GB2253245A

GB2253245A - Control means for a pump

Info

Publication number: GB2253245A
Application number: GB9104290A
Authority: GB
Inventors: Brian Paul Foster; George Edward Carley; Steve Bratcher
Original assignee: Hamworthy Heating Ltd
Current assignee: Hamworthy Heating Ltd
Priority date: 1991-02-28
Filing date: 1991-02-28
Publication date: 1992-09-02
Anticipated expiration: 2011-02-28
Also published as: GB2253245B; GB9104290D0

Abstract

Temperature sensors 24, 26, 32 for a non-positive displacement pump 14 control the operation of the pump in dependence upon variation of a pump temperature or a pump outlet temperature. Thus, the pump can be energised when demand requires it and allowed to run until a temperature variation indicates that the pump is no longer required to operate in the manner of its instant operating condition. In addition, or as an alternative, control means 34, 36, 32 control the operation of a pump 14, 16 in dependence upon the absorbed motor power of that pump or another pump. Thus a first pump can be caused to operate at a faster rate or can be assisted by a second pump if the output power of that first pump indicates that this is necessary or desirable. <IMAGE>

Description

FLOW RELATED CONTROL MEANS FOR A PUMP The present invention relates to a flow related means for a pump. The invention relates particularly to flow related control means for a non positive displacement pump in a variable flow situation, such as, for example, a water booster.

The use of water boosters to satisfy the need for boosting water supplies is well known. Such boosters operate by increasing flow and/or head of local water supply. Typically, but not exclusively, they are used in multi-storey buildings. Typical examples of such buildings are hotels, offices and hospitals.

Conventionally, boosters incorporate non-positive displacement pumps such as centrifugal pumps. The or each pump is controlled so as to be'energised when outlet demand exceeds a respective predetermined threshold level which causes the water pressure to decrease to a trip point level.

Water boosters are typically controlled in dependence upon pressure in the system. However, when pressure is used in this manner, an inherent control logic problem exists. It is known to use a control means which so operates that, if demand is less than the maximum which can be supplied from a given pump, a motor delay timer then controls the booster's operation and causes it to run for a fixed period (generally approximately 3 minutes). After this fixed run period, and assuming no further demands, the pump will shut down as pressure will be satisfied. However, almost immediately after pump shut down the pressure within the system will decay rapidly thus provoking the control means to re-energise the pump, which will then run for a further fixed period. This control logic is inefficient and wasteful of energy and creates accelerated wear on all motor components.

Flow-related control has hitherto been impractical owing to its expense and complexity.

An object of the invention is to relate booster operation more closely to the flow requirements of the system.

According to a first aspect of the invention there is provided control means for a non-positive displacement pump in which the the operation of the said pump is controlled in dependence upon variation of a pump or outlet water temperature.

According to the invention there is also provided control means for a non-positive displacement pump, comprising first and second temperature measuring means for measuring feed water temperature and pump temperature respectively, means for comparing measured feed water temperature and measured pump temperature and detecting a difference therebetween, and means for regulating the operation of the said pump in dependence upon a temperature difference detected between the said feed water temperature and the said pump temperature.

Multi-pump water boosters are known which utilise a so-called 'lead-lag' system of operation. In a lead-lag arrangement, the booster comprises, for example, two pumps in which a first pump operates for up to approximately 50% of maximum demand and in which a second pump cuts-in to operate simultaneously with the first pump in the event that demand exceeds approximately 50%.

System pressure is conventionally used as the control variable for the pumps in a lead-lag arrangement. In order to maintain effective control, it is necessary to use pumps which can adequately cope with the flow variations of the system. However, in order to use pressure control it is necessary to use pumps in which output head varies significantly with flow. Such pumps will, therefore, have a large output head, and will be generally oversized and provide a peak head above what is required. During periods of low demand this type of pump can generate very high pressure head, which is very inefficient.

To achieve more practicable matching of head requirement to maximum pump discharge head, horizontal end suction pumps have been used. However, flow control provides a closer matching of booster operation to system requirement. Typically, flow monitors, which are generally available, are very expensive.

The present invention sets out to provide means for controlling a pump, suitable for use in, for example, a multi-pump booster, in a manner that is energy efficient, practical and inexpensive.

According to a second aspect of the present invention there is provided a water booster comprising a first pump and a second pump, wherein only the said first pump operates during a first operating condition of the water booster and both the said first pump and the said second pump operate during a second operating condition of the water booster, and wherein control means act to switch the said water booster between the first operating condition and the second operating condition in dependence upon the absorbed motor power of the said first pump.

