Smith & Burgess' Blog

Top Ten Things Affecting the Quality of your Relief Validation Study

Posted by Brandon Ring on Feb 24, 2019 12:31:43 PM
Brandon Ring


Pressure relief systems and devices are used to prevent equipment and pipework from over-pressurization which can lead to mechanical failure and associated release of containment.

In order to function correctly, relief systems need to be properly designed to recognized standards, correctly installed and properly maintained. Changes to associated plant and pipework also need to be appropriately managed; often, seemingly unrelated changes can have an impact on the relief case of the system and render it incapable of protecting the system as designed. This is seen particularly on older plants with poor change management history.

The following list provides some details on the areas you should focus on when improving the quality of your Relief Validation Study.


 Equipment Data:

Equipment data is vital to any relief system evaluation. The most important data are the vessel design conditions (MAWP / MAWT) and vessel dimensions. It is imperative to have the current documentation for each equipment item. While some equipment forever remains the same, others have gone through one or more re-rates. Always check to insure that the most recent documentation is available. 


 Flare Design Information:

Flare design information (specifically the flare tip information) can have a tremendous effect on the sizing basis of the flare system and can lead to significant capital investment into the flare header that feeds it. Some facilities do not identify the flare tip as a piece of equipment, so there is usually little to no information (sizing basis, pressure drop, etc.) available. The lack of this information will result in the use of conservative information found in industry literature, found during the evaluation phase. This might possibly lead to an undersized flare or flare header(s).

TIP: If no information is available for the flare tip, contact the manufacturer (if known) to obtain the design information.


Relief Valve Data:

Most relief valves are required to be pulled, inspected, repaired and pop-tested every five years. For the relief valve analysis, this requirement is both good and bad. First, the good: There is recent documentation of the type of the relief device, manufacturer, type, set pressure and so on – all required information for relief devices. The bad: relief device information becomes out of date every five years, so it is easy to use wrong information.

TIP: Some valve testing companies allow for an online reference of relief valve information. Providing the pressure safety consultant with access to this data, ensures that the most up to date relief device information is used.


Scenario Identification:

Overpressure scenario identification, is as important to the success of the validation efforts, as who is doing the evaluation. Working with corporate guidelines, and in conjunction with unit engineer (or operations engineer), the scenario identification should be a shared task with the pressure safety consultant. This ensures that not only the correct "normal" scenarios are taken into account, but that the scenarios that are unique to the specific unit under evaluation are also considered. No one knows the process better than operations. Not using this resource can result in an incomplete analysis, an undersized relief device and possible re-work.



Major changes such as, increasing feed rates or major equipment modifications; can render a relief analysis evaluations obsolete before they are even complete. These types of changes should be kept to a minimum or, at least, identified in the beginning phases of the process so that they are incorporated into the evaluation. The last thing that anyone wants is to spend resources on an evaluation only to have the results negatively impacted due to changes made during the analysis.


Choice of Contractor:

What qualities do you value in a pressure safety consultant? Are all pressure safety consultants the same? Like my grandfather taught me long ago – you get what you pay for. If you want the bare minimum documentation with no thought or consideration for your interest in mind you can find that. However, a good pressure safety consultant is one that has the ability to do the required relief and flare header analysis, provide economical solutions to your findings, and provide you with a tool that allows you to keep this documentation up-to-date.


Over Conservative Guidelines / Process Basis:

The process basis sets the rates for all rate dependent scenarios. The guidelines document is basically the rule book followed during the relief system evaluation. Being overly conservative is not necessarily a good thing. For example: The charge rate to a unit is 1000 GPM but the process basis is 5000 GPM to account for future increases. The undersized devices will require mitigation and will be "adequately sized" until the unit charge rate is 5000 GPM. An oversized relief device is almost as bad as an undersized relief device. Being overly conservative leads to increased mitigation costs, possible re-work, and overall quality issues. Good engineering judgement should be used when developing these items.


PFDs / H&MB:

Process Flow Diagrams (PFD) and Heat and Material Balances (H&MB) provide valuable process information to the engineer doing a relief system evaluation. These "gold mines of data" contain all of the information of a system, such as the operating temperatures, pressures, flow rate in / out, and the process basis for the entire study. Out of date H&MBs can derail an evaluation before it even gets started and inaccuracies can lead to costly confusion and re-work.


The Project Manager's Level of Experience:

A relief system evaluation is not solely a data gathering and documentation effort.Instead, it is a complex engineering evaluation that can involve multiple process units. Each presenting their own unique challenges. This level of complexity requires the same level of experience to manage and execute. Ideally, the representative from the operating company should be well versed in relief system design and have a solid grasp on the operation of the process being evaluated. Assigning a co-op/intern or recent college graduate to this task can lead to multiple problems, including: inefficiencies (higher engineering costs), schedule overruns, and poor assumptions leading to inaccuracies.


Quality of P&IDs:

Piping & Instrumentation Diagrams (P&IDs) contain the most essential information that an engineer will use during the process of sizing a relief device. P&IDs provide information for overpressure scenario identification, process equipment relationships, and process flows. Out of date or incomplete P&IDs can have a tremendous effect on the quality of the relief systems study. Without current and correct P&IDs the relief systems study has no chance of success.

TIP: Provide the pressure safety consultant the latest revision of the P&IDs to begin the validation project.

Topics: Process Safety Optimization, Engineering Review, Engineering Analysis, Relief System Design, Relief Systems Revalidation

 Safety Knowledge Should be Share...

“It should not be necessary for each generation to rediscover the principles of process safety which the generation before discovered. We must learn from the experience of others rather than learn the hard way. We must pass on to the next generation a record of what we have learned.” - Jesse C. Ducommun, Safety Pioneer
As Process Safety engineers and consultants, we believe our unique position creates a responsibility for Smith & Burgess to share our years of accumulated safety knowledge.

>>> Keep up-to-date with our latest Process Safety knowledge, research, and content by subscribing below. >>>

Subscribe Here!

Recent Posts

Post By Topic

See all