What is Choked Flow in a Gas Control Valve?
Choked flow is the point at which decreasing downstream pressure will not increase the flow through a valve.
This typically happens in high differential applications in a high pressure control valve in gas back pressure or pressure reducing service. (What's the difference?)
What Causes Choked Flow?
Choked flow usually happens when there is a high pressure drop across the valve. Choked flow begins when a pressure ratio is reached (1.8 for methane) and the gas velocity is at Mach 1 (767.2 MPH).
There does not need to be a high flow rate for choked flow to occur.
The valve is "choking" the flow back because it has reached sonic flow (Mach 1) and the pressure differential (Δp) is high enough.
Choked flow can happen at any time these factors exist in a valve.
What are the Signs I'm Experiencing Choked Flow?
- Noise—One of the symptoms of choked flow is elevated noise. This is due to the sonic velocity of the gas through the valve.
- Cavitation—In a liquid application cavitation may occur, which can cause excessive noise and vibration as well as valve damage.
- Pitting—If there is pitting damage to your valve trim, choked flow could be the issue.
- Low Flow Rate—If the control valve is not passing the required flow rate, this may be a sign that you are in choked flow or that your valve has not been properly sized for the flow conditions.
How Do I Fix Choked Flow in My Gas Control Valve?
To resolve choked flow, you must lower the differential between upstream and downstream pressure.
For example: If you have 1,000 PSI upstream and 500 PSI downstream, you could increase downstream pressure to eliminate choked flow. The exact amount to exit choked flow is affected by flow rate, temperature and fluid specific gravity.
When sizing the valve in our sizing calculator, pay attention to critical flow warning. This will indicate that you might experience choked flow based on your inputs of flow rate, Cv and pressures.
This does not mean that the valve will not be sized correctly; it is simply a warning that choked flow may occur.
To speak with an expert about choked flow or another issue you're troubleshooting, contact your local Kimray store or authorized distributor.
