Saturday, June 5, 2010

NITROGEN ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL

               

PADDING -- DRYING -- INERTING --  NITROGEN BOTTLE BANK --  BLOCK AND BLEED ( DECK SEAL ) --- PSA  -- MEMBRANE GENERATORS


Nitrogen is a colourless odourless liquid which can cause you to lose consciousness in as little as 20 seconds, death follows rapidly.

MASTERS AND CHIEF OFFICERS PL NOTE-- IF YOUR CREW FALLS UNCONSCIOUS IN A CARGO TANK-- YOU ARE RESPONSIBLE !!!

Nitrogen is not a poisonous gas. It only serves to dilute the action of oxygen in air. Nitrogen exposure causes cyanosis—change of skin colour to dark blue due to lack of oxygen and loss of red blood.

Nitrogen is chemically inert due to its non polar nature and high ionization energy. It is only slightly soluble in water. It is a food antioxidant.

Uses:
For quality control of cargo ( reduced oxidation )
To retard dangerous reactions
To remove moisture from tank
To avoid explosion ( inerting )

Nitrogen is lighter than air and will rise and remain on top of the tank. Do not waste nitrogen by not understanding that heavier oxygen in tank must be removed from bottom by lighter nitrogen from top.

When nitrogen padding, each tank must have its own pressure guage.

Prior to loading cargo requiring either a nitrogen blanket or an inerted tank, the whole tank should be pressurised to approximately the PV valve opening pressure. Any leaks noted should be rectified. (Silicon sealant should only be used where tank packings are suspect and there are no spares on board, or if hatches are not in good condition  as silicon can damage good packings.)

NITROGEN BOTTLE BANK

There are  usually 30 to 40 bottles on board for padding purposes only.  The bottles on board are usually of 68 litres capacity . Storage temp of nitrogen bottles should never exceed 52C. The number of bottles should be > 5% of the total volume of cargo space to be inerted.

Nitrogen you have in the bottle bank on a chemical tanker is industrial dry by fractional evaporation of liquid air. Chemical dry is expensive and not used. There is a filter, regulator and relief valve. The regulator has a drain cock. There is a check nut to be loosened prior adjusting the regulator. The nitrogen on board is reduced from 200 bars to 4 bars and then to 0.15 bars. For topping up at sea or padding in port the 0.15 bars reducing valve is used. The relief valve is set at 0.2 bars. There are two lines –one of them only for emergency use.

The small dia , nitrogen line system should be regularly blown through, to ensure that no contaminants are in the lines. A 68 litre nitrogen cylinder at 200 bars pressure can give out about 13.6 cubic metres of free gaseous nitrogen. Compressed nitrogen gives 200 times free nitrogen .  Liquid nitrogen is not used on chemical tanker as they can be stowed only in insulated tanks ( -196 deg c.)

Inerting - by filling the cargo tank and associated piping systems and, where specified in chapter 15, the spaces surrounding the cargo tanks, with a gas or vapour which will not support combustion and which will not react with the cargo, and maintaining that condition.  Nitrogen purging is usually done prior loading via manifold cargo line. The gas supply must be stopped prior to closing vessel valves.  Vessel manifold valves should be closed on completion of purging.  Always keep the PV valve at manual open position—as soon as the tank comes to required oxygen content—shut the PV valve. As soon as the blanketing is over—blank the manifold lines. Put a tag at the tank—nitrogen blanketed. Nitrogen delivery to tanks must NEVER be able to lift the PV valve .

Padding - by filling the cargo tank (after loading) and associated piping systems with a liquid, gas or vapour which separates the cargo from the air, and maintaining that condition.
The gas should be introduced into the cargo tank through a connection at the top of the tank feeding directly into the ullage space.  Compressed gas must not be introduced into the vessel tank through the manifold or pump stack since doing so invites relatively larger volumes (and higher pressures) of gas, thus creating increased risks of displacing cargo into the venting system and overpressurizing the vessel. Specific carriage instructions must be complied with and the vessel must ensure she has adequate supplies of nitrogen for the period the cargo(es) will be on board. Each tank will require to have an operational manometer and the daily nitrogen log must be completed.

