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http://www.tt-fire.com/All About Fire Safety Valves
One of the more recent developments in industrial tools to prevent fire is the introduction of fire valve. These valves are designed to prevent a fire from spreading if the fuel valve gets damaged or causes a leakage.
A valve is a device that adjusts, directs, or controls the flow of a fluid by opening, closing, or partially blocking various passageways. Valves are technically a type of fitting but are usually discussed separately. In an open valve, fluid flows from higher pressure to lower pressure. The simplest kind of valve is simply a freely hinged flap that drops to obstruct flow one way but is pushed open by fluid flowing the opposite way. This ancient valve is called a check valve, as it stops or “checks” the flow in one direction. Modern valves may regulate pressure or flow downstream and operate on sophisticated automation systems.
Valves have industrial applications for controlling processes, residential uses such managing water flow to dish and clothes washers and taps in the home. They are found in almost every industrial process, including mining, water and sewage processing, power generation, food manufacturing, processing of oil, gas and petroleum, chemical and plastic manufacturing, and many other fields.
Fire-safe valves are spring-loaded valves that are thermally actuated. In case of fire, they close automatically and stop the fuel flow. The fuel flow from the crankcase of the engine and the reserve oil supply gets hindered, thus minimizing the chances of leakage.
Spring-loaded valves help with preventing the flow of fuel such as oil, gas, petrol, and other combustibles, and are useful and effective against any damage. The benefits of using fire landing valve in a system are the protection of human life, lower insurance rates, and the protection of equipment and the building.
It is because of these benefits that the use of these valves is highly recommended and appreciated. These days, many companies offer an extensive range of fire-safety valves to provide protection even in high temperature and high-pressure conditions.
How Fire Safety Valves Work
A fire safety valve aims to keep ignitable fluids away from a flame. These fire gate valve close when exposed to heat, separating a flammable liquid from the heat source. They close in the midst of overheating of the pipeline, brought on by fire or similar events. The obligation to use fire safety valves is subject to any tenets, regulations, and laws concerning the unique methodology, plant, pipeline, or vessel.
Fire-safe ball valves use a combination of a floating ball, graphoil seals, and metal-to-metal seating to provide tight shut-off while preventing external stem leakage. In normal working conditions, the ball rests against two seats, ensuring bubble-tight closure. When the valve is exposed to a temperature above the limits the seats can withstand (for example, +450°F), the seats become deformed and are subject to extrusion. When the seats have been completely destroyed, the ball in the valve will come to rest firmly against the end cap, producing a metal-to-metal closing. Stem seals, which have high temperature-resistant properties, further restrict leakage in conjunction with a blow-out-proof anti-static stem, so that the flammable fluid stays separate from the heat source that may ignite it.
A fire-safe valve may also be made up of four main mechanisms: a spring pack, a trigger assembly, mounting hardware, and a fusible link. The components work in unison to close the valve should a fire be detected within a facility. The fusible link is the key part of the assembly. It keeps the valve open by maintaining tension on a spring pack through the trigger assembly. When a fire breaks out, the fusible link separates once it is heated to a certain high temperature, which releases the spring pack and allows it to close the valve.
A fire-safety valve with fusible links has a primary drop-tight seat, usually made of TFE, along with a second seat made of metal for isolation in a fire. The secondary seat also has graphite seals for further protection. This means that the shut-off valves can be paired with any quarter-turn ball valve, butterfly valve, or fire ball valve.
This article presents an understanding of fire safety valves. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.
The definition of a fire-safe valve has more than one answer since different standards exist for such valves. The major standards are presented in this article and their criteria discussed as an aid in specifying these devices. Fire-safety standards for equipment used in the chemical process industries (CPI) are critical however no single test for fire-safe valves has been developed that covers all of CPI. Since all fires are not alike, safety precautions should not all be the same for all situations. This article attempts to answer such questions as whether the refining industry’s standards cover fire hazards posed by media and processes specific to the rest of the CPI and which criteria come closest to providing proper guidelines for choosing a fire-safe valve for non-oilrefining service.
