A gas letdown valve that screams at 110 dBA can damage hearing, crack welds and shake its own trim loose within months. Good prediction during sizing and the right mix of source and path treatment keep the plant quiet, safe and within its noise specification.
Most loud valves in a process plant are gas pressure reducing valves with a high pressure drop and a small downstream pipe. Understanding where the sound is born and how it escapes through the pipe wall lets you choose a quiet solution at the sizing stage instead of after a complaint.
What Is Control Valve Noise?
Control valve noise is the unwanted sound produced when a liquid or gas is forced through the small restriction of a throttling valve and loses pressure very quickly. It is usually measured in A weighted decibels, written dBA, one metre downstream of the valve and one metre away from the pipe surface, and it is checked during control valve sizing along with Cv and velocity.
The picture above shows a globe valve trim that splits the flow into many small jets and takes the pressure drop in stages. Both ideas attack control valve noise at its source, which is why they appear in most modern control valve trim types.
Where Does the Sound Come From?
Plug, stem or cage vibrates against its guides and produces a rattle or a pure tone.
Turbulence and shock waves in gas or steam leaving the vena contracta at high velocity.
Turbulence in liquid flow, and much louder crackling when the liquid starts to cavitate.
Aerodynamic control valve noise is the big one in gas and steam plants. When the pressure ratio across the valve rises toward choked flow, the jet leaving the vena contracta reaches sonic velocity and forms shock cells that radiate intense sound.
Hydrodynamic noise from plain liquid turbulence is normally quiet, often below 80 dBA. Once vapour bubbles form and collapse, as explained in control valve cavitation and flashing, the sound changes to gravel like crackling and the damage risk rises sharply.
Emerson engineer Mark Nord notes that aerodynamic noise varies roughly with the eighth power of gas velocity. A small increase in velocity therefore produces a surprisingly large increase in sound level.
Why the 85 dBA Limit Matters
Most plant noise specifications ask that control valve noise stay at or below 85 dBA, measured one metre downstream and one metre from the pipe. Mascot Industrial lists this as the usual rule for continuously operating valves, with 90 dBA at four metres for intermittent atmospheric vents.
The 85 dBA figure comes from occupational health rules. OSHA requires a hearing conservation programme when workers are exposed above 85 dBA for eight hours, and the allowed exposure time halves for every 5 dB rise under its rules.
Nord also reports that damage tends to occur above 110 to 115 dBA, because the same vibration fatigues welds, small bore tappings and trim parts.
Always ask which limit applies, the 85 dBA personnel limit or a tighter area limit near control rooms and boundaries. Write the agreed limit and measuring point clearly on the valve datasheet before sizing starts.
Predicting Control Valve Noise With IEC 60534
The IEC 60534 part 8 series is the international basis for noise work. Parts 8 1 and 8 2 describe laboratory measurement of aerodynamic and hydrodynamic noise, while parts 8 3 and 8 4 give the prediction methods used in sizing software.
| Standard Part | Covers | Used For |
|---|---|---|
| IEC 60534 8 1 | Lab measurement of aerodynamic noise | Testing valves on a flow rig |
| IEC 60534 8 2 | Lab measurement of hydrodynamic noise | Testing liquid valves |
| IEC 60534 8 3 | Aerodynamic noise prediction | Gas and steam valve sizing |
| IEC 60534 8 4 | Hydrodynamic noise prediction | Liquid and cavitating service |
The aerodynamic method in IEC 60534 8 3 first decides a flow regime from the pressure ratio, from subsonic up to fully choked flow with shock cells. It then converts part of the jet stream power into sound, predicts the internal sound level in the pipe, and subtracts the pipe wall transmission loss to reach the external level at one metre.
The same valve can be quiet on a thick walled line and loud on a thin one, so the sizing sheet must use the real pipe schedule and the correct flow coefficient.
The pipe behaves as a line source, so each doubling of distance gives only about 3 dB
Example:
Predicted level at 1 m = 96 dBA
Low noise trim gives 6 dBA, heavier pipe and lagging give 4 dBA
Distance from pipe = 2 m, so 10 × log10(2) = 3.01 dB
L = 96 minus 6 minus 4 minus 3.01 = 82.99, about 83.0 dBA
Margin against 85 dBA = 2.0 dBA
Control Valve Noise Level Calculator
The calculator is for quick checks and teaching. Final values should always come from the vendor sizing program or an IEC 60534 calculation, and the decibel arithmetic behind it is explained in decibel in electronics.
Second Worked Example: Two Valves Side by Side
Decibels add as L total = 10 × log10(10^(L1 ÷ 10) + 10^(L2 ÷ 10)), so for two parallel valves each at 85 dBA the combined level is 10 × log10(2 × 10^8.5) = 85 + 3.01 = 88.0 dBA, so each valve alone meeting 85 dBA does not mean the station does.
7 Proven Ways to Reduce Control Valve Noise
Valin specialist Jon Monsen explains that doubling the number of flow openings lowers the level by about 3 dBA, mainly because smaller jets move the sound to higher frequencies. Designs combining flow division with staged pressure drop can reach 30 dBA or more of reduction in extreme service.
A diffuser works by sharing the drop, so the valve sees a smaller pressure ratio and is quieter, while the diffuser itself is designed as a multi hole element. The same idea is used when a restriction orifice downstream of a blowdown valve takes part of the drop.
