TBMs have revolutionized the tunneling industry in recent years and as the technology has developed many contracts are requiring the TBM to have the capability to do compressed air interventions. Contractors have found the regulations governing compressed air work in the US puzzling, and they are right, it is a bit of a bewildering system. This overview will try and address the issue.
The heart of the regulatory system in the US is The Occupational Safety and Health Administration, usually known as ‘OSHA’. OSHA is the federal government agency with the responsibility and the authority to set and enforce workplace health and safety standards. OSHA has authority in every state; however, each state also has the option of creating its own state OSHA system. If a state choses to do this Federal OSHA must approve and monitor each states plan. For a state to get their own program approved they must have safety and health regulations at least as effective as the federal OSHA program. Twenty-two states currently have their own plan covering private sector workers.
Most of the state plans have standards that are the same as the Federal OSHA’s, some have standards just slightly different from Federal OSHA and a couple have standards that are significantly different from Federal OSHA. States may raise the required level of worker protection from that of the Federal OSHA program, but they cannot lower it below the protection required by Federal OSHA. If a contractor is working in a state that has its own plan, then it looks to State OSHA for the regulations it must follow. If it is operating in a state without its own plan, it looks to Federal OSHA for rules.
The Federal OSHA regulations and all other federal rules and regulations are part of the Code of Federal Regulations (CFR). These rules are treated by the courts as being legally binding as law. The CFR is divided into 50 titles with each title representing broad areas subject to Federal regulation. The safety and health regulations for construction are found in title 29, part 1926. The rules specially for compressed air tunneling are found in title 29, part 1926, section 803. This would usually be written as 29 CFR 1926.803.
The compressed air regulations were written for a style of tunnel construction not often used anymore. They were written for a time when a section of the tunnel was isolated with a pressure bulkhead, the tunnel itself was pressurized and the tunnel was essentially dug by hand. Entire work crews would pass into and out of the pressurized area and often work their entire shift under pressure. Modern TBMs have changed that. It is common now for the tunneling crew to spend almost all their time working at a safe one atmosphere pressure, with small crews only occasionally making ‘interventions’ into the pressurized excavation chamber housing the cutting head, usually to inspect or change the cutters. TBMs are not mentioned in the Federal OSHA Regulations. Both tunneling and hyperbaric technology have changed greatly in the years since the regulations were written. Even though the regulations are outdated, they still apply. The contractor must apply for and receive a variance from either Federal or State OSHA for any regulations that it does not comply with.
The general layout of most compressed air work involving larger diameter TBMs is to have a man lock (man lock is another word for decompression chamber) mounted on a bulkhead near the front of the machine. The workers, usually two or three at a time, enter one end of the man lock, are pressurized and then exit the other end of the man lock either directly into the excavation chamber of the TBM or into a pressurized ‘motor room’ (housing the drive motors for the TBM) that is directly behind the excavation chamber (OSHA calls this the ‘working chamber’). If they enter the motor room, they must next carefully open a hatch separating the
motor room from the excavation chamber to access the excavation chamber and begin work.
On the surface, near the entrance of the tunnel, there is a compressor plant supplying the breathing quality compressed air to the tunnel and also a "medical lock". The medical lock is a decompression chamber that is kept ready to treat decompression sickness or, perhaps, other medical emergencies if the worker also has a decompression obligation that must be dealt with.
It has become a common practice on larger tunnels to have a rail mounted hyperbaric evacuation shuttle that can transport workers under pressure from the tunnel to the medical lock. This provides a means to safely transport workers with a decompression obligation if the tunnel must be evacuated for some reason or the worker has an injury that is too serious to deal with in the tunnel. This evacuation shuttle is not directly required by the OSHA regulations, but it has become a common practice and could be required under the General Duty Clause, particularly for tunnels working at higher pressures.
The General Duty Clause
An important part of the Federal OSHA program is a catch-all requirement that covers all conditions. This is the ‘General Duty Clause’. The General Duty Clause reads, "Each Employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or likely to cause death or serious physical harm to his employees."
Sometimes there is a hazard, but OSHA has no specific rule or standard dealing with it. Under the General Duty Clause, the employer has an obligation to protect workers from serious and recognized hazards even when there is no standard. Employers must take whatever action is feasible to eliminate these hazards.
