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| Home | How do you spell relief?: "Pee Ess Vee" of
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Pressure vessels, such as the ones used in our industry, all have a Maximum Allowed Working Pressure (MAWP). Exceed this pressure and you’ll either end up with a much rounder vessel than you initially set out to, or an explosion, or a fire or a combination of these. Of course, the real reason is safety. We owe our people in the field a responsible design that protects not only the equipment, but also a safe work environment. One of the ways to protect pressure vessels is with a Pressure Safety Valve, often abbreviated PSV. Each one is designed for a specific purpose and is set up for a specific job. The PSV is the equivalent of the whistle on your steam kettle. In effect, when water boils in the kettle, the pressure inside the kettle builds up to the point where the kettle whistles. The same can happen inside a separator or similar vessel. Pressure can build up due to overheating in a treater; a well "pressuring up" due to a slug of gas, or simply a slug of gas making its way down a pipeline to a vessel. The end result is an over-pressure on your carefully designed vessel for which your favorite fabricator demanded your first 2 children in return. Before we get into the details, some semantics: "PSV’s can be one of three things" according to Todd Oliphant of Black Gold Valve. Todd describes them like this: "First there is the PRV, or the Pressure Relief Valve. These valves modulate open and closed as the system pressure increases and decreases. These have one adjusting ring. They do not have an audible ‘POP’ when they overpressure. Todd further tells me that the second type is the Safety Valve. "These are automatic pressure relieving devices actuated by static upstream pressure. These are used for air and steam service and usually have two adjusting rings and a lift lever on top of them. These valves make an audible ‘POP’ sound when they give way. The last variety Oliphant identified is the PRV, or the Pressure Relief Valve. " They’re automatic, and are operated by upstream pressure. PRV’s are designed to prevent a rise of system pressure above a specified value and to re-close after normal conditions are restored." Todd states that in the case of the PRV, protection comes from a valve designed to be normally closed due to resistance on it by a spring’s force. As vessel pressure gets closer to the pressure set on the PSV, the pressure tends to equalize the spring’s force, and (in vapor or gas service) the valve may "simmer" before it will ‘pop’. When the vessel pressure increases to within one to two per cent of the set pressure, gas will start to move past the valve’s seat surfaces fast enough that you can hear it escaping. A proper PSV therefore lets pressure gradually build when pressure builds up close to the MAWP of the vessel before it "pops" fully open. The PSV can be used in one of three situations. One is in single valve installations, the second is in, you guessed it, multi valve installations, and finally firesize installations. Todd says that single valve installations occur when "one valve must relieve the entire system". ASME Code dictates that such valves are set no higher than, but may be set lower than the MAWP. This involves calculating valve relieving capacity. Trust me, for this you want to phone Todd. He’ll know which of six formulae to apply to your specific situation. Whenever we are building equipment at OilPro we always get Todd’s people to size the PSV for each case. Multi-valve installations are a bit different. Sometimes manufacturers choose to go with this option because often it is less expensive to fit two smaller PSV’s than a single larger unit. This may cause the end-user to spend a bit more money on piping I the PSV’s, but it also adds an additional measure of safety due to the innate redundancy built in. "The key is that both valves must be in unrestricted communication with the vessel they are designed to relieve" says Todd. In effect, what Multi-Valve installations also allow one to do is to set at least one relief valve at no higher than MAWP, however any subsequent valves may be set at five per cent above MAWP. Again, make Todd do the work, because there are again six more different conditions, governed by set pressure ranges, that apply. Fire Sizing is something I was told about by a friend, Oeltsje Kalsbeek at Tartan Engineering. Oeltsje was racking his brain for quite some time to "firesize" relief valves for a client. What makes things a little less straight forward is that your ordinary battery fire goes through a number of steps. For example, a heated crude oil treater may first contain gas, oil and water. However, light a little fire underneath that treater, instead of in the firetube, and eventually first the lighter ends will boil, then the other hydrocarbons might boil, and finally the water. Each of these happens at different stages of the fire. This means that different quantities of material can come flying out the relief valves at different times. ASME dictates that Fire-Sizing involves multiple valve installations, again in unrestricted communication with the to-be-protected system. The allowable over-pressure for fire-sizing considerations is 21 per cent above MAWP. You can tell these valves because they will have an ASME symbol stamped nameplate reflecting the individual vale set pressure and its relieving capacity at 10% overpressure. In addition, such a fire-sized valve will have another nameplate, without the ASME symbol, which reflects valve set pressure and calculated capacity at its particular overpressure. Oliphant says that in fire-sizing situations it is important to try to use valves already selected for operational overpressure conditions. This is a simple economics-driven measure, which much to Todd’s chagrin, keeps people from buying multiple sets of PSV’s for each application. Says Oliphant:"Fire-Sizing should be considered as an emergency situation, and any fire-sized valve relieving requirements should only be in addition to those already selected." There is a lot more to these valves, such as specific codes, identifying these PSV’s as suitable for use in Power Boilers (ASME Sec. 1), Unfired Pressure Vessels (Sec. VIII), and those used in heating plants (Sec. IV), but those are way too much fun to get into. If you want to know more, call Todd. He’s always there when I need him for sizing situations. The PRV is the "first line of safety, and the last line of defense for the protection of personnel, plant equipment, and property from dangerous overpressure conditions" according to Todd. |
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