Maintain backpressure into a vessel by submerging the outlet?

[skuntz]

We are flowing water into a basin that is 150 psig upstream. The outlet is above the basin. The design now is to use a restriction orifice to create enough backpressure to prevent flashing & cavitation. The new idea is to submerge the outlet deep enough into the basin such that the pressure at the outlet provides sufficient backpressure (so there is no need for the orifice). Intuitively this seem to make sense and it's simple, but is there something I am not considering here? Will this strategy work?

Thanks

 

[LittleInch]

No. The static heads will cancel each other out Draw a section and post it.

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[Compositepro]

Blowing gas though a pipe submerged in water is a very accurate way to control back-pressure but is only practical or desirable for fairly low pressures. 150 psi requires 300 ft submergence. A back-pressure regulator is cheaper and easier.

 

[georgeverghese]

If you were flowing gas through this submerged pipe, you'd develop a backpressure in the pipe equivalent to the depth of submergence. But since this is water flowing down this pipe, LI is right in that the internal pressure in the pipe will still be atmopheric at the elevation which matches external basin water level.

 

[LittleInch]

If you had a pipe 100m in the air which then let the water cascade freely back to your pond it would work.....

Or shot it into the air with a nozzle pressure of 150 psi...

Now can you work out why no one does it this way?

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Also: If you get a response it's polite to respond to it.

 

[MortenA]

All of a sudden ppl started talking about gas, dont know why?

But yes, you could control back pressure this way but remember that it takes 10m. of water to create 1 bar back pressure (US 2 ft water== 1 psi). So it takes a very deep tank to crease any significant back pressure.

Best regards, Morten

 

[LittleInch]

Morten, don't be silly - The only "back pressure" you'll get is the friction in the pipe when flowing down to whatever depth you end up at, but the static pressure, assuming the water in the pipe and the pond is the same SG, will be identical in the pipe and the water.

Unless you're talking about the gas situation of course..... then yes, it's approx. 10m for every bar of back pressure assuming your gas isn't very dense.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

bummer

 

[MortenA]

in fact it will lower since the outlet was above the water level. In my defence it was quite early....

 

[skuntz]

Thanks for the replies so far. Perhaps this sketch will clarify what we are attempting. The thought is to use the hydrostatic head at the end of the pipe to create a backpressure to the valve outlet and limit the differential pressure in the valve and avoid cavitation. We will mount the valve at some elevation "e" from grade, which is yet to be determined.

 

[LittleInch]

Sorry but are you not understanding what we're saying. There is no "hydrostatic head". "e" & "d" are irrelevant. Your only head is the difference between your pipe elevation and the top of the basin, probably only a few metres. The only other pressure drop is the friction in the pipe. If you made that 10km long then you might be able generate some back pressie, but as drawn depth d is irrelevant. It could be 1000M and there is no static back pressure

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Now it looks like your concern is with cavitation at the valve exit and not with subatmospheric conditions at the top of this riser.

Am assuming that youve got the entire pipe liquid filled, or that it will eventually become completely liquid filled.

At low flow when frictional losses are small, the backpressure on the control valve exit would be lowest.

So at low flow, part way through this upriser, where the elevation = the liquid level in the basin, the pressure in the pipe is atmospheric. So the backpressure at the control valve exit at low flow would the static head difference created by the elevation delta between the basin liquid level and the elevation of the control valve.

Dipping this pipe into the basin by d metres doesnt change this backpressure.

 

[Compositepro]

Sorry, I overlooked that it was water flow in the pipe. Never mind.

 

[georgeverghese]

Pls note that the min pressure seen in a control valve in pressure throttling service is at the valve throat in the valve body and not at the valve exit. There is some pressure recovery as the fluid travels from throat to exit. Anticavitation trim is usually selected when the fluid pressure in the valve throat area is below the saturation vapor pressure of the fluid at the operating temp i.e. when vapor breaks out in the valve throat.

 

[LittleInch]

Use a different control valve. The valve should be designed to prevent flashing within the valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gerhardl]

Flow (max, min and medium), pipelengths and diameters, and placement of components are missing, so no ditailed recommondation can be given.

My general suggestion is to change the valve to e suitable type, for instance a needle valve (German 'ringkolben') and place it directly at outlet with air access above water, or some short distance away with air intake tubing with outlet at valve (not pipe!) outlet.

Yes, this will create both noise and a 'geysir' of water (see link to picture) but you will have a very wide possible control range both for pressure and flow, with almost maintenance-free and long lifetime.

https://commons.wikimedia.org/wiki/File:Erhard-needle-valves-premium-rkv-in-action.jpg

Note: Several competitors with similar or slightly differing solutions based on the same principles are available on the market. (Someone does it this way!)