Method and apparatus for continuously monitoring interstitial regions in gasoline storage facilities and pipelines

ES 2 397 610 T3

DESCRIPTION

Method and apparatus for continuously monitoring interstitial regions in gasoline storage pipelines and facilities

Technical scope

In general, the present patent is directed to an apparatus and a mode of interstitial monitoring, and more specifically to a system for continuously monitoring the pressure and vacuum levels within the interstitial space of an underground storage tank system.

Background

Current and proposed US state and federal regulations require that underground tanks for the storage of hazardous substances meet certain environmental safety requirements; in particular, these environmental regulations require that underground storage systems comprise a primary containment unit and a secondary containment unit (see, for example, DE 42 18 830 A1). Likewise, it is required that the primary and secondary containment units comply with the environmental regulations according to which the underground storage tank systems must be hermetic to the product. For the purposes of these environmental standards, “product-tight” is often defined as impervious to the contained substance, to prevent it from seeping through the primary containment unit. Furthermore, for a tank to be hermetic to the product, throughout its useful life said tank cannot suffer physical or chemical deterioration due to the effect of the substance it contains. These regulations also require that the owners or operators of an underground storage tank system, with a single wall component and located no more than 0.3 km (1,000 feet) from a public drinking water well, implement an enhanced leak detection or monitoring program.

A known method of leak monitoring, described in US Patent No. 6,489,894 and entitled "Leak Detection Device for Double Wall Pipeline Systems and Container Systems" uses a leak detector. leaks with a vacuum pump that includes a pressure-dependent switch and an alarm device to detect leaks in a double-walled piping or container system. The leak detector disclosed in said patent has been adapted to simultaneously monitor several containers connected to a collecting pipe and a vacuum pump via vacuum lines. Each monitored vessel incorporates a connector or vacuum valve to fluidly connect a monitoring space to a leak detector. Each vacuum conduit has a first liquid lock provided in the vacuum connector to prevent liquid leaked to the vacuum conduits from a leaking container from entering the control spaces of the containers without leaking. A second liquid lock is arranged in the collecting line to prevent liquid from entering the vacuum pump. Although this method can detect leaks within the control space of a container, it is a mechanically complex system that requires a large amount of materials and time to set up.

Other methods of monitoring interstitial or secondary spaces are well known in the art and include continuous leak detection using both pressure monitoring techniques and brine solutions, in order to determine the presence or absence of leaks between the storage system. and the surrounding environment. However, to effectively calibrate all of these known systems and methods for operation requires a great deal of setup time and system awareness. Specifically, to configure these monitoring systems for operation, the user must enter the volume of the secondary or interstitial space to be monitored, which requires detailed knowledge of the distribution and configuration of the containers and the pipes of the double wall used in the underground storage system.

Summary

An underground storage system comprises a primary containment unit and a secondary containment unit arranged to hermetically encompass the primary containment unit. This underground storage system also includes a leak detection system, fluidly connected to the secondary containment system and adapted to detect fluid leaks in the primary containment system and the secondary containment system.

Brief description of the drawings

For a more complete understanding of the disclosed device, the following detailed description and accompanying drawings should be consulted, in which:

Figure 1 illustrates the basic components of an example pore vacuum monitoring system;

Figure 2 illustrates a flow chart detailing the operation of an exemplary self-learning routine;

Figure 3 illustrates an exemplary interstitial void curve; and Figure 4 illustrates a flow chart detailing the operation of an exemplary monitoring routine.

Detailed description

Figure 1 illustrates an exemplary underground storage system 10 that includes an underground storage tank (DAS) 12 constructed to safely contain a liquid 20, such as gasoline, diesel, or other hydrocarbon. The DAS 12 is a double-walled storage tank, constructed with an outer wall 14 and with a separate inner wall 16 to define an interstitial space 18. In this way, the DAS 12 is divided into a primary containment unit and a secondary containment unit, to provide the underground storage system 10 with redundant leak protection.

