Method for diagnosing the state of wear of a valve arrangement for controlling the flow of a process medium

1. A method for diagnosing an operating state of a valve arrangement for controlling a flow of a process medium through a pipeline, wherein the valve arrangement includes a valve element accommodated in a valve housing and configured to switch the flow of the process medium in accordance with a positioning control system via a pneumatic actuator having a control piston, to at least one side of which control pressure is applied in a drive chamber, and an electronic evaluation unit connected to the positioning control system and configured to monitor the operating state of the valve arrangement, wherein the method comprises:

measuring in a pressure sensor, the pressure in at least one of the drive chamber and the pneumatic actuator operatively connected to the drive chamber;

analyzing the measured pressure in the evaluation unit with respect to a pressure p and a pressure change dp(t) over time to determine at least one of the flow into/from the drive chamber and a ratio of a volume of the drive chamber volume to an effective opening cross section of the valve arrangement as a measure of the operating state of the valve arrangement; and

wherein, for a constant drive chamber volume, the flow is determined, starting from the pressure change dp(t), on the basis of a formal description of the flow.

2. The method as claimed in claim 1 , wherein the measurement of the pressure is carried out while at least one of ventilating and venting the drive chamber.

3. The method as claimed in claim 1 , wherein the formal description of the flow is determined by the following formula:

dm/dt=A *psi( p 1 , p 2)* p 1*sqrt(2 /R/T ).

4. The method as claimed in claim 3 , wherein dm/dt represents the flow of gas, p 1 represents the pressure on a high-pressure side, p 2 represents the pressure on a low-pressure side, R represents a specific gas constant, T represents a temperature of the gas in the valve arrangement, and psi represents a flow function.

5. The method as claimed in claim 1 , wherein, for the constant drive chamber volume, the ratio of the drive chamber volume to the effective opening cross section of a pilot valve of the valve arrangement is determined, starting from the pressure change dp(t), on the basis of a formal description of the ratio of the volume of the drive chamber volume to the effective opening cross section of the valve arrangement.

6. The method as claimed in claim 5 , wherein the formal description of the ratio of the drive chamber volume to the effective opening cross section of the pilot valve is determined by the following formula:

V/A=p 1/( dp/dt )*psi( p 1 , p 2)*gamma*sqrt(2* R*T ).

7. The method as claimed in claim 6 , wherein V represents the volume of the drive chamber volume, A represents the effective opening cross section of the valve arrangement, p 1 represents the pressure on a high-pressure side, p 2 represents the pressure on a low-pressure side, gamma represents an adiabatic exponent, R represents a specific gas constant, T represents a temperature of air compressed in the valve arrangement, dp(t) is a derivative of the pressure with respect to the time t, and psi represents a flow function.

8. The method as claimed in claim 1 , wherein a current position of the valve element is additionally determined via a position sensor, and a state of the position sensor is checked using a signal representative of the determined current position and the ratio V/A for a constant control pressure volume.

9. The method as claimed in claim 1 , wherein the pressure p is an absolute pressure.

10. A valve arrangement for controlling a flow of process medium through a pipeline, comprising:

a valve housing configured to accommodate a valve element;

a position control system configured to control a control piston to control movement of the valve element;

a drive chamber configured to apply a pressure to at least one side of the control piston;

a pneumatic actuator configured to move the valve element accommodated in the valve housing and switch the flow of a process medium in accordance with the positioning control system;

an electronic evaluation unit connected to said positioning control system and configured to monitor an operating state of the valve arrangement;

a pressure sensor configured to measure the pressure in the drive chamber,

wherein the electronic evaluation unit is configured to analyze the pressure measured by the pressure sensor with respect to a pressure p and a pressure change dp(t) over time and to determine at least one of the flow into/from the drive chamber and a ratio of the drive chamber volume to the effective opening cross section of the valve arrangement as a measure of the operating state of the valve arrangement; and

wherein the pressure p is an absolute pressure.

11. The valve arrangement as claimed in claim 10 , wherein the pneumatic actuator comprises at least one digital directional control valve configured to connect the control pressure side to at least one of a feed pressure connection and venting connection in accordance with a manipulated variable specified by the positioning control system.

12. The valve arrangement as claimed in claim 10 , wherein the pneumatic actuator comprises at least one analog proportional valve configured to connect the control pressure side to at least one of a feed pressure connection and a venting connection in accordance with a manipulated variable specified by the positioning control system.

13. The valve arrangement as claimed in claim 10 , wherein the position control system comprises a position sensor for detecting the position of the valve element.