why use shutdown valve together with choke valve ?

[tobebest]

Pls refer to the attachment: there are two shutdown valves---one 4" and the other 2"

Q1: why use different sizes SD valves?
Q2: the purpose of 2" SD valve connecting with a choke valve?
Pls don't hesitate to share your ideas, thanks.

 

[mbt22]

A bypass to allow you to slowly introduce pressure downstream of the 4" valve?

Matt

 

[bimr]

There may be two different operation conditions and the same valve is not capable of operating at the required precision for both cases.

 

[EmmanuelTop]

What does Note 23 say?

It could also be related to the planned production scenarios over the field life. Use 2" line and choke valve when reservoir pressures are high, and switch to 4" unchoked line when the reservoir depletes. Choke valve becomes pretty much senseless device in low reservoir pressure operations.

Obviously, both SDV valves are required for isolation of downstream facilities from the reservoir.

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

mbt22 has it right.

Q1 - The bypass is usually smaller to save money. The bypass can be 2" even if the mainline is 24" or bigger sometimes.

Q2 - As above. Usually this is to either bleed off the upstream pressure if this is too high or to feed flow into the d/stream section following a high pressure trip or where the upstream pressure is too high for downstream. The second SDV protects the D/s system during this manual process. Once pressure is equalised across the bigger valve, the bigger valve is opened and the bypass closed.

Very common. You sometimes need to watch for low temperatures d/s of the choke, depending on the maximum differential pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

What is the purpose of taking the differential pressure across the valve? What process is this?

 

[LittleInch]

This allows a controlled flow into the downstream section to slowly either pressurise it from upstream or slowly de-pack the upstream section before you can reopen the main valve or restart the flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Yes, this arrangement is used for pipeline startup-unpacking when there are safety concerns with opening the DN100 main line SDV with very high pressure upstream.

The DN50 line is sized so that the relief capacity it presents to the downstream RV, as limited by the wide open capacity of this DN50 choke, with upstream pressure being the worst case pipeline packed pressure, is still with this RV capacity. Instrumented startup overrides (not a maintenance override) should be in place to allow the DN50 SDV to be manually opened with high pressure upstream. The startup override should get auto disarmed when dp at DPIAH 3134 drops below a preset value.

Is this piping class 251503Z a low temp (LTCS) piping class ? Presume it is ok for this startup operation after ESD/ blowdown.

Not sure why there is a DBB d/s of this arrangement - the SDV itself should be a valid block valve ? Cannnot tell for sure with this little snippet.

 

[georgeverghese]

What is this FP marker at the 2inch SDV?