generated by the first safety criterion. This can occur, for instance, where the method and apparatus disclosed herein is used to protect a main refrigerant compressor in a LNG liquefaction plant in which the flare system is sized to accommodate the relief load from a propane pre-cooling stage which will be the limiting case.
In a still further embodiment in which the pressure relief valve 250 is sized to at least accommodate the relief flow generated by the first safety criterion, the flare system 300 may be sized to accommodate less than the relief flow generated by the first safety criterion. In this case, if a blocked outlet event was to occur and the pressure relief valve 250 was to open, protection against a loss of containment would be achieved. However, the relief flow generated by the first safety criterion would exceed the capacity of the flare system 300, resulting in excessive flaring and potential thermal radiation damage to the flare system 300, such as the flare stack.
In another embodiment, both the pressure relief valve 250 and flare system 300 have minimum sizes to accommodate at least the relief flow generated by the first safety criterion.
It will be apparent that when the first safety criterion has m valves failing to close, and the number m is less than n, the total number of feed valves, the size of one or both of the pressure relief valve 250 and flare system 300 which is constructed to accommodate the relief flow can be reduced compared to a pressure relief valve and/or flare system sized to accommodate a relief load from a combination of all of the compressor feed streams 10, 20. This can generate significant CAPEX savings because smaller pressure relief valves and flare systems, especially flare stacks, are considerably less expensive than larger sized units.
Thus, in a further preferred embodiment applicable to all aspects disclosed herein, one or both of the pressure relief valve 250 and flare system 300 may be sized to accommodate less than the flow generated from the compression of the n compressor feed streams 10, 20 in the first compressor 100. Still more preferably, one or both of the pressure relief valve 250 and flare system 300 may be sized to accommodate a relief flow of at least the relief flow generated by the first safety criterion and less than the flow generated from the compression of all of the n compressor feed streams 10, 20 in the first compressor 100.
In another embodiment, one or both of the pressure relief valve 250 and flare system 300 may be sized to accommodate at most substantially the relief flow generated by the first safety criterion. For instance, one or both of the pressure relief valve 250 and flare system 300 may be sized to accommodate at most preferably 120%, more preferable 110%, even more preferably 105% of the relief flow generated by the first safety criterion.
Viewed another way, if the method and apparatus disclosed herein is introduced into a plant in which the flare system is designed to handle the entire relief load generated from all of the compressor feed streams, flare system capacity can be advantageously freed for other purposes when the number of feed stream valves m failing to close in the first safety criterion is less than the total number of feed stream valves n.
The first barrier system can be provided by an Instrumentive Protective Function (IPF). This comprises a first monitoring device P1 to monitor for a first indicator of a blocked outlet event of the first compressor 100, and a first controller XC1. The first indicator can be one or more of the group comprising: a selected discharge pressure of the compressed discharge stream 150, a selected discharge temperature of the compressed discharge stream 150, a selected power of the first compressor 100, a selected suction pressure of the one of more compressor feed streams 10, 20 and a selected suction temperature of the one or more compressor feed streams 10, 20. The first monitoring device P1 can be set with a first selected value, such as a pressure, temperature or compressor power, which is a first indicator of a blocked outlet event. When the first selected value is detected, the first monitoring device P1 can transmit a first signal to the first controller XC1.
For example, the first monitoring device P1 can be a pressure sensor in the compressed discharge stream 150. When the pressure reaches a first selected value indicative of a blocked outlet event, a first signal is transmitted to the first controller XC1.
Upon receiving the first signal indicating that a blocked outlet event has occurred, the first controller XC1 can send first instructing signals to the compressor feed valves, 12, 22, which can be electrically actuated valves, instructing their closure.
In addition, the first controller XC1 can send second instructing signals to the recycle valves 112, 122, which can be electrically actuated valves, instructing their opening to allow recycle of the compressed discharge stream 120 to the inlets 18, 28 of the first compressor 100 to prevent surge. However, it is preferred that the first barrier system is entirely independent of any system which may protect the first compressor against surge, or other systems which may protect the first compressor against high discharge temperature, vibration or the like.
If successful, the first barrier system can prevent a further increase in the pressure at the outlet 105 of the first compressor 100. This will occur if all of the compressor feed valves 12, 22 successfully close as instructed.
