GB2091911A - Automatic control of liquid supply - Google Patents

Abstract

A liquid paint circulating system for use in a paint spraying installation includes a pump (13) driving liquid paint (12) from a storage tank (11) around a piping loop (16) having take-off points (18) and back to the tank (11). The pump (13) (e.g. a turbine pump) is driven by a variable speed drive means (19), whose speed is controlled in accordance with the demand for paint from the system as measured, for example, by the pressure of the paint detected by means (23), whereby paint is delivered to spray guns or the like upon demand. <IMAGE>

Description

SPECIFICATION Paint circulating system This invention relates to a liquid paint circulating system for use in a paint spraying installation, the paint circulating system being of the kind wherein a pump drives liquid paint from a storage tank around a piping loop and back to the tank, the loop having take-off points where it passes through each user station, whereby paint is delivered upon demand from the loop to a spray gun or the like at the user station.

In a paint circulating system of the kind specified it has previously been proposed to utilize various alternative types of pump. For example, electrically and hydraulically driven piston and gear pumps have been used. However, a preferred pump type is a constant speed centrifugal pump driven by an a.c.

induction motor, this type of pump often being known as a turbine pump.

Turbine pumps are preferable to gear and piston pumps in a paint circulating system in several respects including efficiency and reliability. However, a paint circulating system using a turbine pump has been found to suffer from problems in low or zero demand situations as a result of the output of the pump exceeding the flow capacity of the piping loop. In such a situation the pressure in the loop rises and paint, not flowing sufficiently quickly through the pump, is subject within the pump to a shearing action between the rotating impeller or impellers and the fixed guide structure of the pump. Such shearing gives rise to heating and degradation of the paint as a result of the mechanical action of shearing the paint.The pigment content of the paint is known to suffer from the shearing action, and for example the flake-like inclusions of metallic paint frequently used in the motor industry suffer severely from any shearing action in the pump. It is an object of the present invention to provide a paint circulating system wherein the aforementioned problems are minimized.

According to the invention, in a paint circulating system of the kind specified, the pump is a turbine pump or the like, and is driven by a variable speed drive means, the system further including means controlling the speed of said drive means, and thus controlling the output of the pump, in accordance with the demand for paint from the system.

Preferably the system includes pressure sensing means providing an output signal when the paint pressure exceeds a predetermined value as a result of the demand for paint decreasing, and said output signal being used to initiate a decrease in the speed of said drive means.

Conveniently the pressure sensing means senses the paint pressure adjacent the pump outlet.

Desirably said variable speed drive means is an a.c. induction motor with an associated speed control unit, said unit being operated to alter the motor speed in accordance with the demand for paint from the system.

Conveniently said speed control unit varies the motor speed by varying the frequency of the a.c. supply to the motor.

The accompanying sketch is a diagrammatic representation of a paint circulation system in accordance with one example of the present invention.

Referring to the accompanying sketch it can be seen that the paint circulation system includes a paint storage tank 11 containing liquid paint 1 2, a turbine-type pump 1 3 which draws paint 1 2 from the tank 11 by way of a feed pipe 14 and which expels paint from an outlet 1 5 into a piping loop 1 6. The loop 1 6 discharges back into the tank 11, there being a restrictor 1 7 at the discharge end of the loop.

Around the loop 1 6 are a series of take-off tappings 1 8 each of which represents a user station, and each of which leads to a spray gun or the like at the user station. The pump 1 3 is driven by an a.c. induction motor 1 9 having an associated speed control unit 21 which, as will be described later, varies the frequency of the standard 50 cycle supply 22 to alter the speed of operation of the motor 19. Included within the loop 16, and for convenience positioned adjacent the outlet 1 5 of the pump, is a pressure sensor 23 electrically connected to the control unit 21.

The combination of pump 1 3 and motor 19 is chosen such that at the normal operating speed of the motor 1 9 the output of the pump 1 5 is sufficient to maintain an appropriate pressure in the loop 16, together with an appropriate flow rate of paint around the loop 1 6 assuming that each of the user stations is operative. Thus the pump 1 3 must be able to provide sufficient pressure and flow to feed, simultaneously, each of the spray guns connected to the loop. It will be recognised therefore that the normal operating speed of the motor 1 9 is also the intended maximum speed of the motor and the pump combination.

