Throttling Valve

[gazepdapi]

What is a better valve for throttling natural gas with a high differential around the valve? Ball, plug or globe? To me, I thing a globe would be the best option but I know a lot of people choose the plug instead.

Could someone please clarify for me.

Thanks!

 

[TD2K]

What size? What pressure and differential pressure are you talking about to give us a feel for what you are looking at?

Some of the areas on the North Slope use ball valves on liquid and gas systems and throttle with them (several hundred psi if I remember correctly) and didn't have problems with them (manual operation, not control valves). It's not what I would recommend. If I need to throttle, I'll go with a globe or some valve that is intended to throttle. Ball and plug valves to me are on/off valves.

 

[gazepdapi]

I'm looking at the 2000 psi range....these valve are 2" and will be around large diamter valves to be used as pressurizing valves so that we don't open the large valve with a high dp across it.

 

[zdas04]

Bypass valves are a tricky issue. First, as a general rule a valve used for throttling cannot be trusted for future on/off use. Second, when pressurizing a line you don't want to slam an on/off valve open.

For bypasses around valves that do not have an ESD function, I'm OK with using a ball valve and throttling it. I don't love the idea, but have had it work just fine.

For bypass valves around an ESD valve I use two valves. One of them is always on or off. The other is a sacrificial valve that I throttle with (generally use a ball valve for on/off and a globe valve for throttling). Occasionally, I use a pair of block valves with a vent and a globe valve between them if the client has a strong double-block-and-bleed requirement (I never call a throttle valve a block).

For other throttling functions I stay away from inherently on/off valves (e.g, gate, ball, or plug) because their linearity is so bad. On those types of valves, you typically have 100% of flow in the first 15-25% of valve travel. That is really hard to manually control and nearly impossible to control with automation.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[gazepdapi]

In this case I would use two valves, one would be a ball valve that would be in the wide open position and the other would be throttling. The ball valve would be second in line which would be wide open and then I'd throttle with either a globe or plug, and that's what I'm trying to determine what is best. The flow from these valves is not my main concern because they'll only be used to pressurize and if the guy has to stand there a little longer it's not a problem.

I'm just worried about eroding the seat away during throttling. I agree with you that the best valve would be a globe but other people I work with think otherwise and I want to prove them wrong.

 

[zdas04]

"Proof" is a slippery concept here. People that use plug valves for anything at all have been under a great deal of pressure to stop that anti-social practice and the ones that didn't stop have gotten really defensive. There is nothing you can do with a plug valve that you can't do better with another valve technology that doesn't require as much maintenance, but plug valves have become a religion.

For throttling applications I look at linearity. First look, is the valve about 50% open at 50% flow? How about 25% open at 25% flow? How about 75% open at 75% flow? Plug valves, ball valves and gate valves fail all of these tests quite dramatically. Globe valves do pretty good at the 50% point, and depending on the shape of the seat do OK at the other points (needle valves do very well at the other points). Getting much more control than that I want to go to one of the exotic valve types like a Vee Ball or a Taylor Choke.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[gerhardl]

I agree with nertil1. But to complete I would for safety reasons have used a complete bypass installation. The only exception not to install any of the components, would be that they would have no necessary contribution to practical operation or safety.

A bypass installation would then consist of (upstream to downstream): One isolation shut off valve, normally open, ball valve, full bore. One throttle valve. Second shut off, isolation valve. Further down (normally on main line): safety valve/device, pressure meter, temperature meter.

Throttle valve: full regulation, high quality valve, especially adapted for media and pressure drop. Could for instance be globe valve with retracted seat construction with skirt(s) protection of seat and cloure surface.

As regulating valves comes in many constructions, possibly suitablein this case, as nertil1 states ( for instance ball(ballshell) valve with v-port,globe cage construction, double or triple eccentric with special discs (for instance holed), needle constructions etc. etc.) it is more interesting to find a commercial available and tested construction already used for similar purposes than recommending a special construction as such.

Good luck with further search in your near market!

 

[stanier]

You may need to consider a tortuous path valve such as CCI's. This are used for HP steam turbine bypass valves.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[gazepdapi]

Thanks for your help guys. I'm convinced the globe valve is the way to go for throttling