Containment failure detection device and method

A device and method for detecting a containment failure. The device has a gas supply means (18) for providing a gas to a containment failure volume (16, 60) and for maintaining a constant gas pressure in the containment failure volume. The device also has gas flow monitoring means (26) for monitoring flow of the gas into the containment failure volume. The flow of gas into the containment failure volume is indicative of a containment failure.

FIELD OF INVENTION 
The present invention is directed to a device for monitoring for 
containment failure in a fluid-containing system. The present invention is 
also directed to a method for monitoring for containment failure in a 
fluid-containing system. 
BACKGROUND OF THE INVENTION 
In chemical processing and other industries, the manufacture, 
transportation, storage, and other operations involving chemicals, 
combustible fluids, and other hazardous materials necessitate containment 
and containment assurance of such materials. Increasingly, environmental 
and safety regulations and laws address hazardous fluids containment and 
containment assurance. Accordingly, there is a continuing need for 
containment systems and devices that comply with the laws and the 
regulations, and the costs associated with compliance can be significant. 
U.S. Pat. No. 4,916,938 describes a leak detection system comprising a 
sealed interstitial space filled with a fluid maintained at an elevated 
pressure. The pressure decay in the space is monitored to provide 
indication of leakage. A disadvantage of this device is that the pressure 
decay characteristics can be affected by ambient temperature conditions, 
possibly leading to false readings or delayed or unresponsive readings. 
U.S. Pat. No. 4,704,897 describes a method for locating a leak in a block 
of underground pipes. The pipes are sealed off and an inert gas introduced 
therein. Suction is then applied to draw air. A flowmeter and oxygen 
sensors are employed to measure the volume of gas being drawn and the 
movement of air from the location of the leak through the pipes, upon 
which the location of the leak can be ascertained. A disadvantage is that, 
although the method may detect a primary system leak, it does not provide 
the means for detecting secondary containment failures. 
U.S. Pat. No. 3,719,070 describes a leak detection system for an underwater 
device for detecting leakage into an area between a primary seal and a 
secondary seal. A pressure gauge is provided to monitor for pressure 
increase between the seals. A disadvantage is that minor leaks can be 
difficult to detect, dependent on leak location and equipment sensitivity. 
Another disadvantage is that fluctuating ambient and service conditions 
can affect the sensitivity of the leak pressure measurement, which can 
cause false, delayed, or unregistered leakage indications. 
U.S. Pat. No. 3,100,392 describes a test apparatus for checking flow 
through a power steering mechanism. Although FIGS. 3 and 4 mention a 
flowmeter, the reference does not elaborate on its use. The device appears 
to utilize a standard pressure indicator for leak detection. Accordingly, 
the device has the same disadvantage of other such prior art devices, in 
which it is subject to spurious readings due to pressure fluctuations 
under nonleakage conditions. Furthermore, the device described is a valve 
testing apparatus and does not monitor a sealed containment volume nor 
address the need for on-line operational capability. 
U.S. Pat. No. 1,096,894 describes a meter tube for measuring steam leakage 
differential pressure to indicate process leakage flow rate through the 
shaft packing of a steam turbine. A disadvantage of the system is that it 
directly measures process leakage of a system fluid, and does not provide 
the capability of remotely sensing a containment failure in a sealed 
containment volume. 
It is an object of the invention to provide a leak detection device and 
method that can detect small seal failures or low-level leaks without 
being susceptible to sensitivity changes due to fluctuations in ambient 
conditions. Another object of the invention is to provide a device and 
method that can indicate a containment failure, whether it is due to a 
failure in primary containment, secondary containment, or both, and in a 
system having a plurality of containment areas. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided, in a fluid-containing 
system having a monitorable containment failure volume, a device for 
detecting a containment failure, comprising: 
gas supply means for providing a gas to the containment failure volume and 
for maintaining a constant gas pressure in the containment failure volume; 
and 
gas flow monitoring means for monitoring flow of the gas into the 
containment failure volume indicative of a containment failure. 
