Noise Mitigation in piping - Downstream of Natural Gas Flow Control Valve 2

[grhead]

We have installed a flow control skid, and have measured noise at the downstream piping in excess of 100 DbA. We will likely be designing a similar installation in the future, and am looking for some input on how to mitigate noise in a gas piping system.

- See attached sketch.
- The skid is located inside a large open building, and 85 dBA max. noise limit is desirable for plant operations that may be working in the area.
- We've added acoustic insulation and cladding to the piping resulting in a reduction of about 5 Db.
- The header skid feeds fuel gas to several burners, the header will modulate various flows and pressures, but we design theoretical noise for a worst case max. flow condition.
- The Flow Control valve is sized by our valve supplier for a max. theoretical noise of 85 DbA at 3' from the valve. However, I believe what we are seeing is noise originating from the flow control valve propagating through the downstream piping, which results in a much higher value at the piping.

Some thoughts I've had:

1) Increasing the straight run of pipe after the pressure reducing skid before the 16" elbow. I'm unsure on how to come up with an estimate of length of this run.
2) Designing for a more gradual pipe bend/transition. Currently we have a 16" standard elbow, perhaps a series of 30 degree or 45 degree elbows?
3) Installing a silencer or conditioning plate downstream of the skid, I'd be curious if anyone has experience with such a device.
4) A noise "trap" (see attached image clip). I came across this in some literature from an ultrasonic meter supplier to mitigate noise upstream of the meter. I was pondering putting in a straight run of pipe with a dead-end, and then install a "T" takeoff which would go to the gas supply. I would likely acoustically insulate the straight run of pipe around the "trap"

Thanks in advance.

 

[davefitz]

The 20 deg was from memory regarding friction losses for expanders, apparently not a very good memory. A little review of boundary layer separation yields this quote "It was found that flow separation occurred for a diffuser expansion angle of 5° for inlet Reynolds numbers less than about 2000. This finding invalidates a prior rule-of-thumb that flow separation first occurs at a divergence angle of seven degrees. " So a much smaller angle is needed for couette flow or to avoid separation. If it is a custom made expander, then perhaps the use of multiple internal "flutes" or fins would break up the large jets to multiple smaller jets. As I recall, Sulzer used to make turbine bypass valves wherein the valve plug contained multiple flutes to reduce the jet noise.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[pierreick]

Hi,
You may consider the document attached to support your work , from Emerson (Fisher) control valve Handbook .
Pierre

 

[davefitz]

Figure 5.9 in the above post by Pierreick may be applicable in this case with a 8x16 outlet expander, to reduce the jet size .

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[LittleInch]

Stupid question, but you have got this valve in the right way around haven't you? The data sheet says its unidirectional. Always worth checking...

But 16:1 is a big turndown for any piece of kit.

As you had to mess with the inlet pressure to get a valve to work then I suspect there were compromises along the way that emerson didn't tell you about.

Also were the noise guesses being made at different flow rates? or only on some? note they will only look at straight pipe in and out. Your fittings a relatively short distance after the valve almost certainly are not helping.

But sure, your 16 x 8 reducer may not help. You might need to go 16 x 12 and then 12 x 8 welded together or make a special conical one.

I've seen 30-40m/sec quoted and used as max actual velocities, but would be rather reluctant to do this due to noise issues.

A couple of other points. That's not a V ball control valve. It's some sort of special ball control valve with a variable area special trim inside. On that front is there any possibility you've got anything inside that valve that is blocking any holes? Paper? plastic? Dirt? Those holes look pretty small to me. I would be looking for something like a 80 or 160 mesh filter immediately upstream.

Most people refer to those fitting as reducers, even if the flow is small to big... Including ASME B 16.9. An "Expander" to me is some sort of turbine like device.



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