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Failed Regulator Characteristic for Relief Sizing

Failed Regulator Characteristic for Relief Sizing

Failed Regulator Characteristic for Relief Sizing

When sizing a relief valve for a failed regulator scenario, what is typically done to determine what the "failed" characteristic of the regulator is? I'm finding it difficult to get a vendor/manufacturer to tell me a "fail Cv" for their regulator, or even what the internal orifice diameter is. I'm also facing skepticism about whether any point on the performance curve from the manufacturer would accurately represent a failure state - the thought being that a performance curve may only represent a region of operation that is recommended for normal operation, and may not necessarily capture a wide-open or failed valve, in particular if one considers that a failure may represent a physical change to the regulator's internals that would not be represented by a performance curve. Furthermore, if given fail-Cv, without an orifice size, how does one determine a choked flow rate, not knowing the area where the choking occurs? I'm assuming one could determine an equivalent orifice size that would mimic the Cv provided, and then calculate choked flow based on that orifice? TIA


RE: Failed Regulator Characteristic for Relief Sizing

AFAIK, "failed regulator" or "failed control valve" refer to failure on the actuator, which would open the valve fully. IMO, if you consider a regulator failure as a mechanical failure that would give you a resultant Cv higher than the valve's original Cv, then this is totally unpredictable and no wonder the manufacturer is unable to provide you that.

In other words, I would not do that. Just use the 100% opening Cv.

Do you have any particular case study where the valve would have a higher Cv than 100% opening (as informed by manufacturer) due to a failure? It would be interesting to see...

Rio de Janeiro - Brazil

RE: Failed Regulator Characteristic for Relief Sizing

I'm surprised that the vendor is unable to supply you with such a figure as this is a very common thing for a designer to know to size his relief system.

You normally just go for what is the max flow at 100% open, not any additional "failure" of the internal parts though so maybe they are confused by what you are asking.

Usually you can either see from the data sheet or calculate the orifice size.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Failed Regulator Characteristic for Relief Sizing

With regulators, they don't tell you where "100%" is - they often just give you a plot of operating curves (reduced pressure vs. flow), where each one represents some arbitrary set pressure, and those plots may traverse all the way to the right-hand side of the chart, or otherwise just end at some other just-as-arbitrary point. There's nothing to tell you that the top curve is necessarily representative of "max adjustment", and there's nothing to show you where along the path from left-to-right the regulator has reached "100% travel". One might surmise that where the plot starts to drop off sharply is where the valve has reached 100%, but this may not be obvious (or included) on the plot provided, and, again, does the top plot shown actually represent the full adjustment, or just a maximum recommended adjustment?

As for a mode of failure, I'm only speculating, but maybe there's a diaphragm that could tear and open up the valve further than normal, or some kind of plug that breaks free and opens up the seat-port more than it would normally be. Such a mechanical failure may make the valve characteristic deviate substantially from its normal operation that would be represented by the curve.

For one manufacturer that does publish a Cv that's explicitly intended for RV sizing, some of those Cv's are larger than the largest Cv shown in the capacity tables for normal operation, which illustrates the potential pitfall I'm concerned about that the published performance data does not capture a "failed" or "wide-open" scenario, because they don't intend their valve to be normally operated in a wide-open condition. Another manufacturer doesn't provide any Cv's, just a series of reduced-pressure vs flow plots at a particular supply pressure.

Consider this manufacturer's explanation for why manufacturers of regulators would intentionally not publish Cv values: https://www.controlair.com/blog/cv-and-your-pressu... This page also shows the kind of capacity curves that I'm referencing that are typical.

So, where I'm at now is that if I need to size an RV based on a regulator failure, and I'm at liberty to specify the regulator in question, I need to use one from a manufacturer that will actually provide that information... but, what do I do if I can't change the regulator and have to do a relief calc? Is an estimate based on maximum published performance acceptable with a certain safety margin applied, knowing that the Cv estimated this way may be something less than "failed" or even "wide-open"?


RE: Failed Regulator Characteristic for Relief Sizing


I think you are asking for the impossible. In the case of a catastrophic unpredictable failure such as the one you describe or e.g. a choke valve failure in a gas well. I think i would examine the drawing of the valve and based on this assess an free area and use an orifice equation to determine capacity at a dP equal to max upstream and downstream P= set point +10% and then you would have to check if your dP from this valve to the PSV is not too high...

--- Best regards, Morten Andersen

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