SAFETY PRACTICES
Safety and Compressed Gases Safety in the transportation and handling o f compressed gases starts with the manufacture of the container. Containers complying with the requirements of the Interstate Commerce Commission specifications are safe only if periodically retested as provided by the ICC regulations, and are properly handled between times. By- using safe containers, a good classification of the gases, and local ordinances and national codes governing the storage o f compressed gases, many serious accidents can be avoided.
JQHN J. CROWE Air Reduction Co.,Znc., Union,N. J.
T
HE Bureau of Explosives defines a compressed gas as any material or mixture having in the container either an absolute pressure exceeding 40 Ib. per sq. inch at 70" F., or an absolute pressure exceeding 104 Ib. per sq. inch a t 130' F., or both; or any liquid flammable material having a Reid vapor pressure exceeding 40 lb. per sq. inch absolute a t 100" F. dlthough this definition is extensive, it is used for this discussion. The containers for the gas are confined to those manufactured mid maintained in conformance wit,h the specifications and regulations of the Interstate Commerce Commission ( 5 ) and the corresponding requirements of the Board of Transport. in Canada. The number one rule for the safe handling of any commodity is a safe container. Limiting this discussion t o portable containers, a range in size is found from less than an ounce capacityfor example, the small sparklets containing carbon dioxide-to the large ICC 3A oxygen cylinders, which are over 20 feet long mounted in multiple on trailer trucks and have a combined capacit,y of approximately 45,000 cu. feet' a t a pressure of 2400 lb. per sq. inch. The ICC regulations require that all containers be carefully inspected during manufacture and hydrostatically tested after manufacture. Only those meeting the rigid specifications of the ICC are acceptable for shipments of compressed gas in interstat,e commerce; in some states, these requirements also apply to intrastate shipments. After the cylinders go int'o service, the ICC regulations require t,hat they he inspected frequently and hydrostatically retested a t stat,ed intervals. For the type 3,4 and 3AA cylinders t,hat, are generally charged to pressures exceeding 2000 lb. per sy. inch, this interval is every 5 years. The hydrostatic test made at a pressure of 1 times the designed working pressure is carried out in a water jacket, and from the readings of the elastic and permanent expansions t'he all thickness of the cylinder is coniputed, Cylinders having a thin rrall are rejected. I n addition t,o the hydrosbatic test in a water jacket, each and every cylinder is a t this time critically examined both internally and externally for pitting or other defect's that may effect the service life of the cylinder, but have not been evident in the pressure test. These tests, backed up with a critical visual inspection and a ring test each time a cylinder is returned for charging, further ensure safet,y in the handling of compressed gases. VALVE CAPS
The cylinder containing the gas, unless of a t,ype that has a recessed valve or has a valve that has been exempted for other reasons, is delivered nit,h a valve cap. This cap, placed over the 230
valve, protects the valve from damage. A leaky valve can cause trouble and a broken off valve even 1%-orse trouble. Therefore, keep the valve protection cap on the cylinder until the gas is ready to be used and replace the cap on thc cylinder as soon as it is empty or is removed from service. VALVE OUTLETS
After many years of effort,, the Compressed Gas Association has come up with tn-o sets of standards for valve outlet connections, standards which cover practically all of thc more widely used gases. These standards make it impossible to attach a cylinder to an inlet intended for a gas having far different properties from that of the gas in the cylinder. The Compressed Gas Association standard (3)for the commercial type cylinder valve outlets has been adopted also by the American Standards rlsxociation as their standard ( 1 ) . For the flush type of valve used largely for medical gases another standard ( 4 )is designed to prevent interchangeability. Where these st.andards are in use, many accidents caused by using oxygen in oil accumulators instead of an inert gas, or of the use of some gas ot,her than oxygen for resuscitation practically have been eliminated. SAFETY DEVICES
Except for containers holding certain poisonous gas x-herc. the t,oxicity hazard has been appraised as being greater than the cylinder failure hazard, all containers of compressed gas are equipped with some form of safety device designed to relieve excessive pressure in t'he case of a fire. Here, again, the Compressed Gas Association has compiled standards ( 2 ) which cover eight different, devices, CG-1 to CG-8, and also lists the gases irith which each device may be used. But herein these are generalized and reduced to three basic types, n-it'h the advantages and shortcomings of each t,ype given in brief. Frangible Disks (CG-1). This device is designed t o rupt.ure and release the gas in the cylinder before the gas reaches the pressure a t which the cylinder was tested. This type of device is not practicable for lon- pressure cylinders because to function successfully at relatively low pressures, the disk must be made very thin or have a very large diameter. They work well on high pressure cylinders except in cases v-here a very intense flame impinges on the side wall and the steel weakens faster than the gas pressure increases. 41s0, if the cylinder is orily partly filled Jvith gas the device may fail to function because t,he st,eel in the cylinder reaches the rupture stress before the gas pressure is sufficiently high t o cause rupture of the disk.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 2
