15. What are the common types of flow characteristics for control valves There are three basic types of control valve characteristics. These are quick opening, linear, and equal percentage. A quick opening valve would flow approximately 70% of its full flow at a valve opening of approximately 30%; a linear valve would flow approximately 50% of its full flow at a valve opening of approximately 50%; and the equal percentage valve (logarithmic) would flow 30% at approximately 70% of its full opening. So the question is - where would you use one style of valve over another • A quick-opening characteristic is recommended for shutoff service only. Not much use for modulating process control except in ON-OFF application. • If the piping and downstream equipment cause significant resistance to the system, equal percentage will provide better control. This is a condition, when the valve pressure drop at the maximum flow rate is less than about one third of the overall system pressure drop (valve + line. • If most of the pressure drop is taken through the valve and the upstream pressure is constant, a linear characteristic will provide better control. Also use linear characteristic when a PLC is used to control the valves in a system. Avoid linear flow characteristics at low flow conditions. Control valves with equal percentage flow characteristics are generally specified for most applications. Both types offer the best throttling control. In case of doubt, use equal percentage.
