PREAMBLE (NOT PART OF THE STANDARD)

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END OF PREAMBLE (NOT PART OF THE STANDARD)

CGA S-7—2005
METHOD FOR SELECTING PRESSURE RELIEF DEVICES FOR COMPRESSED GAS MIXTURES IN CYLINDERS

FOURTH EDITION

PLEASE NOTE:

The information contained in this document was obtained from sources believed to be reliable and is based on technical information and experience currently available from members of the Compressed Gas Association, Inc. and others. However, the Association or its members, jointly or severally, make no guarantee of the results and assume no liability or responsibility in connection with the information or suggestions herein contained. Moreover, it should not be assumed that every acceptable commodity grade, test or safety procedure or method, precaution, equipment or device is contained within, or that abnormal or unusual circumstances may not warrant or suggest further requirements or additional procedure.

This document is subject to periodic review, and users are cautioned to obtain the latest edition. The Association invites comments and suggestions for consideration. In connection with such review, any such comments or suggestions will be fully reviewed by the Association after giving the party, upon request, a reasonable opportunity to be heard. Proposed changes may be submitted via the Internet at our website, www.cganet.com.

This document should not be confused with federal, state, provincial, or municipal specifications or regulations; insurance requirements; or national safety codes. While the Association recommends reference to or use of this document by government agencies and others, this document is purely voluntary and not binding unless adopted by reference in regulations.

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Work Item 06-48
Cylinder Valve Committee

NOTE—Technical changes from the previous edition are underlined.

NOTE—Appendices A and B (Normative) are requirements.

NOTE—Appendix C (Informative) is for information only.

FOURTH EDITION: 2005
THIRD EDITION: 2003
SECOND EDITION: 1996
FIRST EDITION: 1989
© 2005 The Compressed Gas Association, Inc. All rights reserved.

All materials contained in this work are protected by United States and international copyright laws. No part of this work may be reproduced or transmitted in any form or by any means, electronic or mechanical including photocopying, recording, or any information storage and retrieval system without permission in writing from The Compressed Gas Association, Inc. All requests for permission to reproduce material from this work should be directed to The Compressed Gas Association, Inc., 14501 George Carter Way, Suite 103, Chantilly VA 20151 You may not alter or remove any trademark, copyright or other notice from this work.

1 Introduction

This publication presents a method for selecting pressure relief devices (PRDs) for compressed gas mixtures packaged in cylinders having water capacities of 1000 lb (454 kg) (see U.S. Department of Transportation [DOT] Title 49 of the U.S. Code of Federal Regulations [49 CFR] Part 173.301 [f]). This standard also applies to DOT 3AX, 3AAX, and 3T cylinders having water capacities over 1000 lb (454 kg) that comply with the specifications, charging, and maintenance regulations of DOT or the corresponding specifications and regulations of Transport Canada (TC) [1, 2].¹

The task is more involved than the method for determining the appropriate valve outlet connections for gas mixtures. For the latter, as described in CGA V-7, Standard Method of Determining Cylinder Valve Outlet Connections for Industrial Gas Mixtures, mixture component rating numbers are assigned to the various compressed gases based on the physical properties of the gases, i.e., flammability, toxicity, state of the gas, and corrosiveness [3]. The higher the mixture component rating number (1 through 6), the more influential that gas becomes in determining the outlet connection for the mixture in which that gas is a component.

Valve outlet connection assignments are broadly separated into connections for groups of gases having similar properties such as high and low pressure, flammability, corrosivity, toxicity, and inertness. Since 1978 this system has proven satisfactory in avoiding hazardous connections.

The physical characteristics of a mixture consisting of several pure gases could be dramatically different depending on the concentration of each gas in the mixture. Depending on these concentrations, a mixture could be flammable or nonflammable, toxic or nontoxic, liquified or nonliquefied, corrosive or noncorrosive. Since the mixture is released to the atmosphere when the relief device functions, the characteristics of the escaping gas mixture must be well defined.