If one or more of the compressor feed valves fail to close, the first compressor discharge pressure may increase until any second barrier system engages, or the pressure relief valve 250 opens.
The second barrier system disclosed herein is preferably independent of the first barrier system. The second barrier system should also be independent of any compressor safeguarding systems, such as a surge protection system. The aim of the second barrier system is to lower the compression power of the first compressor 100 when a second indicator of a first compressor blocked outlet event is detected. This may include a reduction of the compression power of the first compressor to zero, such that the operation of the first compressor 100 is halted.
The second barrier system monitors for a second indicator of a blocked outlet event of the first compressor 100. The second indicator can be one or more of the group comprising: a selected discharge pressure of the compressed discharge stream 150, a selected discharge temperature of the compressed discharge stream 150, a selected power of the first compressor 100, a selected suction pressure of the one of more compressor feed streams 10, 20 and a selected suction temperature of the one or more compressor feed streams 10, 20.
The second indicator may be the same as or different to the first indicator discussed above. Furthermore, the second and first indicators may measure the same property, such as temperature, pressure or first compressor power, but may be selected to be different values. Thus, selection of the second indicator can provide a second barrier system which is configured to activate: under the same conditions as the first barrier system e.g. if the same activation criteria are used for both systems; before the first barrier system; or after the first barrier system. For instance, the first and second indicators of a blocked outlet event of the first compressor 100 can be selected as the same or different pressure levels of the compressed discharge stream 150.
Upon detection of the second indicator of the first compressor blocked outlet event, a reduction in the compression power of the first compressor 100 is instructed. In a preferred embodiment, the compressor power is instructed to be reduced to zero.
The first compressor 100 can be mechanically driven by first driver 200, which can be an electrical driver as shown in FIG. 1. Alternatively the compressor 100 can be mechanically driven by a gas or steam turbine (not shown). The reduction in compression power can be achieved by one or more of the following procedures: reducing the speed of the first driver 200 and reducing the supply of power to the first driver 200.
In the case that the first driver 200 is an electrical driver, the electrical power supplied to the first driver can be reduced, or, if it is desired to reduce the compression power to zero, the circuit supplying electrical power to the first driver can be broken, as shown by switch 210 in FIG. 1.
If the first compressor 100 is mechanically driven by a gas or steam turbine, the supply of fuel to the gas turbine, or the supply of steam to the steam turbine can be reduced, thereby reducing the speed of the first driver. If desired, the supply of fuel or steam could be stopped entirely, halting the first driver 200 and thereby stopping the operation of the first compressor 100.
A reduction in the power of the first compressor 100 may prevent a further increase in the pressure at the outlet 105 of the compressor.
The second barrier system can be provided by an Instrumentive Protective Function (IPF). This comprises a second monitoring device P2 to monitor for a second indicator of a blocked outlet event of the first compressor 100, and a second controller XC2. The second monitoring device P2 can be set with a second selected value, such as a pressure, temperature or compressor power, which is a second indicator of a blocked outlet event. When the second selected value is detected, the second monitoring device P2 can transmit a second signal to the second controller XC2.
For example, the second monitoring device P2 can be a pressure sensor in the compressed discharge stream 150. In order to provide independence from the first barrier system the second monitoring device P2 is preferably a separate device from the first monitoring device P1. When the pressure reaches the second selected value, a second signal is transmitted to the second controller XC2 indicating that a blocked outlet event has occurred.
Upon receiving the second signal, the second controller XC2 can send a third instructing signal to the appropriate unit to reduce the first compressor power. For instance, the third instructing signal may be transmitted to a valve, preferably an electrically actuated valve such as a solenoid operated valve, to restrict the flow of fuel or steam to the gas or steam turbine first driver 200 respectively. If the first driver 200 is an electric motor, then the third instructing signal can be provided to the motor to reduce power, or to a electrical distribution system to reduce the electrical power supplied to the electric driver, or to an electrical switch 210 in the electric driver power circuit if it is intended to stop the driver completely.
