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 standardi and the test results are considered in this future ISO standard.
This paper will prove that using conservation vents to protect tanks from atmospheric explosion is not a reliable protection method when the vapor/air mixtures in the tank has a concentration between the lower explosion limit (LEL) and the upper explosive limit (UEL).
This is very common for ethanol storage which is gaining a lot of attention globally.
The need for 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 to use most current research work and engineering procedures there is a conflict between some standards, e.g. the 5th Edition of API 2000 and the German TRbF 20 standard.
API 2000 states that a flame arrester is not considered necessary for use in conjunction with a pressure vacuum valve venting to atmosphere because flame speeds are less than vapour velocities across the seat of the pressure vacuum valveii.
On the other hand the TRbF 20 standard clearly calls for flame arresters if the tank contains liquids which can create an explosive atmosphereiii. This would be any liquid which is stored at, above or close to its flashpoint. The FM (Factory Mutual) approval guide requires installation of flame arresters on tanks which store liquids with a flash point at or below 430C or on tanks which heat the stored liquid to its flash pointiv.
Ventsv are seen as a likely place of ignition and it is recommended that flame arresters should be installed to prevent tank explosion.
These contradictions in different globally recognised standards and publications called for clarification during the process of developing the ISO 28300. For this reason German equipment supplier Protego decided to perform research on vents which were tested according to the ISO 16852 flame arrester standard.
The aim of this research was to determine if a conservation vent can truly assure flame transmission through the vent pallet and prevent tank explosion if the tank atmosphere is within the LEL and UEL.
Testing according to ISO 16852
If the interior and exterior 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 tankviii.
To determine if a conservation vent is capable of preventing flash back into a tank if an ignition occurs testing is performed two different tests are conductedvi: an atmospheric deflagration test and a high velocity test procedure.
The atmospheric deflagration test determines if it is possible to assure that a conservation vent will not fail if e.g. a lightning strike ignites a vapour cloud which is present around the conservation vent.
The high velocity discharge test determines if the theoretical approach of some engineering guideline, e.g. API 2210vii, is correct and no flashback through the vent valve is possible.
Test results for atmospheric deflagration
plastic bag Ø 1,2 m, length 2,5 m, foil thickness >0,05 mm
p/v valve (flame arrester)
explosion proof container
mixture inlet with shut-off valve
Atmospheric deflagration test of end-of-line flame arrester as described in ISO 16852 part 188.8.131.52
How the conservation vent was tested atmospheric deflagration
The explosionproof vessel is filled with a propane/air mixture and vented through the conservation vent into the plastic bag until the plastic bag is filled completely. The size of the conservation vents used for testing is DN 100 (4″). Conservation vents of 5 different manufacturers are tested. The set pressures and vacuums are typical values used on API tanks.
To detect a flashback an explosion panel is installed at the explosionproof vessel.
If the conservation vent cannot prevent flash back, the flame will propagate through the p/v valve and an explosion inside the explosion vessel will occur. As a result the diaphragm of the vessel will burst and flames will propagate to the outside of the vessel.
For the test the air to fuel mixture was varied. At the chosen mixture, stoichiometric rich, all conservation vents failed to prevent flame propagation resulting from atmospheric deflagration into the explosionproof vessel.
Test setup and results for the high velocity test
2 p/v valve
3 explosion proof container
4 mixture inlet
5 bursting diaphragm
7 pilot flame
10 shut-off valve
The high velocity vent valve test as described in ISO 16852 part 9.2.
For conducting the tests a conservation vent is installed on top of an explosionproof vessel. A stoichiometric propane air mixture is processed into the vessel and discharged through the pressure side of the conservation vent. As an ignition source a pilot burner is installed close to the discharge side of the conservation vent.
After ignition of the pilot burner a flame was stabilized at the seat of the valve. After a few seconds the flame propagates through the gap between seat and pallet which results in an explosion inside the vessel.
The second series of tests was conducted at a higher volume flow. Again just a few seconds after the ignition a flash back was detected.
This research work proves that conservation vents cannot function reliably as flame arresters if an atmospheric deflagration occurs. Furthermore, the theoretical statement that a flame arrester is not considered necessary with a pressure vacuum valve venting to the atmosphere, because flame speeds are less than vapour velocities across the seat of the pressure vacuum valves, cannot be confirmed.
It seems that some of the statements and guidelines in international standards are based on many years of operational experience. The conclusion that the conservation vents may have prevented flashback could be wrong.
The main reason for this positive experience is the result of the vapour headspace in the tank being above UEL under ambient storage conditions. If a terminal now decides to store a liquid, such as ethanol, which under ambient storage condition allows for a vapour headspace which is constantly within the LEL and UEL, the result could be a catastrophic failure of the tank resulting in million dollar losses.
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.
Protego will be expanding on these issues on day two of StocExpo Turkey & the Black & Caspian Seas in Istanbul on 11-12 November 2008.
i. ISO/ 28300: First Edition 2008-06-15, Petroleum, petrochemical and natural gas industries – Venting of atmospheric and low-pressure storage tanks.
ii. API Standard 2000, Fifth Edition, April 1998, Venting Atmospheric and Low Pressure Storage Tanks
iii. Technische Regeln für Brennbare Flüssigkeiten TRbF 20, Läger , 01.02.2001, BArbBl Nr. 4/2001 S.60
iv. Factory Mutual 2007 Approval Guide, Flammable Liquid Equipment
v. 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.
vi. ISO 16852:2008, Flame arresters – performance requirements, test methods and limits for use.
vii. API Publication 2210, Flame arresters for vents of tanks storing petroleum products.
viii. 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.