RANGE-ABILITY OF EQUAL PERCENTAGE CONTROL VALVES
Q: I'm an engineer employed in operation and upkeep of test facilities within the Indian Space Research Organization. I've got a requirement for a brand new
control valve to become set up in our facilities. Per ISA guidelines, I've calculated the max and min Cv needed through the process conditions. I'm selecting equal percentage characteristics for that control valve, but I have to specify the rangeability for that control valve.
With this, I've calculated the number of needed max Cv to min Cv (10 for that particular situation), and that I have selected the valve rangeability (50:1), that is greater than this ratio. Kindly correct me if my technique is wrong. I'd be highly obliged if you're able to guide me about this point.
A: I possibly could just state that your vendor is creating a false claim (50:1) as well as your valve is most likely OK, but I wish to become more useful, and so I can give a short explanation of the items equal percentage (=%) is, why it's used by us (Figure 1), what its gain is, and just how that gain pertains to stable control.
If your 1% alternation in the control signal produces a 2% alternation in the flow (also known as load) using that valve, it features a gain of two. As long as the pressure drop across the valve (ΔP) is constant, the terms % flow, % load and % valve coefficient (Cv) are interchangeable. The gain of straight line valves is constant the gain of non-straight line valves (=%, quick opening, parabolic, hyperbolic, etc.) varies with load. The gain of the =% valve increases with the flow, so a single percent control signal change may cause a 3% alternation in the flow when the =% worth of the valve is 35. The left side of Figure 2 shows the connection between a number of valve stroke and numbers of flow (Cv) of three valve characteristics on the straight line scale, as the right side shows exactly the same on the logarithmic scale.
With this particular lengthy introduction, now we are able to discuss rangeability, which (at constant ΔP) is the number of that Cvmax/Cvmin where the valve maintains its =% characteristics within 25%. Since the manufacturing tolerance from the plug is all about 10%, to begin with, and typical instability is all about 5%, the valve shouldn't be used close to the two ends (% and 100% stroke). Figure 3 shows one recommendation, suggesting the theoretical (line 1, dotted) and also the actual installed (line 2, solid) gain-to-load relationship of the =% valve ought to be within 25%. We are able to observe that the particular (solid) line stays within 25% from the theoretical (dotted) between about 5% to 70% from the flow (Cv). Therefore, the valve rangeability is 14:1. So, so far as your real question is concerned, a =% valve for the application is alright. Size it so your Cvmin to Cv max range will fall within the 5% to 70% range. Regarding the vendor's claim of fifty:1, forget that. Incidentally, when the ΔP over the valve changes with the flow (pumping systems), or you have flashing, cavitation or sonic velocity (clogged flow) conditions, you'll need a smart positioner. See Chapter 6.7 of Volume 2 of my Instrument Engineer's Guide.