It was concluded that a method would have to be established for assigning the proper relief device for each mixture, just as is done for pure gases in CGA S-1.1, Pressure Relief Device Standards—Part 1—Cylinders for Compressed Gases [4]. To use this method, the proper FTSC code must be determined (see Appendix A). In some cases, determining the proper FTSC number is straightforward. However, in other cases the appropriate FTSC number is not as obvious, and in most cases this determination is the responsibility of the gas mixture producer. It must be understood that some of the considerations made in determining the selection of the PRD used in this publication are based on experience.

The method outlined in this publication is designed to handle the proliferation of mixtures entering the commercial market.

The continued use of previously recognized and installed devices is not restricted by this edition of the standard. However, if a PRD is replaced, the new device shall meet the requirements of this standard.

2 Scope

This method is applicable to the determination of the proper PRD to use with compressed gas mixtures in cylinders defined in Section 1.

This method is limited to those compressed gas mixtures with known flammability, toxicity (LC₅₀), state, and corrosivity. In addition, the DOT/TC rating and dimensions of the cylinder and the final pressure must be known.

For the selection of PRDs for a single component compressed gas, see CGA S-1.1 [4]. For multicomponent compressed gases (mixtures), the method for selection of relief devices in this publication should be used. Where a mixture is predominantly made up of a single component in a mixture, the gas producer shall determine whether the properties of this mixture dictate it being treated as a single component compressed gas or a gas mixture.

¹ References are shown by bracketed numbers and are listed in order of appearance in the reference section.

3 Definitions

For the purpose of this standard, the following definitions apply.

3.1 Algorithm

Series of formatted questions that when answered in sequence will result in the selection of one or more PRDs.

3.2 Blanketing or pressurizing

Pressurization of the vapor space above a liquefied gas or liquid for the purpose of liquid withdrawal.

3.3 Final pressure

Charged settled pressure at 70 °F (21.1 °C).

3.4 Compressed gas

Any material that exerts in a container an absolute pressure of at least 40.6 psia (280 kPa) at 68 °F (20 °C).²

3.5 Gas mixture

Purposeful combination of two or more commodities resulting in a compressed gas.

3.6 Producer

Site where the compressed gas mixture is packaged into cylinders and the personnel who perform the work.

3.7 Lethal concentration fifty (LC₅₀)

Concentration of a substance in air for which exposure for a specified length of time is expected to cause the death of 50% of the entire defined experimental animal population.

NOTE—See CGA P-20, Standard for the Classification of Toxic Gas Mixtures [6].

3.8 Hazard zone A

Material with a toxicity LC₅₀ less than or equal to 200 ppm.

3.9 Hazard zone B

Material with a toxicity LC₅₀ greater than 200 ppm and less than or equal to 1000 ppm.

3.10 Hazard zone C

Material with a toxicity LC₅₀ greater than 1000 ppm and less than or equal to 3000 ppm.

3.11 Hazard zone D

Material with a toxicity LC₅₀ greater than 3000 ppm and less than or equal to 5000 ppm.

4 Types of pressure relief devices

Types of PRDs are designated as follows:

4.1 Type CG-1

A rupture disk.

4.1.1 Limitations

Since this is a pressure-operated device designed to release the entire content of the container, there is no way to prevent the complete release of the content, either as a result of normal functioning or premature rupture of the device.

² kPa shall indicate gauge pressure unless otherwise noted as (kPa, abs) for absolute pressure or (kPa, differential) for differential pressure. All kPa values are rounded off per CGA P-11, Metric Practice Guide for the Compressed Gas Industry [5].

4.2 Type CG-2

A fusible plug using a fusible alloy with yield temperature not over 170 °F (76.7 °C) or less than 157 °F (69.4 °C). Nominal yield temperature is 165 °F (73.9 °C).

4.2.1 Limitations

Since this is a thermally operated device, it does not protect against overpressure from improper charging practices. This device releases the entire lading of the container when it functions, and is limited to use on cylinders of 500 psig (3450 kPa) service pressure or less. This device may be used in higher service pressure cylinders provided that the product pressure does not exceed 500 psig (3450 kPa) at 68 °F (20 °C) and the device type is mandated by this standard or TC regulations.

