Threaded Flange

ASME B16.5 Class 150 flange dimensions

Flange outside diameter, minimum flange thickness, bolt circle, bolting and raised face, in millimetres, for NPS 1/2 to 12 in. These dimensions are set by the size and the pressure class and are common to the flange types on this line; the bore, hub, socket or thread detail changes with the type and is confirmed on the drawing or the quotation. Thickness includes the 1.6 mm raised face.

Data to ASME B16.5; confirm against the project specification before fabrication. Sizes above 12 in and Class 300 to 2500 follow the same layout and are quoted against the specification. Send the size, class, facing and destination port and we will confirm availability and price within one business day.

What a Threaded Flange Is

A threaded flange has a tapped bore instead of a plain or socketed one. The pipe is threaded and screwed into the flange, so the joint is made without any welding at all. In ASME B16.5 the thread is a tapered pipe thread to ASME B1.20.1 (NPT), which seals on the threads themselves with a sealant or tape; the pipe end is normally a standard pipe thread with a wrench-up of a few turns past hand-tight.

Why Choose Threaded Instead of Welded

The reason to choose a threaded connection is that there is no hot work. That matters when the connection has to be made on a plant that is already running, in a classified or hazardous area where a welding permit cannot be obtained, and on small bore connections where a weld would be difficult to fit. A threaded joint can also be unscrewed, which suits connections that have to come apart for cleaning, replacement of a gauge, or a temporary test point. The cost is a joint that is more prone to loosening and to leaking under vibration and thermal cycling, so it is rarely used above small bore sizes.

Size Range and Thread Specification

Threaded flanges are a small bore product: ASME B16.5 tabulates them up to NPS 4, and the highest classes stop at NPS 2 1/2. The thread type must be stated on the order, because NPT and other thread forms are not interchangeable – an NPT flange will not seal on a parallel thread pipe end. Threads are gauged before dispatch and the threaded bore is capped for shipment so that the thread arrives undamaged.

Limitations to Know

Do not use a threaded joint where the service is subject to severe vibration or thermal cycling, or where a leak cannot be tolerated – a threaded connection is the most likely of the small bore joints to work loose. It is also not suitable for very high pressure or for services where the fluid attacks the thread sealant. Where a welded joint is acceptable, a socket weld flange gives a stronger and more leak-tight small bore connection.

NPT, BSPT and BSPP: which thread seals and which does not

The B16.5 threaded flange is tapped with a tapered pipe thread. Tapered threads seal on the threads themselves; parallel threads seal on a face or a washer. A flange bored for one form will not seal against a pipe end cut with another, so the thread form has to be on the enquiry rather than assumed.

Engagement, sealant and re-making the joint

• Engagement. A tapered thread is run up hand tight and then tightened a few turns past hand tight. Under tightening leaks; over tightening splits the flange boss or the fitting.
• Sealant. Tape or paste fills the spiral gap in the thread. Keep the first two threads clean so no tape ends up inside the line, and check that the sealant is compatible with the fluid.
• Re-making. A threaded joint can be broken and remade, which is the reason the type is chosen on a live plant, but each re-make takes the thread a little looser. Threads should be inspected and, where required, re-gauged before the joint is remade.
• Support. A threaded joint does not tolerate vibration or thermal cycling as well as a welded one, so lines that see either should be checked against the piping specification before threaded connections are used.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged flange material grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. The pressure class sets the flange dimensions, so the grade does not set the wall.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• F11
• F22
• F304
• F304L
• F316
• F316L

Download Threaded Flange data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What decides the price

There is no list price for flanges: the same drawing can be quoted at different numbers depending on seven things. Send them and the quotation comes back within one business day.

Reply within one business day. Send the seven items above with the quantity and the destination port. Send the specification or WhatsApp the specification.