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:
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"?
Simplify
