2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Journal, Vol. 50, February 2008. This posting is by permission of ASHRAE. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE s prior written permission. Fundamentals of Safety Relief Systems By Douglas T. Reindl, Ph.D, P.E., and Todd B. Jekel, Ph.D, P.E. ASHRAE made significant changes in 2001 to the calculations required for sizing vent lines for the safety relief systems applied to refrigerating systems in its Standard 15, Safety Standard for Refrigeration Systems. This article provides a review of the fundamental requirements for properly engineering and installing safety relief systems. In addition, the authors review and clarify some of the newly implemented changes to safety relief systems. Pressure relief devices are an engineering control designed to self-actuate and permit flow as a means of protecting system components by preventing their catastrophic failure during operating excursions that create overpressure. ANSI/ASHRAE Standard 15-2007 defines a pressure relief valve as a pressure-actuated valve held closed by a spring or other means and designed to automatically relieve pressure in excess of its setting. By permitting vapor to flow out of the protected component through a pressure relief valve, the pressure in the protected component is reduced, preventing its catastrophic failure. For pressure relief devices to be effective at achieving their intended purpose of protecting components in a safe manner, a properly engineered safety relief system is essential. A safety relief system is comprised of the necessary pressure relief devices (pressure relief valves, rupture disks, fusible plugs, etc.), valves (three-way, stop), fittings (elbows, tees), piping (inlet and outlet), and treatment subsystems (atmospheric diffuser, water tank, flare) arranged to direct the refrigerant vapor About the Authors Douglas T. Reindl, Ph.D., P.E., is a professor and director and Todd B. Jekel, Ph.D., P.E., is assistant director at the University of Wisconsin- Madison s Industrial Refrigeration Consortium in Madison, Wis. 22 ASHRAE Journal ashrae.org February 2008
Component Protection Requirement Reference Comments Pressure vessel with inside diameter, width, height or cross-section not exceeding 6 in. Refrigerating systems Shall be protected by a pressure relief device or fusible plug Protection by pressure relief device or other means to safely relieve pressure due to fire or other abnormal conditions 9.3.1 9.4.1 Liquid-containing parts of the system capable of being isolated from the system during operation or service and subjected to overpressure from hydrostatic pressure due to temperature rise Shall be fitted with a pressure relief device designed to relieve hydrostatic pressure to another part of the system 9.4.3 An addendum to Standard 15 for this section is currently pending Evaporators located downstream, or upstream within 18 in., of a heating coil Shall be fitted with a pressure relief device discharging outside the building in accordance with the requirements of 9.7.8. 9.4.4 See 9.4.4 for exceptions to this requirement Pressure vessels with an internal gross volume of 3 ft 3 or less Pressure vessels of more than 3 ft 3 but less than 10 ft 3 internal gross volume Pressure vessels in excess of 10 ft 3 internal gross volume Shall use one or more pressure relief devices or a fusible plug Shall use one or more pressure relief devices. Fusible plugs are prohibited. Shall use one or more rupture member(s) or dual pressure relief valves when discharging to atmosphere. Dual pressure relief valves shall be installed with a three-way valve to allow testing or repair. 9.7.2.1 9.7.2.2 9.7.2.3 Applies to pressure vessels containing liquid refrigerant with the capability of being isolated by stop valves from other parts the system. The pressure relief devices shall be sized in accordance with 9.7.5. Positive displacement compressors Shall be equipped with a pressure relief device of adequate size and pressure set point 9.8 Appendix F provides methods for calculating capacity requirements for compressor relief protection. Table 1: Summary requirements for equipment requiring pressure relief protection by Standard 15-2007. 