In a preferred embodiment the second barrier system is operated in combination with the first barrier system. For instance, if the first indicator of the blocked outlet event is set at such a level that it would occur before that of the second indicator e.g. at a lower first compressor discharge pressure than the second indicator, then upon detection of the first indicator of a blocked outlet event, the compressor feed valves 12, 22 will be instructed to close. If all of the compressor feed valves 12, 22 successfully close, then any further increase in pressure at the outlet 105 of the first compressor 100 will not be possible, and the first barrier system will have operated successfully.
However, in the event that one or more of the compressor feed valves 12, 22 fail to close, the pressure at the outlet 105 of the first compressor 100 will continue to rise until the second indicator of the blocked outlet event is reached. Upon reaching the second indicator, the second barrier system will activate, instructing a reduction in first compressor power. More preferably, an instruction is given to reduce the first compressor power to zero.
In the case of a gas or stream turbine first driver, instructions will be sent to close one or more valves in the fuel gas or steam streams respectively, such as the stream control valve and the trip valve. Similarly, if the first driver 200 is an electric motor, instructions can be sent to open one or more switches 210 to break the electrical circuit supplying power to the first driver 200. If at least one of these valves successfully close or switches 210 successfully open, the first driver 200 will stop and the first compressor 100 will halt operation. Any further increases in the pressure at the discharge of the first compressor will be halted.
The combination of first and second barrier systems can result in significant reductions in the probability of a pressure increase at the outlet of a compressor above the design pressure after a blocked outlet event.
The first set pressure of the pressure relief valve 250 should be set at a level above that of the first indicator, to ensure that the closure of the feed valves 12, 22 is instructed before the pressure relief valve 250 is opened.
In a preferred embodiment, the first set pressure of the pressure relief valve 250 is set at a level above the first indicator of a blocked outlet event of the first barrier system and any second indicator if a second barrier system is present.
If not all of the feed valves 12, 22 close whilst still meeting the first safety criterion e.g. if this is set at one of the feed valves 12, 22 failing to close, and the optional second barrier system is unsuccessful in reducing the power of the first compressor 100, then the pressure at the outlet 105 of the first compressor will continue to rise until the first set pressure of the pressure relief valve 250 is reached, at which point the relief valve 250 will open and at least a part of the compressed discharge stream 150 will be passed to the flare system 300 as continuing compressed stream 160 and no further pressure increase is expected.
The advantages of the method and apparatus disclosed herein can be exemplified with reference to a LNG export facility. The size of the flare system of a LNG export facility is determined by the relief load which must be dealt with during a blocked outlet of the pre-cooling compressor. If the relief load from this case could be reduced by approximately 50%, then other relief load cases would govern the size of the flare system. In some situations, this could allow a reduction in flare height from approximately 180 m to 120 m, representing a significant size and weight reduction.
In certain situations where the space is limited, such as in off-shore LNG plants, this represents significant cost savings in off-shore vessel or platform construction.
In a preferred embodiment, the method and apparatus disclosed herein is particularly suitable for location on a floating vessel, an off-shore platform or a caisson. A floating vessel may be any movable or moored vessel, generally at least having a hull, and usually being in the form of a ship such as a ‘tanker’.
Such floating vessels can be of any dimensions, but are usually elongate. Whilst the dimensions of a floating vessel are not limited at sea, building and maintenance facilities for floating vessels may limit such dimensions. Thus, in one embodiment disclosed herein, the floating vessel or off-shore platform is less than 600 m long such as 500 m, and a beam of less than 100 m, such as 80 m, so as to be able to be accommodated in existing ship-building and maintenance facilities.
An off-shore platform may also be movable, but is generally more-permanently locatable than a floating vessel. An off-shore platform may also float, and may also have any suitable dimensions.
It will be apparent how the concept described in FIG. 1 can be extended to other embodiments, in which more than 2 compressor feed streams are present. In a preferred embodiment, the method and apparatus disclosed herein can be applied to a refrigeration compressor having four compressor feed streams.
FIG. 2 shows an embodiment in which the first compressor 100 is provided with four compressor feed streams 10, 20, 30, 40. The scheme shown in FIG. 2 represents a simplified refrigerant circuit, such as the propane pre-cooling stage of a propane refrigerant pre-cooling, mixed refrigerant main cooling system (C3MR) disclosed in U.S. Pat. No. 4,404,008.