4.3 Type CG-3

A fusible plug using a fusible alloy with yield temperature not over 224 °F (106.7 °C) or less than 208 °F (97.8 °C). Nominal yield temperature is 212 °F (100 °C).

4.3.1 Limitations

Same as for CG-2 (see 4.2.1).

4.4 Type CG-4

A combination rupture-disk/fusible-plug device using a fusible alloy with yield temperature not over 170 °F (76.7 °C) or less than 157 °F (69.4 °C). Nominal yield temperature is 165 °F (73.9 °C).

4.4.1 Limitations

Since this device is a combination device, it requires both excessive pressure and temperature to cause it to operate. This device will not function due to pressure unless the fusible metal is melted out first. Such a combination device cannot prevent an improperly filled (overfilled) cylinder from rupturing due to hydrostatic pressure at room temperature or any temperature below the melting temperature of the fusible metal, as will devices that contain only a rupture disk (CG-1). There is no way to prevent the complete release of the content when this device functions.

4.5 Type CG-5

A combination rupture-disk/fusible-plug device using a fusible alloy with yield temperature not over 224 °F (106.7 °C) or less than 208 °F (97.8 °C). Nominal yield temperature is 212 °F (100 °C).

4.5.1 Limitations

Same as for CG-4 (see 4.4.1).

4.6 Type CG-7

A pressure relief valve.

4.6.1 Limitations

This device maintains the pressure in the container at a limit as determined by the set pressure of the valve and thus does not protect against rupture of the container when the application of heat weakens the container to the point where its rupture pressure is less than the operating pressure of the device.

WARNING: PRDs might not prevent rupture of a cylinder under all conditions of fire exposure. When the heat transferred to the cylinder is localized, intensive, and remote to the relief device, or where the fire builds extremely rapidly such as in an explosion and is of very high intensity, the cylinder can weaken sufficiently to rupture before the relief device operates or while it is operating.

5 Description of method and procedures

5.1 Algorithm

The algorithm has been designed to assist the gas mixture producer to quickly and correctly determine the appropriate PRD(s) for gas mixtures. The algorithm is shown in Figure 1 (U.S. customary units) and Figure 2 (SI units). Application of the algorithm is illustrated for selected gas mixtures in Examples 1-12.

5.2 Responsibility for selection of the pressure relief device

The responsibility for the selection of the PRD lies with the gas mixture producer. Although the algorithm does permit a degree of flexibility in the selection of the relief device for some cylinders, the flammability, toxicity (LC₅₀), state, and corrosivity of the mixture shall be determined first. Appendix A provides the definitions of the FTSC codes. Appendix B lists the codes for most of the gases that are commonly used in mixtures.

5.3 Guidelines for use of the algorithm

The following guidelines and constraints govern the use of the algorithm:

– The final mixture properties, i.e., flammability, toxicity, state of the gas, corrosiveness, and final pressure are known by the gas mixture producer.

– The producer shall determine the mixture’s flammability. As used in the algorithm, “Flammable” means having an “F” rating of 2, 3, or 5.

– The producer shall determine the mixture’s toxicity. In accordance with DOT (49 CFR 173.115), the toxicity of the mixture can be calculated from the concentrations and toxicity of the components as follows (see CGA P-20 for additional details regarding mixture toxicity calculations) [6]:

Where:

n                       = Number of toxic components

Concentration_i   = Concentration of the i^th toxic component, expressed as a decimal fraction of the whole, e.g., 0.05 for a 5% mixture and 5 × 10^–6 for a 5 ppm mixture. This number is dimensionless

LC_{50, i}                = LC₅₀ of the i^th toxic component, in ppm

LC_{50, mix}            = LC₅₀ of the mix, in ppm

For a mixture with a single toxic component, this simplifies to:

– As used in the algorithm, “Corrosive” means having a “C” rating of 1 or greater.

– Other definitions conform with those of DOT or TC if applicable.

– Pressurization of the vapor space above a condensed liquid for the purpose of withdrawal does not constitute a gas mixture.

– If a 110% fill is authorized, only the CG-1 device is allowed.

