EP2699806B1 - Electropneumatic position controller - Google Patents

Description

The invention relates to an electropneumatic positioner with a supply air connection for connecting a compressed air supply line, with at least one exhaust connection for connecting a pneumatic drive of a control device and with a valve disposed in the positioner controllable valve group according to the preamble of claim 1.

For the control of process valves pneumatic actuators are often used as actuators in which an on an actuator, such as a connected to a closing body lifting rod, acting membrane with the aid of a gaseous medium, for which hereinafter generally the term "air" is used with variable pressure is applied to generate movements. The pressurization can take place on one side, wherein the membrane is loaded on the other side with a spring, or on two sides, with different pressures on both sides of the membrane cause the deflection. The control of the pressures or by an electropneumatic positioner, in which at least one controllable valve pressurized supply air distributed depending on the control via one or two exhaust ports on the one or both sides of the membrane in the actuator.

From the DE 10 2004 048 689 B3 is a two-stage pilot valve for use in an electropneumatic positioner known. In a first stage, four flow resistances, of which at least one flow resistance is variable, are arranged in a bridge circuit. Two opposite circuit points of the bridge circuit are connected to supply air or exhaust air to the environment. Between the two other circuit points is a variable in size and sign pressure difference ready, as a pneumatic output of the so in the first stage realized electropneumatic converter for controlling the downstream second stage is used. In the second stage is a valve group, which has an actuated by the pressure difference control piston. By this a pneumatic valve in the valve group is actuated, either depending on the control via the supply air supplied compressed air to the exhaust port to the actuator or derived via the same exhaust port air supplied to the outside. For example, the pneumatic drive of a control valve is connected to the exhaust air connection. In the event of failure of the pneumatic energy (compressed air), it is often required that the drive controlled by the positioner automatically brings the process valve into a safety position, ie a safe position in the environment of the respective process, usually "open" or "close".

Increasingly, the occurrence of other failures in electropneumatic positioners in the context of functional safety, which is classified in Safety Requirements Level or "Safety Integrity Level" (SIL), requires the most secure attainment possible of the given safety position. It can namely in an error assessment between dangerous errors in which the safety position can not be approached by the error or on a request, and non-dangerous errors in which the device is no longer functional, but is placed in the predetermined safety position, be differentiated. Material wear in valves of the valve group, in particular in the seals or membranes commonly used on valves, can often lead to dangerous errors in electropneumatic positioner.

From the DE 42 39 431 A1 are a positioner and an actuator with such a positioner known. For monitoring purposes, if the actuator has a seal, in particular a sliding seal, a leakage chamber is provided on the side of the seal which is remote from the pressure, which is connected to a pressure switch which, when a set pressure is exceeded, sends a signal to the actuator Positioner generated. As a seal, in particular, the usual in control valves fabric bushing comes into question. With the help of the pressure switch can be monitored whether a leakage above the allowable pressure is formed in the leakage chamber due to leakage. The signaling can be used to generate an alarm or perform a safety routine.

From the US 2003/0208305 A1 For example, a method for detecting a leak by using parameters measured in the control loop of a pneumatic valve is known.

The DE 196 36 418 C2 discloses the use of a gas tight housing for a positioner.

The invention has for its object to provide an electropneumatic positioner, which is characterized by increased functional safety.

To solve this problem, the new electropneumatic positioner of the type mentioned in the characterizing part of claim 1 features. In the dependent claims advantageous developments of the invention are described.

The invention has the advantage that a much larger number of possibly occurring error cases can now be classified as harmless errors. The ratio between dangerous errors and non-dangerous errors is therefore much cheaper.

The invention is based on the recognition that material wear in seals or membranes, which are used in valves of a valve group, usually first expressed as compressed air leakage at the valves and leads to their pressure in a gas-tight housing to an increase in pressure within the housing. If there is a creeping or a sudden leak, for example if one of them breaks Membrane, within the housing, so the resulting pressure increase is detected by means of a pressure switch, which cooperates with the valves of the valve group and this controls so that the valve group is placed in a predetermined safety position.

Since valve groups in electro-pneumatic positioners are usually housed anyway in a separate housing, the invention is associated with only a small additional effort. The existing housing must namely be supplemented only by a suitable housing seal.

