Vendor Selection and Custom Manufacturing

Ammonia is classified as an Extremely Hazardous Substance (EHS). Consequently, the fabrication of any system handling it—from industrial units to large-scale chemical plants—is not a standard manufacturing process. It requires stringent adherence to specialized codes and demands an uncompromising focus on material quality, welding integrity, and vendor expertise to ensure safety, reliability, and regulatory compliance.

I. Criticality of Vendor and Material Selection

Given the toxicity and corrosive nature of ammonia, the selection of vendors and materials is the most critical step in fabrication.

1. Vendor Qualification is Non-Negotiable

• Specialization: Vendors must possess documented experience and expertise specifically in building systems for highly hazardous chemicals, not just general pressure equipment. This includes a deep understanding of OSHA's Process Safety Management (PSM) and the technical requirements of the ANSI/IIAR (International Institute of Ammonia Refrigeration) standards.

• Certification: Fabricators must hold certifications for the specific codes governing the equipment they are producing, such as the ASME Boiler and Pressure Vessel Code (BPVC), particularly Section VIII for pressure vessels, and ASME B31.3 & B31.5 for piping.

2. Material Selection: Avoiding Incompatibility

Anhydrous ammonia is extremely corrosive to certain metals, especially when moisture is present. The fabrication must strictly utilize materials that resist this corrosion:

• Prohibited Materials: Copper, brass, and bronze are strictly prohibited in components that come into contact with ammonia. Even trace amounts of moisture can lead to rapid corrosion and system failure.

• Standard Materials: Systems are primarily constructed from carbon steel or stainless steel, selected based on the specific operating temperatures and pressures to ensure long-term mechanical integrity.

II. Custom Manufacturing and Technical Requirements

Ammonia systems often involve custom-built apparatuses (e.g., vessels, chillers, separators, vaporizers, pump skids, injection skids) that demand advanced fabrication techniques to prevent leaks and catastrophic failure.

1. Advanced Welding Procedures

Welding is the most critical process in pressure vessel and piping fabrication.

• Code Compliance: Welding must be performed by certified welders using documented and qualified procedures that meet the requirements of the ASME or other applicable codes.

• Post-Weld Treatment: Containers exceeding certain size thresholds (e.g., 36 inches in diameter or 250 gallons capacity per OSHA 1910.111) often require stress relieving after fabrication to remove internal stresses introduced during the welding and forming processes. This prevents Stress Corrosion Cracking (SCC), a major failure mechanism in ammonia service.

2. Non-Destructive Testing (NDT)

Rigorous NDT is essential to verify the integrity of every critical weld and component before the system goes into service. This is a vital part of Mechanical Integrity (MI) assurance under PSM.

• Techniques Used: Common NDT methods for ammonia equipment include:

• Ultrasonic Testing (UT): Used to detect internal flaws, gauge remaining wall thickness, and assess weld integrity.

• Radiographic Testing (RT): Employs X-rays or gamma rays to check for internal cracks, voids, and inclusions in welds.

• Liquid Penetrant Testing (PT): Used to find surface-breaking defects on non-porous materials.

3. Protection and Preparation

• Cleaning and Purging: All components must be thoroughly cleaned and dried internally before final closure to remove moisture, scale, and contaminants that could lead to corrosion or system performance issues.

• Protective Barriers: Proper application of external protective coatings and specialized insulation (if applicable) is critical, as insulation failure (in refrigeration systems) can lead to water ingress, external corrosion of the steel (Corrosion Under Insulation, or CUI), and subsequent leaks.

Nameplates: The final fabricated system must bear permanent nameplates detailing the manufacturer, date of fabrication, design pressure, and water capacity, providing the necessary process safety information for the owner's operational and regulatory compliance records.