CGA-G-1.9-2008.pdf

CGA G-1.92008 RECOMMENDED PRACTICES FOR MAINTAINING THE PROPER SOLVENT LEVEL IN ACETYLENE CYLINDERS FIRST EDITION COMPRESSED GAS ASSOCIATION, INC. 4221 Walney Road, 5th Floor Chantilly, VA 20151 Phone: 703-788-2700 FAX: 703-961-1831 E-mail: cgacganet.com PAGE ii COMPRESSED GAS ASSOCIATION CGA G-1.92008 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 web site, 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. A listing of all publications, audiovisual programs, safety and technical bulletins, and safety posters is available via the Internet at our website at www.cganet.com. For more information contact CGA at Phone: 703-788-2700, ext. 799. E-mail: customerservicecganet.com. Work Item 00-58 Acetylene Committee FIRST EDITION: 2008 © 2008 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., 4221 Walney Road, Suite 500, Chantilly VA 20151. You may not alter or remove any trademark, copyright or other notice from this work. -,-,- CGA G-1.92008 COMPRESSED GAS ASSOCIATION, INC. PAGE iii Contents Page 1 Introduction.1 2 Scope and purpose1 2.1 Scope 1 2.2 Purpose.1 3 Definitions.1 4 Construction and function of acetylene cylinders.2 4.1 General2 4.2 Shell.2 4.3 Porous mass (filler)2 4.4 Solvent.3 4.5 Free volume.3 5 Solvent adjustment theory4 5.1 General4 5.2 Cylinder temperature.4 5.3 Cylinder pressure 4 5.4 Cylinder weigh-in.4 5.5 Residual acetylene4 5.6 Solvent addition calculation.5 6 Solvent adjustment in practice .6 6.1 General6 6.2 Example determination of required solvent addition .6 6.3 New cylinder information.7 7 Acetone tables7 7.1 General7 7.2 Acetone residual gas tables 7 7.3 Acetone unitary table.12 7.4 Acetylene solubility in acetone (included for reference)12 8 Dimethylformamide tables13 8.1 General13 8.2 Dimethylformamide residual gas tables 13 8.3 Dimethylformamide unitary table.17 8.4 Acetylene solubility in DMF (included for reference).17 9 References.18 10 Additional references18 Figure Figure 1Typical volume distribution for a 145 ft3 cylinder3 Tables Table 1Residual gas for acetone at 30 °F to 40 °F7 Table 2Residual gas for acetone at 40 °F to 50 °F8 Table 3Residual gas for acetone at 50 °F to 60 °F8 Table 4Residual gas for acetone at 60 °F to 70 °F9 Table 5Residual gas for acetone at 70 °F to 80 °F9 Table 6Residual gas for acetone at 80 °F to 90 °F10 Table 7Residual gas for acetone at 90 °F to 100 °F10 Table 8Residual gas for acetone (MC or B) 11 PAGE iv COMPRESSED GAS ASSOCIATION CGA G-1.92008 Table 9Unitary values table (lb of acetylene per lb of acetone above saturation gas)12 Table 10Solubility values based upon formula (g/kg)12 Table 11Residual gas for DMF at 30 °F to 40 °F.13 Table 12Residual gas for DMF at 40 °F to 50 °F.14 Table 13Residual gas for DMF at 50 °F to 60 °F.14 Table 14Residual gas for DMF at 60 °F to 70 °F.15 Table 15Residual gas for DMF at 70 °F to 80 °F.15 Table 16Residual gas for DMF at 80 °F to 90 °F.16 Table 17Residual gas for DMF at 90 °F to 100 °F.16 Table 18Unitary values (lb of acetylene per lb of DMF above saturation gas)17 Table 19Solubility values based upon formula (g/kg)17 -,-,- CGA G-1.92008 COMPRESSED GAS ASSOCIATION, INC. PAGE 1 1 Introduction This publication is one of a series compiled by the Compressed Gas Association, Inc. (CGA) to meet the demand for information on compressed gases, cryogenic liquids, and related products. 2 Scope and purpose 2.1 Scope This publication covers recommendations for maintaining the proper solvent level in acetylene cylinders. While the background, precautions, and general procedures given apply to all solvents in use with acetylene, special emphasis is given to the use of acetone and dimethylformamide (DMF). Included are residual gas tables based upon the best available data that reflects current industry practice. The expected accuracy of these values is approximately ± 10%, and should be used as a guide only. Note that these tables make use of acetylene solubilities as generated by the appropriate formulas as found in Appendix B of BS EN 1801:1999, Transportable gas cylinders: Filling conditions for single acetylene cylinders 1.1 The guidelines contained in this publication apply to acetylene cylinders manufactured to: U.S. Department of Transportation (DOT) Specifications 8 and 8AL found in Title 49 of the U.S. Code of Federal Regulations (CFR) 178.59 and 178.60 and used per CFR 173.303 as well as older specifications cylinders ICC-8, ICC-8AL, DOT E-6517, DOT E-7542 or DOT E-10320 2; or Specifications TC 8WM and 8WAM found in Canadian Standards Association (CSA) standard B339, Cylinder, Spheres, Tubes and Other Containers for the Transportation of Dangerous Goods, and used in accordance with Transportation of Dangerous Goods Regulations of Transport Canada (TC) found in CSA B340, Selection of Cylinders, Spheres, Tubes, and Other Containers for the Transportation of Dangerous Goods, Class 2, as well as older specifications cylinders CTC-8, CTC-8AL, or CTC-8WC 3, 4, 5. 2.2 Purpose This publication has been prepared as a guide for maintaining the proper solvent level in acetylene cylinders. It should be of interest to acetylene manufacturers, acetylene production and distribution personnel, authorized acetylene cylinder reinspection facilities, welding gas distributors, safety personnel, and users of acetylene. 3 Definitions For the purpose of this publication, the following definitions shall apply. 3.1 Acetylene cylinder shell Metal pressure vessel manufactured for and according to standards reserved for acetylene transport, storage, and usage. 3.2 Filler porosity Water volume of the filler material, expressed as a percentage of the filler volume. 3.3 Maximum permissible settled pressure Maximum gas pressure allowed in an acetylene cylinder defined as: 250 psig (1720 kPa) at 70 °F (21.1 °F) per DOT regulations;2 and 1.9 MPa at 15 °C per TC regulations. 1 References are shown by bracketed numbers and are listed in order of appearance in the reference section. 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 6. PAGE 2 COMPRESSED GAS ASSOCIATION CGA G-1.92008 3.4 Porous mass Highly porous material introduced into the acetylene cylinder to provide for distributing and retaining the cylinder solvent. 3.5 Residual acetylene Amount of acetylene, exclusive of saturation gas, that remains in a cylinder after use. NOTEIt is dependent upon the amount of solvent present, as well as the temperature and pressure of the gas in the cylinder. 3.6 Saturation gas Amount of acetylene gas required to saturate the solvent in the cylinder at the reference pressure and temperature. 3.7 Solvent Liquid introduced into the cylinder to dissolve the acetylene. NOTETypically acetone or DMF is used. 3.8 Tare weight 3.8.1 Tare A Sum of empty mass of the cylinder shell; the mass of the porous substance (see 3.2); the specified mass of solvent; the valve and the mass of all other parts, which are permanently attached, e.g., by clamping or bolting, including the clamps and/or nuts and bolts used to fix the parts to the cylinder when it is filled. 3.8.2 Tare S Tare A plus the acetylene mass required to saturate the solvent at standard conditions. NOTEThis tare weight is required on all cylinders as listed in the scope. 3.9 Water volume Water capacity of the cylinder shell. 4 Construction and function of acetylene cylinders 4.1 General Acetylene is a highly flammable gas and can decompose explosively at pressures above 1 atmosphere. To ensure the safe handling of compressed acetylene without the risk of acetylene decomposition, acetylene cylinders have a unique construction. They use a two-part system to ensure safe transport and use of acetylene: a porous mass that completely fills the internal volume of the cylinder shell and a liquid solvent that absorbs and stabilizes the acetylene. 4.2 Shell The basic function of the cylinder shell is to retain the internal pressure. Shells are designed to withstand a minimum test pressure of 750 psig (5170 kPa). 