Method for remotely approximating the stem thurst of motor operated

Abstract

A method is disclosed for approximating the stem thrust during the operation of a motor operated valve combination including a valve having a valve stem, a valve disk, a valve seat, and a motor operator having a motor drive means interconnecting the motor to the valve stem for movement of the valve stem disk toward and into engagement with the valve seat to close the valve and a control switch for deactivating the motor. The method comprises the steps of sensing the stem thrust of the motor operated valve combination as a function of time during valve closing at a first time and establishing a thrust trace; sensing the real electrical power of the motor as a function of time during valve closing at a second time which is subsequent to the first time and establishing a power trace; relating corresponding points of the thrust trace and the power trace starting at a point on each trace corresponding to the onset of wedging of the valve disk and extending to a point of the power trace at which the control switch deactivates the motor; determining from the power trace the value of the power at the zero thrust point; determining a best fit analytical function that takes into account the zero thrust power point and the changing thrust vs change in power relationship described by the corresponding time points of the two traces from the onset of wedging to deactivation of the motor by the control switch; and approximating from the best fit analytical function a thrust value associated with a point on the power trace at the second time.