Vol. 12 No. 2, 2012

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chan

Vol. 12 No. 2, 2012

In This Issue Sizing Scenarios for Relief 1-14

Vent Systems

Upcoming Ammonia Classes 2

Noteworthy 2

IRC Staff Director Doug Reindl 608/265-3010 or 608/262-6381 dreindl@wisc.edu Assistant Director Todd Jekel 608/265-3008 tbjekel@wisc.edu Research Staff Dan Dettmers 608/262-8221 djdettme@wisc.edu

SCENARIOS FOR SIZING RELIEF VENT SYSTEMS

In the last edition of the Cold Front (Vol. 12 No. 1), we reviewed the sizing basis for pressure relief valves in refrigeration applications and we extended our discussion to include sizing considerations for non-routine situations. In this edition of the Cold Front, we discuss the concept of establishing scenarios that form the basis for properly sizing the safety relief vent systems that are commonly used to protect industrial refrigeration systems. We want to emphasize that the relief valves and vent systems discussed within this article are vapor reliefs discharging directly to atmosphere (or a near-atmospheric pressure treatment system) and not liquid service relief valves or hydrostatic reliefs discharging internally. BACKGROUND The simplest arrangement for a relief vent system is to simply connect piping from the outlet of a single relief valve or dual relief assembly and direct that piping to a safe terminal location. The sizing of single vent lines to atmosphere is straightforward and tables given in standards such as ANSI/IIAR 2 (2010) and ANSI/ASHRAE

IRC Contact Information Mailing Address Toll-free 1-866-635-4721 1513 University Avenue Phone 608/262-8220 Suite 3184 FAX 608/262-6209 Madison, WI 53706 e-mail info@irc.wisc.edu Web Address www.irc.wisc.edu

The Electronic Newsletter of The Industrial Refrigeration Consortium

mailto:dreindl@wisc.edumailto:tbjekel@wisc.edumailto:djdettme@wisc.edumailto:info@irc.wisc.eduhttp://www.irc.wisc.edu/

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15 (2010) provide flow capacity for a range of pipe sizes. For example, Table A.3 in IIAR 2 provides the maximum flow-carrying capacity for a single relief vent line discharging to atmosphere for a range of set pressures, equivalent line lengths and nominal pipe diameters. Table 3 in ASHRAE 15 provides similar information. It is important to note that these tables are strictly limited to a single vent line going to a terminal location at atmospheric pressure. These tables are also built upon an assumption that the vent piping is schedule 40. The results in these tables cannot be used for sizing headered relief vent systems, single vent discharging to a water diffusion tank, or for smaller internal pipe diameters (e.g. schedule 80 piping). The relief vent systems used in industrial refrigeration systems are typically headered (also referred to as manifolded). A headered relief system consists of two or more relief devices discharging to a common main vent pipe. Sizing the individual branch piping as well as header mains in a manifolded relief system requires the use of the following equation provided in both IIAR 2 and ASHRAE 15:

Equation 1

where: L = equivalent line length of discharge piping (ft) f = friction factor d = inside diameter of pipe or tube (in) Cr = rated capacity as stamped on the relief device

(air) or corrected for inlet losses (lbm/min) P0= Pressure at inlet to pipe section (psia) P2= Pressure at outlet of pipe section (psia)

Both IIAR 2 and ASHRAE 15 provide tabular values for friction factors assuming fully turbulent flow. The following relates the fully turbulent friction factor as a

Noteworthy Visit the IRC website to access presentations made at the 2011 IRC Research

and Technology Forum. Mark your calendars now for the 2012 IRC Research and Technology

Forum May 2-3, 2012 at the Pyle Center in Madison, WI. Send items of note for next newsletter to Todd Jekel, tbjekel@wisc.edu.

Upcoming Ammonia Courses

Introduction to Ammonia Refrigeration

Systems October 8-10, 2012 Madison, WI Process Hazard Analysis (Emphasizing

Ammonia Refrigeration Systems) October 19-21, 2012 Madison, WI Intermediate Ammonia Refrigeration

Systems December 5-7, 2012 Madison, WI Process Safety Management Audits for

Compliance and Continuous Safety Improvement

January 16-18, 2013 Madison, WI Introduction to Ammonia Refrigeration

Systems March 6-8, 2013 Madison, WI Ammonia Refrigeration System Safety April 17-19, 2013 Madison, WI Achieving Energy Cost Savings for Ammonia Refrigeration Systems May 22-24, 2013 Madison, WI Register to receive updates on future courses, http://epd.engr.wisc.edu/signup

Noteworthy Mark your calendars now for the 2013 IRC Research and Technology

Forum May 2-3, 2012 at the Pyle Center in Madison, WI. Send items of note for next newsletter to Todd Jekel, tbjekel@wisc.edu.

