GB2599312A - Control system for waste water pumping station - Google Patents

Abstract

A control system for a waste water pumping station (3 figure 1) in a waste water network has a well 6 for containing waste water entering through an inlet 7, a sensor 11 for measuring the level of the well contents, at least two variable speed pumps 8 for pumping waste water out of the well through an outlet 9 and a control panel (10 figure 2). The control panel operates the pumps to maintain the level of the well contents between a normal start level 20 and a normal stop level 21. The control panel (10 figure 2) operates the pumps 8 periodically in an optimisation test mode to determine the most energy-efficient speed for each pump 8, then operates each pump 8 at that speed in normal conditions until the next time the control panel (10 figure 2) operates the optimisation test mode.

Description

CONTROL SYSTEM FOR WASTE WATER PUMPING STATION

This invention relates to a control system for a waste water pumping station, and a waste water network.

Water utility companies operate treatment works for waste water, including sewage and rain water that run into a drainage system. Each treatment works is fed by a network of waste water pumping stations, varying from small installations serving a few properties to medium-sized ones serving a housing estate. These peripheral pumping stations feed terminal pumping stations which are connected directly to the treatment works.

Each pumping station has a well for containing the waste water, a sensor for sensing the level of the contents in the well, at least two electrically-operated pumps for pumping the contents out, and a control panel. Some may also have a flow meter for measuring the flow pumped out. Some pumping stations take only waste water from properties, while others take rainwater as well. In each case the waste water flows in under gravity, and is pumped out to the next point in the network, with the pumps being operated by the control panel in accordance with the level of contents of the well, as indicated by the sensor. Each pump is capable of pumping out the contents of the well at a faster rate than waste can flow in.

The control panel is programmed to operate the pumps to keep the contents of the well between upper and lower predetermined levels, in order to provide a consistent supply to the treatment works. It is obviously important that the well does not overflow or spill, both because of the environmental consequences and the fine that can be imposed on the water utility company. The control panel normally operates the pumps alternately (or sequentially if there are more than two) to ensure that the station is always operational. If a pump fails the control panel can send an alert to a central control, which may be at the treatment works, so that an engineer can visit to carry out a repair. Some systems take account of the cost of energy in operating the pumps, in order to improve the overall efficiency of the pumping station. For example, the pumping stations are subject to an industrial electricity tariff, where energy prices are set for each half hour, depending on consumer demand. Typically, a half hour period on a winter afternoon will be at a peak rate, while a half hour at night-time will be at an off-peak rate. The tariffs are set well in advance according to season, but energy companies can notify triad periods on 12 hours' notice, where the cost is even greater, due to expected consumer demand. It is therefore advantageous to take account of the efficiency of the pumps and the energy costs in operating the pumps.

The system may time the operation of the pumps to reduce the cost of energy needed for operation. As far as possible, in normal operation the pumps are operated in off-peak tariff periods. In exceptional circumstances, the pumps may be operated in peak and triad tariff periods.

Other systems may monitor the inflow rate to try to predict deluge conditions, where there is a sudden increase in the water flowing into the well, and operate the pumps accordingly. The system monitors the rate of filling of the well, and if this exceeds a predetermined rate, a deluge condition is indicated. The system may then calculate IS the most energy efficient operation of the pumps to avoid spill.

If one pumping station in a network experiences a deluge condition, it is likely that others will also be affected. For example, if the deluge condition is due to heavy rainfall, nearby pumping stations may also experience similar rainfall within a short period of time. The pumping stations fed by the pumping stations experiencing the deluge condition may not have the rainfall, but will need to be prepared to receive the extra flow from the stations feeding it.

Currently, the pumps used at pumping stations are of two types: constant speed and variable speed. Variable speed pumps are preferred, as they are more energy efficient.

However, they are normally operated at their maximum speed (that is, at 100% speed) even though this may not in fact be the most energy-efficient speed.

According to the invention, we provide a control system for a waste water pumping station having a well for containing waste water entering through an inlet, a sensor for measuring the level of the well contents, at least two variable speed pumps for pumping waste water out of the well through an outlet, and a control panel, the control panel being operable to operate the pumps to maintain the level of the well contents between a normal start level and a normal stop level, and the control panel operates the pumps periodically in an optimisation test mode to determine the most energy-efficient speed for each pump, and then operates each pump at that speed in normal conditions, until the next time the control panel operates the optimisation test mode.

