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Chemical tankers cargo handling equipments

In order to maintain a proper control of the tank atmosphere and to check the effectiveness of chemical cargo being loaded or discharged a wide range of instrumentation may be fitted on a modern chemical tanker. Only an outline is given here, providing guidance on the safe and efficient operation of the equipment.

It is imperative that apart from the usual maintenance all the chemical equipments are thoroughly examined for any wear and tear prior every use.

chemical tanker navigation at sea
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....

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...

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....

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. Below is listed suitable equipment for operational personnel for various typical cargoes and operations on board:......

Pumproom bilge level alarms

Cargo pump-room – A space housing pumps and their accessories for the handling of the products covered by IBC Code.
Pumproom bilge alarms are to be tested as per PMS, and prior to every cargo operation

Pump suction strainers

The continued efficient operation of cargo pumps requires that any pump suction strainers fitted are kept clean as follows:-
  • Main cargo pump suction strainers are to be opened up for inspection and cleaning after each discharge and may also require inspection/cleaning between grades.
  • Cargo stripping pump suction strainers are to be opened up for inspection and cleaning after each tank cleaning has been completed
  • Ballast pump suction strainers are to be opened up for inspection and cleaning as per PMS.

Submerged cargo pumps: Cofferdam purging routines and cargo pump performance routines to be carried out as per Maker’s requirements and PMS.

Deep well cargo pump for chemical tanker and product carrier
Fig: Deep well cargo pump for chemical tanker and product carrier

Pump safety devices

Cargo and ballast pump safety devices and emergency stops are to be tested at least once per loaded voyage, just prior to the first load and discharge port, and before commencement of discharge. Records of these tests are to be maintained.

Tank cleaning equipment

Tank cleaning machines and gears (Hoses, Wilden Pumps, Air lamps, Chemical spray pumps, etc), when not being used regularly, are to be checked in accordance with the manufacturer’s instructions as per PMS.

Tank cleaning hoses should be tested for electrical continuity in a dry condition prior to every use and in no case should the resistance exceed 6 ohms per metre length. Electrical continuity of tank cleaning hoses is to be checked before each operation.

Pressure gauges and temperature

All manifold and other pressure instrumentation within the cargo system is to be checked before each use and calibration annually and records of such calibration maintained onboard. Vessels are supplied with one test gauge (certified) for temperature & pressure for this purpose and all other gauges are to be checked against the calibrated gauge. Gauges are to be checked within +/- 10% of the certified gauge and a certificate issued by the Chief Engineer. An entry is also to be made in the deck log book.

The certified gauge is to be used only for calibration purposes. Any gauges which cannot be calibrated to within +/- 10% of the certified gauge are to be replaced. Temperature gauges are also to be checked on an annual basis. Calibration should be carried out during repair periods or, if suitable, calibration instrumentation is supplied. In lieu of such calibration, then comparison readings should be taken between local and remote thermometer readings so as to provide a practical cross reference.

Cargo pumping system of the 22,000dwt product tanker CHEMBULK SAVANNAH

The cargo pumping system includes 24 Wärtsilä Svanehoj cargo pumps, driven by electric motors under frequency converter control. Pumps handle cargoes with a maximum density of 1.5t/m3. Using 12 Vacon frequency converters, the system is designed for a maximum of 12 pumps to be operated simultaneously, either under remote control from the cargo control room or under local control from the converter switchboard front panel. The cargo piping system is designed for a maximum pressure of 16 bar, with automatic switch off, if a valve in the discharge system is closed in error.

More cargo handling equipments

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. .....

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

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.

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.

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.

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

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.

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.

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.

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. .....

Reference publications

  • Equipment Manufacturers Instruction Manuals
  • MARPOL – 73/78 (latest consolidated edition)
  • International Safety Guide for Oil Tankers and Terminals (ISGOTT)
  • CFR 33 parts 125 to 199
  • Ship to Ship Transfer Guide (Petroleum)
  • MSDS for particular cargo carried
  • Chemical Tank Cleaning Guide

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