Explosive Range
We have to consider the tank atmosphere which. may contain any one of three gases such as hydrocarbon gas, inert gas and fresh air.
Then we have to pay attention to hydrocarbon gas and oxygen in fresh air which are necessary to explosion. As oxygen content in fresh air is approx. 21 % we can draw the graph shown in Fig. 1-1 which shows hydrocarbon gas with the range from 0% to 100% in the vertical axis and oxygen content with the range from 0% to 21% in the horizontal axis.
Point "A" in Fig. 1-1 means the tank atmosphere which consists of 100% hydrocarbon gas.
Point "O" means the tank atmosphere which consists of 100% inert gas.
Point "E" means the tank atmosphere which consists of 21 % oxygen.
From the above we can consider any kind of tank atmosphere which consists of three gaseous bodies such as hydrocarbon, inert gas and fresh air according to this Fig. 1-1. This means, in other words,,that the tank atmosphere lies within the triangle AOE.
Explosive Range, UEL and LEL (also termed as UFL and LFL)
Under the tank atmosphere which consists of hydrocarbon gas, inert gas and fresh air, the range of mixture of oxygen and hydrocarbon gas which can be ignited by an external ignition source is called the "Explosive Range" or "Flammable Range".
Volume percentage of hydrocarbon gas and oxygen are called "Oxygen content" and "Hydrocarbon gas content".
This explosive range is shown in Fig. 1-1, the area surrounded by points CGD (Flammable envelope).
The highest point of hydrocarbon and oxygen mixture gas in this area is called the Upper Explosive Level (UEL) and the lowest point is called the Lower Explosive Level (LEL).
Non-Flammable Range
Fig. 1-1
No ignition occurs in the area which is outside the explosive range because of inadequate mixture of hydrocarbon gas and oxygen. This area is called the "Non-Flammable Range" or "Non-Explosive Range". This area is shown in Fig. 1-1
A. Too Little Oxygen
Assuming that the oxygen content is less than the point G (less than 11.5%) no explosion can occur even regardless hydrocarbon gas content in a tank. So we call this area surrounded by points AOFB as Too Little Oxygen condition.
The purpose of I.G.S. installation and the main points of I.G.S. operation is to ensure the tank atmosphere is in the Too Little Oxygen condition.
B. Over Rich and Too Lean
Compared with the volume of oxygen, the volume of hydrocarbon gas is too much in the area surrounded by points BGC and no explosion occurs in this area so referred to as the Over Rich condition. On the other hand, the area surrounded by points GFED shows that the volume of hydrocarbon gas is too little compared with the volume of oxygen and no explosion occurs in this area called the Too Lean condition.
How to Use the Explosive Range Diagram
The tank atmosphere which is not supplied with inert gas is to be considered on the line AE (Fig. 1-1) and it may come into the explosive range (Line CD). The vessel which is installed with I.G.S. can keep the tank atmosphere within the Too Little Oxygen condition by supplying inert gas.
The hydrocarbon composition of crude oil is various between the different grade and there is only a small variation in their explosion range. Therefore, in order to have the desired margin of safety, the point UEL is taken as 11.5% by hydrocarbon, LEL is 1.3% and the point of too little oxygen is 11.5% so we can draw boundary of the explosive range as shown in Fig. 1-2.
Keep tank atmosphere less than 8% of oxygen content
Fig. 1-2. is a magnified view of Fig. 1-1. (Area of hydrocarbon content from 0% to 17%).
When we want to know what the tank atmosphere condition is, we find it out by plotting oxygen and hydrocarbon content as in Fig. 1-2.
Assuming at the end of tank washing, a cargo tank atomosphere is measured and found to contain 7% hydrocarbon and 8% oxygen. Plotted on the Explosive Range Diagram this is point (I). If gas freeing was then carried out, the hydrocarbon content would decrease and the oxygen content increase, while passing along line IE. However, on reaching point K on the flammability envelope the tank atomosphere enters the explosive range and remains in this condition to point L. In order to avoid such a condition, it is therefore necessary to purge with Inert Gas before gas freeing with air. Purging the tank atomosphere with good quality Inert Gas will reduce the oxygen content as well as reducing the hydrocarbon content, passing along line IJ. Once at J i.e. 2% hydrocarbon and 5% of oxygen, the gas freeing operation can begin and the addition of air will them keep the tank atomosphere outside the explosive range by moving along line JE.
If we draw a line from point E to the tangent point of explosive range so we get the line EM which is called the Critical Dilution Line. Since the tank atmosphere comes below the Critical Dilution Line there is no possibility of explosion when we supply fresh air into the tanks.
We must therefore carry out gas freeing after confirmation of the tank atmosphere that it is below the Critical Dilution Line after measuring the tank oxygen content and hydrocarbon content.