A submersible turbopump (TBS) 22 such as the TBS model number STP-75-VL2-7 manufactured by FE PETRO, INC. ®, provides a means for pumping the liquid 20 to a manifold 24. The TBS 22 can be mounted to the DAS 12 in a fixed or removable manner to position an inlet nozzle 22a below the surface of the liquid 20. In turn, inlet nozzle 22a provides a fluid path for pumping liquid 20 within the primary containment unit to manifold 24.

A pump manifold 26, which may be a component in one piece with the TBS 22 or a separate component that can be fixedly coupled thereto, regulates the distribution of the pumped liquid 20 to the manifold 24. The pump manifold 26 comprises a siphon inlet 28 adapted to fluidly connect the interstitial space 18 (eg, the secondary containment unit) with the vacuum generated by the TBS 22. Thus, when TBS 22 is active (eg, creating a vacuum), siphon inlet 28 provides a vacuum path to interstitial space 18 to evacuate fluid contained within. A regulating valve 30 can isolate the interstitial space 18 from the siphon inlet 28, to prevent a drop in vacuum when the TBS 22 is inactive and exposed to atmospheric pressure via the primary containment unit.

A vacuum sensor 32 fluidly communicates with interstitial space 18 and siphon inlet 28 to sample and measure vacuum levels therein. The vacuum sensor 32 can be a continuous analog sensor, a discrete digital sensor, a switch-based sensor, or any other device configured to sample the level of vacuum within interstitial space 18. Vacuum sensor 32 may be isolated by regulator valve 30 to prevent atmospheric pressure measurements (ie, zero vacuum measurements) when TBS 22 is inactive. However, when TBS 22 is active and generating a vacuum, regulator valve 30 opens to provide a fluid connection between vacuum sensor 32, interstitial space 18, and siphon inlet 28. In this way, the vacuum sensor 32 samples and measures the change in the vacuum level within the interstitial space 18 generated by the TBS 22.

In addition, the vacuum sensor 32 can communicate communicatively with a control unit 34 having a processor 36 and a memory 38. The control unit 34 and memory 38 receive and store vacuum data, system information, alarm data, etc. from vacuum sensor 32 or any other controlled component. Communications between control unit 34 and, for example, vacuum sensor 32 and throttle valve 30, can be implemented using any desired communications link, such as a wired local area network, a wireless communications link, a direct communication link or a point-to-point wired communication link.

The processor 36 may execute a monitoring routine to direct the configuration and operation of the underground storage system 10. In particular, the monitoring routine may be written in any computer language or process control programming language, such as C⁺⁺, Visual C⁺⁺, Visual Basic or machine language and can be compiled (if necessary) and stored in memory 38. In general, the monitoring routine ensures the integrity of the underground storage system 10 by detecting unwanted leaks. Specifically, the monitoring routine can be run on processor 36 to automatically learn the vacuum characteristics of interstitial space 18. In addition, the monitoring routine may also include additional subroutines adapted to run in processor 36 to continuously monitor the level of vacuum within interstitial space 18 as a function of time.

A leak port valve 40 is fluidly connected to regulator valve 30, vacuum sensor 32, and leak port 42, to provide a vacuum path between interstitial space 18. Leak port valve 40 and leak port 42 may define a removable assembly adapted to be disconnected from the space

ES 2 397 610 T3 interstitial 18 when it is no longer needed for the configuration and use of the underground storage system 10. The leak port valve 40 allows the automatic or manual creation of a calibrated or controlled leak between the interstitial space 18 and the atmospheric pressure beyond the leak port 42. Such controlled leakage causes a drop in the vacuum level within the interstitial space.

