|Pumps and pumping systems|
Pumps and pumping systems
At any one time in a ship's machinery space there will be a considerablevariety of liquids on the move. The lengths of pipework will cover manykilometres, the systems are often interconnecting and most pumps are inpairs. The engineer must be familiar with each system from one end tothe other, knowing the location and use of every single valve. Thevarious systems perform functions such as cooling, heating, cleaning andlubricating of the various items of machinery. Each system can beconsidered comprised of pumps, piping, valves and fittings, which will nowbe examined in turn.
- Pump types
- Bilge and ballast systems
A pump is a machine used to raise liquids from a low point to a highpoint. Alternatively it may simply provide the liquid with an increase inenergy enabling it to flow or build up a pressure. The pumping actioncan be achieved in various ways according to the type of pumpemployed. The arrangement of pipework, the liquid to be pumped andits purpose will result in certain system requirements or characteristicsthat must be met by the pump.
A pumping system on a ship will consist of suction piping, a pump anddischarge piping (Figure 6.1). The system is arranged to provide apositive pressure or head at some point and discharge the liquid. Thepump provides the energy to develop the head and overcome any lossesin the system. Losses are mainly due to friction within the pipes and thedifference between the initial and final liquid levels. The total systemlosses, HTOTAL are found as follows:
HTOTAL — HFRSUCT + HFRDIS + HDISTANK + HSUCTTANK
where HFRSUCT = friction head loss in suction piping
HFRDIS = friction head loss in discharge piping
HDISTANK — height of discharge tank level above pump
HsucrrTANK= height of suction tank level above pump
(negative when tank level is below pump suction)
All values are in metres of liquid.
The system head loss—flow characteristic can be drawn as shown in Figure 6.2. The system flow rate or capacity will be known and the pump manufacturer will provide a head—flow characteristic for his equipment which must be matched to the system curve. To obtain the best operating conditions for the pump it should operate over its range of maximum efficiency. A typical centrifugal pump characteristic is shown in Figure 6.2.
An important consideration, particularly when drawing liquids from below the pump, is the suction-side conditions of the system. The determination of Net Positive Suction Head (NPSH) is undertaken for both the system and the pump. Net Positive Suction Head is the difference between the absolute pump inlet pressure and the vapour pressure of the liquid, and is expressed in metres of liquid. Vapour pressure is temperature dependent and therefore NPSH should be given for the operating temperature of the liquid. The NPSH available in the system is found as follows:
The above values are usually expressed in metres head of sea water.The pump manufacturer provides a NPSH required characteristic forthe pump which is also in metres head of sea water (Figure 6.2). Thepump and system must be matched in terms of NPSH such that PPSHrequired is always greater than NPSH available. An insufficient value ofNPSH required will result in cavitation, i.e. the forming and collapsingof bubbles in the liquid, which will affect the pumping operation andmay damage the pump.