WO1998025122A1

WO1998025122A1 - System and method for leak detection

Info

Publication number: WO1998025122A1
Application number: PCT/US1997/022357
Authority: WO
Inventors: Robert O. Homes
Original assignee: Bell Avon, Inc.
Priority date: 1996-12-05
Filing date: 1997-12-05
Publication date: 1998-06-11

Abstract

A fluid containment and leak detection system and method includes a rigid outer tank (12) with an inner flexible bladder (24) of complementary configuration positioned within the outer tank (12) and adjacent the inner surface of the outer tank (12) such that an interstitial space (26) is formed between the inner bladder (24) and outer tank (12). A vacuum source (36) having a vacuum line (28) is fluidly connected to the interstitial space (26) for periodically applying a pressure to the interstitial space for drawing the inner bladder (24) toward the inner surface of the outer tank (12). A pressure transducer (44) is mounted to the tank (22) for measuring the amount of pressure in the interstitial space and for generating electrical signals proportional to the measured amount of pressure. A controller (46) is operatively connected to the pressure transducer (44) for receiving the electrical signals therefrom for computing a pressure decay slope in response to reiterative electrical signals from the pressure transducer (44) and for generating a warning signal when the pressure decay slope thus computed is greater than a predetermined slope. A pair of annular rings (56, 66) which slope inwardly and downwardly capture the inner bladder (24) at a manhole opening (16) to maintain the seal between the outer tank (12) and the inner bladder (24) when a manhole cover (20) is removed.

Description

SYSTEM AND METHOD FORLEAKDETECTION

BACKGROUND OFTHE INVENTION

Field of the Invention

This invention relates to large, multiple-walled fluid storage tanks, and more particularly to a system and method of detecting leaks in multiple-walled fluid storage tanks. In one of its aspects, the invention relates to a system and method for determining in real time and with a high degree of accuracy the condition of liquid storage tanks that are surrounded by or lined with a fluid-tight envelope with a small interstitial space between the envelope and tank. In another of its aspects, the invention relates to a sealing arrangement at the manhole opening of a liquid storage tank having an inner flexible liner such that a manhole cover can be removed for gaining access inside the liner without disturbing the seal between the liner and tank.

Description of the Related Art

Flexible or semi-rigid liners are positioned within metallic or fiberglass liquid storage tanks such as underground gasoline storage tanks, in order to prevent fluid such as gasoline from leaking out of the tank into the soil surrounding the tank. U.S. Patent No. 5,184,504 to Spring discloses a system and method for detecting leakage from a tank that is surrounded by or lined with a fluid-tight envelope with a small interstitial space between the envelope and tank. A high-vacuum system is used to withdraw liquid from the interstitial space and determine if a leak has occurred when the high- vacuum system is operated for periods of greater frequency and/or longer duration than would be predicted. Alternatively, evacuated volumes are measured and volumes greater than predicted serve as an indication that a leak has occurred. A low- vacuum system is also employed to maintain a negative pressure in the interstitial space and to provide an indication of leakage when the low- vacuum system operates at a greater frequency and/or for periods of greater duration than predicted. Thus, leak detection occurs only after a predetermined frequency is detected in the high and/or low- vacuum systems or when the volume of liquid in the interstitial space reaches a predetermined volume. SUMMARY OF THE INVENTION

According to the invention, a method for determining leakage in a fluid holding tank having first and second concentric containers with a relatively fixed interstitial space between the first and second containers, includes the steps of: reducing the pressure in the interstitial space to a first predetermined level; reiteratively measuring the pressure in the interstitial space over a first predetermined time period to obtain a first measured pressure decay slope; comparing the measured pressure decay slope with a first predetermined slope; and generating a warning signal when the pressure decay slope is greater than the first predetermined slope. The method according to the invention further includes comparing the measured pressure decay slope with a second predetermined slope and generating a warning signal when the measured pressure decay slope is greater than the second predetermined slope. Preferably, the first predetermined slope is less than the second predetermined slope. If the measured pressure decay slope is less than the second predetermined slope, the pressure in the interstitial space is reiteratively measured over a second predetermined time period to obtain a second measured pressure decay slope. The second measured pressure decay slope is then compared with the first predetermined slope and a warning signal is generated if the second measured pressure decay slope is greater than the first predetermined slope. Preferably, the second predetermined time period is shorter than the first predetermined time period. Further according to the invention, the first predetermined slope is obtained by first determining the volume of the interstitial space; then applying a reduced pressure to the interstitial space while the tank is filled with air; and measuring any change in pressure over time due to unavoidable system leakage including permeation of air through the inner container and/or outer container. The volume V, of the interstitial space is determined by measuring a first pressure P, in the interstitial space; connecting a known volume of gas V₂ at a known pressure P₂ with the interstitial space; measuring the pressure P₃ of the combined volume V, and V₂ of gas and calculating the volume V, of the interstitial space in accordance with the formula: {V₂ - P₂ - P₃ - V₂ )

