© irc 2011 all rights reserved

© IRC 2011 All Rights Reserved

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The enthalpy of saturated vapor and the enthalpy of saturated liquid is evaluated at the fully accumulated relief device set pressure (P=Pset * 1.1 + 14.7).

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Set Pressure (psig)

hfg

(Btu/lbm)

150 488

175 479

200 470

225 461

250 453

275 446

300 438

325 431

350 424

375 417

400 410


where:

k is the ratio of specific heats for refrigerant vapor evaluated at the fully accumulated set pressure

Cr is a constant for the refrigerant vapor

Ca is a constant for air (356 – ASHRAE 15-2010 Appendix D)

rw is the refrigerant-to-standard-air-mass-flow conversion factor

Ma is the molar mass of air = 28.97

Mr is the molar mass of refrigerant

Ta is the absolute temperature of the air (ASHRAE 15-2010 Appendix D Ta = 520°R)

Tr is the absolute temperature of the refrigerant at the relieving condition (ASHRAE 15-2010 Appendix D assumes Tr = 520°R)

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Where:

GPM = minimum required volumetric flow rate of liquid (refrigerant) to accommodate the volume

expansion [gal/min]

H = heat addition [Btu/hr]

B = refrigerant volumetric coefficient of expansion [1/°F]

cp = liquid refrigerant specific heat [Btu/lbm-F]

S = refrigerant specific gravity (density of the refrigerant relative to water at 62.4 lbm/ft3)


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where:

Crequired = minimum required relief device discharge capacity rate (lbair/min)

f = for ammonia – f = 0.5 or f=1.25 if combustible materials are within 20 ft of protected device)

D = vessel diameter (ft)

L = vessel length (ft)


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Per ASHRAE 15-2010:

9.8 Positive Displacement Compressor Protection. Every positive displacement compressor with a stop valve in the discharge connection shall be equipped with a pressure-relief device of adequate size and pressure setting, as specified by the compressor manufacturer, to prevent rupture of the compressor or to prevent the pressure from increasing to more than 10% above the maximum allowable working pressure of any other component located in the discharge line between the compressor and the stop valve or in accordance with Section 9.7.5, whichever is larger. The pressure-relief device shall discharge into the low-pressure side of the system or in accordance with Section 9.7.8.

Exception: Hermetic refrigerant motor-compressors that are listed and have a displacement less than or equal to 50 ft3/min (1.42 m3/min).

The relief device(s) shall be sized based on compressor flow at the following conditions:

1. Compressors in Single-Stage Systems and High-Stage Compressors of Other Systems: Flow shall be calculated based on 50°F (10°C) saturated suction temperature at the compressor suction.

2. Low-Stage or Booster Compressors in Compound Systems: For those compressors that are capable of running only when discharging to the suction of a


high-stage compressor, flow shall be calculated based on the saturated suction temperature equal to the design operating intermediate temperature.

3. Low-Stage Compressors in Cascade Systems: For those compressors that are located in the lower-temperature stage(s) of cascade systems, flow shall be calculated based on the suction pressure being equal to the pressure setpoint of the pressure-relieving devices that protect the lowside of the stage against overpressure.

Exception for items 1, 2, and 3: The discharge capacity of the relief device is allowed to be the minimum regulated flow rate of the compressor when the following conditions are met:

a. the compressor is equipped with capacity regulation,

b. capacity regulation actuates to minimum flow at 90% of the pressure-relief device setting, and

c. a pressure-limiting device is installed and set in accordance with the requirements of Section 9.9.

Informative Appendix D describes one acceptable method of calculating the discharge capacity of positive displacement compressor relief devices.

9.4.2 Pressure vessels shall be protected in accordance with Section 9.7. Pressure-relief devices are acceptable if they either bear a nameplate or are directly marked with a “UV” or “VR” symbol signifying compliance with Section VIII of the ASME Boiler and Pressure Vessel Code.

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An example is done in ASHRAE 15-2010 Appendix D


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§9.9 Pressure-Limiting Devices

9.9.1 When Required. Pressure-limiting devices shall be provided on all systems operating above atmospheric pressure, except that a pressure-limiting device is not required on any factory-sealed system containing less than 22 lb (10 kg) of Group A1 refrigerant that has been listed by an approved, nationally recognized testing laboratory and is so identified.

9.9.2 Setting. When required by 9.9.1, the maximum setting to which a pressure-limiting device is capable of being readily set by use of the adjusting means provided shall not exceed the design pressure of the high-side of a system that is not protected by a pressure-relief device or 90% of the setting of the pressure-relief device installed on the high-side of a system. The pressure-limiting device shall stop the action of the pressure-imposing element at a pressure no higher than this maximum setting.

