Method for surveying the condition of a control valve, and a valve apparatus

A method for surveying the condition of a control valve apparatus, the control valve comprising:

• a closing element;
• an actuator which adjusts a position of the closing element;
• an electropneumatic positioner including a pilot valve and which controls the actuator;
• sensors which monitor operation of the valve during operation by reading at reading intervals a control signal, a difference between an input and an output pressure of the actuator and a position of the valve;
• and
• a microprocessor accepting input from the sensors and programmed with deduction rules, the microprocessor calculating from the input and the deduction rules a location of a fault,
• the method comprising:
• monitoring the control valve in a balance state, the balance state comprising a state wherein changes of the valve position, the control signal, and the difference between the input and output pressure of the actuator are continuously smaller than predetermined values for at least a predetermined period of time;
• storing readings from the sensors of the position, the control signal, and the difference between the input and output pressure of the actuator in the balance state as initial readings;
• monitoring the control valve during its operation to determine operational readings of the valve position, the control signal and the difference between the input and output pressure of the actuator;
• comparing the operational readings of the valve position, the control signal and the difference between the input and output pressure of the actuator to the initial readings of the position, the control signal and the difference between the input and output pressure of the actuator;
• producing a fault message indicating that a fault is located in the pilot valve of the positioner when:
• a deviation between the operational reading and the initial reading of the control signal exceeds a first limit value for a first time period,
• a deviation between the operational reading and the initial reading of the valve position is smaller than a second limit value during the first time period,
• no fault message is produced by diagnostic circuits on the positioner, and
• the operational reading is less than the initial reading of the difference between the input and output pressure of the actuator for at least a portion of the first time period;
• producing a fault message indicating that a fault is located in the valve or in the actuator when:
• the deviation between the operational reading and the initial reading of the control signal exceeds a first limit value for a first time period,
• the deviation between the operational reading and the initial reading of the valve position is smaller than a second limit value during the first time period,
• no fault message is produced by diagnostic circuits on the positioner, and
• the operational reading is greater than the initial reading of the difference between the input and output pressure of the actuator for at least a portion of the first time period.