3 replies to this topic

[lokeshmiddha]

I am developing P&ids for some C2-C3 system. I have some doubt over Pump minimum flow system. Please see the attached sketch. My questions is as follows

1)      In Previous projects I have seen that pump MCF tapping taken from three different location

A)     From pump discharge before check valve and isolation valve. Separate tapping for operating and spare pump. generally Used for continuous MCF.

      From pump discharge after check valve and before isolation valve. Separate tapping for operating and spare pump. Generally used for continuous MCF.

C)      From pump discharge after check valve and isolation valve. Common tapping for operating and spare pump. Generally used for instrumented MCF.

I want to know that why pump MCF location depends on whether MCF is continuous or instrumented.

2)      Pump minimum flow tapping join the suction vessel in vapor portion (above HLL) (location B in diagram) or below liquid level (below NLL/LLL) (location A in diagram). And how it will affect the pump operation.

3)      Inventory isolation valve shown in the diagram at the combined pump suction as fluid is light HC and inventory is higher. In case of seal failure inventory isolation valve and pump will close. There are still chances that inventory will lost through pump MCF line (when pump tapping is before check valve.location A in 1^st question) or for safety reason generally we consider single check valve 100% failure. Considering this that if pump discharge check valve is stuck open vessel  inventory will lost through mcf line considering back flow through check valve (location B & C in 1^st question). Is this case applicable and Do we need to add extra inventory isolation valve or check valve in pump mcf line.

Attached Files

•  mcf location.pdf   51.23KB   70 downloads

[fallah]

Hi,

 

1) Min flow tapping location depends on whether there are dedicated min flow lines for each pump or there is a common min flow line for all parallel pumps. In the former case the location should be before the check valve at the each pump's discharge line and in the latter case it has to be at the common discharge header. Of course, there should be adequate protection for common min flow line e.g. considering a FSLL on common discharge header by which running pump and in the next stage all pumps would be tripped on a prespecified min flow value.

 

2) No direct effect on the pump operation, but it will affect the sizing of relevant CV or FO with different downstream pressure based on tapping join location on the vessel.

 

3) For location A in 1st question, you should consider an additional check valve in each by pass connection to stop the back flow.

[farid.k]

I am developing P&ids for some C2-C3 system. I have some doubt over Pump minimum flow system. Please see the attached sketch. My questions is as follows

1)      In Previous projects I have seen that pump MCF tapping taken from three different location

A)     From pump discharge before check valve and isolation valve. Separate tapping for operating and spare pump. generally Used for continuous MCF.

      From pump discharge after check valve and before isolation valve. Separate tapping for operating and spare pump. Generally used for continuous MCF.

C)      From pump discharge after check valve and isolation valve. Common tapping for operating and spare pump. Generally used for instrumented MCF.

I want to know that why pump MCF location depends on whether MCF is continuous or instrumented.

2)      Pump minimum flow tapping join the suction vessel in vapor portion (above HLL) (location B in diagram) or below liquid level (below NLL/LLL) (location A in diagram). And how it will affect the pump operation.

3)      Inventory isolation valve shown in the diagram at the combined pump suction as fluid is light HC and inventory is higher. In case of seal failure inventory isolation valve and pump will close. There are still chances that inventory will lost through pump MCF line (when pump tapping is before check valve.location A in 1^st question) or for safety reason generally we consider single check valve 100% failure. Considering this that if pump discharge check valve is stuck open vessel  inventory will lost through mcf line considering back flow through check valve (location B & C in 1^st question). Is this case applicable and Do we need to add extra inventory isolation valve or check valve in pump mcf line.

dear loke, refer pic below. hope this help.

Attached Files

•  Capture 1.JPG   49KB   15 downloads
•  Capture 2.JPG   42.59KB   11 downloads
•  Capture 3.JPG   46.5KB   12 downloads

[lokeshmiddha]

Hi fallah, farid

 

Thanks for your reply, but the question is still not fully answered. 

 

1) I am still not convince that if there is continuous minimum flow then it will be individual tapping (before nrv and isolation valve) and if it is instrumented then then it is from common line (after individual nrv and isolation valve) As you can see the thumbnail provided by fariq, it shows continuous minimum flow through common line. and why we not consider pump discharge valve malfunction (closes) for pump safety in case of instrumented MCF.

 

2) When we consider continuous MCF, how it will affect pump operation when mcf location is before nrv or after nrv.

 

3) In general it is good practice to increase the suction isolation valve pressure rating (design pressure)  to match with pump discharge because when one pump is running there are chances that spare pump check valve fails (single check valve complete failure) and flow through that nrv will pressurize the suction piping. 

 

Do we need to consider same basis in case of seal failure. If it is continuous inventory will pass through mcf line then through nrv to atm. if we assume that check valve failure (stuck open). and in case of instrumented mcf, mcf valve open when pump stops and inventory can back flow to pump.