Flap Valves: Standards, Applications, and Limitations

Flap valves are relatively new in the North American market and gaining wide acceptance. Although flap valves have been available in Europe for several years, it is only recently that they have been included in NFPA 69 Standard on Explosion Prevention Systems, 2014 Edition. Flap valves are relatively inexpensive as isolation devices go and they are passive, which are definite pluses. On the other hand, they do require more maintenance than is generally appreciated and in-service problems have cropped up with them, the most severe of which have been reports of flame pass through in commercial installations. The objectives of this article are to cover the NFPA 69 Standard as it applies to flap valves, their proper application, and limitations.

History
Flap valves were developed in Europe for use as explosion isolation valves. A need for an inexpensive, passive, mechanical barrier to flame spread during a deflagration is addressed by the flap valve. Originally the flap valve was included in the European EN 15089 Standard that covers all types of explosion isolation devices. Over time, however, reports began to filter in that flap valves allowed flame to pass through them in a few instances in actual service. Based on verification of these reports, the technical committee decided to withdraw flap valves from EN 15089, which covers many types of valves, and to develop a new standard specifically and exclusively for flap valves. The new standard, EN 16447, is now complete and covers the test requirements and limitations of flap valves. Importantly, it addresses the limitations of flap valves that resulted in the aforementioned failures.
    
The NFPA technical committee responsible for NFPA 69, of which this author is a member, advises the user in Appendix A.10.4.2.1 that flap valves require third party testing and approval from an independent, recognized laboratory or certification in accordance with CEN Standards. There is confusion on this point because many claims and counter claims are being made and the European Standards are not generally well understood in North America. It is this author’s opinion that only flap valves that have been certified under EN 16447 meet the requirements for certification, thus the spirit and intent as outlined in NFPA 69.     

Moreover, claims that flap valves that were certified under EN 15089 and are being presented as “ATEX Approved” are no longer approved in the EU, therefore nor are they compliant with NFPA 69. This is a very real caveat and the wise buyer will verify under which EN standard the flap valve under consideration was certified. As this is written there are flap valves being sold in North America that were tested to the old standard.
    
Flap valves are covered in NFPA 69, Chapter 12 Deflagration Control by Passive Isolation. The purpose of isolation is to prevent flame propagation through pipes and ducts, and valves such as flap valves can provide safe and effective flame spread interruption if properly designed, tested, and applied. Taking care to do so is essential under most circumstances and is required under the industry specific NFPA Occupancy Standards, where isolation is applicable. One exception would be metal dusts covered in NFPA 484, because metal dusts often react more quickly than even the fastest explosion isolation valves can respond.
    
In NFPA 69, Chapter 12 flap valves are described and their design is discussed. Only issues that are unique to flap valves will be discussed here. Issues common to all isolation valves will not be covered in this article. Flap valves are susceptible to product buildup inside the housing. So material loading in the piping or duct is very critical. Flap valves are meant for use in light loading applications only. For example, nuisance dust collectors, which typically have light, intermittent dust loading are a good application, but vacuum receivers, which typically have heavy dust loading, more or less continuously, would not be. Physical properties of the material being conveyed such as abrasiveness or adhesiveness should be considered because the flap will be in the flow stream and wear of the flap is a possibility. Furthermore, a sticky material can build up on the flap causing a decrease in response time as well as possibly interfering with tight closure. Turbulence caused by fittings, elbows or similar equipment associated with the convey line could cause excessive turbulence and consequent wear of the flap, the valve seat or the valve body. While true of other types of valves as well, turbulence is particularly detrimental to the life expectancy and performance of flap valves because the flap and seat are in the flow stream. Flow velocity is important, too little and material drop out could occur in the valve body and material buildup could prevent the valve from closing tightly when a deflagration occurs. At the same time, excessive flow velocity can cause undue wear on the flap and seat. Flow direction is only possible such that the flap is held open by the normal process flow. That means that flap valves can only be used on the inlet to the vessel being isolated. Valve orientation is critical. Flap valves must be installed horizontally and in an upright position.
    
Another basic consideration is that there will be a pressure drop associated with the use of flap valves. This factor needs to be accounted for in the design of system. Upon closure during testing under actual deflagration conditions it was discovered that if not locked closed, the flap would flutter. A fluttering flap can allow flame to pass through the valve. Accordingly, the flap must lock in the closed position to prevent flame and burning material from passing through the valve. Flap valves must be inspected and cleaned out periodically to ensure that there is not a buildup of material that could negatively affect function. Either a reduced response time, or the prevention of full closure are possible. In consequence, an inspection lid must be provided to facilitate this maintenance procedure. Obviously, it is important that the lid be located in an accessible location. A valve mounted just below a ceiling would prevent inspection and cleaning. This is an important consideration as material build up inside a valve body could occur very quickly. Upon activation the flap valve should be provided with a signaling device to initiate shut down. Either an automatic sensing and shut down feature, or a documented inspection protocol based on operating experience is necessary to initiate automatic shutdown if there is material buildup in the bottom of the valve.

Summary
As already mentioned, flap valves are meant for applications involving light dust loadings. Several factors can result in excessive material accumulation inside the valve body. Such a buildup can prevent the rapid and complete closure of the flap. If the flap does not completely close and lock during a deflagration, then flaps have been observed to flutter while undergoing tests. This problem has been duplicated at various test facilities and on flap valves from various manufacturers. The combination of incomplete closure and fluttering is believed to be the root cause of earlier reports of flame and burning material passing through flap valves. This phenomenon was the driving force behind the decision in the EU to develop a completely different and unique test protocol for flap valves. Please note that there is no test protocol for flap valves, or for that matter for any other explosion isolation valve type, currently in place in North America. EN 16447 is currently the only established test protocol available that specifically addresses flap valves.  
    So where are flap valves an effective option? Nuisance dust collectors typically have relatively light dust loading. The air induction port to a mill that typically has no dust loading would be an ideal application. Avoid high material loading applications such as product transfer or fill lines from silos or weigh hoppers, in short anywhere that could plug the valve or where the pressure drop caused by the valve could compromise system capacity or performance.
    Flap valves are gaining acceptance in industry at a rapid pace. They are simple, cost effective, and fast. When properly applied, they offer an acceptable level of safety and reliability. If they are improperly applied or installed, or are not inspected and cleaned adequately, they can plug or fail to perform when needed. Make sure the flap valve selected meets all of the requirements outlined in NFPA 69.
    
     Bill Stevenson, VP engineering, CV Technology Inc., has more than 47 years of experience and is a recognized authority in the dust explosion field. He is a senior member of the AIChE and is a member of the NFPA, where he is active on several technical committees for standards pertaining to combustible dust. For more information, visit cvtechnology.com.

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