Keep on running

Regular maintenance of pumps and pumping stations in water and wastewater installations is vital if problems such as leaks, flooding and blockages are to be avoided. Not only do such problems often require emergency action, they can be a major embarrassment when companies are confronted by irate customers.

This was a problem faced by Scottish Water at its Charlotte Street sewage pumping station in Kirkcaldy, the last in the sewerage network before the treatment works and which handles large flows.

The pumping station suffered regular pump blockages, with resultant flooding of local businesses, pollution of the seafront and release of unpleasant odours into the community at large, according to Willie Burnside, an asset planner for Scottish Water: “Eight skips’ worth of ragging are removed from the Kirkcaldy treatment works on a weekly basis, and, as such, Charlotte Street handles the bulk of this waste.”

Operational costs

The frequent blockages at the site required the pumps from the station to be lifted for de-ragging using a crane in a public car park. As a result, operational expenditure was extremely high and the frequent maintenance meant new pumps had to be installed three times in four years.
“Over the last few years the blockages in the pumping station have resulted in repeated replacement of pumps at about £200,000 a time,” said Burnside.

In an effort to overcome a rising level of expenditure, Scottish Water installed a Retroflo RPC2000 pump control system in June 2009. This was integrated with the station’s existing programmable logic controller (PLC) ? without interrupting the pass forward flow to the Kirkcaldy wastewater treatment works (WTW).

The system interfaces with the WTW, enabling onward flow to be reduced to 80%, or stopped entirely when upstream conditions dictate. Signals from the pump-control unit are also transmitted to Scottish Water’s Open Enterprise telemetry monitoring network.

Constant level pumping is the mode determined most suitable for this surface pumping system (SPS). Each time a pump is called to run, a short maximum boost cycle is initiated, ensuring any debris that may have settled in a pump or its pipework is flushed clear.

The flushing feature is also repeated at each pump stop. Under normal conditions ’cycling pump duty’ is selected and the operation rotates around the two pumps based on an integral run-timer, currently set for 60 minutes, alleviating blockages and reducing pump wear.

In its first six months of operation the RPC2000 performed more than 100 pre-blockage detection and self-cleanse routines, all of which returned the pumps to optimum performance. During this period there were no recorded blockages and therefore no need for reactive maintenance call-outs.

The pump control system pre-empts a blockage by comparing real-time data against data benchmarked to actual performance of the pump in the well, rather than to the flow curves.

The system continuously monitors motor power, current, torque and speed, together with wet well level and volumetric flow, and compares this with up to 10,000 pre-determined reference points per pump to identify and correct operational and efficiency issues.

On detection of debris build-up on the pump impellor, the RPC initiates a self-cleansing routine in which the pump is stopped and reversed within site-defined parameters. This ensures that the impeller remains debris-free and maintains pump performance.

Another principal cause of blockages at Charlotte Street was the lack of self-cleansing velocities being generated in the flat rising main, resulting in a build-up of sediment and solids.

An intelligent flushing cycle on the Retroflo system uses historical operating data to predict the incoming flow rate, allowing the well to fill to a pre-set level before initiating a rapid well-emptying sequence.

The condition of the Charlotte Street wet well was inspected by a Scottish Water team three months after the Retroflo installation. The results, said Burnside, were better than expected: “You wouldn’t have thought it was a wastewater wet well at all. There were no scum lines, no ragging, no grease, not a hint. It’s the cleanest I’ve ever seen a wet well.”

After almost a year in operation Burnside concluded: “Since Retroflo was installed there have been no operational call-outs to the pumping station, no cranes required to lift out the pumps and, most importantly, no flooding as a result of a failure in the pumping station. It also keeps the rising main clear, which helps reduce the odour problems.”

The operating expense (OPEX) on the system equates to a minimum of £30,000 annual savings, added Burnside, providing a capital expenditure (CAPEX) payback within 12 months, while the reduction in blockages has also “dramatically” reduced energy costs.

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 significant decrease in power during normal operation.

These benefits have not gone unnoticed by Scottish Water R&D manager George Ponton, who said: “We are currently developing a business plan that could generate up to £500,000 annual OPEX savings, based on the work at Charlotte Street.”