If the most energy-efficient speed is less than 100%, the rate of flow will be reduced, but this will be acceptable in normal conditions, because the pump can still pump out the well contents at a faster rate than waste flows in. The control panel may of course operate the pump at 100% speed if a maximum flow rate is needed. This also contributes to the overall efficiency of the pumping station.

Conveniently the optimisation test mode is enabled when the well level is at a test level, and the energy tariff is the off-peak tariff. A pump is then run for a specified time, at a given speed and the flow rate and energy used are measured. When the well level reaches the test level again the pump is run for a specified time, at a different speed, and the flow rate and energy used are measured. The process may be repeated for up to three further speeds. The flow rate is divided by the energy to give an efficiency value for each speed. The two highest efficiency values are chosen and the pump is run again at a speed half-way between the speeds with the highest values. The efficiency value is calculated, and the speed that produced the highest efficiency value is taken to be the optimally efficient speed until the next test is carried out. The optimally efficient speed is set as the default running speed for the pump. The optimisation test mode is repeated for all the other variable speed pumps.

An embodiment of the invention is illustrated by way of example only, in the accompanying drawings, in which:-Figure 1 is a schematic diagram of a waste water treatment network; Figure 2 is a schematic cross-section through a waste water pumping station; and Figure 3 is a schematic cross-section through a well of the pumping station of Figure 2 showing various levels.

Figure 1 shows a waste water treatment network in schematic form. The network comprises a treatment works I, for treating sewage and rainwater in a known way. The treatment works 1 is fed by a network of pumping stations, comprising terminal pumping stations 2 feeding the treatment works 1 directly, and in turn fed by peripheral pumping stations 3, possibly through intermediate stations 4. The peripheral pumping stations 3 may be small installations serving only a few properties, to medium-sized ones serving a whole housing estate. The network has a central control system receiving information from each pumping station 2, 3 or 4. Because the treatment works 1 operates best when it is fed continuously and at a consistent rate, the pumping stations 2, 3, 4 are operated to provide the required flow in normal conditions.

As shown in Figure 2, a typical peripheral pumping station 3 serves several properties 5, and comprises a well 6 into which waste water and rainwater from the properties 5 flow under gravity, entering through an inlet 7. Two electrically-driven pumps 8 are located near the base of the well 6, and are operated to pump the contents of the well 6 through an outlet 9 up to the next pumping station on the network, or to the treatment works 1. Larger pumping stations 2 work on the same principle, but may have more than two pumps 8, according to the well capacity and flow rates required. Each pumping station 2, 3 or 4 will have at least two pumps 8 to provide resilience. The pumps 8 are operated by a control panel 10, according to the level of the contents of the well 6. One pump 8 is the primary or duty pump, while the other or others are assist pumps. The level of waste water in the well 6 is sensed by a sensor 11 of any suitable type, typically an ultrasonic sensor. The sensor 11 operates continuously, and sends signals to the control panel 10. A backup control float 13 provides a backup to the sensor 11, and also sends a signal to the control panel 10 if the level reaches it. A further float 14 above the backup control float 13 provides a high level action point. The station 3 also has a flowmeter 12 to measure the rate of flow at the outlet 9. The flowmeter 12 also provides data to the control panel 10.

The control panel 10, which is not shown in detail, includes processing means, timer means, memory means, means for receiving signals from the level sensor I I. the floats 13 and 14, and the flowmeter 12, means for transmitting signals to the pumps 8, and means for monitoring the energy used by the pumps 8, together with means for receiving signals from the central control system of the network, and means for transmitting signals to the central control system.

The well 6 has various levels defined, and chosen according to its situation and characteristics. There is a normal upper start level 20 at which the duty pump starts and a normal lower stop level 21 at which it stops, together with a lower, pump down level 22. Above the normal level 20 is an assist upper start level 20a, at which an assist pump starts. The assist pump will stop at the normal stop level 21. The float 14 defines a high level action point 23, and the float 14 a backup control level 25. There is also a deluge start level 24 at which the duty pump starts if deluge conditions are detected, together with a deluge assist start level 24a at which an assist pump starts.

