Where identification of a compressed gas is necessary, safety of the sampling effort becomes a critical concern. Because of the chemical characteristics of some gases, fire or explosion or other release of toxic gas may occur during sampling. When cylinders or valves have deteriorated, it is very possible to have failures resulting in uncontrolled release of the contents.
Procedures and technologies that have historically been used for management of cylinders in poor condition have not always met adequate safety standards. These techniques have included uncontrolled release, valve removal, and uncontained tapping.
Uncontrolled release has historically been the methodology employed for problem cylinders. This includes penetration of the cylinder shell with projectiles or explosives. Variations have provided this penetration in conjunction with fire pits. None of these provide for protection of the environment and rely upon distance for protection of personnel.
One proposed variation of this technique involves explosive detonation inside a "gastight" bomb chamber. With some gases, however, the energy released by the detonation can easily exceed the maximum rating of any chamber that can be built.
Valve actuation inside a containment unit can be accomplished in a foreign-designed overpack vessel. This procedure is ineffective where the valve is blocked, defective, damaged, or failed in a closed position. Experience has also raised concerns over failures associated with gases passing through dynamic seals of the containment unit. Major compressed gas companies with this type of equipment will not permit its use with many hazardous gases or for unknown cylinders.
In a variation of this technique, the valve may be removed rather than actuated. It is possible, however, for the valve to break off or for the cylinder orifice to be completely blocked. Either of these problems will result in a much more hazardous situation following the attempt.
One often proposed technique involves cold or hot tapping of the cylinder. A similar technique is used for low pressure gas lines. Guidelines developed by the American Petroleum Institute (API) clearly demonstrate, however, that it is inappropriate and highly dangerous in situations involving high pressure cylinders with unknown contents.
Guidelines for tapping are published in API Publication 2201 (third edition, October 1985). Burn-through prevention cannot be ensured unless base metal thickness is greater than 3/16 in. Very few cylinders meet this requirement. The procedure requires that the base metal be free of laminations, corrosion, and other imperfections.
Further, hot tapping cannot be done where cylinder contents may contain oxygen-vapor-air mixtures in an explosive range; hydrogen, acids, chlorides, peroxides, or chemicals that may explosively decompose (including acetylene); caustic soda or amines; or unsaturated hydrocarbons (e.g., ethylene). Unless the contents and pressure are absolutely known, this procedure cannot be used.
The cold tapping procedure may be used for known low pressure gases. It is crucial, however, that the cylinder contents be known with certainty. For example, a high pressure inert gas, often used with some liquefied gases, could overpressure the device. Because of the tenuous nature of the gasket seal, a suitable surface must be prepared. As with any extraordinary procedure, provision must be made for emergency containment and response.
A proposed variation of this procedure is installation of a new valve in the body of the cylinder. Along with previously cited technical objections, this is hazardous because the cylinder shell may have been weakened by corrosion.
A major problem with these techniques is graphically demonstrated by reactions involving strong oxidizers. Even in an inert environment, exposure of bare metal to fluorine gas will result in an ignition that destroys the cylinder shell and any associated appurtenances.
Adequate records and labels documenting the origin and use of cylinders may be sufficient for identification of cylinder content. If, however, there is a potential for mixing of different gases (e.g., by common manifolding) or other reason to suspect the accuracy of records, further efforts to obtain confirmatory identification are necessary.
Prior to developing a sampling approach it is desirable to obtain as much information as possible as to possible contents. Inventory records, usage history, and personnel interviews may be useful. This type of information may allow the investigation to be focused on probable contents.
After background information has been obtained, the cylinder should be inspected. Depending upon the circumstances, personal protective equipment, including supplied air and encapsulating suits, may be required. A high level of protection is appropriate when the cylinders potentially contain hazardous materials (this must be assumed unless adequate information to the contrary exists) and if the cylinders are stored in an enclosed area.
The inspection should be conducted in a manner that minimizes disturbance of the cylinders. Cylinders in poor condition may either fail or begin to leak if moved. Simply moving cylinders with unstable contents (e.g., unstabilized hydrogen cyanide or tetrafluorohydrazine) can cause detonation of the contents.
It is especially important to avoid manipulation of the cap or valve until it has been determined that the cylinder is stable and emergency response procedures are in place.
The person conducting the inspection should be familiar with compressed gases and cylinders. The inspector should be sufficiently experienced to evaluate the potential hazards associated with these containers and their contents. As with other potentially hazardous operations, the inspection should not be completed without experienced, professional assistance.
Labels can be used for preliminary indication of contents. The labels will typically be applied to the body or shoulder of the cylinder. Unfortunately, these are not indestructible and may no longer be either readable or even present. Labeling may also be misleading where it has not been updated or contaminants have been introduced.
Some cylinder information is stamped into the body of the cylinder. DOT requires that the cylinder type, service pressure, serial number, test date, and manufacturer by identified on the body of each cylinder. Although these are usually preserved, in some cases corrosion can obscure this information.
DOT has developed specifications for cylinder types based on classes of gases. DOT has also adopted recommendations of the Compressed Gas Association for valve types to be used with most gases.