In principle, the pressure monitor and the control of the valve group, in order to put them in safety position when the internal pressure of the housing is too high, can be realized by electronic or electromechanical means, for example a pressure sensor with a downstream electromagnet. Preferably, however, these are operated exclusively pneumatically, so that advantageously no electrical auxiliary energy is required for their operation, which is available only to a limited extent in particular for field instruments of process instrumentation, which are operated with a 4 to 20 mA interface. In addition, in such a realization, the safety position is approached completely independently of the operability of the electronics of the electropneumatic positioner and thus further increases the functional safety.

Since a small leakage is allowed to a certain limit without operational impairments, a shutter is provided for venting the housing with a predetermined maximum air flow.

The housing interior is then connected via the aperture as a flow resistance with the environment of the electropneumatic positioner. In one embodiment, the aperture may be sized so that an air flow of one liter per minute leads to a pressure increase of 0.8 bar in the housing interior, in which the pressure switch responds and the valve group in the safety position.

In one embodiment of an electropneumatic positioner in which the valve group is preceded by an electropneumatic converter, preferably the exhaust side of the orifice is connected to the exhaust outlet of the converter. This has the advantage that in addition to the monitoring of the valve group on compliance with the permissible leakage and the exhaust air outlet the electro-pneumatic converter is checked for free continuity, since clogging of the output, for example due to icing or contamination of the outlet opening, even with low leakage leads to an increase in the internal pressure of the housing and thus the valve group is placed in the safety position even in this disorder. Thus, a closure of the output of the electropneumatic converter can be classified as a non-dangerous error that could otherwise lead to the inefficiency of the control of the valve group and thus to a dangerous error of the positioner.

According to an advantageous embodiment of the invention, the position of the valve group by a controller electronics, which is usually present in electropneumatic positioner, detectable, so that it outputs a signal indicating an error condition, for example in the form of an alarm message via a fieldbus when the valve group itself is in the safety position, but this state does not correspond to the control of the valve group by the controller electronics and any of these downstream electro-pneumatic converter. In an automation device or a control station as a receiver of the alarm, this can be further processed and output to an operator, so that it can be initiated by this maintenance or other appropriate actions to handle the reported error condition.

Reference to the drawing, in which an embodiment of the invention is shown, the invention and refinements and advantages are explained in more detail below.

In the figure, only the contributing to understanding of the invention parts of an electropneumatic positioner and a connected thereto pneumatic drive 1 for a control device 2, here a process valve shown. The pneumatic positioner has a pilot valve 3 and a controller electronics 4. An air supply connection 5 serves to connect a compressed air supply line 6, via which, for example, compressed air at 6 bar pressure to the pilot valve 3 is supplied. The upper chamber 7 of the drive 1 is connected to an exhaust port 8 of the pilot valve. Depending on the control of the chamber 7 is supplied either by means of a supply air valve 9 compressed air from the compressed air supply line 6 or derived from the pressure chamber 7 by means of an exhaust valve 10 via a valve outlet 11 into the environment. By a suitable locking mechanism, which is not shown for clarity in the figure, it is ensured that the two valves 9 and 10 are never opened at the same time, otherwise compressed air could escape unused into the environment. To control the position of the actuating device 2 via the pressure set in the pressure chamber 7 of the drive 1, the controller electronics 4, an electropneumatic converter 12, which is designed in a known manner as a bridge circuit of flow resistances, and a booster drive 13 for actuating the supply air valve. 9 and the exhaust valve 10 in accordance with the pressure difference set by means of the electropneumatic converter 12.

By a pressure regulator 19, the supply pressure of, for example, 6 bar to a regulated pressure of, for example, 1.4 bar, which is applied to the bridge circuit of the electropneumatic converter 12, reduced.

