THERMAL RUNAWAY

Contents
WHAT ARE CHEMICAL REACTION HAZARDS?
WHAT DO YOU NEED TO DO?
SAFETY MEASURES
SAFETY MANAGEMENT
CONCLUSIONS
KEY QUESTIONS
REFERENCES

CHEMICAL REACTION HAZARDS and the risk of THERMAL RUNAWAY

A chemical reaction that goes out of control and runs away can create a serious incident with the risk of injury to people and damage to property and the environment. This article:

• identifies the main hazards of carrying out chemical reactions;
• provides guidance on how you can ensure a safe operation; and
• identifies some sources of further information and guidance.

The advice is aimed at small and medium-sized companies in the chemical manufacturing industry, although the principles apply equally to all firms.

WHAT ARE CHEMICAL REACTION HAZARDS?
During the manufacture of a chemical, raw materials react together to give the product. Such a chemical process often releases energy, in the form of heat, and the reaction is described as exothermic. A reaction may be exothermic even if you have to heat the reaction mass initially to get the reaction started.
This article concentrates on the hazards arising directly from exothermic chemical reactions. There are other hazards associated with chemical manufacturing - those arising from handling toxic or flammable chemicals and general hazards such as noise or working at heights. These are not covered in this article.

Thermal runaway
An exothermic reaction can lead to thermal runaway, which begins when the heat produced by the reaction exceeds the heat removed. The surplus heat raises the temperature of the reaction mass, which causes the rate of reaction to increase. This in turn accelerates the rate of heat production. An approximate rule of thumb suggests that reaction rate - and hence the rate of heat generation - doubles with every 10°C rise in temperature.
Thermal runaway can occur because, as the temperature increases, the rate at which heat is removed increases linearly but the rate at which heat is produced increases exponentially. Once control of the reaction is lost, temperature can rise rapidly leaving little time for correction. The reaction vessel may be at risk from over-pressurisation due to violent boiling or rapid gas generation. The elevated temperatures may initiate secondary, more hazardous runaways or decompositions.

Effects of thermal runaway
A runaway exothermic reaction can have a range of results from the boiling over of the reaction mass, to large increases in temperature and pressure that lead to an explosion. Such violence can cause blast and missile damage. If flammable materials are released, fire or a secondary explosion may result. Hot liquors and toxic materials may contaminate the workplace or generate a toxic cloud that may spread off-site.
There can be serious risk of injuries, even death, to plant operators, and the general public and the local environment may be harmed. At best, a runaway causes loss and disruption of production, at worst it has the potential for a major accident, as the incidents at Seveso and Bhopal have shown.

Effect of scale
The scale on which you carry out a reaction can have a significant effect on the likelihood of runaway. The heat produced increases with the volume of the reaction mixture, whereas the heat removed depends on the surface area available for heat transfer. As scale, and the ratio of volume to surface area, increases, cooling may become inadequate. This has important implications for scale-up of processes from the laboratory to production. You should also consider it when modifying a process to increase the reaction quantities.

Causes of incidents
An analysis of thermal runaways in the UK has indicated that incidents occur because of:

• inadequate understanding of the process chemistry and thermochemistry;
• inadequate design for heat removal;
• inadequate control systems and safety systems; and
• inadequate operational procedures, including training.

These are some of the key factors you will want to consider in defining a safe process.

WHAT DO YOU NEED TO DO?
In order to deal with chemical reaction hazards first you need to identify them, then to decide how likely they are to occur and how serious their consequences would be, i.e. carry out a risk assessment of your process. You are required to assess the risks that the process presents and, to record the significant findings.

Chemical process risk assessment
A typical assessment will involve:

• defining the process, operating conditions and plant;
• identifying the hazards;
• evaluating the risks arising from the hazards and deciding whether existing precautions are adequate or more should be done;
• selecting and specifying appropriate safety measures; and
• implementing and maintaining the selected safety measures.

