HomeValve ComparisonsAPI 598 vs API 6D Valve Testing: What's the Difference?

Valve Comparison Guide

API 598 vs API 6D Valve Testing: What's the Difference?

Compare API 598 and API 6D valve testing: hydrostatic shell test, seat leakage test, gas seat test, test duration, and when each standard applies.

Overview

API 598 Testing

API 598 (Valve Inspection and Testing) is the general-purpose valve testing standard used for industrial process valves manufactured to API 600, API 602, API 609, API 623, API 594, BS 1868, and ASME B16.34. It defines hydrostatic shell (body) test pressure, back seat test, and closure (seat) leakage test procedures for valves from NPS ½" to NPS 24". API 598 is the most widely referenced valve testing standard in the world and is specified on the majority of industrial valve purchase orders.

NPS ½"–NPS 24" | Shell test: 1.5× rated; Seat test: 1.1× rated | Water or kerosene | Minimum hold: 15 sec to 5 min depending on size

API 6D Testing

API 6D (Pipeline and Piping Valves) includes its own testing requirements that are more stringent than API 598 in several key areas. API 6D testing adds: longer minimum hold times, a gas seat leakage test (in addition to hydrostatic), a body cavity test for DBB valves, and torque measurement under operating conditions. API 6D testing is mandatory for all valves specified under API 6D — primarily pipeline ball valves, gate valves, check valves, and plug valves used in oil, gas, and LNG pipelines.

NPS ½"–NPS 60" | Shell test: 1.5× rated; Seat test: 1.1× rated (liquid AND gas) | Nitrogen or water | Minimum hold: 5–15 min; torque test; anti-static test

Pros & Cons

API 598 Testing

Universally applicable — covers gate, globe, check, ball, plug, and butterfly valves in a single standard
Widely accepted — specified on virtually all industrial valve orders; test certificates are universally understood by end users and inspectors
Graduated leakage allowances — leakage rates are defined by valve size and type (metal-seat vs soft-seat) rather than zero-tolerance
Test durations are short — minimum 15 seconds (small bore soft-seat) to 5 minutes (large bore metal-seat) allows efficient factory testing
Covers backseat test — verifies stem packing gland can be replaced under pressure
No gas seat test in the standard — API 598 seat test uses only water or kerosene (liquid); gas leakage performance is not verified
Shorter test durations than API 6D — API 598 minimum hold time for large bore valves is 2–5 minutes vs API 6D minimum of 5–15 minutes
No DBB (Double Block and Bleed) test — API 598 does not include specific DBB testing protocol
No body cavity test — API 598 does not specifically test the body cavity pressure under DBB conditions
Leakage rates are defined in drops/minute (for liquid) — not easily correlated to volumetric gas leakage

API 6D Testing

Gas seat test — API 6D requires a low-pressure gas seat test (5–7 bar gas) AND a high-pressure gas seat test (1.1× rated using nitrogen or air) for all valves — verifies seal integrity that liquid test may not detect
Longer hold times — API 6D minimum hold times are 2–4× longer than API 598 equivalents, providing higher statistical confidence in tightness
Body cavity pressure test — for DBB valves, API 6D verifies that the body cavity can be pressurised and bled independently (proving DBB function)
Torque testing — valve operating torque is measured at maximum differential pressure; confirms actuator selection is correct
Anti-static device test — API 6D requires <10 ohm resistance between ball/disc and body — mandatory for pipeline safety
Certificate of compliance — API 6D monogram licence and traceability documentation are required with each valve
Longer test cycle — API 6D testing takes 3–5× longer than API 598 for the same valve; increases manufacturing lead time and cost
Requires gas testing facility — gas seat test using nitrogen under pressure requires a dedicated pressure test bay with gas supply, safety barriers, and certified pressure gauges
Only applicable to API 6D scope valves — not appropriate to specify API 6D testing for general process plant valves not manufactured to API 6D
Higher cost — the additional tests (gas seat, torque, anti-static) add 10–20% to the factory test and inspection cost per valve

API 598 Testing vs API 6D Testing — Specification Comparison

ParameterAPI 598 TestingAPI 6D Testing
Shell/Body Test1.5× rated pressure, water — minimum 15 sec to 2 min1.5× rated pressure, water — minimum 5–15 min (longer hold)
Seat Leakage Test FluidWater or kerosene (liquid only) — no gas seat testWater (or kerosene) AND nitrogen gas — both liquid and gas seat tests required
Seat Test Pressure1.1× rated pressure1.1× rated pressure (hydrostatic) + 5–7 bar low-pressure gas + 1.1× rated high-pressure gas
Minimum Seat Test Hold Time15 seconds (small bore soft seat) to 5 minutes (large bore metal seat)5 minutes minimum (all sizes); 15 minutes for Class 600 and above
DBB Body Cavity TestNot included in API 598 scopeRequired for all DBB valves — cavity pressurised and bled independently
Torque TestNot required by API 598Required — operating torque measured at maximum differential pressure
Anti-Static TestNot required by API 598Required — <10 ohm resistance between ball/gate and body/stem
Back Seat TestRequired for gate and globe valves with back seatRequired where applicable (same as API 598 basis, longer hold time)
Applicable Valve TypesGate, globe, ball, check, plug, butterfly (all process plant types)Ball, gate, check, plug valves for pipeline service under API 6D
CertificateHydrostatic test certificate per API 598 signed by QC inspectorFull API 6D data book: MTRs, dimensional reports, all test records, API monogram certificate

