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Technical Paper #6 Relief Vent Piping Per ASHRAE 15-2004 — Don Faust and Brian Peterson

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Technical Paper #6 Relief Vent Piping Per ASHRAE 15-2004 — Don Faust and Brian Peterson Technical Paper #6 Relief Vent Piping per ASHRAE 15-2004 Don Faust and Brian Peterson Gartner Refrigeration & Manufacturing, Inc. Plymouth, Minnesota Abstract Sizing ammonia relief vents, once a simple process, has become more complicated as safety codes have evolved. In recent years, code officials have been scrutinizing vent pipe sizing much more heavily. The latest release of the ASHRAE Safety Standard for Refrigeration Systems devotes considerable ink to the sizing of relief vents, and provides the user with an equation for determining pressure drop in relief piping. This paper will show how to use the ASHRAE equation to solve for the pressure drop in relief vent piping, how to select a relief valve and three-way valve, and also show some strategies to bring existing nonconforming installations into compliance with the code. 2005 IIAR Ammonia Refrigeration Conference & Exhibition, Acapulco, Mexico © IIAR 2005 213 ACKNOWLEDGEMENT The success of the technical program of the 27th Annual Meeting of the International Institute of Ammonia Refrigeration is due to the quality of the technical papers in this volume. IIAR expresses its deep appreciation to the authors, reviewers, and editors for their contributions to the ammonia refrigeration industry. Board of Directors, International Institute of Ammonia Refrigeration ABOUT THIS VOLUME IIAR Technical Papers are subjected to rigorous technical peer review. The views expressed in the papers in this volume are those of the authors, not the International Institute of Ammonia Refrigeration. They are not official positions of the Institute and are not officially endorsed. EDITORS M. Kent Anderson, President Chris Combs, Project Coordinator Gene Troy, P.E., Technical Director International Institute of Ammonia Refrigeration 1110 North Glebe Road Suite 250 Arlington, VA 22201 + 1-703-312-4200 (voice) + 1-703-312-0065 (fax) www.iiar.org 2005 Ammonia Refrigeration Conference & Exhibition Fairmont Acapulco Princess Acapulco, Mexico Relief Vent Piping per ASHRAE 15-2004 — Don Faust and Brian Peterson Introduction Back in the old days, sizing ammonia relief vents was a simple process. Designers would calculate the outlet area of each relief valve in the system and make sure that the relief header had at least that much area, and that was the end of it. No complicated equations, no computerized solutions. However, the codes have evolved, and much more scrutiny has been given to the sizing of relief vents in recent years. The latest release of ASHRAE Safety Standard for Refrigeration Systems, ASHRAE 15, devotes considerable ink to the sizing of relief vents, and provides the user with an equation for determining pressure drop in relief piping. (ASHRAE, 2004) This paper shows how to use the ASHRAE equation to solve for the pressure drop in relief vent piping, how to select a relief valve and three-way valve, and some strategies to bring existing nonconforming installations into compliance with the code. There are three steps to sizing a relief vent system: 1. Determine the required capacity of relief valve for each piece of equipment 2. Select relief valves and three-way valves and determine the actual capacity of the valves selected 3. Size the relief vent piping system In this paper, steps 2 and 3 above are assisted by a design tool, SRVQuick, which is freeware, a beta version of which is available through the IIAR website. Step 1: Determine Required Capacity Throughout this paper, three different relief valve capacities will be discussed. For clarity, they are defined as follows: •Required Capacity: the calculated amount needed to protect the device. •Rated Capacity: the capacity of the relief valve with no restrictions (i.e., the capacity of the valve as read directly from the manufacturer’s charts). Technical Paper #6 © IIAR 2005 215 2005 IIAR Ammonia Refrigeration Conference & Exhibition, Acapulco, Mexico •Adjusted Capacity: the capacity of the relief valve after accounting for inlet restrictions. Where are relief valves required? ASHRAE 15 states that an approved pressure relief device must protect the following devices: • All pressure vessels subject to the ASME Boiler and Pressure Vessel Code (vessels less than 6″ [152 mm] in diameter may use a fusible plug) • Shell and tube evaporators • Shell and tube condensers •Positive displacement compressors (if equipped with a stop valve on the discharge) • Certain evaporators, if located near a heating coil Local codes also may require relief valves on other pieces of equipment (i.e., evaporative condensers). Pressure Vessels and Heat Exchangers The required relief capacity is calculated according to the formula: C=fDL (1) where: C = minimum required discharge capacity of the relief valve, lbs. air/min [kg/s] f = constant, based upon the refrigerant. For ammonia, f = 0.5 [f = 0.041] D = diameter of the vessel, ft [m] L = length of the vessel, ft [m] 216 © IIAR 