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Northumbrian Water’s Skinningrove sewage pumping station has eliminated blockages and reduced operational expenditure (OPEX) through the installation of a pump control system from Retroflo.

The Skinningrove sewage pumping station, located on the north-cast coast, had been of concern to Northumbrian Water because of a risk of discharges as a result of pump blockages.

However, the Retroflo pump control system has maintained consents through a reduction of blockages and has reduced energy costs for the station.

The system was specifically designed to control pumping stations holistically, rather than just the components within.

Before the installation, frequent pump blockages meant numerous reactive maintenance callouts to unblock the pumps.

Through continuous dynamic monitoring of the pump characteristics over a range of wet-well levels and pump speeds, the system is claimed to utilise the full range of pumping data available at Skinningrove to optimise the performance of the station.

It also offers pre-blockage detection, consent security, intelligent flushing cycles, periodic efficiency testing and asset data storage within a single system.

During the first 12-month period from installation of the Retroflo system at Skinningrove, more than 1,000 successful pre-blockage detection routines were implemented, all of which returned the pumps to optimum performance.

This meant that no operator/maintenance intervention was required to remove blockages from the pumps.

The pumps were also regularly inspected during this initial period and an improvement regarding the cleanliness of the impellor was observed.

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 per cent decrease in power during normal operation.

At Skinningrove, this has resulted in a savings of GBP0.45 every 12.2 seconds.

The pump control system has also improved the condition of the wet well because of the initiation of the intelligent flushing cycle.

Retroflo’s intelligent flushing cycle is intended to ensure that wells are kept cleaner, thus alleviating solids settlement and septicity problems.

This, in turn, reduces the threat of blockages to the pumps and lessens the need for routine cleansing.

The scope of modification work undertaken on the Skinningrove site was to retrofit the RPC_2000 controller to the existing infrastructure.

The patented system’s pre-blockage detection provides dynamic monitoring of the pump’s operating characteristics.

When a pump is monitored outside of its normal operating characteristics, indicative of a potential pump blockage event, corrective action needs to be taken to remedy the problem, such as 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.

This saves operating costs by ensuring that pumps are not operated inefficiently.

Following pump lock-out, the standby pump assumes duty and is operated.

Should the duty pump trip, the locked-out pump is reset and can be operated.

This reset action ensures that the station remains pumping, albeit inefficiently, allowing consents to be maintained.

An alarm is forwarded to the regional telemetry system, informing the operators that action is required.

This may be used to indicate that pumps are operating inefficiently and operating costs are increased.

The installation of Retroflo’s intelligent flushing cycle enables selection by volume or time, which ensures adequate dilution of settled solids, allowing the solids to be passed forward.

It also enables rising main self-cleansing velocities to be achieved at regular intervals in order to prevent deposition in the main and fat build-up in the well.

The cycle can be triggered at a period when electricity tariffs are low.

Work at the site also included the optimisation of the existing control system following workflow instrument failure.

The 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 stop level set-points and operates at a fixed speed.

This speed is selected to best deliver the required consent flow rate without compromising the works.

As with the consent alarm, this regime is claimed to reduce reactive maintenance costs by minimising the potential for site visits.

The station operates on a duty/standby basis.

Whenever a pump is called to run, the pump boosts at 50Hz for 30 seconds in order to get velocities through the system.

The pump speed is then controlled to maintain a pre-determined level within the wet well, which is currently set at 1.6m.

This method of control provides the best platform to achieve maximum efficiencies, as pumped flow theoretically matches influent flow.

When inflow increases, the level in the wet well also increases.

As a result, the pump speed is increased and delivered flow increases.

Alternatively, when inflow decreases, the level in the wet well decreases, the pump speed is lessened and delivered flow minimises.

The speed of the duty pump is controlled to deliver a minimum of 60 litres per second.

During times when the influent flow drops below this set point, the level in the wet well will eventually fall below the duty-pump stop level.

At this point, the duty pump boosts at 100 per cent speed for 10 seconds.

This is to prevent both solids depositing 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 is below ‘high level’, if the Retroflo pre-blockage detection facility detects the pump operating outside of its normal operating characteristics, remedial action is taken.

First, 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.

Second, the duty pump is operated in reverse direction at 60 per cent speed for 30 seconds in an attempt to remove the blockage, after which the duty pump is operated in the forwards direction at 100 per cent speed reference for 30 seconds.

Normal operation resumes and pre-blockage detection is re-enabled.

Third, 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 Retroflo 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 and operating costs are increased.

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 fails owing to other reasons, the locked-out pump is reset and operated by the system irrespective of efficiencies and costs in order to prevent compromising the consent.

The speed of the duty pump is controlled to deliver a maximum of 82 litres per second.

During times when the influent is greater than 82 litres per second, the level in the wet well 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 to control the pump to deliver 82 litres 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 as a result of partial blockages.

It is not advisable to perform repeated reversal sequences when in storm conditions because of the increased risk of spillage.

A small blockage in the pump body presents a greater threat to further debris passing through the pump, which tends to collect and add to the original problem.

Pre-modification of this is manifested until the pump can no longer deliver consent flow or the pump protection causes the pump to trip (over-current or over-temperature).

This results in a breakdown situation requiring operation and maintenance personnel attending the site, removing the pump from its location and getting rid of the blockage – a timely and costly procedure.

Removing the smaller debris upon initial pick-up prevents the large build-up that ultimately may lead to a breakdown situation.

Each one of these blockages, if not addressed, could eventually lead to a breakdown situation requiring operator and maintenance intervention.

When in ‘high-level’ conditions, the pumps are controlled to maintain a forward flow to the works of 82 litres per second.

Under optimum operating conditions, each pump is observed operating at approximately a 96 per cent speed reference to deliver 82 litres per second (slight variations exist).

Over time, settlement is intensified 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 being achieved, reversal cycles are not implemented in case of spillage.

Consequently, this debris continues to impede the pumps performance and the control system increases the speed of the under-performing pump in an effort to deliver the required 82 litres per second.

The end result is a pump operating at 100 per cent speed reference (as opposed to 96 per cent speed reference) to deliver the equivalent (and maybe less) flow rate.

The decrease in power consumption may be calculated theoretically; this shows that a four per cent reduction in speed equates to a 12 per cent reduction in power.

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