These are below the normal and assist start levels 20, 20a Two further levels are defined: a deluge preparation level 26 and a test level 30 The control panel 10 is programmed to perform standard operations. It will be appreciated that the inflow to the well 6 has daily and possibly weekly cycles, and the well 6 smooths the variation to assist in feeding the treatment works 1 at a constant rate. The control panel 10 monitors the rate of change of the well level continuously, using the sensor 11. Its primary function is to operate the pumps 8 to maintain the contents of the well 6 between the normal start level 20 and the normal stop level 21, and to send an alert to the central control system if a pump 8 fails to operate. The pumps 8 are chosen so that each can pump at a flow rate greater than the maximum expected inflow rate, to ensure that the well 6 does not spill. The other standard functions of the control panel 10 are to test the pumps 8 periodically to ensure they are both working, to operate the pumps 8 alternately as duty and assist pumps, and to run a periodic pump down, where the well 6 is pumped out to the lower, pump down level 22 to assist in removal of fat. The duty pump is started at the normal start level 20, and if the assist start level 20a is reached the assist pump is started. Clearly, the level is expected to fall after the duty pump is started, so if the assist level is reached it indicates a fault with the duty pump or a very rapid rise in level. The backup control float 13 sends a signal to the control panel 10 if the level 25 is reached. If the level sensed by the sensor 11 does not agree with the control float 13 the sensor 11 may be in failure, and the control panel operates the pumps tl accordingly. if the high level action point 23 is reached this indicates that there is a high risk of overflow, and the pumps are operated accordingly.

The control system provides the ability to measure pump efficiency, and to operate the pumps 8 to take account of pump efficiency, energy costs and abnormal conditions, both for individual pumping stations 2, 3, 4 and for the whole network.

The control panel 10 is therefore programmed to operate the pumps 8 to minimise energy usage and costs. As explained above, pumping stations 2, 3, 4 are subject to an industrial electricity tariff, where energy prices are set for each half hour, taking into account domestic consumer demand. The tariff has off-peak and peak rates, and is set in advance for the coming three-month season. Peak rates typically occur in the morning and afternoon/evening while off-peak will always be in the middle of the night. The energy companies may also notify triad tariff periods due to extremely high demand, with 12 hours' notice, when the price is substantially higher.

The control panel 10 is therefore set to minimise pump operation in peak and triad tariff periods. Thus, any periodic test runs or the pump down to the low level 22 are scheduled in off-peak tariff periods.

In normal operation, instead of simply operating a pump 8 when the normal start level 20 is reached, the system takes into account the current energy tariff, the time until a change in tariff, the rate at which the well 6 is filling and the outflow rate expected, and adjusts the normal start level 20 and normal stop level 21 temporarily.

For example, if the current tariff is off-peak, and the well 6 can be pumped to the stop level 21 before a peak tariff, the pumps 8 will be operated to achieve this. However, if the current tariff is peak rate, the system will compare the time it will take for the well 6 to reach the high level action point 23 with the time before the next off-peak period. if the latter is greater, the pumps 8 will be operated at the start of the next off-peak period. If the former is greater, operation of the pumps 8 in the peak period will be restricted to that required to prevent spill, until the off-peak period starts.

The same principle will apply to triad tariff periods, so that the pumps 8 are operated in a peak period in preference to a triad tariff period.

The system will also be set to operate the pumps 8 before the end of an off-peak tariff period, imless the normal stop level 21 will not be reached before a change to a peak or triad tariff period. In that event the pumps 8 will be operated until the change in tariff, and will stop even though the stop level 21 has not been reached.

In order to measure pump efficiency, the control panel 10 is set to run each pump 8 for a test period, and to take the flow measured by the flowmeter 12 and the energy used by the pump 8, to determine an efficiency value in cubic metres per kilowatt hour. The test is carried out periodically, and preferably daily. The efficiency values are stored in the memory means and the control panel 10 and the control panel 10 then operates the more efficient pump 8 preferentially. The more efficient pump 8 will not be operated exclusively unless the other pump 8 has failed, as it is essential to keep both pumps 8 operational if possible in more detail, before the test is carried out, the system checks that the well level is above the test level 30 and the system is not in peak or triad tariff. A first pump 8 is run for a specified amount of time, while measuring the kWh used and the volume moved. The efficiency value is calculated. When the well has filled back to the test level 30, a second pump 8 is then run for a specified amount of time, while measuring the kWh used and the volume moved. The efficiency value is calculated. If there arc more than two pumps the process will be repeated for the further pumps. The pumps 8 are then ranked in order of efficiency value. The pump 8 with the highest value will be set as the duty pump.