V, wherein P, is the initial pressure of a volume of gas in the interstitial space, V₂ is the known second volume of gas connected to the interstitial space, P₂ is the pressure of the known second volume of gas before injecting into the interstitial space, P₃ is the pressure of the first and second volume of gas in the interstitial space and V, is the volume of gas in the interstitial space.

Preferably, the inner tank is a flexible membrane which forms a liner for the outer tank. The vacuum is sufficient to draw the flexible membrane tightly against the inner surface of the outer tank which is preferably a rigid tank made, for example, of steel.

In accordance with a further embodiment of the invention, a fluid containment and leak detection system comprises an outer tank and an inner tank, preferably a flexible bladder, of complementary configuration to the outer tank positioned within the outer tank and adjacent the inner surface of the outer tank such that an interstitial space is formed between the inner and outer tanks. A vacuum source having a vacuum line is fluidly connected to the interstitial space for periodically reducing the pressure in the interstitial space. A pressure transducer is coupled to the interstitial space for measuring the amount of pressure in the interstitial space and for generating electrical signals proportional to the measured amount of pressure. A controller is operatively connected to the pressure transducer for receiving the electrical signals therefrom for computing a pressure decay slope in response to reiterative electrical signals from the pressure transducer and for generating a warning signal when the pressure decay slope thus computed is greater than a predetermined slope.

According to an even further embodiment of the invention, a fluid containment system comprises a rigid outer tank with at least one access opening and a wall surrounding the opening. An inner flexible bladder is complementary in configuration to the rigid outer tank and is positioned within the outer tank such that an interstitial space is formed between the inner bladder and outer tank. The bladder includes an opening in alignment with the tank opening and a flexible flange portion surrounding the bladder opening. A first annular member has an outer surface in sealing engagement with an interior surface of the wall and an inner surface adapted for sealing engagement with an outer surface of the flexible flange portion. A second annular member is sized to be received within the first annular member and has an outer surface adapted for sealing engagement with an inner surface of the flexible flange portion such that the flange portion is sandwiched between the first and second annular members to thereby form a seal between the outer tank and inner bladder and isolate the interstitial space from atmosphere. The second annular member has a central opening coincident with the access opening for communication with an interior of the bladder. In this manner, a tank cover can be removed from the outer tank opening without disturbing the seal between the outer tank and inner bladder.

In a preferred embodiment of the invention, the inner surface of the first annular member and the outer surface of the. second annular member slope inwardly toward a center of the access opening. One or more grooves are formed in the inner surface of the first annular member and an O-ring is located within each groove. A portion of the O-ring extends outwardly of the groove and into sealing contact with the flexible flange.

Further according to the invention, a method of monitoring a relatively fixed interstitial space between multiple-walled storage tanks comprising an inner tank and an outer tank wherein the change in volume within the interstitial layer is reiteratively measured and compared to determine whether a leak has occurred in either of the inner or outer tanks. In accordance with the invention, the inner tank is filled with a liquid and the interstitial space is sealed from the atmosphere. The pressure of the volume of gas in the interstitial space is measured. The interstitial space is then connected with a known second volume of gas at a known pressure. While the interstitial space is connected to the known volume of gas, the pressure of the first and second volume of gas in the interstitial space is measured. The volume of the gas in the interstitial space is then computed and stored. These same steps are repeated reiteratively to determine subsequent readings for the volume of gas in the interstitial space. The subsequent readings are compared to determine whether a leak has occurred in the interstitial space. Desirably, the change in volume over a particular period of time is measured and compared with a predetermined rate of volume decrease. If the measured rate of decrease of the volume exceeds a predetermined rate, an alarm sounds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings in which:

