No substitute for flame arresters

Published:  24 June, 2011

Testing and applying flame arresters, by Dr.-Ing. Michael Davies & Dr. rer.nat. Thomas Heidermann of Braunschweiger Flammenfilter GmbH (PROTEGO), (Germany).

The latest research, in which conservation vents have been tested in accordance with the new ISO 16852 test method, have proven that conservation vents cannot be used to substitute a flame arrester if potentially explosive atmospheres are present in storage tanks.

This research was conducted during the development of the ISO 28300 standard [1] and the test results are considered in this future ISO standard which will be identically with API 2000, 6th edition.

The first part of this paper shows that using conservation vents to protect tanks from atmospheric explosion is not a reliable protection method – even though this is very common for Ethanol storage (which is gaining a lot of attention globally). Flame arresters provide tanks with protection provided that the right flame arrester is installed.

In the second part of this paper different types of flame arresters are presented, and a parametric analysis shows why choosing a flame arrester is rather complex.

Testing conservation vents to a globally recognised flame arrester standard

For several decades state organisations and engineering societies have published strict engineering guidelines for the design and safe management of storage tanks.

Even though the best effort is made in utilising most current research work and engineering procedures, there is a conflict between some standards, eg the 5th Edition of API 2000 and the German TRbF 20 standard.

API 2000 states that a flame arrester is not necessary for use with a pressure vacuum valve venting to atmosphere, because flame velocities are less than vapor velocities across the seat of the pressure vacuum valve [2].

On the other hand the TRbF 20 standard clearly calls for flame arresters where the tank contains liquids that could create an explosive atmosphere [3] – ie any liquid which is flammable.

According to the UN Recommendations on the Transport of Dangerous Goods, Model Regulations, 14th rev., ed. (2005) and the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) 2nd rev. ed. (2007) a liquid with a flash point of less than 60°C has to be considered as a source of potential flammable vapor inside a tank.

The limiting flash point for explosion protecting measures is often given by national regulations, eg in Germany this requires installation of flame arresters on tanks which store liquids with a flash point at or below 55°C, or on tanks which heat the stored liquid to its flash point [4]. Vents are seen as a likely place of ignition and it is recommended that flame arresters should be installed to prevent tank explosion [5].

These contradictions in different globally-recognised standards and publications required clarification during the development of ISO 28300. Hence it was decided to perform research on vents which where tested according to the ISO 16852 flame arrester standard. The aim for the research was determining whether a conservation vent could truly assure flame transmission through the vent pallet and thus prevent tank explosion.

Concerning the possible ignition sources one remark has to be made: the risk of lightning is highly underestimated as an ignition source. Figure 1 shows the number of lightning per year and km2. Worldwide we record 100 flashes per second, 10% are lightning strikes.

 

Testing according to ISO 16852

To determine if a conservation vent is capable of preventing flash back into a tank, an ignition test was performed in accordance with [6] ISO 16852:2008. Two different tests were carried out:

  • A)  Atmospheric deflagration test
  • B)  High velocity test procedure.

The atmospheric deflagration test (test A) investigated whether it was possible to assure that a conservation vent would not fail if – for example – a lightning strike ignited a vapor cloud which was present around the conservation vent.

Figure 2 shows the typical hazards for which the test procedure for end-of-line flame arresters had been developed. A vessel (tank, reactor, etc) with an explosive mixture in its interior and exterior can be seen. If this explosive mixture is ignited by an ignition source, it is the job of the end-of-line flame arrester to prevent flame propagation into the tank.

The high velocity discharge test (test B) investigated whether the theoretical approach of some engineering guideline, eg API 2210 [7], was correct and no flash back through the vent valve was possible.

Test set up and results [atmospheric deflagration]

Figure 3 shows the test setup for atmospheric deflagration testing in accordance with [6].

The explosion-proof vessel was filled with a propane/air mixture and vented through the conservation vent into the plastic bag until the plastic bag was filled completely. An air dryer was used to enssure a constant oxygen concentration in the fuel/air mixture.

