Technical Paper #6 Relief Vent Piping per ASHRAE 15-2004web.iiar.org/membersonly/PDF/TC/T354.pdf · piping, how to select a relief valve and three-way valve, and some strategies to

© IIAR 2005 213

Abstract

Sizing ammonia relief vents, once a simple process, has become more complicated as safety codeshave evolved. In recent years, code officials have been scrutinizing vent pipe sizing much moreheavily. The latest release of the ASHRAE Safety Standard for Refrigeration Systems devotesconsiderable ink to the sizing of relief vents, and provides the user with an equation for determiningpressure drop in relief piping. This paper will show how to use the ASHRAE equation to solve for thepressure drop in relief vent piping, how to select a relief valve and three-way valve, and also showsome strategies to bring existing nonconforming installations into compliance with the code.

2005 IIAR Ammonia Refrigeration Conference & Exhibition, Acapulco, Mexico

Technical Paper #6

Relief Vent Piping per ASHRAE 15-2004

Don Faust and Brian PetersonGartner Refrigeration & Manufacturing, Inc.

Plymouth, Minnesota

ACKNOWLEDGEMENT

The success of the technical program of the 27th Annual Meeting of theInternational Institute of Ammonia Refrigeration is due to the quality of the technicalpapers 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 theInternational Institute of Ammonia Refrigeration. They are not official positions ofthe Institute and are not officially endorsed.

EDITORSM. Kent Anderson, President

Chris Combs, Project CoordinatorGene Troy, P.E., Technical Director

International Institute of Ammonia Refrigeration1110 North Glebe Road

Suite 250Arlington, VA 22201

+ 1-703-312-4200 (voice)+ 1-703-312-0065 (fax)

www.iiar.org

2005 Ammonia Refrigeration Conference & ExhibitionFairmont Acapulco Princess

Acapulco, Mexico

Technical Paper #6 © IIAR 2005 215

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).

Relief Vent Piping per ASHRAE 15-2004 — Don Faust and Brian Peterson

216 © IIAR 2005 Technical Paper #6

• 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]



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]



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



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



Two methods are commonly used to evaluate inlet restrictions. One uses k, a

resistance factor, and the other uses Cv, a flow coefficient. This paper illustrates how

to solve for inlet restrictions using Cv. Table 4 shows Cv values for flow restrictions

commonly found on inlets.

Using Cv to Find the Effect of Inlet Restrictions: The overall equation for adjusting Cv

for a series of flow restrictions is:

C2v0 = (2)

+ + +...

where:

Cv0 = adjusted system flow coefficient

Cv1,2,3 = flow coefficients for a series of pipes and valves

With the above equation, if we know the Cv of every item in the piping leading up

to and including the relief valve, we can calculate the overall flow coefficient.

To assist with this, Table 5 lists flow coefficients for piping common to relief inlets,

and Table 6 lists the flow coefficients of many of the three-way valves in current use.

Note that most three-way valves have different flow coefficients for the two different

sides. This is because of the shaft that runs through one side and not the other. The

table lists the flow coefficient for the worst-case side of the three-way valve.

For the relief valve, Cv is determined by Equation (13) in the Users Manual for

ANSI/ASHRAE 15-2001, Appendix H (ASHRAE, 2003):

Cv = (3)


1

C2v1

1

C2v2

1

C2v3

1

22.53 Cr

P1


where:

Cv = flow coefficient for the relief valve

Cr = rated valve capacity, lbs. air/min [kg/s]

P1 = valve set pressure (psig) *1.1 + atmospheric pressure

[P1 = valve set pressure (barg)*1.1 + 1 bar]

When you know the adjusted system flow coefficient, Cv0, the adjusted flow, Ca,

through the relief valve assembly can be calculated using the following equation:

Ca = Cr (Cv0/Cv) (4)

where:

Ca = adjusted system flow

Cr = relief valve rated flow

Cv = relief valve flow coefficient determined by Equation 3

Cv0 = adjusted system flow coefficient calculated in Equation 2

Example Inlet Restriction Calculation

Determine the actual capacity of a 250-psig [17.0-barg] relief valve (Henry 5601, 1⁄2 ″ x 1⁄3 ″[13 mm x 19 mm]) with a 1⁄2 ″ [13 mm] three-way valve (Henry 8021A),

connected to a 1⁄2 ″ [13 mm] nozzle on a vessel.

