Relevant relief device considerations keep facilities safer

As technology evolves, most facilities or the brick-and-mortar part of the facility remains the same. This can lead to dangerous situations, as relief systems and devices are outdated and not matched to the current technology. It is vital to plant safety and operability that the design of these crucial systems is kept evergreen.

By Ryan Supple February 20, 2015

Imagine two engineers studying the piping of a process unit trying to troubleshoot a problem. Looking at a pressure relief device installed on a vessel, the first asks, “Why do you think that PSV is there?” The second answers, “I don’t know. It’s probably just there for fire.” Does that sound familiar? Refineries and other process plants of any age have certainly gone through modifications and changes that affect pressure-relief systems and other safety measures. The obvious question coming out of these situations is whether plant designers took all relevant safety scenarios into consideration when making those modifications. Putting that PSV in that location made sense at the time it was installed (we hope), but does it still make sense now that the plant has gone through subsequent changes? 

It is vital to plant safety and operability that the design of these crucial systems is kept evergreen. The best way to do this is by making any potential impact to relief systems part of the regular management-of-change process. That ensures future changes have appropriate analysis, but how should you deal with situations where evidence suggests the initial analysis was not done thoroughly or correctly? A common issue with relief system documentation is that it often doesn’t exist. Someone hunting for that information may find data sheets kept on file that only list “controlling” scenarios, and only show detailed calculations for one applicable set of conditions.

American Petroleum Institute’s (API) Standard 521: Guide for Pressure-Relieving and Depressuring Systems recommends that all reasonable scenarios be identified, and all scenarios deemed applicable have a calculated relief rate. It should be noted that OSHA calls out API 521 as a recognized and generally accepted good engineering practice for design and operation for pressure-relieving and disposal systems.

Analysis is key to golden relief rate

A robust process hazards analysis will help identify key relief scenarios. Some of the more common ones that come into question during this process involve gas blow-by on control valves between high- and low-pressure systems. While these scenarios may seem obvious, if there is little or no documentation, there is no way to tell which scenarios were considered in the design phase. A quick sanity check can help determine whether or not the relief device is designed properly. For example, if the relief device on the low-pressure side is only large enough to accommodate the relief rate for external fire, it would be prudent to investigate its adequacy for gas blow-by from the upstream high-pressure system. The flow rate associated with this scenario can be very large, and may require a large relief device to accommodate the associated relief rate. Such an analysis takes some thought—many variables go into the calculation, such as differential pressure and flow resistance.

Another commonly overlooked scenario is overfilling a vessel with liquid. Initial analysis may have considered this a very unlikely occurrence, and therefore relief devices were not sized to accommodate the associated potential liquid relief rate. Process safety incidents, such as the BP Texas City Refinery explosion in 2005, offer proof that such scenarios are indeed possible and should be evaluated as necessary. In addition to checking for adequate capacity, a device’s ability to relieve liquid should be verified. 

Performing in-depth analysis and calculations is time-consuming, and most operating companies do not have the personnel in house to carry out this task in a timely manner. Third-party studies may be the appropriate solution to fill the calculation and documentation gaps associated with relief systems. Solutions offered by third parties range from simple, stripped down, spreadsheet approaches to complex software packages with all imaginable bells and whistles. Regardless of what the budget allows, the important thing is that competent and conscientious people perform thorough analysis. Extravagant software packages have lots of nice features, but they alone do not ensure a quality study was performed. 

– Ryan Supple is a process safety engineer for ConocoPhillips. He graduated from the University of Toledo with a BS in chemical engineering and serves as an officer on the AIChE Young Professionals Committee. Edited by Peter Welander, content specialist, Oil & Gas Engineering, 

Original content can be found at Control Engineering.