A METHOD OF MONITORING THE FUNCTION OF A CONTROLLED VALVE

The prior art

The invention relates to a method of monitoring the function of a controlled valve, wherein an actuator brings the valve in a position in accordance with the command which the actuator has received from a control system.

Controlled valves are generally known and are used in many connections, e.g. for processing systems or for security purposes. It is evidently important to the course of a process or to the security in a monitoring system that the valve instantaneously reacts as intended to a command from the control unit.

The art includes many arrangements for displaying the function of valves, where the position of the valve body is detected, or where it is measured whether there is a flow through the valve. The position of the valve body may be detected visually or by means of sensors, which may be optical, capacitive or inductive. The pressure drop across the valve may also be measured in order to determine the degree of closure of the valve.

An example of the monitoring of a controlled valve is disclosed in US 5 748 769. The purpose of this known monitoring is to observe errors in the regu- lation of the valve, so that a beginning malfunction is recorded by an underlying control system with a view to correcting the error.

It is complicated - and thereby expensive - valves with a control function, which are typically used in the processing industry, where malfunction may result in serious production errors and thus deterioration or destruction of the production.

Malfunction is referred to a subsystem, which serves to diagnose and thereby facilitate e.g. the replacement of defective parts of the valve.

As the valves are relatively expensive, replacement of defective parts will be an advantage. To this should be added that the system requires considerable computational power in order to be able to detect from small to great malfunctions in the control valve. As a minimum, the system must comprise a server for the positional adjustment. In other words, what is involved is a complicated and thereby expensive system with sophisticated components to ensure a completely monitored process control system.

The object of the invention

The object of the invention is to provide a method of monitoring a controlled valve, where it is absolutely certain that the valve opens and closes correctly, and that the valve body is in the position which is in accordance with the command which the actuator of the valve has received from the control system.

This is achieved according to the invention by a method which is characterized in that the function of the actuator is scanned, and the closed or open position of the valve body is scanned, and that when the function of the actuator and the closed or open position of the valve body are in agreement, a signal is applied, showing that the valve is in order, while when the function of the actuator and the closed or open position of the valve body are not in agreement, a signal is applied, showing malfunction.

Hereby, a surprisingly simple system is provided, which allows the capability of the controlled valve to be monitored. The scannings lead to four pos- sible combinations, viz.:

a) the actuator "closed", the valve closed, b) the actuator "open", the valve open, c) the actuator "closed", the valve open, and d) the actuator "open", the valve closed.

In the first two cases a) and b) an OK signal may be applied for the capability of the valve. In case c) the valve hangs in an open position, and in case d) the valve hangs in a closed position. In the two last-mentioned cases, an error signal is applied.

The method may be performed with simple means, the constituent sensors are simple, as they are purely qualitative, and they are easy to arrange in the system. This means that the preliminary and operational costs are greatly reduced.

If the valve function is detected by a scanning of the movement of the medium, error display may occur if the medium pressure drops or is not present. To avoid this situation, it is possible, as stated in claim 2, to scan the medium pressure on the inlet side of the valve or the differential pressure between the inlet side and the outlet side of the valve. By allowing this scanning to be incorporated in the evaluation it is possible to distinguish between a blocked valve and missing or too low medium pressure.

Claims 3 and 4 define expedient ways of scanning the function/position of the actuator by an electrical and a pneumatic or hydraulic actuator, respectively.

Claims 5 - 7 define expedient scannings of the position of the valve body.

The signal applied in case of error may be a sound signal, flashing light, activation of an electronic relay or a radio or network connection.

The drawing

The invention will be explained more fully below with reference to the drawing which schematically illustrates the invention.

Description of the exemplary embodiment

The drawing schematically shows a length of piping 10 for a medium 1 having an inlet 2 and an outlet 3. A controlled on/off valve 4 may interrupt the flow of medium through the piping 10. The valve 4 is activated by an actuator 5, which may be electromagnetic, hydraulic or pneumatic, and which receives an activation impact 6 via a control unit (not shown), which may be electrical current, liquid or air pressure according to the type of the actuator.

The state of the valve body 4a of the valve 4 (closed or open) is detected by a flow detector 7 in the example shown, said valve 4 being judged to be open if flow is observed, or to be closed if flow is not observed. Two pressure sensors 8, before and after the valve 4, measure the pressure drop across the valve.

The messages from the sensors and the information on the presence of an activation impact of the valve are gathered in a monitoring unit 9, which is adapted to perform the following observations:

a) There is an activation impact 6, and the valve 4 is closed, b) there is no activation impact, and the valve 4 is open, c) there is an activation impact 6, and the valve 4 is open, or d) there is no activation impact, and the valve 4 is closed.

In the first two cases a) and b) the function of the valve 4 is in order. In the

last two cases c) and d) an error or alarm signal is applied, as the valve does not operate correctly.

In case d) the observation may be caused by the circumstance that the valve 4 is blocked in a closed state. However, it may also be due to missing pressure in the medium, as the state of the valve is detected 7 by the flow of medium in the example shown. To distinguish between the two possibilities, the value of the differential pressure is used, which is scanned by the pressure sensors 8. Only if a differential pressure is observed, is the valve judged to be blocked.

The position of the valve 4 may be observed in other known ways, e.g. by electrical, optical or mechanical means, just as the state or the function of the actuator 5 may be detected directly and without the use of the activation means, e.g. in that the movement of the medium may be detected by a sensor arm on the inlet side of the valve 4 which is arranged in the medium.

The invention provides a simple and safe measure for monitoring the function of a controlled on/off valve for use by simple qualitative sensors, which may easily be arranged in the system.