4 causing the overpressure to a safe terminal location. In this article, we review requirements for the application of pressure relief protection on refrigeration systems and outline some of the basic steps to properly engineer a safety relief system. Equipment Requiring Relief Protection Section 9.4 of Standard 15-2007 prescribes equipment requiring pressure relief protection, limitations on the use of isolation valves in a relief vent piping system, materials of construction for relief valves, and maximum changes in the relief valve s set pressure over time. The equipment requiring pressure relief protection includes: Pressure vessels ( 9.4.2 and 9.7.2); Liquid containing parts of a system capable of being isolated ( 9.4.3); Evaporators located within 18 in. (457 mm) upstream or downstream of a heating coil ( 9.4.4); and Positive displacement compressors ( 9.8). Table 1 summarizes pressure relief protection requirements. Although permitted by the ASME Boiler and Pressure Vessel Code, 1 intervening stop valves between the pressure relief device and the component being protected are not permitted by Standard 15-2007 ( 9.4.6). In situations more than one relief device discharges into a common manifold or header, full area stop valves are permitted at the outlet of individual relief devices if the stop valve is car-sealed (locked) open during normal operation ( 9.4.6). In practice, these stop valves are rarely used. Section 9.4.9 of Standard 15-2007 requires that the materials of construction used in the relief valve be compatible and robust such that the valve s set pressure does not change by more than 5% over a five-year span. Additional discussion on materials of construction for vent piping is provided in the Relief Piping section of this article. Pressure Relief Device Certification Pressure relief valves selected for protecting ASME-stamped vessels and equipment must be designed and constructed in accordance with the ASME Boiler and Pressure Vessel (B&PV) Code Section VIII Division 1 (UG-131) per Standard 15-2007, Section 9.4.2. The capacities of these pressure relief devices are then certified by the National Board of Boiler and Pressure Vessel Inspectors. All pressure relief devices that meet the requirements of the National Board s Pressure Relief Device Certification Program carry the NB stamp as shown in Figure 1a. All pressure relief valves that meet the ASME B&PV Code Section VIII Division 1 (UG-131) carry the UV stamp as shown in Figure 1b. Rupture disks meeting the requirements in Section VIII Division 1 (UG-131) carry the UD stamp as February 2008 ASHRAE Journal 23
shown in Figure 1c. When rupture disks are used in series with pressure relief valves (combination relief valve), the assembly must meet the requirements prescribed in the ASME B&PV Code Section VIII Division 1 (UG-132). All of the relief devices receiving National Board approval are listed in NB-18, National Board Pressure Relief Device Certifications. Relief Capacity and Set Pressure Standard 15-2007 provides a prescriptive requirement for determining the minimum capacity for pressure relief devices protecting pressure vessels. The basis for the pressure relief device capacity determination in ASHRAE is a fire condition with the heat generated from the fire radiating on the projected area of the vessel. Equation 1 shows the formula for determining the minimum required capacity for pressure relief valves ( 9.7.5). C r = f D L (1) Advertisement formerly in this space. C r = minimum required discharge capacity of the relief device expressed in lb m /min of air f = a factor depending on refrigerant type and whether combustible materials are within 20 ft (6 m) of the pressure vessel. If combustible materials are within 20 ft (6 m) of the vessel, f is multiplied by 2.5. D = outside diameter of the vessel (ft) L = length of the vessel (ft) All of the calculations for relief capacity determination and vent pipe sizing are on an air basis. In addition, the relief devices themselves are tested and rated on an air basis. This ensures consistency between the selection of relief system components and the capacity requirement calculations. The relief device capacity factor, f, includes an overall heat transfer coefficient attributable to radiation exchange from a fire condition, 2 heat of vaporization for the refrigerant (or other volatile fluid) contained within the vessel being protected, and the conversion of the refrigerant mass flow rate to an equivalent air mass flow rate. Relief device capacity factors are refrigerant-dependent and range from 0.2 (for R-718) to 2.5 (for R- 404A, R-410A, R-410B, R-502, R-507A, and others). Values of the relief device capacity factors for different refrigerants are provided in 9.7.5 of Standard 15-2007. The installed pressure relief device for a given pressure vessel needs to have at least the capacity as determined from Equation 1. If other heat sources are present or contributing to the generation of vapor within the vessel, those need to be considered separately. For example, an intercooler with relief capacity based 24 ASHRAE Journal February 2008
