By HP van Huyssteen, project manager at Energas Technologies
With incorrect flame arresters installed, petroleum storage tank terminals and refineries may be exposed to major explosion hazards.
The very nature of these facilities indicates that flammable mixtures may be encountered very often or even continuously in some areas. A small spark or lightning strike igniting a flammable mixture can have devastating consequences.
Flame arresters are devices designed to stop flame propagation, but they may vary greatly in their design and degree of protection. Flames may be divided into three categories – each with its own set of characteristics. Unconfined deflagrations occur if an unconfined vapour cloud is ignited. The flame will rapidly travel through the vapour cloud, but at sub-sonic velocities.
HP van Huyssteen, project manager at Energas Technologies. Image credit: Energas Technologies
Secondly, a confined deflagration occurs if a flammable mixture in a piping system is ignited. The flame travels at sub-sonic velocities, but will continue to pick up speed as it travels along the piping system. A pressure wave, also increasing in magnitude, precedes the flame. Lastly, a confined detonation is defined as a flame front travelling at sonic velocities and the pressure wave in front of the flame has transitioned to a shock wave.
Understanding deflagrations and detonations
The characteristics of flames travelling down piping systems have not been understood for that many years. Deflagrations travelling sufficiently far down a piping system will turn to detonations and their velocity and pressure will stabilise in a smooth straight pipe. A detonation travelling along a pipe at a stabilised velocity and pressure is termed a stable detonation. Stable detonations are characterised by much higher velocities and pressures than deflagrations; as a result a stable detonation is more difficult to stop than a deflagration.
More importantly though, is that when a deflagration changes into a detonation, it goes through a transition phase during which time the flame front reaches a velocity and pressure that is much greater than that of a stable detonation. During the transition phase, a detonation is termed an unstable or overdriven detonation. Consequently, it is more difficult to stop an unstable detonation than a stable detonation. Pressures experienced during an unstable detonation may reach in excess of 100 bar.
The transition
The transition from a deflagration to a detonation may typically occur within 50 pipe diameters from the source of ignition. For some precarious fuels like hydrogen, the transition can start in as little as 30 pipe diameters from the ignition source. In complex piping systems with many bends and junctions, flame propagation may occur differently and unpredictably.
A flame will accelerate differently around a pipe bend than along a straight pipe. It is still extremely difficult to predict where the deflagration to detonation transition will occur. It is this transition phase that was not well-understood, and is still difficult to predict how far from the source of ignition it will occur. In the past, design engineers may have believed that they had designed adequately by specifying standard stable detonation arresters. If an unstable detonation hits a stable detonation arrester, the flame may pass straight through it. In accordance with ISO 16852 (a standard written for the performance requirements and limits of use of flame arresters) stable detonation arresters require additional protection measures.
Understanding flame arresters
Flame arresters act as heat sinks by partitioning the flame into tiny flamelets and quenching the tiny flames. A flame arrester element will typically consist of a matrix of small holes of a controlled maximum diameter. The maximum experimental safe gap (MESG) is a standard measurement of the maximum distance between two heat absorbing materials through which a gas flame will not pass.
The MESG is different for different types of flammable gases. Flammable gases are classified into different groups and the groups each have a maximum MESG. Higher operating pressures and temperatures in the vapour piping system will make it more difficult for a flame arrester to stop a flame. Not only are flame arresters classified by the type of explosion that they can protect against, but also the type of gas (gas group) and operating conditions.
Unstable detonation arresters, when used within their gas group and permissible operating conditions, have no limitations on their location within a piping system. They are designed to protect against the most violent of flames – unstable or overdriven detonations. Certainly the arresters must still be installed between the source of ignition and the part of the facility or piping system for which they are providing protection. Unstable detonation arresters are heavier and more expensive than other type of arresters, but provide superior protection and peace-of-mind for engineers and facility operators.
Flame arresters can also be classified as short-time-burn or endurance burn depending on the length of time that they can withstand a flame without becoming compromised. The Protectoseal Company manufactures a range of deflagration and unstable detonation arresters for the vapour control industry. Protectoseal so strongly backs the installation of unstable detonation arresters in lengthy piping systems that stable detonation arresters do not even form part of their scope of supply. Protectoseal is represented in South Africa by Energas Technologies.