You should start the assessment as early as possible during the development of the process. The assessment should be sufficient to identify the potential hazards and to investigate their causes. Where possible, hazards should be avoided.
As the process design develops, foreseeable deviations from the normal process, such as equipment failure or operator error, should be considered. You may need to follow a structured method for identifying hazards, such as a hazard and operability study (HAZOP), particularly when the plant or processes are highly hazardous, complex or involve new technology.

Evaluating reaction hazards
In order to determine the hazards of a reaction, you need information on the chemistry and thermochemistry of the reaction. This includes:

• the possibility of thermal decomposition of raw materials, intermediates, products and by-products;
• whether exothermic runaway can occur; and
• the rate and quantity of heat and gas produced by the reaction.

As it is not safe to test unknown reactions in a full-size reactor, various techniques and tests have been developed to provide predictive data. The main methods are:

• literature data and calculation, to give preliminary information;
• basic screening tests, such as differential scanning calorimetry or carius tube;
• isothermal calorimetry (mainly to measure reaction kinetics and heats of reaction);
• adiabatic calorimetry (mainly to examine runaways); and
• relief vent sizing tests.

There is no standard procedure that can be followed for all reactions - the aim is to obtain the data you need to assess the risk adequately. To avoid undue time and effort, any investigation should reflect the complexity of reaction and the size of the risks involved. Further information on assessing reaction hazards is given in the References at the end of this article.
It is important that the assessment of chemical reaction hazards, the selection of suitable test methods and the interpretation of results is carried out by competent, experienced personnel. It may not be cost-effective for a smaller firm to buy specialised test equipment and you may want to use a test house or consultancy.

SAFETY MEASURES
Once you know what the risks are, you can select the measures to ensure safe operation. You can ensure safe operation in a number of ways, by using:

• inherently safer methods, which eliminate or reduce the hazard;
• process control, which prevents a runaway reaction occurring; and
• protective measures, which limit the consequences of a runaway.

Inherent safety
Where possible, you should first eliminate or reduce hazards by inherently safer design. For example:

• replace hazardous materials with safer ones:-have less unreacted material in the reactor, eg using a continuous process instead of a batch reactor;
• use a semi-batch method (in which one of the raw materials is added over time) instead of a batch process; and/or
• use a heating medium which has a maximum temperature that is too low for the reaction mixture to decompose.

As the examples suggest, inherently safer methods can fundamentally affect the process - it will be easier to use such methods if you consider them in the early stages of process development.

Process control
Process control includes the use of sensors, alarms, trips and other control systems that either take automatic action or allow for manual intervention to prevent the conditions for uncontrolled reaction occurring. Specifying such measures requires a thorough understanding of the chemical process involved, especially the limits of safe operation.

Protective measures
Protective measures do not prevent a runaway but reduce the consequences should one occur. They are rarely used on their own as some preventive measures are normally required to reduce the demand upon them. As they operate once a runaway has started, a detailed knowledge of the reaction under runaway conditions is needed for their effective specification. You can:

• design the plant to contain the maximum pressure-fit emergency relief vents and ensure vented material goes to a safe place;
• crash cool the reaction mixture if it moves outside set limits;
• add a reaction inhibitor to kill the reaction and prevent runaway; or
• dump the reaction into a quenching fluid.

Selecting the basis of safety
The basis of safety for a chemical reaction is the combination of measures which are relied upon to ensure safe operation. The measures you choose for a particular case will depend on a number of factors, including:

• how easy it is to prevent runaway;
• how applicable the various methods are; and
• how compatible the measures are with plant operation.

Whatever methods you choose, they must cater for all cases that can foreseeably occur and reduce the risk of runaway to a level that is as low as reasonably practicable.
In practice, you may not be able to eliminate all hazards by inherently safer methods and may choose to add control measures to further reduce risk and back these up with protection, such as a vent, to deal with the residual risk. Such a combination of methods is common. As a runaway incident may affect the environment, you should also consider whether your measures are adequate to comply with environmental law.

