Controlling (liquid) flow and pressure at very low flow rates

In summary, the pump is able to restrict the flow to a very low amount, but they are still able to keep the pressure at the seal up to prevent any dilution of the product.
  • #1
John Archer
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TL;DR Summary
Controlling (liquid) flow and pressure going to a packing that is meant to slowly leak cooling water.
System in question. Centrifugal pump (seal packing is water cooled)
Fluid is water. Main system pressure is 55Psi.
I need the water pressure to be around 25Psi.
I also need the flow rate to be very low (think, maybe 1oz per minute). Basically, I need slightly more than say 30 drops per minute.
The pressure in the system that the pump runs is between 6-10Psi. I need to keep about 1 Bar (14-15Psi) of pressure above that of the system pressure to prevent the fluid the pump is running from getting into the seal packing.
The problem is that I can't have too much cooling water going to the packing as it can thin out the fluid the pump is pumping. an oz. per minute would be an absolute maximum.
So the process allows a very small amount of fluid flow/loss, but I need to keep the pressure up to keep the packing clean. The flow control is mainly to prevent dilution if the packing starts to wear out.
I am using a regulator to get the pressure to 35Psi, and a flow control to limit flow. I have added a 2nd pressure gauge after the flow control, and it reads a lower number (as it must) than the regulator's gauge.
I am wondering if I should put the flow control in front of the regulator instead of downstream.
One more variable to consider; the packing is adjusted by compressing it. the more compression, the more resistance (the closer to 2 gauges will read to each other), but also this generates more heat, needing more flow.
I can also adjust the distance from the pump, flow control, and regulator. At the moment,the regulator is connected to the flow control, then about 18" of hose going to the pump (1/8" ID, that can be changed to a larger or smaller diameter).

Short version, I need to get 25 Psi of water to go to a packing, to cool the packing, too much flow can ruin the product. Where would you put a regulator and/or a flow control?
Thanks
 
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  • #2
You cannot have it both ways. If you need 25 psi at the seal; then, the flow will be determined by the leakage of the pump seal. If the flow is to be regulated; then, the pressure at the seal will be determined by the leakage of the pump seal. It is a matter of deciding what is the worst failure mode between a low pressure at the seal with your system fluid potentially getting into the seal; or, dilution of the system fluid by excessive leakage flow through the seal. Your best choice might be to select one of the two possible control options and then installing an alarm for the other.
 
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  • #3
Have you considered changing from packing to a mechanical seal? If you are not familiar with mechanical seals, a good search term to get started is pump mechanical seal. Mechanical seals reduce leakage to effectively zero. They work well with most fluids, but abrasive slurries can cause problems.
 
  • #4
jrmichler said:
Have you considered changing from packing to a mechanical seal? If you are not familiar with mechanical seals, a good search term to get started is pump mechanical seal. Mechanical seals reduce leakage to effectively zero. They work well with most fluids, but abrasive slurries can cause problems.
Yes. I have quotes for a dual mechanical seal system, but my company doesn't seem interested in spending the money (though it would save a lot of money in the long run.
 
  • #5
JBA said:
You cannot have it both ways. If you need 25 psi at the seal; then, the flow will be determined by the leakage of the pump seal. If the flow is to be regulated; then, the pressure at the seal will be determined by the leakage of the pump seal. It is a matter of deciding what is the worst failure mode between a low pressure at the seal with your system fluid potentially getting into the seal; or, dilution of the system fluid by excessive leakage flow through the seal. Your best choice might be to select one of the two possible control options and then installing an alarm for the other.

I get that it can never be prefect, right now I have the flow control set to allow about 30xthe amount of water it could ever need.
The water going out is at nearly a complete stand still, The flow is at between 1 and 2ml per minute. I have the flow control set to allow up to 30ml per minute if there is zero restriction.
Just wondering if there is a way I can close that window up a little bit.
This is just in case of a seal failure. the pressure would of course drop in that situation, but with a good seal allowing only 1-2ml per minute, I'm hoping I can raise the pressure.
Thank you for the reply!
 
  • #6
UPDATE:
I have pretty much gone away from the flow controls as is the seal starts to fail, pressure will drop, allowing more product into the seal, accelerating the wear on the seal.
Running just a regulator, when the seal starts to fail, I will see more water (not product) leaking. Hopefully in time to make an adjustment.
Just wanted to post this for anyone who might give a hoot. ;)
 
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  • #7
You could out a flow sensor in the feed line to the seal.
 
  • #8
The amount of flow is so small, that I think only a near total failure would show up on most flow sensors.
Generally, it is flowing about 2oz per hour. (.03oz/min)
 
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Likes Tom.G

1. What is the purpose of controlling liquid flow and pressure at very low flow rates?

Controlling liquid flow and pressure at very low flow rates is important for accurately measuring and manipulating small amounts of liquid in scientific experiments and industrial processes. It allows for precise control over the amount and rate of liquid being dispensed, which is crucial for obtaining accurate results.

2. What techniques are commonly used to control liquid flow and pressure at very low flow rates?

Some commonly used techniques for controlling liquid flow and pressure at very low flow rates include using microfluidic devices, syringe pumps, and pressure regulators. These methods allow for precise control over the flow and pressure of liquid, even at extremely low rates.

3. What are the challenges of controlling liquid flow and pressure at very low flow rates?

One of the main challenges of controlling liquid flow and pressure at very low flow rates is the potential for clogging or blockages in the system. This can be caused by small particles or air bubbles in the liquid, which can disrupt the flow and affect the accuracy of the measurements. Additionally, the viscosity and surface tension of the liquid can also affect the flow rate and pressure, making it more difficult to control.

4. How can the accuracy of liquid flow and pressure control be improved at very low flow rates?

To improve the accuracy of liquid flow and pressure control at very low flow rates, it is important to use high-quality equipment and carefully calibrate and maintain the system. Additionally, using filters to remove any particles or bubbles from the liquid can help prevent clogging and ensure a more consistent flow rate. Monitoring and adjusting the temperature and viscosity of the liquid can also help improve accuracy.

5. What are some applications of controlling liquid flow and pressure at very low flow rates?

Controlling liquid flow and pressure at very low flow rates has numerous applications in various fields, including pharmaceuticals, biotechnology, and microfluidics. It is commonly used in drug development and testing, DNA sequencing, and chemical analysis. It also plays a crucial role in the production of microchips and other micro-scale devices.

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