First compressor 100 can be a refrigerant compressor fed by compressor first, second third and fourth feed streams 10, 20, 30, 40 at first, second, third and fourth inlets 18, 28, 38, 48 respectively. The first compressor 100 provides a compressed discharge stream 150 as a compressed refrigerant discharge stream at outlet 105. For simplicity, no recycle streams are shown in FIG. 2.
In this embodiment, the method disclosed herein comprises as step (a) passing a compressor first feed stream 10 having a compressor first feed valve 12 to a first inlet 18 of the first compressor 100; passing a compressor second feed stream 20 having a compressor second feed valve 22 to a second inlet 28 of the first compressor 100; passing a compressor third feed stream 30 having a compressor third feed stream valve 32 to a third inlet 38 of the first compressor 100; and passing a compressor fourth feed stream 40 having a compressor fourth feed stream valve 42 to a fourth inlet 48 of the first compressor 100.
After compression, the compressed refrigerant discharge stream 150 can be passed through one or more cooling devices 400, valves 350, 550, accumulators 500 and/or heat exchangers 600, which may cause a blocked outlet event.
FIG. 2 shows the compressed discharge stream 150 being passed through a discharge isolation valve 350, to provide a controlled compressed refrigerant stream 360. If the discharge isolation valve 350 were to fail in a closed position, a compressor blocked outlet event would occur.
The controlled compressed refrigerant stream 360 can then be cooled in one or more cooling devices 400, such as air or water coolers, to provide a cooled refrigerant stream 410. The cooled refrigerant stream 410 can be used directly to cool a stream 620, such as a hydrocarbon stream, or be further treated prior to the cooling step, such that it is a stream derived from the cooled refrigerant stream 410 which may ultimately provide the cooling. Preferably the cooled refrigerant stream is at least partially condensed, more preferably essentially fully condensed, whereby at least part of the condensed refrigerant is evaporated against stream 620 whereby heat is withdrawn from stream 620.
A cooling failure in one of the cooling devices 400 will result in a loss of condensing capability and an increase in the pressure of the compressed discharge stream 150 at the outlet 105 of the compressor. Such a cooling failure could occur if a cooling stream to the cooling device 400, such as a cooling water stream, were to fail. This can cause a blocked outlet event.
In addition, build up of inert material in any cooling device 400, such as in a condenser, can also lead to a blocked outlet event. Such inert material can be produced by a tube rupture, for instance in a heat exchanger 600, such as a coil wound heat exchanger, which is in fluid communication with the first compressor 100.
The cooled refrigerant stream 410 may be passed to an accumulation device 500 which can provide an accumulator discharge stream 510. The accumulator discharge stream 510 can be passed through an accumulator discharge isolation valve 550 to provide a controlled accumulator discharge stream 560. Should the accumulator discharge isolation valve 550 fail in a closed position, a blocked outlet event could occur.
Controlled accumulator discharge stream 560 can be passed to one or more heat exchangers 600. When there is more than one heat exchanger 600, such heat exchangers may be arranged in series or parallel, and may be supplied by streams derived from the controlled accumulator discharge stream 510, for instance after expansion if the heat exchangers 600 are at different pressures. In the propane pre-cooling stage of a C3MR process, four heat exchangers 600 can be arranged in series, each operating at a different pressure, to provide the four compressor feed streams 10, 20, 30, 40. FIG. 2 shows only a single heat exchanger 600 for simplicity.
Controlled accumulator discharge stream 560 can be heat exchanged against a hydrocarbon stream 620 and/or another refrigerant stream (not shown) in the heat exchanger 600, to provide a cooled hydrocarbon stream 630 and/or a cooled refrigerant stream (not shown) and a heated refrigerant stream 610. Heated refrigerant stream 610 can be further treated before returning to the first compressor 100 as the compressed first, second, third or fourth feed stream 10, 20, 30, 40.
The method and apparatus disclosed herein is preferably used in an LNG plant, such that the cooled hydrocarbon stream 630 can be a cooled natural gas stream, such as a pre-cooled natural gas stream, for example at a temperature of −35° C. or a partly or fully liquefied natural gas stream. A cooled refrigerant stream, such as a mixed refrigerant stream, can also be provided by the one or more heat exchangers 600.
Natural gas is comprised predominantly of methane. In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water, other non-hydrocarbon acid gases, ethane, propane, butanes, C₅+ hydrocarbons and aromatic hydrocarbons. These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.
Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGLs) from a natural gas stream. The methane is usually separated from the NGLs in a high pressure scrub column, and the NGLs are then subsequently fractionated in a number of dedicated distillation columns to yield valuable hydrocarbon products, either as product steams per se or for use in liquefaction, for example as a component of a refrigerant.