Paramagnetic oxygen measurement was used to adjust the fuel concentration.

The size of the conservation vents used for testing was DN 100 (4”), and the conservation vents of five different manufacturers were tested.

The set pressures and vacuums were typical values used on API tanks. To detect a flashback an explosion panel was installed at the explosion proof vessel. The air to fuel mixture was ignited approximately 1m above the point where the valve was connected to the explosion-proof vessel. A chemical igniter with an ignition energy of 160 J was used.

Should the conservation vent be unable to prevent flash back, the flame would propagate through the p/v valve and an explosion inside the explosion vessel would occur. As a result the diaphragm of the vessel would burst, and flames would propagate to the outside of the vessel.

For the first series of tests the plastic bag was filled with a stoichiometric mixture of propane in air (4.2 vol% propane). After closing the shut-off valve the gas mixture inside the bag was ignited. Five major manufacturers of conservation vents were tested. Each conservation vent was set at +10 mbar (+4”wc) and -2 mbar (-0.8“wc).

The second series of tests are performed with 5.5 vol% of propane in air. In the third test a rich mixture was used, 6.0 vol% propane in air. At these air to fuel mixtures all conservation vents failed to prevent flame propagation resulting from atmospheric deflagration into the explosion-proof vessel. In all of the tests the bursting diaphragm ruptured and a large fire ball propagated out of the vessel.

Test setup and results [high velocity]

Figure 4 shows the test setup for high velocity testing in accordance with [6]. For conducting the tests a conservation vent was installed on top of an explosion proof vessel. A stoichiometric propane/air mixture was processed into the vessel and discharged through the pressure side of the conservation vent. As an ignition source a pilot burner was installed close to the discharge side of the conservation vent.

The first series of tests was performed with a volume flow of V= 85 m3/h explosive gas mixture. The pressure valve pallet opened and closed due to the low flow. After ignition of the pilot burner a flame was stabilised at the seat of the valve. After a few seconds the flame propagated through the gap between seat and pallet which resulted in an explosion inside the vessel.

Consequently the rupture panel of the explosion proof vessel broke, and a fire ball propagated to the outside of the vessel. The second series of tests was conducted at a higher volume flow. Now V= 100 m3/h of propane/air mixture was processed through the vessel and discharged on the pressure side of the conservation vent. Again just a few seconds after the ignition a flash back was detected.

Parametric analysis of flame arresters

To solve the safety problem described in chapter 2, the use of flame arresters is necessary, also available as combination with valves. Explosive mixtures can burn in various ways, consequently flame arresters are subdivided into different types.

The following, among other things, can influence the combustion process: the chemical composition of the mixture, possible pressure waves, pre-compression, the geometric shape of the combustion chamber and the flame propagation speed. The relevant combustion processes for flame arresters are defined by international standards, for example:

•            Explosion is the generic term for abrupt oxidation or decomposition reaction producing an increase in temperature, pressure or both simultaneously [EN 1127-1].

•            Deflagration is an explosion that propagates at subsonic velocity [EN 1127-1]. Depending on the geometric shape of the combustion area, a distinction is drawn between atmospheric deflagration (figure 5), pre-volume deflagration (figure 6), and in-line deflagration.

•            Detonation is an explosion propagating at supersonic velocity and is characterised by a shock wave [EN1127-1]. A distinction is drawn between stable detonations and unstable detonations (figure 7).

 

In the same way, flame arresters are subdivided into different types depending on the combustion process and in accordance to the installation (in-line, end-of-line, in equipment) (figure 8).

For all groups special designs are necessary– especially for the flame arrester unit. Additional to these groups the operating conditions strongly also influence the design and dimensioning.

The length of run-up distance, the distance between in-line arrester and the potential ignition source, in relation to diameter (L/D ratio) has a special influence on the development of accelerated flames from pipeline deflagrations up to stable detonations (figure 7).

Detonations have to be expected within pipelines already after comparatively short run-up distances depending on the internal diameter of the pipeline. Only for L/D ratios less than 50 for hydrocarbons and less than 30 for hydrogen gas/air mixtures in-line deflagration arresters can be used.

