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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:-
  1. 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.
  2. 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 back pressures.


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:-
  1. 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.
  2. 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.
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.

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.

Deep well cargo pump for chemical tanker and product carrier
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

stripping-pump
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 all times.

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.



Related Info

Control & operation of centrifugal pumps










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