2005 Technical Paper #6 Relief Vent Piping per ASHRAE 15-2004 — Don Faust and Brian Peterson As a reference to the designer, Table 1 contains values of f for several refrigerants. Additionally, ASHRAE 15 specifies that when combustible materials are used within 20 feet [6.1 m] of a pressure vessel, then the value of f must be multiplied by 2.5. This has lead to some concern because engine rooms are often used to store refrigerants, refrigerant oils, and even flammable fuels. OSHA makes a distinction between flammable and combustible. Most refrigerants, oils and even fuels are not classified as combustible, and thus the 2.5 factor does not apply. Refer to OSHA’s regulations for a complete description and classification of combustible and flammable materials. (OSHA, 1996) It is interesting to note that if a receiver were located above a tar roof, then the 2.5 factor would apply. Wood and paper would be considered to be combustible solids as well. Example Vessel Calculation Find the required relief capacity for a 42≤ [1.07 m] diameter, ASME vessel, 12 ft [3.66 m] in length, containing ammonia refrigerant. Equation 1 applies in this situation: C = f D L C = (0.5) (3.5) (12) [C = (0.041) (1.07) (3.66)] C = 21 lbs. air/min [0.16 kg/s] Note: To convert lbs. air/min [kg/s] to standard cubic feet per minute (SCFM) [l/s], multiply by 13.1 ft3/lb [816 l/kg] (assuming dry air at 60°F [16°C]). Thus, for the above example: C = (21) (13.1) [C = (0.16) (816)] C = 275.1 SCFM [C = 130 l/s] Technical Paper #6 © IIAR 2005 217 2005 IIAR Ammonia Refrigeration Conference & Exhibition, Acapulco, Mexico Positive Displacement Compressors Section 3 of ASHRAE 15 defines a positive displacement compressor as one “in which the increase in pressure is attained by changing the internal volume of the compression chamber.” Screw compressors and reciprocating compressors both fall into this category. When such a compressor is equipped with a stop valve on the discharge line, as most industrial refrigeration compressors are, a relief device must protect it. Section 9.8 of ASHRAE 15 separately addresses compressors that meet, and those that do not meet, all of the following criteria: • Must be equipped with capacity regulation • Can regulate capacity to minimum flow at discharge pressures equal to 90% of the pressure relief setting • Must be equipped with a pressure-limiting device installed and set per Section 9.9. If the compressor cannot regulate flow per the standard, then the relief valve must be sized for the full flow of the compressor. If the compressor does meet the criteria, then the required capacity is the minimum flow of the compressor. In either case, the compressor flow is calculated based upon the following conditions: • High Stage: Flow is based upon 50°F [10°C] saturated suction at the compressor • Booster: Flow is based upon saturated suction equal to the design operating intermediate temperature. For swing compressors, which can operate either as a booster or a high stage compressor, the relief valve should be sized for the worst case. This would typically be the high stage rating. Appendix F of ASHRAE 15 shows an approved method of calculating the discharge capacity of a positive displacement compressor, and the reader is referred to that document for calculations from scratch. The authors requested data from all 218 © IIAR 2005 Technical Paper #6 Relief Vent Piping per ASHRAE 15-2004 — Don Faust and Brian Peterson compressor manufacturers showing the required relief capacity from each of their models of compressors. Data from the manufacturers who responded are shown in Tables 2a through 2d. Step 2: Selecting Relief Valves and Determining their Capacity A look-up table is provided that lists the commonly used relief valves and their ratings in lbs. air/min [kg/s]. (Table 3) Selecting a relief valve could be as simple as finding one in the table with at least as much capacity as is required. However, the designer and owner should be aware that inlet and outlet restrictions could have a significant effect on the actual capacity of the relief device. There are certain combinations of relief valves and three-way valves that can reduce the rated capacity of the relief valve by as much as 40%. Calculating Inlet Restrictions The capacity of any relief valve is reduced by the losses in the piping and valves between the relief device and the equipment it protects. ASHRAE provides a method of calculating the reduction in relief valve capacity imposed by inlet restrictions. Calculating inlet restrictions is important for two reasons: • It assures that the relief valve selected actually has the required capacity. • Code allows the designer to use the adjusted (reduced) capacity of the relief valve to size the relief vent. There are three components of inlet losses to the relief valve: • Entrance losses at the nozzle of the vessel • Pipe and fitting losses in the piping to the three-way valve • Losses in the three-way valve Technical Paper #6 © IIAR 2005 219 2005 IIAR Ammonia Refrigeration Conference & Exhibition, Acapulco, Mexico Two methods are commonly used to evaluate inlet restrictions.
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