Chlorine leakage from bonnet of a valve in a bullet – a case study
R. K. Gangopadhyay, S. K. Das+ and M. Mukherjee*
Chemical Engineering Department
University of Calcutta University 92, A. P. C. Road
Kolkata – 700 009, INDIA
*Department of Polymer Engineering
Birla Institute of Technology, Mesra, Ranchi, INDIA
+Voice : 033 2566 7650 (R). E-mail :
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Abstract
This paper deals with the accidental release of chlorine from bonnet of a valve in a bullet situated in a chloro-alkali industry, the probable causes of the accident and the ensuing sequence of events. Emergency procedures are also discussed. Finally, in the light of the conclusions reached some useful recommendations are drawn for industrial facilities handling chlorine.
Keywords : Chlorine, Valve, leakage
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1. Introduction
Chlorine is extremely irritating to the mucous membranes, the eyes and the respiratory tract. In extreme cases, lung tissues may be attacked resulting in pulmonary edema. Splashes of liquid chlorine on the eyes, skin and clothing may cause immediate irritation and chemical burns as well as severe damages to body tissues. In the presence of moisture, it is highly corrosive and attacks most common metals.
Human exposure to chlorine in the past has resulted in a large number of deaths and injuries. Marco and Santamaria (1998) pointed out from the MHIDAS database of 96 accidents within the period 1964-1996. These resulted in 39 deaths and over 2700 injured. They also showed that mechanical failure (32%) was the main cause of incidents resulting in chlorine release. The threshold values are as follows (Schmittinger et al., 1986)
Odour threshold : 0.31 ppm
PEL (OSHA) : 1 ppm
TLV-STEL (ACGIH) : 3 ppm
TLV-TWA (ACGIH) : 0.5 ppm
IDLH (NIOSH, MSHA) : 2.5 ppm
This paper deals with the incident of accidental release of chlorine from bonnet of a valve in a bullet situated in a chloro-alkali industry and prepare a disaster management plan to identify and mitigate the condition.
2. The chlorine bullet
In a chloro-alkali plant situated in Durgapur, West Bengal there are four dish-end cylindrical shaped horizontal, thermocool insulated liquid chlorine storage tanks. The storage vessels are made of mild steel sheet of 21-mm thickness and of overall 12548 mm in length with inside diameter of 2743 mm. On the top there is one manhole cover on which six valves are fitted with necessary pipe connections. Out of the six valves provided in each storage tanks - two for outlet of liquid chlorine, one for inlet of chlorine, one for safety valve and one each for equalizer and air pressurization. There is one safety relief valve and one rapture disc, one temperature gauge and pressure gauge fitted in each of the four storage tanks.
The internal volume of each storage tank is 71 m3. At any point of time three vessels are meant for use and one vessel is supposed to be in depressurized and empty condition for emergency transfer.
3. Events leading to the accident (Anandabazar Patrika, 1987)
Around 11 pm on 09.06.87 the process controller started degassing of one chlorine tank. After about one hour, chlorine gas nuisance was noticed around the plant. The process controller thought that this was due to the leakage in the degas line and he decreased degassing by throttling the degassing valve. At about 2 am, the plant people again complained about gas nuisance. On checking it was found leakage of chlorine from liquid outlet valve of third chlorine tank. It was tried to rectify the leakage by tightening but these could not be done since the bolts were too jammed and due to excessive chlorine gas leakage on the work spot. To reduce the chlorine leakage the valve was opened using hand-wheel. But on opening the valve, the leak increased and the valve was closed immediately. On checking it was found that there was leakage from the bonnet joint of the valve. At early morning about 7 am it was observed that the drop of liquid chlorine coming out from the valve bonnet and the pressure of the bullet was 2.7 kg/cm2. An operator took a cannister type gas mask and went to the valve, on checking he found one stud nut was freely rotating and saw that the stud was already sheared. Then the higher authority decided that the bonnet should be clamped since other studs may also fail. Then the degassing operation was started at about 7.30 am through equalizer, but it was observed that two tanks could not be degassed at the same time due to common degas line. At about 8 am the pressure of the tank was 2.2 kg/cm2. At about 8.30 am the plant siren began to sound as emergency measure. It was observed that huge leakage of chlorine had taken place and the valve bonnet had already come off and was lying at some distance from the valve.
4. Accident and its aftermath
Water spraying from hydrant points was started at about 8.30 am on the leakage area to arrest the spreading of chlorine gas using three fire service hoses. Water curtaining was further strengthened by five more hose connection.
Second outlet valve of the leakage tank and one outlet valve of the first tank was opened to transfer the contents to the first tank but no effective inter transfer could be done. Therefore attempts were made to plug the cavity by wooden plug but failed. Ultimately at about 5.30 pm the hole was plugged and that time force of leakage had also reduced considerably.
Up to 1 pm, the wind direction was towards the north, which was not a densely populated locality. But after 1.30 pm due to cyclonic weather, the wind veered south and there was widespread panic in the adjoining localities. The residents of the adjoining locality situated at the south and west had to vacate their residences temporarily for a safer shelter. However, a heavy shower for about an hour brought relief by arresting to some extent the spreading of gas. Later the wind direction again reverted towards north.
5. Casualty
About 72 persons were affected of whom 9 were admitted to the hospital. After about 72 hours all were discharged from the hospital. None of the admitted cases was of a serious nature. No person died.
