Fired pressure vessels

The ASTM fired pressure vessels code requires pressure-relief devices to prevent pressures from rising more than 6% above the maximum allowable working pressure. 

Vessels, e g., waste heat boilers, in which steam is generated incidental to the operation of a processing system containing a number of pressure vessels, such as are used in chemical and petroleum products manufacture. (Equipment which may fire a supplemental fuel should be considered as a fired pressure vessel.)... 

Reliable contractors should carry out all boiler treatment and cleaning. Boilers are fired pressure vessels and are subject to mandatory insurance inspections. Significant benefits in safe and economic operation, particularly by reducing unnecessary chemical treatment, can be achieved by monitoring the condition of boilers. 

Fired pressure vessels should be designed and constructed in accordance with Section I (Power Boilers), or Section VIII, Division 1 or Division 2 (Pressure Vessels), as applicable, of the ASME Boiler and Pressure Vessel Code. 

European PED does not differentiate between directly and indirectly fired pressure vessels (steam boilers), nor nuclear nor any other, while ASME differentiates among the following ... 

Direct fired pressure vessels Nuclear power plants Heating boilers Unfired pressure vessels... 

Any economizer which may be shut off from the boiler, thereby permitting the economizer to become a fired pressure vessel, shall have one or more... 

Storage tanks should be designed in accordance with the ASME code for unfited pressure vessels. AH-welded constmction is recommended. Ethylene oxide storage tanks should be electrically grounded, isolated from potential fire hazards, and equipped with pressure rehef devices. New equipment should be cleaned of iron oxide and immediately purged with inert gas. 

MAX. ALLOWABLE WORKING UG-98 TEMPERATURE. DESIGN UG-19, UG-20 PRESSURE VESSELS SUBJECT TO DIRECT FIRING UW-2(d), U-I(h)... 

Pressure vessels subject to direct firing have special requirements relative to weided-joint design and postweld heat treatment. 

Care of Pressure Vessels Protection against excessive pressure is largely taken care of by code requirements for relief devices. Exposure to fire is also covered by the code. The code, however, does not provide for the possibility of local overheating and weakening of a vessel in a fire. Insulation reduces the required relieving capacity and also reduces the possibihty of local overheating. 

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

The ASME code provides the basic requirements for over-pressure protection. Section I, Power Boilers, covers fired and unfired steam boilers. All other vessels including exchanger shells and similar pressure containing equipment fall under Section VIII, Pressure Vessels. API RP 520 and lesser API documents supplement the ASME code. These codes specify allowable accumulation, which is the difference between relieving pressure at which the valve reaches full rated flow and set pressure at which the valve starts to open. Accumulation is expressed as percentage of set pressure in Table 1. The articles by Rearick and Isqacs are used throughout this section. 

Wong, W. Y., Improve the Fire Protection of Pressure Vessels, Chemical Engineering, October, 1999, p. 193. 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

Figure 7. Pressure conditions for safety relief valve installed on a pressure vessel (vapor phase) supplemental valve used for fire exposure only.

Note that for fired boilers, where the safety valve installation must comply with the ASME Code for Power Boilers rather than the code for Unfired Pressure Vessels, the allowable accumulation is only 6% instead of 10%. Reference also should be made to the Kj and A definitions, above. 

The original steam generators were simple pressure vessels that were prone to caiasirophic failures and loss of life. Due to better boiler design, tube-fired boilers, and boiler inspections, the incidence of catastrophic failure is now to a rare event (about once every 100,000 vessel-years). In Great Britain in 1866, there were 74 steam boiler explosions causing 77 deaths. This was reduced to 17 explo.sions and 8 deaths in 1900 as a result of inspections performed by the Manchester Steam User Association. In the United States, the American Society of Mechanical Engineers established the ASME Pressure Ves.sel Codes with comparable reductions. 

Probability of particular storage tanks or process vessels being hit by missiles caused by fires or explosions by fragmentation of rotating machines or pressure vessels, or transport accident s... 

