7 replies to this topic

[farid.k]

Hye all,

 

For PD (recip) pump, for controlling flow, normally will use speed control, and stroke control. But I heard bypass also can be used for control flow (by bypassing a portion of the pumped liquid back to the suction). I just thinking how is the PFD configuration. Is it the control valve at the discharge piping, when need to limit flow, the control valve will close some percentage to recycle flow to the suction OR control valve at bypass piping which is when need to limit flow, valve at the bypass is open to recycle the flow.

 

kindly refer hand sketch for detail 

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[fallah]

farid.k,

 

This is a level control configuration rather than flow control one...

 

The level in suction drum of PD pumps is normally controlled by considering a LV at inlet line. The discharge flow in PD pumps can be controlled by stroke adjustment either manually or automatically...

[breizh]

Hi ,

The by pass line should return to the suction drum ,same for outlet safety valve. Two possibilities to control the level or the flow , by pass  with auto valve or speed control (frequency inverter) .

 

http://modelingandco...nt-pump-part-2/

 

Breizh

Edited by breizh, 08 December 2015 - 07:00 PM.

[katmar]

Your options A and B could both be made to work.  The actual choice would depend on your particular flows and pressures, but as a general rule I would prefer the control valve in the recycle line.  If the valve is in the process line it will probably lead to a higher pump delivery pressure being required, plus the need for a restriction orifice in the recycle line.

 

There are also 3 way valves available where port A opens as port B closes (and vice versa).  It seems an elegant solution, but I have rarely seen it used. Your two options are more generally used.

[shan]

Both your configurations are the suction vessel level control scheme.  When the suction vessel level is low, the control valve will open/close (Case 1/Case 2) to recycle the pump discharge through the on/off gate valve.  You should have FT/FV throttling valve arrange to on the recycle line/discharge line to vary your PD Pump delivery rate. 

[Art Montemayor]

I do not believe it is possible to make the "B" option work with a positive displacement pump - or any pump, for that matter of fact.

 

Option "B" shows that there is a completely unrestricted return line from the discharge port of the PD pump back to the suction port.  That means that while the pump is pumping, this line will always be flowing - regardless of what is downstream of the pump (or upstream).  This cannot be a "controlled" system since there is no manner or method to determine or measure what is flowing in the totally open return line.  It won't work because we don't know if the pump discharge is favored to flow downstream or return to the suction.  Every PD pump has to have a downstream check valve to avoid a return flow from downstream back to the pump.  This check device requires a minimum force (or pressure) to keep it open when flow is required downstream.  This further complicates the attempt to shunt flow back to the suction when the source vessel is detected with low level.  And what is one to do when the suction tank level is too high?  With the 100% open return, the pump will be unable to pump down the tank.

 

Am I dwelling in senility?

[katmar]

Art is correct.  It is such a standard practice to have a restriction orifice in the recycle line that I "saw" it, even though it is not in the sketch!  See my reference to the orifice in post #4 above.  You would have to have this restriction to make Option B work.  The 3-way valve option I mentioned effectively does the same thing by restricting the port to the recycle line when required.

[farid.k]

Dear all,

 

Sorry for the late reply.

 

Thanks all for the very supportive & valuable input. Actually I am trying to have something that is most economical and I found that control valve is cheaper than VSD. I did communicate to the vendor and they suggest to consider the electronic capacitance control (ECC) which is automatically control stroke for the pump once receive signal from transmitter. Normally stroke control is by manual. Thus rather than to have control valve which must add some more piping to the suction (some say to reservoir) at my current system that already congested, I think ECC is simpler and more straight forward in term of design.