An example of the information obtained from valve type is shown by identifying a cylinder as a DOT Type 3AA with a CGA 580 valve. These cylinder and valve types are specified for inert gases such as nitrogen. It should not, however, be assumed that this will always be the nature of the contents. It is not uncommon for cylinders to be filled outside of these guidelines.
These conventions do not apply to cylinders of lecture bottle size. These cylinders contain only small volumes (less than 1/2 L liquid volume).
In recent years a color code has been adopted and used for medical gases. Color of the cylinder is typically not, however, a reliable indicator of contents. Most manufacturers have different color codes, and these may not have been historically uniform.
Some "experts" offer cylinder identification based solely on visual inspection. Relying on this type of identification is risky and negligent. The technical director for the Compressed Gas Association has written that these consultants are like "snake oil salesmen."
The general appearance of the cylinder is important to developing a safe sampling plan. The overall condition of the cylinder should be noted, especially with respect to denting or corrosion. Evidence of exposure to fire may be presented by burn marks. If possible the valve should be examined without unduly disturbing the cylinder.
If the possibility of radioactive gases cannot be eliminated, the initial inspection should include a scan for external radiation. Any detection of radioactivity should be cause to develop appropriate protective procedures.
Evaluation of information obtained during the inspection will permit preparation of a suitable sampling plan. The sampling approach is largely dependent upon the degree of certainty with which the cylinder contents can be deduced and the potential hazards associated with a release.
Only cylinders in good condition should be considered for sampling through the cylinder valve. Handling should be minimized in any case until the contents have been identified.
Safely obtaining a suitable sample is the principal challenge in the identification process. The degree of hazard is largely dependent upon the contents of the cylinder. If there is no indication of the contents, a worst-case approach must be used. As with sampling of other types of unidentified pressurized wastes, sampling must be completed using remote techniques with an explosion-resistant barrier [see regulations promulgated by the Occupational Safety and Health Administration, 29 CFR 1910.120(j)].
Cylinder carts or forklifts with suitable attachments are useful for moving larger cylinders. During any handling the cylinder should be monitored for any temperature increases associated with polymerization of unstable gases.
Unless the nature of the cylinder contents can be determined with reasonable certainty at this stage, remote sampling procedures are required. The mere operation of a cylinder valve has been known to result in detonation. Incidents of this type have included gases such as hydrogen and ethylene oxide and mixtures such as a mixture of deuterium and oxygen.
Accessing the cylinder valve may pose a potential problem. It is common for the protective cap to corrode to such an extent that it cannot be unscrewed.
Upon removal of the valve cap, the valve itself should be carefully inspected. To be considered for the remote valve sampling operation, valve threads must be in good condition with an adequate sealing surface. Discoloration, excessive valve corrosion, or damaged threads are cause for discontinuing the operation.
The construction of sampling apparatus must be compatible with the expected gas. If the contents are unknown, universal compatibility is required. This typically includes the use of stainless steel, passivated steel, or Teflon that has been cleaned of all contaminants. The Compressed Gas Association (CGA) specifies cleaning procedures for use in oxidizing environments (oxygen or fluorine service). Further, all components must be capable of safely containing the maximum pressure that may be exerted by the compressed gas.
It is critical that the atmosphere within the sampling system be inert. Exposure of pyro-phoric gases (gases that can be ignited at room temperatures) to air can result in combustion or explosion.
Obtaining a minimal sample quantity at less than atmospheric pressure is desirable. This can be accomplished by using appropriate controls. Should leakage occur during sample transport, it will be into the sample container.
Provisions should be made for response to valve failures during sampling. It is not uncommon for leakage to occur following valve actuation, especially with packed valves. Treatment or containment options should be provided for this eventuality. It is also possible for a valve to fail during opening, preventing closure. Roughly 1-3% of sampled waste cylinders have experienced some type of valve failure.
For the remote operation to provide protection against potential explosive reactions that could occur during sampling, the cylinder must be isolated. This can be accomplished by erecting barriers such as sandbags or a containment chamber. An example of a mobile sampling system is shown in Figure 1.
A valve opening mechanism consisting of a pneumatically actuated wrench has been developed. The wrench contains adapters to allow it to fit various types of valves. The control is operated from a remote location outside the protective barrier.
Valve movement does not always indicate that the cylinder contents have been accessed. This must be verified by positive means. It is possible that there is internal blockage or the valve stem has broken inside the valve body.
The remote valve opener should be constructed to permit maximum application of torque without overstressing the valve. If this mechanism is incapable of actuating the valve, it cannot be safely opened.
Cylinders or valves that are in poor condition require special handling procedures. These procedures are also required where the valve is inaccessible or cannot be operated.
A patented device has been developed for sampling under these circumstances—the Cylinder Recovery Vessel (CRV). This equipment permits remote release of the gas in an inert contained environment. Figure 2 shows the key components of this sampling system.
The Cylinder Recovery Vessel was designed to control and contain all common compressed gases and liquids. The system provides for remote release and recontainerization of pressurized gases and liquids in a completely contained inert environment.
