Trunnion Mounted Ball Valves Saudi Arabia — API 6D, Class 900 to 2500

In a trunnion mounted ball valve, the ball is supported at the top and bottom by fixed mechanical shafts called trunnions. The ball does not float — it is held in a fixed position. The seats move against the ball under spring preload and line pressure. This design is used in larger sizes (NPS 2 and above) and higher pressure classes (Class 300 and above, typically Class 600+) because the trunnion absorbs pipeline load, resulting in lower operating torque. In a floating ball valve, the ball floats between the seats and is pushed against the downstream seat by pipeline pressure. Floating ball valves are practical in smaller sizes (NPS ½ to 4) and lower pressure classes (Class 150 to 600). For Saudi Aramco ESD and pipeline isolation at Class 900 and above, trunnion mounted designs are mandatory.

Saudi Aramco specifies trunnion mounted ball valves for: (1) Class 600 and above in NPS 4 and larger; (2) all Class 900 through 2500 ball valves regardless of size; (3) Emergency Shutdown Valves (ESDVs) and Safety Instrumented System valves where low operating torque is needed for actuator sizing; (4) pipeline block valves where Double Block and Bleed (DBB) capability is required; (5) all NPS 6 and above ball valves as a general rule per 04-SAMSS-035 and applicable piping class requirements. The distinction is embedded in SAEP-347 Annex 4 material numbers — trunnion configurations have distinct material numbers from floating ball designs.

Double Block and Bleed (DBB) refers to a valve that provides two independent seats that can block line pressure from both sides, plus a connection to bleed the cavity between the seats to atmospheric pressure. In a trunnion mounted ball valve, the upstream and downstream seats independently seal against the ball — enabling both seats to be closed simultaneously while the annular cavity between them is vented through a dedicated bleed valve. This proves both seats hold before opening the downstream side for maintenance. API 6D (ISO 14313) defines the specific leakage test requirements for DBB valves. Saudi Aramco specifies DBB ball valves for process isolation when line venting is required before maintenance.

Trunnion mounted ball valves at Class 900 and above are available in: ASTM A216 WCB (cast carbon steel) for general hydrocarbon and steam service up to 425 °C; ASTM A351 CF8M (cast 316 stainless steel equivalent) for corrosive service; ASTM A352 LCC for low-temperature service down to −46 °C; and ASTM A217 WC6 or WC9 chrome-moly alloy steel for elevated temperature service up to 593 °C. At Class 900, the minimum wall thickness and valve weight are significantly greater than Class 150–600, reflecting the higher pressure containment requirements. Gulf Union Ozone stocks carbon steel (WCB) trunnion ball valves at Class 900 and 1500 from Jeddah — other materials available to order.