Q:-With respect to refrigeration gases used on board vessels answer the following.Explain ozone depleting potential of conventional ref gases.
Name alternative refrigeration gases available and being used on board Explain the steps you will take to ensure that release of refrigeration gases from the plant is minimised during normal operation and during maintenance activities.
ANSWER :-
- The chlorofluorocarbons (CFCs) and hydrochloroflurocarbons (HCFCs) are referred to as ozone depleting substances (ODS), because once these gases are released into the environment and reach the stratosphere, they interact with the ozone layer and destroy ozone molecules. ODS lifetime in the stratosphere is between 100 and 400 years.
- An ODS molecule has potential to destroy ozone molecules during its entire lifetime.Therefore,various CFCs and HCFCs are assigned Ozone Depletion Potentials (ODP) depending on their potential (specified relative to CFC-11) to cause ozone depletion in the stratosphere.
- Ozone is a gas composed of three bonded oxygen atoms (O3).In the Earth’s atmosphere, ozone is formed from molecular oxygen (O2) in the reactions initiated by the UV light.
- Ozone can be found in two levels, at ground level and in the Earth’s upper atmosphere, referred to as the stratosphere. At ground level, ozone is a significant air pollutant, forming smog. In the stratosphere it is referred to as the ozone layer.
- The ozone layer encircles the stratosphere at approximately 10 km above ground level. It filters ultraviolet (UV) radiation reducing the amount of radiation reaching ground level. The depletion of the ozone layer exposes living organisms to high levels of the harmful UV-B radiation.
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OZONE DEPLETION POTENTIAL
Ozone depleting substances (ODS) vary in their capacity to destroy ozone molecules. So scientists have developed a method of characterizing the relative depletion caused by different ODS.
The ODP or Ozone Depletion Potential, is the potential for a single molecule of the refrigerant to destroy the Ozone Layer.All of the refrigerants use R11 as a datum reference and thus R11 has an ODP of 1.0.
The less the value of the ODP the better the refrigerant is for the ozone layer and therefore the environment.
GLOBAL WARMING POTENTIAL
Global warming potential (GWP) is a measure of how much a given mass of a gas contributes to global warming. GWP is a relative scale which compares the amount of heat trapped by green house gas to the amount of heat trapped in the same mass of CO2.
ODP and GWP of conventional refrigerant gas are as follows:-
Gas ODP GWP
R-11 1.0 4000
R-12 1.0 2400
R-22 0.05 1700
Halon 1211 4
Halon 1301 16
Because of much higher ODP halon is banned.
Alternate gases available on board which are used in refrigeration systems are :-
- R 134a (ODP=0)is long term replacement for R-12 and is best performed in medium and high temperature application.
- R 410A (ODP=0) is twice as efficient as R-22 but is recommended for new systems only.
- R 407C (ODP=0) is suitable for medium and high temperature application. And is suitable for new system and for R-22 change over.
- R 404A (ODP=0) is suitable for low and medium temperature application. It is suitable for new marine system.
As per Annex VI , Regulation 12:- Ozone Depleting Substances (ODS)
1.Existing systems and equipment using ODS are permitted to continue in service and may be recharged as necessary. However, the deliberate discharge of ODS to the atmosphere is prohibited.
2.Maintenance, servicing and repair work shall be carried out without releasing any substantial quantity of refrigerant.
3.When servicing or decommissioning systems or equipment containing ODS the gases are to be duly collected in a controlled manner and, if not to be reused onboard, are to be landed to appropriate reception facilities for banking or destruction.
4.Any redundant equipment or material containing ODS is to be landed ashore for appropriate decommissioning or disposal. The latter also applies when a ship is dismantled at the end of its service life.
To minimize the release of refrigerant gas from plant following steps should be taken:
During operation ---
- Most important to maintain daily log of referent commonly called reefer log .This is to be maintain by duty engineer and cross checked by chief engineer to ensure effective monitoring and early detection of any abnormality which can lead to gas leak to atmosphere.
- During normal operation loss of refrigerant from leaking joints, seals, gaskets and cracked pipe should be checked.
- Loss of refrigerant from safety relief valve, to overcome this pressure to be maintain in range.
- Damaged mechanical seals on open type compressor are frequent source of refrigerant leaks. A clean dry system is essential for prolonged mechanical seal effectiveness to eliminate emission. Compressor oils used for HCFC and HFC will absorb moisture readily and must keep dry to prevent refrigerant decomposition.
- Excessive vibration and excess water pressure should not be allowed in the condenser to avoid tube failure.
- Leak testing should be carried out regularly. Testing can be done by bubble testing with soap solution or by electronic leak detection.
During maintenance ----
- Loss of small quantities of refrigerant from charging lines during charging has to be avoid by taken proper care of connection .
- Before doing any maintenance gas should be recovered and not leaked in environment. There is recovery cylinder is there on board and a vacuums pump to recover gas from system.
- During maintenance compatible gaskets should be used which are compatible with the gas and oil used in the system.
- Evacuate the hoses before disconnecting temporary equipment.
- Practice recovery and recycling when recharging dryers and filters.
Records and documents to be maintained:-
a) A list of equipment containing ODS should be maintained.
b) If the ship has any rechargeable system containing ODS, then an ODS record book should be maintained. This record book shall be approved by administration.
c) Check for gas leaks to be carried out regularly and recored.
d) Entries in ODS record book shall be recorded in terms of mass( kg) of substance in respect of-
- Recharge of equipment
- Repair or maintenance
- Discharge of ODS to atmosphere either deliberate or non deliberate
- Discharge of ODS to land based facilities
- Supply of ODS to ship
