Avoiding cargo pumps overload or underload - Chemical tankers procedure
How a cargo pump works ? :
The function of any pump is to transfer liquid from one point to another and this involves the use of
piping. Such a transfer in a tanker can be divided into two parts:-
- The movement of liquid from the tank to the pump. This is a function of the pump
and its installation design. These factors are beyond the control of the ship provided the
design ratings of the pump are maintained.
- The onward movement of the liquid from the pump to its destination. This is an
area where the efficient operation of the pumps is essential if optimum results are
to be obtained.
Normally the design of the pumping system makes the need for careful balancing and adjustment of
the cargo pump controls during bulk discharge essential to avoid problems. This could be with over
heated pumps in the case of high back pressures or overloaded pumps in the case of low
Avoiding pump overload
This is a problem with low back pressures from the shore facility and when the pumps are
incorrectly increased in RPM with fully opened discharging lines in an effort to increase the manifold
pressure with little or no result, the effect on pump prime mover will be as follows:-
Particular problems of overloading can occur with the ballast pumps. If the pumps are used to fill
double bottoms from empty or empty top wing tanks from full then the pumps can be
easily overloaded, causing damage to the prime mover and other components. Careful manipulation
of the pump discharge throttle valves is necessary with these pumps. Double bottom tanks are only
to be filled from empty by gravity, by passing the pump, and similarly the top wing tanks
are to be emptied from full by gravity to pumping level.
- Output is a function of RPM x Torque, therefore, from even low steam turbine speeds the output
from the pump may already be high or maximum, with turbine nozzle pressure (hence torque)
at a maximum. Full output at low RPM means that torque can easily increase above the design
limits. The high torque results in loads outside the manufacturers design limits which in turn
may result in damage to the turbine.
- The gear trains associated with diesel engine prime movers may also be damaged in a similar
fashion if these are similarly overloaded, and indeed bearings and other components in
the pumps themselves may be damaged.
Overload of electrically driven ballast pumps can result in the electrical prime mover and other
electrical installations burning out.
To avoid these types of damages the pump must always be operated within their
permissible operational envelopes, particularly by keeping the discharge pressure versus RPM
within the manufacturers limits by careful use of throttling of the pump discharge valves to create
an imposed discharge head. These discharge valves are normally remotely operated from the
cargo control room.
All centrifugal pumps are to be started with closed or partially open discharge valves to
avoid immediate overloading. This is most critical with diesel and electrically driven pumps, rather
than the turbine driven pumps where the speed of the pump is gradually increased in a
controlled fashion. However, this is good practice for all pumps to ensure that they are always
operating within their characteristic envelope.
Fig: Deep well cargo pump for chemical tanker and product carrier
Avoiding pump underload
Underload is a problem with high back pressures from the shore facility. Underload results in
overheating of pump casings and damage to pump components due to energy developed by the
pump mover being converted into heat rather than in pumping the cargo ashore. Pump balance is
at its most critical when high back pressure from the shore facility is experienced.
Pump Characteristic Diagram
These are diagrams showing pump operational parameters, and contain information, including
volumetric output against RPM, discharge head, power, steam consumption, etc.
Each type of pump will have its own characteristic diagram and all operators must be aware of, and
follow, the limitation
Fig: stripping pump
Balancing/Discharging using more than one pump
When more than one pump is discharging to a common shore line it is essential that the pumps are
correctly balanced so that they meet the parameters of their operating envelopes to avoid
overload or underload
Balancing of the pumps is best achieved by monitoring of the pump
discharge pressure gauges, as the pumps are usually not fitted with remote indicators in the cargo
control room to show whether the pump non return valves are open and consequently each pump is
actually pumping cargo. The RPM in itself cannot be relied upon to balance the pumps, as different
pumps may be operating with different suction pressures. It is therefore important that the pump
discharge pressure indicators, and transmitters are working correctly and are properly calibrated at
During the balance process the pumps are to be monitored locally to ensure that heating of casings is
not occurring so that prompt corrective action can be taken to prevent a pump shutdown by one of
the safety devices.
If this balancing is correctly achieved it can be assumed that each pump will be delivering its own
proportion of the total volume of cargo being delivered ashore, and therefore a check can be made to
ensure that the pumps are operating within their characteristic envelopes. If this is not the case then
the pump discharge throttle valves are to be adjusted until the discharge pressure on the pump is
correctly within design limits.
When using more than 1 pump for discharge, check the manifold pressure to see if the manifold
pressure increases as additional pumps are put on.
Control & operation of centrifugal pumps
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