GB2266956A

GB2266956A - Sensor unit for temperature and conductivity

Info

Publication number: GB2266956A
Application number: GB9210259A
Authority: GB
Inventors: Keith Dewhirst; Anthony Graham Ricketts
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 1992-05-13
Filing date: 1992-05-13
Publication date: 1993-11-17
Anticipated expiration: 2012-05-13
Also published as: JPH06137490A; GB2266956B; JP3373891B2; GB9210259D0; DE4313404A1

Abstract

A sensor unit, which may be used for monitoring the operating condition of a condensate trap, comprises an electrically conductive body (4) which accommodates an insulating element (7). The insulating element (7) supports an electrically conductive tube (10), which houses a temperature sensing device (8), and terminal elements (14). The presence or absence of liquid is determined through measurement of the conductivity between the tube (10) and body (4), and the temperature is determined by means of the temperature sensing device (8). Signals are sent via the terminal elements (14), and there is further provided circuitry to enable the alternate monitoring of the temperature sensing device (8) and of the conductivity between the tube (10) and body (4). <IMAGE>

Description

SENSOR UNIT This invention relates to a sensor unit, particularly although

not exclusively for the monitoring of the operating condition of a condensate 5 trap.

Condensate traps are commonly used in steam systems, in which circumstances they are usually referred to as steam traps. Their function is to discharge condensed water from the system without allowing steam to escape. If steam is lost from the system, this represents a waste of energy. Steam traps thus commonly comprise a valve which is responsive to the presence of condensate or steam in the vicinity of the valve, so that the valve opens when condensate is present, and closes when steam is present.

occasionally, however, a steam trap can fail to close or can close only partially, which permits the continued leakage of steam after the valve should have closed. It is not always easy to detect this condition, and various monitoring arrangements have been devised so that an alarm signal is given if the steam trap malfunctions.

One such monitoring arrangement comprises a sensor chamber which is partially filled with water and is partitioned into inlet and outlet sides by a partitioning wall which projects downwardly from the top of the chamber to a point below the normal water surface. Thus the inlet and outlet sides are connected below water level. Also a small aperture links the inlet and outlet sides above the water level.

Any small flow of steam between the inlet and outlet sides will pass through the aperture. However, as steam flow increases, the corresponding pressure differential will depress the water level in the inlet side. A significant drop in water level can be detected by a level sensor, and this represents a malfunction of the steam trap.

one system which operates in this manner is disclosed in British Patent Specification No. 2231407A.

A further problem may arise when the valve becomes waterlogged. In this condition the valve fails to open or there may be an air lock, or the valve is otherwise blocked, and the condensate collects to an unacceptably high level. When a steam trap is waterlogged, the sensor chamber contains condensate and consequently the level sensor is immersed in the condensate and its output indicates satisfactory operation of the steam trap. However, the condensate temperature will fall, and it has previously been proposed that waterlogging can be detected by monitoring condensate temperature as well as its level. In known systems two separate sensors are required.

According to a first aspect of the present invention there is provided a sensor comprising an electrically conductive body defining a recess which accommodates an insulating element, the insulating element supporting an electrically conductive tube which projects from one face of the insulating element, the tube accommodating temperature sensing means, the unit further comprising terminal elements which are fixed with respect to the body and are electrically connected to the tube and the temperature sensing means for receiving signals representing the temperature at the temperature sensing means and the electrical conductivity between the tube and the body.

The presence or absence of liquid at the sensor can be detected by monitoring the conductivity between the tube and the body. These two elements therefore constitute level sensing means. There may be provided four terminal elements, two of which are electrically connected to the temperature sensing means and the other two of which are electrically connected respectively to the body and the tube.

in a preferred embodiment, there are provided two terminal elements which are electrically connected respectively to the tube and one terminal of the temperature sensing means. In this embodiment, the other terminal of the temperature sensing means is electrically connected to the body, the body itself acting as an electrical connector. In this way the body constitutes a common connector for the temperature sensing means and the level sensing means. The terminal elements of the sensor may be embedded in the insulating element such that the insulating element supports both the electrically conductive tube and the terminal elements. The body of the sensor is preferably provided with a threaded portion to enable the unit to be fitted to a fluid chamber by a screw connection, for sensing the condition of the fluid therein.

