When sourcing industrial valves, one of the most common points of confusion is the difference between API 6D and API 598 — two widely referenced valve testing standards. Both define hydrostatic and pneumatic test requirements, but they apply to different valve types, use different test pressures, and serve different industries.
This guide breaks down exactly what each standard covers, how the test pressure charts differ, and which one your application requires.
What Is API 6D?
API 6D is a specification published by the American Petroleum Institute covering pipeline and piping valves — primarily ball valves, gate valves, check valves, and plug valves used in the oil & gas pipeline industry. It specifies design, material, dimensional, testing, and marking requirements.
Valves certified under API 6D carry the API monogram, which is a mark of quality recognised globally by EPC contractors and oil majors. Novel Valves India Pvt. Ltd. holds the API 6D monogram.
What Is API 598?
API 598 is a valve inspection and testing standard — it does not cover design or dimensions. It defines the acceptance criteria and test procedures (shell test, seat test, backseat test) for gate, globe, check, ball, plug, and butterfly valves of all pressure classes.
API 598 is referenced by most valve purchase orders as the default testing standard when a specific design standard (like API 6D, API 600, or BS 5351) is silent on testing details.
API 6D vs API 598: Side-by-Side Comparison
| Parameter | API 6D | API 598 |
|---|---|---|
| Scope | Pipeline valves (design + test) | Testing only (all valve types) |
| Valve types covered | Ball, gate, check, plug | Gate, globe, check, ball, plug, butterfly |
| Shell test pressure | 1.5× CWP (cold working pressure) | 1.5× CWP |
| Seat test pressure | 1.1× CWP (low pressure: 80–100 psi) | 1.1× CWP (low pressure gas option) |
| Test medium | Water (or air for low-pressure seat) | Water or air depending on class |
| Acceptable leakage | Zero for metal seat; rate-based for soft seat | Leakage rate table by valve size and class |
| Certification mark | API 6D Monogram | No monogram — referenced in PO |
| Industry usage | Oil & gas pipelines, LNG, refineries | General process, water, chemical, power |
Understanding the Valve Test Pressure Chart
Both standards derive their test pressures from the valve's pressure-temperature rating (Class 150, 300, 600, 900, 1500, 2500). Here are the standard shell test pressures by class at ambient temperature:
| Pressure Class | CWP (bar) | Shell Test (1.5× CWP) | Seat Test (1.1× CWP) |
|---|---|---|---|
| Class 150 | 19.6 | 29.4 bar | 21.6 bar |
| Class 300 | 51.1 | 76.7 bar | 56.2 bar |
| Class 600 | 102.1 | 153.2 bar | 112.3 bar |
| Class 900 | 153.2 | 229.8 bar | 168.5 bar |
| Class 1500 | 255.3 | 382.9 bar | 280.8 bar |
| Class 2500 | 425.5 | 638.3 bar | 468.1 bar |
For the full downloadable pressure chart used at Novel Valves, see our Valve Test Pressure Chart PDF.
Which Standard Should You Specify?
- Oil & gas upstream/midstream pipelines: Specify API 6D. Requires monogrammed valves.
- Refineries, petrochemical, general process: API 598 testing with API 600 (gate) or API 608 (ball valve) design standard.
- Power & IBR applications (India): IBR-approved valves with hydro-test per IBR regulations.
- Water treatment / utilities: API 598 or BS EN 12266 depending on project specification.
How Novel Valves Handles Testing
Every valve manufactured at Novel Valves India Pvt. Ltd. undergoes a three-stage test before dispatch:
- Shell (hydrostatic) test — 1.5× CWP with water, minimum hold time per standard
- Seat (closure) test — 1.1× CWP, zero leakage for metal-seated; rate per API 598 Table 6 for soft-seated
- Low-pressure seat test — 5–7 bar air, applicable for double block & bleed and cryogenic valves
All test records, material test reports (MTR), and inspection certificates are provided with each shipment.
Need API 6D certified valves for your project? View our API 6D certificate and compliance details.
View API 6D CertificateFrequently Asked Questions
Can a valve be tested to both API 6D and API 598?
Yes. When a valve is designed and monogrammed under API 6D, the testing requirements of API 6D take precedence. However, most API 6D test parameters align closely with API 598, so dual compliance is straightforward.
What is the difference between shell test and seat test?
The shell test checks the structural integrity of the valve body and bonnet — it confirms there are no leaks through the casting or weld. The seat test checks the sealing ability of the closure element (ball, gate, disc) against the seat in both the closed position directions.
What does zero leakage mean for metal-seated valves?
Under API 6D, metal-to-metal seated valves (such as metal-seated ball valves or gate valves) must show absolutely no visible leakage during the seat test. Soft-seated valves are permitted a small leakage rate per the standard's acceptance table.