When to Use Each

Use API 598 Testing when:

General industrial process valves: gate, globe, ball, check, plug, and butterfly valves per API 600, API 602, API 623, API 609
All valve orders where API 6D is not specified — the default testing standard for process plant valves
Chemical, pharmaceutical, power, and water treatment plant valves
Valves below NPS 24" in non-pipeline service
Any valve order where only liquid-test seat leakage verification is sufficient

Use API 6D Testing when:

All API 6D pipeline valves — ball valves, gate valves, check valves, plug valves for oil, gas, and LNG pipelines
Mainline block valves, scraper trap valves, and pig launcher/receiver isolation valves
Subsea and deepwater pipeline isolation valves where gas-tight shutoff at high pressure is critical
LNG plant isolation valves (cryogenic ball valves) where both gas and liquid leak-tightness must be verified
Any pipeline service where the purchaser's specification requires API 6D testing as mandatory

Decision Guide

API 598 is the correct testing standard for all general industrial process plant valves — gate, globe, ball, check, plug, and butterfly valves manufactured to API 600, API 602, API 623, API 609, and ASME B16.34. Specify API 598 testing on all process plant valve purchase orders unless the valve is specifically a pipeline valve under API 6D scope. API 6D testing is mandatory when: (1) the valve is specified under API 6D (pipeline ball valve, through-conduit gate valve, swing check valve for pipeline service); (2) the purchase order explicitly requires API 6D test certificates; (3) the project specification (e.g. Saudi Aramco SAES-L-108, ADNOC specifications, or similar major company specifications) mandates API 6D testing for all pipeline isolation valves. Do not routinely specify API 6D testing on process plant valves (gate, globe, butterfly valves) not manufactured to API 6D — the longer test duration and gas test requirement adds cost and lead time without benefit, and manufacturers not holding an API 6D licence cannot issue API 6D compliant test certificates.

Frequently Asked Questions

Does API 598 include a gas leakage test?
No — API 598 defines valve seat leakage using only liquid test fluid (water or kerosene). The leakage criteria are expressed in drops per minute (for liquid flow through the seat) or by visual inspection for zero visible leakage. API 598 does not include a nitrogen or air gas seat leakage test. This means that a valve that passes API 598 liquid seat test may still have a detectable gas leakage rate when tested with gas — liquids have much higher viscosity than gas and will not penetrate small leak paths as easily. For critical clean gas service (natural gas ESD valves, instrument gas isolation, breathing air), a supplementary gas seat test should be specified in addition to API 598, or API 6D testing should be specified if the valve is in pipeline service. The supplementary gas test is typically specified as: 'gas seat test with nitrogen at 7 bar (100 psi), 5 minutes minimum hold, zero visible bubble leakage in submerged test' — this is a common addition to API 598 in process gas service.
Can I use API 6D test certificates for a process plant valve?
You can reference API 6D testing criteria for a process plant valve (i.e. require the same test durations and gas tests as API 6D), but the resulting certificate is not technically an API 6D certificate unless the manufacturer holds an API 6D monogram licence and the valve is manufactured to the full API 6D specification. Many process plant engineers specify additional requirements on top of API 598 (longer hold times, gas seat test) without formally requiring API 6D — this is a common approach when greater confidence in seal integrity is needed for clean gas service or high-pressure applications. The valve specification should clearly state: 'Testing per API 598; additionally, gas seat test with nitrogen at 1.1× rated pressure, minimum 15 minutes hold, zero visible leakage' rather than incorrectly citing 'API 6D testing' for a non-pipeline valve. Vajra Industrial Solutions can arrange supplementary testing beyond API 598 — please state requirements clearly in the RFQ.

Browse These Valve Types

Browse API 598 Testings →Browse API 6D Testings →

Other Valve Comparisons

Gate Valve vs Ball Valve

Read guide

Ball Valve vs Butterfly Valve

Read guide

Globe Valve vs Gate Valve

Read guide

Trunnion vs Floating Ball Valve

Read guide

Swing Check Valve vs Dual-Plate Check Valve

Read guide

Globe Valve vs Control Valve

Read guide

Need to Order API 598 Testings or API 6D Testings?

Share your valve specifications — bore, pressure class, material, standard — and we'll respond with pricing and availability within 24 hours.

Request a QuoteWhatsApp