Tapped air in piping during hydraulic pressure test

[sizgalil]

Hi.
I read in "Piping and Engineering" by George Antaki the following example:

Can anybody explain how the trapped air pressure can rise above the hydraulic pressure?

Thanks?

 

[LittleInch]

"To illustrate this effect..." which "effect" was the paragraph referring to??

I can only think this is some sort of surge effect whereby the momentum of the fluid being accelerated by the pump results in a surge effect??

The rather precise nature of the number ( 21 feet, 3 inches etc) makes me think there is more to it than you've given us. The relative size of the "bubble" to the line volume will be important also.

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

 

[sizgalil]

Thanks LittleInch.

Sorry for being so laconic. Will try elaborate.
The author is discussing trapped air pockets in a liquid lines.
He mentions that the water and the air pockets are at the same pressure when performing a pressure test in a pipe against a closed valve. He mentions that the stored energy contained in the air pockets is much higher than that in the surrounding water (stored energy=Gibbs free energy??).
A sudden opening of a valve or start of a pump in a liquid filled line with entrapped air bubbles causes the liquid to rush towards the air bubble, compressing the bubble and causing fluctuations in the line. This is the discussed effect. He then gives an equation relating the volume of the air bubble to the liquid velocity and to the air pressure. Finally, he gives the example above. I can't understand how it is that when compressing water to a certain pressure containing an air pocket, the pressure of that air pocket can suddenly rise above the water pressure surrounding it, causing the air to push the water back.

Hope this clears thing a bit.

Thanks again

 

[zdas04]

Short answer is "it can't". Long answer is "it is amazing how much dreck finds its way on to the internet". Let's say that you have a 5 mile test of 30 inch pipe. You fill it until water is coming out the high-point vents. Then you let it sit for 24 hours to let temperatures equilbrate and any entrained (not disolved) gases to migrate to the vents. Every 8 hours or so you pump in more water to displace the gas. At 24 hours you close the vents and start pumping in at (say)2 gpm to raise the pressure. At 2 gpm in 36 inch pipe the Reynolds number is around 10, deeply in the laminar and even in the creep-flow region. No chance of surge effects.

In the write up you have to wonder how you can "suddenly bring the pressure up to 27 psig. Even starting a pump against a closed valve, the "suddenly" is actually several seconds even with that small of a line. Any arithmetic that gets the gas pressure to 87 psig is simply nonsense. It looks to me like the guy tried to use a steady state equation in a transient situation. In this scenario the action on the gas is absolutely adiabatic, not polytropic, and I don't think that there is any way to determine a polytropic exponent if it was polytropic.

The "energy" they are talking about is most likely enthalpy. The specific enthalpy of the air at 27 psig and 70°F is 180.76 BTU/lbm. The specific enthalpy of water at that pressure and temperature is 38.221 BBU/lbm. The density of the air is 0.22 lbm/ft^3 and the density of water is 62.307 lbm/ft^3 at that pressure and temperature. This says that a cubic foot of water has 0.6 BTU and a cubic foot of air has 822 BTU. A lot more energy, but no way to convert that energy to pressure.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Yup, but my answer is the same - it's all to do with surge / hammer effect.

I think you would have to really shock the pipe to get that level of effect - not one I've seen or been aware of before.

normally the issue with air in during a test is that it invalidates any calculation to with change of temperature to change of pressure. Max percent of air is usually limited in specifications to < 0.2%

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

 

[KevinNZ]

This is all about water hammer. You would be unlikely to see this during a pipeline pressure test (flow too low) but happens often in pumped water lines where the momentum of the water is high and changes in flow stretch and collapse air/vapour bubbles.

 

[zdas04]

Interesting, I've seen water hammers. I've even seen them at locations where I had recording pressure gauges, and a really really bad water hammer would be on the order of 8-15% of static pressure. This report (assuming atmospheric pressure is 14.5 psia) is talking about something on the order of 250%. That would definitely break stuff.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[dik]

I guess as long as you realise that it's the air that's compressible...

Dik