Selecting a Compressed Gas Containment Program
The presence of compressed gases at any industrial facility can impact the safety of a plant's operation. Facilities that use hazardous compressed gases are concerned...
By Gerald F. Connell
The presence of compressed gases at any industrial facility can impact the safety of a plant's operation. Facilities that use hazardous compressed gases are concerned with design and operating conditions not seen at other plants and the presence of these compressed gases create situations that require special considerations.
Containment of these gases in the event of a release is a required plant safety procedure. The designer of these facilities should follow a plan that leads him to the most cost effective and simplest containment method. To attain this objective, the design requirements and operator needs must be determined prior to any other planning.
Containment or safety devices must be evaluated on technical capabilities as well as their practical and economic merits. This must be weighed against the potential risk to the facility, personnel and the surrounding community.
Containment can be achieved from one of a number of choices or combinations of these choices. Here are some of the more popular methods.
Chlorine Institute Emergency "A" or "B" kits are designed to provide a temporary patch or seal so that a leak may be contained from either a 150 - pound cylinder (A Kit) or 2,000 pound (ton) container (B Kit) of chlorine or sulfur dioxide. Leaks at valves, fusible plugs, and cylinder or container walls are contained with these kits. Once secured, the container can then be moved and transported to a disposal site in a safe fashion. "A" and "B" Kits have been in use for over 70 years and have performed well during this time.
Most compressed gas chemical suppliers have containment vessels that will hold a leaking container of gas/liquid. This equipment, a "coffin," is usually portable in nature. Suppliers can come to the site with the coffin and place the leaking container in the coffin. Coffins are pressure vessels that can accommodate the entire cylinder or ton container.
While large and cumbersome, coffins offer the advantage of passive containment. No power is required and there is no exigency to repair a leaking cylinder or container located within the vessel. Some coffins may be a permanent (non-transportable), sarcophagus with process connections. In the permanent type, the container, as well as the gas feeder, is located inside the coffin. Any leak on the cylinder, ton container or the process connection is confined to the coffin.
Scrubbers are designed to exhaust fugitive gases from storage or use rooms for absorption in an absorbing solution. In the case of chlorine and sulfur dioxide, caustic soda is used as the absorbing medium. The absorption process will produce sodium hypochlorite (bleach) from absorbed chlorine or sodium sulfite from absorbed sulfur dioxide. Either solution is easily transported to a suitable disposal facility or can be used as a source of disinfecting or dechlorinating agent.
A scrubber requires suitable start up time to commence operation. The length of time is dependent upon its design. Thus, there may be an accumulation of gas before the initiation of the absorption process. However, on gas release, the scrubber will remove this initial volume of gas from the room and any additional leakage until the leak is stopped.
A dry scrubber using absorbent materials in a packed column is also available. The dry scrubber contains a material that will absorb or react with the released gas emission.
In-Line Ball Valves
Valves that close automatically in a pressure line in response to a gas/liquid leak, fire, earthquake, etc. are used to shut the system down. These in-line valves are equipped with operators (electric or pneumatic) to shut a valve in a liquid/gas line. These in-line valves, most often ball valves, can be installed permanently on either a liquid or gas pressure line downstream of the cylinder or ton container valve. The ball valve that closes at that point only provides downstream and not upstream protection. Any upstream leak continues until the cylinder or container valve is closed.
Automatic Valve Operators
Automatic valve operators (AVOs) are mounted on the cylinder or container valve at the gas/liquid source. The AVOs close the cylinder or container valve either automatically from a signal activated by a gas leak detector, earthquake, fire or remote manual action. These valve operators, sometimes called valve actuators, are either electrically or pneumatically operated.
AVOs offer several advantages over a shut off valve installed in the pressure line. Any leak in the cylinder or container valve, valve packing and pressure line downstream to the process can be stopped. The AVOs are reusable at each cylinder or container change.
With AVOs, the majority of leaks can be terminated without the need for personnel to enter the affected area. In addition, AVOs can be initiated automatically so that unattended facilities can be shut down within seconds of a leak.