FIG. 1 is a cross-sectional view of a multiple-walled storage tank and a leak detection system according to the present invention;

FIG. 2 is an enlarged cross-sectional exploded view of an upper portion of the multiple-walled storage tank of FIG. 1 showing a liner sealing assembly according to the invention;

FIG. 3 is an enlarged cross-sectional view of a portion of the liner sealing assembly of FIG. 2 in its assembled position;

FIG. 4 is an operational logic diagram for initializing the leak-detection system when first placed into operation;

FIG. 5 is a flow diagram of a method for determining whether a leak has occurred; and

FIG. 6 is a flow diagram of a method for monitoring the interstitial space between a multiple-walled storage tank according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 , a leak-detection system 10 constructed in accordance with the present invention is shown. An external storage tank 12 is typically located underground and is used to hold corrosive or volatile liquid materials, such as gasoline or diesel oil. The external storage tank 12 can alternatively be located above the ground, and can be used to store fluids of various types. The external tank 12 is constructed of a rigid material such as metal, fiberglass or synthetic resin. The tank 12 includes a manhole 14 with an annular wall 54 (FIG. 2) and a manhole cover 20 that normally closes a manhole opening 16. The manhole cover 20 is typical in construction and may include one or more through-holes and fittings (not shown) for connection to a liquid retrieving mechanism, such as a gasoline pump. When the cover 20 is removed, the interior of the tank is accessible from outside the tank. The external tank 12 is preferably a cylindrical tank having flat ends 22. However, the external tank 12 may be spherical or of any other shape.

Within the external rigid tank 12 is disposed a flexible liner or inner tank 24 which, in its operative position, is complementary in configuration to the internal surface of the external tank 12. The liner is preferably constructed with an embossed or roughened outer surface which creates an interstitial space or volume 26 when the embossed or roughened surface is in contact with the inner wall of the outer tank 12. This type of liner is well known in the art and therefore will not be described in greater detail.

A vacuum system includes a vacuum line 28 with a free end 29 that is positioned between the outer rigid tank 12 and the inner liner 24 in the interstitial space 26 and extends from a bottom 30 of the tank 12 to a top 32 thereof. The vacuum line 28 is connected to a liquid/vapor separator 34 which is in turn connected to a vacuum source 36 through a second vacuum line 38. A vent line 35 with a valve 37 is connected to the separator 34 to maintain the separator at atmospheric pressure. A return line 40 extends from the liquid/vapor separator 34 to the manhole cover 20 to thereby form a fluid path between the liquid/vapor separator 34 and the interior of the liner.

A manually operated valve 31 is positioned in the first vacuum line 28 adjacent to the tank 12. A normally closed valve 42 is also positioned in the first vacuum line 28 between the liquid/vapor separator 34 and the valve 31 to automatically seal the interstitial layer from the vacuum source 36 when the vacuum source is not operating. When the vacuum source 36 is operating, the valve 42 automatically opens and valve 37 automatically closes and a low pressure vacuum is created in the interstitial volume 26 to thereby press the liner 24 against the inner wall of the outer tank 12. Any liquid that may be present in the interstitial layer 26 due to leakage or natural permeation through the flexible liner 24 tends to migrate during normal operation of the leak detection system to the separator 34 where it is separated from the air and is returned under gravity to the tank interior through return line 40. A valve 43 and pump (not shown) can be provided in the return line 40 to maintain a pressure differential between the liquid/vapor separator 34 and the interior of the liner 24. When the valve 42 is open, the valve 43 is closed. Likewise, when the valve 42 is closed, the valve 43 is open. This construction isolates the vacuum in the interstitial layer from the separator 34 and the inner volume 18 during normal operation. A pressure transducer 44 is connected directly to the interstitial layer through a conduit to detect the pressure in the interstitial layer and to generate a signal representative thereof. The pressure transducer 44 is electrically connected to a controller 46 that has a microprocessor (not shown) and an optional display 48 through control line 47. The valves 42, 43 and 37 are also connected to the microprocessor through lines 45 and 47, respectively, in order to control opening and closing of the valves. An electrical line 50 connects the vacuum source 36, which is preferably a vacuum pump, to the microprocessor for controlling operation of the vacuum source 36 in a manner to be described in greater detail below.