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function of pipe roughness and inside diameter:

= . [ ( ) . ] Equation 2 where: = pipe inside surface roughness (ft) note a surface roughness of 0.00015 ft is assumed for piping d = inside diameter of pipe (in)

As discussed by Reindl and Jekel (2006), the above formulation assumes subsonic isothermal compressible flow through the vent pipe system. Although not in a convenient form, Equation 1 can be used to analyze flow through both a single and headered relief system. For a single vent system, P0, represents the inlet pressure to the vent system (which is identical to the outlet or back-pressure on the safety relief valve) while P2 is the pressure at the vent pipe outlet. For an existing or proposed vent system, the known or assumed variables include the equivalent length of pipe, friction factor, inside pipe diameter, rated relief device flow, and the pressure at the outlet of the vent pipe. The single unknown is the inlet pressure. Unfortunately, Equation 1 cannot be rearranged to explicitly solve for vent pipe inlet pressure, P0; consequently, the inlet pressure to the vent pipe must be found by iteratively solving the Equation 1. For a headered relief vent system, Equation 1 can be used in a piecemeal fashion by analyzing segments of a vent pipe system beginning at a point of known pressure (vent system outlet) and systematically applying the equation to find the inlet pressure to that segment given the pipe diameter, equivalent length, mass flow rate, and friction factor. This process is repeated until the pressures throughout the entire network of relief piping have been determined. One of the critical tests for code/standard compliance of the vent piping system is that the back-pressure on each relief device expected to be operating is less than the maximum allowable back-pressure for that relief device. Besides physical data on piping as well as flow rates obtained from the certified capacity of each relief device, the sizing of a headered relief system requires consideration as to which relief valves are expected to be actuating simultaneously. In this article, we introduce some fundamental concepts and considerations for determining credible relief scenarios that then can be used as a basis for sizing the vent side of a headered relief system. First, lets review basic requirements from codes and standards and then we will consider various relief scenarios that should be considered in the process of establishing a design basis for a relief vent system. Codes & Standards The PSM Standard [29 CFR 1910.1199(d)] requires that employers document the design and design basis for safety relief systems:

(d)(3)(i) Information pertaining to the equipment in the process shall include: (d)(3)(i)(D) Relief system design and design basis;

The PSM Standard does not prescriptively identify the information that should be included in the design and design basis documentation; however, in its VPP Supplement, OSHA recommended that the following items be included as part of the design and design basis documentation (OSHA 2008):

1. Identification/description of each relief device 2. A listing of all relief-protected equipment 3. Equipment design pressure 4. Relief device set pressure

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5. Listing of all sources of overpressure considered 6. Identification of the worst case overpressure scenario or relief design 7. State of material being relieved (i.e. liquid, vapor, liquid-vapor, liquid-vapor-solid, along with an

identification of the material which was the basis for the relief device selection) 8. Physical properties of the relieved materials, vapor rate, molecular weight, maximum relieving pressure,

heat of vaporization, specific gravity and viscosity 9. Design calculations

Items 1-4 above are obvious and unambiguous. Items 5, 6, 7, and 8 are often embedded in the assumptions that form the foundation for the prescriptive methods used to size relief devices for industrial refrigeration systems using either IIAR 2 or ASHRAE 15. Specifically, ASHRAE 15 and IIAR 2 incorporate refrigerant-specific properties and a prescribed external heat load (i.e. fire) on a protected component such as a vessel as described by Reindl & Jekel (2006) and the Cold Front (IRC 2011). What remains unclear in the process of engineering many relief systems is identification of credible scenarios that involve simultaneous relief valve operation so that an appropriately sized relief vent system design can be ensured. The goal of this issue of the Cold Front is to provide further guidance on scenario consideration that can credibly support a proper design basis for a headered relief vent system. First, lets review the codes and standards that form the RAGAGEP (Recognized As Generally Accepted Good Engineering Practice) for relief protection, as applied to industrial refrigeration systems. ASHRAE 15 In 2000, ASHRAE modified its method for determining the allowable length for the outlet piping connected to a relief device as summarized previously by the IRC (2001). Sections 9.7.8.4 and 9.7.8.5 of ASHRAE 15 (2010) identify requirements for relief vent lines connecting two or more relief devices.