Drying - by filling the cargo tank and associated piping systems with moisture-free gas or vapour with a dewpoint of -40 deg C or below at atmospheric pressure, and maintaining that condition.

When inerting/ drying tank with shore nitrogen the tank dome must be kept crack open ( often blown recklessly at 10 bars ) to prevent structural collapse.

During the pre transfer meeting topics such as personnel roles, gas volumetric flow rates (max and min), pressures, method of line clearing,  the amount of ullage space that is available for the line displacement,  connections, valve alignment, event sequence, units of measure, and anticipated stop time must be discussed.
Nitrogen is lighter than air. Do not use dilution method to avoid wastage.

Chemical tankers with cargo tanks of greater capacity than 3000 cum or which has large capacity washing machine ( nozzle 17.5 cum/ hr –machine through put 60 cum / hr –total water flow >110 cum/ hr ) have to be inerted.

Chemical tankers of DWT 30000 mtons carrying oil cargoes or petroleum products of FP<60 deg c have to be inerted. Usually done by IGG by stochiometric combustion of fuel with air. 2% O2  13% CO2  85%  N2  by vol.  There is complete absence of soot. It is important to know the terminal requirement for inerting . Local rules especially, in US west coast ,can vary considerably from what is the international norm.

NITROGEN GENERATORS:
For purging tanks you require a nitrogen generator or shore nitrogen.  Nitrogen generators used for displacement while dischg must have 125% capacity.
There are mainly two types of nitrogen generators.
Membrane separation.
Pressure swing adsorbsion.

MEMBRANE TYPE:
It is capable of separating compressed air to streams of enriched nitrogen and oxygen by means of membranes.
The principle is differential permeation rate of various gases through a polymer membrane.
A typical nitrogen generator description—
The feed air is produced by an air cooled rotary screw compressor. Before it reaches the  membrane the air is cleaned thoroughly by –
Cyclonic water separator
Pre filter (of epoxy resin al silicone) and micro filter
Activated charcoal absorbing tower
Finer microfilter
Heater for feed air.
The first filter mainly removes water, the second filter removes oil, then the last filter removes all foreign material.
The generator can be operated automatically unattended, except when starting the system. During operation, the generator is monitored and controlled by several sensors. The generator stops when a malfunction or failure occurs.

There is a Flow Control Valve to adjust the flow rate of N2 gas extracted from the membrane module. Two valves are installed for 95% mode and 99.9% mode respectively.

Particles of oil will damage the membrane permanently.

Particles of water on the other hand will decrease the capacity of the membrane bundle of thin polymeric hollow fibre. All filters can be drained off condensate. The water separator has an automatic drain with timer. Abrupt pressure changes can damage the sensitive membrane. The oxygen is constantly analysed by an analyzer with a high oxygen content alarm. When this alarm rings the nitrogen is purged / vented away till the oxygen content is within accepted limit.

The membrane bundle is constructed like a tube shell heat exchanger ( fibres are the tubes ). Cleaned air ( 78% nitrogen + 20.8% oxygen + 1% argon ) enters from one side of the millions of membranes bundled together. As it flows through the fibres oxygen and water will permeate through the fibre wall and leave the membrane bundle at the shell. At the other end of the bundle enriched nitrogen leaves the system. Different gases permeate at different rates through the membrane. The slow gases are methane , nitrogen and carbon monoxide. The medium gases are argon and oxygen. The fast gases are water vapour , Hydrogen and Carbon di oxide. Since water vapor permeates quickly it follows that the Nitrogen produced is dry , 99.5 % pure at a low throughput.. The membrane is heat sensitive and so the compressed air must be cooled.  Nitrogen production rate is controlled by throughput---for good quality low throughput---for not so pure quality higher throughput or higher air inlet pressure. Nitrogen can be discharged straight into cargo tanks for purging ( say for carbon tetra chloride ) or into low pressure holding tank for future use. If you want to store it at high pressure in a cylinder then a booster compressor is required.

Remember the oxygen rich permeate is vented to atmosphere and in case your nitrogen generator is next to funnel which can spark, it must be monitored.

With proper design and routine maintenance on the upstream filtration, the membrane life is expected to be ten years.
Membranes will not be permanently damaged by liquid water. The membrane performance can be restored by flowing warm, dry air through the membrane until all the liquid water is removed.  The membranes will be permanently damaged by liquid oil entering or condensing in the membrane. Two coalescing filters are generally recommended upstream of the membrane to ensure all oil aerosols are removed.

If only dryness is required, then nitrogen purity can be sacrificed and higher volume of delivery can be achieved. With automatic humidity controls (like Terra desiccators), this purity level may not even be necessary. Terra systems automatically regulate the nitrogen purge to maintain the most critical humidity levels typically required.

Compact, lightweight design allows set-up wherever there's an oil free  compressed air supply.

Completely reliable, economical operation, virtually eliminates maintenance , provided the pre-filters do their work. Coalescing filters and traps serve to remove oil and liquids, organic contaminants, and other particulates from the feed air The only attention a system typically needs is an occasional recalibration of the oxygen analyzer and filter change.  Because this technology requires no moving parts and consumes relatively little energy, it is surprisingly economical to operate and maintain—the main expense is the energy required to provide a stream of compressed feed air. The  system also has gas pressure control valves and instruments. Once the system is set up, you simply switch to START and push the RESET button to open the feed valve. After about three minutes, when gas purity levels are met, a product valve opens to deliver the nitrogen gas to your process line.

Typical Prefilter specs--One-micron absolute filtration. Dual glass micro-fiber filter beds coalesce and remove water droplets. Remaining oil content: 1 ppm by weight. Includes slide indicator (to indicate need for filter change), liquid level indicator and internal drain.

Typical coalescing filter specs: Absolute filtration of 0.01┬Ám particles; 99.999=% oil removal efficiency. Includes slide indicator (to indicate need for filter change), liquid level indicator and internal drain
Dew point is the temperature at which a given mixture of water vapor and gas is saturated. The dew point and trace contaminants of the nitrogen-enriched product gas are dependent on the water level and quality of the feed air. Operating in the purity range of 95-99% nitrogen, saturated feed air results in a product gas that contains less than 5 ppm water, depending on feed air conditions. The atmospheric dew point equivalent is -65°C (-85°F). The dew point varies slightly with nitrogen purity. At 95% purity, the nitrogen will have at least a –70°F dew point. Operating at 99% will further lower the dew point to below –100°F. Membrane air dryers can be installed in series with nitrogen-generating membranes. On ships nitrogen drying requires only -40 deg C.

Water level in the feed gas is dependent on temperature and pressure. Therefore, if the feed air pressure is reduced, the dew point of the product may increase. If the temperature is increased, the feed dew point increases and the product stream dew point increases. Changes in dew point are minimized by using a refrigerated air dryer to condition the feed air.

Main Control Panel
The main control panel is installed on the nitrogen gas generating system unit. The control panel is equipped With a Programmable Logic Controller (PLC), systems
Start/Stop button. oxygen analyzer and thermometer All control programs are memorised In the ROM inside the PLC.
Do not turn off the power switch of main control panel even during system shutdowns as deck main line pressure should be monitored continuously.
This system can only be operated from the main control panel.

Auto Drain Valve
The auto drain valve installed under the filter automatically discharges drain when drain reaches a specified level of Inner tank.

After separating N2 gas. unwanted compressed air IS safely expelled overboard through a pipe.

Deck Main Valve and Vent Valve
Two deck main valves and a vent valve are installed on the Cargo deck area.
Those valves are air operated automatic valves and are automatically operated by the main control panel. Deck main valves are open when the vent valve is closed. and the vent valve is open when the deck main valves are closed
Fore deck main valve is closed automatically due to the spring return system, when instrument air fails.


Inert Gas Control Valve
The main valve and back pressure regulating valve are provided on the inert gas main pipe.
The main valve controls N2 gas supply at 95%.
The main valve and the back pressure regulating valve are automatically controlled by the inert gas main line pressure.
The main valve is automatically close when instrument air fails. as it is equipped with spring return system.


Monitor Panel
Monitor panels are located in ECR. CCR. and W/H with required indicators and alarms.
Emergency stop switches are provided on the main control panel and the CCR monitor panel.
N2 gas supply switch is located on the CCR monitor panel.
N2 gas supply to cargo tank or N2 gas receiver tank can be operated only by the N2 gas supply switch on the CCR monitor panel.
The vent waste gas (permeate) out of the system is oxygen enriched; it typically contains 30% oxygen but can be as high as 45% oxygen. While oxygen will not burn, oxygen concentrations above 25% will support combustion of other materials much more readily than air. For example, materials that smoulder in air could burn fiercely in this atmosphere. It is imperative that vent gas is discharged only in well-ventilated and safe areas. If the unit is located in an air tight or poorly ventilated room, the permeate stream should be piped to a ventilated atmosphere for release.
After complete shutdown of the system, including power supply, the system remains pressurised! De-pressurise the complete system manually if required and take care that all nitrogen is led to the outside!

Membrane Storage condition:                                                   
minimum temperature 5 °C;
optimum temperature 25 °C (+/- 5°);
maximum temperature 55 °C.
Maximum operating pressure 13 bar(g)
Each feed air compressor operates independent from the N2 system and has its own controls. There is an arrangement of  block & bleed to prevent back flow of a hydrocarbon-nitrogen mixture to the nitrogen generator room (safe area). Additional an excess gas valve is installed upstream the block & bleed valves. The valve is designed for blowing-off about 25% of the total N2 capacity. If the open position of the excess gas valve is indicated (saying over-capacity), the operator is advised to switch-off one of the compressors. This however will be a manual action. A deck transmitter panel takes care of transmission of the deck pressure signals of the deck-main to the Cargo Control Room.

Storage tank filling is possible without supplying nitrogen to deck or during supply of nitrogen to deck.
If no gas is supplied to deck the surplus of nitrogen is purged to outside.
In this case, for energy consumption reason, it is advice to operate maximum 1 compressor, however this is the operator responsibility.
Use of PAO type lubrication oil in compressor
The heater will first be energised after detection of a minimum pressure in the system.
The heater will try to reach the operating temperature of 50°.
A minimum temperature rise by the heater is required. Therefore a differential temperature switch is installed. If the minimum differential temperature is not achieved, the system will shutdown, to protect the adsorption of water vapour by the activated carbon, in the vessel further on in the system.
THE COMPRESSED AIR NEEDS TO BE UNSATURATED BEFORE ENTERING THE CARBON BED VESSEL !

The first 3 minutes after starting the generator any high oxygen alarm is suppressed and the nitrogen cannot be delivered to deck.
The unit switches from delivery to purge automatically, caused by high oxygen level or high deck main line pressure, the unit will switch back  to delivery automatically, when these alarms are not available anymore. Before the deck pressure high alarm will stop delivery, a lamp at the CCRP indicates “over capacity” at the “Bleed/Excess gas vent” line.


Replace filter elements every 1200 running hours. NEW CARBON FILLING EVERY 1200 RUNNING HOURS.
NEVER RUN WITHOUT FILTER ELEMENTS:

When the nitrogen room is outside the engine room it must have its own exhaust ventilation system at 6 airchanges per hour with a low oxygen alarm. Without any direct access to accommodation spaces, service spaces or control spaces. It must have a automatic means to dischg off spec gas to the atmosphere, during start up and abnormal operations.