With reference now to FIGS. 2 and 3, the inner liner 24 has an opening 52 defined by a liner flange 53 that is coincident with the opening 16 in the outer tank 12. An annular ring 56 has an outer surface 59 that is shaped to conform to the shape of the inner surface 58 of the annular wall 54. Preferably, the outer surface 59 is sealed to the inner surface 58 through welding or other well-known fastening techniques. An inner surface 60 of the annular ring 56 extends at an acute angle with respect to the outer surface 59, tapering downwardly and inwardly. Two axially spaced grooves 62 are formed in and extend circumferentially around the inner surface 60. An O-ring 64 is positioned in each groove 62 and sealingly contacts the liner flange 53 when in the assembled positioned as shown in FIG. 3. An annular sealing ring 66 includes an outer surface 68 that extends at an acute angle with respect to an inner surface 70, tapering downwardly and inwardly in complementary relationship with the inner surface 60 of the annular ring 56. Preferably, the acute angle between the surfaces 68, 70 is substantially equal to the acute angle between the surfaces 59, 60 such that the outer surface 68 of the sealing ring 66 is complementary to the inner surface 60 of the annular ring 56. In the installed position, the ring 66 is press-fit into the inner liner opening 52 and against the liner flange 53 to thereby seal the interstitial space from atmosphere even when the manhole cover 20 (FIG. 1) is removed. With this arrangement, the interstitial space 26 is only open to the vacuum line 28. This configuration maintains the sealed integrity of pressure in the interstitial space 26 completely independent of the manhole cover position. Any leak alters the pressure in the interstitial space which is detected by the pressure transducer 44.

Turning now to FIG. 4, initialization of the tank system 10 will now be described. In block 100, the various components of the system described in FIGS. 1 - 3 are delivered to the installation site and assembled into an operational condition. After performing various integrity and system operational checks, the liner 24 is isolated from the rest of the system by closing the manual valve 31 , as represented by block 101. The lines 28, 38 and 40 are then evacuated to a negative pressure as represented by block 102. In a preferred embodiment, the lines are evacuated to approximately 1 Ton^* absolute. The pressure change with time is then monitored in the lines with the pressure transducer 44 and the microprocessor in the controller 46 as represented by block 104. If the pressure change within the lines is greater than a preset limit, as represented by block 106, there may be a possible leak in the system or a faulty transducer. The problem is then corrected, which may include resealing the connections in the pressure lines or replacing faulty equipment as represented by block 108. Once the problem is corrected, the lines are again evacuated to a negative pressure as represented by block 102 and the process continues. In block 110, if the pressure change over time is less than the preset limit, the microprocessor indicates that the lines are properly sealed and ready for operation. The manual valve 31 is subsequently opened, as represented by block 112, to thereby open the liner 24 to the system. When the system is put into operation for the first time, or when the liner 24 is replaced, the interstitial volume (I.V.) between the liner and the inner wall of the tank 12 must be determined, as represented by block 116, in order to properly measure if any leakage is occurring. The interstitial volume must be determined for each installation since tolerance variations in the liner and tank, the type of material used for the liner and the pattern on the outer surface of the liner all contribute to variations in the interstitial volume. The volume of the interstitial space can be determined by first measuring the pressure P, of a volume V_! of gas in the interstitial layer and then connecting to the interstitial layer a volume V₂ of gas at a known pressure P₂ different than P,. The pressure P₃ of the combined pressures is then measured. The volume V, of the gas in the interstitial layer is determined as follows:

The weighted average pressure can be expressed by the following equation:

(V - P^ V₂ - P₂ )

P

( + V₂) where V, = Interstitial Space Volume (unknown) V₂ = Injected Volume (known) P, = Interstitial Pressure (measured/known) P₂ = Injected Pressure (known) P₃ = New Pressure Observed

