Control systems and manual applications utilize various types of valves to turn fluid flows on and off, and also to modulate the rate of fluid flow through the valve. Fluid flow through a valve results from pressure differentials between upstream sources and downstream destinations. Fluid flow is a function of pressure differentials and conduit resistance. Control is generally achieved by varying the resistance to flow by varying the available flow area between zero and a maximum.PLUG VALVES A valve is the conventional method of varying area.

Sliding gate valves present one method of varying flow area. However, in such a valve, the differential pressure from the upstream side to the downstream side multiplied by the area of the obstruction separating each side results in a substantial number. This number represents a load on the guides supporting the gate. This load increases friction in a manner proportional to the area and pressure drop.Globe Valves With increased friction, the amount of force required to move the gate increases, thus requiring more powerful actuators. With greater actuator force requirements, costs escalate. Further, control system dead band becomes larger, which negatively affects system stability.

Plug type valves are an additional method of varying flow area. These valves reduce the flow area by forcing a plug into a hole. When the plug is lowered from the upstream side, typically the result is that the plug slams shut against a valve seat due to upstream pressure and inertia forces pushing the plug toward the hole. This slamming causes hammering which creates noise and valve damage.Needle & Gauge Valves Forcing a plug into a hole from the downstream side can also reduce the flow area. In such a scenario, the obstruction pushes against a substantial opposing force, the force being proportional to hole size and pressure drop between the upstream and downstream sides. With increased opposing forces, the amount of force required to move the plug increases, thus requiring more powerful actuators. Again, with greater actuator force requirements, costs escalate.

In both the gate valve and plug valve instances, the difference in upstream and downstream pressures is the root of their shortcomings. To overcome these shortcomings, balancing of fluid forces is required.

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