4.3 Porous mass (filler) A variety of porous masses have been used in the last century culminating in todays calcium-silicate-based fillers. The materials in use can reach a maximum porosity of 92% of the total volume of the cylinder shell. The porous mass acts to suppress any acetylene decomposition. If no porous mass is present or if a void of significant size in the porous mass exists within the cylinder, the decomposition could progress at a rate that could cause the cylinder to rupture. -,-,- CGA G-1.92008 COMPRESSED GAS ASSOCIATION, INC. PAGE 3 4.4 Solvent The porous mass contains a prescribed amount of solvent, commonly acetone or DMF. When acetylene is charged into a cylinder, it is absorbed by the liquid solvent. Through the use of the porous mass and solvent, it is possible for a cylinder to contain approximately eight times the volume of acetylene that could safely be compressed into the same cylinder without solvent. Acetone is the primary solvent in use with acetylene because it is relatively inexpensive and has a low toxicity. DMF has a much lower vapor pressure than acetone, which translates into a lower solvent loss. It also has a greater solubility for acetylene. These advantages tend to be offset, however, by the higher unit cost of DMF and greater cylinder weights when DMF is used (typically 30% greater for the same size cylinder). As such, the main use of DMF to date has occurred in bulk systems such as bundles or trailers (see CGA G-1.6, Recommended Practices for Mobile Acetylene Trailer Systems) 7. Different solvents should not be mixed within a cylinder. Density variations between different solvents as well as different gas absorption characteristics can lead to confusion about the actual acetylene capacity of the cylinder and to overfilling or underfilling the cylinder with solvent. Additionally, there can be unexpected reactions between the solvents during the heating that accompanies the fill process. 4.5 Free volume Acetylene cylinders are designed to maintain a 10% minimum free volume at 70 °F (21.1 °C). This free volume is the extra space necessary to prevent the cylinder from becoming hydraulically full at elevated temperatures (see Figure 1). Free volume 13% Dissolved acetylene available for use 34% Solvent 42% Porous mass 10% Saturation gas 1% Figure 1Typical volume distribution for a 145 ft3 cylinder PAGE 4 COMPRESSED GAS ASSOCIATION CGA G-1.92008 5 Solvent adjustment theory 5.1 General When an acetylene cylinder is discharged, some of the solvent is carried over as vapor with the acetylene gas. In normal use with acetone as the solvent, this is estimated at approximately 3% loss per fill/discharge cycle, while DMF losses are much less due to its very low vapor pressure (1/60th that of acetone). Additionally, the loss can be much higher due to liquid solvent carryover if the cylinder is discharged too rapidly. The solvent content should be checked and restored regularly. When acetone is used, the solvent level is checked before each refill. Due to the lower solvent loss for DMF solvent cylinders, solvent quantity does not typically need to be adjusted every fill. To adjust the solvent level in cylinders, several properties shall be determined. 5.2 Cylinder temperature The temperature of the gas contained in a cylinder directly affects the amount of acetylene that can be dissolved, and therefore, the amount of residual gas in the cylinder. The average cylinder core temperature should be determined before measuring the pressure and attempting to adjust the solvent level in the cylinder. Cylinder temperatures should be determined by measuring the core temperature of a reference cylinder. A reference cylinder is made from a medium sized (about 10-in diameter) cylinder that is brought to tare with solvent and equipped with a quality dial temperature gauge with at least an 18-in long probe installed in place of the cylinder valve. This cylinder needs to be placed where it receives the same storage conditions as the cylinders to be weighed in, and where weigh-in personnel can read its indication during the day. 5.3 Cylinder pressure The pressure in a cylinder directly affects the amount of acetylene remaining in the cylinder. Each cylinder must therefore be checked for residual pressure. Common practice calls for use of a pressure gauge readable in 5 psig increments. 5.4 Cylinder weigh-in To gauge the status of the solvent charge in a returned cylinder, it is also necessary to obtain the actual cylinder weight. This weight can then be compared to the listed tare weight (Tare S). This difference is used to calculate the status of the solvent charge. Small cylinders (MC and B) can require a scale with a higher accuracy. 5.5 Residual acetylene In normal operation, ac