In another aspect of the invention, there is provided a method of detecting 
a containment failure in a fluid-containing system having a monitorable 
containment failure volume, comprising the steps of: 
providing a gas to the containment failure volume; 
maintaining a constant gas pressure in the containment failure volume; and 
monitoring for flow of the gas into the containment failure volume to 
indicate a containment failure. 
The invention provides a device and method that can readily indicate a 
small failure in a primary seal or a secondary seal and indicate the 
presence of a leak. The invention also provides such a device and method 
that is not subject to substantial fluctuations in operating capability 
due to process system changes or fluctuating ambient conditions. 
These and other aspects, objects, features and advantages of the present 
invention will be more clearly understood and appreciated from a review of 
the following detailed description of the preferred embodiments and 
appended claims, and by reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a containment failure detection system in accordance 
with the invention. A liquid for which containment assurance is desired is 
stored in storage tank 10. Tank 10 has inner wall 12 providing primary 
containment of the stored liquid and outer wall 14 providing secondary 
containment of the stored liquid. Interstitial space 16 between walls 12 
and 14 is a monitorable containment failure volume. An inert gas, such as 
nitrogen, is provided by constant pressure gas supply means 18 via line 20 
to space 16. "Inert gas" as used herein includes any gas that is 
substantially nonreactive with or nonharmful to the contained process 
fluid and apparatus. One skilled in the art can readily select an 
appropriate such gas for a particular system design. 
Under non-gas leaking conditions, as when walls 12 and 14 are intact, line 
20 and space 16 hold the gas at a constant pressure. As used herein, 
"constant pressure" is defined to mean a constant pressure of gas for the 
ambient temperature experienced by the gas at the particular location. The 
actual gas pressure at a location is a temperature and location-dependent 
function, the latter depending on whether the location is upstream or 
downstream of gas pressure regulating means, as is further described 
below. Pressure may also vary depending on how gas supply means 18 varies 
with temperature and whether gas supply means 18 has pressure change 
compensation means associated therewith. The invention is designed to 
operate effectively despite such variations in static pressure conditions. 
Filter 22 is provided as means for filtering the gas supplied by gas supply 
means 18. Pressure regulating valve 24 sets the desired gas pressure in 
space 16. Flowmeter 26, and alarm 28 and light 30 actuated by flow switch 
32, provide the indications of gas flow indicative of a containment 
failure. Pressure regulating valve 24 typically should be set to maintain 
a pressure in space 16 higher than the process pressure plus static head 
pressure within the primary containment space, which in FIG. 1 is the 
process pressure in the primary system enclosed by wall 12. Thus, if a 
failure occurs in primary containment such as a rupture in wall 12, 
process fluid or gas should not flow into line 20, or at least not flow 
into those portions of space 16 or line 20 at which the gas pressure is 
not less than the process fluid static head pressure. If desired, venting 
equipment (not shown) can be provided to vent off leaking primary system 
gases of fluids, in which case the flow of gas through line 20 and 
flowmeter 26 would indicate the existence of the leak. 
The design failure strength of walls 12 and 14 can vary, depending on 
process or equipment conditions such as primary system design pressure, 
temperature, materials of construction, and the like. The choice of gas 
supply pressure can also vary, with pressure regulating valve 24 providing 
the capability to adjust the gas pressure incident on walls 12 and 14. 
Walls 12 and 14 can be provided with rupture seals (not shown) designed to 
fail at pressures during overpressurization events that are less than the 
design failure pressures of wall 12 or wall 14 to protect the integrity of 
walls 12 and 14. Accordingly, it may be desired to maintain the gas 
pressure in space 16 at a level well below the pressure at the gas supply 
means outlet. In a preferred embodiment, a gas supply pressure of about 25 
psi (172 kPa) is lowered by valve 24 to a downstream pressure in the range 
of from about 2 cm of water column to about 15 cm of water column. If 
desired, a pressure relief valve or valves, for example valve 34 and valve 
36, can optionally be provided to alleviate an overpressurization 
condition in line 20, which can be signalled by alarm 38 and indicating 
light 40 actuated by flow switch 42. Similarly, a low pressure indicating 
means (not shown) can also be provided to indicate a too low gas pressure 
in line 20. 