In turn, the vacuum sensor 32 can measure the decreasing vacuum level and communicate the vacuum level data to the monitoring routine running in the control unit 34, via the communication link. In turn, the monitoring routine can manipulate the vacuum level data to establish one or more characteristics of the interstitial space 18. In particular, the monitoring routine may determine a negative vacuum level rate of change based on the decreasing vacuum level data resulting from the introduction of the controlled leak into the secondary containment unit. It will be understood that other vacuum characteristics, eg, a positive vacuum level rate of change, or the time for total voiding of the interstitial space, may be established, additionally or alternatively, based on the vacuum level data.

The DAS 12 can be connected to other components of the underground storage system 10. In particular, the interstitial space 18 can be fluidly connected to a second interstitial space 48 of a distributor tube 46 through a plurality of vacuum intakes 44 -44b. During operation, the double-walled manifold tube 46 can provide the fluid connection between the liquid 20 stored in the DAS 22 and the manifold 24. Thus, the entire underground storage system 10 (including DAS 12 and distributor tube 46) is double-walled and product-tight against penetration and corrosion that may occur during normal operations.

Figure 2 presents a generalized flow chart of the operations of a self-calibration or self-learning subroutine 50, adapted to learn the vacuum characteristics of the interstitial space 18. The self-learning subroutine 50 determines and stores the vacuum characteristics based, in part, on in measured changes in vacuum level as a function of time. The self-learning subroutine 50 learns the vacuum characteristics without the need to determine or calculate the total volume of the interstitial space 18, the vacuum capacity of the TBS 22, the sensitivity of the vacuum sensor 32, etc. In this way, self-learning routine 50 provides a fast and efficient means of calibrating and monitoring interstitial space 18 of any known or unknown volume or complexity. It will be understood that the self-learning routine 50 may act as a stand-alone routine, independent of the monitoring routine or other subroutines. However, the self-learning routine 50 can be integrated with the monitoring routine to satisfy the calibration requirements of the underground storage system 10.

Self-learning routine 50 may run as long as a predetermined criterion is satisfied. In particular, the self-learning routine 50 can be executed manually as part of a regularly scheduled maintenance procedure, or automatically in response to a change in the configuration of the underground storage system 10, as part of the setup and configuration. initials of the underground storage system 10, or to compensate for a change in vacuum level over time.

A block 52 loads the predetermined conditions and stored initial settings that are required to execute the self-learning routine 50 from memory 38 (see FIG. 1). These predetermined conditions and these initial settings may include, among other things, a maximum desired vacuum level Pmax, a minimum allowable vacuum level Pmin, the closure of the throttle valve 30, and the calibration of the vacuum sensor 32.

Although the maximum desired vacuum level can be set to virtually any value, empirical tests indicate that an approximate vacuum level of 254 mm Hg, which represents a vacuum level obtainable and easily distinguishable from atmospheric pressure, may be desirable. . Also, the minimum acceptable vacuum level can be set, for example, to 50.8 mm Hg. Typically, the minimum vacuum level Pmin provides a lower limit or threshold to identify the time when the current vacuum level Pmed within interstitial space 18 decreases towards the level of atmospheric pressure (i.e., approximately 0 mm Hg or vacuum zero).

A block 54 causes the vacuum sensor 32 to sample and measure the current vacuum level Pmed within the interstitial space 18. Typically, the vacuum sensor 32 samples the current vacuum level Pmed at regular intervals At throughout the operation of the self-learning routine 50. The memory 38 may store the vacuum level data representative of the current vacuum level Pmax in a historical database as the stored vacuum level P_tom The stored vacuum level P_tolm can be permanently archived in the historical database (i.e. saved to the database) or temporarily stored for use in calculations / analysis etc., and later deleted or overwritten as new ones are obtained and stored data.

A block 56 compares the current vacuum level Pmed with atmospheric pressure (ie, zero vacuum) to establish a vacuum reference value prior to execution of the remaining steps of self-learning routine 50. Upon sensing a vacuum within the interstitial space 18, a block 58 causes the throttle valve 30 and the leak port valve 40 to open and evacuate the sensed vacuum to the atmosphere. A block 60 results in the

ES 2 397 610 T3 vacuum sensor 32 sample current vacuum level Pmed until atmospheric pressure is detected. When the vacuum sensor 32 detects atmospheric pressure, a block 62 closes the regulator valve 30 and the leak port valve 40 to seal and isolate the interstitial space 18, as a preliminary step for the execution of a portion of the evacuation procedure. self-learning routine 50.

