Saturday, December 11, 2010

CALIBRATION ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL

                         CALIBRATION



Calibrations are done as per the SMS of the company.

The accuracy of instruments critical to the safe and efficient operation of the ship machinery must be verified at regular intervals.

AVE ( ACCURACY VERIFICATION EQUIPMENTS )  must be maintained in full working order. When not in use the AVE must be stored in the protective cases supplied and kept under lock and key in a air conditioned store ,-- not in the machinery spaces.

The AVE consists of the following:

--An electrically powered dry block temperature calibrator with a range of 0 - 600°C/650oC.
--One 400mm mercury in glass Master thermometer, 0 - 500°C.

--A hand operated screw type pressure comparator pump with an operating range capable of 0 - 400bar pressure and up to 800mm Hg vacuum.

--One set of Master gauges covering the ranges as applicable to each ship. These are certified accurate to 0.5% full scale deflection (fsd).

--One set of test gauges covering the ranges as applicable to each ship. These are certified accurate to 1.0% full scale deflection (fsd).

--A set of low pressure capsule gauges complete with hand operated bulb and valve for low pressures, e.g. inert gas pressure, etc. Two gauges will be supplied, both certified to 1.5% accuracy, one of which must be designated the test gauge and the other the Master gauge.

--One hand held digital tachometer.

--One 0-100oC Mercury in glass Master Thermometer.

--A "Length Reference" Master UTI Tape.

All instruments must be operated in accordance with the manufacturer's instructions and no adjustments or alterations to the AVE, other than in accordance with manufacturer's recommendations, are permitted.

The instruments listed below must have their certificates revalidated at least once every three years:-
--The Master Thermometers.
--The Master pressure gauges.
--Both low pressure gauges.
--The digital tachometer.
--The "Length Reference" Master UTI Tape.

Certificates of calibration equipment must clearly state the measured percentage error against full scale deflection for each piece of test equipment in order that any test equipment with an unacceptable percentage error is identified and can be replaced.

The 0-500 deg C Master thermometer must be used to verify the accuracy of the dry block temperature calibrator every three months . The 0-500oC Master thermometer must not be used for any other purpose.

Only fresh water or a low viscosity clean oil is to be as the hydraulic medium in the pressure comparator pump.

The Master pressure gauges are for verifying the accuracy of the test gauges and they must not be used for any other purpose. There is a test certificate for each test pressure gauge. The accuracy of these gauges must be verified against the appropriate Master gauge every three months .

The test gauges are only to be used for the purposes of checking working gauges and must not to be used as working gauges.

In the event of a test gauge being more than plus or minus 2% inaccurate when compared to the Master gauge during the quarterly accuracy verification check, it is no longer suitable for the replacement gauge ordered. In such a case all gauges and instruments that have been checked with the inaccurate test gauge since the previous accuracy verification test must be rechecked when a replacement test gauge has been obtained.

The temperature calibrator and digital tachometer must not be used in hazardous areas.

The AVE can be used at the Chief Engineer's discretion to verify the accuracy of any gauge or instrument to assist with the alarm testing schedule.

Any failure of, or damage to, AVE must be reported to the Company immediately. The Company will arrange for replacement equipment to be supplied to the ship.

The UTI tape used for "length reference" must be set aside in the Critical Instruments storage location. The UTI tape used for "final dips" must be checked for length accuracy against the "length reference" tape every 12 months.  The UTI tape used for "final dips" must be checked for temperature measurement accuracy every three months against the 0-100 deg C glass Master Thermometer.

Verification Procedures The accuracy of gauges and instruments must be verified at least once every twelve months. Certain gauges are to be checked on a more regular basis and these are listed below. Should there be any reason to suspect the accuracy of any gauge, thermometer or instrument, then more frequent verification must be carried out.

Cargo manifold pressure gauges must have their accuracy verified prior to each discharge or every two months, whichever is the longer period. The date of such verification must be recorded.

The maximum permitted deviation of any instrument is plus or minus 5% when compared to the verification equipment. Any gauge or instrument outside these tolerances must be identified using tags or stickers. Such units must be replaced or repaired to bring it back within tolerance at the first opportunity. Replacement gauges must be tested immediately prior to installation.

If for any reason rectification of an inaccuracy is not immediately possible, the Chief Engineer must make due allowance for this inaccuracy when operating the affected equipment. He must ensure that all operators are aware of the inaccuracy and the measures in place to cope with it.

Should a Master or test gauge or instrument be found to be inaccurate all gauges verified against that unit since its last satisfactory test must be re-verified.

