FC Orifice on the Discarge line

[rattyo]

I'm designing the piping on 2 barrel pumps. They are 8" suction and 6" discarge.

My first concern is the suction line because on the P&ID the nozzel is 8" . no reduction at the nozzel. I've spoke to the inexperienced process engineer and he reckons his done the calcs and the will be only a tiny pressure drop across the pumps...

Secondly on the discarge there is an orifice flow meter straight away out the discarge line. So basically I need a 4 m run of pipe and then my check valve and bloke valves which are 5 odd meters away from the nozzel which I thought where ment to be close to the nozzzel to protect the pump.


I've seen a small kick back line from the discarge to the suction before in a small 1" line with an orifce FC in. Maybe these 2 enxperienced process engineers have got it confused....But an FC in a discarge line....




because these are barrel pumps and not centrifugal pumps which Ive only ever piped up before I'm not sure..

can anyone with superior knowledge shred any light on this for me....

 

[monaco8774]

rattyo

Sometimes a flow orifice is put directly in the discharge line of the pump to open a minimum flow line to protect the pump, what you have seen before is a simpler method of achieving the same objective. The orifice and small kick back line are sized to give minimum flow in the event someone closes a valve downstream. Disadvantage of this is you lose some pump capacity. There is something odd about this design though as you havent mentioned a minimum flow line which should also be before the check valve and block valve (as these could be what block and need min flow.) also if it is a flow measurement orifice the size is normally set up to give you around 250mbar pressure drop.If they just want a flow meter for pump condition monitoring then they could put it downstream of the block valve as its not being used to protect the pump form minumum flow, in this case you need something (e.g. a high pressure trip) to do the same upstream of the block valve. Generally you keep the block valve close to the pump to reduce inventory that has to be drained and purged for maintenance. Dont woory about the fact its a barrel pump, common sense princuipals apply in all cases.

 

[rattyo]

Thankyou for replying...


Yes there is a minimum flow line which is routing into the Vacuum tower. A 3" line. It's before the check valve and block valve. Sorry I forgot to mention that. Also between the check and block valve they have a 2" closed drain coneection arrangement aswell.

Why they have not stuck the orifice in the minimum flow line I dont know. Putting it in the discharge line when it needs to be drained there will be alot of build up in there I would have thought with it being such a long line?

 

[rattyo]

Also there are 2 check valves after the minimum flowline and then a block valve. Is this over the top? I don't know?

 

[rattyo]

I asked the question to the process engineer again why not have the orifice in the minimum flow line and he came back with this answer .......

Yes, the orifice is there to open the valve in the min flow line. That's why it's also necessary to have the orifice in the main line, else the orifice won't measure a flow under normal operation and the result will be that the valve in the min flow line always will be open. We have a simular setup at P-820 A/B today.
Of course a sum of the two flow tags can be the control for the control valve and that way solve the problem, but as I said we have a similar setup at P-820 A/B we can look at.