9.7.8.4 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. Where outlets of two or more relief devices or fusible plugs are connected to a common line or header, the effect of back pressure that will be developed when more than one relief device or fusible plug operates shall be considered. The sizing of the common discharge header downstream from each of the two or more relief devices or fusible plugs that are expected to operate simultaneously shall be based on the sum of their outlet areas with due allowance for the pressure drop in all downstream sections.

9.7.8.5 The maximum length of the discharge piping installed on the outlets of pressure-relief devices and fusible plugs

discharging to the atmosphere shall be determined by the method in Normative Appendix E. See Table 3 for the flow capacity of various equivalent lengths of discharge piping for conventional relief valves.

The highlighted provision in 9.7.8.4 above captures the essence of criteria expected for engineering a headered or manifolded relief vent system. In reality, the vent side of a headered relief system must be sized sufficiently to maintain the outlet pressure at each relief device expected to simultaneously operate at a pressure less than the maximum allowable back pressure. Some relief system designers mistakenly believe that sizing a vent header based on the sum of the outlet areas of those connected pressure relief devices is sufficient to guarantee the vent system is able to provide adequate capacity without exceeding the maximum allowable back pressure of any connected relief device. Unfortunately, that is not always the case! In other words, there are cases where the sum of the areas criteria was met but the back pressure at the outlet of one or more relief valves was greater than the maximum allowable. The failure of the sum of the outlet areas guidance is increasingly probable when a given header has a mix of relief valves with different set pressures (e.g. 150 psig and 250 psig set pressures). The sum of the outlet areas is not a reliable criterion to ensure that the back pressure on relief valves will be equal to less than the maximum allowable.

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To arrive at a truly functional and code-compliant relief vent system, the designer can systematically apply the line length equation (Equation 1) to segments of the vent system to demonstrate that the pressure at the outlet of each relief device is less than the maximum allowable back pressure. Normative Appendix E of ASHRAE 15 (2010) defines the following limits on back pressure for pressure relief valves:

For the allowed back pressure (P0), use the percent of set pressure specified by the manufacturer, or, when the allowed back pressure is not specified, use the following values, where P is the set pressure:

for conventional relief valves, 15% of set pressure, P0 = (0.15 P) + atmospheric pressure for balanced relief valves, 25% of set pressure, P0 = (0.25 P) + atmospheric pressure for rupture members, fusible plugs, and pilot operated relief valves, 50% of set pressure, P0 = (0.50 P) +

atmospheric pressure

Unless a relief valve manufacturer provides specific data on the maximum allowable back pressure for their own relief device(s), both IIAR 2 and Standard 15 provide limits the valves back pressure based on its type. Conventional relief valves are the most widely used type of relief valves used in industrial refrigeration systems and the limit on back pressure for these types of valves is 15% of the valves set pressure. IIAR 2 In 2008, IIAR published an updated version of its design standard IIAR 2 with subsequent changes issued in 2010. Section 11.3.4 of IIAR 2 establishes requirements for headered relief systems for industrial ammonia refrigeration applications.

11.3.4 The size of the discharge pipe from a pressure relief device shall not be less than the outlet size of the pressure relief device. The size and maximum equivalent length of common discharge piping downstream from each of two or more relief devices shall be governed by the sum of the discharge capacities of all the relief devices that are expected to discharge simultaneously, at the lowest pressure setting of any relief device that is discharging into the piping, with due allowance for the pressure drop in all downstream sections.

The above IIAR 2 requirements mirror those outlined in ASHRAE 15-2010 9.7.8.4. In addition, normative Appendix A of IIAR 2 (2010) has established limits on the back pressure limits for pressure relief valves identical to those identified in normative Appendix E of ASHRAE 15. The key requirement in IIAR 2 and ASHRAE 15 related to sizing the vent piping for pressure relief devices is to identify scenarios where one or more of the installed pressure relief devices are expected to simultaneously actuate. Once those scenarios are identified, Equation 1 is used to systematically analyze the vent system to determine whether or not the back pressure at the outlet of each relief device is greater than the maximum allowable back pressure. BASIS OF DESIGN FOR VENT PIPING Industrial refrigeration systems are, principally, custom-engineered from multiple components and field-erected. A typical industrial refrigeration system will have a multiplicity of vessels and other equipment that may require the type of overpressure protection afforded by the installation of pressure relief valves. In order to enhance system safety, the outlets of each relief valves are often interconnected in a headered vent piping network to allow discharge vapor from the system to be directed to a safe location in the event that one or more relief valve actuates. Although there is no limit to the number of relief devices that can be connected to a single header, the complexity of the vent piping design process increases as the number of connected relief vent branch lines increases. Part of the complexity involves considering situations or scenarios where two or more relief valves can