SAFETY MANAGEMENT
Your carefully selected safety measures may be ineffective if your operators do not know what to do if an emergency occurs. Safety measures have to be supported by appropriate management systems that deal with factors such as:

• operating and emergency procedures;
• consultation with employees;
• training and supervision of operators;
• maintenance of equipment; and
• control of modifications.

CONCLUSIONS
It does not need to be time-consuming or expensive to assess the risks of your chemical processes and to implement adequate safety measures. It is essential that you can demonstrate that you have carried out a suitable and sufficient assessment and that the systems in place reduce the risk of runaway to a level that is as low as reasonably practicable.
The effort you take to do this should reflect the complexity of the process and the scale of risks involved. Apart from complying with health and safety law, you can benefit by avoiding the disruption, costs and potential damage and injuries that a runaway may cause.

KEY QUESTIONS
Have you adequately assessed the risks of your processes and, if appropriate, recorded the significant findings?
Do you consider inherently safer ways of operating when you develop a process?
Do you know the heats of reaction for the chemical reactions that you carry out?
Do you consider the effect of scale on heat transfer when transferring a process from the laboratory to the plant?
Have you assessed the safe operating limits of your process?
Do you know how the protective measures on your reactors have been designed?
Is the basis of safety for each of your reactions adequate to cope with the event, or sequence of events, that could produce the most harmful consequences?
Would you and your employees know what to do in an emergency?

REFERENCE: http://www.hse.gov.uk/pubns/indg254.htm

What is OHSMS?

Occupational Health & Safety Management System

• A voluntary effort on OH&S management applying PDCA
• Risk identification & control (Process management)
• Internal Safety Audits

Objectives of OHSMS

• Ensure continuity of activities by periodic audits by third parties
• System improvement based on professional recommendations
• Inspire reputation as a safe industry

What is OHSAS 18001?
The Occupational Health and Safety Assessment Series (OHSAS) 18001 and the accompanying OHSAS 18002, Guidelines for the implementation of OHSAS 18001, have been developed in response to customer demand for a recognizable Occupational Health and Safety Management System standard against which their management systems can be assessed and certified.
OHSAS 18001 is compatible with ISO 9001:2000 (Quality) and ISO 14001:2004 (Environmental) management system standards.

Elements of OHSMS (based on OHSAS18001:2007)
General

• OHS Policy

Planning

• Hazard identification, risk assessment and determining controls
• Legal and Other Requirements
• Objectives and Programme(s)

Implementation & Operation

• Resources, roles, responsibility, accountability and authority
• Competence, training and awareness
• Communication, participation and consultation
• Documentation
• Control of documents
• Operation Control
• Emergency Preparedness and Response
  

Checking

• Performance Measurement and Monitoring
• Evaluation of compliance
• Incident investigation, nonconformity, corrective action and preventive action
• Control of Records
• Internal Audit
  

Management Review

If you wish to set up the OHSAS 18000 Safety Management System, visit www.kavservices.sg

Confined space - What is

This clip explains what a confined space, and what to do when working in a confined space

Confined Space

Confined spaces in the workplace include any chamber, tank, pit, pipe, flue or enclosed space in which either or both of the following situations are possible:

• Dangerous fumes are liable to be present to such an extent as to involve risk of fire or explosion, or persons being overcome by fumes.
• The supply of air is inadequate, or is likely to be reduced to be inadequate for sustaining life.

Hazards in confined spaces can be broadly separated into the following two categories:

• Atmospheric hazards, e.g. oxygen deficiency and presence of toxic or flammable gases or vapours
• Physical hazards, e.g. slips and falls, moving machines, exposed electrical components,
  engulfment and drowning

Legal Requirements on Confined Space Work
Safety measures for confined space work are stipulated in Regulation 25 of the Workplace Safety and Health (General Provisions) Regulations. These include:

• Removing any sludge or deposit liable to give off dangerous fumes before confined space entry.
• Preventing entry of dangerous fumes into the confined space.
• Adequately ventilating the space to sustain life before entry and during work.
• Testing of the space for oxygen and any flammable or toxic gases and vapours.
• Wearing suitable breathing apparatus if any space cannot be made safe for entry.
• Wearing a safety harness and lifeline where practicable, and having a standby person keeping watch from outside the space.
• Ensuring there is a sufficient supply of emergency response equipment such as breathing and reviving apparatus, belts and ropes.
• Having a sufficient number of employees trained and practised in the use of emergency response equipment, and in CPR.

