3 replies to this topic

[Sandy2000]

Is it right to take control valve in downstream of reciprocating pumps/compressors for flow controlling?

Thanks

[Art Montemayor]

Sandy:

It is difficult to understand your use of English. What you wrote could mean one of two things:

• Do you mean to say, "Is it correct to install a control valve downstream of a reciprocating pump or compressor's discharge in order to control the capacity of the pump or compressor?"
• Or do you mean, "Is it correct to install a control valve downstream of a reciprocating pump or compressor's discharge in order to control the flow of the fluid going downstream of the pump or compressor?"

It makes a BIG difference on which interpretation you mean. The answer(s) to your query would be much better explained had you furnished a sketch of the arrangement you mean. However, I can easily respond to the above two alternatives:

Yes, but you must locate the installation of the control valve such that it is a RECYCLE control back to the suction source of the pump or compressor. You are, by this method, controlling the net flow of fluid proceeding downstream. Note that this is a rather energy inefficient method - albeit, a very simple and low-cost one. You are controlling the net output of the device going downstream in a reduced capacity; but the device is still putting out its rated capacity - which is more than the net capacity.

You must never attempt to throttle the discharge line of a reciprocating, positive displacement device. The fact that you are literally trying to limit a positive displacement device should indicate to your common sense that the procedure is inherently wrong and DANGEROUS.

I hope this experience helps you out. In the future, always try to employ graphic explanations of your engineering queries. A picture is worth a thousand words.

Also, please note that I moved this thread from the Relief Devices Forum, where you originally placed it. A reciprocating pump or compressor is NOT A RELIEF DEVICE.

[Sandy2000]

thank so much for your kindly attention

i mean that imagine you have a reciprocating pump or compressor then there are a Flow transmitter and a flow control valve in discharge line of equipment that control the discharge flow, I want to know is it practical to control the discharge flow ?

thanks again

[chemsac2]

Sandy2000,

Art is correct in that you should not throttle positive displacement machine's discharge. However, if system needs flow control, it can be provided with few modifications in process scheme.

A typical example is fuel oil feed pump to fired heaters where positive displacemnt pumps are used for viscosity reasons. A pressure control valve is placed in pump discharge that regulates fuel oil pressure to burners and in turn oil flow and heater duty gets controlled. Process temperature changes pressure control valve opening.

However, additional protection is required to avoid pump PSV popping.

Consider above fuel oil case has following specifications:
Pump flow = 100 m3/hr
Heater flow requirement = 50 - 100 m3/hr
Oil flow control valve (generally in the form of pressure) = normally 50% open

Now consider process temperature increases. Control system would decrease fuel oil flow to burners by closing control valve in pump discharge. Assume heater needs 60 m3/hr oil flow for which control valve is just 25% open.

Positive discplacement machines are constant flow machines. Since pump must deliver fixed flow, discharge pressure starts increasing to deliver 100 m3/hr with control valve 25% open (remember flow through a orifice increases with increase in pressure drop). This may lead to pump PSV popping, which is not recommended.

To avoid this, you need to install a bypass with pressure control valve which senses pump discharge pressure and routes excess flow back to suction. In fuel oil example above, out of pump flow of 100 m3/hr, heater flow control valve lets 60 m3/hr pass, and 40 m3/hr would be bypassed back to suction through a pressure control valve.

Regards