A block 64 initiates the evacuation procedure and the self-learning routine 50 begins to learn the vacuum level data necessary to generate a "rising curve" (an example of this curve is line 102 of FIG. 3). In particular, the block 64 activates the TBS 22 and this, in turn, begins to evacuate the interstitial space 18 through the siphon inlet 28. A block 66 opens the throttle valve 30 to establish fluid communications between the TBS 22, the interstitial space 18, and the vacuum sensor 32. Typically, the throttle valve 30 opens after a delay equal to the time required for the vacuum sensor 32 detect the vacuum generated by the TBS 22. It will be understood that the delay associated with the vacuum sensor 32 may also depend on factors such as the sensitivity of the vacuum sensor 32, the vacuum capacity of the TBS 22, and the total volume of the interstitial space 18.

A block 68 causes the vacuum sensor 32 to sample and measure the current vacuum level Pmed within the interstitial space 18 during the interval At. A block 70 causes the processor 36 to set the Palm stored vacuum level equal to the current vacuum level Pmed, and to store the resulting Palm stored vacuum level in the historical database established within memory 38. At this point, the rate of change of the vacuum level of the rising or evacuation curve, within the interstitial space 18, can be calculated based on the difference between the current vacuum level and the stored vacuum level, during an interval of fixed or known time. A rate of change of APevac evacuation can be described mathematically with the formula:

P - P med alm

<img file="ES2397610T3_D0001.tif" />

evac

Dt

The rate of change of the evacuation APevac describes the positive or increasing slope of the evacuation curve representative of an increase in the vacuum level within the interstitial space 18. Alternatively, the evacuation curve can be constructed representing the values ​​of the current vacuum level Pmed and Palm stored vacuum level sampled during execution of self-learning subroutine 50 as functions of time.

A block 72 compares the current vacuum level Pmed with a maximum desired vacuum level Pmax. If the current vacuum level is less than the maximum desired vacuum level, self-learning routine 50 enters a loop 74 and continues to sample and store the current vacuum level Pmed until the maximum desired vacuum level is reached. However, when block 72 detects that the current vacuum level exceeds the maximum desired vacuum level, a block 76 closes the throttle valve 30.

Subsequently, a block 78 deactivates the TBS 22 and the evacuation procedure comes to an end. At this point, the throttle valve 30 seals and isolates the interstitial space 18, and the current vacuum level Pmed remains substantially constant at the maximum desired vacuum level Pmax.

A block 80 causes the vacuum sensor 32 to sample and measure the current vacuum level Pmed within the sealed interstitial space 18 at each time interval At. The current vacuum level Pmed is expected to remain at the maximum desired vacuum level Pmax for a fixed number of time intervals. Also, memory 38 can store the current vacuum level Pmed, which is equal to the maximum desired vacuum Pmax, in memory 38 as the stored vacuum level Palm. At this point, the rate of change of the vacuum level within interstitial space 18 is substantially zero. In other words, the level of vacuum within the sealed interstitial space is constant. A positive or negative change in the vacuum level during this time interval represents an anomaly (eg a leak) that triggers an alarm. It is possible to mathematically describe a rate of change of the maximum vacuum APmax with the formula:

<td>DP, - max</td><td>P _- P _ med alm _ q Dt</td>

The rate of change of the maximum vacuum APmax represents the zero slope line corresponding to the maximum desired vacuum level Pmax. It will be understood that the determination of the rate of change of the maximum vacuum APmax is an optional calculation that can be carried out by the control unit 34.