Fire. The relief valve must be sized to handle the gases evolving from liquids il the equipment is exposed to an external fire. A procedure for calculating this is presented in API RP 520. This condition may be critical for large, low-pressure vessels and tanks but does not normally govern for other pressure vessels. 

Another common problem area is having open and closed drain systems tied together. Liquid which drains from pressure vessels flash at atmospheric pressures giving off gas. If this liquid flows in the same piping as open drains, the gas will seek the closest exit to atmosphere it can find, causing a potential fire hazard at any open drain in the. system. 

Figure 17-6. Hydrocarbon pressure vessel or protected fired vessel in a nonenclosed, adequotely ventilated area. (Reprinfed wilh permission from API RP 500.)...

An old 100-m pressure vessel, a vertical cylinder, designed for a gauge pressure of 5 psi (0.3 bar), was being used to store, at atmospheric pressure, a liquid of flash point 40°C. The fire heated the vessel to above 40°C and ignited the vapor coming out of the vent the fire flashed back into the tank, where an explosion occurred. The vessel burst at the bottom seam, and the entire vessel, except for the base, and contents went into orbit like a rocket [4]. 

D. K. McKibben, Safe Design of Atmospheric Pressure Vessels, Paper presented at Seminar on Prevention of Fires and Explosions in the Hydrocarbon Industries, Institute of Gas Technology, Chicago, June 21-26, 1982. 

The bursting of a large pressure vessel at Feyzin, France, in 1966 was at the time one of the worst incidents involving LFG that had ever occuired but has since been overshadowed by the events at Mexico City (see Section 8.1.4). It caused many companies to revise their standards for the storage and handling of these materials. Because no detailed account has been published, it is described here. The information is based on References 3 through 6 and on a discussion with someone who visited the site soon after the fire. 

Ninety minutes after the fire started, the sphere burst. Ten out of 12 firemen within 50 m were killed. Men 140 m away were badly burned by a wave of propane that came over the eompound wall. Altogether, 15-18 men were killed (reports differ), and about 80 were injured. The area was abandoned. Flying debris broke tbe legs of an adjacent sphere, w hich fell over. Its relief valve discharged liquid, which added to the fire, and 45 minutes later this sphere burst. Altogether, five spheres and two other pressure vessels burst, and three were damaged. The fire spread to gasoline and fuel oil tanks. 

Figure 8-2. Methods of protecting a pressure vessel against fire.

This subject has received little attention in the context of pressure vessel bursts. Pittman (1976) studied it using a two-dimensional numerical code. However, his results are inconclusive, because the number of cases he studied was small and because the grid he used was coarse. Baker et al. (1975) recommend, on the basis of experimental results with high explosives, the use of a method described in detail in Section 6.3.3. That is, multiply the volume of the explosion by 2, read the overpressure and impulse from graphs for firee-air bursts, and multiply them by a factor depending on the range. 

If failure is due to external heat applied to the vessel (e.g., from fire), the vessel s internal pressure rises, and at the same time its material strength drops. 

A boding liquid-expanding vapor explosion occurs when a pressure vessel containing a liquid is heated to a temperature liigh enough to cause tlie metal to lose strength and rupture. The source of tlie heat is nonnally another fire near tlie vessel. The effects of a BLEVE depend on whether tlie liquid in tlie vessel is flammable. If the liquid is flammable, it may eitlier cause a fire, which radiates heat, or fonii a vapor cloud, which could result in a second explosion. 

Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more) used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1 Sizing and Selection, API RP-520, 5th Ed., July 1990, American Petroleum Institute.

When a pressure vessel is exposed to external heat or fire, supplemental pressure relieving devices are required for this excessive pressure. These devices must have capacity to limit the overpressure to not more than 21% above the maximum allowable working pressure of the vessel. (See Figures 7-7A and 7-7B.) A single relieving device may be used to handle the capacities of paragraph UG-125 of the code, provided it meets the requirements of both conditions described. 

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