The Essential requirement of an Explosion Isolation System is to prevent flame and pressure from propagating via connecting pipes or ducts from one part of the installation into other plant areas.
It must be suitable and relate to the process conditions, according to its intended use, without losing its ability to perform its safety function.
All vessel protection systems; containment, venting or suppression, are designed for a deflagration starting in the primary vessel. They may not stop the explosion from spreading to other vessels.
Subscribe to PII and receive:
Our long-established bi-monthly digital magazine *Process Industry Informer*, featuring exclusive articles, news and updates across the Process Industries, plus commentary from our three industry experts:
Sean Moran
Gavin Smith
Dave Green
PLUS:
- Process Industry Update — our weekly e-newsletter
- PII Podcast
- Special messages from our advertising partners
After ignition, flame will propagate within in the primary volume i.e. mills, dryers, ect. at about 4 to 10 m/s growing spherically in all directions. As long as there is fuel to feed it, the pressure could reach 6 to 10 barg.
If the flame enters an interconnecting pipe or ductwork, it will start to accelerate down the pipe or duct due to increasing flow or induced turbulence, as it is now only allowed to grow in one direction. Within a few metres, the flame front could have already accelerated to 100m/s.
However, this could lead to an explosion pressure rate of rise in the secondary vessels magnitudes greater than the original explosion, so any explosion venting or suppression system used to protect the secondary vessels will not be adequate. This is why the correct implementation of explosion isolation is fundamental to safeguarding life, process equipment and business interruption.
Explosion Isolation System Design
What should you consider when designing an explosion isolation system?
The objective is to ensure the safety of anyone in the area is maintained by limiting the possible explosion damage to the process, therefore protecting working areas. Assessment of the origin of an explosion and where it can propagate is critical for the designer to understand.
The fuel may come from the primary volume and/or be fuelled by process dust or contamination due to dust deposits trapped in the pipe or duct. The dust deposits are critical, as dust concentrations as low as 30 grams/m³ may sustain a flame. Therefore, virtually all LEV systems and dust handling systems must be considered.
To ensure an explosion isolation system will provide the correct level of protection, one of which is how fast the flame will move down the connecting pipe/duct. The flame speed is influenced by how fast the fuel will burn and the amount of heat it will generate.
This can be determined by using explosibility data such as Kst (measured in bar.m.s-1). Also a high length-to-diameter (L/D) ratio will mean the explosion pressure generated by the heat of combustion is forced along the pipe quickly compared to a low L/D.
There is a point at which the pipe/duct is able to absorb the heat generated quicker than the combustion reaction is able to produce it, therefore propagation will not occur. This is dependent on the product i.e. fuel ignition sensitivity parameters, Kst, pipe size and the amount of fuel in the pipe.
Despite all this evidence, many OEM’s and end users only request protection for the individual volumes, mills, dryers, mixers, bucket elevators, silos and filters. Explosion protection without explosion isolation could result in catastrophic destruction of plant with possible loss in human life or injury.
There is a wide range of explosion isolation systems available, these are usually grouped as Passive or Active
Passive Explosion Isolation System
Rotary Valves use MESG Maximum Experimental Safe Gap for dusts, calculated from MIE Minimum Ignition Energy and MIT Minimum Ignition Temperature, to prevent flame break-through.
Barrier valves close ahead of the flame front due to the velocity of the forward trapped air.
Diverters, will divert the explosion to a safe area. Note, it prevents flame jet ignition and pressure piling. However, cannot be guarantee to effectively stop explosions from propagating through the device to the second vessel. Only expert knowledge of both the process conditions and the abilities of the passive system(s) can make this a safe protection system.
Material plug and choke, at the end of a silo or inside a screw conveyor, may prevent flame and pressure break-through. It depends on the flowability of the material to stay in place before and during the incident. Sometimes combinations of a rotary valve and material plug can fix this problem.
Note; that this method cannot be performance-tested against a known standard. Therefore, approvals are not possible (e.g., there are no standards available to measure performance). The current guidance given in published British Standards advises the application to be risk assessed, as there are inherent risks that must be assessed to ensure the explosion risk is not increased by using a product plug.
Active Explosion Isolation System
The Control and Indicating equipment monitor the explosion sensors and the explosion protection devices. Quick sliding valves are able to stop explosion pressure and flame, from travelling through pipelines. The chemical barrier system is used to discharge explosion suppressant agent into ductwork, to isolate a flame only and keep it from propagating to other process areas.
