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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.

chemical tanker navigation at sea
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

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

  1. Cargo line leakage countermeasures
    There are many reason that may lead to cargo line failure on board chemical tanker. Galvanic corrosion in the cargo and stripping pipelines may cause several leakage. One of the sources of such corrosion in pipelines is variation in corrosion resistance at adjacent points in the piping.

  2. Checklist for handling dangerous liquid chemicals in bulk
    Is information available giving the necessary data for the safe handling of the cargo and, where applicable, is a manufacturer's inhibition certificate available? Information on the product to be handled should be available on board the ship and ashore before and during the operation.

  3. Recommended temperature monitoring equipments onboard
    Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

  4. Practical example of solving tank cleaning problems
    Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.
    Pre-cleaning /washing of cargo tanks

  5. Pre-cleaning /washing of cargo tanks
    Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on chemical tankers, this cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gas oil and kerosene.

  6. Final cleaning of cargo tanks prior loading
    Method of final cleaning to be used depends on both previous cargo and cargo to be loaded. As a general rule the tanks and piping shall be completely drained of water or residues before loading. The bottom of the tanks may have to be dried up with rags.

  7. Tank cleaning and posoning hazards
    Certain substances affect the tissues locally as an irritant (cashew nut shell oil) or cause grave damage to the eyes, skin or mucous membranes (e g strong acids and caustic). Other substances may be absorbed by contact to the skin without local effects (e g nitrobenzene, aniline).

  8. Testing of tanks and cargoes
    Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness. Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner.

  9. Practical tank cleaning methods for various noxious liquid cargo
    Tanks that may have contained monomer or drying oils should first be cleaned with sufficient cold water quantities to avoid polymerization of cargo residues. In some cases, it is necessary to employ tank cleaning chemicals, but their use is generally limited as it may be difficult to dispose of slops.

  10. Special tank cleaning method
    If a special method involving cleaning agents is to be used, it may create an additional hazard for the crew. Shipboard procedures should ensure that personnel are familiar with, and protected from, the health hazards associated with such a method. The cleaning agents may be added to the wash water or used alone. The cleaning procedures adopted should not entail the need for personnel to enter the tank.

  11. Determining proper tank cleaning by acid wash method
    The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo. The method is also used as a check that a tank is sufficiently cleaned before loading aromatics.

  12. Supervision of all tank cleaning and gas freeing operations
    Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.

  13. Disposal of tank washings, slops and dirty ballast - safe method
    During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges.

  14. PV valves -function and maintenance requirements
    Pressure/Vacuum valves are designed to provide protection of all cargo tanks against over/under pressure and provide for the flow of small volumes of tank atmosphere resulting from temperature variations in the cargo tank(s) and should operate in advance of the pressure/vacuum breaker, where IG system is in use....

  15. Deck seal, tank non return valves and tank gauging requirements
    On vessels fitted with an inert gas system it is a requirement to maintain a positive seal between the cargo tanks and the inert gas generation plant this is usually accomplished by the use of a non - return valve and a Deck Water Seal...

  16. Loading / stress computer
    This instrument is provided to supplement the stability booklet for the vessel. It allows the Officer responsible, to carry out the various complex calculations required to ensure that the ship is not overstressed or damaged during the carriage of the nominated cargoes..

  17. Various cargo handling safety equipments carriage requirements
    It is essential on chemical tankers that everyone knows his ship's safety equipments thoroughly prior handling noxious chemical cargo. Also the master/chief officer must assume responsibility for this.

  18. Vapour emission control requirement for chemical tankers
    Vessels fitted with a VEC system must have an independent overfill alarm providing audible and visual warning. These are to be tested at the tank to ensure their proper operation prior to commencing loading, unless the system is provided with an electronic self-testing capability. Fixed gauging systems must be maintained in a fully operational condition at all times. .....

  19. Draegar Chemical detector tubes use and reading correction guideline
    These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture

  20. Requirements of various grade chemical cargo heating
    : The voyage orders will contain heating information, if heating is required. As a rule the final heating instructions are given by the Shipper in writing to the Master / Chief Officer in the port of loading. If those written instructions are not given, the master should request them and issue a Letter of Protest if they are not received at departure. In the latter case the management office should be immediately informed.

  21. Recommended temperature monitoring equipments onboard
    :Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

  22. Cargo instruments
    :In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, especially prior to tank entry, several different gas measuring instruments need to be available for use. Which one to use will depend upon the type of atmosphere being measured.

  23. Liquid level gauges
    :The accuracy required of chemical carrier level gauges is high because of the nature and value of the cargo. To limit personnel exposure to chemicals or their vapours while cargo is being handled, or during carriage at sea, the IBC Code specifies three methods of gauging the level of a liquid in a tank - open, restricted or closed

  24. Overflow control
    :Certain cargoes require the designated tank to be fitted with a separate high level alarm to give warning before the tank becomes full. The alarm may be activated by either a float operating a switch device, a capacitive pressure transmitter, or an ultrasonic or radioactive source. The activation point is usually pre-set at 95% of tank capacity.

  25. Oxygen analysers
    :Oxygen analysers are normally used to determine the oxygen level in the atmosphere of an enclosed space: for instance, to check that a cargo tank can be considered fully inerted, or whether a compartment is safe for entry.

    Vapour detection
    :Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.

  26. Alarm circuit
    :An important feature of many modern measurement and control instruments is the ability to signal a particular situation. This can be a main operational alarm that gives an indication of a pre-set situation such as liquid level in a tank, or a malfunction alarm indicating a failure within a sensor's own operating mechanism. The designs and purposes of alarm and shutdown circuits vary widely, and their operating system may be pneumatic, hydraulic, electrical or electronic. Safe operation of plant and systems depends on the correct operation of these circuits and a knowledgeable reaction to them.

  27. Venting of cargo tanks safety procedure
    The cargo tank venting system should be set for the type of operation to be performed. Cargo vapour displaced from tanks during loading or ballasting should be vented through the installed venting system to atmosphere, except when return of the vapour to shore is required. The cargo or ballast loading rate should not exceed a rate of vapour flow within the capacity of the installed system. ..... is merely an informational site about various aspects of chemical tankers and safety tips that may be particular value to those working in: Chemical Handling, Chemical Storage, Liquefied Chemical Suppliers, Chemical Shipping, Chemical Transportation, Chemical Terminals, Bulk Chemical Services and Chemical Processing. If you are interested in finding out more about chemical tanker guideline please visit IMO official website. For any comment please Contact us

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