In order to monitor the sensor, there is preferably provided, in combination with the sensor, monitoring circuitry for alternately monitoring a resistance which represents the temperature at the temperature sensing means, and the electrical conductivity between the tube and the body.

A known system uses level sensors to indicate the operating conditions of associated steam traps. Such systems may then have a central control station which is linked to the individual sensors by means of an extensive two wire network. This then enables simultaneous diagnosis of the condition of each steam trap from the central control station.

However, a sensor unit in accordance with the present invention incorporates two sensing means and would therefore normally be associated with a four wire network or three wires if one terminal is made common. The problem then arises in that new networks would have to be laid in existing systems in order to enable the installation of the sensor units, if they were to be centrally monitored.

By using suitable interfacing circuitry, it is possible to monitor the operation of the two sensing means with a two terminal monitoring device. Such an interface is achieved by controlling the flow of current through the temperature sensing means and the level sensing means in turn, by means of two oppositely disposed diodes which, depending upon the sense of the signal present at the input, provide a response from one or other of said means.

Systems which already have extensive two line electrical networks therefore need not be altered when sensor units in accordance with the present invention are installed.

According to a second aspect of the present invention there is provided monitoring circuitry for monitoring the values of two variable resistances, the circuitry comprising a monitoring device having two terminals and means for reversing the electrical polarity of the terminals, the resistances being connected in two branches of the circuitry which are connected in parallel to the terminals, each branch including a diode connected in series with the resistor of that branch, the diodes being connected so that current flows in one of the resistances when a voltage of one polarity is applied at the terminals, and current flows in the other resistance when voltage of the opposite polarity is applied at the terminals.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 shows a steam trap sensor unit; Figure 2 is a diagrammatic sectional view of the sensor unit of Figure 1; Figure 3 shows a sectional view of a sensor of the sensor unit of Figure 1; Figure 4 shows a sectional view of an alternative 5 embodiment of a sensor of the sensor unit of Figure 1; Figure 5 shows a remote mains powered unit for reading the sensor; Figure 6 shows a hand held unit for reading the sensor; and Figure 7 shows an electric circuit for providing the output of the sensor of Figure 3.

The steam trap sensor unit of Figures 1 and 2 is, in use, fitted upstream of a steam trap (not shown) for discharging collected condensate. The sensor unit comprises an outer casing, the interior of which defines a sensor chamber forming a fluid flow path. This fluid flow path is divided into an inlet portion 1 and an outlet portion 2 by means of a partitioning wall 5. This wall extends downwardly from the top of the chamber and in its upper region is provided with an aperture 7, to equalise the pressures on opposite sides of the wall 5 and to permit limited flow of steam under normal levels of leakage through the valve. A sensor 3 is mounted on the casing and projects into the inlet portion 1. The sensor 3 is responsive both to temperature and to the presence.or absence of condensate at the sensor.

As shown in Figure 3, the sensor comprises a cylindrical elongate metal housing 4 which, at one end, is provided with a screw thread for engagement with a threaded bore in the outer casing of the sensor chamber. The screwthreaded region typically has a diameter of approximately 16mm. The sensor 3 has a sensor element in the form of an elongate metal tube 10 which accommodates a temperature sensing device 8. When the sensor 3 is in place, the tube 10 projects into the inlet portion 1 of the fluid flow path. The elongate tube 10 and the housing 4 are both electrically conductive, but are separated from each other by an insulator 7 which is housed within the metal housing 4. The insulator 7 is in the form of a block which is substantially cylindrical in shape and is adapted to be securely retained by the housing 4. The metal tube 10 projects in axial alignment with the insulator, with one portion of the tube 10 projecting beyond a peripheral edge of the insulator, the remainder of the tube 10 being securely housed within the insulator 7. The relative positions of the metal tube 10 and the housing 4 are such that the same detection circuitry used with a known level sensor may be used for monitoring the resistance between the housing 4 and the tube 10. By housing the temperature sensing device 8 within the tube 10, the overall size and thermal mass are approximately the same as those of the known level sensor. Consequently, the sensor 3 may be used as a direct replacement for the know sensor, without requiring replacement or modification of the casing of the sensor unit as shown in Figures 1 and 2. The diameter of the tube 10 may typically be of the order of 3mm and the tube 10 projects approximately 1Omm beyond the peripheral edge of the insulating element 7. In the embodiment shown in Figure 3 the sensor is further provided with four terminal pins 14 which project into a socket 16 exposed to the outside of the sensor chamber. The terminal pins 14 are embedded in the insulator 7 so that they are fixed with respect to the body and project beyond the other peripheral edge thereof, into the socket 16. Two of these terminal pins make electrical contact with, respectively, the tube 10 and the housing 4, and the other two terminal pins make contact with the two terminals of the temperature sensing device 8.