COMPONENTS OF INERT GAS
The exhaust gas which is produced by the boiler is called Flue Gas. In the scrubber, the flue gas is cleaned and cooled and with an oxygen content less than 5% is known as Inert Gas. The equipment which supplies the Inert Gas to the cargo tanks is known as the Inert Gas System.
Automatic Combustion Control (ACC) is the equipment which controls the rate of fuel oil and fresh air automatically in the combustion process.
This equipment should be kept in proper operating condition in order to ensure the supply of Inert Gas of the desired quality. When operating cargo or ballast pumps, communication with the engine room must be maintained. This is particularly important when stopping or reducing the speed of cargo pumps, so that the combustion control operation must be checked to ensure Inert Gas oxygen content is maintained below 5%.
Components of Inert Gas (Fig. 1-3)
Flue Gas Inert Gas
Temperature 300-400 Sea Water Temp. +5 C
Oxygen 2-5% 2-5%
Carbonic acid gas 13-15% 13-15%
Sulphur 0.3% 0.03%
Vapour 250 mg/ml (max) 8 mg/ml (max)
Mist 95% of elimination
Nitrogen Balance Balance
TANK ATMOSPHERE AND INERT GAS
Discharging Cargo (Fig.1-4)
Ullage space in a cargo tank before discharging cargo is generally OVER RICH because of the high content of hydrocarbon gas.
Reference Fig. 1-4. Assume H is a typical content of oxygen and hydrocarbon before discharge begins.
Consider two conditions:-
o Discharging without Inert Gas
o Discharging with Inert Gas
In the first case, the hydrocarbon content will decrease and the oxygen content increase along the line HE, when at the end of discharge of, that tank, the atmosphere could be flammable.
In the second case, both hydrocarbon and oxygen contents will decrease towards J and the tank atmosphere will therefore remain outside the explosive range.
On Passage (Fig. 1-4)
In the absence of positive pressure on a cargo tank either loaded or in ballast air may enter through the breather valve due to climatic changes.
Loaded Passage
On the loaded passage, hydrocarbon gases likely to vaporize so that the ullage space is generally in an OVER RICH condition. However, with air coming in through the breather valve, the hydrocarbon content will be reduced and could possibly reach flammable levels. With Inert Gas protection this can be avoided by maintaining a positive pressure throughout the loaded passage.
Ballast Passage
Cargo tank as supplied with good quality inert gas while discharging cargo are in the TOO LITTLE OXYGEN condition when the vessel leaves port. If tanks are kept under positive pressure throughout the ballast passage, the tank atmosphere will not enter the explosive range. Therefore, it is necessary to record the tank pressure regularly and if necessary, top off with good quality inert gas.
Tank Cleaning (Fig. 1-5)
If good quality Inert Gas i.e., below 5% of OXYGEN is supplied when discharging cargo, the tank atmosphere will be inert and therefore ready for tank cleaning. However, positive pressure needs to have been maintained between the discharging and tank cleaning. If not, air car enter the cargo tanks thereby increasing the Oxygen content along line HI. Before starting tank cleaning, oxygen measurements should be made to confirm it is below 8%. If over 8%, then purging is necessary to reduce the oxygen content from I to J. Fig. 1-5
Deballasting. (Dirty – deballasting Heavy weather ballast) (Fig. 1-6)
Assuming H is point representing a typical tank atmosphere prior to deballasting. If NO inert gas supplied during deballasting, then the tank atmosphere will go along line HE, passing through the explosive range. If however, Inert Gas is supplied during this operation then the tank atmosphere moves along line HI and therefore outside the flammable envelope. Fig. 1-6
OUTLINE OF INERT GAS SYSTEM
Basic Equipmentof Inert Gas System
I.G.S equipment is provided with certain safety features and is designed to clean, cool and supply flue gas under pressure and of satisfactory quality for delivery to the cargo tanks. I.G.S. consists of the following.
Flue gas generated from the boiler flows through the Boiler Up-take Valve and into the Scrubber. There, the gas is cooled down and washed by sea water supplied by the Scrubber Water Pump. Leaving the Scrubber the gas passes through the Demister where water droplets are moved before entering the Blower suction. On the discharge side of the Blower, oxygen content and temperature of the flue gas are monitored. High oxygen content and high temperature activate alarms.
Inert Gas from the blower flows in to the Deck Seal through Inert Gas Pressure Regulating Valve. Main line pressure is automatically controled to keep desired pressure constant. Excessive pressure is avoided by the Inert Gas Pressure Regulating Valve working in conjunction with the Recirculation Valve.