From Table 3, at 250 psig [17.0 barg], this model has a capacity of 57.6 lbs.

air/min [0.436 kg/s].

Cr = 57.6 lbs. air/min [0.436 kg/s]



From Equation 3 the Cv for the relief valve is:

Cv = 22.53 (57.6) / (250*1.1 + 14.7)

Cv = 4.48

From Table 4, the 1⁄2 ″ nozzle has a Cv of 12.6.

From Table 6, the Henry 8021A three-way valve has a Cv of 5.34.

Using these values, we apply Equation 2:

C2v0 =

C2v0 = 10.96

Cv0 = 3.31

The adjusted capacity of the valve is found by applying Equation 4:

Ca = Cr(Cv0/Cv)

Ca = (57.6) (3.31/4.48)

Ca = 42.6 lbs. air/min [0.322 kg/s]

In this example, the relief valve lost roughly 25% of its capacity due to inlet

losses.


1

4.482

1

12.62

1

5.342+ +

1


Use of Rupture Discs on Relief Valve Inlets

Notably, ASHRAE 15 does not address reducing the capacity of the relief valve when

rupture discs are installed in series with them. Some installations include a rupture

disc between the three-way valve and the relief valve along with a non-resetting

pressure gauge. This is useful in helping to determine which relief valve lifted in a

large system. When this arrangement is employed, the capacity of the relief valve is

to be reduced to 90% of its rated capacity. It is ASME Boiler and Pressure Vessel

Code that dictates this, not ASHRAE 15. (ASME, 2004)

To quote the ASME Code, Section VIII – Division 1, Part UG-127: Nonreclosing

Pressure Relief Devices:

(3) Application of Rupture disks

(b 2) The marked capacity of a pressure relief valve (nozzle type) when

installed with a rupture disk device between the inlet of the valve and the

vessel shall be multiplied by a factor of 0.90 of the rated relieving capacity of

the valve alone…

Step 3. Sizing the Relief Vent Piping System

Appendix H of ASHRAE 15 lists the formula to use when figuring out the allowable

equivalent length of discharge piping. The equation is a form of the simplified

isothermal compressible flow equation.

L = – (5)


0.2146d5 (P 20 – P 2

2)

fC 2r

d . ln(P0 – P2)

6f


where:

L = equivalent length of discharge piping, ft [m]

Cr = rated capacity of the relief device, lbs. air/min [kg/s]*

f = Moody friction factor in fully turbulent flow

d = inside diameter of pipe, inches

P2 = absolute pressure at the end of the piping run, psia

P0 = absolute pressure at the beginning of the run, psia

* Note that the adjusted capacity of the relief valve may be used if the calculations

shown in Section 2 are done.

Most of the time, all terms in this equation are known except for P0, the pressure at

the beginning of the pipe run. It is not possible, algebraically speaking, to isolate P0

on one side of the equation. In order to solve for the pressure drop in the pipe, the

pressure drop in the pipe must be already known! The only way to solve this

equation is to iterate: guess what the pressure drop might be, plug it into the

equation, see if the result is close to the guess, then guess again at the pressure drop

and continue the cycle until the guess and the actual result are reasonably close to

being equal. This is where the computer program SRVQuick comes in, as it doesn’t

get bored and can try hundreds of solutions in a split second and determine the

answer to more decimal places than should ever be needed.

The total allowable backpressure in the piping system is defined as a percentage of

the set pressure (Ps) of the valve.

