GB2449841A

GB2449841A - System for flushing a submerged pump

Info

Publication number: GB2449841A
Application number: GB0706969A
Authority: GB
Inventors: Andy Laundon
Original assignee: RETROFLO Ltd; Byzak Ltd
Current assignee: RETROFLO Ltd
Priority date: 2007-03-29
Filing date: 2007-03-29
Publication date: 2008-12-10
Anticipated expiration: 2027-03-29
Also published as: AU2008234705B2; EP2137355A1; WO2008119933A1; US20100135823A1; GB0706969D0; AU2008234705A1; GB2449841B

Abstract

A method of flushing a wet well operating under a constant level pumping regime in a sewage system 10, in which the wet well level is monitored and controlled along with the pump operation in response to a predicted period of reduced inflow. The method operates such that in advance of the predicted period of reduced inflow, the wet well level is allowed to rise to a 'flush' level and during the predicted period of reduced inflow, the wet well level is reduced from the flush level to a normal level.

Description

Improvements in and Relating to Sewage Pumping The present invention

relates to improvements in sewage pumping.

In a usual arrangement waste water and sewage flows out of homes, businesses and surface water drains into a sewer system through which it is directed to a treatment works. In the treatment works the water is separated and purified for discharge into the water course. In a typical sewage system, the waste water and sewage flows primarily under gravity. This may not be possible throughout the whole network. Accordingly, at some points, pumping stations are provided. The pumping stations are operable to raise waste water and sewage from the low end of one part of the system (the wet well') to the high end of an adjacent part of the system. Once in the adjacent part of the system, flow under gravity may resume. In certain systems smaller pumping stations may discharge into larger stations as so on until the flows reach the treatment works.

There are various pumping regimes that may be employed to pump waste water from the wet well such as batched pumping and constant level pumping. In * I. * . S * .5 batched pumping, once the level in the wet well exceeds a predetermined pump start *5S* :. level, the pump is started and runs at constant speed until the level in the wet well drops below a predetermined stop level. In constant level pumping the pump is run at *: a variable speed to transfer flows entering the wet well directly to the adjacent part of : .: 20 the system. Constant level pumping provides for a more regular flow volume than batched pumping. This is beneficial as sewage systems function best with a relatively constant throughput of fluid rather than pulsed surges. Constant level pumping is also more efficient to run. Unfortunately, in a sewage system, constant level pumping does provide a greater opportunity for solids in the wet well to settle and thus increases the possibility that the pump will become blocked. This reduces reliability and thus increases reactive costs.

In order to combat the increased occurrences of blockages in a constant level regime, after a preset time period the pump is started and runs at constant speed until the level in the wet well drops below a predetermined stop level as in batch pumping.

The pump then returns to constant level pumping mode. This helps to flush solid material out of the wet well and reduces the likelihood of pump blockage, but does result in distinct surges of water to running through the sewage system at regular intervals, which is detrimental to the efficiency of operation of the system as a whole.

It is therefore an object of the present invention to provide a method of pumping sewage that alleviates or overcomes the above problems.

According to a first aspect of the present invention there is provided a method * 15 of flushing a wet well operating under a constant level pumping regime in a sewage system, the method comprising the steps of: monitoring and controlling the wet well level and the pump operation in response to a predicted period of reduced inflow, :. such that: in advance of the predicted period of reduced inflow, the wet well level is allowed to rise to a flush level; and during the predicted period of reduced inflow, the wet well level is reduced from the flush level to a normal level.

This flushing method allows the operators of a sewage system to take advantage of predictable fluctuations in flow volume to enable a wet well to be flushed whilst reducing the magnitude of flow volume fluctuations downstream of the wet well. Tying the flushing to periods of low inflow also helps to prevent settling of solids during this time and thereby reduces the potential for future blockages.

The flushing method may be initiated at a specific local time. The specific time may be identified by monitoring inflow to identify periodic variations. This monitoring can be carried out directly or by comparison of the outputs of wet well level and pump speed monitors. The variations can be identified by use of a processor. Alternatively, the specific periods of variation may be identified by an operator. Relating flushing to local time enables periods of reduced inflow to be predicted in advance as most sewage systems exhibit a characteristic daily variation in inflow. For example, fewer users generally produce waste water at night and thus the inflow typically reduces overnight. Additionally if the system has multiple pumping stations within it the flushing cycles of these pumping stations may be analysed and coordinated to achieve maximum efficiencies of flow transfer.

