8 replies to this topic

[Ghasem.Bashiri]

Dear Sir, Madam
I need to know:
Is it practical to use Flow element in suction of P&ID?
As you can see in attached file we have one design with FE in suction of pump.
Fluid is Condensate (C4-C7 Hydrocarbon) from Tank (Atmoshperic).
If I want to relocate Flowelement in out side of the pump, is it possible?
(I should have separate DCS and ESD Flow element as process Basis, Therefore exisitng 2 FE should be kept).
Also by knowing that pump discharge manual valve should be close to pump and also FE have some limitation (Some inlet length without piping element to avoid measurement error), is it possible to use FE after pump discharge manual valve.
As general point: What is optimum arrangement for this design?

Attached Files

•  PID Sample.doc   139.5KB   91 downloads

[Zauberberg]

You don't need to have two flow elements - one for the DCS and other one for the SIS for (whose logic is that?) - but two different transmitters. That's how it works.

I am surprised to see flow element on the suction side of pump, and unless there is really an excessive suction head available for the pump, I would remove that thing. Is there any description e.g. in the operating manual or as a note in the P&ID, why the flow element is there in the first place? Low flow you can see on the discharge flow element in a matter of seconds, I wouldn't say that flow element on the suction side is required for that purpose.

[Art Montemayor]

Zauber is once again correct, in my opinion.

The normal and usual reason for investing in a flow element for a centrifugal pump is to ensure that your process downstream is getting the flow that you require. That, normally, is the scope of the flow element. Therefore, one never installs a flow element in the suction side of a centrifugal pump - if you are to be guided by your scope of work. You don't do that because the flow that goes into the suction eye of the pump doesn't necessarily come out of the discharge and goes directly to process. A centrifugal pump has other requirements besides supplying the process downstream: it must also ensure that it has sufficient flow for its control of minimum flow - i.e., a return to suction source is normally used to take the flow pumped to ensure that a minimum flow is always going through the pump. Therefore, it is always more prudent to place a flow device on the discharge side (the part going to process) of the pump.

You certainly don't need TWO devices. The suction side device is another net NPSHa negative contributor and, therefore, is a hinderance or a detriment for good pump performance. The suction side of a centrifugal pump is sacred ground when one is trying to honor and abide by the NPSHr needs of this type of pump. I would not do it unless I was forced to or if there were no other options for measuring the flow.

[fallah]

Lack of adequate interlock information on the PID,but at the first look seems one common FT on discharge line would be adequate for ESD and DCS systems.

Indeed,in such system usually a SDV would be considered in suction line to be closed at LOW LOW Level in the storage tank following that the pumps would be tripped.

[JMW]

The risk of installing flow elements on the suction side that of cavitation or gas breakout.
Flow elements often benefit from installation in the discharge (higher pressure) of pumps, sufficiently far downstream to avoid disturbed flow profiles where appropriate, and upstream of control valves.
In a similar vein, unless care is used, avoid flow elements that are open to atmosphere on the discharge side.
A useful guide is to install where the back pressure is twice the vapour pressure.
But in all situations, refer to the manufacturer's specific guidance on installation or any relevant industry standards. If you do this you are in most cases going to achieve a good installation.

[ogpprocessing]

For some critical services flow metering at suction side can be used to warn against fouled strainer conditions provided that FE is installed downstream of strainer.

[Zauberberg]

For some critical services flow metering at suction side can be used to warn against fouled strainer conditions provided that FE is installed downstream of strainer.

For such a purpose I would provide a DP gauge/transmitter across the strainer, and a flowmeter at the pump discharge. I don't need a flowmeter in the suction to warn me about low flow - I will see it on the discharge side just as quickly.

[Dev 009]

Dear All, with due respect , I would like to point out few important points on the configuration specified in the P&ID,

If not wrong the FE in the suction side is being used for Shut Down service. My gut feeling says that this is a wrong location for sensing a low low signal. Ideally it should be downstream of strainer shown. Also the type of FE used is wrong, the orifice will create further pressure drop and Net NPSHa will reduce down.

Basically it is not recommended to put any kind of restriction in suction line, but what i understand from configuration is that " The FE used in suction line is to protect pump in case of low low flow if the pump discharge fails to open". Downstream FE after pump will protect the pump in case of low flow upto the minimum flow of pump as specified by pump vendor in case of discharge valve closure. But what about if pump discharge valve struck at 5% open condition ( actual case of MOV in pump discharge).

Various pump manufacture recommends that low energy pumps upto 160 kW can run against closed discharge upto 5 seconds, and high energy pumps upto 2 sec.

The crux is that, the FE ( orifice type) wont be able to protect the pump, instead of that we have to provide certain other means like
1. Low flow switch ( Thermal dispersion type , the same i have used for one of our project in Oman)
2. Motor trip via measuring load current
3. PT with PAHH trip

awaiting for any clarification or kind suggestions.

best regards

Dev

[asade abiodun]

All information given before now are correct.

Installing FE on the pump suction line could affects the pump operation and cause cavitation and impeller damage. FE can be used to measure and control flow, it is logical to have it installed downstream of the pump discharge line.

The pump can be protected against minimum flow by the recirculation CV downstream of the discharge line and the pressure swith high instrument.

This is my opinion about the pump control.