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be expected to simultaneously lift. For the purposes of this article, a relief vent line is satisfactorily sized if the back pressure at the outlet of each relief device coincidently operating is less than the maximum allowable back pressure for that valve. We will functionally characterize relief scenarios into the following three categories: maintenance, fire condition, and operational upsets as illustrated in Figure 1. If installation of safety relief valves for your situation involves other categories, they would should considered accordingly. Lets look at each of these categories and identify a range of different relief scenarios that can arise in the design process of engineering a relief vent system.

Figure 1: Categories of overpressure situations for industrial refrigeration systems relief scenarios.

Maintenance The simplest case that must be considered in engineering the relief vent system for an industrial refrigeration system involves the actuation of pressure relief valves protecting each individual piece of equipment to which they are attached. This situation has the potential to occur when the individual pieces of equipment are isolated from the system for maintenance purposes coincident with an external or internal heat load creating overpressure. For each individual piece of equipment, the maintenance relief scenario needs to be considered in the context of the basis for sizing the relief for protecting the equipment. For vessels, the most common sizing basis is an external heat load. For positive displacement compressors, the relief valve sizing basis is often the minimum regulated flow of the machine. For heat exchangers, the sizing basis would be the larger of an external or internal heat load. Details on relief sizing bases were discussed in the last edition of the Cold Front (Vol. 12 No. 1) and additional information can be found by referring to Reindl and Jekel (2009 and 2006). Considering separately the relief valve lifting on each individual piece of equipment establishes a minimum branch line size. If one or more of the branch segments creates excessive outlet or back-pressure on operating relief devices, the diameter of the offending branch pipe(s) must be increased to reduce the vent line pressure drop which decreases the relief valve outlet or back pressure. Keep in mind that if a relief valve cannot pass the maximum allowable back-pressure test when operating by itself, it will not pass when simultaneously operating with other relief devices on a header. In some cases, it may be possible to maintain the size of an existing vent pipe branch if a lower capacity relief valve can be installed. Keep in mind that any proposed relief valve

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alternative must have sufficient flow capacity to protect the vessel or component. Fire Condition As noted above, the most common basis for sizing relief valves on pressure vessels is a radiative heat load from a fire causing overpressure due to liquid refrigerant evaporating (or by the specific volume increase in vapor-only vessels ). The relief device sizing methods included in ASHRAE 15 and IIAR 2 do not take any credit for heat load reduction on vessels due to the presence of insulation or metallic jacket materials both of which could theoretically reduce the effective heat load on the vessel. As a result, the fire condition scenarios will consider the relief valves protecting one or more vessels or other equipment operating at their rated flow coincidentally. Lets explore when and where this may occur. Machinery Room A fire condition within a machinery room can create overpressure in a number of situations. Depending on how the machinery room is configured, the fire condition could be limited to a geographic area of the space as shown in Figure 2. If the machinery room has some form of a fire protection system, this may be a reasonable assumption. In this case, the machinery room fire scenario may only consider the simultaneous relief of the recirculator and Booster 1. A more conservative approach would be to assume that a fire in the machinery room affects all refrigerant containing vessels/equipment within the room.

Figure 2: Machinery room fire scenario with limited geographical impact due to the presence of a fire

protection system.

Another question that often arises in machinery room scenarios involving a fire condition is whether or not one or more positive displacement compressors co-located in the room should be considered as relieving coincidentally with vessels. The answer to this question is probably not as it would be highly unlikely for compressors to continue their operation during a fire condition. In addition, compressors are required to be fitted with engineering controls in the form of a high-pressure cutout that shuts down the machine at discharge pressures no higher than 90% of the pressure relief valves set pressure. This engineering control reduces the likelihood of compressors continuing to operate. Two types of equipment that we have not discussed yet in the context of a fire scenario in a machinery room are oil separators and the refrigerant-side of thermosiphon oil cooling heat exchangers. Frequently, the pressure relief protection affixed to an oil separator is sized to accommodate the positive displacement compressor to which it is connected1. In this case, the capacity of the relief valve is far in excess of the minimum required to protect the vessel due to an external heat load based on the traditional sizing approach for a vessel (C=fDL).