– When the CG-2 device is indicated, it allows use of the CG-3 device where the final mixture properties permit. Likewise when the CG-4 device is indicated, the CG-5 device may be used where the final mixture properties permit.

NOTE—The decision of whether the CG-2 or CG-3 device is used as well as whether the CG-4 or CG-5 device is used shall be made by the gas mixture producer as authorized by the latest edition of CGA S-1.1 based on the producer’s classification of the final mixture according to its properties [4].

– Where two or more types of PRDs are indicated, only one of them is required. Care should be taken that the selection made conforms to all requirements in 49 CFR and CGA S-1.1 [1, 4]. For example, see CGA S-1.1 regarding the requirements for relief devices at both ends of the cylinder if the cylinder length exceeds 30 in (762 mm) [4].

5.4 Procedure for assigning a pressure relief device for a mixture

With an understanding of the guidelines in 5.3, the procedures below shall be followed to assign a PRD for a mixture (see Appendix C for a worksheet format):

1. In the worksheet, list the components and concentration thereof in the proposed gas mixture in the first two columns;
2. Enter the FTSC code for each mixture component directly from Appendix B in the next four additional columns;
3. Add the LC₅₀ in the last column from Appendix B, if available;
4. Verify that the DOT/TC rating and dimensions of the cylinder to be filled as well as the final pressure of the mixture are compatible with each other and are in compliance with pertinent regulations contained in 49 CFR 173.301(a)(8) [1];
5. Calculate the toxicity (T) of the mixture as described in 5.3 and assign F, S, and C codes based on experience and the properties of the components of the mixture;
6. Using the mixture’s FTSC code established in step e), answer each question, beginning at the “Start” of the algorithm, until a determination of the proper PRD is accomplished; and
7. Size the PRD in accordance with the requirements of the latest edition of CGA S-1.1 [4].

Figure 1—Algorithm for gas mixtures (U.S. customary units)

Figure 2—Algorithm for gas mixtures (SI units)

Worksheet for Example 1
100 ppm hydrogen, balance nitrogen
(U.S. customary units)

STEPS ON ALGORITHM: (See Example 1)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied but pressure is > 300 psig.
3. Cylinder has an O.D. > 4.5 in.
4. Mixture nonliquefied at 70 °F.
5. Mixture is nonflammable (as determined by gas producer).
6. Pressure is > 500 psig.
7. Mixture is not corrosive (as determined by gas producer).
8. PRD is CG-1 or CG-4.

OBSERVATIONS:

1. Either device is suitable.
2. Since the length of the cylinder is < 65 in, only one PRD is required.
3. The pressure in the cylinder at 130 °F will be < 5/4 times the service pressure of 2015 psig.

Example 1—100 ppm hydrogen, balance nitrogen (U.S. customary units)

Worksheet for Example 2
5% trimethylamine, balance sulfur hexafluoride
(U.S. customary units)

STEPS ON ALGORITHM: (See Example 2)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied and pressure is < 300 psig.
3. Cylinder is not DOT 39.
4. No PRD is required.

OBSERVATIONS:

No PRD is required.

Example 2—5% trimethylamine, balance sulfur hexafluoride (U.S. customary units)

Worksheet for Example 3
5% trimethylamine, balance sulfur hexafluoride
(SI units)

STEPS ON ALGORITHM: (See Example 3)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied and pressure is < 2070 kPa.
3. Cylinder is not DOT 39.
4. No PRD is required.

OBSERVATIONS:

No PRD is required.

Example 3—5% trimethylamine, balance sulfur hexafluoride (SI units)

Worksheet for Example 4
0.5% oxygen, 0.5% nitrogen, balance methane
(U.S. customary units)

All mixture components have LC₅₀ values above 5000 ppm, but oxygen is < 19.5%, therefore the T code is 1.

STEPS ON ALGORITHM: (See Example 4)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied but pressure > 300 psig.
3. Cylinder has an O.D. > 4.5 in.
4. Mixture is nonliquefied at 70 °F.
5. Mixture is flammable (as determined by gas producer).
6. Pressure > 500 psig.
7. PRD is CG-4.

