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Velocity Inlet to a Pump

09/30/2021 7:56 AM

Hello everyone,

I have a 1/4 inch line flowing with liquid CO2 with flow velocity of about 2m/s flowing into a cooler at 25 C. The cooler reduces the temperature upto 19 C which increases the density of the fluid, resulting in a decrease in volume. This flow goes to a positive displacement pump. My question is, if the flow velocity to the inlet of the pump is less than 1 m/s, will it affect the functioning of the pump? Or will there be a decrease in the efficiency of the pump?

Thank you very much.

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#1

Re: Velocity inlet to a pump

09/30/2021 8:26 AM

CO2 density at 25°C & 1 bara= 1.784 kg/m3 ...density at 19°C and 1 bara = 1.821

So the volume doesn't change that much....your incoming volume has to match your outgoing volume or the system won't function properly...to drop the flow to 1 m/s you would have to increase the pressure which would then raise the temperature...

https://www.engineeringtoolbox.com/carbon-dioxide-density-specific-weight-temperature-pressure-d_2018.html

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#2
In reply to #1

Re: Velocity inlet to a pump

09/30/2021 8:33 AM

Sorry, I forgot to write. The pressure is 60bar. THe CO2 is flowing out of a CO2 cylinder

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#3
In reply to #2

Re: Velocity inlet to a pump

09/30/2021 8:49 AM

The CO2 is only going to flow as fast as the pump can pump...the pump determines the flow rate, and the flow rate should match the cooling ability of your cooler...now if your line size at the pump is increased, then the velocity will be lower but the flow rate will be the same, so you have to calculate the volume at both temperatures to determine the flow rates necessary....

https://www.sensorsone.com/flow-velocity-and-diameter-to-volume-flow-rate-calculator/

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#5
In reply to #3

Re: Velocity inlet to a pump

09/30/2021 9:16 AM

You will want to use actual pipe sizes available...instead of nominal sizes...

for copper...

https://www.petersenproducts.com/Copper-Tubing-Sizes-s/1979.htm

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#13
In reply to #5

Re: Velocity inlet to a pump

10/02/2021 7:26 AM

The difference between"Pipe Size" and "Tubing Size";

Pipe sizes are nominal trade sizes.

The internal diameter will vary according to the schedule of the pipe and is not precise.

(The OD will remain the same.This will vary when using very large size,such as 10 inch diameter and above)

Tubing sizes are actual external sizes and the ID is usually very precise.

For more info,here is a link:

https://pipeandhose.com/book/pipe-vs-tubing

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#7
In reply to #3

Re: Velocity inlet to a pump

10/01/2021 1:51 AM

thank you for the input. I have considered the temperature and pressure effects and the flow and velocity calculations at these points. I was worried that if my flow in the line is too slow, maybe it will cause some problems at the pump.

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#4

Re: Velocity Inlet to a Pump

09/30/2021 8:56 AM

The mass of CO2 entering has to equal the mass exiting (mass/time). So the flow rate (volume/time) has to be inversely proportional to the density (mass/volume).

If the CO2 is in the liquid state, the density will not increase significantly with a reduction in temperature, definitely not a factor of 2.

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#8
In reply to #4

Re: Velocity Inlet to a Pump

10/01/2021 1:53 AM

Actually the density increases by almost 3 times within the temperatures I have mentioned.

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#6

Re: Velocity Inlet to a Pump

09/30/2021 7:48 PM

My main concern would be the inlet pressure, not velocity. If the pressure is too low, the liquid will flash ("boil" in other words) inside the pump and cause cavitation, which will affect your pump's efficiency and cause damage to your impellers and/or seals.

regards,

Vulcan

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#9
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Re: Velocity Inlet to a Pump

10/01/2021 1:57 AM

Yes I have considered that too. I have a margin of almost 3 bars of pressure drop. If the pressure drops to 55 bar after the cooler then flashing will occur. Considering that, the length of the pipe is quite less with fewer fittings. What I am worried is that I have an inlet velocity of 0.66 m/s to the positive displacement pump and how it will affect the functioning of the pump.

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#10
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Re: Velocity Inlet to a Pump

10/01/2021 2:42 AM

If you are limiting your flow rate or fluid velocity going into the pump and reduce it too much, you will reduce the pressure at the inlet of the pump and, yes, you will cause flashing and thereby affect the functioning of the pump. I cannot tell you if 0.66 m/s is too low because there are other factors. I would much rather monitor the pressure at the inlet than monitor the flow rate or fluid velocity. It is the pressure (or lack of it) that will kill your pump.

regards,

Vulcan

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#11

Re: Velocity Inlet to a Pump

10/01/2021 3:59 AM

No, assuming you are not cavitating at that velocity.

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#12

Re: Velocity Inlet to a Pump

10/01/2021 10:59 AM

The important thing to note, as implied above, is that you cannot say yes or no based solely on the flow rate, you have to consider all the NPSH parameters as a whole. The pump manufacturer will give you the figure for NPSHr, and you can calculate NPSHa from the following. Note that NPSH is usually expressed in terms of feet or metres Head (ie pressure).

=available NPSH
=absolute pressure at the inlet
=average velocity at the inlet
=

fluid density

=

acceleration of gravity

=vapor pressure of the fluid
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