Should a blocked outlet event occur in relation to first compressor 100, the first indicator of a blocked outlet event will be detected by the first monitoring device P1. For instance a pressure of a level indicative of a blocked outlet event will be detected by a first pressure sensor P1 in communication with the compressed refrigerant discharge stream 150. The first barrier system will then instruct the closure of compressor first, second, third and fourth feed valves 12, 22, 32 and 42.
Thus in the method of the embodiment of FIG. 2, step (d) comprises instructing the closure of the compressor first, second, third and fourth feed valves 12, 22, 32, 42 if the first indicator of the blocked outlet event of the first compressor 100 is detected.
Five first safety criteria may be applicable to the disclosed refrigeration system, in which either none, one, two, three or four of the four feed valves 12, 22, 32, 42 fail to close. Which safety criteria to be selected will depend upon the probabilities of these scenarios occurring.
Should the probability of one or more of the feed valves remaining open be tolerably small, then the first safety criterion could be set at zero valves failing to close and the apparatus provided without a pressure relief valve 250 or flare system 300.
Should the probability of two or more of the feed valves remaining open be tolerably small, but the probability of one of the feed valves remaining open be unacceptably high, then the first safety criteria can be set at one feed valve failing to close and the flare system 300 sized to accommodate the flow generated by the compressor feed stream having the largest flow rate.
Should the probability of three or more of the feed valves remaining open be tolerably small, then the first safety criteria can be set at two feed valves failing to close and the flare system 300 sized to accommodate the flow generated by the two compressor feed streams having the two largest flow rates. It will be apparent that such a flare system 300 will be larger than that in the embodiment of the previous paragraph, but still smaller than a flare system sized to handle the flow generated from the three or four compressor refrigerant feed streams 10, 20, 30 with the highest flow rates.
Should the probability of all four of the feed valves remaining open be tolerably small, then the first safety criteria can be set at three feed valves failing to close and the flare system 300 sized to accommodate the flow generated by the three compressor feed streams having the three largest flow rates. It will be apparent that such a flare system 300 will be larger than that in the embodiment of the previous paragraph, but still smaller than a flare system sized to handle the flow generated from all four compressor refrigerant feed streams 10, 20, 30, 40.
Alternatively should the probability of all four of the feed valves failing to close be unacceptably high, then the first safety criteria can be set at all four feed valves failing to close and the flare system 300 sized to handle the full flow from the four compressor feed streams.
Should any of the four feed valves 12, 22, 32, 42 fail to close under instruction of the first barrier system, the pressure at the outlet 105 of the refrigerant compressor 100 will continue to rise. If a second barrier system is present with the second indicator chosen to occur after the first indicator, the second indicator of a blocked outlet event will then be detected. If the second monitoring device P2 is a pressure sensor in communication with the compressed refrigerant discharge stream 150, then this would be set to occur when a pressure higher than that of the first indicator is reached.
FIG. 2 shows a steam turbine 200 b, powered by a high pressure steam stream 240 passing through a control valve 220 and a trip valve 230. Upon detection of the blocked outlet event by the second barrier system, third instruction signals can be sent from second controller XC2 to close control valve 220 and trip valve 230. Should either of these valves close, the high pressure steam stream 240 will be prevented from reaching the steam turbine 200 b and the turbine stopped, thereby halting the first refrigerant compressor 100.
In an alternative embodiment, the steam turbine 200 b may be halted by removing the cooling duty of the steam condenser 240 of steam turbine 200 b, thereby reducing the power to the first refrigerant compressor 100.
Should the second barrier system be successful, any further increase in the pressure at outlet 105 of the first refrigerant compressor 100 will be prevented. Opening of the pressure relief valve 250 can be avoided such that no flaring of the inventory of the first compressor 100 and related circuit occurs.
If the first and second barrier systems are unsuccessful, while still meeting the first safety criterion, then the pressure relief valve 230 will open when its set point pressure is reached and pass the compressed refrigerant equal to the flow from the refrigerant feed streams 10, 20, 30, 40 with feed valves 12, 22, 32, 42 remaining open to the flare system 300.
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, it will be apparent that the present invention can be utilised with any compressor system, and not just the refrigerant circuit specifically disclosed. In addition, although embodiments utilising 2 and 4 compressor feed streams have been discussed in detail, the present invention can also be applied to any compressor having 1 or more compressor feed streams.

