When do you use a Ball Valve followed by a Globe valve? 3

[Juujee12]

Hello,
I'm looking at a P&ID with an inlet ESD Valve with a bypass around it that contains a ball valve followed by a globe valve.

This may be a very basic question but, can someone please tell me what the basis is for having a ball valve in series with a globe valve?

thank you.

 

[DRWeig]

Hi Juujee12,

Is the globe valve perhaps for modulating control, with the ball valve simply to isolate it for maintenance?

Can't tell for sure without looking at the P&ID...

Good on ya,

Goober Dave

 

[Juujee12]

Hi DRWeig,
Thanks for the response.

Sorry I can't upload the P&ID, but just picture a normally fully open ESD valve on a line with a bypass around it for maintenance. The bypass line has a ball valve and globe valve in series. that's it. no other blinds or hardware.

The ESD is normally full port open and is not modulated. There is no control required at this point, it's not like a bypass around a control valve.

thanks again,
Juujee12

 

[Chance17]

I would say that a ball + globe in series is not typical practice.
There must be some system specific reason.
One guess is to manually throttle flow around the ESD when it is closed.
Of course this defeats the purpose of the ESD.

 

[pmover]

without observing the P&ID and since you mention ESD inlet valve, the bypass piping with ball & globe valve are most likely used to purge and pressurize the plant piping after an ESD event. simply opening a large diameter ESD valve without purging & pressurizing the plant in a controlled fashion may very well be catastrophic. the globe valve will be used to control the amount of gas for purge and to control the pressurization rate. the ball valve is either full-open or fully closed - definitely no throttle control. so, likely open the ball valve and then control purge and pressurization via the globe valve.

hope this helps.
-pmover

 

[Juujee12]

pmover you've given me a very logical reason for the arrangement. much appreciated. I'm sure that's why a globe valve was installed for re-pressurization after a shutdown event, or for start-up.

thanks again to everyone for your help!

 

[MortenA]

Also at a fairly low dP a large size ball valve will be impossible to open - even with a powerfull actualtor (the most common reason for installing a bypas). Besides - i would say its a very common thing - at least for system with valve larger than say 6-8" and any signifiant pressure.

The reason why you cant move the valve is in he way the seal work that prevents the valve from leaking.

The globe valve is of course for a controlled release - sometime a RO is used instead.

Best regards

Morten

 

[TechThreads]

Ball valve shutoff > Globe valve shutoff

 

[tickle]

As above

Assuming the arrangement to be on a high pressure gas facility. The pressurisation is manally controlled to prevent (minimise) the seat erosion on the ESD valve. Also J-T cooling can result in temperatures below the MDMT. On some arrangements, eg compressors with auto start, the pipework downstream of the globe valve is stainless steel. The globe valve will be on the low pressure side (or the maintainable side).

The larger ESD valve can be a rotating seat so that the same bit of seat is not subjected to the wear in the same point.

 

[BigInch]

I agree with pmover. Its a black start-up arrangement for after an ESD event. If its a natural gas line, its probably providing fuel to start the generator.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso