ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
PRESSURE VESSEL HAZARD REDUCTION
Pressure vessels must comply with all regulations, industry codes, and standards to keep vessels in safe condition to handle design pressures and temperatures. Areas to review could include, but are not limited to, the following:
At a minimum, pressure vessels should be designed in accordance with the American Society of Mechanical Engineers (ASME) Code for material contents of varying characteristics. Facilities should address any added concerns about the temperature and characteristics of vessel contents (e.g., toxic, corrosive, reactive, or flammable contents). When the vessel contents are changed from those the vessel as designed for, a risk analysis should be conducted to determine if it is still safe for the new materials.
Certification of Vessels
Many states have a boiler law, but others do not. In states with a pressure vessel law, all pressure vessels must be certified by the relevant state authority as meeting requirements of the ASME Code. When a pressure vessel cannot be constructed to comply fully with the ASME Code, however, the National Board Inspection Code (NBIC) provides a procedure by which the pressure vessel may get state approval without bearing the ASME symbol. This procedure includes submittal of drawings, calculations, welding procedures, service conditions, welding qualification and performance tests, and professional engineering certifications. This should be done before any construction begins.
When a facility finds an unmarked vessel or is about to bring one into a state, similar information plus the repair history should be submitted to the state pressure vessel authority for review and approval before use begins or continues.
On the other hand, when a pressure vessel is located in a state without a pressure vessel law, is not marked with the ASME symbol, and there are doubts about the safety of the vessel, the information listed above should be submitted to a pressure vessel consulting engineer and authorized inspector for a safety review.
Inspection of Vessels
The NBIC and American Petroleum Institute (API) 510 require that vessels be periodically inspected externally and internally. External inspections are made more frequently and Involve visual and nondestructive examination. An internal inspection is more difficult to perform because it usually requires a confined space entry and the vessel must be taken out of service, cleaned, and prepared. General or localized thinning of the internal walls due to corrosion or erosion is a potential problem and must be monitored, with records kept of the rate of thinning. When the vessel is reaching the end of its useful life, the period between inspections is shortened so that the vessel may be taken out of service before it can become dangerous. An internal test may also reveal stress corrosion, cracking, pitting, embrittlement, and other defects that could weaken the vessel. In addition to the vessel itself, the relieving devices must also be tested. When practical, this can be done in place for vessels containing non-hazardous substances, but for vessels containing hazardous substances without special controls (e.g., scrubbers), safety relief valves must be taken off to ascertain whether their settings are correct. How this can be done safely and conveniently should be considered.
In addition to maintenance requirements, the NBIC and API 510 include specific preheating and post heating requirements. Large temperature differences between the outside and inside surfaces of the vessel – during repair or other welding - must be avoided to minimize embrittling or stressing the metal. Nondestructive examinations may include radiographic, ultrasonic, liquid penetrant, magnetic particle, eddy current, visual checks, and leak testing.
Operation of Vessels
Operators should consider process start-up and shutdown conditions, possible process upsets, and any other unusual conditions that might cause overpressure problems. The ASME Code includes recommended pressure differentials between safety valve set pressures and maximum allowable working pressure, as well as the pressure differential settings of the relieving devices when there are multiple devices.
ATMOSPHERIC TANK IS NOT A PRESSURE VESSEL
On June 21, 2001, a worker died when a 500-gallon atmospheric storage tank he was emptying of waste oil and water exploded from its base. He was pressurizing the contents of tank using compressed air to speed up draining. Compressed air at 120 psi was used. The tank was not approved for use as a pressurized vessel. According to a co-worker, the practice of pressurizing the tank had been going on for 6 years without management’s awareness. The procedure had been passed from maintenance worker to maintenance worker.
The force of the explosion propelled the tank 500 feet in the air over the plant fence and a nearby bank parking lot onto a busy road. The event resulted in OSHA issuing one serious citation under the General Duty Clause for failing to establish, train on and enforce use of a procedure to empty waste oil containers which would prohibit application of air pressure to a container not designed or rated as a pressure vessel.