A block 82 starts the decay procedure and the self-learning routine 50 begins to learn the vacuum level data necessary to generate the "down curve" or "decay curve" (an example of this curve is line 106 of the figure 3). In particular, the leak port valve 40 opens in response to an instruction issued by the monitoring routine that is running in the control unit 34. During operation, the leak port valve 40 (which may be a manual valve and require intervention

ES 2 397 610 T3 human to open) provides a fluid path between the current vacuum level Pmed in interstitial space 18 and the zero vacuum level of the atmosphere. In other words, the leak port valve 40 provides an equalization path between the high vacuum level of interstitial space 18 and the zero vacuum level of atmospheric pressure. The decrease in the current vacuum level Pmed in the interstitial space 18, caused by the controlled leak, provides a method of rating the performance of the secondary containment unit in the presence of an actual uncontrolled leak.

A block 84 causes the vacuum sensor 32 to sample and measure the current decreasing vacuum level Pmed within the interstitial space 18 during each time interval At. A block 86 instructs the processor 36 to store the current decreasing vacuum level Pmed in memory 38 as the stored vacuum level Palm. At this point, the rate of change of the vacuum level of the descending or decreasing curve within the interstitial space 18 can be calculated based on the difference between the stored vacuum level Palm and the current vacuum level Pmed during a fixed interval of time At. It is possible to mathematically describe a rate of change of the decrease AP decrease with the formula:

ΔΡ ...

decrease

P - P alm med

Dt

The rate of change of the decrease AP decrease represents the negative slope of the decrease curve, which is the line defined by the decreasing values ​​of the current vacuum level Pmed measured with the vacuum sensor 32 during the decrease procedure of the self-learning routine. fifty.

A block 88 compares the current vacuum level Pmed with a minimum desired vacuum level Pmin. It will be understood that the minimum desired vacuum level Pmin could be set to zero vacuum (ie, atmospheric pressure), but will typically be set higher to reduce overall system setup time. In other words, the closer the set minimum desired vacuum level Pmin is to atmospheric pressure, the longer it will take to equalize the interstitial space 18. If the current vacuum level Pmed is greater than the minimum desired vacuum level Pmin, the self-learning routine 50 enters a loop 90 and continues to sample and store the current vacuum level Pmed until the vacuum sensor 32 detects the vacuum level. Minimum desired vacuum level Pmin within interstitial space 18. However, if the current vacuum level Pmed in block 88 is less than the minimum desired vacuum level Pmin, a block 92 causes the throttle valve 30 to close. At this point, the step-down procedure of self-learning routine 50 comes to an end and the learned rates of change APevac and APrecall can be combined to produce the total vacuum characteristics curve of FIG. 3.

Figure 3 presents a curve of total vacuum characteristics 100 as an example that incorporates the learned change rates APevac, AP decrease, and APmax (obtained optionally), measured and obtained by executing the routine self-calibration 50. As already indicated, line 102 represents the APevac learned evacuation rate of change that is obtained during self-learning routine 50 and, in particular, shows a positive increase in the interior vacuum level of interstitial space 18 as a function of the weather. In physical terms, line 102 represents sealed interstitial space 18, fluidly connected (via throttle valve 30) to active TBS 22. A maximum time Tmax represents the time required for TBS 22 to increase the current vacuum level within interstitial space 18 to the maximum desired vacuum level Pmax.

An upper margin defined by line 102a, and a lower margin defined by line 102b, establish the allowable variation of the vacuum level from the learned line 102 during the evacuation procedure. An alarm subroutine can be activated when the current vacuum level Pmed exceeds the acceptable limits established by the upper and lower margins defined by lines 102a and 102b. For example, the alarm subroutine may determine the existence of a leak in interstitial space 18 if it is determined that the current vacuum level falls outside the upper and lower ranges defined by lines 102a and 102b, or if the maximum desired vacuum Pmax is not reached before the expiration of the Tmax time.