Solving the equation for V,:

(v_ - p_ - p_ -v₂)

Once the interstitial volume V₂ is determined, the interstitial space 26 is again evacuated to the predetermined vacuum pressure, as represented by block 118. The pressure in the interstitial volume is then monitored over time in block 120. If the change in pressure over a predetermined time is greater than a preset limit in block 124, there exists the possibility that the liner or tank is defective or the connections between the liner, tank, valves, or vacuum lines are not adequately sealed. Preferably, the preset limit is approximately 7.3 x 10^{" 3} Torr hour^χft² for an interstitial volume of about 25 liters. This limit can vary greatly depending on the volume of the interstitial space 26, the size of the vacuum lines, material properties of the components, etc. In any event, the problem is corrected in block 126 and the interstitial space 26 is again evacuated in block 118 to the predetermined vacuum pressure. If it is determined in block 128 that the change in pressure over the predetermined time is less than the preset limit, the system 10 can then be placed in normal operation as represented by block 129.

With reference now to FIG. 5, a process for monitoring the pressure in the interstitial layer will now be described. The system is initially started manually, as represented by block 130, by throwing an appropriate starter switch (not shown) on the controller 46. The microprocessor first determines if there is an appropriate vacuum in the interstitial space 26 based on one or more pressure readings measured by the transducer 44. If the pressure is not within a predetermined range, the vacuum pump 36 is actuated and controlled by the microprocessor to evacuate the interstitial space 26 to a predetermined negative pressure, after which the pump 36 is turned off. This pressure will decay over time due to temperature changes (which are usually negligible if the tank 12 is located underground), changes in the total iiquid volume or in the hydrostatic head of the liquid in the tank, the rate of allowable slow leakage through joints and seals, permeation of fluid through the liner frorri the inner volume 18, and even gaseous leakage through the ligid skin of the external tauk 12 as a result of the inherent porosity of certain metals, such as steel. The low-pressure decay over time due to the above-described changes is minor, however, in comparison to the low- pressure loss caused by a major leak. The rate of permissible leakage or slope of pressure loss versus time curve is reflected by the rate of low pressure decay in the interstitial volume 26, which can be predetermined through testing of the system 10. Any sustained deviation in the predetermined rate of pressure decay is indicative of an unacceptable leak in the system. Such deviations are monitored by the microprocessor through readings from the pressure transducer 44 taken at predetermined intervals. Preferably, a pressure reading is recorded in five minute intervals and a total of 20 data points, as represented by "y" in block 132, taken at the five-minute intervals are collected and stored in a memory associated with the microprocessor. The microprocessor then calculates an average slope or rate of pressure decay based on the 20 data points. Although five minutes is a preferred time interval and 20 data points is a preferred number for computing the rate of decay, it is to be understood that data points can be taken in intervals of fractions of seconds, seconds, minutes, etc., and that any number of data points can be recorded and used to calculate the decay slope depending on the amount of accuracy and degree of sensitivity desired. In block 134, the average calculated slope from the pressure data points is compared to a predetermined set point to determine if the measured slope is greater than the predetermined slope. If not, the microprocessor determines that no leakage has occurred and subsequent pressure data points are obtained in block 132. This process forms a sliding window of 20 data points wherein a new data point is added to the data base and the earliest data point is eliminated. A new average slope is then calculated. Thus, a rate of pressure decay can be calculated and compared to the predetermined rate in real time.

