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Globe versus Gate Valves 3
- Thread starter ADNPD
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- #2
no, but the globe valve can be repaired in place while nearly all gate valves need to be removed from the piping system to have leakage problems repaired. Also, a globe valve can be opened against a high differential pressure, while a gate valve will bind and cannot be opened. If you try to open againast a high DP, the seat will score and leak. Also, a globe valve has a near linear characteritic of Cv vs. % open, while a gate valve has a severe parabolic characteristic and thus cannot be used for throttling.
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*Why do globe valves seal more efficently than gate valves?
I guess that depends on your definition of "efficient". Gate valves can provide a very tight shutoff, which gets tighter with increased pressure. For globe valves, it depends on the orientation. With flow from under the disc (the "normal" configuration for globe valves), increasing pressure results decreased sealing performance (ie, possibility of leakage). With flow from over the disc, increased pressure produces a tigher seal, but the valve can be hard to open. Globe valves, at least in my training, are not generally used as shut off valves, only for throttling.
*What to do if there's a high differential pressure?
With a gate valve, higher delta P means more torque is required to open. Use a gear (or electric) operator. As an alternative, use a bypass configuration: the torque required to open a small bypass valve is minimal, open that valve first and the let the pressure across the big gate valve equalize, then open the big gate valve.
With a globe valve, again it depends on orientation. With flow under the disc, tight shutoff requires high torque: again, think gear or electric actuator, or possibly hammer-blow handwheel. Opening is easy. For flow over the disc, closing is easy but opening requires more torque (actuator), or a bypass.
*Flow Curve
I must disagree with davfitz's assertion that globe valve flow curves are essentially linear. At least in my experience, the curve is more S-shaped and tapers off once the valve is open to about 1/3 of the pipe size. It depends on the disc design, but most globe valves should not be counted on for very fine flow control (that's what a control valve is for.) But davefitz right that gate valves should not be used for throttling.
Jonathan Nieuwsma
I guess that depends on your definition of "efficient". Gate valves can provide a very tight shutoff, which gets tighter with increased pressure. For globe valves, it depends on the orientation. With flow from under the disc (the "normal" configuration for globe valves), increasing pressure results decreased sealing performance (ie, possibility of leakage). With flow from over the disc, increased pressure produces a tigher seal, but the valve can be hard to open. Globe valves, at least in my training, are not generally used as shut off valves, only for throttling.
*What to do if there's a high differential pressure?
With a gate valve, higher delta P means more torque is required to open. Use a gear (or electric) operator. As an alternative, use a bypass configuration: the torque required to open a small bypass valve is minimal, open that valve first and the let the pressure across the big gate valve equalize, then open the big gate valve.
With a globe valve, again it depends on orientation. With flow under the disc, tight shutoff requires high torque: again, think gear or electric actuator, or possibly hammer-blow handwheel. Opening is easy. For flow over the disc, closing is easy but opening requires more torque (actuator), or a bypass.
*Flow Curve
I must disagree with davfitz's assertion that globe valve flow curves are essentially linear. At least in my experience, the curve is more S-shaped and tapers off once the valve is open to about 1/3 of the pipe size. It depends on the disc design, but most globe valves should not be counted on for very fine flow control (that's what a control valve is for.) But davefitz right that gate valves should not be used for throttling.
Jonathan Nieuwsma
In my 30 yrs experience , nearly all process control valves have been globe or angle-globe style valves. The linear charaacteristic trim is the normal default trim. For low pressure air or gases , some applications have used butterfly valves, and for critical applications ( cavitation , very high noise) multistage trim style valves have been used. But the Globe dominates control and throttling service. A quick look at a Fisher valve catalog should suffice.
Wear, that’s a simple answer wear. Bits of nasty hard stuff getting between the gate and the seat face, localised cavitation with liquids, areas of low pressure inducing jet streaming and then a large area of metal rubbing on a not so large area of metal which happens to be the sealing face and large loads requiring, in some cases, very high shock loads to get the valve moving.
Having said that gate valves work, just make sure the application suits the valve.
As fare as the rest of the stuff that others have pointed out about globe valves, well it all depends whose product and what application. All of what’s been said isn’t necessarily so and some of what’s been said is incorrect, so just be careful and don’t rely on a single source.
Having said that gate valves work, just make sure the application suits the valve.
As fare as the rest of the stuff that others have pointed out about globe valves, well it all depends whose product and what application. All of what’s been said isn’t necessarily so and some of what’s been said is incorrect, so just be careful and don’t rely on a single source.
It has not been my experience over the past 30 years that Globe Valves seal more efficiently than Gate Valves. I agree with much of the advice given by Davefitz and Jnieuwsma. I use Gate Valves for isolation with bypass valves to equalize upstream and downstream pressure to save the seats and the operator thus increasing the life of the valve. I use Globes Valves for throttling (control). However, I always install a gate valve upstream of the globe for positive shutoff because the globe valves do not tend to hold tight.
J. Alton Cox
J. Alton Cox
jsummerfield
Electrical
In my experience, (and I am getting old) throttling control valves are often globe and other patterns such as high-performance butterfly and v-ball style valves are used where the capacity is high and cavitation is not a serious issue. Typical process control valves have equal percentage trim charaacteristics as the normal default trim.
Default ANSI/FCI Class II globe style control valves leak like a sieve. Control valve leakage classifications reflect the leakage in percentage terms based upon the valve sizing coefficient. Class III are normal with Class IV still leaking, Class V still leaking and Class VI still leaking. Do not compare control valve leakage rates against gates or balls as the requirements differ quite dramatically. Shutoff valves are gates and ball valves not globes.
John
Default ANSI/FCI Class II globe style control valves leak like a sieve. Control valve leakage classifications reflect the leakage in percentage terms based upon the valve sizing coefficient. Class III are normal with Class IV still leaking, Class V still leaking and Class VI still leaking. Do not compare control valve leakage rates against gates or balls as the requirements differ quite dramatically. Shutoff valves are gates and ball valves not globes.
John
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