Energy losses associated with a gas regulator

[James_Frogan]

Referring to: http://www.cunicocorp.com/images/2250.gif

I'm looking at some of the regulator/shut-off valves in use today, and as far as I can tell, every one of them kind of redirects the gas as they go through the plug in an awkward and flow-disturbing pattern.

As with something as simple as even a bend in a pipe, there is head loss as the fluid moves around. How then does a fluid regulator stay efficient, especially when it goes toward higher mass flowrates where having a tiny cross sectional area for fluid flow might possibly make it locally supersonic.

What other losses are associated with gas regulators? Are there any good resources I can pick up about regulator and shut-off valve operation? (by good resources I mean something that I can read through and finally design a shut off valve)

One more question, how does the above regulator valve keep the fluid inside the regulator, given that the tolerance is never 100% accurate? (is there some grease to apply or something that keeps the fluid inside.

Thanks in advance!

 

[Q_Goest]

Hi James,

Referring to: http://www.cunicocorp.com/images/2250.gif

I'm looking at some of the regulator/shut-off valves in use today, and as far as I can tell, every one of them kind of redirects the gas as they go through the plug in an awkward and flow-disturbing pattern.

As with something as simple as even a bend in a pipe, there is head loss as the fluid moves around. How then does a fluid regulator stay efficient, especially when it goes toward higher mass flowrates where having a tiny cross sectional area for fluid flow might possibly make it locally supersonic.

What other losses are associated with gas regulators?

If a regulator can't add or remove work, and can't add or remove heat, then what kind of 'efficiency' should one expect? Apply the first law of thermo - isn't the flow through a regulator (or any similar restriction such as a hand valve, pipe, elbows, etc...) isenthalpic? (Think about this. Draw a control volume around any piping component and apply the first law.) How do you want to evaluate the "efficiency" of a regulator or other restriction? Isn't the flow pretty close to 100% isenthalpic? Any increase in velocity (ie: increase in kinetic energy) of the fluid, moves this towards an isentropic process, but pipe isn't intended to be isentropic. That would require all the internal energy to be converted to kinetic energy.

Are there any good resources I can pick up about regulator and shut-off valve operation? (by good resources I mean something that I can read through and finally design a shut off valve)

Lyon's valve designer's handbook is a good one. There are others. But why would you want to design your own? Valves are relatively inexpensive, and you would learn most about them by working at a manufacturer. Designing a valve is more than just making it work, it has to meet industry standards as well.

One more question, how does the above regulator valve keep the fluid inside the regulator, given that the tolerance is never 100% accurate? (is there some grease to apply or something that keeps the fluid inside.

Thanks in advance!

Not sure what you're getting at here... The .GIF you pointed to doesn't show seals or a diaphragm but they're only left out of the picture because it's a very simplified sketch. Seal design is a big part of the design of fluid components.

 

[James_Frogan]

Q_Goest, thanks for your detailed reply

I'm not so good at understanding practical thermodynamics, so if I'm spouting rubbish please be patient with me. With regard to the isentropic nature of a regulator, there are odd nooks and crannies in the regulator design, won't this give rise to some sort of drag due to flow separation or some other phenomena?

Theoretically drawing a CV around a bunch of pipes, I can see the near-isentropic-ness of the system, but in my head I always imagined that if I had a segment of pipe that zigzagged everywhere, it would be different from a straight pipe. (now that you question my line of thought, i can't see how it would mess things up, except maybe turbulence at the bends)

I'm tasked to design a regulator valve (engineering accuracy) as a uni project and have problems finding detailed explanations of these valves and the necessary theory, construction etc behind them, hence the post here. I will check out Lyon's handbook and let you know if i have extra questions, thank you very much :)

 

[Q_Goest]

Sorry, you misunderstood. Flow through regulators and piping is not isentropic at all, it's isenthalpic. You should convince yourself of that by applying the first law to the regulator (or any piping component) and seeing what comes out.

Regulators are fairly simple devices, and the "nooks and crannies" have no impact on function. There are a few regulators that are designed to reduce those nooks and crannies but that's only to minimize particle shedding. You'll find these only in ultra high purity systems such as those used in the manufacture of computer chips. They are no more 'efficient' than any other regulator.

The basic principal of a regulator is that you have a diaphragm directly attached to your valve which moves up and down depending on pressure. The pressure acts on the diaphragm in one direction and either a spring or a reference pressure acts on the opposite side. If they are equal, the valve doesn't move. Depending on the valve's position then, it may be open or closed.

In the case of the spring loaded regulator for example, the spring has a constant, k, which changes the force on the diaphragm depending on how compressed it is. With the valve in the closed position, the spring load has to be equal to or less than the load produced by the pressure on the diaphragm (F = PA where A is the area of the diaphragm). As pressure decreases, and assuming A stays constant, force on the diaphragm decreases, so the spring extends per F=kx and finds a new equilibrium by opening the valve.

So lower pressure on the diaphragm means the spring extends, which per kx, reduces the force exerted by the spring, and the two forces come into equilibrium by opening the valve as pressure drops.

Hope that helps.

 

[James_Frogan]

Thank you very much, you have been very helpful :)

 

[Bob S]

Natural gas expanding through a pressure regulator at constant enthalpy cools due to the Joule-Thomson effect:

"The cooling which occurs when a compressed gas is allowed to expand in such a way that no external work is done. The effect is approximately 7 degrees Fahrenheit per 100 psi for natural gas."

http://www.aga.org/Kc/aboutnaturalgas/glossary/default.htm?id={F06A61F5-196B-4BA3-B2C8-C7F278DC374F}

http://en.wikipedia.org/wiki/Joule–Thomson_effect

Bob S