Issue of explosion protection classification for Category C media; request to temporarily make it the top topic of discussion~
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These past two days, I almost got into an argument with the older staff members at the company over the issue of class C medium explosion-proof zone classification. The project involves a Class C fluid with a flash point of 110°C; it is volatile. Since it is located too close to the cabinet room, management does not want to classify that area as an explosion-proof zone, as that would require the construction of explosion-proof walls in the cabinet room. The old man said that for Category C media with a flash point above 45°C, there is no need for explosion protection measures, as this is clearly stated in certain regulations regarding oil depots; moreover, this medium is used at normal temperatures. I disagree with this statement. My explanation is that fire protection and explosion protection are two separate sets of standards, with NEC and NFPA having different focuses. Although GB50058 specifies that flammable liquids with an operating temperature above their flash point must be explosion-proof, it does not state that Class C substances with an operating temperature below their flash point do not require explosion protection. My thinking is: 1) In this filling room, we have pumps, fillers, and even continuous release sources; especially when the medium is highly volatile, it’s easy to create an environment prone to explosions, and prolonged operation could lead to reaching the lower explosion limit ； 2) Although the operating temperature is at room temperature, this does not mean that the conditions for flash ignition are absent; the surface temperature of the motor, static electricity, and even arcs can all trigger an explosion ； 3) If the operating temperature is above the flash point, it indicates that an explosion could occur under normal operation due to the flash point temperature; if it is below the flash point, other possibilities of ignition cannot be ruled out. 4) During safety inspections, explosion protection is not checked based on the factory building, but rather the medium in question is the focus of the inspection. My principle is that as long as a gas or vapor has an explosive limit, explosion prevention must be given full consideration. Some people always cite diesel as an example; it belongs to Category C, with a high flash point. Of course, some components in diesel have a lower flash point and may fall into Category B. So in some design institutes, the diesel plant areas are not classified as explosion-proof areas. However, a person from the safety supervision agency pointed this out clearly – it’s related to the fourth point I mentioned earlier. His reasoning was simple: explosions occur due to vapors, and flash fires also involve vapors; so what does that have to do with categories A, B, or C? Who can guarantee that diesel stored for an extended period will not evaporate to the explosive limit? There’s a need to consider the saturated vapor pressure; what happens when gases leak in areas such as low-lying spots and accumulate over time? Please, senior members, discuss this issue together. Some people have talked about it on the forum; in just a few words, this time we have the opportunity to have a thorough discussion using this platform. Thank you
Why isn’t it specified what type of Class C medium it is – volatile yet with a high flash point?
It is an infrequently used intermediate in pesticide production; its name is spermine. It has a flash point of 110°C, but it is highly volatile, and the smell generated during reactions at the site is quite strong.
Safety is an ongoing pursuit; risks need to be taken into account, but not to an extreme extent. The cost of making everything explosion-proof could increase dramatically, so it’s necessary to find a balance
A strong odor does not necessarily mean high volatility; for example, malodorous substances can have an extremely strong smell even in small amounts. The flash point indicates the volatility to a certain extent; a flash point of 110°C means low volatility. An explosive hazardous environment refers to gases (or gas phases in equilibrium with a liquid phase), or mists; in your case, only mists need to be considered. You should carefully consider the last two explosive hazardous environments mentioned in point 3.1.1: both involve the transformation of a liquid phase into a gas phase or the formation of mist. The operating temperature is well below the flash point; considering the second scenario, another factor to consider is the ability to form a mist or to evaporate. Based on the flash point, the evaporation capacity is not strong, so the probability is low. Although you despise diesel as a reference, this place can indeed take diesel as a reference. But you need to understand the principle. In summary, it seems unnecessary to arrange it according to explosion hazard zones.
Your colleague cited the regulatory provisions; you should also use those provisions to support your argument. Is spermine volatile? To what extent is it volatile? What is the explosion risk? Please base your arguments on data, not just subjective feelings.
Thank you for the discussions by the seniors above. Today, the owner mentioned that an explosion occurred in the spermine storage tank, caused by static electricity. Therefore, they want to make the filling area explosion-proof, as the filling equipment is located inside enclosed cabinets, where the explosive limit is even lower. But the designers disagreed, as it would cost money. The two parties continued to argue, and in the end they called the person in charge of safety assessment. The explanation given by that person was that category C substances have a high flash point, so explosion protection is not necessary, and their operating temperature is very low. On the contrary, fire prevention needs to be properly addressed, as Class C materials are not flammable, but once they catch fire, it is very difficult to extinguish them. Class C explosions are based on combustion, similar to deflagration. Ensure proper ventilation and maintain the secondary load of the fans. As a result, this small filling room ended up without any explosion-proof electrical equipment and without any zones designated as explosion-hazardous areas. I hope it will serve as a reference for colleagues working on subsequent designs.
Whether explosion protection is required is determined in accordance with the provisions of GB50058 regarding explosive gas environments and explosive dust environments. In the case of explosive gas environments, it is necessary to determine whether a mixture of combustible gases, liquid vapors, and air can form an explosive atmosphere. Generally, substances of Class C have a high flash point, low volatility, a high lower explosion limit, and a low upper explosion limit; for example, if the lower explosion limit is above 30%, it is almost impossible to create an explosive atmosphere under normal operating conditions, so there is no need to classify such areas as explosion-proof zones. Generally speaking, substances of Classes A and B, as well as those of Class C whose operating temperature is above their flash point, require explosion protection measures. The specific requirements for explosion protection depend on the temperature category of the gas and the grade of the gas mixture; Appendix C of GB50058 can be referred to for this purpose. As for strong oxidizers and substances that are prone to thermal decomposition and explosion, the building codes should be consulted, but currently there is no clear list available for reference.
A work attitude that supports such debates
On-site safety inspections may not be based on your design, but rather on the actual materials you use. If those materials pose safety risks and there is evidence to support this, it’s useless to try to convince the authorities otherwise; you will have to make the necessary corrections as ordered by them. However, if it is possible to ensure safety through practical measures and precautions, it may not be necessary to use explosion-proof electrical equipment, as the cost of such equipment is extremely high.