What Can You Do
Conduct job safety analyses and establish standard operating procedures for routine maintenance tasks and train the maintenance personnel in these procedures.
A job hazard analysis is a procedure used to review each job, identify potential hazards, and design actions and procedures to eliminate or control the hazards. Input from workers who usually perform the tasks is important. Of primary importance is the recognition that hazards exist. Even though maintenance work is complex and constantly changing, there are routine tasks. The transferring of waste oil and water was a routine task. A job hazard analysis may have identified the potential for employee injury pressurizing a tank that was not approved as a pressurized vessel.
Do not pressurize a container not approved as a pressure vessel.
This fatality involved the improper use of equipment. The task was commonly conducted, yet no one associated with the practice was aware of the consequences of pressurizing a vessel not approved for use under pressure. OSHA has regulations regarding the use and inspection of pressure vessels. ASME, the Compressed Gas Association, and the American Petroleum Institute among others have standards and guidelines governing the use of pressure vessels.
Train maintenance personnel to anticipate conditions that could jeopardize their safety or the safety of others.
A Maintenance Supervisor indicated he did not know pressurizing the tank was a dangerous practice or that anything could go wrong. Working with pressurized equipment requires a level of care and engineering knowledge which was not available in the Maintenance Department. Maintenance workers need safety training because they are continually presented with a complex and constantly changing set of activities. Maintenance workers travel throughout the entire plant in the course of their work. Properly trained, they can provide information that is beneficial to the other plant activities while conducting their work.
Inform employees that no equipment is to be altered or retrofitted. Establish a procedure for a qualified person(s) to review proposed equipment changes. Conduct periodic plant audits specifically for non-standard use of equipment.
When equipment is retrofitted, altered or used in a way for which it was not designed or for a purpose other than originally intended, unintentional consequences may result. A procedure for a qualified person(s) to review equipment change, modification or use should be developed and implemented. Since there is always pressure to “make do” with what is at hand or adapt what is available, periodic plant audits to detect unapproved equipment change or modification should be conducted.
LIQUEFIED GAS CYLINDER FAILURE
A liquid nitrogen cylinder in a university chemistry laboratory catastrophically failed due to over pressurization, causing substantial damage. Fortunately the incident occurred at 3 AM and the building was not occupied, so there were no injuries. The over pressurization blew out the bottom of the cylinder and propelled the cylinder upwards. The cylinder pressure relief valve and rupture disc had been replaced by two brass plugs at some time in the past by an unknown person. Before the incident, the cylinder may have been leaking through an old gasket, providing sufficient release of gas to prevent over pressure.
Approximately twelve hours before the explosion, the leaking gasket had been replaced and the cylinder refilled with liquid nitrogen. With the new gasket, the cylinder was now completely sealed, and pressure could build up. The cylinder ruptured when its internal pressure rose above 1000 psi. The catastrophic failure of the nitrogen cylinder was a result of the removal of the pressure relief devices.
Did you know?
- Liquefied and pressurized gas cylinders are commonly used in laboratories and in manufacturing plants.
- In this incident, the force released by the failure of the cylinder was estimated at 250,000 pounds (~ 113,000 kilogramsforce).
- Cryogenic storage must either be refrigerated to maintain the low temperature and pressure, or slowly bleed off enough vapor to maintain pressure and cool the remaining inventory.
- An incident this powerful can release other hazardous materials in nearby containers, vessels, and piping, causing an even more severe incident.
What can you do?
- Never modify any equipment containing hazardous materials or energy without qualified engineering evaluation, and always conduct a management of change review.
- If you observe a high pressure or liquefied gas cylinder that appears to have been modified, or is corroded or otherwise damaged, report it to supervision immediately so it can be removed from service.
- Ensure that cylinders are properly maintained and periodically inspected, including the pressure relief devices.
- If you use pressurized gas cylinders, make sure you are properly trained in the safe handling of high pressure cylinders.
- Share this incident with your colleagues in the laboratory who may use pressurized gas cylinders.
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Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers.With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.
Fabricated Projects Include:
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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.
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