Under the Workplace Safety and Health (Risk Management) Regulations, employer and contractor must conduct a risk assessment in relation to the safety and health risks posed to any person who may be affected by his undertaking, and take all reasonably practicable steps to eliminate any foreseeable risk.
In addition, safe work procedures (SWP) must be implemented to control the risks. The SWP must include the safety precautions to be taken in the event of an emergency.

Employees or any other persons at the workplace who may be exposed to a risk to their safety and health must be informed of:

• The nature of the risk involved
• The measures implemented to control the risk; and
• Applicable safe work procedures.

Oxygen Deficient Atmospheres


The level of oxygen inside a confined space can be reduced to a dangerous level due to:

• Chemical reactions (rusting, decomposition and fermentation)
• Absorption by porous materials (e.g. activated carbon)
• Displacement by inert gases (e.g. nitrogen and carbon dioxide)

Oxygen is vital for sustaining life. Many physiological effects emerge when the oxygen content is below the minimum safe level. The symptoms range from mild headache to permanent brain damage or death at a highly deficient level.

A confined space may be entered only if it contains at least 19.5% but not exceeding 23.5% oxygen by volume

Toxic Atmospheres

Air contamination inside a confined space occurs when hazardous substances inside the space become airborne. Depending on the type of contaminants, the effects can be irritation, asphyxiation, or systemic poisoning, even at low concentrations.

Common toxic/poisonous gases:

• Solvent vapours (e.g. acetone, toluene, trichloroethylene, xylene)
• Carbon monoxide
• Hydrogen sulphide
• Petroleum vapours (e.g. naphtha)

SolventsSolvents are petroleum derivatives and are commonly found in products such as paints, cleaning agents and adhesives. Due to their highly volatile nature, solvents can rapidly accumulate at dangerous levels in unventilated confined spaces. Acute exposure usually results in narcosis as many of the vapours depress brain function and the central nervous system. Chronic exposure can cause systemic poisoning and damage the organs.

Carbon Monoxide (CO)
Carbon monoxide is a colourless and odourless gas that is produced from incomplete combustion. The gas is a chemical asphyxiant. It binds strongly to red blood cells, preventing the flow of oxygen to the brain. In the absence of oxygen, the brain cells die, leading to unconsciousness and even death.
The Permissible Exposure Level (Long Term) for carbon monoxide is 25 ppm.

Hydrogen Sulphide (H2S)Hydrogen sulphide is a rapidly acting systemic poison which, at high concentrations, paralyses the respiratory function and causes asphyxiation. When breathed in for prolonged periods at low concentrations, it dulls the sense of its characteristic rotten-egg odour.

At high concentrations, the sense of smell is readily deadened, so odour cannot be used as an early warning sign.

The PEL for hydrogen sulphide is 15 ppm (Short Term), and 10 ppm (Long Term).

Flammable Atmospheres

Flammable substances in a confined space can cause fire and explosions in the presence of an ignition source e.g. open flame and sparks. Flammable sources include:

• Residual gases or vapours e.g. petroleum vapours
• Leaks from gas cylinders or pipelines e.g. acetylene, liquefied petroleum or natural gas
• Underground marsh gas (methane)
• Vapours evaporated from solvents e.g. toluene, xylene

The concentration of any flammable gas or vapour in a confined space must not be more than 10% LEL. If hot works is to be carried out in the space, the space should be free from any flammable substance.
Other Hazardous Gases
Acid FumesSkin reddens and blisters when exposed to acid fumes. When inhaled, a sore throat and shortness of breath result. Severe exposure can cause pulmonary oedema with a potentially fatal result when fluid accumulates in the lungs.