The efficiency values are stored in the memory and analysed over time to track any decline due to wear, so that servicing or replacement can be planned Further, while each pump 8 is operating, the flow and energy are monitored continuously, so that a short term drop in efficiency can be detected. Such a drop may indicate failure, and the other pump 8 can take over. if both pumps 8 suffer a short term drop in efficiency this may indicate a blockage downstream of the outlet 9.

The control panel 10 may send suitable alarm signals to the central control system in the event of short and long term efficiency drops.

As the flow is monitored continuously when a pump is running, it is also possible to detect whether the flowmeter 12 is worn or damaged or needs re-calibrating. A tolerance range is chosen for the flowmeter 12, depending on the characteristics of the pumping station 2, 3, 4, and is input into the control panel memory, if the measured flow rate is outside the tolerance range the flowmeter 12 may be giving unreliable results, and the control panel 10 sends an alarm signal to the central control. An engineer can then visit to check the flowmeter 12. The control panel 10 is also programmed so that sending the alarm signal also disables the efficiency testing of the pumps 8 until an engineer enables it again. This avoids spurious efficiency values.

The control system is also configured for a deluge condition, where there is a sudden increase in inflow, usually due to heavy rainfall. A deluge condition is detected when the rate of rise of the level in the well 6 is greater than a predetermined deluge value. The control system then operates at a network level as well for individual pumping stations 2, 3, 4. For example, if one pumping station 3 experiences a deluge condition because of a storm, nearby stations 3 may be similarly affected. The downstream stations 2, 4 may not be directly affected by the storm but will receive the extra flow from the upstream stations 3 and can prepare for it.

Thus, a pumping station 3 that detects a deluge condition by the increase in the rate of inflow will operate as follows to minimise energy costs while avoiding a spill. Firstly, the normal start levels 20, 20a will be reduced to deluge start levels 24, 24a, and the pump or pumps 8 operated as described above to prevent a spill. The control panel 10 of that pumping station 3 will send an alert to the central control system, which may use meteorological data in conjunction with that alert to send signals to neighbouring peripheral stations 3 that may also be affected, and to downstream stations 2, 4. Once the rate of rise of the well level falls below the predetermined deluge value, the system will return to the normal start levels 20, 20a However, the neighbouring and downstream stations can operate in a deluge preparation mode. In this mode the normal start levels 20, 20a will temporarily be reduced to the deluge start levels 24, 24a, so that (subject to energy cost considerations) the pumps 8 will be started at the deluge start level 24, in order to allow for the extra inflow. In the deluge preparation mode, assuming that the well level is above the normal stop level 21, the first action is to run a brief test for all pumps 8 to ensure they are operational. If there is a failure the central control system will be sent an alert. The next action depends on the current energy tariff. If it is a triad period the pumps 8 are not operated. If it is a peak period the pumps 8 are operated until the deluge preparation level 26 is reached. If it is an off-peak period the pumps 8 are operated until the normal stop level 21 is reached. The well level is monitored continuously, and the pumps 8 may be operated at a change in tariff as necessary. The system waits for a specified amount of time for the deluge. If the expected deluge is not detected, the system returns to a normal mode of operation, with the normal start levels 20, 20a. The amount of the reduction from the normal start level 20 to the deluge start level 24, and the setting of the deluge preparation level 26 will normally be determined by the characteristics of the well 6 and the pumps 8. For example, if a pumping station has a well 6 with a larger volume, or pumps 8 with greater capacity than an otherwise similar pumping station, the normal start and stop levels may not be reduced as much, and the deluge preparation level 26 may be higher. in some cases the deluge preparation level 26 and the normal stop level 21 may in fact be the same.

