Control Valve Outgassing
Fisher severe service control valves are suitable for outgassing applications
Fisher severe service control valves are suitable for outgassing applications
Outgassing is a severe service phenomenon in which dissolved gas in a process liquid is released as pressure decreases. Process liquid enters the control valve as a liquid, and exits the control valve as a liquid and a gas. This two-phase flow produces excessive gas and typically causes two types of damage, cavitation and erosion. Excessive vibration and noise may occur as well.
Outgassing is one of several severe service phenomena that are encountered in refining, petrochemical, and oil and gas applications involving control valves. The ability to identify an outgassing application is important because it is handled very differently than any other application.
Outgassing occurs in many combinations of liquids and gases. So how can you tell if your application may have an outgassing problem? Listed below are some good indicators that outgassing is occurring:
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.