The Chief Engineer is responsible for ensuring that the verification records for gauges and instruments is maintained up to date as required by the Company.

Percentage error of gauge under test can be is found by subtracting the actual reading from the applied test temperature/pressure, dividing by the full scale deflection temperature/pressure of the gauge under test and multiplying by 100.

Temperature Gauge Test

Methods
Thermometer and temperature probes must be verified against the dry block temperature calibrator provided. Manufacturer's operating instructions are to be closely followed.

This instrument is supplied with standard size brass inserts which will cover most sizes of thermometers and probes. However should a ship have non-standard size probes then the ship's engineering staff are to fabricate suitable inserts onboard using the standard supplied insert as a template. When using the inserts, the fit to the thermometer or probe under test should be as close as possible to ensure that the air gap between the thermometer or probe and the insert is at a minimum.

When a verification check is carried out on a temperature indicator it should be checked at three temperatures, i.e. ambient, normal working temp and 75% of full scale or 600°C max.

The dry block calibrator is capable of heating thermometers or probes up to 600°C. Care should be taken when removing items from the dry block calibrator that has been heated to elevated temperatures.

Pressure Gauge Test Methods:--

Each ship is supplied with a range of Master and test pressure gauges to cover the range of gauges . The portable hand operated screw type pressure pump is supplied complete with 1/2 inch BSP, valves, standard size adapters (to suit 3/8 inch, 1/4 inch, 1/8 inch, male BSP connections) and a 1000mm long fine bore copper capillary tube suitable for pressure up to 400bar. The capillary tube can be used where gauges are not easily removed from their mounting or where the pressure readout is obtained using a pressure transmitter.

Should any pressure gauges not have standard size connections then the Chief Engineer is responsible for ensuring that any purchased or onboard manufactured adapters are suitable for the test pressure to be applied. When a verification check is carried out on a pressure gauge or instrument it should be checked up to 75% of full range or 250kg/cm2 maximum (except for the gauges referred to in the following paragraphs).

For low pressure testing (up to 4000mm water pressure) a low pressure capsule gauge complete with bulb, valve and push on rubber hose is supplied. This low pressure capsule gauge must be used with the bulb and valve provided and must not be used with the hydraulic screw pump. The rated pressure of these low pressure capsule gauges must never be exceeded as they are susceptible to damage from over pressurisation.

For vacuum testing (and low pressure testing up to 3bar) a gauge is supplied with a range of -1bar to +3bar. This can be used with the comparator pump which is able to pull a vacuum of up to 800mm Hg (- 0.8bar).

Fuel valve pressure test gauges must be verified up to the maximum range possible with the AVE instruments. In addition, at each major repair period their accuracy must be verified at 75% of full range.

Also, at some time in the three months preceding or during the MRP (also PRIOR to main engine work commencing, where this unit would be required) the main engine pneumatic/hydraulic pump unit pressure gauge is to be verified at 75% of full range.

Certificates must be maintained in the Verification Record folder.



Master Calibrator:

Here is the explanation of a typical class type approved master calibrator provided on Goodwood chemical tankers.

Temperature:

The idea of calibrating is to know the value so that the index error can be applied and to check if it is within acceptable limits.

Adjusting the calibrated object to remove the error must be done by a qualified person. You must remember that the calibrator could be at error too—as there is always variance between the thermal contact between the probe being calibrated and the calibrator well.

Make sure the voltage is within the range of the info on the label of the set. Calibrator dry well should be clean. The probe must fill properly in the well –so choose the appropriate hole. If your probe has another diameter other than those pre drilled orifices in the well—use an adaptor. While using the adaptor ensure that the probe reaches the bottom of the well repeat bottom of well—or you rather not go through this meaningless exercise which can cause needless expense to the the company.  

You MUST read the manual to check out where exactly the sensor is located.  Enter the desired set point or the desired calibration temperature on the controller. There will be a toggle switch to alternate between the set point and the actual well temperature. The well temp MUST stabilise repeat must stabilise at the set point ( better to wait 5 minutes extra ) —the take the well reading as well as the temperature of the thermometer which is being tested. Set the next set point at a higher level and do the same thing.

When all desired calibration points are reached, set the set point to ZERO for cooling and let the well temp reach 50 C before you store the master calibrator away. The default temp while powering up will be the last set point set ( zero in our case  ) while powering down.