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That being the case, it is natural to ask: Is it reasonable to include the simultaneous relief of the PRVs on the oil separators along with other vessels in the machinery room during a fire scenario? The answer to this question is no if the separator is not isolated from the system for maintenance but connected to high-side with piping and associated downstream equipment such as evaporative condensers, HPR, thermosiphon pilot receiver, etc. which are located outside of the machinery room. In this situation, heat gained to the oil separator will cause the refrigerant vapor to expand and pressure to build. When the pressure of refrigerant vapor in the oil separator exceeds the prevailing high-side pressure, the compressors discharge check valve will naturally relieve vapor from the separator to high side of the system where the higher pressure gas will have a greater volume to occupy or it may be condensed as it gives up heat to the ambient environment. In the case where the plant anticipates having one or more of its compressors routinely valved out of the system, it would be prudent to include the simultaneous relief of the pressure relief valve connected to that compressors oil separator for fire condition cases. In a similar fashion to that described above for the oil separators, the fire condition would not warrant inclusion of the pressure relief valves protecting the refrigerant-side of the oil cooling heat exchangers in a simultaneous relief scenario with other vessels in the machinery room. Individually, the oil coolers should be evaluated to ensure they have sufficiently sized vent piping but they would not be expected to relieve simultaneously with pressure vessels because they would normally be open to the high-side of the system. If more than one oil cooling heat exchanger can be expected to routinely be left valved out of the system, those oil coolers should be included in the fire scenario with the other vessels simultaneously relieving. Plant Production Areas Sometimes, a single relief vent header for a refrigeration system may include protected refrigerant-containing equipment that is located outside of the machinery room. For example, a single vent header could include all equipment within the machinery room as well as refrigerant-containing vessels or heat exchangers located in an adjacent production area as shown below in Figure 3 or outside of the machinery room. From a relief scenario standpoint, is it reasonable to assume that all equipment connected to the vent header is simultaneously relieving? The answer is probably not. Architecture practices for code-compliance will have a fire-rated wall constructed to separate the machinery room from the plant. In this case although not impossible, it would be highly unlikely for the same fire to spread from the machinery room to the plant and involve over-pressuring equipment outside of the machinery room space.

Figure 3: Production area fire scenario effecting process equipment with PRVs but not machinery room

equipment with PRVs where both share a common vent pipe system.

1 The most notable exception would be the case where there is an intervening stop valve between the compressor and the oil separator. In this case, separate relief valves would need to be installed: one on the upstream side of the stop valve to protect the compressor and one on the downstream side to protect the oil separator.

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For a fire scenario where the protected equipment resides in an area separated from the machinery room but sharing a comment vent pipe system, we recommend evaluating a scenario where the protected equipment located within the separate area (e.g. the plant) would be relieving coincidentally and independently of the equipment located within the machinery room. Operations-Related Scenario Probably the most difficult of categories for establishing all possible overpressure scenarios is related to facility operations. The challenge, in part, is due to the diversity of refrigeration-equipment within a plant that may be connected to a headered vent pipe system but the difficulty is also attributable to the designers clairvoyance in identifying those situations where one or more relief valves protecting pieces of equipment in the refrigeration system may simultaneously lift. Lets look at a couple of examples. INTERNAL HEAT LOAD SCENARIOS A number of pieces of protected equipment can have their relief valve sizing basis established by internal heat loads that create overpressure. For example, the largest heat load on a refrigerant-to-process fluid heat exchanger (i.e. a chiller) could be due to warm or hot clean-in-place (CIP) fluid being passed through the tubes of the chiller. If the shell of the chiller contains refrigerant and the refrigerant-side of the chiller is isolated from the system, the evaporation of refrigerant due to the internal heat load attributable to the warm CIP fluid or product can create overpressure when the source temperature of secondary fluid is greater than the saturation temperature for the components maximum allowable working pressure (MAWP). Table 1 lists the maximum process fluid temperatures in order to prevent internal heat gains from causing equipment overpressure for a range of maximum allowable working pressures (MAWPs). For example if a plate-and-frame heat exchanger equipped with pressure relief protection has a MAWP of 250 psig, supplying secondary fluid to the heat exchanger in excess of 114.6F (45.9C) would exceed the set pressure of a relief device assuming the set pressure was equal to the MAWP.