• Conventional relief valves: 15% of Ps

• Balanced relief valves: 25% of Ps

• Pilot-operated reliefs, fusible plugs, rupture members: 50% of Ps

For each of these cases, atmospheric pressure (14.7 psi) must be added to the

allowable backpressure in psig [barg] to convert to psia, the units normally used for

P0. Thus, the equations to calculate allowable backpressure in psia would be:



Conventional: P0 = 0.15 x Ps + 14.7 (6a)

Balanced: P0 = 0.25 x Ps + 14.7 (6b)

Pilot, etc.: P0 = 0.50 x Ps + 14.7 (6c)

It is interesting to note that the 15% allowable backpressure for conventional relief

valves is not a universal recommendation. The ASME Boiler and Pressure Vessel

Code (in the non-mandatory appendices) recommends that the allowable

backpressure be only 10%.

Example Calculation

What is the allowable backpressure in psia [bar] for a 250-psig [17.0 barg]

conventional relief valve?

Equation 6a may be applied to this situation:

P0 = (0.15) (250) + 14.7 = 52.2 psia [3.55 bar]

The vast majority of relief valves used in industrial ammonia systems are the

conventional type of relief valve; thus, for this paper it is assumed that the total

allowable backpressure in the system is 15% of the set pressure.

There are two points to keep in mind here. First, the relief vent must be sized based

upon either the rated capacity or the adjusted capacity of the relief valve, not the

required capacity as calculated in the section on vessels, heat exchangers, and

positive displacement compressors. Thus, oversized relief valves will require the

system to have larger relief vents, even though the required capacity may be

considerably smaller. Secondly, a 150-psig [10.2-barg] relief valve has half of the

allowable pressure drop of a 300-psig [20.4-barg] relief valve. In analyzing relief vent

systems, those that have 150-psig [10.2-barg] vessels will have the most difficulty

conforming to the code.



Table 3 of ASHRAE 15 lists pressure relief valve discharge line capacities for various

set pressures, pipe sizes, and lengths of run. If the system being analyzed has each

relief valve individually piped with a single pipe size to its own atmospheric vent,

these tables are quite useful. If, however, the system has more than one relief valve

piped to a common header, then a different approach to solving the problem must

be used.

Chapter 6 of IIAR’s Ammonia Refrigeration Piping Handbook, as well as the ASHRAE

15 User’s Guide, lists a method of sizing a common discharge line for two or more

relief valves. (IIAR, 2004; ASHRAE, 2003) The method is based on an algebraic

rearrangement of Equation 5, and assumes a constant friction factor

(f = 0.02) for all sizes of pipe. Using that simplification, the method creates a

dimensionless resource that can be consumed by each section of piping. In practice,

this reconfiguration of the ASHRAE formula yields good results for pipe sizes around

11⁄2 ″ [38 mm]. As pipe sizes get further away from 11⁄2 ″ [38 mm], the approximation

of the constant friction factor adversely affects the accuracy of the results.

Example Calculation

What is the maximum allowable length of 6 ″ [152 mm] pipe carrying

1,420 lbs/min of air [10.8 kg/s] from a 300-psig [20.4-barg] relief valve? Assume

a maximum pressure drop of 45 psi [3.1 bar].

From Table 3 of the Standard, the maximum allowable length is 100′ [30.5 m].

The Standard uses a Moody friction factor of 0.0149 for 6 ″ [152 mm] pipe.

However, if this problem is recalculated using a friction factor of 0.02, per the

IIAR Ammonia Refrigeration Piping Handbook method, then the maximum

allowable length of 6 ″ [152 mm] pipe is 75′ [22.9 m]. The assumption in the

IIAR method of a constant friction factor will cause pipes larger than 2 ″ [51 mm]

to appear to have less capacity than they actually have, and pipes smaller than

1 1⁄2 ″ [38 mm] to appear to have more capacity than they actually have.



One could argue that there are not many systems with 6 ″ [152 mm] relief vents, but

for those that are that big, one should at least be aware of the errors.

