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http://www.shinjovalve.com/news/api-6d-ball-valve-vs-api-608-ball-valve-how-are-they-different.html

API 6D “specification for piping and pipeline valves” while API 608 ” specification for flanged, threaded and welded metal ball valves” provide detailed requirements for ball valves in terms of structural design, performance requirements, test methods and other aspects. API 6D and API 608 constitute together a complete specification of ball valves in the petrochemical field, and each has its own characteristics according to different working conditions and requirements. API 608 adds the requirements like design, operation and performance based on ASME B16.34 “flanged, threaded and welded valves for general industrial use”. API 6D is more used for long-distance pipeline engineering and specified the differs from API 608 in terms of structure and function.

Applications and Structure
API 608 ball valves is used for the petrochemical industry pipeline media opening or cutting off, which is under the environment such as high temperature and high pressure, inflammable and explosive, corrosive and continuous operation, where require more requirements on valve sealing, material, corrosion. API 608 ball valve has a fixed ball structure and floating ball structure and mainly floating ball structure.
API 6D ball valves are specially used for long-distance pipeline transportation. In addition to switching on or cutting off the medium, the ball valve under this standard also has functions such as blowdown, emptying, overpressure relief, grease injection and on-line leak detection. API 6D ball valves are almost fixed ball construction. In consideration of environmental protection and economy, the blowdown/empty-out of the pipeline ball valve is more important.
API 6D ball valve can choose other structure design or materials to ensure the sealing performance of the valve, such as using the body structure with large storage space, increasing the diameter of the body cavity, etc., to avoid sand and stones and other foreign matters in the pipe to stay in the cavity for a long time and prevent damage to the seat and the ball.

Inspection and Testing
API 608 provides for the inspection, inspection and pressure testing of ball valves in accordance with API 598” inspection and testing of valves”. As a supplement to ASME B16.34, API 608 ball valves shall also fully meet ASME B16.34” inspection and test requirements”. ASME B16.34 and API 598 are basic specifications for general purpose valves.
API 6D provides more detailed requirements for the inspection and testing of pipeline valves, which are more demanding than ASME B16.34 and API 598, such as longer pressure duration, more test items, and more complex operating procedures.API 608 ball valves usually test the seal by pressurizing one end and observing the seat at the other end during the seal test, while API 6D ball valves test the seal from the middle chamber by pressurizing one end.
The latest API 6D 2014 version has added the requirements of QSL. QSL includes detailed requirements for nondestructive testing (NDE), pressure testing and manufacturing procedure documentation. Each QSL required API 6D ball valve inspection and test items are also different, QSL-1 is the minimum quality specification level specified by API 6D, the higher the QSL grade, the more stringent the requirements, the buyer can specify that the ball valve should conform to QSL- (2 ~ 4) quality specification level.

Installation and Maintenance
API 608 ball valves can be installed in the factory, easy to store and transport. usually small size , low cost , easy to repair compare to APD6D ball valves .

API 608 Ball Valve
API608 Ball Valve figure

API 6D ball valve is used for long-distance oil and gas pipeline, with a large diameter and harsh environment, and daily maintenance needs to be strengthened.
API 6D Ball Valve
API6D Ball Valve figure
The API 6D ball valve is difficult to replace and has high maintenance cost due to factors such as caliber, buried installation and welding connection with pipelines. Therefore, the API 6D ball valve of the long-distance pipeline requires higher safety reliability, tightness and strength than API 608 ball valve to ensure long-term safe and reliable operation of the long-distance pipeline.
In general, the API 6D ball valve is primarily used in oil and gas industry pipeline systems, including long-distance oil and gas pipelines including ASME B31.4 and B31.8, with a diameter range of NPS (4 ~ 60) and pressure levels of 150, 300, 400, 600, 900, 1500,2500. Generally fixed ball structure, sealed at the inlet. API 608 ball valves are used in petroleum, petrochemical and industrial applications, mainly for ASME B31.3 process pipeline, diameter range NPS (1/4 ~ 24), small diameter, pressure class 150, 300, 600, 800 pounds, generally floating ball structure, sealed at the outlet.