A valve group of the electropneumatic positioner, which consists in the embodiment shown from the supply air valve 9, the exhaust valve 10 and the booster drive 13 is disposed in a gas-tight housing 14 within the positioner. By a pressure switch 15, the pressure prevailing within the housing 14 is monitored for maintaining a predetermined threshold value of, for example, 0.8 bar. All free internal spaces within the gas-tight housing 14, the pressure chambers of the booster drive 13 opposite areas, the area around the valves 9 and 10, located in the valve group spring chambers and the area around the locking device of the valves 9 and 10 are pneumatically interconnected. Occurs in the valve group a leak, such as a crack in a membrane of the booster drive 13, so escapes compressed air through the leak in the gas-tight interior of the housing 14 and leads to a pressure increase, which is detected by the pressure switch 15. As long as the pressure does not exceed a threshold value of 0.8 bar, the pressure monitor 15 does not intervene in the regulation of the position of the actuating device 2. However, as soon as the pressure rises above the limit value, the pressure monitor 15 opens the exhaust air valve 10. The supply air valve 9 is closed at the same time by the locking device, not shown in the figure, so that the pressure in the pressure chamber 7 of the drive 1 drops rapidly and the actuating device 2 in the safety position, which corresponds to an "opening" of the process valve in the embodiment shown, is offset. The required switching state of the valve group is also referred to as a safety position.

Notwithstanding the described embodiment, it is of course possible in an alternative embodiment, the process valve to be arranged so that it "closes" at pressure drop in the chamber 7. In a further modification of the exemplary embodiment, additional components, for example the electropneumatic converter 12, the pressure regulator 19 and / or the pressure monitor 15, can additionally be integrated into the housing 14.

Small leaks on the valve group in the housing 14 do not necessarily lead to functional impairment. In order to avoid that the safety position of the actuator 2 is taken even at low leakage, a diaphragm 16 is provided, which can also be referred to as throttle or flow resistance and is connected between the interior of the housing 14 and an exhaust outlet 17 of the electropneumatic converter 12. The orifice 16 is for example adjusted so that a leakage of one liter per minute leads to a pressure increase of 0.8 bar in the housing 14. The pressure switch 15 therefore only responds when the air flow through the throttle 16 exceeds one liter per minute or if the exhaust port 17 of the converter 12 is clogged due to, for example, icing or soiling.

To carry out the control, the current position of the valves 9 and 10 is taken into account in the controller electronics 4. In order to detect a response of the pressure switch 15 and thereby caused starting the safety position by the valve group with the valves 9 and 10, is additionally checked in the controller electronics 4, whether the position of the supply air valve 9 and the exhaust valve 10 with the by the control electronics 4 via the electro-pneumatic transducer 12 corresponds to the predetermined position. A mismatch while simultaneously taking the safety position by the valve group evaluates the controller electronics 4 as a fault condition and generates a corresponding alarm message, for example, via a fieldbus 18, with which the electropneumatic positioner is integrated into an automation network of an automation system, for example, to a higher-level control station so that appropriate action can be taken to remedy it in response to the error that has occurred.

Claims (4)

1. Electropneumatic position regulator
   having a feed air port (5) for the connection of a compressed-air supply line (6),
   having at least one exit air port (8) for the connection of a pneumatic drive (1) of a servo unit (2), and
   having a controllable valve group (9, 10, 13) which is arranged in the position regulator and which has at least one valve (9, 10), by means of which valve group, during normal operation and depending on the actuation of the valve group, either compressed air supplied via the compressed-air supply line can be conducted to the exit air port (8) or air supplied via the exit air port (8) can be discharged to the outside, characterized
   in that the controllable valve group (9, 10, 13) is arranged in a gas-tight housing (14) within the position regulator,
   in that means (15) are provided for monitoring the pressure prevailing in the housing and for actuating the valve group (9, 10, 13) such that the valve group (9, 10, 13) is placed into a predefined safe position if the pressure in the housing exceeds a predetermined threshold value, and
   in that an aperture (16) is provided for ventilating the housing (14) with a predetermined maximum throughflow.
2. Electropneumatic position regulator according to Claim 1, characterized in that the means (15) for monitoring and actuation are realized in pneumatic form and are designed to pneumatically actuate the valve group (9, 10, 13).
3. Electropneumatic position regulator according to Claim 2, characterized in that an electropneumatic transducer (12) is connected upstream of the valve group (9, 10, 13), and in that the exit air side of the aperture (16) is connected to the exit air outlet (17) of the transducer (12).
4. Electropneumatic position regulator according to one of the preceding claims, characterized in that the position of the valve group (9, 10, 13) can be detected by regulator electronics (4), and in that the regulator electronics (4) are designed to output a signal (18) for displaying a fault state if the valve group (9, 10, 13) is in the safe position and this does not correspond to the actuation of the valve group (9, 10, 13) by the regulator electronics (4).

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