Document QXMZEYEBG6XjmvVNenxELdoGv

732 CHAPTER 45 crearingty successful, despite the possibility of damage by large pieces of foreign matter in the systems. Applications In compressor discharge Una, check valves are used to prevent flow from the condenser to the compressor during tb off-cycle, or from an operating compressor to one which is idle. While 2 to 6 psi pressure drop is tolerable, the check valve design must reristspulsations of-the compressor, tem perature of discharge gas, and must be bubble^tight to prevent accumulation of liquid refrigerant at the compressor discharge - valves or in the iranlrpaat . ' In liquid Una, check valves are used to. prevent reverse flow through the unused expansion device on heat pump systems, or to prevent back-up into the low pressure liquid Dneof a redrcuteting sy8tem;during a' defrost period. While 2 to 6 psi pressure drop is usually acceptable, the check valve seat must be virtually bubble-tight. In tire suction line of a lower temperature evaporator check valve may be used to prevent the transfer of refrigerant vapor from one evaporator to another evaporator which is at a lower temperature on the same suction main^-In-.this case the pressure drop must be less than 2 pri, .the .valve seating must be reasonably tight, and the check valve must be reliableiat low temperatures. * . ....... : Nonnaliy-open pressure. operated - chafka, are ^.frequently used to close suction lines, gas, or liquid legs in gravity redrcu? lating systems during defrost . In hot gae defrost Una, check valves may- be used.in the branch hot gas lines winnerting the individual evaporators, to prevent crora-feed of. refrigerant during the cooling cycle when toe defrost operation is not..taking place. In addition, check Valves are used in the hot gas liiie, between thehot'gas heating 'coil in the drain pan and toe evaporator, to 'prevent 1965 Guide And ;Data- Book pan coil'sweating during, the refrigeration .cycle! Tolerable pressure drop is.typically 2 to 6 psi, seating must be nearly bubble-tight, and seat materials must withstand high tem peratures. . .To prevent chatter or pulsation, check.valves nbmild bg sired for the pressure drop which will assure that they are in the wide-open position at the desired flow rate. RELIEF DEVICES .' Refrigerant relief devioes are usually safety or functional in usage. A sb/efy relief device is designed to positively relieve at its ret pressure for one crucial occasion without prior leak age. This relief may be to the atmosphere or to the tow ride. A functional relief device is a control valve which may be called upon'to. open, to modulate, and to re-close, with re peated accurate performance. Relief is usually from a portion of the system at higher pressure to a portion at tower pres sure, for reasons of system control. Design refinements of the functional relief valve usually make it nniifaridA 0r uneco nomical as a,safety relief device. ; Safety Relief Valves .. These are most commonly pop type dwagna which abruptly open ;when the inlet pressure exceeds the outlet pressure by the valve setting pressure (see Fig. 28): Seat configuration is such that once lift begins, the resulting increased active.seat area causes the valve seat to pop wide open against the force of the setting spring. Since the flow rate is measured at a pres sure of 10 percent above.the setting, it is necessary that the valve open within this 10 percent increase in pressure. ' This'type of relief valve operates ona fired pressure dif ferential from inlet to outlet. Because the valve is affected by back pressure, the installation of a rupture member at the valve'outlet is. not permissible. } Relief valve seats may be made of metal, plastic, lead alloy,' or* synthetic1 rubber. The latter two are very popular because' of their greater resilience *"d consequent probable reseating tightness. For valves having lead 'alloy seats, an emergency manual reseating stem is occasionally provided, in order to permit reforming of the sparing surface by tapping the stem lightly with a hammer. The advantages of the pop type relief valve are: simpli city of design, tow initial cost, and high discharge capacity. Other Safety Relief Devices Two devices performing rimilar safety relief operations are the fusible plug-(see Fig. 29) and the rupture member. The former contains a fusible member that melts at a predeter mined^ temperature corresponding to the safe saturation Refrigerant .Control Devices 733. mwsuie of the refrigerant,, but is limited, in application to