The flush method may be initiated: once or more during each predicted period :* ::* of reduced flow; once or more during alternate periods of reduced flow or any other specified fraction of periods of reduced flow; during a specified number of predicted reduced flow periods during a predetermined monitoring period; or at any other

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* suitable frequency. In a typical implementation, the flushing method may be initiated :1..: 20 once during every twenty four period. Alternatively, the flush method may be *5* SS * initiated in response to a user control; in response to a wet well level exceeding a preset level or in response to an indication that there is an increased risk of pump blockage.

The wet well level may be allowed to rise to the flush level over any suitable rise period ahead of the predicted period of reduced inflow. The wet well level may rise substantially steadily over the whole of the rise period. Alternatively, the pump may be controlled to allow the wet well level to rise more rapidly during some parts of the rise period than others. In particular, the wet well level may rise more slowly at the start and end of the rise period than in the middle section of the rise period.

Control of the wet well level may be achieved by reducing the speed of the pump over the rise period. The speed reduction may be by a specified fraction of the normal speed. This fraction may be calculated with reference to a prediction of the inflow rate over this period. Alternatively, this may be achieved by raising the desired wet well level over time. In a further alternative, the wet well level may be allowed to rise by stopping the pump completely for the rise period.

The wet well level may be reduced from the flush level to the normal level over any suitable flush period. Preferably, the pump operates at a speed equal to or in excess of the required flow rate for self cleaning during the flush period. Preferably, the flush period is substantially shorter than the rise period. The pump may operate at S...

a constant speed during the flush period. Alternatively, the pump may operate at a variable speed during the flush period.

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: * The flush period may be initiated directly after the rise period. Alternatively, * : 20 there may be a hold period between the end of the rise period and the initiation of the flush period. During the hold period the wet well level is maintained at the flush level.

The flushing method may be implemented as a flushing mode comprising part of a pumping regime. In default mode, the pumping regime may be operable to control the pump operation in response to the wet well level so as to maintain the wet well level at or within a predetermined range of a desired normal level.

The flushing method may be applied to systems with more than one pump. In particular, at least some of the variations in wet well level may be achieved by controlling the number of operational pumps in addition to or in place of controlling the operating speed of the pumps. For instance, if more than one pump is provided only one pump may be normally operable during constant level operation but more than one pump may be operable during the flush period to increase the throughput and thus improve the flushing operation. Additionally, increasing the velocities of the flows will aid in cleansing the upstream pipework in the system. The additional pump or pumps may be operable for the whole of or part of the flush period. In one preferred scheme the additional pump or pumps start operating after a predetermined time interval has elapsed since the initiation of the flush period and stop when the wet well level drops below a predetermined level. * S. S...

S According to a second aspect of the present invention there is provided a control unit for a sewage pumping station comprising: wet well monitoring means for

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* monitoring the wet well level; pump operation monitoring means for monitoring the pump operation; a processor operable to receive input from the wet well monitoring *.S.S * means and pump operation means, to determine that a period of reduced inflow is predicted and to output control signals to the pump such that: in advance of the predicted period of reduced inflow, the wet well level is allowed to rise to a flush level; and during the predicted period of reduced inflow, the wet well level is reduced from the flush level to a normal level.

The control unit of the second aspect of the present invention may incorporate any or all of the features described in relation to the method of the first aspect of the invention as desired or as appropriate. As discussed above, the prediction can be programmed to accommodate any other flushing cycles that may be present on the sewage system.

According to a third aspect of the present invention there is provided a sewage pumping station operable in accordance with the method of the first aspect of the present invention.

According to a fourth aspect of the present invention there is provided a sewage pumping station incorporating a control unit in accordance with the second aspect of the present invention.

