Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
(OP)
1) It is my understanding that if a control valve is installed downstream of a centrifugal pump, then the following will occur for minimum flow:
namely, the ACTUAL inlet pressure will be maximum and the ACTUAL pressure drop across the control valve will be maximum.
Similarily, the following would be true at maximum flow:
namely, the ACTUAL inlet pressure will be minimum and the ACTUAL pressure drop across the control valve will be minimum.
In other words, for increasing load (flow), the ACTUAL pressure drop across the valve is decreasing.
Question A:
Are the statements in (1) above correct?
Question B:
Will someone please explain to me why, in some situations,
the pressure drop across a control valve would INCREASE with INCREASING load?? What are these situations?
Question C:
When a process engineer fills in my valve spec sheet with the MAX deltaP occurring at MAX flow and the MIN deltaP occurring at MIN flow, how do I determine if this is the actual case or if indeed this is not correct and in actual fact the MAX deltaP occurs at MIN flow and the MIN deltaP occurring at MAX flow ????
Will some expert please please enlighten me. Thanking you profusely in anticipation.
namely, the ACTUAL inlet pressure will be maximum and the ACTUAL pressure drop across the control valve will be maximum.
Similarily, the following would be true at maximum flow:
namely, the ACTUAL inlet pressure will be minimum and the ACTUAL pressure drop across the control valve will be minimum.
In other words, for increasing load (flow), the ACTUAL pressure drop across the valve is decreasing.
Question A:
Are the statements in (1) above correct?
Question B:
Will someone please explain to me why, in some situations,
the pressure drop across a control valve would INCREASE with INCREASING load?? What are these situations?
Question C:
When a process engineer fills in my valve spec sheet with the MAX deltaP occurring at MAX flow and the MIN deltaP occurring at MIN flow, how do I determine if this is the actual case or if indeed this is not correct and in actual fact the MAX deltaP occurs at MIN flow and the MIN deltaP occurring at MAX flow ????
Will some expert please please enlighten me. Thanking you profusely in anticipation.





RE: Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
B) When the valve is fully open then an increase in the flow for instance by turning on a second pump) would lead to an increase across the valve
Max/min flow for spec sheets is usually not related to a specific pump and the way i read what you write min could actually be larger than max because they are related to different flows
Best Regards
Morten
RE: Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
G. Gordon Stewart, P.Eng.
Gas & Oil Process Engineering Consultant
http://www.ggordonstewart.com/
ggstewar@telusplanet.net
RE: Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
e.g. at high flow there may be a branch stream going to somewhere which is at lower pressure. Therefore you will have to increase the control valve dp. There may be lot of other reasons too. Therefore thorough review of the system and discussion with appropriate people is must.
RE: Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
gpm = K x sqr.root of P
when gpm increases p increases upstream of valve. The delta P of the valve is fixed though the flow rate has increase. Therefore you will have a higher pressure difference accross the valve. Keep in mind that there will be high pressure on downstream of the valve to due to high flow rate.
Asghar Khan
RE: Control Valve Sizing: deltaP increases/ decreases w/ increasing load?
may be this will help,,,
for fixed restriction system ...dp will increase across as flow increases ...talking about typical liquid ..this will follow q=kSQRT(dp),,
example of fixed restriction system is pipe , fitting ,equipments like heat exchangers ..filters.....etc.
now for any system there will be available maximum pressure drop call it dpt that will enforce maximum possible flow (as per the relation highlighted above)...now
if you need to decrease flow through this system .,from relation hilighted above ,you have to decrease total available dpt ...how to do it ?
you install a (variable resistence device ...as opposed to fixed restriction component like pipe ,,exchanger ,,,)that will behave opposite to fixed resistance device following that relation i,e will take more dp for less flow and vice versa ...this is your control valve ..as the valve opens it will take less dp from available dpt ...and leave the balance (more dp ) will drive the higher flow in the system ..
so as long as the valve is opening or closing (variable restriction ) then flow will be inversible to dp unlike the fixed restriction components...(now you have restriction hydraulic dia changing every time ).
if valve is fully open or fixed at one position ( control valve on manual or open on/off valve )then its no longer avariable restriction device but a fixed restriction one like a fitting or any other equipment in the system ..and in that case if you manage to increase flow in the system ( increasing upstream or decreasing downstream pressure ) then flow as it increases will require higher dp across the valve ...
confused ?! hope not ....:)
regards