The design tool mentioned previously, SRVQuick, solves the compressible flow

equation for sections of pipe, and calculates equivalent feet of pipe and fittings,

using the ASHRAE recommended Moody friction factors, thus eliminating the error

from the friction factor approximation. SRVQuick has the advantage of calculating

the pressure drop in each section of pipe in psi, and is easy to use.

To begin solving for pressure drop in the system, start at a point where the pressure

is known. For a relief vent system, that point is the outlet of the vent, which is

referred to as the terminal pressure. It either vents to atmosphere (14.7 psia, 0 psig

[0.0 barg]) or into a tank of water, with a defined height of water over the outlet of

the relief valve.

To figure out the terminal pressure P2 (in psia) [bar] of a relief vent that discharges

into a water tank of height H, use the following equation:

P2 = + 14.7 (7)

Example Calculation

Determine the terminal pressure of a relief vent that discharges into a tank of

water with 10′ of water over the relief vent outlet.

Applying equation 7 to the problem:

P2 = + 14.7 = (10) / (2.31) + 14.7 = 19.0 psia [1.29 bar]


H

2.31

H

2.31


Once the terminal pressure is known, then the calculation is a matter of going

backwards through the system, determining the resultant pressure at each node,

until the relief valve is reached. If the calculated pressure drop is less than the

allowable 15%, then the system conforms. If not, then some of the pipe sizes must

be increased to accommodate the flows.

Relief Vent System Calculation: Summary Example

Size the relief system for the following ammonia refrigeration machinery room:

• Vessel V-1: 42 ″Ø x 14′ vertical HP Receiver, 300 psig [barg], ASME

• Vessel V-2: 30 ″Ø x 12′ vertical, +20°F recirculator vessel, 250 psig [barg], ASME

• Comp. C-1: Frick RWF-100 High Stage Compressor, Liquid Injected

• Vent pipe discharges to atmosphere

• Assume all relief valves lift simultaneously

Step 1. Determine Required Capacities

Vessel V-1: HP Receiver

C = f D L = 0.5 (3.5) (14)

C = 24.5 lbs. air/min [0.186 kg/s]

Vessel V-2: +20°F Recirculator

C = f D L = 0.5 (2.5) (12)

C = 15 lbs. air/min [1.02 kg/s]

Compressor C-1

Reading from Table 2a, the required capacity is 25.8 lbs. air/min [1.76 kg/s].



Step 2. Select Relief Valves and calculate actual capacities

Results were obtained using SRVQuick.

V-1 HP Receiver

Select R/S SR21⁄2 ″ x 1 ″ 300-psig relief valve [13mm x 25mm, 20.4 barg] with R/S

M1 three-way valve. Assume 1⁄2 ″ [13mm] nozzle and a 90° elbow on the inlet

Required Capacity 24.5 lbs. air/min [0.186 kg/s]

Rated Capacity 36 lbs. air/min [0.27 kg/s]

Adjusted Capacity 33.1 lbs. air/min [0.251 kg/s]

V-2 +20°F Recirculator

Select Hansen H5600R 1⁄2 x 3⁄4 250-psig [13mm x 19mm, 17.0-barg] relief with

Hansen H8021 three-way valve. Assume 1⁄2 ″[13mm] nozzle and a 90° elbow on

the inlet.

Required Capacity 15 lbs. air/min [0.11 kg/s]

Rated Capacity 17 lbs. air/min [0.13 kg/s]


C-1 Compressor

Manufacturer supplies Shank Model 813 1⁄2 x 1 ″ 300-psig [13 x 25mm, 20.4-barg]

relief valve with Shank 843 1⁄2 ″ [13mm] three-way valve. Assume 1⁄2 ″ [13mm]

nozzle and no 90° elbow.

