Analysis of the life cycle costs for pumping systems provides an invaluable basis for decision-making as well as offering insights into resource use. Here, Mariusz Cieslak of Enterprise Rail examines the real case of a group of pumping stations operating as part of the local watersupply system to provide water to high-rise dwellings.
Calculation of life cycle costs (LCC) makes it possible to take decisions based on the total analysis of purchase and usage costs over the whole period of a pumping station’s lifetime. The literature on LCC is very abundant. Articles, monographs and standards describe various aspects of the analysis in detail and indicate those industry disciplines in which this analysis can be used.
Pump designers as well as companies buying and operating pumping systems still lack suffi cient knowledge of the issues connected with LCC. The reason for this situation is the lack of readily available and independent results of
analyses carried out either for existing facilities or those being designed, which indicate the relationships between particular cost elements. The aim of this article is to show the real costs incurred during their lifetime by pumping stations supporting the local water-supply system. These are pumping sets (of water pumps) supplying individual high-rise buildings or groups of such buildingsadd .
Typical water pumping station supporting the water_supply system
Tests were carried out for 13 pumping stations located in a large urban area. They are supplied with water from the local water-supply system. The pumping stations support the water-supply system by providing water (and increasing the pressure) to the occupants of high-rise buildings, namely buildings with between 11 and 16 fl oors.
Costs were analysed connected with the design, start up and operation of each pumping station, and for the following elements in particular: pump units, hydraulic fi ttings and electric devices used to power the pump engines and control their operation. Costs connected with the purchase of land (to site the stations) and the construction of the buildings to house the pumping stations were considered minor enough to be omitted from the calculation. A lifetime of 15 years was assumed for the water pumping stations on the basis of the pump manufacturers’ recommendations.
Data, essential to calculate the LCC, were collected as a result of tests carried out and on the basis of information obtained from the building administrators. Costs were determined taking into account invoices, balance sheets and examination of analytical accounts, operation books for the pumping stations and workers’ notes. Interviews with maintenance staff helped to fi x the time necessary for supervision and maintenance of each water pumping station; it takes about 20 minutes every second day.
It was therefore possible to calculate annual operating costs using the man-hour rate. Energy costs were determined on the basis of energy meter readings or, if they were absent, readings from the frequency converter panels. Energy costs were calculated for periods in which the price of energy was constant. The analysed pumping stations are currently in operation; therefore, the cost of their possible decommission and disposal was omitted.
The costs of decommissioning a pumping station are often included in the purchase or assembly costs for new devices. During the analysed lifetime, most of the pumping stations operated without failure; a few minor failures had no infl uence on the LCC value. Failures concerned frequency and pressure converters, which were subsequently changed. The costs connected with replacement had such a small infl uence on the overall LCC values that they could be omitted.With the aim of determining lifetime costs, the cost distribution structure established by the Hydraulic Institute (HI), Europump1 and the Offi ce of Industrial Technologies (OIT)2 was adopted.
Life cycle cost – LCC – is calculated as follows:
LCC = Cinv + Ce + Co + Cm + Cs + Cenv + Cd (Eqn 1)
where Cinv = Cic + Cin: with Cinv the sum of investment costs; Cic the initial costs and purchase price (pump, system, pipe);
and Cin the installation and commissioning costs (including training)
Ce = energy costs (including operation of the pump driver, controls and any auxiliary services)
Co = operating cost (labour cost for normal system supervision)
Cm = maintenance and repair costs (routine repairs)
Cs = downtime costs (loss of production)
Cenv = environmental costs (contamination from pumped liquid and auxiliary equipment)
Cd = decommissioning/disposal costs(including restoration of the local environment and disposal of auxiliary services).
for the analysed water pumping stations.
All the costs mentioned above are incurred over a period of time. For that reason, discounting was used in order to compare those economic values that appear in different time periods. The discounting technique makes use of two tools:
1) The interest coeffi cient, pt (base point is in the future): pt = (1+k)j (Eqn 2) where k is the interest rate and j the number of periods (years)
2) Coeffi cient discounting future values that are compared to the present time:dt = 1/[(1+k)j] (Eqn 3) Values of dt (for a given k) can be obtaineds from discount tables or calculated.
The discount rate (k) includes three elements: the expected infl ation rate (p), the safe deposit interest rate (i) (e.g. investment into government bonds, treasury bills), and bonuses for undertaking risk. Economic infl ation makes it necessary
to fi x the real interest rate (discount), which was calculated as follows:
k = (i – p)/(1 + p) (Eqn 4) where k = real interest rate (discount); i = nominal interest rate; and p = inflation rate.
It was possible to determine the cost elements and include them in the LCC on the basis of the data collected and calculations made.
From data on the quantity of water used by occupants (assuming that water loss in installations in buildings is so small that it can be omitted) and values of energy supplied to the pumping station, it was possible to determine indexes of energy consumption. The index of unit energy use, e1 [kWh/m3], is defi ned as the amount of electric energy necessary to pump unit liquid capacity: e1 = ∑E/∑V [kWh/m3] (Eqn 5) where ∑E = the sum of electric energy used [kWh]; and ∑V = the sum of liquid capacity pumped [m3].
and energy use for the local pumping stations.
The effi ciency of the pumping sets has a decisive infl uence on the unit energy use, e1. Index e1 is also highly correlated with the height through which the pumping station must lift the water; therefore, it can have diff erent values for systems with diff erent lifting heights. Energy use index e2 was used to assess pumping stations having diff erent lifting heights. Index e2 [kWh/(m3•bar)] is defi ned as the amount of electrical energy used by the pump to lift unit liquid capacity through unit height.
e2 = ∑E/(∑V•H) [kWh/(m3•bar)] (Eqn 6) where ∑E and ∑V are as defi ned above and H is the head lifting height in bar.
It was possible to calculate the real index of unit water use, qj,Mk, for the actual number of occupants provided with water by pumping stations no. 10, 11 and 13. For the other pumping stations, only an approximate index of unit water use, qj (assuming from three to four people per fl at), was indicated due to a lack of data concerning the number of occupants.
These indexes appeared to be similar when an occupancy of three people per fl at was assumed. The calculated unit water and energy use indexes for the local pumping stations are given .
The tests carried out and the analyses of costs incurred during the assumed lifetime make it possible to formulate a number of conclusions.
1) Investment costs play the crucial role as the cost driver for pumping stations supporting the local water-supply system. They constitute about 50% of total LCC and are obviously paid in the fi rst year of pumping station use.
2) Energy costs account for about 30% of LCC and are spread across the whole period of the pumping station’s lifetime.
3) The costs of supervision and service are similar for the analysed pumping stations. The cost depends on the salary rates for the individual maintenance staff (which vary according to the range of additional tasks each does); that is why it varies for pumping stations being exploited by diff erent users.
4) In general, the analysed pumping stations operated without failure, or the cost of repairs was small enough that it
could be omitted. There was one emergency case noted where one of four pumps at a pumping station failed. However, it was removed without generating any costs and was therefore omitted from the LCC calculation.
5) There were neither environmental costs nor costs associated with withdrawal from use for the pumping stations investigated.
6) The tests carried out made it possible to evaluate the operational efficiency of the pumping stations by means of the analysis of energy consumption indexes.