The effectiveness of fire air release valve has been investigated when offshore process equipment is exposed to a fire. Simulations of several typical offshore pressure vessels have been performed using the commercial software VessFire. The pressure vessels are exposed to a small jet fire, large jet fire, and a pool fire on both the wetted and unwetted part of the vessels. Rupture times of the vessels are calculated by comparing the pressure in the vessel with the tensile strength of the material. Rupture times are then compared for the vessels, with and without a PSV, in order to see the effect of the installed PSV. It is found that when a fire affects the unwetted part of a vessel, the PSV offers only minor or no additional protection. When a fire affects the wetted part of a vessel, the PSV relieve the inventory as designed. It is argued that PSVs provide insufficient fire protection for typical offshore fire scenarios and that Blowdown Valves and Passive Fire Protection should be considered as alternatives.
In order to protect process equipment from a possible overpressure scenario a PSV is installed as a mechanical barrier. Often, when other credible over-pressure scenarios such as process upsets are ruled out, either due to the vessel in question being protected from over-pressure by upstream or downstream equipment, and/or the design of the vessel is to full pressure spec, the remaining credible scenario is a fire case. This is according to normal industry practice and according to code (API Standard 521, 2014, API Standard 14C, 2007). It is the authors’ experience that many vessels are equipped with a fire PSV as the main/only relief case. The main concern is that a pressure vessel exposed to a fire may cause a Boiling Liquid Expanding Vapor Explosion (BLEVE) upon rupture, leading to a significant escalation in consequences. The BLEVE scenario is relevant for vessels carrying significant components of light volatile liquid hydrocarbons such as separators and not for vessels only containing gas or non-volatile liquids. Standards such as API 521 (API Standard 521, 2014) (ISO 23251) and API 14C (API Standard 14C, 2007) (ISO 10418) discuss the requirement for PSVs for fire protection and how to size such PSVs. The use of fire PSVs, in accordance with API 521, has been developed for pool fires on onshore refineries. However, on offshore oil and gas installation a more likely fire scenario will often be that of a jet fire. According to API 521 fire PSVs do not offer proper protection against jet fire, but other measures such as shutting down the jet fire source and depressurizing process inventories should be considered the primary protection against a jet fire. Pool fire heat loads applied in API 521 for offshore applications have also been questioned (VESSFIRE, 2003). Even in the case of pool fire a fire PSV may not provide adequate protection in accordance with API 521 if the pool fire exposes the unwetted part of the pressure vessel. Despite this, fire PSVs are often installed on offshore oil and gas installations to protect even completely gas filled pressure vessels.
Installing safety equipment such as a PSV that does not provide any or insignificant protections represent a significant lifecycle cost and it may also increase risk of operating the offshore installation as well as provide a false sense of security. The PSV will require testing, inspection and maintenance which will expose personnel to hazards, the PSV will be a source of potential leakage, and human errors in connection with the PSV can lead to increased risk of failure. In fact, there have been numerous examples in the offshore oil and gas industry where incidents and accidents have occurred in relation to use of PSVs. The best example, illustrating the risk associated with PSVs, is the Piper Alpha disaster where a PSV was taken out for maintenance together with a condensate pump without positive isolation. Experiments carried out by Birk et al. (2006), demonstrates that a pressure vessel, which is exposed to a flame on the unwetted part, will rupture before the pressure is high enough to trigger the PSV. As discussed by Dalzell and Chesterman (1997) safety systems should only be installed if they provide a safety benefit and not merely because it is recommended in standards or because it is the normal industry practice. Another issue with installing a fire PSV that cannot prevent rupture, is that the PSV may be regarded as a sufficient safety barrier in accordance with standards and best practice, when in fact, it provides no, or no significant, barrier. The purpose of this article is to investigate in which cases a fire PSV is expected to provide a safety benefit offshore by performing a case study with the state of the art software tool VessFire (VESSFIRE, 2003). Seven typical offshore pressure vessels are analyzed for three different fire scenarios: a large jet fire, a small jet fire and a pool fire. The simulations have been performed for the fire exposing both the wetted and unwetted part of vessels.