Required Capacity 25.8 lbs. air/min [0.195 kg/s]

Rated Capacity 52.4 lbs. air/min [0.397 kg/s]




Step 3. Size Vent Piping

Figure 1 shows a diagram of the piping. Nodes were labeled A, B, C. The only

place where the pressure is known is at the outlet of the relief vent, which would be

0 psig [0 barg] (discharges to atmosphere). Start at that point and work backwards

through the main. Results were obtained using SRVQuick. (Table 7)

Checking the results, we need to determine if P0 is higher than allowable at each

relief valve. For V-1 and C-1, (300-psig [20.4-barg] ASME design) the allowable

backpressure is:

(300) * 0.15 = 45 psig [3.1 barg]

Both V-1 and C-1 pressures are lower than that.

For V-2 (250-psig [barg] ASME design), the allowable backpressure is:

(250) * 0.15 = 37.5 psig [2.55 barg]

V-2 pressure is lower than that. Thus, the piping system meets code as designed.

Piping Relief Valves – Dos and Don’ts

There are a number of items that the code specifically prohibits in the relief valve

piping system. First, there can be no stop valves between the relief valve and the

item it is protecting. Three-way valves are not considered to be stop valves. In

general, there may not be stop valves on the outlet of the system, but there is an

exception if the valve is locked open, a full area valve, and only closed if there is a



parallel relief valve system protecting the equipment or the system has been

depressurized and vented to atmosphere.

A second area of confusion pertains to the use of reducers around relief valve

systems. For example, if a vessel has a 1 ″ relief connection on it, does that mean

that a 1 ″ [25mm] three-way valve and a 1 ″ [25mm] inlet relief valve must be used?

The answer is “not necessarily.” Section 9.7.6 of ASHRAE 15 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.

This means that if a 3⁄4 ″ [19mm] inlet relief valve has the required capacity, then at

some point between the vessel and the relief valve, the pipe size must reduce from

1″ to 3⁄4 ″ [25mm to 19mm]. However, a 1 1⁄4 ″ [32mm] inlet relief valve could not be

installed, because the 1 ″ [25mm] nozzle on the vessel is smaller than the inlet to

the relief valve. Locating the reducer would be up to the piping designer, but it

would be best, due to inlet pressure losses, to use a 1 ″ [25mm] three-way valve, and

reduce to 3⁄4 ″ [19mm] at the inlet to the valve.

Further, outlet (vent) piping may not reduce in size (per Section 9.7.8.4). The piping

must be at least as large as the relief vent outlet. To meet the current code, most

relief valve outlets pipes will have to be immediately increased in size. Many local

codes require, and the author considers it good practice, to install a drip leg on the

outlet of the relief valve. This will prevent condensation from collecting on the seat

of the relief valve, which will accelerate corrosion possibly leading to premature

failure of the relief valve.



Bringing Existing Systems into Compliance

The majority of refrigeration systems installed before 1999 do not meet the current

code. There is no requirement within the current version of ASHRAE 15 that

grandfathered systems be brought into conformance. However, there are some

scenarios under which an existing system might need to be brought up to current

standards:

• New construction that adds equipment to the relief vent system

• A safety review or mechanical integrity inspection that recommends or requires

that the system meet ASHRAE 15-2001 or -2004

There are two strategies that can be employed to minimize the amount of re-piping

necessary to update older systems:

1. Properly Size the Relief Valves

2. Examine Relief Scenarios

These strategies are examined below.

Properly Size the Relief Valves

Many relief valves on older vessels are severely oversized. This is especially true of

smaller vessels and heat exchangers, as reduced capacity relief valves have only

recently become available. Remember that the relief vent is sized for the actual

capacity of the relief valve, not the required capacity. Thus an oversized valve

unnecessarily taxes the capacity of both the riser and the relief main. As shown

above, it is a straightforward calculation to determine the required capacity for the

vessel, and Table 3 shows the capacity of relief valves commonly used in ammonia

refrigeration. If the existing relief valve is considerably oversized, select and install a

relief valve closer to the required capacity. Note that this will likely require a

Management of Change (MOC) procedure for plants subject to the requirements of

OSHA’s PSM regulations, and should be duly noted by those implementing IIAR’s



Ammonia Refrigeration Management program, and that paperwork and analysis

should be completed prior to replacing the valve. (OSHA, 1992; IIAR, 2004)

For compressors, it is best to check with the manufacturer to verify the required

capacity of the specific model and serial number of compressor under consideration.