Some primary systems can be subject to varying process conditions and 
pressures, such as pressures ranging from atmospheric or low shutdown 
pressures to high operating pressures. Containment devices in such systems 
can be provided that have soft, unidirectional seals or packing to protect 
system components. In order to maintain system integrity and avoid 
damaging the soft seals, a lower gas pressure in space 16 may be desired. 
A high primary system pressure event, however, can rupture a primary 
containment such as wall 12, and it may be desirable to protect the leak 
detection system from the high pressure. Accordingly, check valve 44 is 
provided to prevent reverse flow through line 20 toward gas supply means 
18. The high pressure condition is indicated by pressure switch 46 
actuating light 48 and alarm 50, indicating the containment failure even 
absent gas flow from gas supply means 18 to the location of the failure. 
Additional leak monitoring means can be provided, such as vapor sniffer 52 
and liquid detector 54, as secondary or backup indicators of leakage into 
space 16. 
FIG. 2 illustrates an embodiment of the invention having a plurality of 
secondary containment devices and monitoring means. Double stem seal 
valves 56, and double wall piping 58 each have containment failure volume 
60 adjacent to a primary seal, a secondary seal, or both as described 
above for tank 10. Gas supply means 18 is in communication with each 
containment failure volume 60. A primary or secondary seal failure 
adjacent to any one or more of containment failure volumes 60 leading to 
gas flow thereto from gas supply means 18 will thus be indicated by 
flowmeter 26, alarm 28, and light 30. Isolation valves 62 are provided 
should it be desired to seal off one area of the system from another, such 
as for leak detection, leak isolation, and maintenance. Pressure gauge 64 
can be provided as assurance that gas-containing lines are pressurized and 
are at the desired set static pressure. Pressure relief valves 66, and 34 
and 36, respectively, are means for preventing overpressurization of the 
gas-containing lines or other components positioned between gas supply 
means 18 and containment failure volumes 60, and between gas supply means 
18 and space 16, respectively. Such means help maintain system integrity 
and avoidance of damage to system components, the containment failure 
system, or containment seals downstream of check valve 44 due to pressure 
transients, over-pressurization conditions due to primary system leakage 
into the gas lines, and the like. If desired, means responsive to pressure 
or flow changes, such as pressure switch 46 (described above) and flow 
switch 68, can be provided to alert the operator of such system 
fluctuations. Flow switch 68 actuates indicating light 70 and alarm 72. 
Also, further gas pressure regulating means such as precision pressure 
regulating valve 74 can be provided to establish a precise gas pressure in 
the lines to the containment failure volumes. Other monitoring devices 
such as pressure gauge 76 can be provided where desired. 
Referring again to FIG. 1, upon a failure in wall 12, wall 14, or a failure 
in a wall rupture seal, and when primary system pressure is less than gas 
pressure in space 16, gas flows from gas supply means 18 through line 20, 
filter 22, regulating valve 24, flowmeter 26, and flow switch 32 into 
space 16 and through the breach in containment. Although the gas flow can 
be continuous or noncontinuous, depending on the location of the 
containment breach, even a temporary flow condition is capable of 
detection. Flowmeter 26 or flow switch 32 can indicate the occurrence of 
the gas flow, alerting the operator of the containment failure. 
The invention therefore provides a useful containment failure detection 
system and method which can be readily used in conjunction with leak 
detection and leak isolation equipment. The invention avoids the prior art 
problem of false readings caused by pressure changes due to ambient 
conditions since its operating principle is based not simply on a change 
in static pressure but rather on gas flow into the containment leakage 
volume. The invention can detect containment failures that are difficult 
to detect with other detection systems, such as those that rely on visual 
leak detection, measurable pressure changes, and constant ambient 
conditions. Visual detection is not reliable when a leak is not readily 
observable or is occluded by environmental conditions such as dirt, ice, 
ambient temperature variations, or the like. Leak detectors relying on 
pressure measurements can be prone to false readings and, in cases of 
small leaks or under changing ambient conditions, may not register at all. 
The device and method of the invention satisfy various government laws and 
regulations requiring leak containment, monitoring, and detection in 
fluid-containing systems. 
The invention has been described in detail with particular reference to a 
preferred embodiment thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.