Regarding the backpressure correction factor for balanced bellows PSVs in liquid service, Kw:

Has anyone seen values for Kw for backpressure/setpressure ratios greater than 80%?

I have a situation where I have a ratio greater than 80%, but I have never seen Kw values published for this range.

I have not and you shouldn't either. Your tail pipe (and downstream header, if applicable) is way too small. If this is a new design, increase your pipe size. You may want to re-evaluate your credible scenarios. You may also want to check if you bought (or are going to buy) too big of a relief valve or are buying the wrong type of valve. Bigger valves mean larger rated (capacity) flows.

If this is a check for an existing system, again, re-evaluate your scenarios. Check to see if your relief valve is the correct size (too large?) or type. You may have to improve your piping.

Check with the vendor you are interested in. They may be able to advise further.

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