Masterclass Pressure Relief Safety Devices Part 1. The last article covered liquid refrigerant receivers. The containment of refrigerant in vessels and refrigerant lines demands that safety be given due consideration to eliminate risk to persons due to refrigerant release or explosion and to minimise refrigerant losses thereby avoiding environmental damage and financial loss associated with replacement. This month we review various methods to meet the aforementioned requirements. Types of relief devices. There are three general types of relief devices commonly used, namely the fusible plug, the rupture member and the pressure relief valve. Safety Relief Valve Handbook Free DownloadThe fusible plug contains a fusible member which melts at a predetermined temperature corresponding to the safe saturation pressure of the refrigerant. The rupture member contains a frangible disc designed to rupture at a predetermined pressure. The pressure relief valve is a pressure actuated valve held closed by a spring or other means and designed to automatically relieve at a predetermined pressure. Relief device selection. Four requirements to select the correct relief device 1. Type of refrigerant. Size of vessel. This determines type of relief device which can be used and the discharge capacity requirement of the relief valve. Design pressure of vessel to be protected. This determines the pressure setting of the relief device. WRITTEN FIRE SAFETY ANALYSIS CHECKLIST SECTION 1 Facility Information 1. Facility name 2. Physical address. The 36th SHPA National Conference. Melbourne 1114 November 2010 Melbourne Convention Exhibition Centre. Valve Leakage Classifications Reprinted courtesy of Control And Instrumentation. Types of Valve Leakage There are two types of leakage from a valve, namely. A valve is a device that regulates, directs or controls the flow of a fluid gases, liquids, fluidized solids, or slurries by opening, closing, or partially. Length of discharge piping required from outlet of the relief valve. Correct size required to prevent back pressure from building up in the discharge piping, and preventing the relief valve from discharging at its rated capacity. Grainger/5DLY5_AS01?$lgmain$' alt='Safety Relief Valve Handbook' title='Safety Relief Valve Handbook' />The Safety Relief Valve Handbook DownloadANSIASHRAE 1. Selection of types. Pressure vessels with internal gross volume of 3ft. Pressure vessels of less than 6in inside diameter can be protected by a pressure relief device or a fusible plug from fire or other abnormal conditions. Pressure vessels over 3ft. Pressure vessels of 1. Each relief valve shall have sufficient capacity to protect the vessel, because only one relief valve shall be under system pressure at a time. No stop valve may be located between the vessel and any relief device except the three way type with two relief valves permitting only one relief valve to be shut off at a time. Discharge capacity. The ANSIASHRAE 1. Code provides a formula for calculating the required discharge capacity of the relief device C f. DLwhere C Minimum required discharge capacity of the relief device in lbs of air per minute kgsD Outside diameter of the vessel in feet mL Length of the vessel in feet mf Factor dependent upon type of refrigerant. When used on the low side of a limited charge cascade system We recommend that the latest edition of ANSIASHRAE 1. This formula was based on a fire condition and developed by assuming a flame temperature to calculate the BTU heat input radiated to the vessel. As pressure rises in the vessel, the discharging relief device acts as an expansion valve to provide a cooling effect. Since the latent heat of vaporisation of the refrigerant is known, it is made equal to the heat input from the fire to determine the amount of refrigerant which must be discharged to maintain uniform temperature and pressure in the vessel. Since the heat input depends on the size of the vessel exposed to the fire, the diameter D and length L of the vessel become part of the formula. The factor f provides for the difference in latent heats of the various refrigerants and converts the flow of refrigerant to the equivalent flow of air. This permits the rating of relief devices to be expressed in terms of discharge of air in lbsmin regardless of the type of refrigerant used. For example a pressure vessel 1ft in diameter by 5ft long containing R2. For ammonia f 0. This smaller capacity requirement for ammonia is due to its high latent heat of vaporisation with greater cooling effect. Capacities of pressure relief valves are determined by test in accordance with the provisions of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 Relief Valves, approved by the National Board of Boiler and Pressure Vessel Inspectors, and are stamped with the code symbol. This symbol consists of the letters UV in a clover leaf design with the letters NB stamped directly below the symbol. In addition, the pressure setting and capacity are stamped on the relief valve. Capacities of fusible plugs and rupture members, if not stamped by the manufacturer, may be determined by the following code formula C 0. P, d. 2 where. C rated discharge capacity in lbs of airmin kgsd internal diameter of bore of fusible plug or rupture member in inches. For rupture members P1 1. PSIG k. Pa Gage 1. For fusible plugs P1 absolute saturation pressure corresponding to the stamped temperature melting point of the fusible plug or the critical pressure of the refrigerant used, whichever is smaller, PSIA k. Pa. The capacity of a relief device will increase proportionately with an increase in pressure setting. Therefore, the capacity stamped on the relief device by the manufacturer is correct only for the pressure setting stamped on the device. The ANSIASHRAE 1. Code formula for determining the required capacity is independent of the pressure setting. To obtain the maximum rated capacity of the relief device, the setting should be as high as the design pressure of the vessel will permit. Safety Relief Valve Handbook Marc Hellemans Free Download' title='Safety Relief Valve Handbook Marc Hellemans Free Download' />Pressure setting. Pressure vessels are normally manufactured to a safety factor of 2. LANDLORD HANDBOOK 55 W. Mission Ave., Spokane, WA 99201 5093282953 Fax 5093275246 TDD 711 www. The primary purpose of a safety valve is the protection of life, property and environment. This page descibes the function of pressure relief valves. Importantly, the maximum pressure setting for a relief valve is limited by the design working pressure of the vessel to be protected. The relief device must also have enough discharge capacity to prevent the pressure in the vessel from rising more than 1. Since the capacity of a relief device is measured at 1. To prevent loss of refrigerant through pressure relief devices during normal operating conditions, it is necessary that the relief device setting be substantially higher than the system operating pressure. Unless specified otherwise by local governing codes, the relief valve should be set at a pressure equal to the design working pressure of the vessel as furnished by the manufacturer and stamped on the vessel. For proper functioning of the relief valve, the design pressure of the vessel and corresponding relief valve setting should be at least 2. This factor of safety will provide sufficient spring force on the valve seat to maintain a tight seal and still allow for setting tolerances and other factors which cause settings to vary. Although relief valves are set at the factory within a few pounds of the stamped setting, the variation may be as much as 1. Discharge piping from relief devices. The discharge of toxic refrigerants or large quantities of non toxic refrigerants through relief devices must be vented to the outside atmosphere or in the case of ammonia into a tank of water. Buck Converter Mosfet Driver on this page. While the relief device is discharging, back pressure will build up in the discharge piping which may prevent the device from performing properly or from discharging its rated capacity. For relief devices with fixed openings, such as fusible plugs and rupture members, this back pressure can build up to approximately 5. This is due to the phenomenon known as critical flow through a fixed orifice, whereby a decrease in outlet pressure below 5.