According to a further aspect of the invention, there is provided flow control means for a water pump, wherein the operation of the pump is controlled by the control means, which control means operates in dependence upon the absorbed motor power of the said pump.

The total absorbed motor power of a pump can be said to be a function of the flow rate and can therefore be used as a control signal representing the state of water demand.

The relationship between the absorbed motor power and flow rate does not require the head of either of the pumps to vary significantly with flow. In fact the relationship between absorbed motor power and flow will still exist even if the output head of the pump is substantially constant with respect to variation in water flow.

It is therefore possible to monitor absorbed motor power and use this as a basis for controlling the operation of a pump. This means of control is accurate and energy efficient because it does not require the use of an oversized pump or a pressure reducing valve.

This in turn allows end suction pumps to be used, thereby eliminating the need for close hysteresis switch control, and hence eliminating spasmodic pressure surges within in the system.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which Figure 1 is a schematic block diagram showing a multi-pump water booster system incorporating flow control means according to the first and second aspects of the invention; Figure 2 is a schematic block diagram showing a water booster system including a flow control means according to the first aspect of the invention; Figure 3 is a graph of temperature differential with respect to time constant in respect of a water pump forming part of a water booster system such as that shown in figure 1 or 2; and Figure 4 is a graph showing a power/flow curve for a centrifugal water pump at various flow rates.

Figure 1 relates to a first embodiment of the invention and shows a lead-lag water booster system which comprises two pumps 14 and 16, each of which is provided with a respective pump temperature sensor 26, 28. The pump temperature sensors 26, 28 measure the temperature of the water within the respective pump bodies. The two pumps are connected in parallel and share a common fluid inlet 20 and a common fluid outlet 22. A temperature comparator, forming part of a control unit 32, monitors the temperature differential between the temperature recorded by an inlet temperature sensor 24, which measures the temperature of the feed water to the pumps, and the temperatures recorded by the respective temperature sensors 26 and 28 of the pumps 14 and 16.

Both of the pumps are further connected to an energy sensor 36 which is in turn connected to a power transducer 34. The power transducer 34 transmits a signal to the control unit 32.

This water booster system, therefore comprises two water flow control means which respectively relate to two aspects of the invention; namely: (i) temperature sensitive control means for controlling each pump individually in low water flow conditions; and (ii) motor power sensitive control means for co-ordinating the operation and control of both pumps in high water flow conditions.

Operation of the temperature sensitive flow control means, which is the first aspect of the invention, will now be described.

Because each of the pumps 14, 16 is a non-positive displacement pump, a low flow condition in the system produces a semi-stalled condition in either of the pumps 14, 16 when that pump is not switched off. In the semi-stalled condition water is agitated within a pump 14, 16 without actually leaving the pump. As a result of this agitation, kinetic energy is converted into thermal energy and the temperature of the pump body rises. As the temperature of the pump body rises, the temperature comparator detects an increase in the temperature differential between the temperature recorded by the inlet temperature sensor 24 and the temperature recorded by the pump temperature sensor 26 or 28. When the temperature differential reaches a predetermined level, the control unit 32 causes the pump to switch off or to reduce speed.

The temperature comparator includes a suitably compensated electrical bridge circuit. The above described temperature differential causes the bridge to become out of balance and this is amplified to trip the respective pump motor in operation at such a time when the temperature differential reaches the predetermined level. The trip point may be fixed or adjustable to suit the specific low water flow conditions required.

In a predetermined low water flow condition at which the temperature differential reaches a predetermined high level, one operational pump will normally be switched off or have its speed reduced as described above. This is because for reasons given below, in most instances only one of the pumps 14, 16 would normally be operating in a low water flow condition.

Figure 2 shows a temperature dependent control according to a second embodiment of the first aspect of the invention, in which the temperature sensitive control means is applied to a single pump water booster. Its operation corresponds exactly to that of the temperature sensitive control means described above.

Figure 3 is a graph showing a typical variation of temperature differential, between inlet water and a pump, with respect to time, for a series of different pump flow rates. From this graph the temperature differential increase with a decrease in pump flow can clearly be seen.

Operation of the power sensitive control means, which is the second aspect of the invention, will now be des cribed.

Figure 4 shows the variation of total absorbed motor power with respect to flow for a two pump system, such as that shown in figure 1.

One of the pumps 14 is designated by the control unit 32 as the primary pump and the other 16 is designated as the secondary pump. When a pressure trip indicates an insufficient water pressure condition, the control unit 32 activates the primary pump 14. The load on the primary pump 14 is then monitored by energy sensor 36 which sends a corresponding signal to the control unit 32 and, if it exceeds a predetermined level, the control unit 32 causes the secondary pump 16 to activate.

