FIELD OF THE INVENTION

The present invention relates to rotary apparatus and in particular concerns a rotary valve having rotor contact monitoring wherein the state of the rotary valve is continuously monitored to detect a rotor/housing contact condition or an open circuit condition indicative of a monitoring circuit fault.

BACKGROUND Rotary valves are generally compact devices which are typically used for transferring powders and granular products under gravity from one part of a storage or processing system to another. Typically the transfer occurs with the assistance of a gas (commonly air) pressure differential. Rotary valves comprise a vaned rotor running at low speed, usually between 10 to 30 rpm, in a housing which has a product feed inlet at the top and outlet at the bottom. The valve can also be required to control the rate of product flow, and/or act as an explosion/flame barrier. In the latter case it is designated as an 'Autonomous Safety Device' under the EC ATEX Directive requiring certification from an independent institution referred to as a Notified Body.

To achieve any or all of the above functions the rotor/housing running clearances are maintained within close limits, which for most applications may be in the range of between 0.13mm to 0.3mm. These clearances are easily lost if any of the following conditions are experienced:- i) Rotor overload from entry of debris material; ii) System over pressure; iii) Excessive product forces; iv) Over temperature; v) Component failure; vi) Bearing failure; vii) Poor maintenance; viii) Poor reassembly after cleaning/maintenance.

Any resultant contact between the rotor and the housing can be very damaging to the valve especially when manufactured in all stainless steel when there is a strong likelihood of galling and shedding of material into the product being handled.

The consequences of such an event can be significant to the user, contaminating the product and rendering the valve void as an Autonomous Safety Device. Product contamination is particularly critical in the pharmaceutical, food and cosmetic processing industries where such valves are often found.

Known systems have been designed to detect rotor/housing contact. Known contact detection devices are connected to the valve motor control so that on contact detection, the drive motor is stopped instantaneously, in time to avoid damage to the valve. In addition, if contact exists after incorrect maintenance the valve is prevented from being restarted.

Known systems are generally prone to nuisance tripping for reasons which the present inventor has recognised as being incorrect. Nuisance tripping has generally been assumed to be due to the sensitivity of the monitoring system to product conductivity, debris within the valve or static electricity discharge. As the rotary valve is a key component in any process system, spurious tripping stops the process run at some cost and disruption to the user. For reasons explained below known monitoring systems are limited to use with low conductivity products and have to be isolated when the rotary valve undergoes cleaning when flushing with common conductive clean-in place (CIP) fluids. This last requirement is particularly important as it normally involves elevated temperatures when the valve is at a higher risk of component contact due to differential thermal expansion of the rotor and housing.

Known systems use what are known as active barriers to provide an intrinsically safe circuit. An active barrier achieves safety through the use of galvanic isolation and energy limitation rather than purely the use of energy limitation in a passive barrier.

Figure 1 shows a typical arrangement of a known rotor contact monitoring system. A resistor is positioned in the loop between the terminals of an active barrier and the rotor and rotor housing of the valve. The resistor is provided for detection of the lead-breakage or circuit fault condition. It provides a fixed load to the barrier to ensure a current continuously flows. If this current is unable to flow due to loss of contact, wire breakage etc. then this is detected by the barrier and a fault trip occurs.

The barrier implements fixed output impedance in a similar fashion to a passive barrier by use of a Zener diode and a fixed output resistor. This is a relatively simple arrangement that effectively permits a constant power output (or energy rate) to the safe side of the system.

As more current is drawn from the output (e.g. due to increased load/lower resistance) the voltage drops proportionally.

This approach is simple, inexpensive and effective toward an intrinsically safe circuit. However, it is not as appropriate with regard to condition monitoring as it uses components, such as the resistor, of inherently poor tolerance which introduce variability and thus limits on the ability of the system to sense the signal accurately. The value of the resistor also has an impact on circuit fault thresholds.

A typical barrier may have an open circuit voltage of say 8V and a short-circuit current of say 8mA, and therefore an output impedance of 1 kΩ.

The operating current of a typical circuit may therefore be as follows:

Open Circuit Voltage/fOutput Impedance + Load Resistor) = 8V / (l kΩ + l l kΩ) = 667μA, where a 1 1 kΩ load resistor is used.

If the Circuit Fault threshold is 0.1 mA; there is 567μA of margin. The normal operating voltage of the circuit is Normal operating current * Load Resistor = 667μA * 1 1 kΩ = 7.33V.

If the switching threshold for detecting a contact fault condition in an arrangement of the tyre shown in Figure 1 is between say 2.I mA and 1.2mA, then the upper and lower voltage thresholds are as shown in the drawing of Figure 2, which typically shows the contact threshold tolerance range for a known type of rotor/housing contact system.