The Deck Seal isolates the boiler up-take from the deck line by using sea water and to prevent the backflow of the hydrocarbon gas. Inert Gas from the deck seal flows into the deck supply line through the Non-retum Valve and Deck Isolating Valve and then enters each tank through the Inert Gas Supply Valve (some vessels do not have individual valves). The Inert Gas System can also be used for Gas Freeing by opening the Fresh Air Inlet Valve.
The P/V breaker is installed to protect the cargo tanks from excessive pressure or vacuum.
Functions of the Inert Gas System Unit
BOILER UP-TAKE VALVE
Flue Gas generated by the boiler flows into the Scrubber Unit through the Up-Take valve. This valve is opened by remote control on Blower start up and when the Blower stops this valve has to be closed in order to avoid Flue Gas entering the Scrubber Unit.
SCRUBBER UNIT
This is installed to clean and cool the flue gas and to reduce the sulfur dioxide (SO2 ) from the flue gas. The temperature and composition of the flue gas is schown in Components of Inert Gas (Fig. 1-3).
DEMISTER
The demister is provided to remove water droplets contained in the inert gas which have passed through the scrubber. Since the inert gas is cooled and cleaned at the scrubber, the gas at the scrubber outlet inevitably contains water droplets as a result of its direct contact with the sea water used for cleaning it and could thus overload the blower and damage and increase corrosion on the blower impellers.
BLOWER
Total blower capacity is more than 125% of total cargo pumps capacity. And combination of two blowers is;-
o two blowers together giving 125% (i.e. 62.5% each) of total cargo pump capacity. Thus two blowers must be used during normal cargo discharging
o one blower having 125% capacity plus one standby/ auxiliary with either 30/ 60/ 125% of total cargo pump capacity
DECK SEAL UNIT
The IGS connects the boiler up-take indirectly with the cargo oil tanks, and while the system is not in operation, the backflow of the oil vapour under pressure from the cargo tanks must be protected against. The Deck Seal is provided for this purpose.
RECIRCULATION LINE
This allows the blower to operate when the pressure.regulating valve is being closed. Gas flows back through this line to the Scrubber and thus avoids pressure built up on the discharge side of the blower.
P/V (Pressure Vacuum) BREAKER
Under normal conditions, the Breather Valve controls the cargo tank pressure/vacuum automatically when the I.G.S. is off. As a back-up safety device, a Pressure/Vacuum Breaker is fitted to the deck main piping and is designed to release pressure from this piping and cargo tanks to atmosphere in the event that the Breather Valve capacity is exceeded while operating the I.G.S.
The P/V Breaker does not have any moving parts and is filled to be required level by oil or fresh water containing an antifreeze solution.
Detectors
OXYGEN ANALIZER (FIXED TYPE)
This contineously measures the oxygen content of the flue gas on the discharge side of the blowers. Oxygen content is effected by the combustion efficiency of the Boiler, or the proportion of surplus air. The alarm lamp flickers when the oxygen content is more than 5% and the alarm buzzer sounds when the content is more than 8%.
THERMOMETER
This is fitted on the blower outlet and continuously measure the temperature of the inert gas. The temperature of the inert gas at the blower suction is normally 5C higher than the scrubber water but it is increased while passing through the blower. The temperature alarm lamp flickers at 60C and the blower is tripped at 65C.
PRESSURE DETECTOR
This is fitted in order to measure the pressure in the main inert gas line common with all cargo tanks. The pressure at the outlet of the deck seal is detected by the pressure transmitter, which operates the pressure switch in the cargo control room panel.
TANK INERT GAS SUPPLY VALVE and VENT RISER MAIN VALVE
When using the Inert Gas System, Tank Supply Valve (if fitted) should be open and the Vent Riser Main Valve should be closed.
OPERATION AND APPLICATION OF INERT GAS SYSTEM
One cycle of tanker operation is as shown in Fig. 2.
Tank atmosphere is safe from explosion with an O2 content of below 8%.
The alarm buzzer of the O2 content analyzer will sound when the O2 content is more than 8% and the supply to the deck seal will stop and the recirculation valve will open. However, in normal operation, O2 content of supplied I.G. should be below 5%.
Tank atmosphere is most important therefore measurement of O2 content and Hydrocarbon gas should be carried out with strict attention according to sections.
TANKER OPERATION/ONE CYCLE
GENERAL OPERATION AND PRECAUTIONS
Generally, operating procedure for I.G.S. is as undermentioned.
However, operation of I.G.S. should be considered in conjunction with individual maker's instruction manuals.
