Understanding Choked Flow in Control Valve

First, we shall understand what is Choked Flow. Choked flow is a condition where the flow rate through the control valve becomes constant even though we increase the pressure difference (ΔP) between the control valve inlet and outlet.

Note: This definition assumes a fixed flow area and unchanged Pressure inlet.

Note: The valve can choke in a liquid application and gas application as well.

How does the chocked flow condition occur?

For liquid Application

Let's take a closer look at what happens when liquid flows through a valve or nozzle. We can do this by plotting a curve of pressure vs. velocity. As the liquid flows through the valve, the pressure will decrease across the valve and then recover at the outlet. The opposite for the velocity, it will increase as the liquid flows through the valve and then recovers at the outlet.

Now if the pressure drops below the vapor pressure of the liquid, the liquid will vaporize. This phenomenon is known as Cavitation. Compared to the liquid, the vapor needs more space, therefore vapor will act as a restriction to the flow. This means that the flow will remain constant and cannot be increased. "The flow is choked"

For Gas Application

Choked flow can also happen in gas applications. It is similar to choked flow in liquid applications, but instead of looking at the pressure curve, we need to look at the velocity curve. When the velocity of the flow across the valve reaches the speed of sound (1 Mach), shock waves will form. These shock waves act as a restriction to the flow, and the flow will not be able to increase any further.

Conclusion

Choked flow can occur in both liquid and gas applications. To avoid it, it is important to ensure that the pressure drop across the valve is above the vapor pressure of the liquid in liquid applications and that the velocity of the flow is less than 1 Mach in gas applications.