The operating parts of a valve which are normally exposed to the process fluid are referred to as 'valve trim'. Usually these parts are subject to wear and degradation and hence are replaceable. Usually parts like stem, closure member, seating surface etc. are called as valve trim. The parts such as body, bonnet, yoke etc. are not considered as trim. Trim in a control valve also influences its flow characteristics.
Following table represents a list of parts, which can be considered as trim for different types of valves.
|Type of Valves||Valve Trim|
|Angle Valves||Stem, Seat, Disc, Disc nut, back-seat bushing|
|Disc Valves||Disc, Disc nut, back-seat bushing|
|Gate Valves||Stem, Seat, wedge, back-seat bushing|
|Globe Valves||Stem, Seat, Disc, Disc nut, back-seat bushing|
|Lift type check Valves||Seat ring, Disc, Disc guide|
|Swing type check Valves||Seat ring, Disc, Disc holder, Disc holder pin, Disc nut, Disc nut pin, side plug, carrier pin|
Cross section of a ball valve showing - stem, ball, seat etc. - valve trim parts
Trim in control valve
The trim plays a specially important role in case of a control valve. In a control valve the trim determines 'characteristic of a valve' or in other words how the flowrate through the valve changes depending on the valve opening.
The relationship between the control valve opening (also known as ‘valve travel’) and the flow through valve is known as the flow characteristic of that valve. An inherent flow characteristic is the relation between valve opening and flow under constant pressure conditions. There are 4 main types of control valve characteristics, which are sometimes also referred to as the 'control valve trim'. Trim and characteristic terms are used interchangeably.
- The linear flow characteristic has a constant slope, meaning that valves of this type have constant gain through complete range of flows. These valves are often used for liquid level control and certain flow control operations requiring constant gain.
- Equal percentage valves are known by that name because whenever the valve opening is changed, the percentage change in flow is equal to percentage change in the valve opening. This means the change in flow proportional to the flow just before the incremental valve opening is performed. This can also observed in the following figure. This type of valves is commonly used for pressure control applications. They can be considered for applications where high variations in pressure drop are expected.
- Quick opening type of valves does not have a specific mathematical definition. These valves give a large increment in flow for relatively smaller valve opening, as can be observed in the following figure. These valves usually find use for on-off service applications.
- Modified parabolic valves lie somewhere between the linear valves and equal percentage valves. As can be observed in the following figure, they can be used for throttling at low flow levels and have almost linear characteristics at higher flows.
Following figure shows the control valve trim or characteristic for different possible valve types.