This blog is to provide fundamental information and comprehension of the ASME code governing the design of pressure vessels in the chemical and process industry. This code is applicable in the United States, as well as most of North and South America.
Hazard Reduction for Pressure Vessels:
To ensure the safe operation of pressure vessels under design pressures and temperatures, it is imperative to comply with all regulations, industry codes, and standards. Various aspects that should be reviewed include, but are not limited to, the following:
Pressure vessels should be designed in accordance with the American Society of Mechanical Engineers (ASME) Code, considering the characteristics of the materials involved. Facilities should also address any specific concerns related to the temperature and characteristics of the vessel contents, such as toxicity, corrosiveness, reactivity, or flammability. Whenever the vessel contents are changed from the originally intended materials, a risk analysis should be conducted to assess the safety implications.
Certification of Vessels:
In states with pressure vessel laws, all pressure vessels must be certified by the relevant state authority to ensure compliance with the ASME Code. However, when a pressure vessel cannot fully adhere to the ASME Code, the National Board Inspection Code (NBIC) provides an alternative procedure for state approval without utilizing the ASME symbol. This procedure involves submitting drawings, calculations, welding procedures, service conditions, welding qualification and performance tests, as well as professional engineering certifications. It is essential to complete this process before commencing any construction.
In situations where an unmarked vessel is discovered or when bringing a vessel into a state without a pressure vessel law, the aforementioned information, along with the repair history, should be submitted to the state pressure vessel authority for review and approval prior to use.
Inspection of Vessels:
Both the NBIC and the American Petroleum Institute (API) 510 require periodic external and internal inspections of pressure vessels. External inspections, involving visual and nondestructive examinations, should be conducted more frequently. Internal inspections are more challenging as they necessitate confined space entry and require taking the vessel out of service, cleaning it, and making necessary preparations. Monitoring of internal wall thinning caused by corrosion or erosion is crucial, and records should be maintained to track the rate of thinning. As a vessel approaches the end of its useful life, the interval between inspections should be shortened to ensure it is taken out of service before posing any danger. Internal inspections may also reveal defects such as stress corrosion, cracking, pitting, embrittlement, and other issues that could weaken the vessel. Additionally, relieving devices must be tested. While it is feasible to conduct these tests in place for vessels containing non-hazardous substances, vessels containing hazardous substances without special controls require the removal of safety relief valves to verify their correct settings. The safety and convenience of conducting these tests should be carefully considered.
In addition to general maintenance requirements, the NBIC and API 510 prescribe specific preheating and post-heating requirements. It is crucial to avoid significant temperature differences between the vessel's outer and inner surfaces during repairs or welding to prevent embrittlement or stress on the metal. Nondestructive examinations may involve techniques such as radiographic, ultrasonic, liquid penetrant, magnetic particle, 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 may lead to overpressure issues. The ASME Code provides recommended pressure differentials between safety valve set pressures and maximum allowable working pressure, as well as pressure differential settings for relieving devices in situations.
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