A line 104 represents the maximum desired vacuum level Pmax and the rate of change of the maximum learned vacuum APmax equal to zero (ie, the vacuum is constant). In physical terms, line 104 represents the constant current vacuum level measured when the interior of interstitial space 18 is sealed and isolated from TBS 22 and leak port valve 40. The isolated interstitial space 18 ensures that the current vacuum level Pmed remains virtually constant at Pmax for the set number of time intervals.

As already indicated, line 106 represents the rate of change of the learned decrease AP decrease that is obtained during the self-learning routine 50. Line 106 shows a decrease in the vacuum level measured in interstitial space 18 as a function of time . In particular, line 106 corresponds to a system configuration in which a controlled leak has been introduced into the underground storage system 10, and the

ES 2 397 610 Τ3 current vacuum level P_Id decreases when the vacuum within interstitial space 18 equals atmospheric pressure (ie, a zero vacuum level).

As seen in Figure 3, permeability margin 108 is defined by an upper line 108a and a lower line 108b descending from line 106. Permeability margin 108 represents the exemplary void profile for interstitial space. sealed 18 as a function of time. In other words, during normal operations (for example, steady state operations, without leaks or other variations) it is expected that the current vacuum level P_Id is measured within the permeability range 108 defined by lines 108a and 108b. The permanent vacuum decrease represented by the permeability margin 108 is attributable to the natural permeability properties of the underground storage system 10, rather than a leak or other anomaly. However, if the current vacuum level P_Ic the rate of change of the current vacuum level AP_actuai deviates from the range defined by lines 108a and 108b (that is, if it falls outside the permeability range 108), a leak or other abnormality in interstitial space 18 is assumed, and the alarm subroutine may be activated.

Figure 4 presents a detailed flow chart of the operation of an exemplary monitoring routine 120 using the total vacuum characteristics curve 100. A block 122 causes the vacuum sensor 32 to sample and measure the level. current vacuum P_Id within interstitial space 18. A block 124 compares the current vacuum level P_Id with the minimum allowable vacuum level P_m¡_n (for example, 51 mm Hg or zero vacuum). If the current vacuum level P_Id is less than the minimum allowable vacuum level P_min, a block 126 activates the TBS 22 which, in turn, begins to evacuate the interstitial space 18 as indicated generally by the evacuation curve 102 in FIG. 3.

A block 128 causes the throttle valve 30 to open to establish fluid communication between the TBS 22, the interstitial space 18, and the vacuum sensor 32. Typically, the throttle valve 30 opens after a delay equal to the time required to that the vacuum sensor 32 detects the vacuum generated by the TBS 22. A block 130 instructs the vacuum sensor 32 to sample and measure the current increasing vacuum level P_Id within interstitial space 18 in each of the time intervals ΔΤ

A block 132 compares a rate of change of the current vacuum level Pactuai with the rate of change of the AP learned evacuation_eVac, determined during self-learning routine 50. It will be understood that the rate of change of the current vacuum level AP_actuai can be determined based on the difference between the current vacuum level Pmed and the stored vacuum levels P_toi_m as a function of time. It is possible to describe a rate of change of the current vacuum level AP_actuai using the formula:

) _ med ^x current alm _TO ,

If it is determined that the rate of change of the current vacuum level AP_actuai is less than the AP learned evacuation rate of change_eVac, a block 134 may activate the alarm routine. However, if the rate of change of the current vacuum level AP_actuai exceeds the rate of change of AP learned evacuation_eVac, a block 136 instructs the processor 36 to store the current increasing vacuum level P_Id in memory 38 as the stored vacuum level P_toi_m.

A block 138 compares the current vacuum level P_Id with a maximum desired vacuum level P_mmax- If the current vacuum level Pmed is lower than the maximum desired vacuum level P_mmax, the monitoring routine 120 enters a loop 140 and continues to sample and store the current vacuum level P_Id until the maximum desired vacuum level is detected P_mmax- However, if the current vacuum level P_Id is greater than the maximum desired vacuum level P_mat x, a block 142 causes the throttle valve 30 to close.

