Catch-up today on technology and risk management via published webinars by ioMosaic
Do you understand PRV instability, the risks associated with it and how to perform an engineering analysis to assess it? Watch this webinar to find out. This 60-minute ioMosaic-sponsored webinar outlines the risks associated with potentially unstable PRV installations and reviews the latest developments in PRV stability. Take this webinar to review practical methods for identifying and evaluating high-risk relief system installations, examine how to perform an engineering analysis, explore PRV stability screening methods, and understand why speed of sound estimates are important. Case studies on vapor systems, liquid systems, and two-phase systems will demonstrate the application of what you learn in the real world.
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Q. The use of force balance methods is not a consensus in the codes and industry. Can you explain how and which cases the force balance method can be used? For instance, is the force balance method acceptable to justify that release valves with IPD above 3% are stable?
A. According to the 6th Edition of API 520 Part II released in March of 2015, the use of force balance methods are now a recommended engineering analysis in the industry, although more detailed dynamics methods also exist. We are working with a number of companies that have accepted the force balance approach. I would say it’s definitely accepted within industry and there has been a lot of research conducted to validate this approach. It varies from company to company but the trend I am seeing is that it’s being accepted more and more.
Q. When would you recommend performing a force balance?
A. In an ideal world, I would say all the time for every relief system because even systems that don’t exceed 6% in pressure loss can exhibit unstable behavior. But I would recommend prioritizing the order in which devices are analyzed and that the highest priority are devices that fall between 3% and 5% in the pressure losses. A lot of companies opt to go directly to mitigation for anything that exceeds 5%.
Q. Is it possible that the fluid dynamic looks okay but the force balance analysis indicates an issue?
A. They tend to go hand in hand so if the fluid dynamics look ok then the force balance should be ok. We haven’t come across cases where they contradict each other.
Q. How do you proceed if you don’t have all of the key force balance input?
A. Some of the inputs can be approximated from published correlations. For example, I mentioned the Grolmes’ correlation during the presentation. There’s also Cremer’s correlation. Published correlations will help you determine inputs such as valve opening time, spring coin stem, or valve mass and motion. ioMosaic can also provide guidance on determining those inputs.
Q. Can chatter occur in liquid full relief systems or liquid full systems?
A. I would say so, yes.
Q. How often has performing a force balance calculation resulted in a device with high inlet pressure loss becoming adequate due to the expected behavior being stable?
A. I would say that approximately 75% of the systems that we have analyzed end up being stable or acceptable. Now that does depend on what we are analyzing. And in a lot of cases, those systems are between 3% and 5% in the pressure block to begin with. We have also seen that bellow devices and pulp action pilots interfere better than conventional systems, but I’d say we definitely see the investment being valuable in the sense that a significant percentage of the valves are deemed to be stable action.
Q. Which valves don’t necessarily have to follow the 3% rule, if any?
A. I would say pilots or remote sensing pilot devices and external relief devices, as mentioned in the API standard.
Q. Does the force balance have to be performed only at the 10% overpressure? For example, can you use a higher percent overpressure, such as a device sized for fire at 21% overpressure?
A. In API 520 part 2 it does specifically recommend 10%, so that’s what I would stick with.
Q. For valves that are installed in the field, who typically advises that chatter is a serious issue?
A. You would be relying on your relief valve shop or maintenance and inspection primarily, as we would be getting input from maintenance and inspection if there is any evidence of chatter or any potential damage to the valves. Operations may be able to tell you if the relief devices have gone off and they might have experienced hearing the relief valve go off, which could indicate the chatter as well.
Q. Can you do a force balance on a system that has multiple devices at different sizes and set pressures?
A. Good question. That’s actually something that ioMosaic has been putting a lot of work into recently. The pieces where you got multiple relief devices on a manifold are a very complex system but that’s something we have made a lot of progress in. We are going to be issuing a white paper on that in the fairly near future and so, yes, that’s something that can be modeled.
Q. You showed a chart with relief valve devices showing an inherently higher susceptibility to chatter, can you explain why?
A. It’s all dependent on the beta ratio, and that’s been understood for a long time. This is based on how the beta ratio is defined and the inlet pipe actual diameter, divided by actual discharge diameter.