The system must have minimum 2 compressors which cannot be less than1/3 of the total required  capacity.  If there is only one compressor then there must be sufficient spares.
Instrumentation :
Permanent recording of oxygen content

Audible and visual alarms with automatic shutdowns for—
Low feed air pressure from compressor
High air temperature
High condensate level at automatic drain of water separator
Failure of electrical heater
Failure of power supply

PSA TYPE:

PSA generators can produce N2 with oxygen of only 4 ppm.
It works on the principle that the two major constituents of air (nitrogen and oxygen ) are adsorbed to a different extent when passed over a carbon adsorbant molecular sieve. The quantity of gas adsorbed depends on time of exposure. The sieve adsorbs the oxygen and allows nitrogen to pass through and be collected. The oxygen is then desorbed ( returned to gas form ) and vented out, thus regenerating the sieve. The air must be from an oil free compressor. The nitrogen generated is very pure and below the limit of dewpoint required for purging ( minus 40 degrees ).
Typical values:
Purity of nitrogen/ 99.9995%
Oxygen/ < 10ppm
CO< 1 ppm
CO2< 1 ppm
HC< 0.1 ppm
A unique PFX control valve allows purity level selections.
Zero air module/ oxidizes HC from air inlet supply.
Uses high performance molecular sieving carbon (MSC ). Input of compressed air is reduced.
Path: Screw air compressor/ oil filters/ activated carbon filters/ water cooled heat exchanger / water separator/ air drier/ airbottle/ 2 nos aDsorpsion columns around 650 litres each with pressure guage/ oxygen analyser with solenoid inlet valve/ flow control valve/ nitrogen tank in room / buffer tank in pumproom/ deck
MSC adsorbs oxygen and CO2 and extracts nitrogen as a continious supply.
The adsorbed oxygen and CO2 is desorbed and restored to normal pressure and ejected to atmosphere via a silencer.
Nitrogen starts filling the tank only when the oxygen content drops to the specified value.
Make sure no oil/ water or impurities are fed to MSA –as it will get irreversibly damaged and performance will deteriorate.
It takes about 20 to 40 minutes for the generator to stabilise before the deck starts receiving nitrogen—depends on del pressure , flow rate, air temp etc
If the generator is not used for a long time run the system every 2 weeks or purge 99.99% nitrogen from a bottle through the MSC to prevent it from deteriorating. The MSC must have nitrogen always between 1 to 2 bars
Parameters of PSA nitrogen generator on a typical 20000 DWT tanker :
Air compressor supply/ 7 kg
Nitrogen flow-- 100 cum/ hr
Nitrogen outlet pressure/ 5 kg
Purity/ 99.9%
Dew point/ < minus 50C
Particles removed tp 0.01 microns and oil carry over <0.01 ppm before entering MSC.
Ensure oil and water filters are renewed and oxygen analyser calibrated as per the manual

     BLOCK AND BLEED ARRANGEMENT ( IN LIEU OF DECK SEAL )

Company  policy requires that the Block and Bleed valves, be require properly  maintenaned at regular intervals and all the valves (Regulating valve, Block valves, Bleed valve, Non-Return valve and the Deck Main Isolating valve) be regularly tested for proper operation and tightness.

Unlike the fool proof deck seal, this arrangement requires close monitoring and testing alarms for safety.

When chemical tankers with a Nitrogen plant carries both chemical and Annex 1 cargoes, a BLOCK AND BLEED arrangement is used instead of a deck seal, to prevent backflow from cargo tank into IG plant and safe non-cargo spaces.

 It is an arrangement consisting of two shut-off block valves in series with a venting to atmosphere bleed valve in between (double block and bleed).  It is between the DMV and the GRV . The operation of the valves is to be automatically executed. Signals for opening/ closing is to be taken from the process directly, e.g. inert gas flow or differential pressure. Alarm for faulty operation of the valves is to be provided, e.g. the operation status of “Blower Stop” and “supply valves open” is an alarm condition”. 

The size of the bleed valve should be such that it will not allow pressure build-up from back-flow of hydrocarbons in the event that the block and bleed arrangement and the non return valve) fail to achieve their function. Block and Bleed arrangement is triggered by zero flow condition. The differential pressure arrangement will sense the difference between the pressure in the line upstream of the pressure regulating valve and the pressure in the IG main.