Table 1: Maximum secondary fluid temperatures to prevent ovepressuring equipment.

MAWP

(psig)

Tmax 2

(F) [C]

150 84.3 [29.1] 250 114.6 [45.9] 300 126.5 [52.5] 400 146.7 [63.7]

From a relief scenario standpoint, it will often be the case that protected equipment will relieve individually and not simultaneously. If there are situations where the refrigerant-side of multiple pieces of equipment are valved-out from the system and subjected to internal heat loads that lead to over-pressuring, a scenario where the PRVs on those pieces of equipment relieving simultaneously should be analyzed. UPSET CONDITIONS The term upset condition is used to broadly describe system operating excursions outside of a normal range. The underlying factors that contribute to upset conditions are varied but could include: abnormal weather, production starts or stops, loss and/or restoration of commercial power, temporary operations, equipment mechanical failure, controls (sensors, actuators, control logic) failure, human error, or any other situation that can drive the system outside of its normal operating range of pressures, temperatures, or flows. It is important to think broadly about the refrigeration system layout, equipment, and operations to identify

2 Values assume ammonia as the refrigerant and temperatures are based on MAWP as well as an allowed 10% overpressure.

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those factors that have the potential to cause upset conditions that lead to overpressure. The process hazard analysis (PHA) process is a useful tool to uncover possible upset conditions as well as their likelihood and consequence. Applying the PHA process directly to operating and maintenance procedures (e.g. What if a procedural step is skipped?) can also help define other possible scenarios that should be considered in a relief systems analysis. Any upset condition scenarios identified either in the PHA processes or during operations (as near misses) should feed into the analysis of safety relief systems to ensure the functional performance of relief devices and vent systems. OPERATING MISHAPS Operating mishaps can lead to upset conditions that result in overpressuring equipment. Consider a system design that uses a dedicated evaporative condenser for heat rejection of a thermosiphon oil cooling system as shown in Figure 4 and described in more detail in a past edition of the Cold Front (Vol. 3 No. 1). In this case, the oil cooling circuit has a dedicated refrigerant charge not shared or cross-connected with the main refrigeration system. Should an operator inadvertently shutdown the TSOC circuit evaporative condenser fan (or pump) while compressor oil coolers are rejecting heat to the circuit, the pressure in the entire oil cooling circuit will rise rapidly leading to an overpressure condition. Although the oil temperature in each operating compressor will also rise, the refrigerant pressure in the fixed oil cooling circuit will rise at a faster rate. In this design, an overpressure scenario included with the vent piping analysis would have all of the connected thermosiphon oil cooler pressure relief valves (e.g. PRVs 1-3 shown above) lifting simultaneously and flowing at their stamped capacity. This is just one example of an operator mishap. For each installation, it is important to identify situations that involve advertent or inadvertent actions operations staff that can lead to overpressuring equipment. Analyze all overpressure scenarios identified accordingly.

Figure 4: System-segregated closed circuit heat rejection system for thermosiphon oil cooling.

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Note that in this rather unique case, we highly recommend the addition of one or more separate pressure transducer(s) be installed on the refrigerant-side of the oil cooling heat rejection circuit and wired to shut down the compressors in the event the circuit pressure rises above a defined threshold level. We have seen more than one case where the refrigerant-side of that dedicated thermosiphon oil cooling circuit was overpressured without actuating a related safety, high oil temperature, to shutdown compressors. To be consistent with the engineered safeties on the compressors themselves; the cutout threshold should not be greater than 90% of the MAWP of the equipment within the oil cooling circuit. If this safety is added and maintained, the scenario outlined above may be relaxed to an external fire scenario where the total heat absorbed by the oil coolers and the pilot receiver would need to be relieved. ALL Relief Valves Operating This simultaneous relief scenario is quite simple it assumes that every relief valve connected to a common vent header is coincidently relieving at each valves rated capacity. Although this scenario is simple, it is not necessarily credible or realistic. If a scenario where all relief valves simultaneously lifting is not realistic, then dont include this in the list of scenarios for relief systems analysis! Table 2 provides a summary of various pressure relief scenarios that incorporate maintenance, fire, and operational upsets. The summary includes examples of protected equipment, relief device set pressures, equipment locations and descriptive scenarios. It is intended to capture the more common relief scenarios but each installation and facility needs to be considered in its own context. REQUIRED CHANGES In the course of analyzing all credible relief scenarios, it is essential to identify any case where the calculated back pressure at the outlet of a relief valve is greater than the maximum allowable. When this occurs, changes are required to bring the vent system into compliance. The most logical change is to increase the diameter of the vent pipe branch, sub-main, or main; however, there may also be an opportunity to reduce the length of pipe as well or right-size the relief device. Because there are multiple paths that can be taken to achieve a compliant relief system, designers have some degree flexibility. Here are a few approaches to consider as you try to bring a relief system into compliance:

1. Branch piping: Increase the size of the branch line (i.e. the piping connected to the outlet of the relief device experiencing excessive back pressure).

2. Right-size the relief device: If the installed relief device capacity is greater than the minimum required to protect a given vessel, compressor, or other piece of equipment, there may be an opportunity to select a valve with lower capacity. When the capacity of the installed relief device is reduced, the ability for a given vent pipe size to carry flow increases. The increase in flow carrying capability of the vent line translates into reduced relief device back pressure. With this approach, the designer must ensure that the proposed relief device capacity is equal to or greater than the minimum required capacity for the protected component. Remember to check the capacity of the proposed replacement relief device when accounting for relief device inlet pressure losses. With inlet losses, the corrected relief device capacity must be equal to or greater than the minimum capacity required to protect the component. Although ASHRAE 15 and IIAR 2 allow the vent pipe on a relief valve to be sized based on its diminished capacity due to inlet losses, we highly recommend that the vent system be sized based on the stamped capacity on the relief valve. This approach is somewhat conservative but it eliminates future problems that may arise if modifications are made that result in reduced inlet pressure losses. Such modifications could include the removal of rupture disks on a combination relief arrangement or replacement of a three-way manifold with a valve that has lower pressure drop.

3. Sub-main and main piping: Increase the size of any sub-main or main headers. This strategy is often not entirely helpful because mains tend to be the largest pipe in a vent system with the least pressure drop. Nonetheless, there can be circumstances where bottlenecks occur in main parts of a vent system

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Table 2: Examples of common scenarios for relief vent piping analysis.

Case Protected Equipment Set pressure Relieving Location Scenario

1

Low-side vessels 150 or 250 psig Yes

Machinery room

External heat addition due to fire

High-side vessels (CPR, HPR, or TS pilot located in the machinery room) 250 psig (or higher) Yes

Compressors All No

2

Low pressure vessels 150 or 250 psig Yes

Machinery room Medium pressure vessels 250 psig (or higher) Yes High-side vessels (CPR, HPR, or TS pilot located in the machinery room) 250 psig (or higher) Yes

Compressors All No 3 Vessels & other equipment All Yes Production areas

4 Positive displacement booster compressors All Yes3

Machinery room

Assumes compressor is started with discharge stop valve closed, refrigerant-side of oil cooler valved out, and compressor discharge pressure cutout failing

5 Positive displacement high-stage compressors All Yes3

6 Positive displacement swing compressors All Yes3

7

TS oil cooling heat exchangers All

Yes (each individual thermosiphon oil cooler)

Machinery room

Internal heat addition due to compressor starting with refrigerant-side of the TS oil cooler valved-out

8 TS Pilot receiver is relieving

External heat addition due to a fire on the pilot plus the heat addition (C=fDL) on the oil coolers

9 Vessels (TS Pilot, HPR, other that are located outdoors) All Yes (each vessel individually) Outdoors External heat addition due to fire

3 Each positive displacement compressor relieves individually. For thermosiphon oil-cooled compressors, this scenario would also include the simultaneous relief of the refrigerant-side of the oil coolers when the compressor standing maintenance procedures require the refrigerant-side of the oil cooling heat exchanger to be valved out along with the compressor during maintenance.

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upsize them if their pressure drop is high. Undersized mains result from designs that only considers individual relief valves lifting (i.e. no simultaneous relief scenarios considered).

4. Vent main outlet location: The location of the relief vent outlet to atmosphere can play an important role in the back pressure developed as a result of relief device actuation. To the greatest extent possible, it is desirable to locate the outlet of a relief vent system nearest to lower set pressure reliefs (on a system with a mix of set pressures). It is also desirable to locate the vent outlet near the largest capacity relief valves on the vent header in order to reduce its effect on the installed capacity of the connected relief devices. This is often a strategy that is most applicable to vent systems that are under design for new installations. It is a less helpful strategy if one is attempting to rehabilitate an existing relief vent system.