During operation of the two pumps, the power absorbed by the pumps is monitored by the energy sensor 36, and the operation of the pumps is controlled by the control unit in dependence upon the information supplied by the energy sensor. When the load upon the pumps decreases sufficiently to enable the primary pump to operate alone, the control unit 32 causes the secondary pump to cut out.

The temperature sensitive control means described above continues to operate throughout, and causes the primary pump to cut out when water flow decreases further, to the effect that a low water flow condition occurs and operation of any of the pumps 14 and 16 is unnecessary.

When neither of the pumps is in operation, the control unit switches the designation of the primary pump and the secondary pump. Thus, pump 14 becomes the secondary pump and pump 16 becomes the primary pump. It is to enable this switch to be performed that a temperature sensor is fitted to each pump. This is because the primary pump will usually be the pump running when the system returns to a low water flow condition. The benefit of switching the designation of the pumps in this manner is that pump wear will be evenly distributed between the two pumps. If this switch was not performed, the primary pump would wear out considerably more quickly than the secondary pump.

Although embodiments of the invention have been described in use in with centrifugal pumps in water boosters, many other modifications and suitable applications will be readily apparent to a skilled person upon making reference to the above. For example, it should be noted that many types of non-positive displacement pump can be used with the temperature sensitive control means, and that many additional types of pump could be used in a system where only absorbed power sensitive control means are included.

Furthermore, it should also be noted that the lead lag system is not limited to include two pumps only, but could include several more pumps. By adjusting the control means and including corresponding additional temperature sensors, all aspects of the invention could be used in such a multi-pump lead-lag system.

The present invention will find many further applications in many further types of externally variable flow situation.

Claims

1. Control means for a non-positive displacement pump in which the operation of the said pump is controlled in dependence upon variation of a pump temperature or a pump outlet water temperature.

2. Control means according to claim 1, wherein the said control means causes the said pump to cut out when the said pump temperature or pump outlet water temperature reaches a predetermined value.

3. Control means for a non-positive displacement pump according to claim 1, comprising first and second temperature measuring means for measuring feed water temperature and pump temperature respectively, means for comparing measured feed water temperature and measured pump temperature and detecting a difference therebetween, and means for regulating the operation of the said pump in dependence upon a temperature difference detected between the said feed water temperature and the said pump temperature.

4. Control means according to claim 3, wherein the said control means causes a motor of the said pump to trip at a trip point which occurs when the temperature difference reaches a predetermined level.

5. Control means according to claim 4, wherein the trip point has a fixed value.

6. Control means according to claim 4, wherein the trip point has an adjustable value.

7. Control means according to any one of claims 3 to 6, wherein the said means for comparing the feed water temperature and the pump temperature comprises an electrical bridge circuit, the circuit being such that it becomes out of balance when a temperature differential occurs.

8. Control means for a non-positive displacement pump substantially as herein described with reference to features 24, 26 and 28 of figure 1 and figure 2 of the accompanying drawings.

9. Control means for controlling the operation of a water pump, in which the operation of the said pump is controlled in dependence upon the absorbed motor power of the said pump.

10. Control means according to claim 9 in combination with an end suction pump, wherein the control means controls the said end suction pump.

11. Control means substantially as herein described withreference to feature 36 of figure 1.

12. A water booster comprising a first pump and a second pump, wherein only the said first pump operates during a first operating condition of the water booster and both the said first pump and the said second pump operate during a second operating condition of the water booster, and wherein control means act to switch the said water booster between the first operating condition and the second operating condition in dependence upon the absorbed motor power of the said first pump.

13. A water booster according to claim 12, wherein the said control means switches the said water booster to the second operating condition when the absorbed motor power of the said first pump reaches a predetermined value.

14. A water booster according to claim 12 or 13, wherein at least one of the said pumps is an end-suction pump.

15. A water booster according to claim 12, 13 or 14, wherein the said control means reverses the designation of the said first pump and the said second pump, so that the said first pump becomes the said second pump and the said second pump becomes the said first pump, in the event that neither pump is operating.

16. A water booster according to claim 15 when dependent upon claim 13, wherein each of the said pumps is a non-positive displacement pump, and wherein said control means further comprises first temperature measuring means, for measuring feed water temperature, second temperature measuring means, for measuring the temperature of the said first pump, third temperature measuring means, for measuring the temperature of the said second pump, means for comparing measured feed water temperature and measured temperature of either or both of the said pumps and detecting a difference therebetween, and means for regulating the operation of either or both of the said pumps in dependence of upon a temperature difference detected between the said feed water temperature and the temperature or temperatures of the said pump or pumps.

17. A water booster substantially as herein described with reference to figure 1 of the accompanying drawings.