64 Double Ball Valves For An FPSO Project in Brazil: 2-Inch Class 1500 A105 LF2, NACE MR0175 Sour Service

Case specification

Specification Line, Decoded

"2 inch, Class 1500." Small bore at high class is the standard combination for injection, drain and instrument isolation lines on topsides: the lines are 1 to 3 inch, the design pressures are high, and the flange class follows the line rating, not the pipe size. For A105/LF2 (material group 1.1 under ASME B16.34), Class 1500 carries a cold working pressure of 3,705 psig - about 255 bar at 38 °C. The rating falls as temperature rises, so the class is checked at design temperature rather than at ambient; and on a soft-seated ball valve it is the seat, not the body, that caps continuous service at roughly 200 °C. Raised face ends per ASME B16.5, matched to the class, dominate small-bore topside valves because removal for maintenance outweighs the weight saved by welded ends - the trade-off runs the opposite way on large-diameter lines.

"A105 LF2." A105 is the general forged carbon steel for pressure piping components; A350 LF2 is the same family with Charpy impact testing at −46 °C. Naming both tells the shop that the pressure boundary is forged carbon steel with low-temperature toughness verified heat by heat - what an offshore installation needs, where a winter shutdown can take topside lines below ambient design temperature.

"NACE MR0175." The sour-service requirement. Wet H2S cracks carbon steel from the inside, and the standard's answer is a hardness ceiling of 22 HRC (≈237 HBW) on pressure-containing parts, achieved through chemistry control, post-weld heat treatment of welds and hard spots, and hardness verification on the certificate. Sour service is a manufacturing discipline, not a test that can be added afterwards.

What a DBB Valve Is, and Why Topsides Standardize on It

A double ball valve is one body, two balls in series, one bleed between them. API 6D defines double block and bleed as a single valve with two seating surfaces that, closed, seal against pressure from each side independently, with a means to vent the cavity between the seats. One unit therefore replaces two isolation valves, the spool between them, and two extra flanged joints - the joints that leak, and the weight topsides never have. It also matters which philosophy the line class calls for: API 6D distinguishes DBB (each seat seals independently from both directions) from DIB (one seat may be unidirectional). Ordering one where the design intent was the other is a functional substitution, not a formality.

On an FPSO every kilogram competes with payload and stability margin, so consolidating each isolation point into one valve removes two flanged joints and a spool, cuts installation hours and hydrotest boundaries, and reduces the leak points that offshore inspection trends over a 20-year life. That is why the same small-bore sour-service DBB specification repeats across offshore projects - and why we hold a Class 1500 body envelope and the NACE MR0175 paperwork as standard, not as a special build.

Test and Document Scope

One vocabulary point decides disputes, so it is worth stating: seat leakage belongs to ISO 5208 - Rate A (zero visible leakage) for soft seats, Rate D for metal-seated valves. Quotations promising "API 598 Class VI" are mixing standards; API 598 is an inspection standard and contains no Class VI classification.

Delivery: Two Lots, Against the Module Piping Schedule

An offshore valve package is rarely wanted in one drop. Modules are piped up one at a time, and the contractor needs the valves for a module while that module is being fitted out - not months earlier, when they sit in a yard store where flange faces get knocked and soft seats sit in the sun, and not weeks later, when the module cannot be closed out. This order was split accordingly: 40 sets in September 2020 and 24 sets in November 2020. Splitting 64 valves costs a second set-up, a second paint batch and a second inspection pass, which is why a phased offshore order carries more man-hours than the same quantity in one lift.

Two conditions matter more than the split. Certificates must follow the lot - a 3.1 certificate is issued heat by heat, and mixing heats across two deliveries makes the module's material traceability file harder to reconcile at commissioning, so each lot was tested, certified and crated as its own unit. And storage protection is part of the scope - flange faces and bore protected for a sea crossing followed by months of yard storage, soft seats kept out of direct light and heat, and the nameplate engraving legible after any paint touch-up.

Sourcing Double Ball Valves for Offshore or FPSO Duty?

Send us the size, pressure class, end connection, body and trim materials, seat type and leakage rate, and the sour-service requirement - we will confirm the producible range, the type-test package your project file requires, and a production schedule within 24 hours.

FAQ

What is a double ball valve, and how does API 6D define DBB?

A double ball valve is a single body containing two balls in series with a bleed port between them. Under API 6D, double block and bleed (DBB) means a single valve with two seating surfaces that seals against pressure from each side independently in the closed position, with a means to vent the cavity between the seats. One valve therefore does the work of two isolation valves plus a vent spool, in one envelope and one document set.

Why specify Class 1500 on a 2-inch valve?

Small-bore lines that carry high pressure - chemical and MEG injection, drains, instrument isolation on topsides - are usually built in 1 to 3 inch sizes at high class. For A105/LF2 (material group 1.1), Class 1500 gives a cold working pressure of 3,705 psig (about 255 bar) at 38 °C, and the rating falls as temperature rises, so the class must be checked at the design temperature, not only at ambient.

What does NACE MR0175 require for a carbon steel valve in sour service?

NACE MR0175 / ISO 15156 limits carbon steel pressure-containing parts to a maximum hardness of 22 HRC (about 237 HBW) to resist sulfide stress cracking in wet H2S service. In practice this means controlled chemistry, post-weld heat treatment of welds and hard spots, and hardness verification on the certificate. Sour service is a manufacturing requirement - it cannot be retrofitted onto a valve already made without it.

What seat leakage class should a double ball valve meet?

Seat leakage is specified under ISO 5208: Rate A for zero visible leakage with soft seats, which is the normal requirement for resilient-seated DBB valves, and Rate D for metal-seated valves with a defined maximum leakage. Quotations that promise Class VI leakage under API 598 are mixing standards - API 598 is an inspection standard and does not contain a Class VI classification. Ask the supplier to state the ISO 5208 rate and the seat material instead.