The CRV is an ASME-rated pressure vessel. The waste cylinder can be pierced by a drilling mechanism housed within the vessel. Prior to drilling, a vacuum is obtained and an atmosphere of inert gas is introduced. All internal components are hydraulically actuated, removing possible sources of ignition.
The interior of the vessel and its associated systems are composed of passivated steel, stainless steel, or Teflon. All hydrocarbons and other reactive materials are excluded from possible contact with cylinder contents. The hydraulic fluid used for CRV components is an inert fluid.
A secondary containment chamber houses the CRV and its systems. This reinforced steel chamber is sealed to contain any release from the primary system. All of the equipment in the chamber is suitable for operation in a Class I, Division II explosive environment. Ports are attached so that any released gases can be withdrawn and treated in the unlikely instance of a leak in the primary system. Temperature inside the chamber can be controlled by a heating and cooling unit.
All operations are controlled from a panel located outside the trailer. In this manner personnel are isolated from the sampling operation.
After contents of the cylinder are released inside the CRV, a sample can be withdrawn through a port extending to the exterior of the trailer. An evacuated sample cylinder attached to the port is opened to obtain a small volume of the contents for analysis. On-site analytical equipment provides an identification of the contents within minutes.
The CRV provides for recontainerization of both gases and liquid cylinder contents. The cylinder contents are transferred to a new DOT-approved cylinder for subsequent disposal. The contents can also be transferred to an appropriate treatment system.
Appurtenances to the vessel include a cylinder clamping and locking mechanism and a roller mechanism for rotating liquid cylinders. Diaphragm compressors complete the purging and recontainerization process.
Included in the operation are controls to verify the functioning of the equipment. The entire operation is monitored by a remote video camera that shows the interior of the CRV. Pressure and temperature are monitored at numerous points in the process system. These systems allow the operator to effectively control the process.
When the cylinder processing has been completed, the empty target cylinder is removed from the chamber. The cylinder is then cut into halves with a power saw. Any solid residue inside the cylinder is removed and containerized. The clean, empty remains of the cylinder is containerized and staged for disposal.
Analyses of gases can be completed with a variety of instrumentation. Typical techniques used include mass spectroscopy (MS), Fourier transform infrared spectroscopy (FTIR), gas chromatography (GC), and wet chemical testing.
Gas chromatography is useful when the identity of the gas is known or suspected. The technique provides for selective absorption and elution in a column. Typically a standard of the test gas is required.
The mass spectrometer (or residual gas analyzer) is a vacuum analyzer that will measure total pressure and partial pressure. The analyzer is typically capable of separating the ions
formed in an electron impact source according to their mass-to-charge ratio. The signal collector may be either a Faraday cup or a secondary emission multiplier.
The FTIR can be used for identification of unknown gases based on absorbance spectra. The infrared spectrum contains characteristics that permit identification of the functional groups or "working parts" of molecules. Through the use of an interferometer, infrared wavelengths are passed through a sample simultaneously.
After obtaining the sample transmittance, the spectrum is mathematically converted to absorbance. The absorbance spectrum is then compared to spectra contained in various libraries. This can be accomplished through computer programs.
Wet chemical methods can also be used. Typically this will involve reaction with a variety of reagents and calorimetric materials. These are inexpensive procedures that can be used if the gas is known. With unknown gases the methodology can be extremely hazardous.
Information generated in the analytical process may be insufficient to provide an identification of the cylinder contents. In many cases it is necessary to have associated observations to reach a reasonable interpretation of the results.
A typical problem in obtaining an analysis is related to the reactivity of the sample gas. Many gases will react during the sampling or analytical process to form other compounds. The analysis will generally show the reaction products and not the cylinder contents.
An example of the problems associated with obtaining a sample of a reactive gas is illustrated by fluorine. Because of its extreme reactivity, it is likely that the sample will react with some contaminant (residual water or air or unpassivated surfaces) during the process. The gas can similarly react within the analytical equipment, even to the point of damaging the instruments. Analysis may show reaction products such as hydrogen fluoride.
Most analytical libraries of spectra are incomplete with respect to compressed gases. Further, depending upon environmental conditions, the spectra may appear to be different. Complicating the analysis are the many potential mixtures that may be present. This can lead to some ambiguity in interpretation.
After tentatively identifying a compound, it is important to relate it back to the physical characteristics observed during sampling. For example, it is easy to confuse the variety of hydrocarbon spectra. If significant pressure was noted during sampling, hydrocarbons with a low vapor pressure can be eliminated. The same conclusion cannot, however, be reached where there are additional components indicated (e.g., metal anhydrides may be dissolved in an organic solvent).
All of the observations made during the analytical process should be combined with experience with compressed gases and cylinders to reach a reasonable interpretation.
The sampling and identification process is fraught with potential hazards for the unsuspecting. It is crucial that those involved with the sampling be professionals with extensive experience in handling compressed gases.
Unfortunately there have been examples of contractors misrepresenting their experience and capabilities. Given the ramifications and liabilities associated with an accident, careful screening of contractors is imperative.
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