Highlighting the hidden extras
According to Hidrostal, some pump manufacturers' whole life cost models fail to place sufficient emphasis on maintaining long-term efficiency and the cost of clearing blockages
One way of reducing costs has been to cut the pump supplier base, typically around 25 to less than five manufacturers, with water companies regularly entering framework agreements with pump suppliers. To benefit from these agreements, suppliers are working on minimising their equipment's whole life costs. Successful companies are chosen on the basis of the pump costs estimated over 20 years, rather than the cheapest initial cost. In return water companies gain a guaranteed supplier, at reduced but fixed prices, as part of an agreement which often includes maintenance provision.
However, there is no universal life-cycle cost model in the UK water industry. Each pump manufacturer has an in-house model, often built around its own priorities. Details are rarely shared, but in general costs are taken to include purchase price, energy use and maintenance. These are the most obvious costs, and are easy to quantify and track. But they do not tell the whole story, particularly for sewage and wastewater pumps. There are other issues, which at the moment represent significant hidden costs, and these need to be incorporated into any future comprehensive life-cycle cost model according to Derek Jackson, managing director of Hidrostal.
It is essential energy costs arising from loss of efficiency caused by blockages or wear are taken into account. Energy frequently represents more than 80% of the lifetime costs, so any evaluation of this expenditure will be heavily influenced by energy costs and pump efficiency.
Many companies base their efficiency statistics on data for a new pump working at the specified duty point. This model is too simplistic for wastewater pumps. The specified duty point, which is nearly always the maximum output of the station with all pumps running, may only be reached for a small percentage of the time. It is far more accurate to use the efficiencies at the usual operating points on multiple pump stations. It is more time consuming to gather the data, but good preparation will save money over the lifetime of the station.
Sometimes pumps become partially blocked. This not only reduces efficiency, but also cuts output. Consequently running hours increase to pump the same volume, thus raising energy consumption. This extra energy cost will be increased further after April 2001, when the Climate Change Levy is added to all energy bills. This increase has been estimated at an extra 11% for a company paying 4p/kWh.
A common mistake is to assume maintenance costs apply purely to routine maintenance and spare parts. The reality is that unscheduled maintenance for unblocking pumps represents a significant extra cost.
When pumping wastewater it is essential to have a large free/ball passage so the pump can handle large solids easily. It is often assumed that a pump with the ability to pass a 100mm diameter solid will also be able to handle rags and fibre easily. In many cases this is not the case, and experience has shown that rag handling is the biggest problem.
With a conventional sewage/wastewater impeller, flow approaching the leading edge is approximately at right angles to the blade and continues to push or hold rags onto the blade, where they accumulate. A 'ragged-up' impeller reduces efficiency, leading to higher energy consumption. Output is also reduced resulting in longer operating periods. The cost implications for unscheduled visits to unblock pumps also need to be taken into account. Ragging causes vibration, which may lead to premature seal or bearing failure and opening up of wear ring clearance.
Hidrostal's single blade screw centrifugal impeller prevents blockages by having a large ball passage and a specially shaped inlet for handling rags and fibre. The pump's externally adjustable liner maintains optimum clearances and high efficiency over prolonged periods. At the same time the pump's hard liner reduces wear. A non-blocking impeller maintains a high efficiency and output, thus minimising energy consumption. Unscheduled visits are kept to a minimum and the lack of ragging and blockages means the pump continues to run smoothly. This extends the time between repairs, increases plant availability, and reduces spares and repair costs.
An example of the potential hidden costs associated with unblocking pumps was presented by Derek Jackson at the recent VDMA pump congress in Karlsruhe, Germany. Assuming it takes two men two hours to make the round trip and make good the repair, unblocking a single pump will cost an estimated £158. If the failure occurs once a month the annual cost for unscheduled maintenance would be around £1,900. If blocking continued at this level over the 15-year life of the pump, costs for unscheduled maintenance at current values would be £23,000 for a pump originally bought for around £1,500. Discussions with water companies have shown that, in some cases, these hidden costs could be as high as £250-300 per month, per blocked pump.
Hidrostal has found smaller organisations, such as municipalities, will often accept blocked pumps as a fact of life. Larger organisations, such as water companies are now realising that collecting data for inclusion in a cost database will help them analyse these hidden costs and they are actively evaluating their pumps and systems - replacing them where necessary - to avoid the cost of unscheduled maintenance.
Some pump designs incorporate a cutting device, this lowers the efficiency and eventually becomes ineffective due to wear. Experience has convinced Hidrostal's engineers that the ability to handle rags is a function of the blade geometry at the inlet edge. The single impeller design means rags and fibre are gradually drawn through the pump without snagging on the leading edge so a build up of rags does not occur and the passage remains free. In trials, Hidrostal 150mm pumps have easily passed a pair of workmanÕs trousers and common blocking agents such as nylon tights and stockings.
Pump blockages can also arise from a poorly designed sump which allows rags to accumulate, from the sewer characteristic and from sewers handling both sewage flows and stormwater. During a storm, it is not unknown for flow meters to register a drop in flow of between 5-25% due to blockages. This is not acceptable to the public and storm pumps must be able to deliver the required output. During autumn 2000, residents along the south coast learnt first hand what happens when pumping stations are unable to handle the combined inflows resulting from storms. Houses were flooded with storm water and sewage when the pumps were overwhelmed. It is vitally important not to lose capacity under storm conditions.
"I would encourage water companies to embrace the least whole life costs argument entirely, and not just to grasp the obvious" said Derek Jackson. If water company managers do a more in depth analysis, then the benefits of a more efficient pump will become clear. However, it is important to balance the need for both efficiency and the pump's fitness for its intended use. "When looking at life cycle costs, the analysis is only meaningful if an accurate assessment has been made of the pump's utilisation, its long-term energy use and its reliability" concluded Jackson.