Balancing is the term used when the resultant force on a plug is neutral. This means that the plug is neither forced up or down by the pressure of the flow stream. The advantage with balancing is that the actuator force required for controlled movement is greatly reduced. This allows for smaller and cheaper actuators. Balancing is applied to single-seated and double-seated valves in different ways.
The double-seat valves were originally designed to balance. These valves use opposite forces from the two plugs to minimize the actuator force required for the control movement. That is, the pressure of the flow stream acting on the upper plug is intended to cancel the pressure acting on the lower plug. The force in the top plug is in the opposite direction to that of the bottom plug and, as such, the result must be zero. However, because the sizes of the plugs are different, the forces
They are not the same and the result is an unbalanced force. The double-seat valves are actually semi-balanced.
In a single seated valve, the forces of the flow stream can push against the plug, requiring greater actuator force to operate the valve movement. To balance a single-seated valve, balancing holes are added to equalise the pressure
on both sides of the plug. This eliminates any unbalanced force on the plug, however further seals are required for the extra leakage path between the plug and the cage.
An unbalanced valve has better shut off capability because there is only the problem of leakage between the seat and the plug.
A balanced valve however, has a total leakage of the sum of the following:
– leakage between the seat and the plug
– leakage between the plug and the cage
The seal for the seat and the plug is a closing seal. This applies to balanced and unbalanced valves. But the seal between the plug and the cage is a dynamic seal and only applies to balanced valves. Being a dynamic seal, it is necessary to take into account maintenance and service life, as well as the operating conditions in the sealing material. Maintenance on unbalanced valves can involve machining the valve seats to rectify leakage problems. If leaks with balanced valves occur, there are two types of seals responsible. It is not uncommon for the seat to be mechanized to rectify the problem when, in fact, the cause of the leak is due to the adjustment. Although Teflon is limited by temperature compared to graphite, it does have better sealing properties. Better sealing can beachieved, by not over specifying operating temperature ratings