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Positive Displacement Pump Curve


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#1 van8888

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Posted 28 December 2018 - 08:42 AM

Dear All,

 

Could you help me with one concern regarding positive displacement pump flowrate/ curve as follows:

 

- Pump type: Positive displacement

- Existing: Pump is designed at 30m3/hr and 800 kPa diff pressure.

- Revamping: System friction loss at 30m/hr is 950kPaG. From new pump curve (at new viscosity), pump diff pressure at 30 m3/hr is 1050kpa. 

Question: With new viscosity, pump transfer rate is still 30 m3/hr or the transfer rate (> 30m3/hr) is corresponded with pressure (950 - 1050 kPa) as per pump curve.

 

It is understood that pump is not changed (no change of area, speed, etc.), thus there is no change of pump rate. However, I can't explain it with pump curve.

 

Could you kindly correct me?

 

Thank you very much for your advice.

 

Attached Files



#2 latexman

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Posted 28 December 2018 - 03:53 PM

Simplistically, a constant speed PD pump is a constant volume machine.  It will pump 30 m3/hr and the differential pressure (back pressure - suction pressure) is whatever the pipe, fittings, and fluid result in.



#3 Bobby Strain

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Posted 28 December 2018 - 05:28 PM

What is the origin of the pump curve you attached?



#4 breizh

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Posted 29 December 2018 - 05:08 AM

https://blog.craneen...ment-pump-curve

 

Hello,

 

You may find some pointers  in the link above

 

Breizh



#5 van8888

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Posted 29 December 2018 - 06:27 AM

Hi All,

 

Thanks for your reply.

 

Hi Bobby Strain, original curve is referred to black line (below new red curve).

 

Hi Latexman, it is agreed that pump rate depends on pump speed. However, i understand pump rate is determined based on pump speed working against pump discharge. Therefore, pump flow should be checked with pump curve.

 

Please kindly correct me if i am wrong.

 

Thank you. 



#6 Chemitofreak

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Posted 31 December 2018 - 03:13 AM

Hi,

 

1) I would like to rephrase what Boby said, please provide the vendor curve. The curve that you have attached does not seem like a vendor curve.

 

2) A positive displacement pump can provide infinite head and hence there is always a PSV at the discharge of the same. So, in case there is a blocked outlet, the PSV pops and there is a relief path for the fluid and the system is subjected to limited overpressure and thus safety of the system.



#7 latexman

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Posted 31 December 2018 - 07:49 AM

True.  What type of PD pump is it?  Some types also depend on the pumpage viscosity.  Other types, not so much.



#8 van8888

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Posted 01 January 2019 - 08:28 AM

Happy new year, everyone.

 

Thank you for your reply.

 

Sorry, I can't copy the vendor curve here, but the curve slope is like as attached file.

 

Hi Chemitofreak, It is agreed that a positive displacement pump can provide infinite head. In my opinion, a constant flow rate through positive pump is for idea positive pump which pump curve (pressure vs flow) is parallel with pressure axis. For actual positive pump, the curve is not parallel with pressure axis, so the pump rate is little bit changed as per pump curve at corresponding differential pressure.

 

Hi Latexman, the pump is screw type. 

 

Please kindly correct me if i am wrong

 

Thank you



#9 fallah

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Posted 01 January 2019 - 01:44 PM

Hi,

 

Please be informed the deliverable flow rate of a screw pump will be increased if there would be an increase in the fluid's viscosity.  Hence the pump curve will vary accordingly. 



#10 Art Montemayor

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Posted 01 January 2019 - 03:46 PM

Van8888:

 

Just as Latex and Fallah state: Although you don't tell us the TYPE of positive displacement pump you are using (a piston pump will have essentially no pump curve; it's displacement is constant if both valves work), the pump's volumetric efficiency will increase as the viscosity increases.  Rotary type pumps depend on internal clearances (pistons don't) in delivering a specified liquid volume per time.  As the viscosity increases, the pump increases its effectiveness due to the thicker fluid.  So, a rotary pump's pump curve will be different for different viscosities.

 

ON A MORE PERSONAL NOTE:

Happy New Year to all our members and a special salute and recognition to our good Breton friend, BREIZH, who has announced his retirement.  I hope he persists in joining us here on the Forums and I like his latest personal photo  ........ although, like me, he seems to be losing a few hairs.

 

My personal well wishes go out to all our members and contributors.  May this year be one of happiness and prosperity to all.



#11 SyedAhmed

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Posted 02 January 2019 - 01:36 AM

Hi,

 

Please be informed the deliverable flow rate of a screw pump will be increased if there would be an increase in the fluid's viscosity.  Hence the pump curve will vary accordingly. 

 

Fallah

 

Please explain this statement because of my limited knowledge.

 

As I know that Viscosity has impact on pump size and speed. High viscosity required high speed and some cases large pump to accommodate the required flow at those speed.

 

If the speed is fixed @ low viscose fluid to pump Xm3/hr amount of fluid but at that fixed same speed high viscose fluid will not be pumped same flow rate (Xm3/hr).

 

Please correct me.



#12 fallah

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Posted 02 January 2019 - 02:22 AM

 

Please explain this statement because of my limited knowledge.

 

As I know that Viscosity has impact on pump size and speed. High viscosity required high speed and some cases large pump to accommodate the required flow at those speed.

 

If the speed is fixed @ low viscose fluid to pump Xm3/hr amount of fluid but at that fixed same speed high viscose fluid will not be pumped same flow rate (Xm3/hr).

 

Please correct me.

 

 

Hi,

 

At first it's reminded that we are discussing about the effect of fluid viscosity on PD pump deliverable flow rate...

 

Going in depth in technical standpoint, as you forced us to do so, PD pumps have slip factor which is based on the viscosity of the fluid is to be handled...as follows:

 

At a fixed speed, PD pump will experience less slip when the fluid viscosity increases and vice versa...hence the flow rate will increase with an increase in fluid viscosity...

 

The design slip factor will be specified by PD pump manufacturer based on the given fluid viscosity in relevant data sheet by the customer. It's obvious the deliverable flow rate should be modified for new fluid with different viscosity based on specified design slip factor reported by manufacturer.



#13 latexman

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Posted 02 January 2019 - 07:36 AM

Vendors usually give flow curves at 3-4 selected pumpage viscosities.  For example, 1, 10, 100, 1000, 10,000, 100,000, and 1,000,000 cP.






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