The point (speed of sound) at which air flow through an orifice can not increase regardless of pressure drop.
Sonic flow can cause noise and vibrations in a Control Valve.
A typical flow curve in relation to pressure is shown in Fig. 1. The straight horizontal line on the left side of the graph represents the maximum flow rate of the test component. Right at the point this line starts to fall, it enters the subsonic flow condition. This point is the critical pressure ratio b. As the pressure ratio increases, the flow rate decreases. When we reach a pressure ratio of 1, flow has stopped (e.g., an actuator reaching its end position).
As per the simplified setup of ISO 6358, compressed air is supplied to a pressure regulator set to 6 bar (87 psi). The compressed air flows through the test component. The output pressure is monitored on the downstream side of the test component. As the test begins, the flow control valve is fully open (no back pressure), so that a maximum flow is achieved. The flow rate is measured by the flow meter, shown in our example as 100 l/min.
As the flow control valve starts to close, the flow rate starts dropping. Fig. 3, with an 80% open flow control valve, shows that P2 has increased to 1.5 bar and the flow has just started to decline to 99.9 l/min. This is the point of ‘b-Value’.
b-Value = P2abs / P1abs [(1.5+1)/(6+1)] = 0.36
That means this test component has a rated ‘b-Value’ of 0.36. Any pressure ratio below 0.36 indicates a sonic flow condition, and any ratio higher than 0.36 implies a subsonic flow condition.
To plot the graph, we continue to reduce the flow by continuing to close the flow control valve. On the way to a fully closed flow control valve, we collect P1 and P2 data and calculate pressure ratios accordingly. When the flow control valve is fully closed, supply pressure and output pressure will be equal; hence, the pressure ratio will be 1 with no flow occurring.
When the fluid flow velocity in a choke reaches the velocity of sound in the fluid under the localized temperature and pressure, the flow is called “sonic flow.”
At subsonic velocities the flow is characterized by turbulent mixing, and this is responsible for the noise produced. This noise is best described as a “hiss” for small jets or as a roar for larger jets but has no discrete dominating frequency. Its spectrum is continuous with a single, rather flat maximum.
As the pressure ratio increases past the critical ratio and the fluid reaches its sonic velocity, the sound emanating undergoes a fundamental change, while the roaring noise due to the turbulent mixing is still present, it may be almost completely dominated by a very powerful “whistle” or “serooch” of a completely different character. This noise is rather harsh, becoming much more like a pure note.