Stainless Steel Pipes & Flanges

In stainless steel piping, pipe class, wall thickness, flange type, pressure rating and gasket face are evaluated together to ensure safe flow and removable maintenance connections. This group brings together two different worlds of pipe: metric OD seamless hydraulic tube for hydraulic lines with fittings, and pipe defined by DN/NPS and Schedule for welded process lines, along with the flanges that connect to these lines.

Stainless pipe and tube selection: hydraulic tube vs process pipe

• Hydraulic tube: defined by OD × wall thickness (example designation: 12 × 1.5 mm); used with 24° cone compression fittings and weld nipples. The OD tolerance and hardness must suit the fitting type.
• Process pipe: defined by DN or NPS together with Schedule (wall thickness series). At the same NPS, as the Schedule increases, the OD stays the same, the wall thickness increases and the ID decreases.
• Seamless or welded: Seamless tube is preferred for high-pressure hydraulic lines; longitudinally welded pipe is also used for large-diameter, medium-pressure process lines. The selection is made according to the project specification.

Types of stainless flanges

Pressure rating (PN / Class) and gasket face

The European standard EN 1092-1 classifies flanges by PN (e.g., PN16, PN40), while the American standard ASME B16.5 classifies them by Class (e.g., Class 150, Class 300). The flange dimensions and bolt circles of the two systems are generally different; PN and Class flanges should not be connected to each other. The gasket face (flat face or raised face) must be the same as that of the mating flange, and the gasket must be selected according to this face, the fluid and the temperature.

Flange installation

Flanges must not be forced into alignment with the bolts; if there is misalignment, the pipe support is corrected first. The gasket is centered, and the bolts are tightened in a criss-cross pattern and in stages. On welded joints, the heat tint is cleaned off, and tools that have been in contact with carbon steel are not used. Placing removable flanged points in front of and behind pumps, filters and valves that need servicing avoids having to cut the line during maintenance.

Which industries use them?

Process and transfer lines in chemical plants, cleanable stainless steel lines in food and beverage plants, auxiliary systems exposed to seawater in ship engine rooms, pump discharge and skid piping in fire suppression systems, and lines exposed to salty environments at offshore facilities. Pipes and flanges in stock are typically scheduled within 1-3 business days, and project-specific lines within 10-20 business days after technical approval.