It will be recognised that as the demand for paint from the loop 1 6 progressively decreases, as a result of an increasing number of the user stations ceasing to operate, then the volume of paint returned rom the loop to the tank 11 will increase. The turbine pump is a centrifugal pump in which one or more rotary impellers driven by the motor 1 9 and co-operating with stationary guides, effect a centrifugal pumping action on the paint. As the demand for paint from the system progressively decreases a point will be reached in which the return of the loop 1 6 including the restrictor 1 7 can no longer cope with the increasing flow of paint back to the tank 11.

Thus the paint within the pump 1 3 will not be able to leave the pump sufficiently quickly, and there will be a tendency for the rotating impeller and fixed guide arrangement of the pump to perform a shearing action rather than a pumping action on the paint within the pump. However, prior to such stalling there will have been a pressure rise at the outlet of the pump, and this pressure rise, beyond a predetermined chosen value, will have been sensed by the sensor 23.

As the pressure at the outlet of the pump 1 5 rises above the predetermined value the sensor 23 supplies a signal to the control unit 21. The control unit 21, in response to receipt of the signal from the sensor 23, reduces the frequency of the alternating current supplied from the standard 50 cycle supply 22 to the motor 19 so that the motor 19 is caused to run at a lower speed.

The operating characteristics of centrifugal (turbine) pumps are known to include an operating range wherein a reduction in impeller rotational speed causes a significant reduction in the volume delivered by the pump in unit time, but with only a relatively small drop in the output pressure. Thus provided that the pump 1 3 is utilized within the appropriate operating range then the reduction in speed will cause a reduction in the output from the pump without greatly reducing the pressure.

The reduction in pressure will be sufficient to reduce the pressure to below the predetermined value at which the sensor 23 operates, but will remain sufficiently high to ensure efficient operation of the spray guns at the user stations. However, the reduction in flow from the pump is sufficient to ensure that no stalling of the paint within the pump occurs, and thus the possibility of a shearing action is prevented.

Should the demand from the system continue to decrease then again a point will be reached at which the pressure exceeds the predetermined value and the control unit 21 will then effect a further reduction in motor speed.

A point may be reached where no user station is operating, and thus there is zero demand from the system. Such a situation can arise when there is a break in working for any reason for example at a weekend, or during lunch breaks and the like. In such circumstances it is desirable to maintain the paint circulating system operative but since there is no demand from the system then the required flow rate, to avoid shearing, is very low, and since the system is effectively in a shut-down mode there is not the need for the operating pressure to be maintained in the loop 1 6. In such circumstances the motor speeci can be reduced to a very low level whereby the output flow, and output pressure are low, and the power consumption of the motor is also extremely low. Thus a very economic 'idling" condition can be established.

The change from a low flow, high pressure operating condition to a low flow low pressure idling condition can, if desired, be effected manually rather than automatically. The change in flow rate between these two conditions is relatively small, but the change in pressure and motor power consumption is very significant.

It will be understood that during the normal operating range, that is to say while the flow is varied but the pressure is maintained relatively high, should the demand increase, as a result of more user stations being operated, then the pressure in the loop 1 6 will drop as a result of the increase demand. When the pressure falls below a further predetermined value the signal applied by the sensor 23 to the control unit 21 will cause the control unit 21 to increase the frequency of the a.c.

supply to the motor 1 9 thus increasing the speed of the motor 1 9.

The control unit 21 will cause changes in the speed of the motor 1 9 in a stepwise manner, that is to say the motor will always be operating at one of a number of predetermined speeds. The control unit 21 may include micro-processor control technology whereby the parameters of the control unit 21 for each predetermined speed of the motor 1 9 are stored, and are selected in sequence either in increasing speed, or decreasing speed dependent upon the nature of the signal received from the sensor 23. If a low pressure signal is received then the next higher speed will be selected, similarly if a high pressure signal is received then the next lower speed will be selected.