Ensure that the concentrations of toxic gases or vapours in the confined space are below the Permissible Exposure Levels (PEL).

Useful points to note when assessing the conditions of a confined space:

• Contents - Previous contents in the space
• Reactions - Possible reactions that can happen inside
• Operations - Nature and type of operations to be carried out inside, including the type of materials to be used
• Potential hazards - Inadvertent introduction of contaminants from outside environment

Control and Preventive Measures

The following measures should be taken where appropriate to prevent deaths and injuries from
confined space work.

Risk Assessment
Prior to commencement of work, a risk assessment must be carried out to identify the hazards associated with the work, assess the risk of accident that may occur, implement SWP and take appropriate measures to eliminate the hazards, or to reduce the risk.

Isolation
Before entry, valves and pumps to all pipes leading to confined spaces must be locked and tagged to prevent the entry of hazardous materials.

Gas Check
A competent person must test and certify the atmosphere of confined spaces for oxygen, flammable and toxic gases or vapours prior to entry. Confined spaces can only be certified safe for entry if:

• The oxygen level is within 19.5% to 23.5%;
• The level of flammable gas is less than 10% of the LEL; and
• The concentration of toxic gases or vapours is below the PEL.

While a person is inside the space, continuous or regular gas testing should be conducted to
ensure that the space remains safe.

Calibration of Gas Meters
Gas monitoring devices must be regularly maintained and their accuracy verified with functional (bump) tests and full calibrations. A functional test is a brief exposure of the gas monitor to a known gas for the purpose of verifying sensor and alarm operation. If the instrument fails to operate properly following any functional test, a full instrument calibration should be performed.

A full calibration involves the use of (certified standard) calibration gases, and requires verification that the gas concentrations listed on the cylinder label match the concentration setting for calibration in the instrument. Calibrations must be carried out by trained personnel, and records kept.


Entry Permit


Before entering a confined space, a permit must be issued by a competent person certifying that all hazards have been assessed and precautionary measures have been taken to ensure the safety of entrants.



Ventilation
When a confined space is occupied, suitable and adequate ventilation should be provided at all times to provide fresh air and/or to dilute and remove any contaminants to a safe level. It is recommended that a combination of forced or supplied and exhaust ventilation be used to ensure adequate ventilation of the space.

Forced or supplied ventilation introduces fresh air into the space typically through the use of a fan or blower while exhaust ventilation removes contaminants from the space by drawing air out using an extractor.

Standby PersonA standby person should be stationed outside the confined space to keep a look out and render help in the event of an emergency.

Safety Appliances
When entering a confined space, a safety harness and lifeline should always be worn. This will facilitate retrieval during an emergency. Suitable respirators should be worn where toxic gases or vapours are known to be present. Air supplied respirators must be used if the space is likely to be deficient in oxygen or contain unknown or high concentrations of air contaminants.

Rescue Plan and Equipment
A written rescue operation plan should be established and on-site equipment such as retrieval devices and breathing and reviving apparatus should be readily available for emergency use.

TrainingTraining should be provided to all persons involved in confined space work, including worker (entrant), standby person, supervisor and gas tester. The training should cover the following
areas:

• general hazards associated with confined space
• safety and health precautions with respect to entry into confined space
• entry permit system and safe work procedures
• emergency response

Other Safety Measures

Diesel-driven and petrol-driven engines such as pumps and compressors should never be placed inside a confined space.

Host employers should ensure that contractors are competent for work involving a confined space. They also need to brief their contractors on any precautions or procedures to be implemented.

Procedures for emergency response must be established and communicated to all personnel on-site.

For more safety requirements on entry into and working in confined spaces, please refer to:

• The Workplace Safety and Health (General Provisions) Regulations
• Singapore Standard - CP84: Code of Practice for Entry Into and Safe Working in Confined Spaces.
  

(Reference: Technical Advisory for Confined Spaces by WSH Council)