Sewage blockages cut by pump control

How a sewage pumping station virtually eliminated blockages and vastly reduced operational expenditure by fitting a pump control system from Retroflo.

Nothumbrian Water's Skinningrove sewage pumping station
Nothumbrian Water's Skinningrove sewage pumping station

Skinningrove Sewage Pumping Station is on the UK's north east coast and is situated in an area of natural beauty. As such it has been of concern to Northumbrian Water because of the risk of discharges through pump blockages. The introduction of Retroflo's pump control system has not just maintained consents through reduction of blockages, but has also dramatically reduced the station's energy costs.

This pump control system was specifically designed to control pumping stations as a whole, rather than just the components inside. Through continuous dynamic monitoring of the pump characteristics over a range of wet well levels and pump speeds, the system was able to use the full range of pumping data available at Skinningrove to optimize the performance of the station. It is also thought to be the first product on the market to offer pre-blockage detection, consent security, intelligent flushing cycles, periodic efficiency testing, and asset data storage, all within a single system.

Blockages occur in sewage pumping because of the gradual build-up of debris on pump impellers. This causes inefficient pump operation and eventually leads to a blockage, resulting in time consuming and costly call-outs.

During the first 12 month period from installation of the pump control system at Skinningrove, over 1000 successful pre-blockage detection routines were implemented. All returned the pumps to optimum performance, so no operator/maintenance intervention was required to remove blockages. The pumps were also regularly inspected during this initial period and a massive improvement regarding the cleanliness of the impellor was observed.

Improved conditions

As the site was already fitted with variable speed drives (VSDs), the principal reason for installing the pump control system was blockage alleviation. However, by initiating pump reversal cycles on detection of partial blockages and returning the pump to optimum operating conditions, the improved pump performance equates to a 12% decrease in power during normal operation. At Skinningrove this has resulted in a savings of 0.45p every 12.2 sec. This energy saving – coupled with the savings from no call-outs – meant the capital expenditure of installing the system was repaid within the first year.

A further benefit was a vast improvement in wet well condition because of the initiation of Retroflo's Intelligent Flushing Cycle. This ensures that wells are kept cleaner, so alleviating solids settlement and septicity problems. This, in turn, reduces the threat of blockages to the pumps and reduces the need for routine cleansing.

Scope of work

The scope of modification work undertaken on the Skinningrove site was to retrofit the company's RPC_2000 controller to the existing infrastructure. The patented system's Pre-Blockage detection provides dynamic monitoring of the pump operating characteristics. When a pump is monitored outside of its normal operating characteristics, indicative of a potential pump blockage event, corrective action is taken to remedy the problem – i.e. reversal of the suspect pump. If successful, the blockage is removed and normal operation resumes. However, following a maximum of three unsuccessful reversal cycles, the suspect pump is locked out – so saving operating costs by ensuring that pumps are not run inefficiently. Following pump lockout, the standby pump assumes duty.

Should the duty pump trip, the locked-out pump is reset and operated. This reset ensures that the station will remain pumping, albeit inefficiently, allowing consents to be maintained. An alarm is forwarded to the Regional Telemetry system, informing the operators that action is required.

The Intelligent Flushing Cycle enables selection by volume or time, which ensures adequate dilution of settled solids, enabling the solids to be passed forward. It also allows regular rising main self-cleansing velocities to be achieved to prevent deposition in the main and fat build-up in the well. This can be triggered when electricity tariffs are low.

Work at the site also included the optimization of the existing control system following works flow instrument failure. The works flow meter is located at the treatment works, the flow signal being generated to the transfer station PLC via the communications network. Originally, when the flow signal to the transfer station PLC was lost, the pumps were inhibited to protect the works from unknown flow rates. Immediate manual operator intervention was required to prevent spill events. Under the current modified regime, upon flow meter failure, the pumps remain operational. The duty pump responds to the normal duty pump start and duty pump stop level set points, and operates at a fixed speed selected to best deliver the required consent flow rate without compromising the works. As with the consent alarm, this new regime reduces reactive maintenance costs.

Operating philosophy

The station operates on a duty/standby basis. Whenever a pump is called to run, the pump boosts (50Hz) for 30 sec in order to get velocities through the system.

The pump speed is then controlled to maintain a pre-determined level within the wet well – currently set at 1.60 m. This control method best achieves maximum efficiencies, as pumped flow theoretically matches influent flow. When inflow increases, the level in the wet well also increases. Therefore, the pump speed is increased, as is delivered flow. Alternatively, when inflow decreases, the level in the wet well decreases, the pump speed is decreased and delivered flow also decreases.

