Maintaining? Any of the above. Close it all the way and you get a great dP across the valve.
I suspect you mean throttling though.
Valve Magazine had/has a series called "Back to Basics" which covers this. Otherwise, "Valve Selection Handbook" by Smith and Zappe (ISBN 978-0750677172) is a good resource.
This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.
Of only those choices the globe is suitable for pressure control purposes.
You can get some pressure differential at nearly closed positions with the other valve types, but too close to fully closed to do you much good. They will also be subject to erosion and filling the seal areas with dirt when in that position. It's by far better to use those types of valves in Open or Closed Positions only. Some patterns of plug valves and butterfly valves would be more effective for throttling, if for some reason you don't want to use a globe type.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
BigInch is correct. Normally the globe valve is well suited for throttling. Above NPS 6 it is common to consider a segmented ball or a high performance butterfly valve. Some applications such as cavitation service are not well suited for the throttling balls or butterfly valves. A standard full or reduced port ball valve or gate valve is a very poor choice for throttling.
Download the free Fisher Control Valve Handbook from the Emerson web site for a primer.
Thanks all of you,
StevenHPerry, No doubt all of the valves can provide dp across at closed position. Your guess is nearly right. I mean capability of maintaning dp across at Fully opened position.
BigInch & JLSeagull, I agree with you that the globe is well suited for throttling purpose. But what about the ON/ OFF control purpose? For example, in the case of on/ off drain liquid from the high pressure system to the low pres. does the ball work as well as globe? or which of them is preferred?
JLSeagul, is there any reason to prefer using of the segmented ball or butterfly compared to the globe in the case of above NPS 6?
For on/off the ball valve or gate can be superior, because it is not unknown that some particle, or cavitation erosion will occur, or dirt will also damage the seals of a globe valve used for throttling, as it does spend most of it's life in some %open position. Because of those possibilities, I do not consider any type of control valve to be a %100 shutoff, ever, ever.
Typically balls are easier to operate taking only 1/4 turn O/C. Gates can take many, many more full turns to O/C, but gates can be even cheaper than balls, so make your decision according to cost, or ease of operability for those applications that only need an ON/OFF function.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.
BigInch made an important point above--you can NEVER rely on a throttle valve for shutoff. They often work, but they may not work the next time you try--there is no way to predict when trash will foul the seat, and it doesn't take much.
For on/off "control", an actuated ball valve gives more predictable results than the globe valves with diaphram operators that are often used for dump valves or wellsite ESD valves. I've done the accident investigation on at least 3 fires that the root cause was a diaphragm operated globe valve in on/off service (as a spec break) that leaked through and over pressured downstream equipment. This is common practice in Oil & Gas, and it is a bad practice.
Total agreement from me zdas. A control valve is never, ever in my bag of tricks to be used as a spec break. They should always be backed up by an automatic positive shutin (ball preferred for fast action) in case of runaway pressure downstream of a fail-closed control valve that didn't seat properly.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
Within certain ranges, one can use a "V-notch" ball valve for control. They provide close to proportional flow over the "percent" open or closed of the 1/4-turn valve. They also provide positive shut off. There are, of course, trade-offs. There is more pressure drop than a standard ball valve, there is the likelihood of build-up on both sides of the ball, etc. They are more likely used in utility service, but I have used them in very limited cases with chemical flow control.
Ultimately, I Thank all of you for your efforts, from which it can be concluded that for on/off operation the Ball valve is more reliable than other types and also, installation an orifice down stream the valve makes the down stream low press. system more safe.
It's best not to assume that an orifice will make the downstream segment more safe. An orifice contributes only pressure drop when fluid is flowing through it at a relatively high rate and has no effect on the absolute values of either the upstream or downstream pressures. The pressures in either the up or downstream segments are a function of your boundary and control conditions at the inlet or the outlet respectively and have nothing to do with the orifice plate's, or pipe pressure drops in between. Inlet pressure - pressure drops = outlet pressure; Outlet pressure + pressure drops = Inlet pressure. You can see that you must have one pressure known (your boundary condition) either at the inlet or at the outlet, and you must know the flow to calculate all the pressure drops in between. An increased pressure drop from the orifice will either reduce the downstream pressure only when the upstream pressure is controlled constant. Likewise an increased pressure drop will raise the upstream pressure only when the downstream pressure is controlled constant. If neither is controlled, an increased pressure drop might raise one and lower the other, but might not. An orifice plate will do nothing to stop a slow buildup of pressure (from changing boundary conditions) when there is negligable flow in the line.
"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.
I'll add that I have a practical dislike of gate valves due to their sensitivity to debris. They can jam in some part-closed position due to debris and thereby fail to accomplish their intended purpose.
Using a gate valve for throttling is a practical absurdity. All that will be accomplished is the rapid damaging of the sealing surfaces.
In very clean services, gate valves can provide good shut-off performance with increasing upstream pressure increasing the forces on the sealing surfaces. This can result in greater actuating force requirements.
Because of their sensitivity to the accumulation of debris, I would never install a gate valve in a horizontal line with its stem in a horizontal orientation.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
First, one of the things that a young engineer learns early in life is to NEVER use a gate valve in throttling service. People do it, but never with good results.
Second, you can buy globe style control valves that are built to class umpty-umph drip tight leakage rates in the dribbles per hour/year/lifetime range, but they always have SOME leakage rate associated with them. As BigInch has so succinctly noted, at some point in its life, the valve seat will get cut, will wear down from its original tolerances, or will just get some trash wedged in it and it won't shut off. Use devices whose only function is to shut off and that are good at it when called on to do so.
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