There are always options in the management of explosion risks! These measures are classified as either prevention or protection.
Each application requires unique assessment that includes the following considerations:
- Quantify the risk; define the deflagration characteristics for the process material (Testing may be required)
- Determine whether active or passive technologies should be employed
- Consider whether a hybrid of prevention & protection measures should be employed
Direct explosion venting is economically attractive, however, the full impact of a vented discharge must be assessed in order to determine the right safety measures to be implemented:
Can the flameball developed during a deflagration be accepted?
When protecting indoor equipment, can a vent duct to atmosphere be accepted?
How will connected systems be isolated to prevent propagation of the explosion?
Can the equipment accept the reaction forces generated by the vented release?
Can buildings or equipment in the free vent path accept the shock loading of the explosion pressure pulse & accept the momentary high temperature of the fireball?
The answers to these questions will often determine that alternative protection measures should be implemented such as:
- Suppression whereby the full explosion event is prevented and no material is released to atmosphere.
- Determine how to prevent Fire + Explosion Ignition from occurring: (Housekeeping to limit combustible load)
- Management of electrostatic build-up, especially for low MIE ducts
- Management of spark ignition sources (e.g. grinding, milling) by use of Spark Detection & Extinguishing technology.
What does it take to have an industrial explosion?
An explosion results from ignition of a combustible material (dust, gas or mist) when mixed with oxygen present in the air. When this takes place inside a process or storage enclosure, the rapid rise in pressure developed will exert destructive forces within a few milliseconds that will place both personnel and equipment at risk. Most materials handling, processing and storage equipment is not designed to resist the pressure of a deflagration*. Only equipment designed to resist the maximum pressure (Pmax) developed by the combustion process will survive – such design pressures typically exceed 75psig (5.2 bar), making design for containment of a deflagration prohibitively expensive in most cases.
- Deflagration: explosion that proceeds below the speed of sound in air, as compared to a detonation which exceed the speed of sound in air. Typically deflagrations can be protected.
What does it take for prevention & protection of a deflagration?
Usually a hybrid of technical measures is required to manage the deflagration process safety risk. The technology employed in deflagration management can be divided into passive and active. Typically passive technology functions by mechanical means, automatically, without external energy requirements. Explosion vents (also known as explosion panels) are a passive protection technology. Active systems require one or more sources of energy to function. Explosion suppression systems are an active protection technology. Spark detection and extinguishing systems are an active prevention technology designed to detect and eliminate sources of ignition that could lead to fire or explosion.
Assess the Risk / Manage the Risk
Successful management of deflagration risks requires that the characteristics of the material (such as Pmax and Kst – the deflagration index of a dust) are known along with the process conditions, such as dust concentration, airflow velocity, operating pressure, temperature and humidity.
Economic considerations favor the use of explosion vent technology both in terms of cost of equipment and simple periodic inspection requirements. Consider however, the following application factors:
Can the flameball ejected from an open vent be accepted? It may exceed 10/20 meters in length, and about half this in diameter. Simple free venting must be to a safe location where personnel will not be present and other equipment cannot be damaged.
If venting equipment installed indoors, can a vent duct to a safe outdoor location be provided? Vent ducts will always increase the required vent area and may not be acceptable for protecting smaller process volumes at higher Kst values.
Can the required vent area be accommodated? As well as requiring the space for vent installation, can the reaction forces during venting be sustained and for tall equipment, can a near thrust neutral vent arrangement be achieved to prevent collapse during relief.
If there are process inlets and outlets to the protected equipment, are these protected to prevent propagation of the deflagration to other equipment or work areas? Codes and Standards are now very clear in requiring isolation of vented equipment to prevent secondary explosions. Secondary explosion risks are typically much greater in their potential for damage and destruction.
Can the clean up of a vented explosion be accepted? Depending upon the design basis adopted, a vented explosion may require replacement of equipment components that have become damaged by the pressure wave, resulting in loss of production while delivery is awaited.
What will the neighbors think? A vented explosion is a spectacular event that will draw considerable community attention. What if the process material is toxic or hazardous? A vented release must simply be avoided for certain materials.
As illustrated by the above series of questions posed for a vented deflagration application, each process needs to be considered both alone and as a component of a production facility to ensure implementation of the right explosion protection technology. BS&B’s local representative , POGC Sensor Technology, will guide you following the practices required by the latest Codes and Standards.
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