Thus, any debris, product, clean-in-place solution, or other effect that can influence at the Kilo-Ohm level will trip the system. In practice this is highly likely, whether it be from material conductance or charge build up on plastic products for example. It will also be sensitive to any electrical noise; Moreover, if the barrier has a short integration time, say in the region of 10-20ms. The system is likely to be prone to tripping due to transients.

It is possible to fine tune the known system shown in Figure 1, but only by a few tens of percent and at the expense of circuit fault margin. For example, if the 1 1 kΩ load resistor is increased to 47kΩ, the final contact resistances reduce to 6.44kΩ and 2.99kΩ respectively. Circuit fault current then drops to 167μA but this is still within limit.

There is a requirement therefore for a rotary apparatus having a fault monitoring system which is less prone to nuisance tripping than hitherto known arrangements.

SUMMARY

According to an aspect of the present invention, there is provided rotary apparatus comprising a housing, a rotor mounted in the housing and at least one intrinsically safe monitoring circuit for detecting rotor/housing contact or a monitoring circuit fault condition, said monitoring circuit including intrinsically safe amplifier barrier means having a maximum voltage and substantially fixed constant current output, said barrier being electrically connected to both the rotor, by rotary contact means, and the housing, said circuit including first resistor means arranged in parallel with said rotary contact means and second resistor means electrically connected in series between said housing and said rotary contact means and said first resistor means, wherein the impedance of said first resistor is such that in a normal operative non-contact state current substantially flows through said rotary contact means to the said second resistor, in a contact fault state flows through said rotary contact means said rotor and said housing and in a circuit fault condition through said first and second resistors with said rotary contact being shorted such that the state of the said apparatus is determined by the resistance of the circuit.

The method of energy limitation in the apparatus of the present invention is provided by the adaption of an alternative barrier, which uses an active output that delivers a fixed constant current to the valve. An upper limit exists on the voltage that can be output to generate the constant current in order to maintain intrinsic safety levels. This approach provides for the current to be very tightly controlled and thus it is possible to achieve highly accurate low level sensing. It also makes the measurement of resistance directly proportional to voltage, which provides for accurate sensing.

In preferred embodiments the barrier comprises a programmable barrier, preferably comprising an analogue to digital converter (ADC). This readily enables the barrier to measure the electrical characteristics of the monitoring circuit preferably by means of an ADC such that the programmable barrier can use the measured parameters to determine the state of the system by comparison with predetermined data. A particular advantage of this arrangement is that by using a high resolution, preferably self calibrating, ADC the barrier can reliably detect very strong voltages which is necessary when there is a requirement to detect very low resistances with a current that meets the intrinsic safety requirement of the apparatus.

The use of an ADC and programmable logic further provides for introducing digital signal filtering, by means of an integration period. This feature essentially averages out the input signal over a fixed time period, thus rejecting insignificant transient events or noise spikes. The integration period may be in the region of 250-350ms, preferably in the region of 300ms, which is an appropriate compromise between rejection and risk of valve damage.

In preferred embodiments the output impedance of the programmable barrier is adjustable. In this way it is possible to fine tune a system to its particular installation in the event that nuisance tripping occurs when using the factory default settings. In preferred embodiments the barrier includes relay means for isolating the power supply to the motor drivingly connected to the rotor of the rotary apparatus for operationally driving the rotor within the housing. In this way it is possible to stop the rotor instantaneously in response to a rotor/housing contact fault condition being detected and/or in response to a circuit fault condition being detected.

In preferred embodiments the resistance/impedance of the first resistor means is great than then the resistance/impedance of the second resistor means. The resistance/impedance of the second resistance means is determined such that a predetermined voltage is developed which the barrier recognises as a "no fault" condition so that the relay or relays are held in a functioning state. The resistance/impedance of the first resistance means is preferably much higher than that of the second resistance means so that in use the fixed constant current of the barrier is shunted away and flows through the rotor contact means and the second resistor means.

In preferred embodiments the rotary contact means comprises at least one slip ring for electrically connecting the rotor and the barrier.

In preferred embodiments of the invention the rotor and housing comprise respective parts of a rotary valve, preferably a rotary valve having a vaned rotor of the type used for transferring powders and granular products in a processing system.

In preferred embodiments the operational rotational speed of the rotary valve is in the range 1 to 50rpm, preferably 10 to 30rpm, and the operational clearance between the respective vanes of the rotor and the housing is in the range 0.1 to 0.4mm, preferably 0.13 to 0.3mm.