6. Probable causes of the accident
It was first noticed the gas nuisance at 12 O' clock at night on 09.06.87 and presumed a leak at the gland. As no one experienced a bonnet leak before and as light on the spot was not sufficient workers never thought a leak could be there through body seal of the bonnet. Workers attended the leak of the gland at the flange and did not see the bonnet leak and tried to tighten the bolts on the gland flange. Tightening was not possible and it was too jammed and also difficult to withstand in the workplace due to chlorine leakage.
Leak from the bonnet joint continued till 3.45 am of 10.06.87, when bonnet leak was detected. Liquid chlorine vaporized at the bonnet joint to bring down the bonnet and the body nut to a temperature much below 0° C.
At a low temperature the ductility of body nut was reduced and the stud became brittle. Someone wanted to tighten the bolt and with little force of brittle nut sheared to increase the leak. As the leak increased temperature was brought down further and frosting was there all around the bonnet and the bonnet stud. Ultimately all the studs failed at 8.30 am of 10.06.87. Failure looked like a failure by shear as the material failed under cryogenic condition when the material was brittle.
The release of liquid chlorine through the large hole as the valve was thrown out of its position gave rise to huge spillage of liquid chlorine from the tank. A part of the liquid chlorine evaporated immediately with large expansion in volumes (One volume of liquid chlorine expands approximately 450 volumes of gas at normal pressure). It is reasonable to expect that the major portion of the liquid chlorine released at high velocity will spill over to the floor of the liquid chlorine storage tank house and will settle in the form of crystalline solid of chlorine hydrate, Cl2,8H2O, in presence of water mist created by water spray blanket. The chlorine hydrate so formed will release acidic vapour for a long time even after the leakage was stopped. It was necessary to neutralize the chlorine hydrate on the floor with lime water or caustic soda solution. Such neutralization was, however, done at a much later stage.
The spillage of liquid chlorine on the insulation of the adjacent storage tanks is likely to affect the materials of construction of the tanks, since severe corrosion is likely to be encountered in case of chlorine penetration into the insulation of the tanks.
Chlorine is very slightly soluble in water reaching its maximum at approximately 1% at 9.6° C. At the normal water temperature of 25 - 27° C the solubility of chlorine is around 0.6gm/100gm of water.
However, creation of water blanket helps formation of solid chlorine hydrate at low temperature of liquid chlorine, which can then be treated with lime or better with caustic solution.
7. Disaster Management Plan
Fig. 1 shows a out line of a disaster management plan which is based on the hazard identification, prediction and prevention. The disaster management plan of any process industry must contain the following points
§ Identification of hazardous chemicals, processes and the operations
§ Release scenarios, consequences in term of heat generation, over pressure and toxic release etc.
§ Preparation of site plan for damage control
§ Identification of the vulnerable zones
§ Classification of unit or units which have the most potential for creating on-site as well as off-site emergency
§ Identification of the important facilities available in the vulnerable zone
§ Identification of the requirements of various departments in-site as well as out-site the process plant for coping emergency situation
Several techniques are available for the identification and evaluation of hazards in the chemical process industries, like (i) safety audit including check list procedure, most extensively used in India and mandatory to carry out every year; (ii) The more creative and open-ended procedure is the Hazard Operability Study (HAZOP).
8. Rescue and evacuation
For short term release, the useful action for the nearby residents would be to remain indoors with door, windows and other ventilators etc closed and shut off air conditioner system. The evacuation from the area must be through of only as a last resort and if the emission continues for more than 30 minutes or more (Ramabrahmam and Swaminathan, 2000). Decisions on the necessity of evacuation as well as evacuation distances are depending on the intensity of the release. The damage distance determine by using the local area map and the population likely to be affected around the plant is estimated. The evacuation must be carried out as quickly as possible. An emergency plan is required to serve this purpose.
9. Public information
The District Collector / District Emergency Authority or his representative will act as the chief coordinator. After raising the on-site emergency alarm, the on-site emergency control room should inform the District Authority, Pollution Control Officer, Police Superintendent, Divisional Fire Officer and District Medical Officer about the accident and the probable extent of damage and the action taken report. On the advice of the on-site emergency controller, the District Authority will declare an emergency and arrange to inform the public concerned using a siren, public address system, radio/TV etc. The District Authority will be responsible for announcing the end of the emergency on the advice of the on-site emergency controller. Fig. 2 shows the network of communication between all the various agencies.
10. Roles and Responsibilities
Roles and responsibilities of various agencies organizations and individuals are to be clearly spelt out. The key personnel are as follows,
· District Emergency Authority/District Collector/Chief Emergency coordinator
· Police Services
· Fire Services
· Medical Services
· Transport Services
· State Pollution Control Board
· Inspector of Factories
11. Testing the plan
The plan needs to be tested with the help of standard procedure and using actual mock drill. The shortcoming and insufficiencies in the plan could be identified and need to modify and fine tuned.
12. Conclusions and recommendation or suggestion of preventive measure
The chlorine release was an extremely serious accident that could have had severe consequences under less favourable circumstances. It is clear that the residents at the neighboring areas seems to lack information regarding the potential hazards related to the activity in the plant, warning procedures in case of emergency and the actions that they should take in that event. The recommendations are as follows,
1. One liquid chlorine storage tank should be kept empty and in completely depressurized condition so that the transfer of chlorine can be done in an emergency.
2. Bolts, nuts and studs, other pipe connections etc. of proper material specification strictly be used where there is chance of chlorine leakage.
3. A proper system of periodic inspection of all plants and equipment including cocks, valves and pipelines and degassing system should be introduced and followed jointly by the process and maintenance department. Preventive maintenance should be planned in a manner to synchronize gradually with periodic routine shut down of equipment and plant.