Container valve mounted vacuum regulators that are provided as part of gas feeder systems automatically shutoff chlorine gas flow when downstream vacuum is lost. Some code interpretations allow the vacuum regulator to serve as a shutoff valve since shutoff occurs at the vacuum regulator mounted at the outlet of the container valve. When vacuum regulators close, the gasket connection, fusible plug, valve packing and packing nut can continue to leak. Additionally, vacuum regulators can vent chlorine gas if the regulator is pressurized or otherwise exposed to contamination.
Choosing an Option
The designer wants the best equipment or method for each location. The reliability of any equipment is reduced as the complexity of the containment method is increased. The frequency of maintenance and testing required by a particular containment system can have a negative impact on the desire to use the system. The maintenance required to sustain a given level of reliability increases dramatically with equipment complexity. These points and others should be evaluated for all systems.
A scrubber can be described a mini chemical processing plant. The successful performance of the scrubber is dependent on the proper functioning of the exhaust fan and motor, circulating pump and motor, liquid seals and nozzles, instruments, etc. A malfunction or poor efficiency of any one of these items can cause the scrubber to perform poorly and fail to fulfill its intended purpose. A malfunctioning scrubber may actually exacerbate the severity of a chlorine gas leak. For example, if the caustic circulation were to fail with the exhaust fan operating, chlorine gas would be discharged into the ambient air and surrounding area.
The most effective means to verify scrubber reliability and readiness is testing under actual emergency conditions. However, since it is not practical to generate a chlorine gas leak, verification of the function and absorption capacity of a scrubber is difficult. The only test that can be employed is to "dry cycle" the pumps, fan and motors, using ambient air as the gas to be scrubbed. This is not a thorough test because conditions caused by factors such as caustic solidification, plugged nozzles or foaming are not encountered.
Automatic Valve Operator
The automatic valve operator (AVO) or actuator can be fully tested each time a cylinder is changed. The valves and hardware are precisely those that would be used to isolate any leak subsequent to the test. Test results can be observed and verified under actual operating conditions. The observations with the control system diagnostics verify the actual function.
The AVO consists of a single electric motor directly acting on the valve stem via a gear train and rigid mechanical components. Some AVOs are pneumatically operated. The electrically operated AVO is failsafe since a battery supplies the system's power. During times of power outage, the AVO will close the valve. Scheduled maintenance consists of periodic testing and annual battery replacement.
If the actuator, controls or battery should fail the self-test, each can be replaced in minutes and the system retested on the spot by operators. Only the replacement of the control circuit board and battery charger requires the skills of an electrician.
AVOs offer a practical and economic secondary safety enhancement in accordance with the intention of the UFC and IFC fire and building codes. They do not perform the same function as a scrubber, which absorbs a leak in progress. However, by terminating a leak quickly these operators provide a similar level of safety enhancement.
The greatest risk reduction can be achieved by installing a combination of containment systems. For example, both a scrubber and an AVO are activated automatically and will provide maximum protection against a release. This combination offers a degree of redundancy that might be desired in a particularly sensitive facility such as a highly populated area.
Choices often depend on site requirements and operator capability. A system that is too complicated can defeat its purpose by not working when needed or requiring a higher degree of operator capability than currently available at the site.
The better philosophy might be to provide the simplest system that offers the least complexity. Invariably this could also be the least expensive option since a minimum amount of parts is required to keep the system operational and in a 100% ready mode.
The best combination under these circumstances would be to use the vacuum regulator mounted on the container valve combined with an automatic valve operator. Data suggests that the simplest, most cost effective method is the AVO with a vacuum regulator followed by the AVO only. The most effective, from a security standpoint, would be combination of the scrubber, vacuum regulator and AVO.
About the Author:
Gerald F. Connell is a consultant in chlorine use and handling and chlorination of water and wastewater. He is the author of The Chlorination/Chloramination Handbook (AWWA) and the Chlorination/Dechlorination Handbook (WEF).