a. b. c. Figure 1: (a) National Board symbol stamped on pressure relief devices designed and manufactured in accordance with National Board-recognized practices. (b) Official symbol stamp to denote compliance with the ASME B&PV Code for pressure relief valves. (c) Official symbol for stamp to denote compliance with the ASME B&PV Code for rupture disks. Advertisement formerly in this space. Figure 2: Field installation of dual reliefs on a three-way valve. solely on the heat load from an external source (Equation 1) may need additional relieving capacity to accommodate the vapor flow from one or more booster compressors operating and discharging vapor to the vessel. Although the capacity of the installed relief device needs to have at least the minimum required capacity, it is important to avoid oversizing pressure relief devices. Oversizing leads to larger required inlet piping, outlet relief vent piping, and can result in abnormal relief valve operation, i.e., valve chatter. For information on the requirements of pressure relief protection for positive displacement compressors, see Section 9.8 and the newly revised Appendix F in Standard 15-2007. Apart from applying a relief valve with at least the minimum required flow carrying capability (capacity), the relief valve s set pressure cannot be greater than the design pressure of the parts of the system being protected ( 9.7.5). Standard 15-2007 defines set pressure as the pressure at which a pressure relief device or pressure control is set to operate. The set pressure of the relief device applied to protect a vessel cannot be greater than the vessel s maximum allowable working pressure (MAWP), as stamped on the vessel s data tag, but it can be less. February 2008 ASHRAE Journal 25
Required Quantity of Relief Devices Standard 15-2007 defines the minimum number of relief devices for pressure vessels based on the size of the pressure vessel being protected ( 9.7.2). For vessels with a gross internal volume not greater than 3 ft 3 (0.085 m 3 ), one or more pressure relief devices or a fusible plug with at least the minimum required capacity determined from Equation 1 can be used. For vessels with a gross internal volume greater than 3 ft 3 (0.085 m 3 ) but less than 10 ft 3 (0.285 m 3 ), Section 9.7.2.2 requires the installation of one or more pressure relief devices (no fusible plugs permitted). For those vessels with a gross internal volume greater than 10 ft 3 (0.285 m 3 ), one or more rupture disks or dual relief valves discharging to atmosphere are required ( 9.7.2.3). The dual Line Length Limit (Equivalent ft) 200 100 pressure relief valves are required to be installed on a three-way valve in order to provide continuous relief protection for the vessel while allowing operations and maintenance personnel to inspect, test, and/or service individual relief valves. Note that each relief valve installed on a single three-way valve must have sufficient capacity to meet the requirements in 9.7.5. Figure 2 shows a field installation of a three-way valve on an oil separator the outlets of both relief valves are connected to a single branch vent line. Relief Piping Proper selection and sizing of a pressure relief device is a necessary but not sufficient condition to achieve a safe and codecompliant relief system. An appropriately sized pressure relief device must be properly connected to the component it protects (including the installation of a three-way valve, if required) and then correctly piped using an adequate size and material capable of conveying the vented refrigerant to a safe location. The first step in engineering the relief vent piping system is to specify appropriate materials of construction for the relief vent line(s). Section 9.1.5 of Standard 15-2007 states: Piping material used in the discharge line of a pressure relief device or fusible plug shall be the same as required for refrigerants. The standard provides an exception to this requirement to permit the use of Type F buttweld pipe for vent systems discharging to