Answer EKG Question 15

Q15. With reference to Flue gas Inert gas system:

a. Sketch a line diagram showing a typical ‘Inert Gas System’ used for inerting the cargo tanks of oil tankers, labelling the component parts.

b. Describe the system.

c. State what oxygen content you would expect in the flue gases if good combustion is achieved.

Answer:

Cargo tank protection

Fire and explosions in the cargo tank of oil tanker can occur due to a wide variety of causes including static electricity, pyrophoric ignition and certain metal impact, if the atmosphere inside the tank is within flammable limits.

The flammable limits vary for different petroleum liquids.

In practice the lower and upper flammable limit of oil cargoes can be taken 1% and 10% hydrocarbon by volume respectively. When an inert gas is added to a hydrocarbon gas and air mixture the result is to increase the lower flammable limits concentration and to decrease the upper flammable limit concentrations.

Hydrocarbon and their mixture without inert gas lie on the line AB the slope of the line shows the reduction in oxygen content as the hydrocarbon content increases.

Points to the left of AB represent mixture with their oxygen content for the reduced by addition of IG.

From this figure it is evident by adding inert gas in the hydrocarbon and their mixture the flame level range decreases until oxygen content reaches about 11% by volume at which no mixture can burn.

8% by volume specified by the IMO regulation gives safety margin.

SOLAS regulation for cargo tank protection:

In accordance with the regulation fixed inert gas system shall be equipped on the following tankers. 

 New tanker of 20,000 deadweight and upward.

Tankers of 20,000 is deadweight and upward constructed before 1st September 1984 which is engaged in the trade of caring crude oil shall be fitted with inert gas system not later than

A: Tanker of 70000 tonnage deadweight and upward 1st September 1984 the date of delivery of the ship Which ever Rockers letter and

B: For a tanker of less than 70000 is deadweight 1st May 1985 of the date of delivery of the ship which never occurs later.

Tankers of 40000 is deadweight and upward constructed before 1st September 1984 which are engaged in a trade of carrying coil other than crude oil and any such tanker of 20000 is deadweight and upward engaged in the trade of caring oil other than crude oil fitted with tank washing machine having individual throughput of greater than 60 metre cube per hour shall be fitted with another gas system within the allowable provided as above.

All tankers operated with a cargo tank cleaning procedure using crude oil washing shall be fitted with inert gas system.

Method of gas replacement.

    a)   Inerting

    b)   Purging

    c)    Gas freeing

Each of these replacement operations one of two processes can predominate.

(i) Dilution which is a mixing process

(ii) Displacement which is a layering process.

Inert gas is oxygen deficient

Its Main function is to inhibit fire and explosion risk by removing one side of the fire Triangle.

Other advantage of inert gas is the reduction in pumping time as always positive pressure is maintained in the tank.

 A). Sketch a line diagram showing a typical ‘Inert Gas System’ used for inerting the cargo tanks of oil tankers, Describe the system labeling the important component parts. 

Inert gas installation for tankers from funnel gas:-

Engine exhaust is not used as it has too high percentage of oxygen especially at light load

(A)Boiler uptake: - point positioned

- It should not be too near to funnel top as there is risk of Ingress of air.

- Not too low uptake that will cause too hot gas and soot ingress, also it may cause hard deposit on the flue gas isolation valves.

Funnel gas - temperature is 300 degree Celsius approximately contains 12 to 14% CO2 and 2-4% oxygen 0.3 - 0.5% of SO2 (SOx and NOx unburned hydrocarbons and nitrogen).

(B) Flue gases isolation valve:

There is an indication given for open and close for the valve. If valve is open - soot blower in the boiler cannot be operated. Valves provided with on suit blowing facility.

Material: - corrosion resistant heat resistant cast iron less than (220 degree Celsius).

(C) Scrubber: - internal part should be constructed in corrosion resistant material.

Material:  wash water and inlet box are of MS plate lined with two layer of glass fibre and reinforced epoxy resin coating. The flue gas from boilers is lead to the scrubber gas inlet duct where flue gas is cleaned from sulphur oxide and suit and cool down approximate 70 degree Celsius with pre cooling water spray as first step.

Afterwards the gas pass through the water sealing area in the bottom part of the scrubber where the gas is drastically cool down to normal temperature, design not more than 3 degree Celsius above sea water temperature. While the gas is coming up through this scrubber element of a corrugated plate type further cleaning and cooling of the gas takes place with the water sprays fitted at the top side.

Water sealing area serving two purposes.

-First to prevent hydrocarbon gases from flowing back to the boiler side. -Second to prevent boiler flue gases from coming into the scrubber.

The gas inlet duct is sealed with sealing depth of 150mm. The scrubber is provided with the high level switch of a floating type with which the high level alarm can trip both the cooling water pump and the inert gas system operation this alarm is most important to avoid the water backflow to the boiler side.

For maximum contact between gas and water one or more arrangement adopted (i) spray nozzle (ii) tray of packed stone or plastic chippings (iii) impingement plates (iv) Venture nozzle and slot adequate opening and sight glasses (impact and heat resistance type) for inspection cleaning and observation purposes.

(D) Demister: - the structure is made up of mild steel coated with tar epoxy greater than 250 microns thick corrosion-resistant. It removes solid and any entertained water and dried. (98.5% of suit particle larger than 1 Micron is removed as to contain reduce less than 150 PPM) demister may be corrugated plate type or vortex type with a drain. it can be cleaned by back flushing.

