12 replies to this topic

[luny]

Hi everybody.
How are the minimum flow of pumps controled? I have noticed sometimes the flow of the discharge is used and sometimes it is the pressure that controls the amount of minimum flow. Which control system is preferable and why?

[fallah]

Hi everybody.
How are the minimum flow of pumps controled? I have noticed sometimes the flow of the discharge is used and sometimes it is the pressure that controls the amount of minimum flow. Which control system is preferable and why?

Flow control is preferred,because you haven't to convert pressure signal proportionally to flow signal may lead to increasing error/decreasing the accuracy.

[luny]

Flow control is preferred,because you haven't to convert pressure signal proportionally to flow signal may lead to increasing error/decreasing the accuracy.
[/quote]

For small pumps i mean flow under 3 or 4 m3/h, I believe flow control is used and I think it is because of accuracy as you mentioned
But there are many cases where pressure is used.Especially in header lines. It is said that pressure changes aresensed with less delay than flow.But I haven't read any standard for that and I think there must be cost consideration for choosing any of the systems.By the way is there any standard for pumps?
Thanks

[luuquocdai]

Dear Luny
I remember that, there is a similar topic in this forum which discussed about this problem.
From my point of view, there are types of minimum flow control.
- Using ARV (Automatic Recirculation Valve)





- Using On/off valve






- Using RO (Restriction orifice)



Could you introduce me the second type of control, control the minimum flow by pressure?
I have never seen any drawing which has this type of control system.
Looking forward the information from you.

Regards.

Luu Quoc Dai
Email: luuquocdai@gmail.com

[manchester]

Hi All,

This is interesting. I also have the same refference as luuquocdai said,there are 3 types of minimum flow.
However, I've seen a design which use pressure control.

Not "control" actually. It using a self regulating valve that open on certain pressure. I've discussed this design with a senior engineer where I work, and he said that this design is quite unique,because he never saw a design like this before. But he also told me that this design is allowable. And pretty much interesting, since you only have to put a self regulating valve, without any additional control element. You won't waste energy with this design, not like when we use RO for minimum flow.

[fallah]

And pretty much interesting, since you only have to put a self regulating valve, without any additional control element. You won't waste energy with this design, not like when we use RO for minimum flow.

Risk of pressure regulating valve to be in closed stuck position!

Usually in minimum flow control with FCV,there is a LL alarm on relevant FT triggered pump shut-down.This is pump protection against FCV being closed stuck while discharge line is blocked.

Edited by fallah, 25 February 2010 - 05:08 AM.

[luny]

Hi all,
The pressure in discharge of the pump is read and the following valve opens or closes according to the read pressure. According to the pump curve, when the flow in pump suction decreases, it will cause an increase in pressure in discharge line (since the pump is supposed to give us a fixed head, decrease in inlet flow causes increase in outlet pressure). The control valve is a fail to open valve which means in case of any failure, it opens. I have seen such systems in water services. maybe because of the fact that the pressure of the water header is supposed to be constant and consequently the pressure changes are in a limited range for which the relation between pressur and flow can be mathematically explained with an acceptable accuracy.

[Zauberberg]

See the articles below:

http://www.chemicalp...s/2007/109.html
http://www.driedger...._cp/CE1_CP.html
http://www.mcnallyin...2-html/2-7.html

[luny]

Thanks everybody
Dear Zauberberg,
The articles were very helpful. thank you but by the way do you know any standard such as API dealling with pump minimum flow?

[Qalander (Chem)]

Thanks everybody
Dear Zauberberg,
The articles were very helpful. thank you but by the way do you know any standard such as API dealling with pump minimum flow?

Dear
Sorry for intervention here,I only know API610 as most related to centrifugal pumps standard;hoping this helps

[Root]

Hi,

NRV is being provided for minimum pump flow for the safety of pump and PRV is being provided for the safety of equipemnt, eg, for Compressor Lube oil systems and Turboexpanders. indeed where you need low presure and you have pump discharge pressure hi, there you need to reduce it.

I have seen on Turbowexpander where PRV is provided.
Cheers
Toor

[GAaREDSTATE]

Hi everybody.
How are the minimum flow of pumps controled? I have noticed sometimes the flow of the discharge is used and sometimes it is the pressure that controls the amount of minimum flow. Which control system is preferable and why?

It depends on what you are trying to accomplish. Are you preventing pump cavitation, and protecting the pump from damage, or are you trying to regulate flow to either dispense or meter tha fluid.

[S.AHMAD]

Hi luny

The others have explained on the flow control for centrifugal pump minimum flow. I have nothing else to add.
Pressure control for minimum flow is not a common practice but operationally feasible for centrifugal pump. This is because the pump characteristics (performance curve) where pump head (differential pressure) varies with flow. Normally those installed this type of control, the main purpose is to control the pressure but it can also serve as minimum flow at the same time (kills two birds with one stone). However the back flow is normally higher than the minimum flow required since the main purpose is not for minimum flow control. If the sole purpose is for minimum control, this type is not reliable since the discharge pressure varies with density and suction pressure so the back flow rate will also vary. So the pressure setting need to take into consideration worst case scenario where the discharge pressure will be the lowest e.g. minimum level at suction vessel or lowest possible density.

Please take note that, pressure control for minimum flow will give problem for flat pump curve.

Edited by S.AHMAD, 05 March 2010 - 02:44 AM.