PUMPS AND PUMPING SYSTEMS
Pumping station stands the test of time
Sustainability may be a relatively new concept, but a 20-year old Scottish Water installation demonstrates that the best systems run and run. Andy Wilson of Hidrostal explains
ack in 1992, Lothian Regional Council (LRC) was faced with the challenge of installing a new wastewater pumping station (PS) at Prestonpans at which time using a variable speed drive (VSD) to operate a sewage pumping station was a bit like science fiction.
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The new PS had to accept an inflow ranging between 70 – 720l/sec raw unscreened sewage, from both the local catchment and network PSs at Cockenzie and Tranent. There were also concerns about odour and available space. Design thinking at the time was centred on different combinations of fixed-speed pumps. Variable speed technology was considered expensive, unproven and risky. There were two main choices:
1.
Install the minimum number of pumps, all of the same size, in a duty/assist/assist/ standby configuration. However, this offered no flow turn down under dry weather flow (DWF) conditions. To avoid excessive start-stops per hour, a bigger wet well surface area was required to allow a large enough effective pumping volume between pump start and stops. The advantages of this option were that it required the minimum number of pumps, spares were easier to keep as they were all for the same model, it was an economic option. The disadvantages were that it would not be able to achieve
much flow turndown and a large wet well was required, which can lead to the accumulation of settled and floating solids and increased odour issues.
2. A cascade solution using 15%, 35% and 50% of flow pumps. This offers a turndown with good efficiency and lower energy consumption during DWF. With standby requirements, six pumps needed to be installed in the wet well. The advantages were: better flow turndown with higher efficiency across the flow range and less chance of blockage by working pumps closer to best efficiency point (BEP) more often. The disadvantages were that is was more expensive, required two more pumps, needed a larger footprint PS to accommodate the additional pumps and required more pump models and variables for spares and maintenance. It also needed a larger control panel with more starters.
LRC considered the advantages of combining both options - high efficiency with a large flow turndown, but using a minimum number of installed pumps. Additionally there is no inlet screen so a large free passage is desired to handle the raw sewage.
The engineers came across Hidrostal’s Prerostal system. This simple and effective technology effectively offers eight pumps
Actuated sluice valve chamber at Prestonpans
in four, using twin-speed motors, a proven technology at this time. Flow turndown is now easily achievable. The pumps also feature an impressive 145mm free passage with a 79% hydraulic efficiency, a rare combination in 1992 and even today.
Actual 20-year whole life cost of Prestonpans PS, based on 80% in station usage time at low speed - 22KW, with no recorded unscheduled maintenance
wwtonline.co.uk
Prerostal works on the basis of aiming to remove sewage as fast as it enters the wet well. This then delivers flow matching and a continual cleaning cycle on every pump down. At high wet well level, sewage flows over a weir wall. At low level, an inlet channel in the pump basin guides the flow in a single direction. Pre-rotation (pre-swirl) then occurs at lower sump levels. This then reduces the flow output from the pumps and flow matching occurs. At high inflow and high wet well level, full pump performance is delivered. Conversely at low wet well level and low inflow, reductions in flow output of up to 50% are naturally possible. At the same time self-cleaning of the pumping chamber is taking place assisting with blockage free operation and energy savings. By using twin-speed motors Prerostal has a minimum number of pumps to handle the flow turndown efficiently. This allows a reasonable size for the wet well, without the concerns of too many start-stops per hour. The wet well basin is part of the system package, saving on design time and all pumps are the same model simplifying maintenance requirements. Operating levels are typically very close on a Prerostal system, negating the need for a deep pumping station, reducing engineering costs. Also by keeping the station static lift to the minimum further energy savings are made. The advantages are clear. Energy savings achieved through running blockage free and operating closer to BEP, particularly on flow turndowns as well as energy saving from a minimum static head designed pumping station.
August 2013 Water & Wastewater Treatment 19
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