Internal matters

One big challenge faced by maintenance engineers, according to AxFlow, is not knowing what the condition of a pump’s internal components will be until the complete unit is stripped down. Externally the pumps may look to be either in a reasonable state or not worthy of being rescued.

In one recent example, the company’s engineers were called in by the Environment Agency’s contractor AMCO to the East Hull pumping station. A previous external inspection of the four Gwynnes 24in and 48in mixed-flow, vertical, land drainage pumps, operating at the pumping station since 1949, had indicated that the pumps were old and in need of replacement and had exceeded their design life capacity.

When later removed from service by AxFlow for evaluation, however, it was clear that they could be renovated to a standard that would meet the future requirements of the Environment Agency. Moreover, AxFlow found that it could undertake the renovation project in a timeframe quicker than a full replacement programme and at a more competitive price.

“In late 2009 we undertook an on-site evaluation of all four pumps in the East Hull works,” said AxFlow’s Mark Redgrove, “and made recommendations as to what work would need to be undertaken to put them all back into a reliable and efficient operating condition.

“In mid-January 2010, with assistance from AMCO, we removed the four pumps and brought them to our Huddersfield works where we did a full strip-down to allow dimensional checks to be made, along with an overall condition report.”

The two 24-inch pumps, which were the first to be refurbished, were found to be in a reasonable condition, according to Redgrove. The bronze bushes, sleeves and grease-lubricated bearings were fairly well worn but were serviceable, and the impellers had been repaired at some stage, so there was no reason for additional work.

Refurbishment work

The two 48-inch pumps needed more work and it was necessary to cast a new wear ring for the back of the impeller on the third pump as the original was cracked. When the impeller race was removed on this pump it completely disintegrated.

AxFlow has balanced the impellers and replaced all of the wearing parts, such as bushes and sleeves on the shafts. By their design, each of the pumps contains three or four shafts as the water flow from top to bottom of the sump is seven metres, so the shafts are in sections.

In addition, all the white metal Michell bearings have been overhauled, the pads re-metalled, thrust collars skimmed and all the internal surfaces of the housing shot-blast and repainted. Running together, the refurbished pumps will a have the capacity to handle 7.5m3/second and they will be expected to run for periods up to three hours at a time, twice a day.

“The key to the success of this contract was that we were able meet the Environment Agency’s request to work all four pumps at once, as opposed to instigating a rolling programme,” said Redgrove.

“Because of the size of these pumps, weighing up to12 tons, it was necessary to dismantle them on site then rebuild them back in our workshops once the refurbishment jobs had been undertaken to check tolerance and dimensions. We then had to dismantle them again for returning them to the pumping station and once again rebuild them on site.”

To improve efficiency and reduce downtime at its sewage-treatment works in Dorking, Thames Water has installed an EZstrip maintain-in-place pump, from NOV Mono,

The previous equipment at the site, which transferred sludge from the primary settlement tank into a picket fence thickener, was failing and thus causing significant downtime. Mono’s EZstrip sewage pump, which can be fully stripped down and maintained in just 30 minutes, has overcome the problem.

The water company says it would normally have taken a full day just to disassemble the previous equipment, so after de-ragging the pump and putting it back together, it had lost around two-and-a-half days in total. And as the Mono pump requires no dismantling space, it was easily installed into the same space as the existing pump.

Gavin Parker, sewage works operator at Thames Water, said that “maintenance and downtime at the works has been significantly reduced”.
The chamber of the pump has a two-piece design that can be opened in less than one minute, the suction chamber and rotating parts de-ragged and the chamber reassembled with a spanner and an Allen key. The operation can take less than two-and-a-half minutes, depending on the severity of the rag blockage.

ITT Water & Wastewater has secured a £1.5-million direct contract with Yorkshire Water (YW) for the repair and maintenance of ITT’s Flygt pumps for ’shining star, critical and large pump sites’. The 12-month initial contract has a potential five-year extension and covers all pump maintenance, including product replacement, and, for the first time, pump repair and rental.

For the past 15 years YW has outsourced all maintenance to a third party, but now has set up a pump repair framework to deal directly with suppliers to achieve efficiency savings and exert more control over activity and spend.

According to ITT field sales engineer Mark Collinson: “This is a significant result for us and a great achievement for the team. With ITT Water & Wastewater’s total capabilities we can offer customers efficiency and cost savings combined with a reliable, high-quality service.”