CHOOSING THE RIGHT FLAME ARRESTER
Deciding when to use a deflagration and when a detonation arrester can be confusing. Therefore, it is important to first answer these questions:

1. Where is the ignition source?
Turbulent flow enhances the mixing of the combustible gases, greatly increasing the combustion intensity.
This can result in increased flame speeds, higher flame pressures and higher flame temperatures than those found in laminar flow conditions. Such
piping configurations must be taken into consideration when selecting an arrester. If flashback should occur, the flame arrester could fail before the system or personnel have time to react.

2. What needs to be protected?
Using the flame arrester as a means of protection with a vacuum inside the storage area might at first glance appear inapplicable for the case when a vacuum valve of the conservation vent opened. However, the usual flame exposure to a storage tank conservation vent involves an external fire. The heat transferred from such a fire would warm the gas space inside the tank and cause the gas to expand. Thus, at times of fire exposure to a conservation vent, the gas flow through the conservation vent would be expected to be outward to the atmosphere. Flame propagation backward through the conservation vent is thereby prevented.

3. Are there any flow restrictions on the protected side of the arrester?
During low and medium level deflagrations, pressure waves, travelling much faster than the flame front, travel through the arrester and are reflected by a restriction such as a partially closed valve. The reflected wave can cause significant pressurisation within the flame arrester element channels, which can enhance conditions for flame transmission. Furthermore, the following conditions must be thoroughly evaluated prior to
selecting the correct deflagration or detonation arrester:

Gas/vapour composition
As part of the selection process, a classification must be identified for the product passing through the arrester.
Maximum explosion pressure, minimum ignition temperature and the maximum experimental safe gap (MESG) are all used to formulate an EN explosion
group rating.

Flame & detonation velocities/pressures
The characteristics of a flame front as it encounters an arrester is affected by numerous factors. In addition to initial pressure, temperature and vapour composition, the pipeline plays a critical role. The distance between an ignition
source and an arrester, often referred to as the ‘run up’ distance, has a dramatic effect on the flame speed and pressure seen by the arrester.

Flame speed tends to increase as it travels down a confined piping run. This is a result of unburnt gas ahead of the flame front being preheated and compressed by the escaping combustion products.

Turbulence provides a mechanism for acceleration of the flame front. Irregularities in the piping configuration (obstructions, bends, etc.) tend to increase the turbulence in the unburned gases, resulting in higher flame speeds.

Pressure drops
The pressure drop associated with an arresting device must be known in order to ensure it is properly sized for a given application. Minimising the pressure
drop across the flame arrestor is in part accomplished by increasing the overall bank diameter to provide increased free area and by increasing the length of the
flame arrestor element which allows a larger hydraulic diameter of the element passageways to be used.

WHAT IS AN IN-LINE FLAME ARRESTER?
Sometimes it is desirable to install flame arresters in the middle of a long pipeline. They are particularly suited for installation inside a tank house where vent pipes or lines are extended through the tank house roof or walls to the outside. Locating the unit inside eliminates difficult inspections on the storage tank roof and protects the vent from the freezing of condensate. Combustion in a pipeline can readily make the transition from deflagration to detonation; in the case of some designs a detonation can be stopped, whereas a deflagration may pass through.
Therefore, an in-line flame arrester must withstand and stop both types of combustion from both downstream and upstream in a pipeline. Since the pressure developed in the combustion can be high, the in-line arrester and pipeline must be properly designed, constructed and tested. The flame arrester element itself must be capable of withstanding substantial pressure differentials in both directions as a result of the combustion.
Please note that Storagetech flame arrester production, assembly and tests are in accordance with EN ISO 16852:2016.

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Storagetech is an Äager brand that serves the bulk liquid tank storage sector along with the oil & gas, mining, water, chemical, and petrochemical industries.