Pressure Regulator problem 1

[mantan77]

Hi All,
I have a big tank (6,603 cu.ft.) that I am pressure testing with air to a max. pressure of 6.25 psig. The hose running from my compressor to the top of the tank is 3/4". The compressor is probably 30hp, 100 CFM @ 175 psi. The situation now is that we are not using any pressure regulator from the compressor to fill the tank to save the time and fill the tank quickly. In order to avoid over pressure, we have safety devices in the tank but just to be more safe, I want to install a pressure regulator on the compressor so that we can have some control over the pressure. Ideally, we should set the pressure at the regultor @ 6.25 psi + the losses thru the hose. But that takes an awfully long time to fill the tank and bring it to the required pressure. I am pretty sure that flow coming out of the compressor is choked and probably can put a regulator that would lower the pressure till the flow still remains choked, so that my time for filling the tank doesn't increase substantially and we have some margin for safety instead of just letting it run wild at the compressor. Any suggestions/ideas?
Thanks.

 

[BigInch]

You've got the wrong compressor. You need a low pressure high volume unit. You've got a relatively high pressure-low volume unit and the compressor is completely stopped up with your first requlator.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[iainuts]

You should be able to pressurize the tank in 28 minutes:
Pressure: 6.25/14.7 = .425 cubic feet @ STP
Tank is 6603 Cu Ft.
Fill time is 6603/100*.425= 28 minutes

A 3/4" hose probably has ~ 1/2" ID. If this is 20 feet long, your pressure drop is at least 30 psi.

So you'd need to set your reg to around 40 psi, which doesn't help you at all. You need a bigger hose. I'd suggest a 1.5" hose (~1.2" ID). Then the pressure drop through the hose is on the order of 1 or 2 psi. Set the reg about 2 psi higher than tank pressure.

The other issue regards droop in a reg which is the pressure below set pressure needed for the reg to fully open. A reg with a Cv = 5 should be plenty big enough, but that doesn't mean the reg will be open to a Cv of 5 given some small pressure drop below the set pressure. Your regulator's Cv will increase gradually from 0 to maximum opening over some pressure range which may be considerable depending on how well it's designed. I've seen a lot of crappy regs that have very high droop and many manufacturers can't even give you the droop information. A reg with a Cv of 15 to 30 might be able to deliver the flow more easily because such a reg would only need to open a relatively small amount to give you the necessary flow.

 

[BigInch]

iainuts,
You've got no fear about pressurizing a tank being tested to 6.25 psig to 40 psig. I think first, the vessel relief valve should be set to 6.25 psig and the valve checked for the full flow capacity of the compressor at that pressure.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[pmover]

absolutely correct BigInch + a star!

not much more need be stated other than to re-iterate and emphasize the safety & health of personnel/equipment before pressurizing a closed vessel.

highly recommend BigInch recommendations be implemented.

-pmover

 

[iainuts]

Hi BigInch, It's always good to have multiple options. If it were me, I'd not use a lower pressure, higher volume compressor unless I could ensure myself that compressor could never overpressurize the tank. A regenerative blower or similar compressor might work if a perfect match for the tank could be found (ie: the blower would deadhead at just above the 6.25 psi limit by no more than 1 or 2 psi), and that 'might' be a better solution than providing the gas more slowly using a regulator, but that's debatable.

The idea of using the reg as originally suggested by the OP is a decent one but one that obviously has a flaw; they're using a 3/4" hose, and that's the first problem. The 3/4" hose results in a large pressure drop, so the reg can't be set at a safe level. The right thing to do if implementing this solution is to provide a larger "BigInch" hose as I'd suggested above. That way, given a reg with low droop, you could set the reg 1 or 2 psi above test pressure. This should be a manned operation any way, and per the OP, they already have relief devices on the tank so the reg is a 'belts and suspenders' approach which should be a considerable improvement over what they're doing now.

Either method is an improvement over the existing set up, and having additional options is always better than having just one.

 

[BigInch]

Sorry, but there should be no difference of opinion when safety is involved. How can you tell him to set the regulator pressure at 40 psi, when you don't know the pressure or capactiy of the relief valve? If you had to guess that the OP had proper settings now, you'd have to guess the relief pressure was set to 6.5 psi. No assumption about capacity is possible. I'd guess thermal relief volume only, if anything.

Your method potentially tests the tank at 7 X test pressure AND the relief valve at the same time. Not good. And nobody should be near it if you chose to do it in that manner.

If I was doing this, I'd seal atmospheric air in the tank at 6 am and wait for the sun to warm it up to 6.5 psi. No compressor needed.



Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[iainuts]

Ah... you missread my post. I didn't say any such thing.

I'd suggest a 1.5" hose (~1.2" ID). Then the pressure drop through the hose is on the order of 1 or 2 psi. Set the reg about 2 psi higher than tank pressure.

So the reg should be set at 7 - 8 psi, the pressure drop through the hose is negligable, and the reg shuts off when it gets to roughly 7 or 8 psi.

Either way (your method or mine) someone has to shut down the system to prevent that last 1 or 2 psi from going into the tank.

 

[BigInch]

I hope so.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com