A block 144 deactivates the TBS 22 upon completion of the evacuation of the now sealed interstitial space 18. Thus, the monitoring routine 120 has recharged the vacuum level within the interstitial space 18. During operation, the evacuation or vacuum level increase of the interstitial space 18 proceeds along the learned evacuation curve 102, and the monitoring routine 120 continually checks that the current vacuum level Pmed remains within the predefined range that they define lines 102a and 102b. Simultaneously, the time required to recharge the interstitial space 18 to the maximum desired vacuum level P_mmax can be compared to the maximum time T_mmax- If the current recharge time exceeds the maximum time T_mmax, a leak or other abnormality is assumed and alarm routine 134 is activated.

A block 146 restarts the monitoring routine 120 for the vacuum sensor 32 to sample and measure the current vacuum level Pmed in block 122. In block 124, the current vacuum level P is compared._Id that has just been recharged with the minimum allowable vacuum level P_m¡_n (for example, 51 mm Hg or zero vacuum). Because the vacuum level

ES 2 397 610 T3 current Pmed that has just been recharged is higher than the minimum allowable vacuum level Pmin, a block 148 compares the rate of change of the current vacuum level Pactual with the rate of change of the learned decrease AP decrease, which was determined during self-learning routine 50.

As already discussed, the interstitial space 18 is sealed and the monitoring routine 120 measures the current vacuum level Pmed to determine if the decrease in the current vacuum level Pmed is attributable to the natural permeability properties of the underground storage system 10 or a leak. Likewise, the comparison of the learned vacuum curve with the current vacuum level Pmed can be based on the difference between the rate of change of the decrease AP decrease and the current rate of change AP current, or simply on the difference between the vacuum level current Pmeas and the vacuum curve itself learned.

A block 150 instructs the processor 36 to store the current vacuum level Pmed in memory 38 as the stored Palm vacuum level. At this point, the monitoring routine 120 enters a loop 152 and continues to sample and store the current vacuum level Pmed until the minimum allowable vacuum level Pmin is detected, at which point the TBS 22 is activated to evacuate the space. interstitial 18.

Similar monitoring of the vacuum level during evacuation can also be used to check for faults. The system uses the APevac learned evacuation rate of change, or upward curve, as seen in line 102 of Figure 3, to determine if liquid has entered the secondary containment unit. This is accomplished by comparing the upward curve learned from memory with the currently measured upward curve. If the slope of the currently measured upward curve is greater than the slope of the learned upward curve by a threshold factor greater than that defined by line 102a (Figure 3) (i.e., to evacuate the containment space it took a sufficient time lower than originally learned), then it is suspected that liquid has entered the secondary containment unit. This is due to the fact that the ingress of liquid has effectively reduced the containment area available for the vacuum. Additionally, if the slope of the currently measured ascending curve is less than the slope of the learned ascending curve by a threshold factor higher than that defined by line 102b (figure 3) (that is, to evacuate the containment space it took time sufficiently higher than originally learned), it is then possible that fluid is entering through a leak in the vacuum suction line. In either case (a currently measured slope that is sufficiently higher or sufficiently lower than the learned slope) an alarm will be triggered. In this way, a physical liquid collection chamber or liquid sensor is not required, reducing the cost and complexity of the system.

Although the embodiments described herein have focused on analysis and measurements of vacuum levels, it will be understood that an overpressure in interstitial space 18 may be used to provide a suitable pressure gradient for a measurement. by self-learning routine 50 and supervision by supervision routine 120. It will also be understood that the current vacuum level Pmed and the calculated gear rates can be determined manually. For example, control unit 34 may be manually instructed to sample and store the current vacuum level Pmed within interstitial space 18. In addition, an operator can use the gear shift concepts and formulas discussed above, in conjunction with stored Palm vacuum levels, to manually calculate the desired gear shift rates.