ISO 15761 is a standard for small-bore steel valves used in the oil and gas industry, covering sizes from DN 15 to DN 100 and pressure classes from Class 150 to Class 2500. It applies to gate valves, globe valves, and check valves.

 

These valves are not produced in a single step, but through a sequential manufacturing chain. The quality of each stage directly affects the next. Understanding this chain helps identify critical issues more efficiently during valve selection, compliance review, and supplier evaluation.

 

Complete Manufacturing Process

 

Step 1: Material Selection

 

Material determines the applicable service conditions and is the starting point of the entire process.

Common materials under ISO 15761 include:

●  Carbon steel for general oil and gas service

●  Low-temperature carbon steel for cryogenic or low-temperature conditions (e.g., LNG applications)

●  Stainless steel for corrosive media

If the service contains hydrogen sulfide (H₂S), materials must also comply with NACE MR0175 / ISO 15156 to prevent sulfide stress cracking. This requirement is applied independently of ISO 15761.

Incorrect material selection cannot be compensated by subsequent process control.

 

Step 2: Forging

 

This step determines the internal quality of the valve body.

Forging involves forming heated metal under pressure, resulting in a dense internal structure with a lower probability of defects. It is typically preferred for high-pressure or high-reliability applications. For Class 800 and above, forged bodies are commonly selected in engineering practice to reduce internal defect risks and improve structural reliability, although final selection depends on project specifications.

 

Step 3: Machining

 

After forming, precision machining is performed to meet dimensional and sealing requirements.

Sealing surface machining is a critical control point. The contact surfaces between the seat and disc must undergo multiple machining and lapping processes to achieve specified flatness and surface roughness, directly affecting shut-off performance.

The stem surface must also meet low roughness requirements to ensure long-term packing sealing stability. Excessive roughness accelerates packing wear and may lead to external leakage during operation.

 

Step 4: Welding (Hardfacing of Sealing Surfaces)

 

This process is used to enhance sealing surface performance.

For wear or corrosion-resistant applications, sealing surfaces are typically overlaid with hard alloys such as Stellite to improve resistance.

During welding, heat input and dilution rate must be controlled to prevent excessive mixing of the base material, which would reduce surface hardness. The hardfacing layer is usually required to meet a specified hardness range (e.g., Stellite typically ≥ HRC 35–45).

This process must be performed by qualified welders, with welding procedure specifications (WPS), procedure qualification records (PQR), and traceable documentation.

 

Step 5: Heat Treatment

 

Heat treatment improves material properties and relieves residual stress. It is a mandatory process.

Post-forging heat treatment ensures the material meets required mechanical properties and removes internal stress. Without it, strength and toughness remain uncertain.

Post-weld heat treatment (PWHT) is typically required to relieve welding residual stress. In H₂S-containing environments, PWHT is often mandatory to prevent cracking in weld zones.

If heat treatment records or furnace traceability cannot be provided, material performance cannot be verified.

 

Step 6: Surface Treatment

 

This step prevents corrosion during storage, transportation, and service.

●  Carbon steel valves are typically sandblasted and coated with anti-corrosion paint

●  Stainless steel valves are usually not painted but undergo pickling and passivation to form a stable protective layer

Without passivation, stainless steel is more susceptible to pitting corrosion in chloride environments.

 

Note: Low-temperature carbon steel valves are not suitable for hot-dip galvanizing. Zinc may cause embrittlement at low temperatures, reducing material performance.

 

Step 7: Assembly

 

All components are assembled after machining. Assembly quality directly affects final performance.

Key control points include:

●  Seat interference fit

●  Packing compression

●  Bonnet bolt tightening sequence

 

Excessive packing compression increases operating torque and may cause sticking. Insufficient compression may lead to leakage even before shipment. Improper bolt tightening (e.g., not following a diagonal sequence) can result in uneven flange stress and compromised sealing.

These issues cannot be identified visually and must be verified through assembly records and testing data.

 

Step 8: Pressure Testing

 

This is the final mandatory verification before delivery.

Testing is typically conducted in accordance with ISO 5208, API 598, including:

●  Shell test: verifies no leakage under pressure

●  Seat test: verifies sealing performance in closed position

●  Backseat test: verifies upper stem sealing

 

Leakage rates are classified from Class A to D under ISO 5208. If not specified in purchase documents, the manufacturer may apply a higher allowable leakage class, which complies with the standard but may not meet actual service requirements.

 

Conclusion

 

The manufacturing quality of ISO 15761 valves is not determined solely by final pressure testing, but by the entire process.

Material suitability, internal integrity of the blank, sealing surface machining quality, and completeness of heat treatment cannot be verified by visual inspection. They rely on traceable process documentation.

 

In supplier evaluation, complete process records are typically more valuable than a single test report.

 

FAQs

 

Q1: How to distinguish forged and cast valve bodies during procurement?

In addition to material certificates, suppliers should provide:

●  Forgings: forging records and heat treatment reports

●  Castings: RT or UT reports

Certificates alone are insufficient to confirm the actual manufacturing process.

 

Q2: Can heat treatment be omitted?

No. Without heat treatment records, mechanical properties cannot be verified. In H₂S service, the absence of PWHT represents a clear safety risk.

 

Q3: What happens if leakage class is not specified in the order?

The manufacturer may apply a higher allowable leakage class. Although compliant with standards, it may not meet actual sealing requirements.

Specifying leakage class in technical documents is the lowest-cost way to avoid disputes.