In preferred embodiments the apparatus is capable of detecting a rotor/housing contact condition when the measured resistance is less than lOOOOhms, preferably less that 500Ohms, most preferably less than lOOOhms. In this way any debris, product, clean in place solution or other effect that can influence at the KiloOhm level will not trip the apparatus of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

Figure 3 is a schematic representation of a rotary apparatus and associated monitoring circuit according to the embodiment of the present invention; and Figure 4 is α graphical representation of circuit fault and contact fault noise tolerances.

DESCRIPTION OF THE EMBODIMENTS

Referring to Figure 1 , there is shown an intrinsically safe environment 10 having a safe side 12 and a hazardous side 14 in which rotary apparatus according to an embodiment of the present invention is located. The apparatus is divided by the boundary 16 between the safe side 12 and hazardous side 14.

A rotary device 18, which in this embodiment is a rotary valve, comprises a housing 20 and a vaned rotor 22 rotatably mounted within the housing. A slip ring assembly 24 is provided on the rotor shaft 26 for electrically connecting the rotor to a programmable intrinsically safe amplifier barrier 28 on the safe side via a valve junction box 30 on the hazardous side of the intrinsically safe environment. The housing 20 is similarly electrically connected to the barrier

28 via the valve junction box 30. The rotor 22 of the rotary device 18 is driven by an electric motor (not shown) and the barrier 28 is provided with first and second relays 32 and 34 which are connected to an appropriate control circuit to ensure that the motor driving the rotor is isolated in the event of a fault as will be described in more detail below.

The barrier input is provided by an intrinsically safe monitoring circuit, generally indicated at 40, between input terminals 42 and 44 on the programmable barrier such that the monitoring circuit constitutes a field device on the hazardous side 14. The monitoring circuit provides an analogue input to the programmable barrier which includes an analogue to digital converter for converting the analogue input to a digital signal for subsequent digital signal processing. The input terminal 42 is electrically connected to the junction box 30 where the circuit divides with one branch being connected to the low impedance slip ring assembly 24 with the other branch being connected to a high impedance shunt resistor 46 arranged in parallel with the slip ring assembly. The two branches rejoin within the valve junction box 30 where the circuit continues through a second resistor 48 which has a much lower impedance than the first resistor 46. The other side of the resistor 48 is electrically connected from the junction box 30 to the housing 20 which is electrically connected to the input terminal 44 of the barrier via the junction box 30.

Power supply to the barrier 28 is provided by an optional power supply unit 50, or other means (not shown), via a power supply terminal on the barrier.

The barrier feeds a safe, fixed constant current, into the rotary valve 18 and detection circuitry 40. The barrier uses this and the returned signal to determine the state of the system. In the event that the barrier senses a higher current above tolerance being circulated it trips showing a contact fault, suggesting that rotor/body contact has occurred lowering the circuit resistance. Where it sees a drop in current below tolerance it trips showing a circuit failure which has given rise to a higher resistance e.g. connection or component failure. The latter provides for fail-safe operation, in the sense that if a circuit fault condition is detected the valve is stopped.

The barrier applies a constant current of XmA which is shunted away from the high impedance Rl resistor 46 and flows virtually entirely though the low impedance slip ring assembly 24 and R2 resistor 48. This current develops a voltage related to the value of the settings. The barrier sees this as no fault condition and relays 32, 34 are held in a functioning state.

Under a contact fault condition, the constant current continues to take the low impedance path through the slip ring, but now due to contact, flows from rotor to housing thus shorting out the R2 resistor 48. If the resistance at the point of contact is below a sensing value, the voltage developed is now essentially zero, 0Ω x XmA = OmV. The barrier detects this and sets the contact fault relay 32 to a failed state.

To provide for coping with highly conductive products being handled, the embodiment of Figure 3 provides for setting the contact resistance between the rotor and housing components to be able to accurately set a low level below which the barrier will sense a contact fault. Setting values may be as low as 10Ω.

The circuit fault is an indication as to whether there are any dormant failures in the circuit that leaves the essential contact fault protection potentially disabled. In the event of a circuit failure, such as broken wiring or loss of contact in the slip-ring, then in both instances the slip ring no longer provides the low impedance path that shorts out the Rl resistor 46.

Current now flows through this resistor. The resultant voltage is higher than the set point which is seen by the Barrier and sets the circuit fault relay 34.

The embodiment of Figure 3 has the ability to deal with most normal operational levels by having the ability to accurately set trip levels 54, 56 apart and position the normal sensing current level 52 anywhere between the two to provide a bias in favour of the best condition. This is best understood with reference to Figure 4.

As shown in Figure 4 the standard default setting 52 is biased in favour of contact fault tolerance 56 as this is where most electrical noise is seen.