The pumping station 3 shown in Figure 2 has a flowmeter 12 to measure the outflow from the well 6. This means calculation of the pump efficiency in cubic metres per kilowatt hour is straightforward. However, not all pumping stations have a flowmetcr. It is still possible to calculate the relative efficiencies of the pumps 8, but it is not as straightforward or accurate. in this case, the control panel 10 is set to run tests of each pump 8, and the time taken for each pump 8 to pump between two levels is measured, together with the energy used, to calculate an efficiency value. The levels may be the normal stop level 21, and the test level 30, or any other appropriate levels. The tests are repeated periodically to monitor the relative efficiencies of the pumps 8 so that as with the embodiment including the flowmeter 12, the more efficient pump 8 can be used preferentially. The efficiency values are stored in the memory means so that wear can be detected over time.

The invention as claimed relates to assessment of the efficiency of a variable speed pump 8. Older pumps 8 are constant speed pumps, whereas newer ones may be variable speed. These have the advantage of reducing energy use by varying the operating speed, in practice they are operated at 100% speed, as this is the speed recommended by the manufacturer for maximum efficiency. it may be however that a lesser speed is more energy-efficient, and that the efficiency varies as the pump wears.

The system therefore has an optimisation test mode to determine the most energy-efficient speed for a variable speed pump, both on installation and throughout its life. The maximum efficiency speed is determined by running a pump 8 in a test period at several different speeds, and measuring the outlet flow and energy consumption for each speed. The efficiency in cubic metres per kilowatt hour is calculated for each speed, and an intermediate speed calculated as the average of the two speeds with the two best efficiency values. The pump 8 is then run at the intermediate speed, the efficiency value calculated, and the most efficient speed chosen as corresponding to the best of the three values. This speed is used as the default operating speed of the pump 8 unless a higher flow rate is required, for example in deluge conditions.

Typically, the system uses the test level 30 for this operation. Firstly, the system checks that the well level is above the test level 30, and that the tariff is not peak or triad. if either of these conditions is not met, the test is postponed until an off-peak tariff, and the well is above the test level 30. A pump 8 is run for a short time, such as a minute at say 60% speed, and the efficiency measured (as described above) as cubic metres per kilowatt hour. The process is repeated, for four further speeds -for example, 70%, 80%, 90% and 100%, as long as the well 6 is above the test level 30 each time. The two speeds with the highest efficiency value are chosen, and the test is run again at a speed half-way between the two most efficient speeds. The speed that gives the best efficiency value is stored in the memory as the optimal speed for that IS pump until the next test is carried out. That pump is operated at the speed unless a greater speed is required to deal with the flow. For each test run of a pump the system also stores the flow output from the flow meter and notes which speed gives the greatest flow output. This speed is stored for use if the pump is an assist pump and is brought into operation as a backup. This normally occurs when there is a deluge or the duty pump has failed, and maximum outflow is needed for overflow prevention.

Claims (4)

1. CLAIMS1. A control system for a waste water pumping station has a well for containing waste water entering through an inlet, a sensor for measuring the level of the well contents, at least two variable speed pumps for pumping waste water out of the well through an outlet and a control panel, the control panel being operable to operate the pumps to maintain the level of the well contents between a normal start level and a normal stop level, and the control panel operates the pumps periodically in an optimisation test mode to determine the most energy-efficient speed for each pump, and then operates each pump at that speed in normal conditions, until the next time the control panel operates the optimisation test mode.
2. 2. A control system as claimed in claim 1, in which the optimisation test mode is enabled when the well level is at a test level, and the energy tariff is the off-peak I5 tariff.
3. 3. A control system as claimed in claim 2, in which the optimisation test mode comprises: (a) running a pump for a specified time at a given speed; and measuring the flow rate and energy used; dividing the flow rate by the energy to give an efficiency value; (b) waiting for the well level to reach the test level again; (c) running the pump for the specified time, at a different speed, and measuring the flow rate and energy used; dividing the flow rate by the energy to give an efficiency value; (d) repeating step (b) and (c) for up to three further speeds; (e) choosing the two highest efficiency values; (f) repeating step (b) at a speed half-way between the speeds with the highest values (g) taking the speed that produced the highest efficiency value as the optimally efficient speed and setting this as the default running speed for the pump until the next test is carried out.
4. 4. A control system as claimed in claim 3, in which each pump is operated at the default running speed unless a greater speed is required to deal with the flow.A control system as claimed in claim 3 or claim 4, in which the system includes a flow meter, and for each test run of a pump the flow output from the flow meter is also stored, and the system determines which speed gives the greatest flow output, and stores it for use if the pump is an assist pump and is brought into operation as a backup.