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

[John Archer]

TL;DR

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

 

[JBA]

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.

 

[jrmichler]

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.

 

[John Archer]

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.

 

[John Archer]

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!

 

[John Archer]

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. ;)

 

[Tom.G]

You could out a flow sensor in the feed line to the seal.

 

[John Archer]

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)