General Operating Procedure
Starting procedure | General check & confirmation |
Communication with engine department | (1) a.Inform engineers when system will be required. (2hrs notice) b.Check : Possibility of soot blowing before starting the I.G.S. c.Confirming generater capacity/readiness for operating blower and scrubber water pump. |
Scrubber water pump START (at least half hour before starting system – dpending on Manufacturer)
| (2) a. Confirm Cooling water supply. b.Check : Flow rate by flow meter or pressure gauge. c.Confirm Scrubber drainage by visual inspection. |
O2 Analyzer switch ON | (3) a. Check : Zero adjust and Span adjust. Confirm : Supply water. b. A characteristic of the magnetic 02 analyzer is that it reaches normal measurement condition in about 2 hours after being switched on. Therefore, it should be switched on at least 3 hours before starting the I,G.S. |
Control panel switch ON | (4) a. Lamp and buzzer test should be carried out to confirm electric circuit is operating correctly |
CONFIRM | (5) a. Open : Scrubber drain valve Demister drain valve (Where appropriate) Blower drain valve Deck seal drain valve b. Confirm : Water level in deck seal unit. Water supply to deck seal unit. |
Fresh air inlet valve CLOSED | (6) a. It should always be closed except when gasfreeing. |
I.G. supply valve OPEN | (7) a. Confirm : I.G. supply valve for individual tanks (if fitted). b.Closed tank hatches, ullage ports, I.G. supply hatches and tank cleaning ports. |
Deck isolating valve OPEN | (8) -- |
Blower inlet valve OPEN | (9) |
Boiler up-take valve OPEN | (10) a. Confirm Opened completely. b. Confirm Seal air valve closed. |
No. 1 (or No.2) blower START | (11) a. Check Blower operating normally. b. Confirm Indication of ammeter normally. c.When using two blowers, the 2nd blower should be started 30 seconds after the 1st blower is started. |
Blower outlet valve OPEN | (12) a.It should be opened after blower is started. b.Confirm : Pressure attained. On blower start up, I.G. is recirculated automatically through the scrubber and recirculation line. |
Pressure regulating valve OPEN
| (13) a. It should be gradually opened. b.Check : Blower dischargeO2 content and temperature. c.Bringing up to required pressure. |
INERTING | (14) Inert Gas will be supplied into the tanks. |
B. Stopping procedure GeneralGeneral check & confirmation
INFORM Engine department | (1)- - - |
Blower STOP | (2)- - - |
Valves of I.G. plant CLOSE | (3) a. Boiler up-take valve b. Blower inlet valve c. Blower outlet valve d. Pressure regulating valve |
Deck isolating valve CLOSE | (4) - - - |
Deck seal water level CONFIRM | (5) a. Confirm: By observing water level through sight glass. |
Cooling down of scrubber CONFIRM | (6) a. Keep scrubber pump running after system shut down for about 30 minutes. |
Scrubber water pump STOP | (7) a. Confirm Water seal level in scrubber by visual inspection through sight glass. |
O2 content Analyzer STOP | 8)- - - |
Control panel switch OFF | 9)- - - |
General Precautions
1) Do not start up the I.G.S. without first informing the engine control room. Keep engine control room informed of start/stop requirements.
2) Inert gas supply valve of each cargo tank should be kept open at all times except gas freeing, tank repairing.
3) I.G. supply valves should not be operated without permission of the duty officer ( it generally has an arrangement to lock the valves – key with duty officer). The duty officer must pay careful attention to ensure the I.G. supply valves are open before any cargo movement either by cargo pump or gravity.
It is important to be aware to recognize that if I.G. supply valve is CLOSED and any cargo movement made, structural damage could occur.
4) The fresh air inlet valve must be kept closed at all times except when the I.G. blower is being used for gas freeing.
5) Seal water of deck seal unit should be supplied at all times whether the I.G.S. is operating or not, to ensure the seal water is kept at the proper level. The supply has one dedicated pump and a back up.
6) The operator must confirm that the outlet valve of the I.G. blower is closed before starting the I.G. blower.
(The blower outlet valve will automatically be opened on certain maker's systems.)
7) Before operating the I.G.S., the Supply of water into the Scrubber should be confirmed by visual checking of the pressure gauge and/or flow meter. Blocked spray nozzles can result in indicating the correct pressure but not the required flow. It is therefore important to keep the spray nozzles clean.
8) The liquid of the P/V breaker must be kept at a proper level at all times. The gauge glass should have spring loaded valves.
9) Prior starting the IGS the O2 analyser should be callibrated with Nitrogen.
While operating the I.G.S., the duty engineer must check the flxed O2 analyzer by means of comparing it with a measurement made by portable O2 meter from the blower outlet.
10) The impeller of the I.G. blower should be washed with fresh water as soon as possible after each time the system has been used.
(This is to prevent corrosion of the impeller and wash away soot deposits which might cause imbalance of the impeller and excessive vibration.)
11) The I.G. blower should be started by operating the switch installed beside the blower.
12) There are cases when the temperature of the blower outlet will rise during recirculation of I.G.
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