Claims

1. A method of operating a compressor, for the reduction of relief load during a blocked outlet event, comprising at least the steps of:

(a) passing one or more compressor feed streams to one or more inlets of a first compressor, each compressor feed stream passing through a compressor feed valve;

(b) compressing the one or more compressor feed streams in the first compressor to provide a compressed discharge stream at the outlet of the first compressor;

(c) monitoring for a first indicator of a blocked outlet event of the first compressor;

(d) instructing the closure of the compressor feed valves upon detection of the first indicator of the first compressor blocked outlet event.

2. The method of claim 1, wherein:

in step (a) there are n compressor feed streams passing through n compressor feed valves, in which n is an integer greater than or equal to 1;

in step (c) a first monitoring device monitors for the first indicator of the blocked outlet event of the first compressor; and further comprising the step of:

(e) providing a first safety criterion in which m of the compressor feed valves fail to close in step (d), wherein m is an integer in the range of from 0 to n.

3. The method of claim 2, further comprising the step of:

(f) providing a pressure relief valve in fluid communication with the compressed discharge stream, said pressure relief valve set to open at a first set pressure, which is reached after the first indicator of the blocked outlet event of the first compressor;

said pressure relief valve being connected to a flare system; and

wherein one or both of said pressure relief valve and said flare system is sized to accommodate at least the relief flow generated by the first safety criterion in which m of the compressor feed valves fail to close in step (d), wherein m is an integer in the range of from 1 to n.

4. The method of claim 1, wherein step (a) comprises: passing a compressor first feed stream having a compressor first feed valve to a first inlet of the first compressor; and passing a compressor second feed stream having a compressor second feed valve to a second inlet of the first compressor.

5. The method of claim 1, further comprising the steps of:

(g) driving the first compressor with a first driver;

(h) monitoring, for a second indicator of a first compressor blocked outlet event with a second monitoring device; and

(i) instructing a reduction in the compression power of the first compressor when the second indicator of the first compressor blocked outlet event is detected.

6. The method according to claim 5, wherein the reduction in the compression power of the first compressor is achieved by one or more of the group comprising: reducing the speed of the first driver and reducing the supply of power to the first driver.

7. The method according to claim 5, wherein the first driver is a steam turbine and the reduction in the compression power of the steam turbine is achieved by one or more of the group comprising: reducing the flow of steam to the turbine and removing the cooling duty of a steam condenser of the steam turbine.

8. The method according claim 5, wherein the first set pressure is reached after the first and second indicators of the first compressor blocked outlet event are reached.

9. The method according to claim 5, wherein:

(i) the first indicator and the second indicator are the same; or

(ii) the first indicator is selected such that it occurs before the second indicator; or

(iii) the second indicator is selected such that it occurs before the first indicator.

10. The method of claim 1, in which the indicator of a first compressor blocked outlet event is selected from one or more of the group consisting of: a selected discharge pressure of the compressed discharge stream, a selected discharge temperature of the compressed discharge stream, a selected power of the first compressor, a selected suction pressure of the one of more compressor feed streams, and a selected suction temperature of the one or more compressor feed streams.

11. The method according to claim 1, in which the first compressor is a refrigerant compressor, the two or more compressor feed streams are refrigerant feed streams and the compressed discharge stream is a compressed refrigerant discharge stream, further comprising the steps of:

(j) cooling a stream derived from the compressed refrigerant discharge stream to provide a cooled refrigerant stream; and

(k) heat exchanging the cooled refrigerant stream, or a stream derived therefrom against a hydrocarbon stream or a further refrigerant stream to provide a cooled hydrocarbon stream.

12. The method according to claim 11, wherein the hydrocarbon stream is a natural gas stream and the cooled hydrocarbon stream is a cooled natural gas stream, such as an at least partially liquefied natural gas stream.

13. An apparatus for the reduction of relief load during a blocked outlet event of a compressor, comprising at least:

a first compressor having an outlet for a compressed discharge stream and one or more inlets for one or more compressor feed streams, each compressor feed stream passing through a compressor feed valve; and

a first controller, the first controller in communication with a first device to provide a first indicator of a blocked outlet event of the first compressor, said first controller in further communication with each compressor feed valve, such that the first controller transmits a first signal to close each compressor feed valve upon detection of the first indicator of the blocked outlet event of the first compressor.

14. The apparatus according to claim 13, wherein there are n compressor feed streams passing through compressor feed valves, in which n is an integer greater than or equal to 1; and further comprising:

a pressure relief valve in fluid communication with the compressed discharge stream, said pressure relief valve being sized to open at a first set pressure, which is reached after the first indicator of the first compressor blocked outlet event; and

a flare system in fluid communication with the pressure relief valve, wherein one or both of pressure relief valve and the flare system is sized to accommodate at least the relief flow generated by a first safety criterion in which m of the compressor feed valves fail to close in step (d), wherein m is an integer in the range of from 1 to n.

15. The apparatus according to claim 13, further comprising:

a first driver mechanically connected to the first compressor to drive the first compressor; and

a second controller in communication with a second device, the second device to provide a second indicator of a first compressor blocked outlet event, such that said second controller transmits a third signal to reduce the compression power of the first compressor when the second indicator of a first compressor blocked outlet event is detected.