However, the sensor may be provided with only three pins if one terminal is made common between the temperature sensing device 8 and the tube 10 or housing 4. The sensor may even be provided with only two pins if in addition the metal housing 4 is used as a terminal. This embodiment of the sensor is shown in Figure 4, in which one pin makes contact with the metal tube 10 and the other makes contact with one of the terminals of the temperature sensing device 8. The other terminal of the temperature sensing device 8 makes electrical contact with the housing 4 which itself acts as a connector terminal.

The sensor may be electrically connected to a remote mains powered unit as shown in Figure 5. The remote unit comprises inputs 18 which enable electrical connection to a plurality of sensors 3, each sensor corresponding to the operation of a steam trap. The system may then monitor a plurality of these sensors to detect a waterlogged condition or a condition in which there is steam leakage, as a result of malfunction of one of the steam traps. The remote unit has a means 19 for indicating which trap is being monitored. Furthermore indicators 21 are provided to indicate the results of the test for waterlogging or steam leakage.

These may be in the form of green diodes and red diodes to indicate a satisfactory or unsatisfactory response respectively to the tests carried out.

In existing steam trap monitoring systems the outputs of the level sensors are fed to the unit of Figure 2 via a two line network. In order to utilise existing wiring installations after replacement of the level sensors by sensors in accordance with the present invention, as shown in Figure 3, the four terminal pins 14 of each sensor are connected to the two wires of the existing wiring by the circuitry represented in Figure 7.

The circuit comprises the temperature sensing device 8 which is represented by a variable resistance RT, and the level sensor which is represented by a variable resistance (i.e. variable conductivity) RC, which is the resistance between the tube 10 and the housing 4. There are two oppositely disposed diodes D1, D2 which permit flow of current through the variable resistors in opposite senses. At the input 22 of the circuit, a voltage Vin may be applied by the main control unit in a positive or a negative sense. By applying a d.c. voltage in one direction, a current will be generated to flow around the circuit from the positive terminal of the input 22 to the negative terminal. One diode will therefore conduct and current will pass through one of the two variable resistances. This resistance can then be determined, for example by the measurement of current. If the voltage applied at the input of the circuit is reversed, the current will flow in the opposite direction, and the other diode will conduct. In this case the current will flow through the other variable resistor in the opposite direction. In this case, this second resistance can therefore be determined, again by measurement of current. 25 The circuit of Figure 7, when used in conjunction with a sensor having four terminal pins, is connected with the positions in the electrical circuit of the terminal pins being given by connections 26 to 29. Connections 26 and 27 represent the two terminals which are connected to the temperature sensing device 8 (RT). Connections 28 and 29 then represent the terminals which are connected to the tube 10 and the metal housing 4.

However, if the circuit is used in conjunction with a sensor having only two terminal pins then connection 30 represents the connection that is common to both sensing means and which is in the form of the metal housing 4, and connections 26 and 28 represent the two terminal pins.

In the use of a sensor monitoring system in accordance with the present invention, the two resistances RC and RT will be measured one after the other through the application of a voltage to the input 22 of the circuit in both possible senses. The change over in polarity is performed automatically by the monitoring unit shown in Figure 5. This enables there to be provided a diagnosis relating to a waterlogged condition of the steam trap, or a condition of steam leakage through the trap in the system.

Figure 6 shows a hand held unit for periodic monitoring of the sensor of Figure 3. In this system the hand-held unit is provided with a four-wire lead 20 having an appropriate plug 23 for connection to the four terminal pins 14 of the sensor 3. Alternatively, the hand-held unit may have a two wire lead which is provided with a socket, to enable connection to the circuit of Figure 7 which in turn is connected to the four terminal pins 14 of the sensor 3.