According to a fifth aspect of the present invention there is provided a sewage 15 system incorporating one or more pumping stations according to the third or fourth aspects of the present invention. p* * *

In order that the invention can be more clearly understood it is now described further below with reference to the accompanying drawings: * Figure 1 a is a schematic diagram of part of a sewer network showing a pumping station; Figure lb is a schematic diagram of the pumping station of figure la; Figure Ic is a schematic diagram of part of a sewer network showing multiple pumping stations; Figure 2 is a schematic block diagram of a pump control system for the pumping station of figure 1; Figure 3a is a schematic representation of the variation of the wet well inflow in the pumping station of figure 1 over a 24 hour period; and Figure 3b is a schematic representation of the variation of the wet well level in the pumping station of figure 1 over a 24 hour period.

Referring now to figure la, waste water flows under gravity in a first part II of a sewer network 10. At the low end of this part 11, the waste water is raised by a pumping station 100 to the high end of a second part 12 of the sewer network 10 where flow under gravity can resume. The sewer network 10 may be provided with a * ** s. 15 plurality of such pumping stations 100, as required. **.. I. **

* * Referring now to figure lb. the pumping station 100 comprises a wet well 101 *.* * which waste water from the first sewer section 11 enters via an inlet 110. The waste water collects in the wet well 101 and is pumped out of the wet well 101 by a pump * 102 into sewer section 12. The pump 102 is controlled by a pump control unit 200 in response to the water level in the wet well 101 and the pump 102 operation speed.

The pump control unit 200 is illustrated schematically in figure 2. It comprises a processor 201 which is operable to receive signals from a pump monitoring means 202, a wet well level monitoring means 203 and an operator interface 205. In response to the signals from the monitoring means 202, 203 the processor 201 is operable to control the operation of the pump 102. The operator interface 205 is operable to display information relating to the operation of the control unit 200 and may be used by an operator to vary the operating parameters of the control unit 200.

The pump control unit 200 is nonnally operable to maintain the level in the wet well 101 at a pre-selected normal level N, shown in figure lb. This is achieved by comparing the wet well level detected by the wet well monitoring means 203 with the pre-selected level N and adjusting the pump 102 speed in response thereto. In the event that the pump speed still exceeds the inflow, the pump 102 will stop completely when the wet well level reaches a low boundary Ni. Once the pump 102 is stopped, it is not restarted until the wet level reaches a high boundary N2 at which point normal operation is resumed. * .. 0 0 * .0 S...

Pumping in this manner, rather than in periodic batches results in a relatively I* * *. even throughput and is more efficient. Unfortunately, it is more prone to blockages * S..

than batch pumping methods as the solids in the wet well have a greater opportunity to settle. Accordingly, the present invention provides for a method of flushing the wet * well 101 that results in a minimum variation in throughput.

In a sewage system, inflow varies over the course of a day. Typically, inflow is higher during the day than the night as more users are awake and using water. A simplified schematic illustrating this variation in inflow is shown at figure 3a. Of course it is possible that in some localities there may be other different or more complex patterns of inflow variation, perhaps related to particular local industrial plants.

The relative inflow levels over the course of a day can be predicted by looking at the inflow pattern of previous days. Accordingly, periods of reduced inflow can be predicted. This prediction can be carried out automatically by the processor 201.

Alternatively, as the variation of inflow rate with respect to real time is likely to be relatively constant each day, the times at which the wet well level is allowed to rise and fall can be preset by a human operator.

In the present invention, once the daily inflow variation pattern is known, the control unit 200 is operable to predict a period of reduced inflow by reference to local time. Accordingly, ahead of a predicted period of reduced inflow the control unit 200 is operable to allow the wet well level to rise to a flush level F (shown in figure ib). a,..

a,,-', the control unit 200 is then operable during the period of reduced inflow to operate the pump 102 reduce the wet well level back to normal. By allowing the level to rise U..