Note that the dry well calibrator if used mindlessly can give you severe burns –so never remove hot probes or inserts from the well. It can even cause a fire if something gets into the well at 600 deg C. check the air inlet to the instrument at the bottom is not clogged by dust. Read the manual well to see if the dry well calibrator absolutely need oil of safe auto inginition temperature, inside the well for better thermal contact—

Please remember the oil can dry up in the well as a carbon cake and render the well ineffective as it is hard to remove. If the oil in the well explodes the probe can be projected like a MISSILE or emit dangerous fumes. Don’t try to do this job when the ship is rolling heavily.

Please be honest with calibrations. Nothing is achieved by filling up company calibration forms in a dishonest manner.

Pressure:

Typically can test to 700 bars. Do not try to operate this instrument without be absolutely familiar—lest you get severely injured.

The factory setting may be for ABSOLUTE pressure—read the manual—also to check out the set ranges. Remember a pressure guage on a ship reads in excess of atmospheric pressure. So for sealed gauge pressure measurements activate zero set at ambient pressure.

The instrument is likely to be powered by a Lithium 3V battery for the digital readings. Always remove the air pressure by operating the pressure release valve before removing the connectors from the one hand operated scissor action hydraulic hand pump. This release valve can be used to reduce the pressure too and must be fully closed prior to pumping..

There will be a selector switch for pressure/ vacuum. There will also be a fine adjustment to increase or decrease the generated pressure minutely which must not be manhandled and kept at midlevel—read the manual. The pressure relief valve can be adjusted to set the max output pressure.

The seals or O rings of the flexible hoses must be airtight. The stroke lengths of the pump can be adjusted. The hydraulic reservoir must have oil filled up to the desired level as marked. There will be a dedicated pressure relief valve which must not be touched.

Read the manual to check how the air in the hyd pump can be expelled—remember air is compressible and will spoil the show.  For low pressure testing upto 60 bars the  pneumatic selector switch can be activated and the same pump can be used..


Gas detection instruments:-

Calibration checks of gas detecting equipment shall only be carried out by the Chief Officer or by an officer under the supervision of the Chief Officer strictly as per the manufacturer’s instructions.

All instruments shall be frequently calibrated in accordance with the manufacturer's instructions

The composition of the calibration gas must be known . In multi gas detectors detecting 4 gases like O2/ HC/ CO/ H2S—the 4 runner span gas canister is used.

Portable instruments shall be calibrated immediately prior to each period of use or at monthly intervals, whichever is less

When oxygen detectors are calibrated for 21% oxygen it is essential that this is carried out in clear air conditions

Sufficient supplies of required calibration and span gases must be carried on board. These are part of the  minimum critical spares list.

Any defective portable units shall be returned to an authorised repair company after making the appropriate landing report.

All sample lines shall be maintained in a clean and unobstructed condition such that a leak tight connection can be made to the instruments

The correct sampling point, upper/lower for the fixed gas detection equipment shall be correctly set in respect of the cargo being carried.   If the vapor is different from that used for calibration ( which is always the case) the appropriate conversion factor should be used.  Or your reading is just a useless figure.  For example for an instrument calibrated by Iso Butane of factor one the corresponding multiplier for Methanol vapor is 0.72.    Remember the LEL of Iso Butane is 1.8% by volume  and that of Methanol is 6% .

When using portable equipment care must be taken to ensure that an adequate number of samples are taken from different points so as to verify that the entire area is represented in the readings taken

When fixed gas detection readouts are on the bridge or in the control room etc. care must be taken that the exhaust from the detection equipment is lead to atmosphere and operating correctly

Any local and remote readouts shall be regularly checked and compared for accuracy

Maintenance of gas detection equipment on board is generally about calibration, changing of batteries and ancillary components like sensors, filters  etc.

Calibrator gas bottles must have a regulator , adaptor and flow meter

The digital mutimeter for checking resistance of hoses must not be used for any other routine jobs.

Span gases usually held on chemical tankers for –
Portable oxygen analyser
1)99.99% Nitrogen for Zero calibration
2)10% oxygen , rest Nitrogen for span calibration.

Explosimeter for HC
1)40% LEL IsoButane for span.

Tankscope for HC
1)8% by volume IsoButane in Nitrogen

4 runner
Four runner standard gas cans for all 4 gases in a single canister. HC/ O2/ H2S/ CO


A pumproom gas detection system is a simple “ go- no go” warning and does not require regular calibration. The system must however be tested with the standard gas.


CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )
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2 comments:

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    Temperature calibration

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  2. Dear Master Ajit ,

    Whatever I search in internet about tankcleaning, gas detectors etc..., only on your blog I can find all I need...no any info on the websites...
    THANK YOU VERY MUCH AND GOD BLESS YOU , YOU ARE A VERY GOOD MAN!

    Artur Mantashyan

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