Exception: On systems using non-positive displacement compressors, the maximum setting of the pressure limiting device is not required to be less than the design pressure of the high-side of the system provided the pressure-relief device is (1) located in the low-side and (2) subject to low-side pressure and (3) there is a permanent (un-valved) relief path between the high-side and the low-side of the system.

9.9.3 Connection. Pressure-limiting devices shall be connected between the pressure-imposing element and any stop valve on the discharge side. There shall be no intervening stop valves in the line leading to the pressure-limiting device.


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Note that the example in ASHRAE 15-2010 Appendix D uses a refrigerant temperature of 50oF (510oR) for the analysis. Another possibility that may be more reflective of the actual conditions would be to establish the mass flow conversion based on a saturation temperature at the relieving pressure of 275 psig (250 psig set pressure plus 10% overpressure), which for ammonia at sea level atmospheric pressure of 14.7 psia is approximately 20oF. This change makes the multiplier 1.324 instead of 1.284.


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Cr,plate,HX = minimum required discharge capacity of relief devices (lbm/min of air)

f = a factor that depends on the type of refrigerant and whether or not there are combustible materials within 20 ft of the pressure vessel. For ammonia, f=0.5 when no combustible materials are within 20 ft of the vessel and f=1.25 when combustible materials are located within 20 ft of the vessel.

L = length of the plate pack (ft)

W = width of the plate pack (ft)

H = height of the plate pack (ft)


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IIAR 2-2008 states:

11.2.7 The required discharge capacity of a pressure relief device for each pressure vessel shall be determined by the following equation:

C = 0.5 DL (lb/min)

[C = 0.04 DL [kg/s]]

where

C = required discharge capacity of the relief device, lb air/min [kg/s]

D = outside diameter of vessel, ft [m]

L = length of vessel, ft [m].

When one pressure relief device is used to protect more than one pressure vessel, the required capacity shall be the sum of the capacities required for each pressure vessel.

In the case of a plate heat exchanger, replace the DxL portion of the equation with a term equal to half the overall external surface area in square feet (m2).

In the case of a double-pipe condenser, replace the DxL portion of the equation with a term equal to half the overall external surface area in square feet (m2).

NOTES:

a. When combustible materials are used within 20 ft (6.1 m) of a pressure vessel, the formula becomes C = 1.25 DL [C = 0.1 DL].

b. The formula is based on fire conditions. Other heat sources shall be calculated separately.


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Where:

A = area for heat transfer (ft2)

U = overall heat transfer coefficient from the refrigerant to process fluid (Btu/hr-ft2-°F)

TCIP = Temperature of fluid creating potential overpressure situation during internal load scenario (°F)

Tsat,ref = Saturation temperature of refrigerant evaluated at the fully accumulated relief device set pressure – P = Pset * 1.1 + 14.7, (°F)

hvapor = Enthalpy of saturated vapor at the fully accumulated relief device set pressure, (Btu/lbm)

hliquid = Enthalpy of saturated liquid at the fully accumulated relief device set pressure, (Btu/lbm)

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The conversion from ammonia to an air basis mass flow rate is based on the following air properties:

Ca = 356

Ta = 520°R

Ma = 28.97

and the following ammonia properties:

P = 289.7 psia (250*1.1+14.7)

Dh = 488 Btu/lbm

k = 1.575

Tr = 580.8°R (120.8°F – saturation temperature at 289.7 psia)

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The conversion from ammonia to an air basis mass flow rate is based on the following air properties:

Ca = 356

Ta = 520°R

Ma = 28.97

and the following ammonia properties:

P = 289.7 psia (250*1.1+14.7)

Dh = 488 Btu/lbm

k = 1.575

Tr = 580.8°R (120.8°F – saturation temperature at 289.7 psia)

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QOC,max = design heat load on the oil cooling heat exchanger (Btu/min)

Tsuction = saturated suction temperature (°F)

mrefr,TS = mass flow rate of refrigerant vapor produced by the oil cooler at maximum heat load (lbm/min)

hvapor = saturated vapor refrigerant enthalpy at the fully accumulated relief device set pressure – P=Pset * 1.1 + 14.7, (Btu/lbm)

hliquid = saturated liquid refrigerant enthalpy at the fully accumulated relief device set pressure – P=Pset * 1.1 _ 14.7, (Btu/lbm)


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