Tandem trim is a high performance specialty trim that is cage guided. The standard design is a uniquely balanced port throttling trim designed to solve difficult high temperature, high-pressure differential applications that would require oversized, heavier actuators if other standard trims were utilized.
Tandem trim provides excellent control, rangeability, and tight shutoff to Class V or MSS-SD-61 in high temperature applications.
This trim is designed with the main and pilot plugs working in tandem. The Tandem trim may be customized to use Raven, Hush, or any standard cage can be used with this trim design.
The Tandem Trim consists of a cage and balancing cylinder which is gasketed and trapped between the valve bonnet and body web. The large plug is the control device. Inside the main plug, is an inner plug which is connected to the valve stem.
When the valve is closed, the balancing chamber above the main plug is at the inlet pressure due to flow past the piston rings. Full inlet pressure acts on top of the main plug which generates a high contact load on the seat and provides positive shutoff.
When the valve stem lifts, the inner plug is opened and the pressure in the balancing chamber is vented to the downstream side of the valve through a port in the main plug. The equalization of pressure reduces the unbalanced forces on the main plug. As the inner plug continues to lift, the inner plug shoulder contacts the main plug to open it. The main plug is opened without excess force since it is essentially pressure balanced.
As the stem positions the main plug to modulate the flow, the unbalanced force remains constant (with a constant differential) allowing smooth control and ensuring that the inner plug remains open.
When the valve is closing, the main plug first contacts its seat. Further downward movement of the stem then closes the inner plug. Flow past the piston rings then pressurizes the balancing chamber to the upstream pressure, holding the main plug tight against the seat.