Blockages occur in sewage pumping because of gradual build-up of debris on pump impellers. This causes inefficient pump operation and eventual breakdown.

Blockages are virtually eliminated by Retroflo's Pre-blockage Detection, which ensures that the pumps operate efficiently at all times. As is clear, this pump is very clean.

The speed of the duty pump is controlled to deliver a minimum of 60 l/s. When the influent flow drops below this set point, the level in the wet well will eventually fall below the duty pump stop level. Then, the duty pump boosts at 100% speed for 10 sec to prevent both solids deposition in the line and rag balling in the riser pipe. Duty is then rotated.

During normal conditions, with the duty pump in operation and the monitored wet well below high level, if the Pre-Blockage Detection facility detects the pump operating outside its normal operating characteristics, remedial action is taken:

• The duty pump is stopped. The system does not perform a boost operation prior to stopping to prevent compounding the potential blockage event. At this point Pre-Blockage Detection monitoring is disabled.

• The duty pump is operated in reverse (at 60% speed) for 30 sec to try to remove the blockage, after which, the duty pump is operated forwards at 100% speed reference for 30 sec. Normal operation resumes and Pre-Blockage Detection is re-enabled.

• If the blockage is still evident, the reversing cycles continue for a maximum of three attempts, after which the suspect pump is tripped and locked-out by the system. An alarm is forwarded to the Regional Telemetry system.

• The standby pump assumes duty and is operated as the duty pump. Should the control system detect this pump operating outside of its normal operating characteristics, a maximum of three pump reversal sequences are implemented, after which the pump continues to operate and an efficiency alarm is forwarded to the regional telemetry system. This provides indication that, despite there being no current threat to the consent, the pump is operating at reduced efficiencies. This enables action to be taken to address the problem.

• If at any time a pump is locked-out by the system, and the operating pump becomes failed for other reasons, the locked-out pump is reset and operated by the system, irrespective of efficiencies and costs, to prevent compromising the consent.

The duty pump's speed is controlled to deliver a maximum of 82 l/sec. When the influent is greater than this, the wet well level will rise to the ‘High Level’ set point. At this point, a high level alarm is forwarded to the regional telemetry system. Pre-blockage Detection is disabled and Consent Alarm monitoring is effective. The system continues deliver 82 l/sec for the duration of the high level event. This is to ensure that greater emphasis is placed on the importance of processing flows during storm events, rather than reduced efficiencies and increased operational costs through partial blockages. It is not a good idea to perform repeated reversal sequences in storm conditions because of the increased risk of spillage.

System benefits

A small blockage in the pump body means a greater risk of further debris passing through, which tends to collect and add to the original problem. Pre-modification, this happened until the pump could no longer deliver consent flow, or the pump protection caused tripping. This resulted in a breakdown requiring pump removal for costly un-blocking. Removing the smaller debris upon initial pick-up prevents the large build up.

Over a year, the control system has performed over 1000 Pre-Blockage Detection pump reversal sequences. Not all of these potential blockage events would have resulted in maintenance intervention – however each of the reversals allowed the pump to be returned to its efficient state automatically. Early stage blockages are eradicated, which also avoids any related health and safety issues.

When in high level conditions, the pumps are controlled to maintain a forward flow to the works of 82 l/sec. Operating optimally, each pump works at around 96% speed reference to deliver 82 l/sec with slight variations. Over time, settlement intensifies and the pumps pick up debris. Ordinarily, (when not in high level conditions), Pre-blockage Detection would detect these partial blockages and perform reversal cycles; however provided that consent flows are achieved, reversal cycles aren't implemented because of the increased risk of spillage. Consequently this debris continues to impede the pump's performance and the control system increases the speed of the under-performing pump to deliver the required 82 l/sec. The result is a pump operating at 100% speed reference (as opposed to 96%) to deliver the equivalent flow rate.

Theoretically, the decrease in power consumption from 100% pump speed to 96% pump speed can be calculated.

The calculation shows that a 4% reduction in speed is in fact a 12% reduction in power. Without Pre-Blockage Detection, and the facility to detect and counter partial blockages, when partially blocked the pumps will eventually be operated at maximum speed, resulting in increased operating costs. This situation would go undetected until a catastrophic blockage and breakdown occurs. Pre-blockage Detection ensures that the pumps operate efficiently at all times, whilst also reducing the potential for breakdown.