(E) Inert gas blowers casing is made up of mild steel plate with epoxy coating. It is a single stage centrifugal type as driven by electric motor. With which inert gas or fresh air is supplied into cargo tanks under optimum positive pressure on the Deck main line.

Impeller is made up of aluminium bronze which should be stress relieved.

Tested by overspeeding to 20% of the designed the speed of electric motor for 10% above the speed at which speed trip of turbine operate.

Fresh water washing arrangement is provided to remove deposits. Deposits can cause vibration.

Blower characteristics:- minimum pressure 200 mm-wg when cargo discharged.

At least two blower shall be provided which together shall be capable of delivering inert gas to a cargo tank at least 125% of the maximum rate of discharge capacity of the ship by volume.

One large blower and a small blower for topping up purpose.

Or two blowers can meet above requirement. Advantage is that change over the blower in case the blower is defective.

 

(F) Pressure control valve (gas pressure regulating valve and recirculation valve)

- Main pressure control valve it regulate the flow of inert gas to the inert gas deck men.

- purge control valve it automatically prevent backflow if cargo tank pressure exceeds IGI main pressure and mechanical non return valve and take water seal fails it also closes if inert gas blower scrubber pump extra fails.

(G) Vent valve: - non return valve and regulating valve vented when non return valve/ deck isolation valve manually closed.

(H) Non return device: - it prevents backflow of hydrocarbon vapour cargo found the cargo tank to the machinery space uptake other safe area.

*deck water seal should present a resistance to back flow of not less than the pressure setting of the pressure vacuum breaker. Water flow to be maintained, steam heating pipe provided in the tank of the Deck water seal to ensure function even in cold ambient condition.

- Wet type: Demister to be fitted to reduce any carry over

- semi dry type: Instead of bubbling through the water trap the inert gas float draw the seal water into separate holding chamber by Venture action avoiding/ least water droplets.

- dry type: It is fitted with water when the IG plant is either shut down for the tank pressure exceeds the IG blower discharge pressure fitting and drain are performed by automatically operated valves are controlled by levels in the water seal and drop tanks by the operating state of the blower.

Advantage: - water carry over prevented.

Disadvantage: - risk of failure of auto valves

* The Deck seal is designed so that back seal of 2500 mm Aq. Can be maintained.

Or a mechanical non return valve can be fitted automatically if backflow.

(I) deck isolation valve: - to be used during maintenance

(J) Pressure vacuum breaker (liquid filled): - the pressure vacuum breaker is provided to prevent the cargo tank from be subjected to an excessive positive pressure during loading and discharging operation. Capacity is equal to IG system capacity correct level of water and glycol 20% is used.

Excess pressure means positive pressure in excess of tank test pressure. Negative pressure means -700 mm wg.

 

(k) Mast riser: - incorporated PV valve and Bypass valve. The PV valve formed of ring valve and solid valve to being superimposed and bevelled angle 45 degree both valves fitted with light springs. Cage surrounded by gauge screen set to operate 1900 mm wg positive and 400 mm of wg negative.

When filling both solid and ring valve lifts but when pumping out solid valve depressed only.

Instrumentation and alarm: -

- Indicate and record gas pressure forward of non-return device and oxygen content

- indicate temperature at I.G fan discharge.

- Audio and visual alarm

1. Low water level in scrubber it will auto shutdown blower and gas regulating valve

2. High water level in scrubber will auto shutdown blower and gas regulator valve.

3. Highest temperature will automatically shut down blower and gas regulating valve.

4. Fan failure automatically shut down gas regulating valve.

5. Power failure in control system in regulating device will give alarm in machinery spaces and cargo control room.

6. Low level and Decor dress IL will auto shutdown of her go pump or independent alarm.

7. Oxygen content is between 5% to 8% will give alarm.

8. Gas pressure lower than 100 mm of wg will auto shutdown of cargo pump or independent alarm.

9. High gas pressure will give audible in visual alarm.

Cargo tanks must be protected against both pressure and vacuum.

Pressure vacuum valves which are set to operate at 1900 mm of wg water positive pressure and 400 mm of wg negative pressure, moderate pressure acting on large surface area of cargo tank can cause damage either in the form of bulging or inward collapse small volume of vapour pressure caused by natural aspiration of the cargo tank and relieved by means of PV valves. They can also release moderate change in tank pressure due to variation of temperature and vapour quantity.

High velocity vents:

No tanks should be vented during loading or ballasting through high velocity vent and mast head vents as practice of vending through open tank is dangerous particularly during thunder conditions a simple design event has two weighted flaps which are pushed open by pressure built up to achieve ejector effect with an estimated velocity of per second will dilute the fume. Flame trap gives protection against flame travel. This gauge flame trap to be cleaned regularly to ensure venting.

If IG system become defective and repair cannot be made and another arrangement cannot be made the following to be observed.

- For crude oil tankers cargo tank must be maintained in an inert condition to prevent pyro phosphoric oxidation which can cause exothermic oxidation can cause explosion.

- For product tanker detailed emergency processor are listed includes severe restrictions on tank dipping ullaging and sampling.

b. State what oxygen content you would expect in the flue gases if good combustion is achieved. 

For good combustion the percentage by volume of the composition of the inert gas will be as follows:

(1) Nitrogen—–85 %.

(2) Carbon di oxide——11%.

(3) Oxygen—————–4%.

When the combustion is good there cannot be any carbon monoxide in the flue gases. If the combustion is good there cannot be any un-burnt hydro carbons in the flue gas