The hand-held unit has two switches 24 to carry out the tests for waterlogging or steam leakage respectively. There are also light emitting diodes 25 to indicate the results of these tests. As with the remote unit of Figure 5, these light emitting diodes are red and green to represent normal operation or a condition where there is a failure in the steam trap.

Using the hand-held unit, each steam trap of a system may be monitored individually.

When installed, the steam trap sensor unit as shown in Figures 1 and 2 will be fitted at a lower region in a pipe run of a steam system, with the inlet portion 1 and outlet portion 2 of the sensor chamber defining the flow path of the pipe run. Condensed water forming within the steam system will drain through the sensor chamber. Under normal operation of the trap, the level of condensate present in the sensor chamber will be above the sensor. The presence of condensate in the vicinity of the steam trap will cause it to open to discharge some of the condensate. When the condensed water has been discharged, the steam trap will close again to prevent the loss of steam. When the steam trap is closed, and is operating properly, the tube 10 will submerged in condensate, and there will be steam present at a slightly higher level in the sensor chamber. The temperature of the water in this sensor chamber will therefore be close to that of the steam. This temperature is detected by the temperature sensing device 8. Furthermore, the condensate will provide a conductive path between the tube 10 and the housing 4. This condition is therefore detected by the level sensing means. A small amount of leakage is acceptable, and owing to the provision of the small aperture 7 in the partitioning wall, this does not significantly affect the level of condensed water in the sensor chamber.

However, if severe leakage occurs, the resulting relatively large pressure drop across the partitioning wall 5 will result in a greater flow rate of steam through the sensor chamber than can pass through the aperture 7. The pressure drop will cause the level of the condensate in the region of the sensor 3 to drop below the level of the tube 10 and steam will bubble beneath the partition wall 5. This condition is represented by an increase in the resistance RC. The temperature sensing device 8 in this condition will be detecting a relatively high temperature, since it is in the steam.

In a waterlogged condition, the valve has failed to open or there is an air lock. In this case, there will be an excessive collection of condensate in the sensor chamber, which will cool with time, and this change in temperature can be detected by the temperature sensing device 8. In this way the waterlogged condition may be detected, even though the level sensing means provides a normal output since the tube 10 is submerged.

Claims

1. A sensor comprising an electrically conductive body defining a recess which accommodates an insulating element, the insulating element supporting an electrically conductive tube which projects from one face of the insulating element, the tube accommodating temperature sensing means, the unit further comprising terminal elements which are fixed with respect to the body and are electrically connected to the tube and the temperature sensing means for receiving signals representing the temperature at the temperature sensing means and the electrical conductivity between the tube and the body.

2. A sensor as claimed in claim 1, in which there are four terminal elements, two of which are electrically connected to the temperature sensing means, the other two of which are electrically connected respectively to the body and the tube.

3. A sensor as claimed in claim 1, in which t---iere are two terminal elements which are electrically connected respectively to the tube and one terminal of the temperature sensing means, the other terminal of the temperature sensing means being electrically connected to the body. 25

4. A sensor as claimed in any preceding claim in which the body is provided with a threaded portion to enable the unit to be fitted to a fluid chamber by a screw connection, for sensing the condition of the fluid therein. 30

5. A sensor as claimed in any preceding claim in which each terminal element is embedded in the insulating element.

6. In combination, a sensor as claimed in any preceding claim and monitoring circuitry for alternately monitoring a resistance which represents the temperature at the temperature sensing means and the electrical conductivity between the tube and the body.

7. A sensor unit substantially as described herein with reference to and as shown in the 5 accompanying drawings.

8. Monitoring circuitry for monitoring the values of two variable resistances, the circuitry comprising a monitoring device having two terminals and means for reversing the electrical polarity of the terminals, the resistances being connected in two branches of the circuitry which are connected in parallel to the terminals, each branch including a diode connected in series with the resistance of that branch, the diodes being connected so that current flows in one of the resistances when a voltage of one polarity is applied at the terminals, and current flows in the other resistance when voltage of the opposite polarity is applied at the terminals.

9. Monitoring circuitry for monitoring the values of two variable resistances substantially as described herein with reference to and as shown in Figure 7 of the accompanying drawings.

10. A sensor arrangement comprising a sensor as claimed in any one of claims 1 to 7 and monitoring circuitry as claimed in claim 8 or 9 in which the variable resistances are constituted by the temperatUre sensing means and the resistance between the body and the tube.