SAFETY PRACTICES For high pressure flammable or toxic gases, where the premature failure of the disk could be serious, the frangible disk should be backed up with a fusible alloy plug having a yield temperature of 165" F. for some gases and 212' F. for others. This type of safety device is also being used with gases that are neither flammable nor toxic but t o a lesser extent now than formerly. The development of the reinforced frangible disk used without a fusible metal backup, has done much to obsolete the older flat type in which premature failures were frequent and unpredictable. The frangible disk type of safety device is generally installed as part of the cylinder valve assembly. Fusible Alloy (CG-2, CG-3). This is usually a hollow metal plug with 1/4-inch external threads and a hexagonal head, which is filled with a fusible alloy that is designed to yield and blow out when i t reaches a temperature of 165" F. (CG-2) or 212" F. (CG-3). These plugs, usually placed in the top section of small containers and in both the top and bottom of the larger containers and occasionally in the cylinder valve, function a t the designed temperature regardless of the pressure in the cylinder, but have the disadvantage that the heat must reach the plug before it functions. Intense heat applied t o the cylinder wall, remote from the fusible plugs, can cause failure of the cylinder by building up the gas pressure within the cylinder and, a t the same time, reducing the rupture strength of the steel before the plugs reach the softening temperature. Safety Relief Valve (CG-6, CG-7). These devices used with compressed gas cylinders are much the same as those used for other purposes. While they relieve the excessive pressure in the cylinder caused b v a fire, they close and trap residual gas and a c t as a regulator in maintaining the pressure in the cylinder a t a value only slightly below the blow off setting of the valve. This pressure may, when the container walls have been heated to a high temperature, cause failure. T o overcome this objection, use is sometimes made of an auxiliary safety device of the fuse alloy type which, when it functions, relieves all of the pressure. Like the frangible disk type, the safety valve, with or without the addition of the fusible device, may fail to function when an intense source of heat is applied to a localized area. The frangible disk and safety relief valve have the advantage over the fusible alloy in that each prevents overfilling of the container with liquefied gases where a small increase in temperature would result in a high hydrostatic pressure. MARKING AND LABELING
All ICC containers are marked with a specification number such as ICC-3A2200, indicating in this particular case t h a t it is of seamless construction and designed for a filling pressure of 2200 lb. per sq. inch a t 70" F., and if certain test requirements are observed, may be filled to a pressure 10% higher than the marked pressure. I n addition to any color coding that the manufacturer may use to identify the contents of a container, the consumer should insist that every container be legibly labeled with the name of the gas, and refuse to use the gas from any container that is not properly labeled. STORING
Gas containers not in active use should be stored i n properly ventilated rooms or areas away from sources of heat, surh as radiators. Flammable gases should not be stored in the same room with oxygen, and in every case the storage facilities should comply with all local, state, and municipal requirements, and the standards of the Sational Fire Protection Association where applicable. HANDLING AND USE
Handling and use start with the unloading from the truck. If an unloading platform of truck level height is not available February 1956
Precautions for Safe Handling of Compressed Gases Start with a safe container Handle the container with care Store cylinders in well ventilated areas, maintaining a separation between oxidizing gases and flammable gases Keep cylinders away from radiators and heat ducts to avoid pressure increase Identify the gas by the label on the container Firmly secure cylinders in use to prevent falling Eliminate leaks-all more so than others
are dangerous, some
Use standard connections and avoid the use of adaptors
B y means of check valves, prevent the back flow of corrosive gases or liquids or other gases into the regulator and container
Do not use corroded regulators or other equipment, as they may be too weak to stand pressure Use special care in handling flammable or toxic gases, those with either very low or very high specific gravities, and those without odor Use a regulator to reduce the container pressure to working pressure; needle valves allow pressure to build up to container pressure if consumption i s stopped Protect the system supplied by a compressed gas container with a pressure relief valve-do not depend on the safety relief device on the regulator (If there is one, it is for the protection of the regulator and not for the system it is supplying) Keep containers away from electric circuits-a small arc burn on a cylinder may cause an incipient crack and eventually cause the cylinder to fail The obvious solution of many of the problems involved in the handling and use of compressed gases i s to manifold the cylinders a t a remote location and supply the gas to the point of use by means of a low pressure pipeline
