Determine the Rate of Change of Pressure Across a Valve

[Purple_Dan]

Hello all!

On closer inspection of the valve specifications, the orifice size is only 1.57mm (0.062") at max, that's tiny!

Anyway, we've decided to go with proportional control. If we're way off the target, open/close the valve quickly. If we're closeish to the target, open/close the valve slowly.
That is until the client decides this is not effective enough and decides to do it the right way!

At the moment, they're dropping the pressure at about 2000 psi/sec after a test! That's mental! I think any improvement is an improvement and the control valve should definitely improve the way we control this!

In any case, when the customer decides they don't want to do it this way, I'll use what I've learned from all of you as my guide to improving the system.

But there's still one small question that hasn't been answered:

I don't think that will work. At least not measurably by practical means.

Why wouldn't Boyle's Law work? If I knew the temperature and the pressure, could I not work out the volume? Is it a special case at 23000psi? Or am I just being silly?

Also, thank you all for contributing to my first post and making it last week's most popular Mechanical Engineering post!

 

[montoyas7940]

Boyle's ideal gas law won't work for you in this application. It is for gases only.

Someone else here may be able to help devise a practical way to implement your idea. I don't know...

 

[Purple_Dan]

Boyle's ideal gas law won't work for you in this application. It is for gases only.

Someone else here may be able to help devise a practical way to implement your idea. I don't know...

Okay, so that's interesting. No one has come up with an equation for the relationship between Volume, Pressure and Temperature in a liquid. Someone should do that now, get to work scientists!

I spoke with the client today and there's no trapped gas because they pump water through until they see it coming out the other end and close the end valve whilst it's still pumping. So that's not a factor anymore, which is nice.

But I won't know any more about the system to shed some light on it, and hopefully figure this thing out, until I go on site in a few months. Which is annoying...

 

[Tatersoup]

I am trying to maintain a specific rate of pressure drop, opening the valve would increase the rate the pressure drops, and vice versa.

Also, the pressure drop across the valve is variable at a maximum of 23000 psi, I already know what this is, I'm just trying to figure out what it will be.

I understand your point, it's your nomenclature that is confusing. Instead of saying, "opening the valve would increase the rate the pressure drops", I would say, "opening the valve would reduce the pressure drop"

The system pressure has nothing to do with it. The rate of flow through the valve does. If you go look at my original post, I tried to suggest a method to estimate the flow for different valve openings using a basic orifice equation.

 

[Purple_Dan]

I understand your point, it's your nomenclature that is confusing. Instead of saying, "opening the valve would increase the rate the pressure drops", I would say, "opening the valve would reduce the pressure drop"

The system pressure has nothing to do with it. The rate of flow through the valve does. If you go look at my original post, I tried to suggest a method to estimate the flow for different valve openings using a basic orifice equation.

Ah, I see where the confusion could arise. I have the flow coefficient at different valve openings, from which I can calculate the flow rate without using orifice equations.

It's where I go from there that I'm having trouble with.

 

[russ_watters]

You should also look into the issue of cavitation:
http://www.valmatic.com/pdfs/Cavitation_in_Valves_7-22-08.pdf

 

[jim hardy]

I still sense a disconnect here.

Clearly from previous posts they're depressurizing a test rig that's been filled with a test fluid(water?) nearly purged of air.
It's depressurized through an orifice that's tiny compared to the size of the test chamber.
In petro world, water is considered "slightly compressible", and when compared to pure water steel itself is compliant.

So to exaggerate for purpose of seeing what's going on, refer way back to PurpleDan's water bottle analogy. He's depressurizing a water bottle through a hypodermic needle.

With so much not known about the system , the only approach i can envision would be to record pressure vs time for a known valve opening and catch the fluid released in a beaker.
One could then come up with a term for the total compliance ΔV_olume/ΔP_ressure of the combined water-steel-entrained-air system.

A measurement of flow during the depressurization would give a good number for compliance vs pressure.

Here's an academic looking paper on fluid mechanics in wellbores.
Since this consultation is in petro field maybe it'll have some vocabulary words you could use to prime the conversation pumps.
http://petrowiki.org/Fluid_mechanics_for_drilling