* during a period of relatively high inflow and then pumping the level back to normal during a period of relatively low inflow, the overall variation in throughput is reduced. Additionally, flushing during a period of low inflow helps to prevent settling and thus reduces the potential for future blockages. -10-

An example of this system in operation is set out below with reference to figure 3a and figure 3b showing the variation of wet well level over a typical day. In the present example, at from l.3Oam to 7pm, the normal period, the pump 102 is controlled by the control unit 200 to maintain the wet well level at or around N. 7pm is the start of a rise period RP wherein the wet well level is allowed to rise to F. Typically, as in the present example, the wet well level is allowed to rise substantially steadily over the full rise period RP. This can be achieved by slowing or stopping the pump 102 for all or part of the rise period RP. The start time and duration of the rise period RP may depend upon the expected inflow. Typically, they would be set in response to the expected variation in inflow at the pumping station 100. In order to achieve a substantially even rise over the whole rise period RP, the control unit 200 reduces the speed of the pump by a suitable fraction. Alternatively, the control unit can continue to act in the same mode as the normal period but gradually raise the position of level N to F over the course of the rise period RP.

After the rise period RP, there is a hold period 1-IP. In the present case this is shown as 1 hour from 12am. In the hold period HP, the wet well level is maintained * *e * * * * at F. This is achieved by the same method as the level is maintained at N during the * S* normal period. In some implementations the hold period HP may be longer or shorter. It may even be omitted, if desired. Typically, the duration of the hold period HP (if any) will be set in response to the expected pattern of inflow variation. * *v

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* At lam in the present example, the flush period FP is initiated. During the flush period FP, the pump 102 is run at a relatively high speed to reduce the wet well level from F at the start of the flush period FP to N3 (scourer level) at the end of the flush period FP. The flush period FP may have a set duration, in which case the pump speed selected will depend partly on the expected inflow. More commonly, the flush period FP will not have a set duration but will involve operating the pump 102 at a set speed until the wet well level has fallen to N3. After the wet well level has fallen to N3, the operation of the normal period is resumed.

In pumping stations with a more complex inflow pattern, flushing operations may be performed more or less often as required. In such circumstances, the duration of the rise period RP, hold period HP and flush period FP may also be varied as well as the set normal level N and flush level F. These parameters may be set by the processor or by an operator and are tailored to the conditions at a particular pumping station 100.

The operator may be able to override the control unit 200 by use of the interface and thereby order or cancel a flushing operation at any time. In some embodiments, the control means 200 may be automatically able to implement a flushing operation if the wet well level exceeds a preset warning level A. Level A may be above or below level F as required by the characteristics of the pumping station 100.

In many typical pumping stations 100 there is more than one pump 102.

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: * References to pump above should therefore be understood to refer to pumps in the collective where appropriate. In one typical implementation, three pumps 102 are I...

provided, the pumps 102 being used in rotation as duty pump with one of the other pumps 102 designated as standby pump operable to take over should the duty pump -12 -fail. In such a system, two pumps 102 may be operated during the flush period FP, the duty pump and an assist pump, with the third pump designated as standby pump.

The standby pump is operable to take over if the duty pump or the assist pump fails or becomes blocked. In a preferred implementation, the assist pump is started a preset time after the initiation of the flush period FP and is stopped prior to the end of the flush period FP, when the wet well level drops below a preset level.

It is of course to be understood that the invention is not to be restricted to the details of the above embodiments which have been described by way of example only.

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Claims

-13 -Claims 1. A method of flushing a wet well operating under a

constant level pumping regime in a sewage system, the method comprising the steps of: monitoring and controlling the wet well level and the pump operation in response to a predicted period of reduced inflow, such that: in advance of the predicted period of reduced inflow, the wet well level is allowed to rise to a flush level; and during the predicted period of reduced inflow, the wet well level is reduced from the flush level to a normal level.
2. A method as claimed in claim 1 wherein the flushing method is initiated at a specific local time.
3. A method as claimed in claim 2 wherein the specific time is identified by monitoring inflow to identify periodic variations.
4. A method as claimed in claim 3 wherein this monitoring is carried out directly or by comparison of the outputs of wet well level and pump speed monitors.
5. A method as claimed in claim 4 wherein the variations are identified by use of a processor.

