11 replies to this topic

[gunjan]

Hello Friends...

I was checking P & ID's of one of the refinery unit, there I need some clarifications. In centrifugal pump downstream across NRV/Check Valve one bypass is shown of 1" for 12" main discharge line. I was wondering why it is needed ? One thing I am sure that it is not warm up line because warm up line usually taken from downstream of isolation valve after NRV n link before NRV.
One more thing I would like to mention is there is a gate valve in this 1" bypass line.

Thanks & Regards,

Gunjan

[JoeWong]

Gunjan,
Is the fluid hot and you are having at least one pump not in standby mode (this pump will be kicked in whenever duty pump failed)...Please advise.

If so, the 1" by pass is for continuous heating of standby pump to avoid thermal shock.

JoeWong

[Nirav]

You need to give follo.process data as minimum for someone to help you.

Fluid being handled :
Op.Temperature :
Suction Press :
Discharge press :
Flow rate :


Thanks,

[gunjan]

Hie JoeWong...

Well the fluid is hot. I was also thinking that only that it may be warm up line. But as I said in my question if it would be an warm up line the tapping would have been taken from downstream of Isolation valve in pump discharge but here the tapping is taken from NRV downstream. Since for standby pump the isolation Valve in pump discharge is Closed so it cannot be used for warm up purpose.

Attached drawing describes the arrangement I am talking abt.

Attached Files

•  Attachment.doc   24.5KB   206 downloads

[Art Montemayor]

I've often resorted to a small return line around a check valve located on the discharge of a centrifugal pump - very much like that described.

The reason(s) I've done this is that I have required a means to:

1. Prime the pump with the discharge fluid that is on the downstream side of the check valve; and,

2. Drain the discharge fluid that is on the downstream side of the check valve.

There may be various reasons why you may need to drain back to the pump casing. These reasons depend on the specific process and the operational procedures.

[JoeWong]

gunjan,

Well the fluid is hot. I was also thinking that only that it may be warm up line. But as I said in my question if it would be an warm up line the tapping would have been taken from downstream of Isolation valve in pump discharge but here the tapping is taken from NRV downstream. Since for standby pump the isolation Valve in pump discharge is Closed so it cannot be used for warm up purpose.

If you can decribe more about the service, quantity of pumps, operation mode, etc, i am sure many of us can provide good advice...however, i try my best to guess...

Please confirm to me that your system having
i) standby pump
ii) standby pump will kick in automatically whenever the duty pump failed

If the pump is started-up automatically whenever the duty pump failed, then the standby pump discharge isolation valve shall always open. The bypass gate shall always open to allow small amount of hot fluid from discharge manifold back to suction manifold. Allowing hot fluid passing the standby pump will always keep the pump hot. Whenever the standby pump kick in, it is ready to receive hot fluid avoid pump crack due to uneven thermal expansion.

Sometime designer drill a hole at check valve to perform similar function...

JoeWong

[DUmesh]

Hi Friend,

I am working in MEG plant . We use to keep standby pump hot by keeping

discharge valve slightly open. We keep NRV bypass full open so as per my experience it is right.

Regards
Umesh

[Pronab]

Gunjan

Same subject was discussed in this forum and raised by me on March 24, 2006. This type of valves are used for keep ready the stand by pump all the time and make warm condition to avoid any thermal shock as stated by JoeWong. We have also same types of arrangments of small centrifugal pumps with same by-pass valve arround a NRV.

Regards.
Pronab.

[chirag29in]

Gunjan,
We are using such gate valve (across NRV) for emptying out fluid between NRV and discharge valve of pump for releasing it for maintenance. When there is " U " loop between NRV and discharge valve, we have separate drain valve at the lowest point of U loop. But when the line between NRV and discharge valve is horizontal or going upward, we have gate valve across NRV.

[Samir Gupta]

Hi Gunjan,
The centrifugal pumps (when feeding to some unit/equipment) is equipped with a minimum circulation line along with a control valve. If the receiving unit suddenly stops taking this feed for some reason, the liquid from pump's discharge is routed thru this line back to the suction of the pump and thus maintaining a minimum circulation across the pump.
Regards
samir

[Dharmesh]

This type of valves are used for keep ready the stand by pump all the time and make warm condition to avoid any thermal shock as stated by JoeWong.
Beside this it also helps sometimes for the priming of the CF pump.

[pawan]

Good to see that people are commenting without understanding the drawing.

If the pump is started-up automatically whenever the duty pump failed, then the standby pump discharge isolation valve shall always open. The bypass gate shall always open to allow small amount of hot fluid from discharge manifold back to suction manifold. Allowing hot fluid passing the standby pump will always keep the pump hot. Whenever the standby pump kick in, it is ready to receive hot fluid avoid pump crack due to uneven thermal expansion.

Then its a wrong engineering practice as in case of manual closer of isolation valve the purpose is defeated while if it taken from downstream of Isolation as pointed out by Gunjan, it will always be foolproof.

We are using such gate valve (across NRV) for emptying out fluid between NRV and discharge valve of pump for releasing it for maintenance

Generally the distance between an isolation & NRV is so close compared to other volumes that it is not required. Moreover it doesnt have any drain.

The centrifugal pumps (when feeding to some unit/equipment) is equipped with a minimum circulation line along with a control valve. If the receiving unit suddenly stops taking this feed for some reason, the liquid from pump's discharge is routed thru this line back to the suction of the pump and thus maintaining a minimum circulation across the pump

I could not see any minimum circulation line - bcoz the definition of minimum circulation is somewhat different.

Gunjan has raised a very good point on the good engineering practice & what we are doing since pragmatic age......We can't take everything as it is.

The answer depends on his actual service as pointed out by Art.