Replacing PSL with gas detection

In order to understand a lot of what is written in standards that define instrumentation and control equipment, one needs to look back into its history.

Edward M. Marszal
Kenexis

In order to understand a lot of what is written in standards that define instrumentation and control equipment, one needs to look back into its history. A lot of the background on why things are designed as they are is lost on engineers just entering the field. Most often, they have no concept of the limitations that prior generations of instrument engineers and technicians were required to work under. Unfortunately, industry also tends to have trouble letting go of some portions of standards that have outlived their usefulness.

This author was recently asked to consult with the API 14C fire and gas subcommittee to offer some suggestions on how to improve the standard to accommodate some of the newer technologies that are available. One area of particular interest is making use of the gas detection equipment already in place on production facilities in the Gulf of Mexico. Specifically, many operators have made significant investments in gas detection equipment to safeguard their facilities. It makes sense that they should be given credit for the implementation of this equipment as a means of leak detection, in lieu of the traditional pressure safety low (PSL) leak detection mechanism.

API 14C acknowledges that leaks in pieces of process equipment are undesirable events that require the standard's typical treatment of primary and secondary protection measures. Traditionally, the primary protection has been provided through PSL measurement (which detects that the leak is occurring), coupled with activation of inflow shutoff and use of a FSV to minimize backflow. The secondary protection is then provided by "adequate ventilation" or an emergency support system if adequate ventilation is not possible.

The standard's choice of protection measures seems a bit curious from the perspective of current technology, but historically speaking makes perfect sense. At the point in time when standards like API 14C were being written, gas detection technology was not sufficiently mature to takeOffshore in a reliable fashion. In addition, typical operating pressures were sufficiently lower than they are today, as the industry seeks to develop ultra-deep reservoirs. This confluence of factors made the "PSL" approach appropriate. Using this approach, if a leak was very large, i.e. large enough to cause the system pressure to significantly drop, the process section would effectively be isolated, thus limiting the extent of the leak. In addition, the relatively low-pressure gas would be expected to dissipate fairly quickly in an open area with limited risk of vapor cloud fire or explosion.

At the time of the initial release of the standard, the decisions made by the standards committee made perfect sense. But, time marches on and it is incumbent upon us as industry practitioners to take another look at the standard and determine if those initial decisions still make sense in the light of new technology for leak detection and substantially different operating conditions. Whereas in the distant past gas detection technology was untested and not rugged enough forOffshore deployment, current technology is rugged, accurate, and proven. Furthermore, a wide variety of different detection technologies are available from a wide variety of equipment vendors. Current gas detection technology is much better at accurately detecting very small leaks. These detectors have the ability to detect leaks that a PSL would have no chance of being able to detect, because the leak rate would not significantly change the operating pressure.

Operating pressures in the Gulf have also risen and continue to rise. One result of that increase is that leak rates are greatly increased. Whereas an argument could be made that small leaks from lower pressure vessels do not pose a great risk in a well ventilated area, this cannot be credibly claimed for a high-pressure release. At an operating pressure exceeding 1,000 psi, even a small leak (e.g., 0.25-in. diameter) will, within a matter of seconds, result in a gas cloud sufficiently large that it could destroy large sections of a process facility if ignited. Furthermore, the PSL would have little or no chance of being able to detect this condition.

Instrumentation has improved and operating conditions have become more severe. As a result, some in the industry are considering change. The first step being contemplated is the inclusion of the use of gas detection systems in addition to (or, as an alternative where effective) the use of the PSL as the primary protection against gas leaks. This change would allow credit for the systems that some operators are already using. The next step might be (gulp) the requirement for use of gas detection systems as the primary protection against gas leaks. It is recognized that the second change will be a lot harder for operators of older, smaller, and lower-producing facilities to accommodate.

This is especially true since many of these are not equipped with gas detection systems; and they are often also lower pressure/lower-risk facilities. As a result, for the forseeable future, it is expected that gas detection will be an option but not a requirement.

One point of concern in moving to gas detection equipment in lieu of PSL is that the determination of the required number of detectors and placement of those detectors has been something of a black art. The standard committee hopes that this problem can be addressed by the proper specification and use of grid layout systems, or the use of fire and gas mapping technology and software. With the mapping technology/software, operators can calculate detector coverage and compare that against performance targets that are determined using a risk-based approach.

All of these things are currently under consideration by the API 14C committee. The API 14C fire and gas subcommittee wants and needs your input on this topic. If you have an opinion on this topic, you can make your opinion known by contacting the representative from your organization that sits on the API 14C; contacting an API 14C committee member; or you can contact this author, who will forward your comments on to the committee.

The author

Edward M. Marszal is President and CEO of Kenexis, a global consulting and engineering firm that specializes in the design of engineered safeguards such as safety instrumented systems or fire and gas systems. He can be reached at edward.marszal@kenexis.com.

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