At least 10 workers have died from natural gas piping explosions in the past 12 months. The most recent disaster, which occurred during gas system purging at the Kleen Energy Systems plant, claimed five lives and injured 27 workers. It’s time the industry understood the unique design and safety requirements for working with and purging natural gas piping.
Gas piping design, operation, and safety require a very specialized level of expertise. Recent events graphically illustrate the inherent hazards of improperly designed natural gas piping that is commissioned in a poorly planned and haphazard manner. Beyond the obvious risk of death and injury, explosions on the scale of the Kleen Energy Systems plant in Connecticut can cause extreme property damage worth hundreds of millions of dollars, cause significant lost production time, and irreversibly damage a company’s reputation (Figure 1).
1. Preventable tragedy. Workers at the Kleen Energy Systems plant in Middletown, Conn., attempted to clean debris in the plant’s fuel gas line with 650 psi natural gas. The gas purge, vented inside the plant building, found an ignition source and exploded. Six workers were fatally injured. Courtesy: Chemical Safety Board
To avoid a repeat of this and similar disasters, engineers and designers must be thoroughly trained in the code and practical design requirements for natural gas piping. Contractors should be especially cognizant of the safety requirements and worker training required when purging natural gas piping. In our experience, few engineers and contractors have the necessary knowledge or experience to safely purge and start up natural gas piping systems. It’s time to take a deep breath and reevaluate our commitment to safely handling natural gas in utility plants (Figure 2).
2. Never completely clean. Weld slag and contamination are rarely completely removed from a piping system. A well-designed pipe purge configured with strainers and sediment traps can be safely accomplished with the right planning and training. Courtesy: CEC Combustion Safety Inc.
What’s So Special About Gas Pipes?
We have also found that many designers, engineers, contractors, and maintenance workers do not understand that natural gas piping is quite different from the more familiar fluid or air piping, and therefore requires special knowledge and training. There is often a huge and dangerous lack of knowledge among those who routinely manage or perform gas piping repairs.
Six major steps are taken when performing a gas piping repair. Each of these steps should be the subject of hours of careful consideration. In general, these six important steps are:
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Planning/layout
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Isolation
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Making the repair
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Pressure testing
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Post-repair purge
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Re-introduction and light-off (the most dangerous part)
Step 6 may be the most important step because it is clearly the most dangerous. Whenever gas piping is repaired, extended, or newly installed, it must be purged. However, a safe and successful re-introduction of natural gas into the pipe and start-up of the equipment only occurs when good planning and execution occurred during the first five steps (see sidebar).
Even those conducting the work often misunderstand the fundamental rules and best practices for handling gas piping system start-up. But if a good plan is prepared by engineering experts and the correct precautions are taken by the contractor, you can have an incident-free project.
Consider this short list of important tasks and key questions when planning a gas-piping repair:
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Is there a written purging plan (pre-repair and post-repair) that identifies all of the pressure-testing needs and standards that must be complied with? Does everyone on the team understand the plan and the role they play in its execution?
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Has a gas re-introduction plan been prepared? Has re-introduction and start-up been discussed with the start-up team, including any unique hazards?
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Were the local emergency services or fire department involved in the planning? Is fire fighting equipment centrally located for workers?
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Has the gas piping design been reviewed for mechanical issues, including piping materials, rated valves and fittings, purge points, and blinds? Where are the isolation points, and how will isolation be safely achieved?
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Is there a natural gas isolation and equipment lockout plan? Is there a plan for cleaning lines and re-testing automatic valves after the project has restarted? Verify that all special shutoff valves are serviced. Verify that all piping is properly marked.
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Is the gas utility involved in the purge process (or should it be)? Do they have any special requirements?
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Have you determined the amount of nitrogen needed for the purges and leak checks and how it will be introduced to the piping system? Does everyone understand nitrogen hazards? Conduct safety training on the use of nitrogen as part of the planning process.
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Where will the purge be directed? What are the prevailing winds? Are there building vents or other equipment air inlets nearby? If a high-pressure pipeline blowdown or pressure release is to occur, modeling of the plume may be helpful (Figure 4).
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Are all electrical services secured in the area of the gas purge and where the venting of natural gas may occur? An explosion can only occur when an ignition source is nearby.
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The portion of the project site where gas purges take place should be cleared of all workers, except those performing the gas purge. Spectators should not be allowed.
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Document all of the pressure testing results (such as test pressures and how long they were held).
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