Introduction

For sales engineers and procurement specialists in the valve industry, understanding API 598 is critical for specifying, testing, and validating valve performance. This standard defines acceptance criteria for valve leakage rates, ensuring reliability in oil & gas, chemical, and power plants.

At STV, we manufacture API-certified valves that exceed these requirements, giving your projects a competitive edge in quality assurance.

In this 2000-word guide, we’ll cover:
✔ What is API 598? Scope & Importance
✔ Valve Leakage Classes (Rate Comparisons)
✔ Testing Procedures (Shell, Seat, Backseat Tests)
✔ How STV Valves Comply & Exceed API 598
✔ Key Takeaways for Buyers

1. What is API 598?

API Standard 598 is the industry benchmark for valve inspection and pressure testing, published by the American Petroleum Institute (API). It applies to:

• Gate, globe, check, ball, plug, and butterfly valves

• Pressure classes 150 to 2500 (ASME B16.34)

• Metallic and soft-seated valves

Why It Matters for Buyers

• Ensures leak-tight performance under operating conditions.

• Defines accept/reject criteria for procurement contracts.

• STV’s valves are API 598 certified, reducing risk in critical applications.

2. Valve Leakage Classes (Acceptable Rates)

API 598 specifies maximum allowable leakage for two test types:

A. Shell Test (Pressure Integrity)

• Checks valve body & seals for leaks.

• Acceptance Criteria: Zero visible leakage at 1.5x rated pressure.

B. Seat Test (Sealing Performance)

• Evaluates disc-to-seat sealing under pressure.

• Leakage rates vary by valve type and seat material:

STV’s Triple Eccentric Valves achieve Class V (tighter than standard Class IV), ideal for zero-emission zones.

3. API 598 Testing Procedures

Step 1: Shell Test (Body Integrity)

• Test Medium: Water, air, or kerosene.

• Pressure: 1.5x rated pressure (e.g., 225 psi for Class 150).

• Duration: ≥60 sec for liquids, ≥30 sec for gases.

• Pass Criteria: No visible leakage.

Step 2: Seat Test (Closure Tightness)

• Test Medium: Air (soft seats) or water (metal seats).

• Pressure: 1.1x rated pressure or 60 psi (whichever is higher).

• Measurement:

  • Liquid test: Drip counting (max 1 drop/min per inch of seat diameter).

  • Air test: Bubble counting (zero bubbles for soft seats).

Step 3: Backseat Test (Stem Sealing)

• Applies to gate and globe valves with backseat features.

• Ensures stem packing does not leak under pressure.

4. How STV Valves Exceed API 598 Standards

A. Enhanced Seat Machining

• Metal-seated valves: Lapped surfaces (Ra ≤0.8 µm) for Class V performance.

• Soft-seated valves: PTFE/EPDM with backup metal seals (fire-safe API 607).

B. Automated Pressure Testing

• STV uses computerized test rigs with 0.001% resolution for leakage detection.

• Every valve is 100% tested before shipment.

C. Case Study: STV’s API 598-Certified Triple Eccentric Valve

Model: 48″ Triple Offset Valve (WCB Body, SS316 Seat)

• Test Result: 0.01% leakage (Class V) vs. API 598’s 0.05% limit.

• Applications: Refinery crude oil lines, LNG terminals.

5. Key Takeaways for Buyers

1. Always specify API 598 testing in valve RFQs.

2. Class VI (soft seats) = Zero leakage (best for toxic fluids).

3. Class V (metal seats) = Near-zero (best for high-temperature steam).

4. STV provides full test reports with every order.

Conclusion

API 598 isn’t just a formality—it’s your insurance against valve failures. STV’s API 598-certified valves deliver proven leak-tightness, reducing downtime and maintenance costs.

📞 Need API 598 Documentation? Contact STV Today!