6 replies to this topic

[M.Sadiq]

I m studying about chemical dosing metering pump, I have some confusions, first; what’s the effect of flow rate on discharge pressure & if I vary the flow rate, is the discharge pressure remains the same? Please elaborate about the effect of Stroke length and stroke speed effects on discharge pressure.
Further; please define the position of pulse dampener and pressure relieve valve at the pump discharge line.

[Padmakar Katre]

I m studying about chemical dosing metering pump, I have some confusions, first; what’s the effect of flow rate on discharge pressure & if I vary the flow rate, is the discharge pressure remains the same? Please elaborate about the effect of Stroke length and stroke speed effects on discharge pressure.

_{In simple words the centrifugal or dynamic pumps are constant head and variable flow machines where as positive displacement pumps are contant flow and variable head machines. The discharge pressure is the back pressure from the system to your pump discharge. In case of the centrifugal pump it will develop the discharge pressure to maximum of shut-off pressure with minimum or no flow but in case of the positive displacement or metering pump it will establish the discharge pressure withing it's and downstream line/equipemnt design pressure and for the same reason the PSV is provided on the outlet to restrict the dischargae pressure within allowable limits.}

Further; please define the position of pulse dampener and pressure relieve valve at the pump discharge line.

_{In case of positive diplacement pumps the flow is not contineous and hence it is provided with dampener. I will suggest you to go through the pump theory and you will undertsand much better.}

Edited by Padmakar S Katre, 30 December 2011 - 05:29 AM.

[sheiko]

Hi Mama,

See this as an introduction:
http://www.pumpschoo... vs centrif.pdf

Also consider this website in which you can find free video tutorials:
http://www.lightmypump.com/

Edited by sheiko, 31 December 2011 - 09:51 AM.

[kkala]

Agreeing to previous posts, covering the subject, a summary with some explanations might be useful. Dosing (metering) pumps are positive replacement pumps, so that set flow rate is independent of developed discharge pressure for accurate metering. Centrifugal pumps have not been seen for this function http://en.wikipedia....i/Metering_pump.
Suppose a metering pump is connected to some piping ending to liquid tank or vessel.
1. If you increase set flow rate, discharge pressure will increase due to increase of frictional ΔP in the piping.
2. Increasing stroke length or speed will increase set flow rate, with consequential increase of discharge pressure as above.
3. I believe proper location for dampeners is immediately downstream pump; PRV has to be placed after this, upstream of first discharge valve.
I have not met metering pump combined with dampener (only big positive displacement pumps), yet this can occur; see http://www.pulse-dampers.com/.

Edited by kkala, 01 January 2012 - 05:33 AM.

[M.Sadiq]

Thanks sir,

If you have any graphical representation of Flow Vs Discharge Pressure, i will be very thankful to you.

I have a system for injection which has capacity flow range 5 - 10 gpm as min. and max. flow repectively, and discharge pressure range 100-4000 psig. If i require low flowrate for instance 5.5 gpm with discharge pressure of 3800 psig. further more, if i also require high flow as 9 gpm at discharge pressure of 200 psig. Since you have said that decreasing stroke length will decrease flow rate and consequently discharge pressure, then how i can achieve the conditions as mentioned above. Please elaborate.

[kkala]

1. For flow versus discharge pressure of any positive displacement (PD) pump, see first link of post No 3 in thread http://www.cheresour...diaphragm-pump/, "controlling positive displacement pumps" (cases seem to assume dampener already installed - note 1).
2. Above case as well as post No 4 here assumes constant pressure at destination, e.g. atmospheric or whatever. Hence increased flow will increase discharge pressure due to higher frictional pressure drop in discharge pipe.
In fact you cannot control both flow and discharge pressure in a PD pump. You set flow, then discharge pressure comes as a consequence.
Your requirement of 5.5 gpm at 3800 psig and 9 gpm at 200 psig is understood to concern two different cases, or a case quite specific. Explanations on this point would be useful.
Suppose a metering pump of 5 - 9 gpm that can withstand a discharge pressure of 3800 gpm. It will deliver 9 gpm at any discharge pressure supported by its mechanical strength, that is up to 3800 psig. So it can deliver 9 gpm at a discharge pressure of 200 psig. No problem is anticipated from technical point of view.

Note 1 (4 Feb 2012): Dampeners are needed for piston and diaphragm pumps, not e.g. for rotary pumps.

Edited by kkala, 04 February 2012 - 04:24 AM.

[S.AHMAD]

1. Positive displacement pump is a fixed volume. The flowrate is controlled or adjusted by changing the frequency (rpm) of the stroke.
2. Let say you have a PD pump of 1 liter volume (e.g. cylinder) ie 1 lit per stroke. If the frequecny is set at 100 rpm, thenthe flowrate is 100 liter/min. If you change the frequency to 1000 rpm then the flowrate is 1000 lit/min.
3. PD pump discharge pressure is not a controlled parameter. It is determined by the system pressure and ppiping configuration (pipe length, valves, fittings etc)
4. Supposing the system pressure is 3800 psig, the pump discharge pressure, Pd is given by:
Pd = DP + 3800 psig
Where DP is the line pressure drop which is given by kV^2/2g x J where J is the unit conversion factor for unit consistency.
5. The DP is proportional to the square of velocity that is the square of flowrate.
6. Determining the PD edischarge pressure is essential for determining the design pressure of the pump and piping sytem and the PSV set point.

Edited by S.AHMAD, 27 January 2012 - 03:56 AM.