BS&B Sigma / Sigma EXL reverse buckling disk

The Sigma is the top of the Sta-Saf range. It is a solid metal reverse buckling disk that is loaded in compression, so the metal is never stressed towards its tensile limit during normal operation. That is what allows an operating ratio of 95 % of the marked burst pressure under ASME, and 100 % of the minimum burst pressure under CE/PED: the process can run at, or very close to, the burst pressure without shortening the life of the disk.

Because the dome faces the process, the Sigma withstands full vacuum without a separate vacuum support and it tolerates pressure cycling and pulsation extremely well. On activation it opens cleanly along a controlled pattern and stays in one piece, which makes it the preferred choice for installation directly under a safety relief valve. The Sigma EXL extends the same design to larger burst pressure and size combinations.

Why choose the Sigma

• Highest operating ratio in the range, so no need to oversize the burst pressure
• Full vacuum and back-pressure resistance without supports
• Best fatigue life in the BS&B line-up
• Non-fragmenting, ASME UD and CE marked
• SAF (structural apex forming) on the dome for burst accuracy; every Sigma EXL is proof-pressure tested to its maximum operating pressure before delivery
• Fail safe: a damaged disk bursts at or below its marked pressure (damage safety ratio below 1)
• Metal tag with burst pressure, temperature, lot number and full traceability

Selection data we need

Size, required burst pressure and coincident temperature, normal operating pressure, medium, whether a vacuum or back pressure can occur, and the flange rating of the companion flanges. If the disk is installed under a safety valve, tell us the valve set pressure. For replacements, the lot number on the disk tag is enough.