atmosphere. In the case of refrigerant piping for industrial refrigeration systems, small diameter carbon steel pipe (i.e., less than 2 in. [51 mm]) is often installed in Schedule 80 wall thickness. Although allowed, Schedule 80 pipe is not required for vent piping since the operating pressures are low. In evaluating the required size of piping for a vent system, it is important to differentiate whether Schedule 40 or Schedule 80 piping is assumed. When a relief valve lifts, it allows a sudden rush of vapor from the vessel (or component) being protected into a relief 10 1 0.1 0.01 Unchoked 0.001 10 15 20 25 30 35 40 45 50 Figure 3: Influence of choked flow condition on line length limit. 26 ASHRAE Journal ashrae.org February 2008 Choked Mass Flow Rate (lb m air/min) 1 in. Schedule 80 Pipe Conventional Relief Valve 15% Maxiumum Allowable Back Pressure 250 psig Set Pressure Choked Flow Constraint No Choked Flow Constraint vent system. Once established, the velocity of the gas flowing through the piping will be large, as will be the changes in gas pressure. These two factors result in significant changes in gas density as it moves from the vessel being protected to the discharge point of the relief vent system. Because the density changes are significant, the analysis for gas flow must be treated as compressible flow. Two limiting cases arise in analyzing compressible flow in closed conduits with friction. The first case is adiabatic (perfectly insulated) flow, which is reflective of flow through short piping systems. The second case is isothermal (constant temperature) flow, which is typical of long pipe runs. The actual flow conditions present in relief vent piping systems lies some between that of an adiabatic and isothermal. Standard 15-2007 uses an isothermal formulation of the governing equation for compressible flow, which is simpler and more conservative than the adiabatic formulation. 2 Standard 15-2007 provides the following line length limit for safety relief vent systems: P 1 0 2 (2) 0.2146 d 5 2 2 d ln L = (P 0 P 2 ) P 2 f C 2 r 6 f L = equivalent line length of discharge piping (ft) f = friction factor d = inside diameter of pipe (in.) C r = rated capacity as stamped on the relief device (air) or corrected for inlet losses (lbm/min) P 0 = pressure at inlet to a pipe section (psia) P 2 = pressure at outlet of a pipe section (psia) This formulation is convenient for single relief vent lines to establish a maximum equivalent length of a vent pipe permitted given a relief device capacity (mass flow rate), inside pipe diameter, piping system inlet pressure (equivalent to
Figure 4 (left): Illustration of inlet vent pipe sizing in accordance with Standard 15-2007, Section 9.7.6. Figure 5 (right): Illustration of outlet vent pipe sizing in accordance with Standard 15-2007, Section 9.7.8.4. relief device outlet pressure) and piping system outlet pressure. Standard 15-2007 limits the pressure developed at the outlet of each relief device (i.e., the back pressure) during full flow at the relief device s rated condition to no more than 15% of the set (gauge) pressure for conventional relief valves, 25% of set pressure for balanced relief valves and 50% for rupture members, fusible plugs and pilot-operated relief valves (Standard 15-2007, Appendix H). On a single vent pipe system the back pressure is equivalent to the vent pipe inlet pressure, P 0, the 15% limit for conventional relief valves is given by: P (3) 0 = (0.15 P set ) + P atm P 0 = limit on conventional pressure relief device back pressure (psia) P set = relief device set pressure (psig) P atm = local atmospheric pressure (psia) Equation 2 is general and applicable for isothermal flow. However, it does exhibit one critical flaw. Equation 2 by itself cannot recognize a choked flow condition. 