Valve Linearization

• posted

Hey there,

I have a valve I have linearized on a pipeline inlet into our sour gas plant. Typically this 8" butterfly valve runs wide open, and when it does, the pipeline pressure upstream of the valve is just under 100 kpa above the downstream pressure (plant pressure). When the valve begins to close the pipeline pressure goes up, and then obviously the valve gain goes up. I have a H2S slug controller configured in the DCS to close the valve to 20% when the slug is detected, then control the rate in of the slug, and then disable, and go back to flow control when the slug is over. Depending on the length of the slug, there can be 1000+ kpa differential across the valve when the slug is over, so what was the flowrate with the valve wide open before the slug is now flowing with the valve around 50% open until the pipeline pressure is brought down. So anyway, I figured the way to linearize the valve was to have the valve wide open, with minimum dp across the valve, and move the valve to 10%, let the flow stabilize somewhat and then move to 20%, and keep going that way. By the time I finish, I'm flowing a lot more than at first because I've created line pack in the pipeline, so I just have to kind of figure the best fit for the top numbers.

So my question is, is there a better way to linearize this valve? I did get the valve profile I expected out of a butterfly, and it seems OK. The flow controller is kind of tuned for about a 500 kpa differential, so I spit the difference and get adequate control. Thanks.

Curtis

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• posted

What you need is a linear INSTALLED flow characteristic; i.e. dM/dx (or dQ/dx) (small-signal gain, or slope of the curve relating flow to valve opening) to be approximately constant across the valves opening range under installed conditions. Generally speaking: a) A situation in which pressure differential across the valve is constant at all flows calls for a valve with linear INHERENT flow characteristic, to produce a linear INSTALLED flow characteristic. b) A situation in which the pressure differential across the valve decreases with increasing flow and valve opening calls for an equal-percentage INHERENT flow characteristic, so as to compensate and produce a linear INSTALLED flow characteristic.

I may have misunderstood; but this latter situation sounds like yours. Butterfly valves have an approximately equal-percentage INHERENT flow characteristic, so I'd expect the INSTALLED flow characteristic of you valve to be approximately linear. Use plant data to make phase plots of normalised flow (0-100%) vs. stem position (0-100%) over time. How close to a

45-degree line is the curve?

Kelvin B. Hales Kelvin Hales Associates Limited Consulting Process Control Engineers Web:

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• posted

I haven't worked on the big industrial stuff, but here's what I think:

Assuming roughly atmospheric pressure on the downstream side of the valve put a pressure sensor on the upstream side. You'll have to do some PLC programming but at reduced valve openings your flow should be roughly proportional to the valve opening * sqrt(pressure). At wide openings the flow will be controlled by the pipeline; it'll be up to you to figure out where the valve stops making a difference and the pipeline head drop takes over.

If you can't assume atmospheric pressure on the downstream side then think of a differential pressure sensor, or two sensors.

An approach that is adequate in theory and will range anywhere from perfectly adequate to grossly malfunctioning in practice is to build a little dynamic model of the pipe pressure as a function of flow and time. If you can't make it work with a simple low-pass filter with perhaps a squared term in there for flow then it's probably too complex. Use this model to predict the pipe pressure given the valve position and use the valve position to predict flow. Then servo your valve position to get the flow that you want.

This is a WAG. YMMV. UAYOR. IMHMHUMA. Etc.

• posted

There's obviously something I don't get. Is there a reason not to just use feedback? Sense the flow with a prop, Pitot-static, venturi, or other flow gauge and adjust the butterfly valve to achieve the desired value? I know it works for sewage; why not gas?

Jerry

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Because it's MONDAY and I'm not THINKING yet!

Oh damn -- it's tuesday.

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...

James Watt didn't invent steam engines. They had been around for generations when he produced his first one. He certainly contributed many refinements, such as extracting energy from both pressure and condensation in the same space, and the eccentric-driven slide valve. His great invention, the one that attached his name to the mechanism and relegated Newcomen* to a footnote, isn't really part of the engine at all: the flyball governor. It's principle is simple: use the engine's speed to close a butterfly valve called a throttle. Without a governor to control the throttle, an automatically-valved steam engine is unstable. With a governor, it's speed is steadier than a horse's.

It is significant that, although steam engines were used in France and Germany as well as England, the governor is an English invention. A case can be made that it wouldn't have happened in a country without a Parliament.

Consider the poultry incubator. These were in use with moderate success in several countries, marvels of clockwork. In order to maintain reasonably constant temperature after stoking, dampers were operated by clockwork and the firewood size, dryness, and quantity was carefully controlled. With properly shaped cams and careful clockwork regulation, the system succeeded more often than not. From a social perspective, the scheme was monarchical. Intelligent instructions, scrupulously carried out, was the recipe for success.

In England, with its limited rather than absolute monarchy, the social paradigm of using analysis of events to guide future action led to a different technical approach. The temperature in the incubation chamber was directly controlled the dampers. The firebox could be restoked any any time with a variety of reasonable fuels. Indeed, doing that depended on the invention of the bimetallic strip, but many historians of technology contend that the *idea* of feedback called the invention forth, just as it later called forth the flyball governor. The idea of feedback is not a natural one in an authoritarian society. With an authoritarian mindset, a controller *controls*, it doesn't accept input.

Jerry _______________________________

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