If the average decay slope, is greater than the predetermined limit (x), e.g., approximately 7.3 x 10^"3Torr/hour ft² for an interstitial volume of about 25 liters, but less than a second predetermined limit (z) at block 1 35, e.g. approximately 1 Tθιτ/minute for an interstitial volume of about 25 liters, ancώer set of data points are gathered at block 136 and a new average slope is calculated. Preferably, the number of data points gathered at block 136 is half of the data points that are present in the data window, as represented by "1/2 Y" in block 136. At block 138, if the average slope is less than the first predetermined limit, the cycle starts over again at block 132 and a new average slope is calculated. However, if the average slope is again measured to be greater than the predetermined slope (x) at block 138, an alarm signal is generated at block 140. The alarm signal can actuate a visual display, an audio signal, or a combination of both to indicate that a leak has developed in the system. Once a leak has been detected, the leakage detection system is preferably shut down, i.e. electrical power is cut off from the vacuum pump and valves. System shut-down in this manner prevents the transfer of liquid from the interstitial space to the collector and therefore prevents collector overload and the continuous cycle of liquid through the leakage detection system, while permitting liquid to be drawn from the tank in the usual manner. Air or moisture may enter the interstitial space through a hole in the tank, thereby breaking the vacuum in the interstitial volume. Likewise, liquid from the tank may enter the interstitial space through a hole in the liner, thereby breaking the vacuum in the interstitial volume.

In the event that the measured slope is greater than the second predetermined slope (z) at block 135, indicating a rather large leak in the tank, liner, etc., the alarm signal is generated in block 135 and sent to the alarm at block 140. The leakage detection system is then immediately shut down. It is to be understood that the limits (x) and (z) are given by way of example only and can change significantly based on the actual interstitial volume, the size of the vacuum lines, material properties of the components, etc. The predetermined slopes are based on a leak-free tank. In the preferred embodiment as described above, approximately 18 hours pass between successive actuation of the vacuum pump due to acceptable low-pressure losses as previously described.

It will be appreciated that the system of the present invention has several high- performance features. Very minute leaks above the predetermined acceptable leak rate can be detected on a continuous or real-time basis, depending on the frequency of pressure measurement in the interstitial space. With pressure decay slope measurement and comparison, the leak rate can be determined with a high degree of accuracy. Thus, very minute leaks above the acceptable predetermined limit can be detected and action taken to repair the leak prior to the development of a major leak and possible subsequent environmental contamination.

Another method for monitoring the interstitial space for leaks is by using the weighted average pressure and monitoring the volume in the interstitial space. The volume of gas in the interstitial layer is determined by the formula set forth above.

The interstitial volume can be monitored for changes over a period of time. If the rate of change of volume exceeds a predetermined limit, an alarm sounds and the system is shut down to check for leaks. The same averaging techniques as described above with respect to pressure monitoring can be used to determine when a leak occurs.

This volume measuring system is illustrated in the flow diagram of FIG. 6 to which reference is now made. The system is initialized as shown in the block 142 and the pressure in the interstitial layer is measured as represented in block 144. The pressure change over a period of time is then determined as represented in block 146. If the change in pressure is above a predetermined pressure decay rate, then the system is examined for a potential problem or the system is allowed to stabilize as represented in blocks 148 and 150. The steps of blocks 144 and 146 are then repeated until the pressure decay is less than the predetermined value which, for example, can be 1.0 Torr/minute for certain storage tanks. A known volume of gas (air) at a known pressure is then injected into the interstitial layer as represented in block 152. The known pressure of the volume of injected gas is generally different than the pressure in the interstitial layer. It can be greater or less than the existing pressure in the interstitial layer. The new pressure in the interstitial space is then measured as represented in block 154. If the pressure change is less than a predetermined limit, for example, 5 Torr, then anothei volume of gas in connected to the interstitial layer at a different pressure or different volume or both, as represented in block 160 and the interstitial layer pressure is then measured again as represented in block 154. If the change in pressure over time is greater than the predetermined pressure change, then the interstitial volume is calculated as represented in block 158 in accordance with the formula set forth above. This process is carried out at least 3 times and the calculated values are averaged to arrive at a reliable value of the interstitial volume. This process is carried out on a periodic basis, for example, every hour, every 12 hours. The calculated volume is then compared to the previous volume calculation to determine whether a leak has occurred. The volume calculated is the volume of gas in the interstitial space. To the extent that liquid fills the interstitial space, the volume of the space will decrease. To the extent that gas enters the interstitial space, the pressure in the space will increase and thus tend to indicate a smaller interstitial gas volume, also indicating a possible leak

The volume monitoring technique is especially useful in relatively large above-ground tanks in which the interstitial volume tends to be large and temperature changes tend to be significant. Reasonable variation and modification are possible within the spirit of the foregoing specification and drawings without departing from the scope of the invention. For example, the monitoring system of the invention can be used to detect leaks in above-ground as well as below-ground tanks. Further, the invention can be used to monitor leaks in tanks with an external flexible bladder and internal rigid tank as well as an internal flexible bladder and external rigid tank. Further, the monitoring system of the invention can be used to monitor leaks in an interstitial space between two rigid tanks.

