Why Is Reduced Trim Required In Control Valves?
What Is The Trim In A Control Valve?
The trim of the control valve is the parts that are in direct contact with the fluid that is controlled. The edge of the valve consists of all the parts that will get wet, apart from the body and the bonnet.
• The compensation is the heart of the valve with a primary function to proportional the valve orifice in such a way that there is an established relationship between the capacity of flow and the elevation of the plug of the valve.
• The secondary function can be closed hermetically.
• Fit includes seat, plug, stem, gland follower, gland nut, plug, guide bushings, and cage
• The components of the packing box considered as trim are the packing, the follower, the spring, the ring of the flashlight and the packing retainer.
• The secondary molding parts are derived from the sumps, the seat retaining ring, the seals of the seat body and the spacers.
Why Is Reduced Trim Required In Control Valves?
• The control valves are sized according to the requirements of the application and must meet both the Cv and speed criteria.
• The reduced setting is used when it is necessary that the valve has a Cv capacity lower than the maximum possible in that valve size.
• The most common reason for a reduced cut is that the flow rate is low for the required valve size, particularly when the 25 mm valves have been specified as the smallest size to be used. Some plants stipulate that no control valve should have less than two sizes smaller than the size of the line and that the valve should not be less than half the size of the line.
• The second reason is that in gas or high-pressure steam applications, the valve is invariably sized at the speed limits of the output port and the required CV is much smaller than the total internal diameter Cv.
3 types of control valve trim
The figure below shows the trim characteristics and illustrates the relationship between the percentage of flow and valve stem travel from 0% to 100%. There are 3 types of control valve trim:
• **Quick setting (quick opening): the ** quick release of the valve opens quickly and is used for on/off service. The main applications include liquid discharge, pressure relief and measurement.
• Normal (Linear): the nominal configuration of the valve is used to throttle liquids, control the fluid level and in applications where water hammer has been a problem.
• Equal%: used to control the pressure or flow of gases and vapors in throttling applications.