OBSERVATIONS:

1. Since the length of the cylinder is > 65 in, a PRD is required at each end.
2. The pressure in the cylinder at 130 °F is < 5/4 times the service pressure of 2400 psig.
3. Since the mixture has been deemed to be predominantly methane, then CGA S-1.1 permits a CG-1, CG-4, or CG-5 device [4]. The CG-7 device may not be used since the final fill pressure is > 500 psig.

Example 4—0.5% oxygen, 0.5% nitrogen, balance methane (U.S. customary units)

Worksheet for Example 5
12% ethylene oxide, balance dichlorodifluoromethane (R12)
(U.S. customary units)

STEPS ON ALGORITHM: (See Example 5)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is liquefied but pressure is < 300 psig.
3. Cylinder has an O.D. > 4.5 in.
4. Mixture is liquefied at 70 °F.
5. Mixture is nonflammable (as determined by gas producer).
6. Pressure is < 500 psig.
7. Mixture is not corrosive (as determined by gas producer).
8. PRD is either CG-1 or CG-7.

OBSERVATIONS:

1. Either device is suitable.
2. Since the length of the cylinder is < 65 in, only one PRD is required.
3. The pressure in the cylinder at 130 °F is < 5/4 times the service pressure of 240 psig.

Example 5—12% ethylene oxide, balance dichlorodifluoromethane (R12) (U.S. customary units)

Worksheet for Example 6
5% carbon dioxide, balance oxygen
(DOT 3AA2015 cylinder)
(U.S. customary units)

ALL mixture components have LC₅₀ values above 5000 ppm, and oxygen is > 19.5%, therefore the T code is 0.

STEPS ON ALGORITHM: (See Example 6)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied but pressure > 300 psig.
3. Cylinder has an O.D. > 4.5 in.
4. Mixture is nonliquefied at 70 °F.
5. Mixture is nonflammable (as determined by gas producer).
6. Pressure is > 500 psig.
7. Mixture is not corrosive (as determined by gas producer).
8. PRD is CG-1 or CG-4.

OBSERVATIONS:

1. Either device is suitable.
2. As the length of the cylinder is < 65 in, only one PRD is required.
3. The pressure in the cylinder at 130 °F is < 5/4 times the service pressure of 2015 psig.

Example 6—5% carbon dioxide, balance oxygen (DOT 3AA2015 cylinder) (U.S. customary units)

Worksheet for Example 7
5% carbon dioxide, balance oxygen
(DOT 3E1800 cylinder)
(U.S. customary units)

All mixture components have LC₅₀ values above 5000 ppm and oxygen is > 19.5%, therefore the T code is 0.

STEPS ON ALGORITHM: (See Example 7)

1. Mixture LC₅₀ is > 200 ppm
2. Mixture is nonliquefied but pressure > 300 psig.
3. Cylinder O.D. < 4.5 in, length < 12 in, final pressure < 1800 psig and not DOT 39.
4. No PRD is required.

OBSERVATIONS:

The pressure in the cylinder at 130 °F is < 5/4 times the service pressure of 1800 psig.

Example 7—5% carbon dioxide, balance oxygen (DOT 3E1800 cylinder) (U.S. customary units)

Worksheet for Example 8
Multicomponent gas mixtures
(U.S. customary units)

STEPS ON ALGORITHM: (See Example 8)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture is nonliquefied, pressure > 300 psig.
3. Cylinder has an O.D. > 4.5 in.
4. Mixture is nonliquefied at 70 °F.
5. Mixture is flammable (as determined by gas producer).
6. Pressure is > 500 psig.
7. PRD is CG-4.

OBSERVATIONS:

1. Since the length of the cylinder is < 65 in, only one PRD is required.
2. The pressure in the cylinder at 130 °F is < 5/4 times the service pressure of 2015 psig.

Example 8—Multicomponent gas mixtures (U.S. customary units)

Worksheet for Example 9
20% arsine, balance hydrogen
(U.S. customary units)

Both components are flammable, so the mixture F code is 2 (as determined by the mixture producer).

