Figure 7.8 Combined ship and shore cargo pumping characteristics — single pump
Discharging by centrifugal cargo pumps, either alone or in series with booster pumps, is the method adopted by most ships and an understanding of the centrifugal pump characteristic (as outlined in 4.2) is essential for efficient cargo discharge. Figure 7.8 shows a cargo pump Q/H curve (flow against head) superimposed on a system resistance curve (or system characteristic). The graph shows the head or back pressure in mlc (metres liquid column) in the terminal pipeline system against flow rate measured in cubic metres per hour. Increasing the flow rate increases the back pressure. This varies approximately as the square of the flow rate, giving the shape of system characteristic curve as shown. The point where the two curves intersect is the flow rate and head at which the pump will operate.
Some of the above points are further demonstrated by inspection of Figure 7.9. This diagram shows a gas carrier alongside a jetty discharging to shore storage set at some elevation. The elevation of the tank introduces the concept of static head — this being the back pressure exerted at the pump even when pumps are not running. It can be seen that the static head changes as the ship moves up and down with the tide and as the level in the shore tank alters. The diagram also indicates that the friction head loss is largely dependant on the length of the pipeline system.
Figure 7.9 Illustrations of static head and friction head
Consider now the situation where pumps are run in parallel, as would be the normal case for a gas carrier discharge. Figure 7.10 shows the pump characteristics using one pump and when using two, three or four similar pumps in parallel. (This family of curves is derived from the principles discussed in 4.2).
Superimposed on the pump characteristics are a number of system characteristics labelled 'A', 'B' and 'C'. System characteristic 'A' indicates a small diameter shore pipeline, 'B' a larger diameter pipeline and 'C' a very large diameter pipeline with shore tanks situated nearby. The latter provides the least resistance to cargo flow.
The actual system characteristic applicable at any terminal should be known to shore personnel and they should have such curves available. In preparing such graphs, personnel should note, as mentioned above, that the system characteristic can vary with the size of the chosen pipeline and with variation in the pipe-lengths from the jetty when alternative shore tanks are used. If a range of pipelines and tanks are available at any one terminal, then, it may be appropriate for terminal personnel to have a number of system characteristics, already pre-calculated and available, for use during pre-transfer discussions.
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In any case, during the pre-transfer discussions (see 6.4), such matters should be covered and the optimum transfer rate should be agreed.
Figure 7.10 Combined ship and shore cargo pumping characteristics — parallel pumps
To clarify some of these issues, two of the system characteristics, as shown in Figure 7.10, are covered in detail below.
If a ship, having the pumping characteristics as shown in Figure 7.10 (numbered 1, 2, 3, and 4), is discharging to a terminal presenting only minor restrictions to flow, then the shore system characteristic may be equivalent to 'C'. The operating point of the ship/shore system moves from points C1 through to C4 as the number of cargo pumps in operation is increased from one to four. Under such conditions, the total flow achieved (when using four pumps) is only marginally less than the total theoretical flow (assuming no resistance). With such a shore pipeline system, it is therefore probable that all four pumps (and maybe more) can be run to good effect.
In the case of system characteristic 'A', where flow restrictions are high, it can be seen how little extra flow is achieved by running more than two pumps. By running three pumps the operating point moves from A2 to A3, achieving some extra throughput. By running four pumps the operating point moves from A3 to A4, achieving an increased flow of virtually zero. In such cases, much of the energy created in the additional pumps is imparted to the cargo. This is converted to heat in the liquid and results in an increase in cargo temperature. This increases flash-gas boil-off as the liquid discharges into shore storage and this excess must be handled by the shore compressors. If the shore compressors are unable to handle the additional flash-gas, the terminal will require a reduction in flow rate to avoid lifting the shore relief valves. Therefore, the net effect, in restricted circumstances, of running an unnecessary number of pumps can be to decrease rather than to increase the overall discharge rate.
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Observing pressure gauges at the manifold will give a good indication if it is worthwhile running, say, four pumps or six pumps. The discharge rate should not be reduced by throttling valves at the ship's cargo manifold if the shore cannot accept the discharge rate. Throttling in this manner further heats up the cargo. However, those gas carriers with only limited recirculation control may have to use manifold valves to throttle pumps.
Figure 7.11 Discharge without vapour return
Figure 7.12 Discharge with vapour return
It also may be desirable to throttle a cargo pump discharge when it is used in conjunction with a booster pump. This may be done in order to reduce the pressure in the booster module. Any additional control of flow, however, should be carried out by throttling the booster pump discharge, by opening the main pump recirculation or by a combination of the two. It should be noted that control of flow solely by throttling the main pump discharge may cause loss of booster pump suction.
As liquid is being pumped from the ship, tank pressures tend to fall. Boil-off due to heat flow through the tank insulation takes place continuously and this generates vapour within the tank. The boil-off is usually insufficient to maintain cargo tank pressures at acceptable levels but this ultimately depends upon discharge rate, cargo temperature and ambient temperature. Where vapours produced internally are insufficient to balance the liquid removal rate, it is necessary to add vapour to the tank if discharge is to continue at a constant rate. This vapour may be provided, either by using the ship's cargo vaporiser (see 4.4), or from the terminal (via a vapour return line). When using the cargo vaporiser, the liquid is normally taken from the discharge line and diverted through the vaporiser. Figure 7.11 shows a discharge operation without the vapour return facility; Figure 7.12 depicts a similar operation but with a vapour return in use.
7.7.3 Discharge via booster pump and cargo heater
Where cargo is being discharged from a refrigerated ship into pressurised storage, it is necessary to warm the cargo (usually to at least 0°C). This means running the cargo booster pump and cargo heater in series with the cargo pump. To operate the booster
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