INDUSTRIAL AND ENGINEERING CHEMISTRY
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and the cylinders must be unloaded to ground level, t,he containers should be eased onto a mat, but, not dropped. Haring received the cylinders in good shape and checked t,lie labeling, the cont,ents should be called by the proper name-for example, not air for oxygen or gas for acetylene. After making the necessary connections for the discharge of the gas (always using a regulator for reducing the pressure and carefully cleaning all mating parts I d o r e making up the connection), if leaks should be detected a t the valve stem or anywhere else 011 the cylinder, t,he cylinder should be prompt,ly taken out of doors, a tag at,tached noting the defect, and the supplicr notified. This may not seem to he particularly import,ant except in the case of flammable or toxic gases, hut the content,s oE an inert gas cylinder leaking into a small room could be a serious hazard by reducing the oxygen content, or the contents of less than one cylinder of oxygen could increase the flamma,bility :iiid decrease the ignition temperature of the room contents. 113th the connections made betn-een t.he cylinder and thc point of use, the cylinder valve should bc opened slo\\-ly and the syst,eni test,ed for leaks Lvith a solution of soap in water. This caution is particularly applicable t o oxygen, because in an at,nioPphere of oxygen most materials are flammable and the igi~ition temperatures are loxer than in air. I n the adiabatic compression of oxygen from atmospheric pressure at, room teniperat,ure to 2000 lb. per sq. inch the t,heoreticid temperature rearlied is over 1700" F. = TI
7'2
ignited by the flame a t the leak. This type of acrident, which is reported as a safety dcvicr faiiure, may occur hours after t,hc personnel have left the p1:tnt. .hother typical accident n-ith flammable gases is one in d i i c h the gas :it the lcak does not burn but the leaking gas is ignited by a source remote from the leak. The explosion which follows can be bad. Inert Gases. The greatest danger with this class of gascs i s That a leak will cause the oxygen concentration to be reduced t o a point where the attending personnel will black out or be asphj-siatrd. For example, recentl>-,nitrogen was being used t o drive a n i o i i ~ ethrough an electric condiiit. The mouse nent through) hilt the nitrogen following the mouse as siifficieiit to blticit o u t tl\vo n-orkmen in the receiving niarihold; one recovcrcil. ii caylinder of carhon dioxide accitle~~tally discharged in n cellar u - a q suficient, to cause a complete black-out in a man entc,iiiry to dctcrniirie m-hat n'as wrong. LITERATURE CITED
(1) . h i . Standards dssoc., S e w Ymk, S.Y . ,AS.%B5i.l (2) Compressed Gas Assoc.. Xew Yolk, S . Y . , Pamphlet, S-1 (3) Compressed Gas Aosoc.. Ne, Yoi.k, LGT. P., Pamphlet V-1 ( 4 ) Compressed Gas dssoc., Ten- Tork, l\i. P., Paiiiphlet V-3 ( 5 ) Bur. of Explosives, New Y o r k N. Y., C a m p l d l ' s Tariff 1-0. 9.
RECEITED for review hIarcl; 1 7 , ic1.i.i.
ACCEPTED Ilccember 2 3 , 1%3,5.
(5;) (-)
n - 1
T I and T2 = absolute temperattires M o r e and after coniprcssion PI and PZ = absolute pressures before ~ n aft d cr compression n = ratio of specific heats at const,ant pressure antl constant volume. for oyygen )L = npproximatrly I .4 80lving t,he equation:
T,= 2180" F. ahsolute t
=
1720" F
By opening the oxygen valve slo~vly,the rate of compression is held down (thereby affording an opportunity for the heat generated to be drained off) and the resulting temperature due to the compression is much Iou-er, A study of past accidents oftentimes suggests means of avoiding similar accidents, and in this connection, the discussion of a few accidents typical of the various types of gases is of interest. Oxidizing Gases. By far, the majority of accidents occurring in this class is with burned out regulator and valve seals caused by rapid compression of oxygen in the high pressure passages leading to the seats. Fires starting in the regulators or viilvcs oftentime spread to other parts of the system. Another type ol accident is caused by using oxygen instead of an inert gas S1li.h as nitrogen or carbon dioxide for pressurizing systems containing oils or other readily oxidizable materials. Still another type of accident, occurring less frequently but usually with serious consequences, is that resulting from fires in oxygen enriched areas. For example, a small tube conveying oxygen t o a recorder in a superintendent's office developed a leak iind increased t'hc oxygen concentration i n the room. Later, a watchman iliaking his rounds stepped on a book of safety ma,tc!ics; he was very lucky t o get off with 6 months in the hospital. Flammable Gases. Obviously, leaks can be dangerous with any gas but they are particularly serious with flammable gases for the reason t h a t small leaks at connections t o the cylinder valve or regulator may be a?cidentally lighted from sparks or other sources of ignition and burn undetected for hours without causing trouble. However, eventually the safety devices function and the large volume of escaping gas is almost immediately 232
C O U R T E S Y M I N E S A F E T Y APPLIANCES CO. A N D I S M
Explosimeter detects presence of cornlPustible gases antl
signals any dangerous concentration
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 2