nressure vessels with internal gram volume of 3 cu.ft or less. Therupfwe member (see Fig. 30) contains a frangible disc, designed, to rupture at a predetermined pressure. Discharge Capacity The ASA B9.1 Code provides a formula for calculating the discharge capacity required by the relief device: C -fDL where . q IT'?nimiim discharge capacity of the relief device, pounds of air per minute. D >= outside diameter of the vessel, feet. - L -- length of the vessel, feet. factor depending on the kind of refrigerant: for ammonia J (Refrigerant 717), f 0.5; for Refrigerants 12, 22, and ., 500,/" 1.6; for all other refrigerants,/." L0. Capadties'of pressure relief valves are determined by test in accordance, with the provirions of the ASME Boiler and Pressure Vessel Code. Relief valves, approved by toe National Board'of Boiler and Pressure Vessel Inspectors, are stamped with the code symbol, which consists of. the letters UV in a clover leaf design^ with the letters NB stamped directly below this symbolsIn addition, the pressure setting and capacity are stamped on the valve. Pressure Setting ... The maximum pressure setting for a relief device is limited by the design' working pressure of the vessel to be protected. Pressure vessels normally have a factor of safety,of 5, so that the maximum-bursting pressure is 5 times the rated design working' pressure.'The relief device must-have enough dis^ charge capacity to prevent the pressure in the..vessel, from rising more than 10 percent above its design pressure: Since the capacity' of a relief device is measured at 10 percent above its stamped retting, the setting cannot exceed" the design pressure of toe vessel. The exception to this in'toe ASA B9.1 Code applies.to vessels of 3 cu ft,or less, where' the bursting pressure;of the vessel need be only 2\ times the relief device retting. In the of fusible plugs this would be 2} times the refrigerant saturation pressure (psia)- corresponding' to the stamped temperature, or 2\ times the critical pressure of the refrigerant, whichever is smaller. .To prevent toss of refrigerant through pressure .-relief de vices during normal operating conditions, it is necessary that the relief device retting be substantially higher toan the system operating pressure. . For;mpture members,, the-retting-should,be 50 percent above a static system presriire, and 100 percent above a maxi-, mum .pulsating system pressure. Failure to provide this mar gin .of safety will cause fatigue of toe frangible member, and rupture will occur well below the stamped retting. For refief valves the retting should be 25 percent above the maximum system pressure. This factor of safety will provide sufficient spring force on the valve-seat!to maintain a tight seal and still allow for retting-tolerances and other factors which came settings'to vary. Although relief valves,are set at'toe factory within'a few pounds .of the.'stamped setting, the .variation may be as much as 10 percent after the .valves Lave.been stored or placed in service for a period of.tiihej' Relief Device Summary : : "' .1. Select a 'relief device which' has riifficieht capacity for code requirements sod which is suitable for the type of refrigerant bred. 2.-:Use the proper size and length of discharge tube or pipe. " 3. Do not discharge the relief device prior to installation or when pressure testing the system: ,4. For systems containing large quantities of refrigerant, use a thrto-way valve and two relief devices. ' 5. Provide a pressure vessel which will-permit the relief valve to be set at least 25 percent above the maximum system pressure. Functional'Relief Valves . .. Functional relief valves are usually diaphragm type in which the system pressure acts upon a diaphragm-which lifts the valve disc from the seat (see Fig. 31). The other side ofthe diaphragm is exposed to the adjusting spring and to atmos pheric pressure. The ratio of effective diaphragm area to seat area is high,' so the outlet pressure has very little effect'upon the operating point of the valve. ' -` ; - Since the lift of toe diaphragm is not great, the diaphragm valve'is frequently-built as the.pilot'br-servo of'a-larger piston-operated main valve, thereby providing fine senritvity and high-flow capacity. Construction; and'performance are similar to the- pilot-operated evaporator pressure regulator described previously in this chapter except the- diaphragm valves are constructed for higher pressures. They are cons; latXT . -.- <Rg. 29 .... Fusible Plug , , Rg. 30 .... Rupture Disc