Claims

The embodiments for which an exclusive property or privilege is claimed are defined as follows: 1. A method for determining leakage in a liquid holding tank having a first container and a second container, the walls of the first container being at least in proximity to the walls of the second container with a relatively fixed interstitial space between the first and second containers, the method comprising the steps of: reducing the pressure in the interstitial space to a first predetermined level; reiteratively measuring the pressure in the interstitial space over a first predetermined time period; calculating a first measured pressure decay slope from the measured pressure over the first predetermined time period; comparing the measured pressure decay slope with a first piedetermined slope; and generating a warning signal when the measured pressure decay slope is greater than the first predetermined slope.

2. A method according tc claim 1 and further comprising the step of comparing the measured pressure decay slope with a second predetermined slope before the step of generating a warning signal; and wherein the step of generating a warning signal comprises generating a warning signal when the measured pressure decay slope is greater than both of the first and second predetermined slopes.

3. A method according to claim 2 wherein the first predetermined slope is less than the second predetermined slope.

4. A method according to claim 3 and further comprising the steps of: reiteratively measuring the pressure in the interstitial space over a second predetermined time period when the first measured pressure decay slope is less than the second predetermined slope; computing a second measured pressure decay slope from the measured pressure over the second predetermined time period; comparing the second measured pressure decay slope with the first predetermined slope; and wherein the step of generating a warning signal comprises generating a warning signal when the second measured pressure decay slope is greater than the first predetermined slope.

5. A method according to claim 4 wherein the second predetermined time period is shorter than the first predetermined time period.

6. A method according to claim 5 wherein the reiterative measuring steps include measuring a pressure value at discrete time intervals during each predetermined time period.

7. A method according to claim ¹ wherein the reiterative measuring step includes measuring a pressure value (?,) at lime intervals during the first predetermined time period,

8. A method according to claim 1 wherein the first predetermined pressure decay slope is obtained by the steps of: first, determining the volume of the interstitial space; then, applying a reduced pressure to the interstitial space; and measuring any change in pressure over time.

9. A method according to claim 8 wherein the step of determining the volume of the interstitial space comprises the steps of: first, connecting a first volume of gas V₂ at a pressure P₂ with the interstitial space; then, measuring the pressure P₃ of the interstitial space; and computing a volume V, of the interstitial space in accordance with the formula: {v₂ - ^p ₂ - ^p, -v₂) v. =

10. A method according to claim 9 and further comprising the step of evacuating the interstitial space to a predetermined pressure before the connecting step.

11. A method according to claim 1 wherein the second container is a flexible bladder which forms a liner for the first container and the first container is rigid.

12. In a fluid containment system comprising a rigid outer tank having inner and outer surfaces, an access opening extending between the inner and outer surfaces, a wall with an interior surface surrour.ding the opening, and a cover adapted for mounting to an upper portion of the wall for closing the access opening: an inner flexible bladder complementary in configuration to the rigid outer tank positioned within the outer tank and adjacent the inner surface of the outer tank such that an interstitial space is formed between the inner bladder and outer tank, the bladder having an opening in alignment with the access opening of the outer tank, the improvement comprising: the bladder including a flexible flange portion surrounding the opening; a first annular member having an outer surface in sealing engagement with the interior surface of the wall and an inner surface adapted for sealing engagement with an outer surface of the flexible flange portion; and a second annular member sized to be received within the first annular member and having an outer surface adapted for sealing engagement with an inner surface of the flexible flange portion such that the flange portion is sandwiched between the first and second annular members to thereby form a seal between the outer tank and inner bladder and isolate the interstitial space from atmosphere, the second annular member having a central opening coincident with the access opening for communication with an interior of the bladder; wherein the cover can be removed from the outer tank without disturbing the seal between the outer tank and inner bladder.