Outlet velocity matters because aerodynamic noise rises so steeply with velocity. Many valve vendors guide designers to keep the gas outlet Mach number low, often around 0.3, and the method is covered in control valve outlet velocity.
For the path, Nord reports that going from standard to extra strong pipe wall cuts about 2 to 3 dBA at low cost. Thermal lagging gives about 3 to 5 dBA per inch up to 12 to 15 dBA, while proper acoustic insulation gives about 8 to 10 dBA per inch up to 24 to 27 dBA.
Monsen points out that a noisy pipe acts as a line source, so moving twice as far away gives only about 3 dB of relief. A small point source would give about 6 dB for the same move.
Source Treatment or Path Treatment?
- Stops the sound and vibration at their origin.
- Protects the valve, pipe and supports from fatigue.
- Works along the whole downstream line.
- Large reductions, often 10 to 30 dBA or more.
- Low cost for small reductions of a few dBA.
- Easy to add to an existing installation.
- Vibration inside the pipe remains unchanged.
- Lagging must cover a long pipe run to work.
A simple rule helps. If predicted control valve noise is above about 110 dBA, treat the source because the pipe and fittings are at risk, and use path treatment only to trim the last few dBA.
Before paying for a special trim, check whether the downstream pipe can be one size larger with a heavier schedule. A cheaper expander and pipe change sometimes gives the few dBA you need.
Control Valve Noise Myths
Installation and Commissioning Tips
Support the downstream pipe properly and avoid long unsupported small bore branches near the valve. Vibration monitoring with a vibration sensor during the first weeks of operation often reveals weak points early.
- Confirm the noise limit and measuring point on the datasheet.
- Use the real downstream pipe size and schedule in the calculation.
- Check predicted levels at minimum, normal and maximum flow.
- Flush the line before fitting a multi hole trim.
- Support small bore tappings and instrument tubing near the valve.
- Measure dBA at 1 m after start up and record it.
- Recheck noise after any change in operating pressures.
Troubleshooting a Noisy Valve
Start by identifying the type of control valve noise. A pure tone that changes with travel points to mechanical vibration, a hiss or roar points to aerodynamic noise, and crackling in liquid service points to cavitation; the general routine in control valve troubleshooting then applies.
Next, compare actual process conditions with the sizing sheet, because a higher pressure drop than designed raises noise, and a review of control valve selection factors may show that a different trim is needed.
Where Control Valve Noise Is a Problem
Emerson Guide on Reducing Noise
Valve Noise Explained on Video
Control Valve Noise FAQ
It is the sound produced when a fluid loses pressure quickly through the restriction of a throttling valve. The level is usually quoted in dBA one metre downstream and one metre from the pipe.
Most of the energy travels inside the downstream pipe and leaves through the pipe wall. That is why pipe size, schedule and insulation strongly affect the level people hear in the plant.
Aerodynamic noise comes from turbulence and shock waves in gas or steam leaving the valve at high velocity. It is predicted with the method given in IEC 60534 8 3.
Hydrodynamic noise comes from liquid turbulence and, far more strongly, from cavitation. Its prediction method is given in IEC 60534 8 4, which uses the pressure ratio and cavitation index.
It links to occupational health rules on hearing protection for an eight hour working day. OSHA requires a hearing conservation programme when exposure goes above this level.
Most plant specifications therefore ask each valve to stay at or below 85 dBA at one metre. Intermittent atmospheric vents are often allowed about 90 dBA at four metres.
A multi hole cage with flow division alone gives a moderate reduction of a few decibels. Doubling the number of openings lowers the level by roughly 3 dBA according to Valin.
Trims that combine flow division with staged pressure drop can give 30 dBA or more in severe service. The exact figure comes from the vendor sizing program for your conditions.
Yes, it is a useful path treatment when the reduction needed is modest. Emerson reports roughly 8 to 10 dBA per inch of acoustic insulation, up to about 24 to 27 dB in total.
It must cover a long length of downstream pipe to work well. It does not reduce the vibration inside the pipe, so it cannot protect the piping from fatigue.
Decibel levels combine logarithmically, not arithmetically as ordinary numbers do. Two equal sources give a total about 3 dB higher than one source alone.
For example, two valves each at 85 dBA give about 88 dBA together. A second source that is 10 dB quieter than the first adds less than half a decibel to the total reading.
Use a calibrated sound level meter set to A weighting and slow response. Measure one metre downstream and one metre from the pipe surface while logging flow, travel and pressures.
Compare the result with the sizing prediction at the same operating point. A large gap often means changed process conditions, debris in the trim or a mechanical fault.
Related Articles
- Control Valve Cavitation and Flashing
- Choked Flow in Control Valves
- Control Valve Trim Types
- Control Valve Outlet Velocity
- Control Valve Sizing Basic Requirements
External References
- Reducing the Noise from Control Valves, Emerson
- Valve Aerodynamic Noise Reduction Strategies, Valin Corporation
- Noise Control, Wikipedia
What We Learn Today
- Control valve noise is mostly aerodynamic in gas service and hydrodynamic in liquid service, and IEC 60534 8 3 and 8 4 give the prediction methods.
- Most plants limit each valve to 85 dBA at one metre, while levels above 110 to 115 dBA can damage the valve and its piping.
- Source treatment with multi hole and multistage trims gives the largest reductions, while heavier pipe, lagging and silencers trim the final few dBA.