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Top 7 Mistakes Lurking in your Relief Valve Calculation Database (and How to Avoid Them)

Posted by Dustin Smith, P.E. on Jun 23, 2017 12:26:00 PM
Dustin Smith, P.E.



#7) Relief Valve Database is Outdated Due to Changes in Process

So, you receive your relief valve data base from your Process Safety Consultant and just like a computer bought anytime in history, it is out of date before you open the box. Changes happen at a pretty high pace in most facilities. Dealing with Management of Change must include the effect of the change on the Relief System Database and documentation. Not keeping this documentation up to date makes your relief system documentation almost as valuable as that "new" computer.

TIP: Dedicate resources to keep this relief valve database up to date. These resources could be a team of process engineers, a dedicated relief system expert, or a Process Safety Consultant. Process Safety Consultants can provide this service on a T&M basis, so you only pay for what you use and they bring their experienced knowledge base to perform this function efficiently.

#6) Relief Valves / Process Equipment Not Included

There can be many reasons why this error may occur: an engineer missed the equipment during the analysis, equipment was deleted from original scope, miscommunication between engineers and the facility, or a piece of equipment is truly not protected from overpressure. While these are some really good reasons, none are good enough. At a minimum, this omission can lead to code violation / citation during OSHA review.

TIP: As with Item # 3, have the engineer (Pressure Safety Consultant) provide a diagram or sketch that identifies all the pressure sources for a given system.This will make for an easier review.

#5) Environmental Credits

In this context, an "environmental credit" refers to an allowance for reduction of required flow rate for the external fire scenario on "liquid containing process vessels." This environmental credit can be taken if the facility has: i) an onsite fire-fighting capabilities, ii) the vessel is in an area that has good drainage, or iii) if the vessel is insulated with fire-proof insulation and approved jacketing material. (Credits can only be used in the relief system evaluation if the specific criteria are met.) There is a chance that this credit can be used without meeting the full requirements to do so. This would result in a relief system being undersized by at least a factor of two.

Prevention: Clearly state in the relief system guidance document the circumstances of the environmental credit being used. Ask the engineer or Pressure Safety Consultant for a query of the environmental factor used for all fire scenario calculations - as a quick way to check if the correct factor was used.

#4) Incorrect Unit Charge Rate or Basis

This issue is one of the hardest to identify once the relief system analysis is complete and can also render a large percentage of your relief system analysis useless as any rate dependent scenario is now incorrect. How does this happen? There are several ways: a change in unit charge MOC not considered, incorrect data from the facility, or miscommunication between the engineer and the facility. This error can not only lead to problems in the unit that the error occurred but also lead to a lack of confidence in any additional analysis performed.

Prevention: Clearly identify the unit charge rates (normal, PSM, typical) prior to any analysis. If the unit charge rate / basis is going to be changed, a revalidation of the work may be required as a part of the MOC process.

#3) Equipment Not Simulated Properly

An example of this issue is as follows: Pump A charges to Reactor B - however, during the relief analysis Pump X was used instead of Pump A. Pump X has a larger capacity than Pump A. Here is the issue. This error can happen anywhere along the relief analysis work flow and be caused by something as simple as a typographical error. Once the wrong equipment information is there, it can be propagated through the entire calculation process.

TIP: Have the engineer (Pressure Safety Consultant) provide a diagram or sketch that identifies all the pressure sources for a given system. This will make for an easier review. 

#2) Incorrect System Contents

While the chances of incorrectly completing this task are low, the impact of doing so can be great and has been witnessed first-hand. The engineer completing the relief system analysis is human, and of course prone to error. Since the engineer is most likely working on several systems at one time, it is very easy to have the wrong stream input into the calculations. For example, a Kerosene Feed Drum was once simulated with a light naphtha stream leading to an undersized relief device finding. This error propagated through the process, and significant capital was spent to fix an issue that should never have been an issue to begin with.

TIP: Take the time to review what streams are used in which calculation. Some relief system software allows you to query all systems and stream data. This allows you to look for inaccuracies quickly.

#1) Credible Overpressure Scenarios

One of the first steps in the process of evaluating a relief system is determining which overpressure scenarios are credible for the system being analyzed. However, these scenarios can automatically be identified for "like systems," or the scenarios are defined manually by the engineer performing the analysis. Due to the amount of time required, this analysis is not always reviewed with the process engineer or operations representative for the given unit. This action can lead to scenarios being overlooked – especially unique scenarios that may have occurred in the past.

TIP: Insist on reviewing and challenging the credible scenarios that the Process Safety Consultant has identified as credible.

Topics: Did You Know?

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