In one example of the arrangement illustrated in the drawing the pump is a Worthington-Simpson multi-stage turbine pump known under their code number 8WMV16 and it is found using such a pump that when the motor drives the impeller at 2900 r.p.m. then the pump can deliver 5.3 cubic metres of a predetermined paint per hour at a pump outlet pressure equivalent to a head of 11 2 metres of water. By reducing the speed of the pump to 2320 r.p.m. the delivery of the pump can be reduced to a flow of 1.2 cubic metres per hour and the output pressure will only be reduced to 94 metres of water. Thus there is a very significant reduction in flow with only a relatively minimal reduction in pressure. The pressure is still adequate to operate spray guns connected to the loop at the user stations, but the flow rate is sufficiently low that the loop can cope with the whole of the flow without stalling of paint within the pump. The power rating of the motor when operating the pump within tne above range varies from 3.8 K.W. at 2320 r.p.m. to 7 Kv\i al 2S0C) r.p.m. Iioweser, Dy reducing the speed from 2320 r.p.m. down to 1 200 r.p.m. the output flow from the pump is virtually unaltered, but the output pressure drops to approximately 22 metres of water.

The power consumption also drops significantly, down to approximately 0.5 KW thus in the idling condition there is a considerable saving in power.

It is to be understood that while it is preferable to use an a.c. induction motor with a speed control unit to drive the pump other variable speed drive arrangements are possible. For example it would be possible to drive the pump by means of an hydraulic motor, the speed of the motor being controlled in accordance with the demand for paint from the system in such a manner that the shearing action on the paint arising from a stall condition, can be avoided. Moreover while a stepwise control of speed is provided in the above examples it is to be understood that a continuous stepless adjustment of pump speed (over a predetermined range) can be utilized if desired.

A shearing action on the paint is disadvantageous both from the point of view of raising the temperature of the paint, and from the point of view of degradation of the pigment and other inclusions in the paint. The pigment is in effect subject to a grinding action, and when the paint is a metallic finish paint then the flake-like inclusion can rapidly be degraded by a shearing action in the pump to a point of which the metallic effect is lost.

The circulation system illustrated in the sketch is simplified, and a more practical system may well include filters and a pressure relief valve. Moreover the take-off tappings 1 8 illustrated in the sketch represent a "dead" length when their respective gun is not operative. In some respects this is undesirable, and a system wherein each gun has a feed and return line connected respective parts of the loop may be substituted. In such an arrangement paint not expelled through the gun flows back from the gun along the return line and back into the return part of the loop 16.

Furthermore it will be recognised that certain tappings from the loop 1 6 instead of directly feeding spray guns may constitute connections to secondary loops having their own take-off tappings supplying spray guns.

Claims (7)

1. A paint circulating system of the kind specified, wherein the pump is a turbine pump or the like, and is driven by a variable speed drive means, the system further including means controlling the speed of said drive means, and thus controlling the output of the pump, in accordance with the demand for paint from the system.

2. A system as claimed in claim 1, wherein there is provided pressure sensing means providing an output signal when the paint pressure exceeds a predetermined value as a result of the demand for paint decreasing, and said output signal being used to initiate a decrease in the speed of said drive means.

3. A system as claimed in claim 2, wherein the pressure sensing means senses the paint pressure adjacent the pump outlet.

4. A system as claimed in any one of the preceding claims, wherein said variable speed drive means is an a.c. induction motor with an associated speed control unit, said unit being operated to alter the motor speed in accordance with the demand for paint from the system.

5. A system as claimed in claim 4, wherein said speed control unit varies the motor speed by varying the frequency of the a.c. supply to the motor.

6. A system as claimed in any one of the preceding claims, so arranged as to provide an idle condition wherein the pump output volume and pressure and the pump power consumption is low, the output pressure being too low to operate spray guns at the user stations but is sufficient to circulate paint around said loop.

7. A paint circulating system substantially as hereinbefore described, with reference to the accompanying drawings.