- 9 May 2007 -
Is the additional cost of original spare pump parts worthwhile?
The decision regarding the purchase and use of original spare parts or cheaper parts from replica manufacturers is one that all pump operators face at one time or another. For one user in Germany , the operator of a sewage treatment plant, the decision is clear cut, as Jörg Gertz reports.
Sewage works utilize progressing cavity pumps in numerous process stages. These pumps are always the first choice if the pumped medium is contaminated with solids. Within the waste water treatment process, this is particularly the case at the plant inlet for raw sewage and the cleaning stages for the transport of the sludge. At the inlet the pumps must be able to cope with the most diverse solids. At the same time the capacity of the pumps must be able to adapt to variations in flow of the waste water. High discharge pressures are, however, of secondary importance.
Sludge cleaning
In the subsequent cleaning stages sewage sludges have to be transported. These are initially still of very low viscosity; however, since the sludges will have started to rot, they have a tendency to generate gas. As well as the ability to pump low-viscosity sludges, the pumps therefore must not be sensitive to the development of gas in the medium.
During the further course of cleansing, the sewage sludge is thickened, fully rotted and then drained. The slurry pumps used here must not only be capable of building higher outlet pressures but also cope with the increasing viscosity of the sewage sludge. Mechanically drained sewage sludge frequently has a dry substance content of between 18% and 45%. Afterwards, the pumped liquid is sometimes already 'slump-free' - the solids content being so high that it can be cut and shovelled. Only pumps that are especially designed for these media ('high-viscosity sludge pumps') are then able to work reliably.
During the chemical cleaning process, special progressing cavity pumps act as dosing pumps to convey additives such as acids, alkali, lime or polymers. Exact dosing is necessary for the optimum reaction as well as to ensure economic operation of the plant.
Pump choice
For almost all these applications, the treatment plant operator has a choice between different equipment manufacturers. In later operation he or she is often confronted with the decision of whether to use original spare parts or parts from replica manufacturers.
Operators of sewage plants are advised not to base their decision regarding equipment solely on the purchase price. The suitability of the pump, durability of materials, reliability, maintenance friendliness, competitive operating costs and long service intervals are more important. Whether the parts of a pump assembly have to be replaced every 1000 operating hours or every 8000 operating hours quickly decides over many thousands of Euros of annual maintenance costs. Constant efficiency during long-term operation is also a decisive characteristic of a reliable and economical pump. Inferior designs lose performance when solid deposits build up over a period of time. Poorly designed bearings are susceptible to wear caused by solids and abrasion. Here it is worthwhile closely inspecting the technical and design criteria in detail. Are impellers and mouth openings 'immune' to the deposition of solids and to blockages? Which constructional features prevent the bearings from wearing? Optimized impellers, retainers and swirl regulators as well as flush channels in the bearing are examples of designs that prove their worth under operating conditions.
Choosing spare parts
Similar criteria apply to the use of spare parts. In progressing cavity pumps these are mainly the rotor and the stator. Both parts are subject to wear and have to be regularly maintained or replaced. Equally regularly, the operator is faced with the decision of utilizing more-expensive original spare parts from the manufacturer of the pump or cheaper parts from other suppliers.
For Peter Hentschke, the operator of the sewage treatment plant at Fürstenfeldbruck near Munich , Germany , this decision is now clear. The sewage plant has used pumps from Allweiler AG for the entire sludge line for more than 20 years. Equally, the sewage plant obtains wear parts from this manufacturer and not from other suppliers. In both cases the purchase price is not the sole deciding factor. As Hentschke explains: "Of course every supplier must fall within a price range. As far as we are concerned, however, of equal importance at least are the quality of spare parts, how quickly they can be supplied and the service on site". A higher price can be justified by superior quality and the resultant longer service life. In his experience original spare parts have even more advantages: on the one hand, the manufacturer's warranty for the complete assembly remains valid and, on the other, at least as far as Allweiler is concerned, competent individual support is provided.
There are further aspects to be considered, in particular for progressing cavity pumps. Only Allweiler is able to supply ductile coated rotors for its pumps. These rotors have a significantly longer service life particularly where abrasive additions are present. The same applies for the slotted stators available as a factory-fitted option. This type of pump is used in the Fürstenfeldbruck sewage works. As a result of re-tensioning, the service life is increased by approximately 50%. In Hentschke's experience: "The higher price justifies itself especially when media with abrasive particles are present".
The Fürstenfeldbruck sewage plant is constructed to cater for 100 000 person equivalents and cleans on average
3 650 000 m 3 of waste water per year. That means approximately 8000 m 3 per day in dry weather. Since the town of Fürstenfeldbruck maintains a predominantly mixed system of sewers, a maximum of 1100 litres/s can reach the sewage plant. From that amount, 500 litres/s are processed in the sewage plant and 600 litres/s are taken off into rainwater basins. The average sewage freight at the inlet is about 2500 kg/day. 3600 tonnes of rotted sewage sludge are produced annually. The gas produced is used to generate the treatment plant's own electricity supply and for heating energy. In addition to other pumps, 13 progressing cavity pumps and a macerator from Allweiler operate in the plant. As a result of the pumps' modular system, with its numerous identical parts, the sewage plant only needs to hold a few wear parts in store.
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