Motor operators can utilize built-in data loggers coupled with highly accurate torque sensing mechanisms to record data on the valve as it moves through its stroke. The torque profiles can be used to monitor changes in the operating conditions of the valve and to predict when maintenance is required. They can also be used to trouble shoot valves.

Forces on a valve can include the following:

1.       Valve seal or packing friction

2.       Valve shaft, bearing friction

3.       Valve closure element seat friction

4.       Closure element in travel friction

5.       Hydro-dynamic forces on closure elements

6.       Stem piston effect

7.       Valve stem thread friction

      Most of these are present in all types of valves, but in varying degrees of magnitude. For example, closure element travel friction in a butterfly valve is negligible. Where as a non-lubricated plug valve has significant in travel friction. Valve actuators are designed to limit their torque to a preset level using a torque switch, usually in a closing direction. An increase in torque above this level will stop the actuator. In the opening direction, the torque switch is frequently bypassed for the initial unseating operation. The resulting torque profile is useful in analyzing the valve condition.

Figure 6: Gate valve torque profile

       Different types of valves have different profiles. For example, a wedge gate valve has significant torque at the opening and closing positions. During the remaining portion of the stroke the torque demand is made up of packing and thread friction on the acme threaded shaft. On seating, the hydrostatic force on the closure element increases the seating friction, and finally the wedging effect of the closure element in the seat causes a rapid increase in torque demand until seating is completed. Changes in torque profile can therefore give a good indication of pending problems and can provide valuable information for an effective predictive valve maintenance program