Tables 2a through 2d list the required capacity of popular screw compressors, but

there are other factors to consider when replacing a relief valve on a compressor.

The type of controls and safeties the compressor is equipped with, and the method

of unloading the compressor, come into play. For example, some screw compressors

use a plug valve to unload, rather than a slide valve, and thus may have higher

required capacities than the table suggests.

Examine Relief Scenarios

The simplest way to examine a relief vent system is to assume that all relief valves

lift simultaneously. However, in larger systems, this assumption may lead to very

large relief vent mains. Section 9.7.8.4 of ASHRAE 15 requires that:

The sizing of the common discharge header downstream from each of the two or

more relief devices…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.

The key phrase is “that are expected to operate simultaneously”. This leaves the door

open to establish relief scenarios, where a hazard analysis is performed to establish

which relief valves are expected to operate simultaneously, and to subsequently size the

vent system to accommodate the various scenarios. For example, during a fire in the

engine room, would the compressors still have electrical power? The answer would

depend on many site-specific factors, but these can be reviewed in a Process Hazard

Analysis. This approach may be more work than it is worth in small to medium sized

plants, but for larger plants or new installations the effort may be worthwhile.



Conclusion

Sizing relief vent systems has gone from being very simple to becoming one of the

more complex tasks in designing a refrigeration system. The safety relief system must

be able to perform as intended to avoid potentially disastrous consequences. The

equations required to design the system lend themselves very well to computerized

solution, and it is hoped that the use of the SRVQuick program saves designers and end

users time in designing or verifying compliance with the Standard.

Relief valve manufacturers have responded to the Standard, and many are designing

new valves with reduced capacities to fit smaller heat exchangers and vessels better.



References

ASHRAE. ANSI/ASHRAE 15, Safety Standard for Refrigeration Systems. American

Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). 2004.

ASME. Boiler and Pressure Vessel Code: Section VIII, Pressure Vessels. American

Society of Mechanical Engineers (ASME). 2004.

Fenton, D., and W. Richards. User’s Manual for ANSI/ASHRAE 15-2001, Safety

Standard for Refrigeration Systems. American Society of Heating, Refrigerating, and

Air-Conditioning Engineers (ASHRAE). 2003.

IIAR. Ammonia Refrigeration Piping Handbook. International Institute of Ammonia

Refrigeration (IIAR). 2004.

IIAR. Ammonia Refrigeration Management Program. International Institute of

Ammonia Refrigeration (IIAR). 2004.

OSHA. Process Safety Management (PSM) Regulation, 29 CFR 1910.119.

Occupational Safety and Health Administration (OSHA). 1992.

OSHA. Process Safety Management of Highly Hazardous Chemicals, 29 CFR

1910.1200. Occupational Safety & Health Administration (OSHA). 1994 (amended

1996).




Figure 1: Relief Vent System in Summary Example

Table 1: Refrigerant Data



Table 2a: Required Relief Capacities (Frick)



Table 2b: Required Relief Capacities (FES, smaller sizes)



Table 2c: Required Relief Capacities (FES, larger sizes)



Table 2d: Required Relief Capacities (M&M)



Table 3: Relief Valves



Tab

le 4

: C

vo

f In

lets



Table 5: Flow Coefficients for Pipe and Fittings



Table 6: Cv of Common Three-way Valves

Table 7: Results for Summary Example


Notes:


Documents

Technical Paper #6 Relief Vent Piping per ASHRAE 15-2004web.iiar.org/membersonly/PDF/TC/T354.pdf · piping, how to select a relief valve and three-way valve, and some strategies to