Running pumps in parallel and in series

During a gas carrier discharge, cargo pumps are usually run in parallel but, when a refrigerated ship discharges to pressurised storage, cargo tank pumps are run in series with a booster pump, as explained in 7.7.3.

When pumps are run in parallel, their individual pump characteristics can be com­bined to give, for example, a flow/head curve for two, three or four pumps when running together. Taking the pump characteristic as given in Figure 4.3, the flow/head curve for running two pumps in parallel can be easily plotted by doubling the flow rate at the appropriate head for a single pump. This is shown in Figure 4.4. Similarly, when running three pumps in parallel, the flow rate at the appropriate head can be obtained by multiplying the single pump flow rate, at the same head, by three. Thus, a series of curves can be built up from the pump characteristic curve of a single pump.

 

Figure 4.4 Centrifugal pumps in parallel — combined characteristics

Figure 4.5 Centrifugal pumps in series — combined characteristics

When pumps are run in series, again the individual pump characteristics curves can be combined to give the appropriate curve for the series configuration. Figure 4.5 shows how this can be done using, for example, two similar pumps in series (see again Figure 4.3). This time, for each value of flow rate, the appropriate head developed by a single pump is doubled to give the resultant head.

The foregoing arguments relate only to pump performance. For a full assessment of a ship's discharge performance the effect of head difference from the cargo tank to the manifold and of pipeline resistance between cargo pump and manifold should be subtracted from pump performance.

The cargo flow rates achieved by any pump or combination of pumps will depend upon the back pressure encountered due to static head (difference in liquid levels of receiving tank and tank being discharged) and the resistance to flow in the pipe-line. To determine the flow rate for a particular pipeline set-up, the shore pipeline flow characteristic must be superimposed upon the ship's pumping characteristic. This is dealt with in 7.7 but it should be noted that the system resistance may be steep enough to restrict the flow shown in Figures 4.4 and 4.5.

The minimum necessary pumping power should be used in order to reduce heat input to the cargo and to limit the rise in saturated vapour pressure of the delivered cargo (see 7.7.2).

Deepwell pumps

Deepwell pumps are the most common type of cargo pump for LPG carriers. Figure 4.6 shows a typical deepwell pump assembly. The pump is driven electrically or hydraulically (through a sealing arrangement) by a motor which is mounted outside the tank. The drive shaft is held in carbon bearings inside the cargo discharge tube and these bearings are lubricated and cooled by the cargo flow.

The centrifugal impeller is mounted at the bottom of the cargo tank and frequently comprises two or three stages together with a first stage inducer: this latter is used to minimise the NPSH requirement of the pump. Shaft sealing at the cargo tank dome

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