 If the pressure in the IG main is higher than the pressure upstream of the control valve, the system will close the gas regulating valve and shut down automatically. Indication of this condition should be provided in the Engine Control Room and the Cargo Control Room. The measuring points should be arranged to preclude bypass of hydrocarbons from the tanks into the engine room via the sensing lines by, for example, using secondary 3 to 15 p.s.i. loop signal in lieu of direct signal or by using other suitable electronic sensors. A timing mechanism, providing a few seconds delay, should be included into the starting sequence to permit the blower to start while the pressure in the tanks is higher than the pressure at the blower discharge.

The effectiveness of the Block and Bleed devices depends on their ability to sense the “process” fluid (Inert Gas) pressure or flow. In some instances, deficient systems have been built with Block and Bleed arrangements which do not sense the “process” fluid but rely on the blower operational “on” / “off” status to operate the Block and Bleed valves and achieve the sealing function.

For Block and Bleed arrangements which do not have differential pressure or flow sensing devices, it is possible that, during periods of cargo discharge operation with the blower operational, the tank pressure may increase to a point higher than the inert gas generator discharge pressure. In this case, cargo gases back-flow into the machinery space could occur if the Non-Return valve was defective. Thus, when the Block and Bleed arrangements depend on the blower “on” / “off” condition, there is effectively only one protective non-return device in the system, namely the Non-Return valve. Such arrangement does not conform as an alternative arrangement providing a measure of safety “Equivalent” to that of a water seal.

The minimum hardware recommended for proper Block and Bleed arrangement are indicated below.
1. Block and Bleed control to be based on signals from a differential pressure or flow sensing detector of the “process” fluid, Inert Gas or Nitrogen.
2. Provision of an audible and visual alarm for low differential pressure or low flow if the block valves remain open.
3. Provision of an audible and visual alarm for faulty operation of the block and bleed valves. Blower “stop” and supply valves “open” should be an alarm condition.
4. Audible and visual alarm to be provided if valve position is mismatched ((i.e. if Block Valves and Bleed Valve are closed) – this function to be provided with a time delay.
5. Provision of indication in the Cargo Control room of “open” and “closed” status of the block and bleed valves, the Gas Regulating valve, and Bleed to Atmosphere valve.
6. Provision of control air for the Block and Bleed Valves to be via an independent solenoid valve arrangement from the solenoid supplying the Regulating Valve (Bulkhead Valve).
7. The pressure sensors used for the block and bleed control should be independent from the pressure sensor required by SOLAS Regulation 62.16.1.1 (equivalent Regulation 14.1.1 on Reference A)
8. Provision of interlock to ensure that Blower shutdown will close the Block and Bleed arrangement and that the Block and Bleed arrangement cannot be opened unless blower power is on. Also, the blower should be arranged so that it cannot start unless the block valves are closed.

The bleed ( vent ) to atmosphere is best operated by power.

Flow diagram schematics:
N2 plant—valve---vent to atmosphere---pressure transmitter---GRV/ safe area to dangerous area/ bleed to atmosphere with flame screen ---  block and bleed arrangement ( in lieu of deck seal )--- NR valve—pressure transmitter of differential pressure device.











-------CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )
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3 comments:

  1. sir,
    can u pls explain about the venting arrangement in parcel tanker???

    ReplyDelete
  2. hi rajeev,

    when you load liquid cargo you can expel air from tank via-

    1)PV cone by pressure lift +2000 mm aq, in case cargo is toxic /inflammable/ corrosive
    2)in case the vapor is harmless , by purge pipe or by PV vent open vaccum side (as both have flame arrestors ).

    when you discharge you take in air to replace liquid by--

    1)nitrogen generator via VRL
    2)in case the cargo is non-flammable/unreactive by PV vent open vacuum side or purge pipe. never allow vacuum side to lift at -350mm aq during discharge. do NOT keep tankdome and butterworth ports open, as standard practise .

    capt ajit vadakayil
    ..

    ReplyDelete
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    ReplyDelete