Some Federal OSHA Highlights
The physician
"There shall be retained one or more licensed physicians familiar with and experienced in the physical requirements and the medical requirements of compressed air work and the treatment of decompression illness. He shall be available at all times while work is in progress in order to provide medical supervision of employees employed in compressed air work." 1926.803(b) (1)
"No employee shall be permitted to enter a compressed air environment until he has been examined by the physician and reported by him to be physically qualified to engage in such work." 1926.803(b) (2)
The medical lock
"A medical lock shall be established and maintained in immediate working order whenever air pressure in the working chamber is increased above the normal atmosphere." 1926.803(b) (9)
The medical lock must "have 6ft (1.9m) of clear headroom at the center…." 1926.803(b) (10) (i)
"Be kept ready for immediate use for at least five hours subsequent to the emergence of any employee from the working chamber." 1926.803(b) (10) (iv)
"Be designed for a working pressure of 75 p.s.i.g." 1926.803(b) (10) (vii)
"Be equipped with a manual type sprinkler system that can be activated inside the lock or by the outside lock tender" 1926.803(b) (10) (x)
"Be provided with oxygen lines and fittings leading into external tanks. The lines shall be fitted with check valves to prevent reverse flow. The oxygen system inside the chamber shall be of a closed circuit design as to automatically shut off the oxygen supply whenever the fire system is activated." 1926.803(b) (10) (x)
"The medical facility shall be equipped with demand-type oxygen inhalation equipment…" 1926.803(b) (10) (xiv)
"Be provided with sources of air…which are capable of raising the air pressure in the lock from 0 to 75 p.s.i.g. in five minutes." 1926.803(b) (10) (xvi)
The compressor plant and air supply system
"…air compressor units shall have at least two independent and separate sources of power supply and each shall be capable of operating the entire low air plant and its accessory systems." 1926.803(h) (3)
"The capacity, arrangement, and number of compressors shall be sufficient to maintain the necessary pressure without overloading the equipment and to assure maintenance of such pressure in the working chamber during periods of breakdown, repair, or emergency." 1926.803(h) (4)
"Duplicate low pressure air feedlines and regulating valves shall be provided between the source of air supply and a point beyond the locks…" 1926.803(h) (6)
The man lock
"Except where air pressure in the working chamber is below 12psig, each man lock shall be equipped with automatic controls which…shall automatically regulate decompressions. It shall also be equipped with manual controls…" 1926.803(g) (1) (iii)
"The man lock shall be large enough so that those using it are not compelled to be in a cramped position, and shall not have less than 5ft (1.6m) clear headroom at the center and a minimum of 30 cubic feet of air space per occupant." 1926.803 (g) (1) (ix)
"A…continuous recording pressure gauge with a four-hour graph shall be installed outside of each man lock… A copy of each graph shall be submitted to the appointed physician after each shift." 1926.803 (g) (1) (v)
"Man locks shall be equipped with a manual type fire extinguisher system that can be activated inside the man lock and also by the outside lock attendant. In addition, a fire hose and portable fire extinguisher shall be provided inside the and outside the man lock." 1926.803 (l) (9)
"Equipment, fixtures, and furniture in man locks …shall be constructed of noncombustible materials." 1926.803 (l) (10)
The special decompression chamber
"A special decompression chamber of sufficient size to accommodate the entire force of employees being decompressed at the end of a shift shall be provided whenever the regularly established working period requires a total time of decompression exceeding 75 minutes." 1926.803 (g) (1) (xvi)
"The headroom in the special decompression chamber shall not be less than a minimum 7ft (2.2m) and the cubical content shall provide at least 50 cubic feet of airspace for each employee." 1926.803 (g) (2) (i)
OSHA defines this "special decompression chamber" as "A chamber to provide greater comfort of employees when the total decompression time exceeds 75 minutes." It is a remnant of old style tunneling, however, it is still a current regulation and still a requirement.
Decompression issues
"No employee shall be subjected to pressure exceeding 50 pounds per square inch except in emergency." 1926.803 (e) (5)
It is not uncommon for modern compressed air work to approach or exceed this 50psi limit.
"Decompression to normal condition shall be in accordance with the Decompression Tables in Appendix A of this subpart" 1926.803 (f) (1)
The OSHA decompression tables in Appendix A are obsolete. They use a slow, continuous, reduction in pressure until atmospheric pressure is reached to accomplish the decompression. The workers breathe only air during the decompression. They have a history of producing both an unacceptable rate of decompression sickness and also dysbaric osteonecrosis, a potentially crippling condition caused by death of portions of the bone.
Modern decompression tables use staged decompression where the worker stops at a specific pressure for a specific amount of time, than the pressure is reduced and he spends a specified amount of time at the lesser pressure. The process continues, following the decompression table, until he reaches atmospheric pressure. Modern decompression tables also use 100 per cent oxygen as the preferred breathing gas during parts of the decompression. Oxygen decompression results in both a more efficient and a much shorter decompression.
The state of California has adopted the oxygen decompression tables from Revision Six of The U.S. Navy Diving Manual as mandatory for compressed air work.
The variance process
A variance is a regulatory action that permits an employer to deviate from the requirements of an OSHA standard under specified conditions. It is common in compressed air work for contractors to request variances from parts of the OSHA standard that they feel do not apply on their project or they find unreasonable or inadequate for some reason. Some examples of variance requests might be the requirement for the special decompression chamber, the 50psi pressure limit and the requirement to use the OSHA decompression tables.
In order to get a variance the contractor must demonstrate that the proposed alternative means of compliance provides its workers with safety and health protection that is equal to, or greater than, the protection afforded to them by compliance to the standard from which they are seeking a variance.
It will probably be much easier to obtain a variance in states that have their own OSHA plan. In states that rely on Federal OSHA the variance process is more involved and may take six months to a year even if the process goes smoothly.
Pressure vessel standards
Although it is not part of the compressed air work regulations, OSHA does require that decompression chambers used in tunneling, both man locks and medical locks, comply with the American society of Mechanical Engineers Boiler and Pressure Vessel Code or equivalent. The ASME Safety Standard for Pressure Vessels for Human Occupancy, usually called ASME PVHO-1, is the applicable code.
The ASME PVHO-1 standard covers almost every aspect of the construction of the decompression chamber. It gives the standards for the pressure vessel shell, the examination of the welds and the testing of the pressure vessel. The only windows allowed are acrylic plastic designed, manufactured and tested to the code requirements. It specifies the piping system design and piping material. Most piping must be non-ferrous, such as stainless steel or brass.
Many states will require inspection by a state pressure vessel inspector before allowing the chamber to be used.
Change may be coming
Federal OSHA recognizes that its standards for compressed air work need updating, but the process of change is onerous. Even so, change is happening. Federal OSHA has asked the ASME to review the pressure vessel requirements associated with tunnel boring machines and the ASME has formed a task group to do this, Washington state recently held a ‘stakeholders’ meeting to give interested parties a voice in what changes should be made to their state OSHA program, and California has adopted the U.S. Navy decompression tables. All these are small changes but, taken together, they indicate a growing awareness that the regulations badly need updating and an interest in doing so.
The next few years will be interesting and could influence the course of compressed air work in the United States for many years to come.