13. A fluid containment system according to claim 12 wherein the inner surface of the first annular member and the outer surface of the second annular member slope inwardly toward a center of the access opening.

14. A fluid containment system according to claim 13 and further comprising at least one groove in the inner surface of the first annular member and an O-ring located within the at least one groove, a portion of the O-ring extending outwardly of the groove and into sealing contact with the flexible flange.

15. A fluid containment system according to claim 14 and further comprising: a vacuum source having a vacuum line fluidly connected to the interstitial space for periodically applying a pressure co the interstitial space for drawing the inner bladder toward the inner surface of the outer tank; a pressure transducer fluidly connected to the interstitial space for measuring the amount of pressure in the interstitial space and for generating electrical signals proportional to the measured amount of pressure; a controller operatively connected to the pressure transducer for receiving the electrical signals therefrom, for calculating a pressure decay slope based on the electrical signals received from the pressure transducer, and for generating a warning signal when the pressure decay slope is greater than a predetermined slope.

16. A fluid containment system according to claim 15 wherein the controller is operatively connected to the vacuum source for controlling operation of the vacuum source when the pressure in the interstitial space rises above a predetermined value.

17. A fluid containment system according to claim 12 and further comprising: a vacuum source having a vacuum line fluidly connected to the interstitial space for periodically applying a pressure to the interstitial space for drawing the inner bladder toward the inner surface of the outer tank; a pressure transducer fluidly connected to the for measuring the amount of pressure in the interstitial space and for generating electrical signals proportional to the measured amount of pressure; a controller operatively connected to the pressure transducer for receiving the electrical signals therefrom, for calculating a pressure decay slope based on the electrical signals received from the pressure transducer, and for generating a warning signal when the pressure decay slope is greater than a predetermined slope.

18. A fluid containment system according to claim 1 / wherein the controller is operatively connected to the vacuum source for controlling operation of the vacuum source based on the electrical signals from the pressure transducer.

19. fluid containment and leak detection system comprising: an outer tank having an inner surface; an inner tank of complementary configuration to the outer tank positioned within the outer tank and adjacent the inner surface of the outei tank such that a relatively fixed interstitial space is formed between the inner tank and outer tank; a pump having a vacuum line fluidly connected to the interstitial space for periodically reducing the pressure to the interstitial space; a pressure transducer mounted to the interstitial space for measuring the amount of pressure in the interstitial space and for generating electrical signals proportional to the measured amount of pressure; a controller operatively connected to the pressure transducer for receiving the electrical signals therefrom for computing a pressure decay slope in response to reiterative electrical signals from the pressure transducer represents reiterative measurement of the pressure in the interstitial space over a period of time and for generating a warning signal when the pressure decay slope thus computed is greater than a predetermined slope.

20. A fluid containment system according to claim 19 wherein the controller is operatively connected to the vacuum source for controlling operation of the vacuum source based on the electrical signals from the pressure transducer.

21. A method of monitoring a relatively fixed interstitial space between multiple-walled storage tanks comprising an inner tank and an outer tank comprising the steps of: (a) filling the inner tank with a liquid; (b) sealing the interstitial space from the atmosphere; (c) measuring the pressure ?_ of a volume of gas in the interstitial space; (d) connecting the interstitial space with a known second volume V₂ of gas at a known pressure P ; (e) while the interstitial space is connected to the known volume V„ measuring the pressure P₃ of the first and second volume of gas in the interstitial space; (f) computing the volume V, of gas in the interstitial space; (g) storing the volume V, of gas in the interstitial space; (h) repeating the steps (c) - (f) to obtain a volume V₃ of gas in the interstitial space; and

(i) comparing the computed volume V₃ with the volume V, to determine whether a leak has occurred in either of the inner or outer tanks.

22. A method of monitoring the interstitial space between multiple-walled storage tanks according to claim 21 wherein the volume V, is computed in accordance with the formula:

23. A method of monitoring the interstitial space between multiple-wall storage tanks according to claim 21 wherein the rate of change of the volume of gas over a period of time is computed and compared with a predetermined rate of change of the volume of gas in the interstitial space to determine whether a leak has occurred.