Minimum Flow to Lift a Check Valve

[more22]

Colleagues,
I have a question regarding minimum flow required to lift a check valve in nitrogen gas service. I am using a rotameter with a needle valve to set a low flowrate (1 SCFH) to sweep a seal chamber at a pump seal. Due to the risk of backflow into the nitrogen header, we have to install a check valve downstram of the rotameter. Can 1 SCFH of nitrogen in a 3/4" line be enough to lift a check valve? Are there any guidelines I could follow?
I would really appreciate your input,
more22

 

[MariusChE]

I think that the check valve will be opened by pressure as opposed to flow. Whether the flow is sufficient to keep the check valve open is another question though. If you are not too concerned with on/off flow then you could probabbly just go ahead and install it.

A better idea could be to install a check valve that works like a float i.e. a ping pong ball that would begin to float and close the nitrogen inlet line as soon as there is back-flow. This means that the check valve is installed in an unconventional way which ensures unrestricted nitrogen flow.

remove.marius@mailbox.co.za

 

[katmar]

This a standard parameter that you will find on the specification sheet of the valve. Check with your supplier or in the catalog.

 

[djack77494]

I think you answered your own question. If you get a flow of 1 SCFH to flow through your system, then you've lifted the check valve. (At least enough to flow 1 SCFH.) Initially it is a pressure differential that causes a swing type check valve to start opening, but it is the pressure differential at flowing condition, i.e. the pressure loss, that keeps the valve open. An alternate way of looking at this is to say it's flow that keeps the valve open. I think the same is pretty much true of other types of check valves as well. Never make the mistake of blindly using a reference like Crane to calculate pressure loss through a check valve when it is NOT fully open. At low flow conditions, the valve may not fully open, and DP may be much higher than you expect.
Good Luck,
Doug

 

[lastone]

More22, read page 2-7 in Crane 410. It explains the answer to your question and shows you how to determine the answer. Most dry gas seals that I have encountered employ the following: 3/4" line from the nitrogen header in your unit to a 3/4" block valve, then 2 3/4" check valves, then a swedge down to 1/4" SS tubing, then a filter/adjustable regulator, then a restriction orifice, then a rotameter, then a pressure gauge to determine how the regulator should be adjusted per the seal requirments, then a tubing check (probably not necessary, but we use it), and last an enlargement up to the tubing size on the gas seal. Also, use your seal vendor for their recommendations or go to one of the seal vendor's websites for a standard drawing. My company's policy is to use dual checkvalves to prevent the possibility of process gas backing into the plant nitrogen system. I think it is a good policy and you should look at this. There have been incidents in the chemical industry with nitrogen system contamination. Hope this helps.