3 Unlike adiabatic compressible flow, which chokes at a Mach number (Ma) equal to unity, isothermal compressible flow chokes at a Mach number equal to1/ k, 3 k is the ratio of specific heats at constant pressure and constant volume (i.e., c p /c v ). Under a choked flow condition, the ability of a given pipe to carry gas flow will be unchanged as the downstream pressure continues to drop further. Codes and standards, such as Standard 15-2007, base the entire analysis for relief vent systems on air (ideal gas) k is 1.4; consequently, choking can be expected to occur at a Mach number of Ma = 0.84. Figure 3 shows the line length limit for a 1 in. (25 mm). Schedule 80 pipe with varying mass flow rates of air for both constrained choked conditions and unconstrained choked conditions. The unconstrained choked condition is identical to applying Equation 2 without restricting flows greater than choked conditions (Ma = 0.84). In this situation, choked flow conditions are reached at a mass flow rate of 21.6 lb m /min (air). By imposing a choked flow condition, the line length limit monotonically decreases. Without imposing a choking condition, the line length limit will decrease rapidly and result in line length limits that are not physical (i.e., negative) at higher flow rates. When using Equation 2 for relief vent calculations, it is important to check the Mach number at the vent outlet and limit the velocity to not exceed Ma 1/ k or Ma 0.84 for air. Alternatively, the pressure in the vent piping that corresponds to a choked condition can be determined using the following equation. C P r choked = 0.6226 (4) d 2 P choked = absolute pressure (psia) corresponding to choked flow C r = mass flow rate of air in the piping segment (lb m /min) d = internal diameter of the piping segment (in.) The relief piping consists of the inlet piping, valves, fittings, and vent line piping. Standard 15-2007 requires that each relief device have an inlet connection size no smaller than the pipe connection size on the component being protected. Section 9.7.6 of Standard 15-2007 states: All pipe and fittings between the pressure relief valve and the parts of the system it protects shall have at least the area of the pressure relief valve inlet area. Figure 4 illustrates inlet piping configurations that violate or meet the 9.7.6 requirement. It shows inlet piping less than the relief device inlet connection size (left), inlet piping equal to and greater than (center and right) the relief device inlet connection size. Section 9.7.8.4 of Standard 15-2007 requires that the outlet pipe connected to a relief device not be smaller than the relief 28 ASHRAE Journal ashrae.org February 2008
device outlet connection size: the size of the discharge pipe from a pressure relief device or fusible plug shall not be less than the outlet size of the pressure relief device or fusible plug. Figure 5 shows relief device outlet piping configurations that both violate and meet the 9.7.8.4 requirement. It shows outlet piping less than the relief device outlet connection size (left), outlet piping equal to and greater than (center and right) the relief device outlet connection size. Where does the relief vent line need to be piped? The vent line needs to be terminated in a safe manner. Section 9.7.8 of Standard 15-2007 provides a number of refrigerant or system criteria that, if met, necessitates that the relief vent pipe terminate to the atmosphere at a location not less than 15 ft (4.57 m) above the adjoining ground level and not less than 20 ft (6.1 m) from any window, ventilation opening, or exit in any building. Section 9.7.8 requires that the discharge of the relief vent line shall be terminated in a manner that will prevent the discharged refrigerant from being sprayed directly on personnel in the vicinity and foreign material or debris from entering the discharge piping. To meet the last provision, many systems use some type of diffusing device that prevents rain and other debris from directly infiltrating the vent piping. References 1. ASME. 2004. Boiler and Pressure Vessel Code, Section VIII, Division I. 2. Fenton, D.L. and W.V. Richards. 2003. User s Manual for ANSI/ASHRAE Standard 15-2001, Safety Standard for Refrigeration Systems. Atlanta: ASHRAE. Advertisement formerly in this space. 3. White, F. 1986. Fluid Mechanics. New York: McGraw Hill Publishers. 4. Reindl, D.T. and T.B. Jekel. 2006. Engineering Safety Relief Systems. Industrial Refrigeration Consortium, University of Wisconsin- Madison, Madison, Wis. Conclusion Pressure relief protection is an important part of refrigeration system safety. This article reviewed the basic requirements for relief systems to comply with the provisions of Standard 15-2007. Here are some of the common mistakes made in relief systems: Outlet vent lines too small; Inlet connection on the relief device greater than the connection on the vessel being protected; and Sizing of vent lines not consistent with Equation 2. Avoid these and other mistakes by reviewing those provisions relating to pressure relief protection in Standard 15-2007. February 2008 ASHRAE Journal 29