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6. A method as claimed in claim 4 wherein the specific periods of variation are * identified by an operator.
7. A method as claimed in any preceding claim wherein the flush method is * 20 initiated: once or more during each predicted period of reduced flow; once or more during alternate periods of reduced flow or any other specified fraction of periods of reduced flow; during a specified number of predicted reduced flow periods during a predetermined monitoring period; or at any other

suitable frequency.
8. A method as claimed in claim 7 wherein the flushing method is initiated once during every twenty four period.
9. A method as claimed in any preceding claim wherein the flush method is initiated in response to a user control; in response to a wet well level exceeding a preset level or in response to an indication that there is an increased risk of pump blockage.
10. A method as claimed in any preceding claim wherein the wet well level is allowed to rise to the flush level over a rise period ahead of the predicted period of reduced inflow.
11. A method as claimed in claim 10 wherein the wet well level rises substantially steadily over the whole of the rise period.
12. A method as claimed in claim 10 wherein the pump is controlled to allow the wet well level to rise more rapidly during some parts of the rise period than others.
13. A method as claimed in claim 12 wherein the wet well level rises more slowly * s at the start and end of the rise period than in the middle section of the rise period.

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14. A method as claimed in any one of claims 10 to 13 wherein control of the wet well level is achieved by reducing the speed of the pump over the rise period.
15. A method as claimed in claim 14 wherein the speed reduction is by a specified fraction of the normal speed.

-15 -
16. A method as claimed in claim 15 wherein the fraction is calculated with reference to a prediction of the inflow rate over this period.
17. A method as claimed in any one of claims 10 to 16 wherein control of the wet well level is achieved by raising the desired wet well level over time.
18. A method as claimed in any one of claims 10 to 17 wherein the wet well level is allowed to rise by stopping the pump completely for the rise period.
19. A method as claimed in any preceding claim wherein the wet well level is reduced from the flush level to the normal level over a flush period.
20. A method as claimed in claim 19 wherein the pump operates at a speed equal to or in excess of the required flow rate for self cleaning during the flush period.
21. A method as claimed in claim 19 or claim 20 wherein the flush period is substantially shorter than the rise period.
22. A method as claimed in any one of claims 19 to 21 wherein the pump operates at a constant speed during the flush period.
23. A method as claimed in any one of claims 19 to 21 wherein the pump operates at a variable speed during the flush period.
24. A method as claimed in any one of claims 10 to 23 wherein the flush period is initiated directly after the rise period.
25. A method as claimed in any one of claims 10 to 23 wherein there is a hold period between the end of the rise period and the initiation of the flush period.
26. A method as claimed in claim 25 wherein during the hold period the wet well level is maintained at the flush level.
27. A method as claimed in any preceding claim wherein the flushing method is implemented as a flushing mode comprising part of a pumping regime.
28. A method as claimed in claim 27 wherein in default mode, the pumping regime is operable to control the pump operation in response to the wet well level so as to maintain the wet well level at or within a predetermined range of a desired normal level.
29. A method as claimed in any preceding claim wherein the flushing method is applied to systems with more than one pump.
30. A method as claimed in claim 29 wherein at least some of the variations in wet well level are achieved by controlling the number of operational pumps in addition to or in place of controlling the operating speed of the pumps.
31. A method as claimed in claim 30 wherein an additional pump or pumps start operating after a predetermined time interval has elapsed since the initiation of the flush period and stop when the wet well level drops below a predetermined level.
32. A control unit for a sewage pumping station comprising: wet well monitoring means for monitoring the wet well level; pump operation monitoring means *., for monitoring the pump operation; a processor operable to receive input from the wet well monitoring means and pump operation means, to determine that a period of reduced inflow is predicted and to output control signals to the pump such that: in advance of the predicted period of reduced inflow, the wet well level is allowed to rise to a flush level; and during the predicted period of reduced inflow, the wet well level is reduced from the flush level to a normal level.
33. A control unit as claimed in claim 32 wherein the control unit is operable in accordance with the method of any one of claims 1 to 31.
34. A sewage pumping station wherein the sewage pumping station is operable in accordance with the method of any one of claims I to 31.
35. A sewage pumping station incorporating a control unit as claimed in claim 32 or claim 33.
36. A sewage system incorporating one or more pumping stations as claimed in claim 35.
37. A sewage system as claimed in claim 36 wherein if the system has multiple pumping stations within it the flushing cycles of these pumping stations are coordinated to achieve maximum efficiencies of flow transfer.

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