For the purpose of classification of each product the so-called MESG (Maximum Experimental Safe Gap) was defined and internationally fixed (figure 9).

According to this MESG the inflammable materials (gas, vapor or liquid) are classified into the different explosion groups that characterise the capacity for flashback.

Common practice is to use MESG value tested flame arresters for standard conditions and most plants do not consider the impact of pressure, temperature and oxygen concentration on the reactivity of a processed explosive gas mixture.

Flame arresters tested at ambient conditions can be used up to operational temperatures of 60°C (140 F) and operational pressures up to 1,1 bar absolute (15,95 psia). If the working conditions are above these values, special tested flame arresters should be used. The user can improve the general and operational safety of his plant by using flame arresters approved according to the ISO 16852 standard.

Conclusion

This research work proves that conservation vents cannot reliably function as flame arresters if an atmospheric deflagration occurs. Furthermore, the theoretical statement that a flame arrester is not considered necessary for use in conjunction with a pressure/vacuum valve venting to atmosphere because flame velocities are less than vapor velocities across the seat of the pressure vacuum valves, cannot be confirmed.

The performed testing and the results explain why the TRbF 20 standard clearly calls for flame arresters if the tank contains flammable liquids. Furthermore it makes complete sense to install flame arresters on tanks which store liquids with a flash point at or below 60°C, or on tanks which heat the stored liquid to its flash point.

For this reason the new ISO 28300 standard recommends to use a flame arrester as an effective measure to reduce the risk of flame transmission.For the right selection of flame arresters also as combination with valves the specific conditions of the application are important. Flame arresters tested at ambient conditions can be used up to operational temperatures of 60°C (140 F) and operational pressures up to 1,1 bar absolute (15,95 psia).

If the working conditions are above these values, special tested flame arresters shall be utilized. Another important point for the selection of flame arresters is the vapor group. Each flammable substance has a defined MESG and vapor group. Flame arresters are tested and approved for special vapor groups, like IIA (NEC group D), IIB3 (NEC group C) or IIC (NEC group B). A tank filled with hexane shall be protected against atmospheric deflagration resulting from a lightning strike. Hexane has a MESG of 0.93 mm, therefore Hexane is a group IIA vapor and a deflagration arrester for IIA shall be used. Additionally flame arresters for a higher vapor group can be used for a lower group, but arresters approved for lower group are not suitable for higher vapor groups.

References

[1] ISO/ 28300: First Edition 2008-06-15, Petroleum, petrochemical and natural gas industries – Venting of atmospheric and low-pressure storage tanks.

[2] API Standard 2000, Fifth Edition, April 1998, Venting Atmospheric and Low Pressure Storage Tanks.

[3] Technische Regeln für Brennbare Flüssigkeiten TRbF 20, Läger ,  01.02.2001, BArbBl Nr. 4/2001 S.60.

[4] Factory Mutual 2007 Approval Guide, Flammable Liquid Equipment.

[5] James I. Chang, Cheng-Chung Lin, A study of storage tank accidents, Journal of Loss Prevention in the process industries, 19, (2006) page 51-59.

[6] ISO 16852:2008, Flame arresters - performance requirements, test methods and limits for use.

[7] API Publication 2210, Flame arresters for vents of tanks storing petroleum products.

[8] Förster,H., Flame Arrester Testing and Qualification in Europe, Proceedings of the 10th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, Stockholm, Sweden, 19-21 June 2001.

[9] H. Steen: Handbuch des Explosionsschutzes. Wiley-VCH Verlag GmbH, Weinheim, New York, 2000.

[10] K. Schampel: Flammendurchschlagsicherungen. expert verlag, Ehningen bei Böblingen, 1988.

[11] R. Jeschar, R. Alt, E. Specht: Grundlagen der Wärmeübertragung. Viola-Jeschar-Verlag, Goslar,1990.

 

This paper was first printed in the Q3 2011 issue of IFJ. To read the whole issue click here.

  • Operation Florian

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