Control Valve Outgassing

Outgassing can cause problems in a control valve because of the sudden increase in flow volume. When the gas comes of solution, it has a much larger volume than the liquid. The control valve’s flow area needs to be larger or else the velocity of flow will increase.

A restricted flow area can create a stream of high-velocity liquid droplets with the potential to be erosive to any surface they impinge upon. The droplets can cause extensive erosion to the internal surfaces of the control valve and its trim.

Another problem is the potential for high vibration levels. If a liquid’s outgassing percentage is high, the release of energy subjects the control valve and piping to high vibration and noise levels.

In outgassing applications, specialized control valve sizing and selection practices are needed to ensure capacity needs, vibration risk, and erosion concerns are addressed. Selecting incorrect control valves for outgassing applications can result in restricted flow, increased frequency of maintenance, and unnecessary shutdowns.

Production processes with complex fluids containing dissolved gases complicate control valve sizing since the equations are based on pure fluids. The standard application of ANSI/ISA S75.01, IEC 60534-2-1, or other proprietary liquid level control valve sizing equations do not accurately account for outgassing.

In order to obtain a Cv that is neither undersized nor oversized, contact an Emerson sales office.

Emerson uses a proprietary “bracketing” method for sizing control valves for outgassing applications. It takes into account the amount of vapor content at the outlet of the control valve and estimates the downstream area needed to accommodate the volumetric expansion at the outlet of the control valve.