Sizing a regulator - how do I read these charts?

[NuclearNerd]

I'm hoping someone can walk me through this process, because I'm missing something. I'm trying to size a filter/regulator/lubricator package. I have some difficult specs to meet though. The supply pressure could be anywhere from 90 to 150 psi. The downstream pressure should be no less than 80 psi at 90 scfm.

I figured I could calculate the Cv required at the minimum supply pressure, but few regulators come with a "wide open" Cv spec. Instead, you usually get this chart showing a series of curves showing outlet pressure vs. flow for a given inlet pressure and a set of "setpoint" pressures, like this:

http://www.cunicocorp.com/images/2253_chart.gif

My question is, how do I extrapolate for inlet and "setpoint" pressures not listed on the chart? Does the top line necessarily represent the "wide open" condition?

 

[BigInch]

Don't use inlet and setpoints at the edge or not on the chart.

The idea is to allow these things to regulate accurately and if you're out there at the end, or off, the curves, they won't work properly. Stay within the mid 50% of the charts.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[NuclearNerd]

Thanks for the advice. The problem is that the charts seem to be printed for "typical" uses - ie, setpoint pressure no more than about 80% inlet pressure. For my application the regulator might have to operate at anywhere from 50% to 90% of the inlet pressure (!), and at inlet pressures potentially lower than those typically specified, while still passing a high flow. Hence my need to extrapolate.

 

[ports394]

Why is your air inlet pressure swinging so much?

 

[NuclearNerd]

Why is your air inlet pressure swinging so much?

In reality it won't swing very much at all, but my customer won't specify a narrower range. They want our tool to operate properly whether their air system delivers full design pressure, or barely acceptable air pressure

 

[BigInch]

That's because it needs to get some dP from somewhere. If your set pressure is equal to 80% or more of inlet pressure, then you're only going to be able to get a max dP of 20% of inlet pressure to push whatever flow you need across the regulator. You must have sufficient dP to get flow.

You should use a regulator output set pressure lower than 80% of inlet.

And... You can't extrapolate into areas where you run out of dP. If you try, you'd better be using a quadradic equation because you'll lose flow by at least the squareroot of available dP, so you can see you'll be getting nowhere fast.

You may also need a pressure control valve upstream in that inlet pressure line. At those low pressures, a 100% change in inlet pressures is not going to do nice things to your regulated flow output. Or perhaps two regulators, one preregulator that kicks in when things get high.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[NuclearNerd]

You should use a regulator output set pressure lower than 80% of inlet.

I wish I could. These specs are pretty inflexible. The best I can do maybe is install a bypass line and 3 way valve in case the inlet pressure drops too much. I was hoping that I could just get a large enough regulator that it will pass enough flow at even small pressure drops. Unfortunately, the data is hard to come by.

I have another need for the wide-open Cv: I have to size a relief valve with adequate capacity to reduce the pressure to a safe range were the regulator to fail open.

 

[ports394]

If dP is too low, it won't flow very fast. Mechanically it probably won't be enough to get through the regulator.

Think of Bernoulli. Flow rate on the right side increases if dP on the left side increases. (I think I got that right)

I like BigInch's idea of a pre-regulator. I was thinking something similar with perhaps just a very large regulator upstream.
Unfortunately, I don't use a lot of controls in the work that I do because of electrical regulations. But the easiest way to do what you want is to have a large regulator upstream with a pressure transducer and a controller from Omega. Let that drop you to 100 psi or whatever you need. And then use a regular-static, larger, FRL to get your CFM, etc from the charts.

 

[BigInch]

Another useless piece of equipment is a wide open (or closed) control valve. These also must be sized to provide needed flowrates when working within 30% to 80% percent open.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[ports394]

BigInch, what do you think about using a 3 way valve with 2 regulators? A higher pressure FRL, and a lower pressure FRL on the other leg of the 3-way. And using a controller to switch between to the two and supply air to the machine?

Both supply lines would have check valves and tie into the same air inlet on the machine.

 

[BigInch]

Might be heading in the right direction.
I would tend to think about using two parallel regulators before adding a switching valve, but that might depend on how much flow you need and at what outlet pressure you need to hold as the inlet pressures change. Do you need to hold the same outlet flows at a constant outlet pressure at whatever the inlet pressures will be, or can flow and/or outlet pressures vary with inlet pressures. That may have some bearing on whether you can use one valve at low pressure and switch to the other at hi P, use one at lo P and then simply open up the inlet to the 2nd, using two valves at high pressure, or if you might be able to use 2 parallel valves with "average" inlet and outlet set pressures that will work at all inlet pressures. I would only add extra controls if you can't make it work without it.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[ports394]

Nuclearnerd, are you in a position to say that, "look here are the proper operating conditions for our equipment. We're putting this regulator on it for you, and its up to you to figure out how to supply it? And if you don't operate our equipment in the proper operating conditions, then we won't honor the warranty."

 

[hydtools]

You should select the FRL that has your required range of outlet conditions somewhere in the middle of its operating range. The regulator will control downstream pressure within limits. Flow will depend on what your device consumes at that pressure. In the performance charts you show, your single point operating is at the left end where the controlled pressure begins to drop off. Then your customer must provide sufficient capacity to drive your system to get the device performance he expects.

What range of pressure to your product demands what range of flow at your design product performance.

Ted

 

[NuclearNerd]

are you in a position to say that, "look here are the proper operating conditions for our equipment. We're putting this regulator on it for you, and its up to you to figure out how to supply it?

It might come to that. I can't be in the "middle of the range" for a supply pressure of both 150 psi, and 90 psi. (not to mention, I can't find curves for these extreme supply pressures).

 

[hydtools]

For instance Clippard -4 on page 341, setpoint 80psi midrange flow 100scfm with 100psi input. At 80psi setpoint it can handle flow 50 to 150scfm. If your device absorbs 90scfm at 80psi that one will work; 90scfm is ~midrange. Maximum operating pressure 150psi.

http://www.clippard.com/downloads/g...atalog by Section/325-344 Air Preparation.pdf

No?

Ted

 

[NuclearNerd]

Thanks for the link hydtools

So here's my difficulty with these charts: On the chart you reference there is no curve above 80 psi at a flow rate of 100 scfm. I assume that means the valve is wide open at this point (thus cannot reduce its pressure drop any further) not in the "midrange", as you suggest. Unfortunately none of the other F.R.L.s in the catalog are any better.

If I can live with the valve being wide open and I raise my specified minimum supply pressure to 100 psig, that F.R.L. does seem to at least work for my operating condition. Thanks.

 

[SEP87]

Can you not ask the regulator supplier to size/select the correct regulator. The chart you refer to in your first message is nowhere near detailed enough to give you sufficient information. I regularly size regulators for customers using internal sizing graphs which are not available available outside the company. Any good supplier should have no hesitations doing the calculations for your providing you supply all the relevant information. Any graphs in product brochures/data sheets are only a guide and should not be relied upon in my opinion. Finding accurate product specific graphs will be very difficult.