High Noise Through Nitrogen Line-Increase Line Size??

[RJB32482]

We are having noise problems near a nitrogen line that flows through a 2" pipe. The nitrogen goes through a presure reducing valve that causes the flow to be choked (80 psig coming in and 3 coming out of the valve). How could we reduce the noise? Would increasing the line size help reduce it? The flow is choked, so flow rate would not decrease if we increase the line size correct? This should decrease velocity and therefore decrease noise.

Thanks

 

[sshep]

RJB32482,

In this case you must focus your efforts on reducing the single stage pressure drop ratio at the valve- your ratio of 95psia/18psia tells the whole story. Reducing the aerodynamic noise is usually done using staged trim, outlet diffuser, or both. Check with the representative of the valve to see what your options are. Increasing the line size will not help.

best wishes, sshep

 

[RJB32482]

Thanks for the help. What exactly is staged trim?

 

[sshep]

Typically this a cage trim with an aeropath designed to divide the stream and take the pressure drop in steps.

example

http://www.emersonprocess.com/fisher/products/severeservice/products_toc.html

 

[RJB32482]

Would using the diffuser decrease noise in the pipe downstream of the control valve? Is a diffuser just like a muffler? Why wouldn't an increase in line size not work? Choked flow states that velocity is not changed even if the pressure downstream of the CV is changed. Does the opposite also hold true (increasing pressure by reducing pressure drop through the piping, aka. increase line size, will not reduce the flow)? Thanks.

 

[liberoSimulation]

In similar cases we have in previous plants, acoustic insulation was usually used to reduce noise level to acceptable level keeping the line size and associated components are unchanged.

Hope this would help

Cheers

 

[ApC2Kp]

RJB32482,
The line size of 2" would seem to have a moderate cost for a two or even three stage set of regulators, i.e first regulator 80 psi drop to 20 psi, second regulator 20 psi drop to 3 psi. It might be less cost than staged trim in a single regulator.
Flow velocities over approximately 150 feet per second will tend to have more noise. The acoustical insulation on piping will reduce transmission of noise, as noted by liberoSimulation. Heavier wall pipe will also reduce the noise. The major noise source is in the regulator / control valve disk - seat area. The piping might generate some noise from flow turbulence in the pipe and fittings if velocities are high. That would be where a larger downstream pipe size would have benefit. Given the ratio of the pressure drop, the downstream pipe size could be 3" or 4", depending on mass flow required.

 

[RJB32482]

So for choked flow, if I remeber correctly, decreasing downstream pressure would not increase the velocity of the gas. But wouldn't increasing the downstream pipe area decrease the velocity of the gas? This could help in reducing the noise levels.

Thanks.

 

[StoneCold]

RJB32482
The noise is generated at the valve. If you took off the down stream piping you would still have the noise. You have to attack the problem where it is occuring to have an impact.

Stonecold