Flow Restricting Orfice for Oxygen

Here look here: http://en.wikipedia.org/wiki/Orifice_plate

Use the oriface to measure the flow and a valve to restrict it. In any case the line pressure will need to be hiher than the regulated pressure

What is the pressure downstream of the orifice? Need this to size orifice. If it's between 0 and about 35 psia (note"a") flow is sonic and does not change with pressure. Above 35 psia the flow (for a given orifice dia) falls as upstream pressure is approached.

Cheers........Codey

Codey,

The downstream pressure is 50 -55 psig. Once I exceed the 2500 scfh flow rate I am looking for a minimum 10 psig pressure drop to activate a low pressure alarm to let me know that we are exceeding the system design flowrate. This is why I am looking for a flow restricting orifice.

Thanks

Hello again Medgas

Still not quite sure what you're saying, but I don't think there's enough information. Do you mean the normal d/s pressure is fairly constant at 50 - 55 psig, but when something happens further d/s (presuimably at the point of use) the flow increases and the pressure falls?

If that is correct, you need to specify what the flow is allowed to rise to when the d/s pressure has fallen by 10 psi. Given that, it is possible to calculate the upstream pressure required (assumed constant, but must be > 50 - 55 psig) and the orifice dia. Interesting calc, let me know and I'll have a go.

BTW, pressure drop is psi, not psig (or psia).

Cheers.........Codey

Hello Codey,

My application is where the upstream supply pressure is regulated to a constant of 50 to 55 psig. The oxygen is from a cryogenic source where the vaporizer is only rated for 2500 scfh. I am looking at using the orifice restriction to cause a minimum 10 psi drop (in order to activate a low pressure alarm) when the users flow requirements exceed the 2500 scfh vaporizer rating.

Thank You

Hello Medgas

In #6, you said the downstream pressure is 50 - 55 psig!

Now you've given more detail, I don't think you need an orifice at all. As the vaporizer is only rated for 2500 scfh, if higher flow is demanded the pressure both up and downstream of the pressure regulator will fall off rapidly and downstream pressure can give an alarm.

Even if the flow were not limited by the vaporizer, it would be by the regulator. As flow increases, the downstream pressure stays constant until the regulator is fully open. After that, as demand increases further the pressure falls and the flow increases to a maximum determined by the characteristics of the point of use. Of course the actual flows depend on regulator size.

Cheers.......Codey

Hi,

try with this link, http://www.lmnoeng.com/Flow/SmallOrificeGas.htm

You must be registered first.

Regards

Ernesto

With flow through an orifice from 55 psig (69.7 psia) to atmosphere.

2500/54662 = d² d = .214" Most orifices have a discharge coefficient between .6 and .9 depending upon shape and finish. If your orifice is not too crude with a Cd of .8 or d² = .046/.8 you may find that .24" dia is just right.

However you may want to start smaller. It is easier to start small and open the orifice a little than to start large and refill.

B-Well, Tom

For a limited flow situation, subsonic, the full flow orifice would be .243" dia.

again the coefficient of discharge, Cd will reduce the flow below 100% in most cases.

A crude orifice may have a Cd of .6 while a professional, well shaped, rounded and polished orifice my have a much higher Cd of .9+. Etimate the quality of the orifice and select a size that if not correct can be opened rather than refilled.

.243" dia. ² / Cd = actual diameter You will require a flowmeter to finalize the orifice or a small volume chamber.