Hi all.
I have got a system with an arrangement as follows:
A 4bar pressure supply, connected with a pressure regulator, connected to a container for water and then a flow meter.
I reduce the pressure fro 4bar to, say, 0.5bar by the pressure regulator and use the pressure to drive the water in the container to flow through the flow meter and then finally into a measuring cylinder. The tubing used in this setup is 6mm and the flow meter is aruond a few ml/min.

What happens is that the pressure gradually drops according to the pressure gauge on the pressure regulator. I expect that the pressure and flow rate delivered by this system would be very steady or constant since the variation of height in the container shall be negligible compared with the 0.5bar pressure.

There shall be nothing varying during the operation except the reduction in water level in the container....

So, why would the pressure gradually drops from 0.5bar and eventually to a very very low value as time goes? Do you guys have any idea? Thanks

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FredGarvin
First guess is droop in the regulator. What regulator are you using?

First guess is droop in the regulator. What regulator are you using?
It is of this type:
http://www.northerntool.com/images/product/images/4519303_lg.jpg [Broken]
It is newly bought. I don't know...

What I am wondering is that:
is the pressure drop normal? I mean when pressure is used to drive a flow, must the pressure be gradually dropping? I am not sure about this...

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FredGarvin
The drop is normal. It's part of the beast. However, droop can be minimized by the selection of regulator. You are using a cheap, Chinese knock off regulator. You really need to look at a precision regulator like a Fairchild Model 10 or 30. Here's a good article on dealing with droop:

http://www.chemicalprocessing.com/articles/2002/29.html

Regulators have a rather involved process for proper selection and use. Welcome to that world.

By the way, when you set your pressure set point, do you set it when everything is static or when things are flowing?

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The drop is normal. It's part of the beast. However, droop can be minimized by the selection of regulator. You are using a cheap, Chinese knock off regulator. You really need to look at a precision regulator like a Fairchild Model 10 or 30. Here's a good article on dealing with droop:

http://www.chemicalprocessing.com/articles/2002/29.html

Regulators have a rather involved process for proper selection and use. Welcome to that world.

By the way, when you set your pressure set point, do you set it when everything is static or when things are flowing?
Oh...I did not expect the "droop" problem...actually this is the first time I hear this word.

I set the pressure before adjusting the flow rate from zero to the desired value via a valve. When the water is flowing, I also tried adjusting the pressure regulator again to see if it would stay at the same value.

How shall I really solve this problem...Cost is a prime issue in this system, so I can't afford buying an expensive regulator actually...

By the way, from the website you quoted (thank you), it seems that the "droop" effect will cause the pressure to decay up to a certain level, right?

But in my case, the pressure tend to decrease to zero...how come..

FredGarvin
It's tough without knowing all of the details of your system. I am assuming that your source for the compressed air is large enough to be considered infinite in regards to the rest of your system. You do have a constant source pressure, correct?

Spitballing here....Two things that are coming to mind that I would first look at are:

1) As your vessel empties the water out, the air is expanding to fill the void in the tank. To keep that entire expanding cavity pressurized to your set point, the regulator needs to open up and flow more. I would guess that you are surpassing the max flow rate of the regulator and it simply can not keep up. It seems like you have a pretty decent delta P across the regulator though. Do you have the specs on the regulator you are using?

2) You have some kind of siphon happening on the downstream side???

It's tough without knowing all of the details of your system. I am assuming that your source for the compressed air is large enough to be considered infinite in regards to the rest of your system. You do have a constant source pressure, correct?

Spitballing here....Two things that are coming to mind that I would first look at are:

1) As your vessel empties the water out, the air is expanding to fill the void in the tank. To keep that entire expanding cavity pressurized to your set point, the regulator needs to open up and flow more. I would guess that you are surpassing the max flow rate of the regulator and it simply can not keep up. It seems like you have a pretty decent delta P across the regulator though. Do you have the specs on the regulator you are using?

2) You have some kind of siphon happening on the downstream side???
I do have some tube connected downstream after the flowmeter.

What do you mean by surpassing the max flow rte of the regulator? Is there a maximum or limit on the flow rate that a regulator can sustain?

NoTime
Homework Helper
Yes there will be a max flow rate.
And not just for the regulator, but the tubing and coupler combination as well.
Not to mention potential kinks or other obstructions in the lines.

stewartcs
Is there a maximum or limit on the flow rate that a regulator can sustain?
Absolutely! This is normally called when you specify the regulator to use.

CS

Q_Goest