The mixture is a nonliquefied gas between 600 psig and 3000 psig, so the mixture S code is 6 (as determined by the mixture producer).

Neither component is corrosive, so the mixture C code is 0 (as determined by the mixture producer).

STEPS ON ALGORITHM: (See Example 9)

1. Mixture LC₅₀ is ≤ 200 ppm.
2. PRD is prohibited.

Example 9—20% arsine, balance hydrogen (U.S. customary units)

Worksheet for Example 10
2% arsine, balance hydrogen
(U.S. customary unit)

Both components are flammable, so the mixture F code is 2 (as determined by the mixture producer).

The mixture is a nonliquefied gas between 500 psig and 3000 psig, so the S code is 6 (as determined by the mixture producer).

Neither component is corrosive, so the mixture C code is 0 (as determined by the mixture producer).

STEPS ON ALGORITHM: (See Example 10)

1. Mixture LC₅₀ is > 200 ppm.
2. Mixture LC₅₀ is > 200 ppm, ≤ 5000 ppm, and the mixture contains a hazard zone A component.
3. No PRD is required. If one is used, it shall be CG-4.

Example 10—2% arsine, balance hydrogen (U.S. customary units)

Worksheet for Example 11
0.2% arsine, balance hydrogen
(U.S. customary units)

Both components are flammable, so the mixture F code is 2 (as determined by the mixture producer).

The mixture is a nonliquefied gas between 500 psig and 3000 psig, so the mixture S code is 6 (as determined by the mixture producer).

Neither component is corrosive, so the mixture C code is 0 (as determined by the mixture producer).

STEPS ON ALGORITHM: (See Example 11)

1. Mixture LC₅₀ > 200 ppm.
2. Mixture LC₅₀ is > 5000 ppm, and the mixture contains a hazard zone A component.
3. No PRD is required. If one is used, it shall be a CG-4.

Example 11—0.2% arsine, balance hydrogen (U.S. customary units)

Worksheet for Example 12
5% arsine, 10% phosphine, balance hydrogen
(U.S. customary units)

The mixture contains three flammable components, one of which is pyrophoric, so the mixture F code is 3 (as determined by the mixture producer).

The mixture is a nonliquefied gas between 500 psig and 3000 psig, so the mixture S code is 6 (as determined by the mixture producer).

Neither component is corrosive, so the mixture C code is 0 (as determined by the mixture producer).

STEPS ON ALGORITHM: (See Example 12)

1. Mixture LC₅₀ is < 200 ppm.
2. PRD is prohibited.

Example 12—5% arsine, 10% phosphine, balance hydrogen (U.S. customary units)

6 References

Unless otherwise specified, the latest edition shall apply.

[1] Code of Federal Regulations, Title 49 (Transportation) Parts 100 to 180, Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. www.gpoaccess.gov

[2] Transportation of Dangerous Goods Regulations, Transport Canada, Canadian Government Publishing, Public Works and Government Services Canada, Ottawa, ON K1A 0S9, Canada, www.tc.gc.ca

[3] CGA V-7, Standard Method of Determining Cylinder Valve Outlet Connections for Industrial Gas Mixtures, Compressed Gas Association, Inc., Compressed Gas Association, Inc., 4221 Walney Rd., 5th Floor, Chantilly, VA 20151. www.cganet.com

[4] CGA S-1.1, Pressure Relief Device Standards—Part 1—Cylinders for Compressed Gases, Compressed Gas Association, Inc., 4221 Walney Rd., 5th Floor, Chantilly, VA 20151. www.cganet.com

[5] CGA P-11, Metric Practice Guide for the Compressed Gas Industry, Compressed Gas Association, Inc., 4221 Walney Rd., 5th Floor, Chantilly, VA 20151. www.cganet.com

[6] CGA P-20, Standard for the Classification of Toxic Gas Mixtures, Compressed Gas Association, Inc., 4221 Walney Rd., 5th Floor, Chantilly, VA 20151. www.cganet.com

Appendix A—FTSC numerical code for gas classification (Normative)

Appendix B—List of gases (Normative)

Appendix C—Sample worksheet (Informative)