Frequently Asked Questions for Relief Valve Sizing

Frequently Asked Questions for Relief Valve Sizing

Cognascents’ relief valve sizing and flare header design experts have lead projects across the oil, gas and chemical industries. The studies include precision-strike PSV sizing efforts, MOC-related project updates, third-party relief and flare header compliance audits, and full-facility relief valve sizing and flare header studies. Throughout their years of experience, our experts have been asked countless questions including the following.

Question 1) For my facility, how often do we need to update our relief valve sizing and/or flare header design?

Updating the sizing basis for both relief valves and the flare header should be done as a part of the MoC process for each change made to a facility determined to have an effect on these systems. It is in our experience that changes to a facility occur without the relief and flare header sizing being updated. This oversight that occurs at facilities leads to vulnerabilities as changes to flowrates, equipment, and connections can render the relief and flare systems inadequate. As more and more of these changes to the facility are made without corresponding updates to the relief and flare header analysis, the more likely an entire revalidation of the relief and flare header will need to be performed.

Question 2) What are some of the most common deficiencies in a facility’s relief valve sizing?

As methodology advances, some topics come into focus as the industry understanding becomes more refined. Some re-occurring topics we find that were overlooked in previous relief valve sizing studies are, but not limit to: high inlet pressure drop, high outlet pressure drop, liquid overfill scenarios, gas blowby scenarios, pocketed piping, check valve installations in relief valve piping, etc.

Question 3) My facility has satisfied all deficiencies from the previous relief system and flare header study, should we expect to find deficiencies in a new study? Why?

Our relief sizing and flare design experts have worked on many projects for many clients across the entire oil, gas, and chemical industries without ever doing a project that resulting in zero findings. The amount of findings strongly correlates with the strength of your facilities MoC process. It is not uncommon for relief valve sizing to be overlooked during project MoCs. Additional reasons our experts may have findings are inaccuracies in the previous relief sizing study, updated methodology, increased access to information, and many more.

Question 4) What kind of software does your company use?

Cognascents has generated our own Excel based templates that we use if our client does not have a preference. One of the benefits our clients have found with this approach, is that once the project is over, they can update their relief valve sizing on their own. We however, are willing to use whichever software a client is currently using upon their request.

Question 5) There are a lot of relief valves at my facility, some that go to flare and some that go to atmosphere, is it possible to do a unit at a time for budgetary purposes?

Cognascents is a flexible company that works directly with our clients to develop Scopes of Work catered to the client’s particular needs. Relief valve sizing is no different.

Question 6) I am not sure if my facility’s relief system and flare header are adequately designed and/or documented, is there a way you can perform an audit of our system to see how we should proceed?

Cognascents is equipped to perform full facility PSM compliance audits. If your company is specifically interested in the gaps you may have in relief valve sizing / flare header design, we do perform third party compliance audits on just those elements as well.

Relief Valve Sizing and Flare Header Design Whitepapers

Cognascents encourages you to take the opportunity to read a few of our whitepapers related to relief valve sizing and flare header design.

Question 1) Will It Really Make that Much of a Difference? Broad Effects of Operational Changes on Relief System Design

Throughout the life of an operating facility, changes to the process are inevitable and potentially affect the systems that keep personnel and equipment safe. The Management of Change (MOC) process is intended to evaluate proposed changes prior to implementation to assess and address any risks that might be introduced as a result of this change. The MOC process typically includes a process hazards analysis (e.g. Hazard and Operability Study, Layer of Protection Analysis), which evaluates safeguards and independent protection layers (IPLs) for the proposed changes.

Of the many IPLs, pressure relief devices are often overlooked and a re-evaluation of the relief system design basis is sometimes not performed. The authors have observed that personnel do not always recognize the operational change may affect the relief system.

In this paper, the authors explore what types of changes should trigger a relief system design review, exploring why minor modifications may have major ramifications. They also provide specific examples of the most common changes that demonstrate how the relief system design can be affected.

The target audience for this paper is anyone whose responsibilities include (1) pressure relief analysis, (2) process safety management, (3) management of an operating facility, (4) process engineering, and (5) process safety information management.

Evergreen, flare system analysis, Hazard and Operability Study (HAZOP), Independent Protection Layer (IPL), Management of Change (MOC), overpressure analysis, pressure relief analysis and design, Process Hazard Analysis (PHA), process modes of operation, Process Safety Information (PSI), relief device design basis, relief rate calculation and sizing, safeguard.

Question 2) Overpressure Protection Assurance Through Management of Change

Pressure relief analysis and design basis integrity are paramount to the safe operation of any facility handling a highly hazardous chemical or operating a process system under potentially hazardous conditions. As one of the process safety information elements of the OSHA 1910.119 Process Safety Management mandate, facilities handling highly hazardous chemicals must establish and maintain their overpressure protection design basis. “Relief Systems” and “Blowdown Drums and Vent Stacks” are also on OSHA’s National Emphasis Program Static List of Inspection Priority Items, which was implemented in June, 2007.

Data from numerous pressure relief analysis efforts provide evidence that deficiencies continue to exist in overpressure protection systems, including pressure relief devices and effluent handling systems. One of the primary reasons for these deficiencies is the shortage of technical personnel with sufficient experience to identify when the pressure relief analysis design basis must be reviewed and perhaps modified as changes to the process are made. The authors contend that comprehensive integration of pressure relief analysis expertise with a facility’s management of change (MOC) program is necessary to establish and preserve overpressure protection integrity.

This paper shows how to achieve accelerated MOC program improvement when modifying existing processes and equipment by including key questions regarding pressure relief design aspects of the change under review. The focus is on developing an assessment checklist for the project design team. Responses to the checklist provide guidance regarding potential involvement of a pressure relief analysis subject matter expert as an integral element of the proposed modification.


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