5. Watch set your pressures: It is possible to engineer a compliant relief vent system with connected relief valves of varying set pressures. With that said, lower set pressure relief valves have lower maximum allowable back pressures. This means that less pressure drop can be tolerated in the portion of a vent pipe system serving the lower set pressure relief valves. In some cases, it may be desirable (or necessary) to split up a vent pipe system and have separate vent lines serving the lower set pressure relief valves.

Of course these are not the only strategies that can be applied to arrive at a compliant relief system. Be creative but not crazy! CONCLUSIONS Achieving a compliant pressure relief system requires design professionals to identify and evaluate a range of situations that can lead to equipment overpressure. These overpressure conditions are an important part of ensuring that the relief devices themselves have sufficient capacity but that the vent pipe system will also allow one or more of the relief valves to operate coincidently without adversely effecting valve operation. The scenarios assumed for the purpose of evaluating a vent pipe system need to be internally consistent with the assumptions made for sizing the relief devices themselves. In this edition of the Cold Front, we focused on identifying overpressure situations that are rooted in fire conditions, maintenance, and operational upsets. We discussed a range of scenarios where multiple relief valves may be actuated. These scenarios would then be included in a vent system analysis to ensure that no operating relief valve has a back pressure greater than its maximum allowable back pressure. REFERENCES ANSI/ASHRAE 15, Safety Standard for Refrigeration Systems, ASHRAE, Atlanta, GA (2010).

ASME B31.5, Refrigeration Piping and Heat Transfer Components, American Society of Mechanical Engineers, (2010).

ASME Section VIII Div. 1, Boiler and Pressure Vessel Code Rules for Construction of Pressure Vessels, American Society of Mechanical Engineers, (2010).

ANSI/IIAR 2, Equipment, Design, and Installation of Closed-Circuit Ammonia Mechanical Refrigerating Systems, 2008 edition including Addendum A published in 2010, International Institute of Ammonia Refrigeration, (2010).

IRC, Engineering Relief Systems Relief Valve Sizing Considerations, Industrial Refrigeration Consortium Cold Front newsletter, Vol. 12, No. 1 (2012).

https://www.irc.wisc.edu/?/file&id=355

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IRC, Closed Refrigerant Circuit for Screw Compressor Oil Cooling, Industrial Refrigeration Consortium Cold Front newsletter, Vol. 3, No. 1, (2003).

IRC, Code Changes for Relief Vent Lines, Industrial Refrigeration Consortium Cold Front newsletter, Vol. 3, No. 1 (2001).

IRC, Relief Vent Lines TechNote - An Overview of Standard Changes, Industrial Refrigeration Consortium technical note, (2001).

Reindl, D. T. and Jekel, T. B, Pressure Relief Device Capacity Determination, ASHRAE Transactions, Vol. 115, No. 1, pp. 603-612, (2009).

Reindl, D. T. and Jekel, T. B., Engineering Safety Relief Systems Guidebook, Industrial Refrigeration Consortium, University of Wisconsin-Madison, (2006).

OSHA, VPP Application Supplement for Sites Subject to the Process Safety Management (PSM) Standard, Occupational Safety and Health Administration, http://www.osha.gov/dcsp/vpp/psm_app_supplement_final.html (2008).

https://www.irc.wisc.edu/file.php?id=89https://www.irc.wisc.edu/?/file&id=23https://www.irc.wisc.edu/?/file&id=59http://www.osha.gov/dcsp/vpp/psm_app_supplement_final.html

BackgroundIn This IssueIRC Contact Information Mailing Address Toll-free 1-866-635-4721 1513 University AvenuePhone 608/262-8220 Suite 3184FAX 608/262-6209 Madison, WI 53706e-mail info@irc.wisc.edu Web Address www.irc.wisc.edu

IRC StaffDirectorDoug Reindl 608/265-3010 or 608/262-6381Assistant DirectorTodd Jekel 608/265-3008Research StaffDan Dettmers 608/262-8221

NoteworthyNoteworthyASHRAE 15IIAR 2Basis of Design for Vent PipingMaintenanceFire ConditionMachinery Room

Plant Production AreasOperations-Related ScenarioInternal Heat Load ScenariosUpset ConditionsOperating MishapsALL Relief Valves OperatingRequired ChangesConclusionsReferences