How To Tighten a CF Flange with a torque wrench correctly

Werner Sölken, is a piping and static equipment expert with 40+ years of experience in the petro and chemical industry.

In his Explore the world of piping website, Werner states:

“A bolted flange connection is a complex combination of many factors (Flange, Bolts, Process, Temperature, Pressure, Medium). All these various elements are interrelated and depend upon one another to achieve a successful result.” 1

While all 6 of these elements are critical in assuring the integrity of a bolted joint, in this post we are going to focus solely on the flange.

What is a flange?

A flange is simply a way of jointing pipes, pumps, valves and other similar equipment together in order to create a uniform piping system.

What standards are used to assure integrity of bolted joints?

Most flanges are united via bolted fasteners or welded.

Welded joints are held to the quality regulations of ISO 25.160.40 and ASTM F722 standards.

Bolted jointed are guided by industry guidelines, such as those recommended by the Bolt Council‘s Guide to Design Criteria for Bolted and Riveted Joints 2nd Edition 2 published by the Institute of Steel Construction, Inc.  and other ASTM standards related to bolting.

For example, in March 2010, ASTM updated its Pressure Boundary and Bolted Joint Assembly guidelines 3

Guidelines will sometimes vary based on the type of flange that is beginning used to create a union between to pipes.

Six common types of flanges

According to ASME B16.5 There are 6 types of flanges typically used to bring joints in alignment with one another. They are as depicted in Figure #1 by

6 types of flanges - AMG Bolting Solutions - How To Correctly Tighten a CF Flange with a torque wrench
Image Credit:

As you can image, flanged joints are made by fastening together two flanges, typically with a gasket between them to provide a secure vacuum seal.

Sandia Report 4 Guideline for Bolted Joint Design and Analysis: Version 1.0 states:

For the case of a bolted flange of a pipe with the bending applied to the neutral axis of the pipe, the actual load on the bolt will be more like an axial load and less like a bending load.
There is an additional concern with this method because it is probable that the actual load on the bolt due to bending will be higher than what this theory predicts (i.e., this does not produce conservative results).
This is a major concern and great care must be taken when considering bending loads on bolted joints with this method.

What is the best fastener material?

Fastenal, North America’s largest fastener distributor of light-duty OEM and MRO products including nuts, bolts, and impact wrenches has an Engineering and Design team that is responsible for the R&D of over a half dozen fastener brands. On their support website they state;

There is no one fastener material that is right for every environment. Selecting the right fastener material from the vast array of those available can be a daunting task.
It is clear from this statement that just like a flange has varying element which determine is the best type to use, fastener materials have various elements that must be consideration as well.
These are things like strength, temperature, corrosion, vibration, fatigue, and many other variables. These same materials are used in manufacturing flanges. That said, with some fundamental knowledge and understanding, an college-educated guess can be made to evaluate the best options.

What about CF Flanges?

The”ConFlat” is a registered trademark of Varian, Inc, which was acquired by Agilent on May 14, 2010.

As a result “CF” is commonly used by other flange manufacturers to avoid litigation.

The Conflat flange is available is various sizes as seen below:

CF Flange Dimensions

CF Flange Dimensions
Image Source: MKS Instruments, Dimensions of CF Flange
Flange Size A
Nominal Tube Diameter
Flange Outside Diameter
Bolt Circle Diameter
Number of Holes
Tapped Holes – Thread Thru Holes Diameter F
Face to Counter Bore
11/3 .75 in
(19 mm)
1.33 in
(34 mm)
1.06 in
(27 mm)
.30 in
(8 mm)
6 8 – 32 .172 in
(4.3 mm)
.18 in
(4.6 mm)
21/8 1.00 in
(25 mm)
2.12 in
(54 mm)
1.62 in
(41 mm)
.5 in
(13 mm)
4 1/4 – 28 .265 in
(6.7 mm)
.17 in
(4.3 mm)
23/4 1.50 in
(38 mm)
2.75 in
(70 mm)
2.31 in
(59 mm)
.50 in
(13 mm)
6 1/4 – 28 .265 in
(6.7 mm)
.21 in
(5.3 mm)
33/8 2.00 in
(51 mm)
3.38 in
(86 mm)
2.85 in
(72 mm)
.69 in
(18 mm)
8 5/16 – 24 .332 in
(8.4 mm)
.22 in
(5.6 mm)
41/2 2.50 in
(64 mm)
4.50 in
(114 mm)
3.63 in
(92 mm)
.69 in
(18 mm)
8 5/16 – 24 .332 in
(8.4 mm)
.38 in
(9.6 mm)
45/8 3.01 in
(76 mm)
4.61 in
(118 mm)
4.03 in
(102 mm)
.75 in
(19 mm)
10 5/16 – 24 .332 in
(8.4 mm)
.38 in
(9.6 mm)
6 4.00 in
(102 mm)
6.00 in
(152 mm)
5.13 in
(130 mm)
.78 in
(20 mm)
16 5/16 – 24 .332 in
(8.4 mm)
.43 in
(10.9 mm)
8 6.00 in
(152 mm)
8.00 in
(203 mm)
7.13 in
(181 mm)
.88 in
(22 mm)
20 5/16 – 24 .332 in
(8.4 mm)
.50 in
(12.7 mm)
10 8.00 in
(203 mm)
10.00 in
(254 mm)
9.13 in
(232 mm)
.97 in
(25 mm)
24 5/16 – 24 .332 in
(8.4 mm)
.50 in
(12.7 mm)

Some companies make CF Flanges even larger, for example:

MDC Vaccum Products, features a 16.5″ rotable flange. Here are the specs:
  • UHV rated to 1×10-13
  • High temperature rated to 450ºC
  • Rotatable and nonrotatable geometries
  • OFE Copper or optional Viton® elastomer gaskets
  • Tapped or clearance bolt holes
  • Conflat® compatible design


Flanges 304ss
Gaskets OFE copper
Bolts 300ss and 300ss silver plated
Bolt Type Hexagonal head
Nut Type Hexagonal
Size 0.375-24 UNF
Torque 26 lb-ft
Vacuum Range 1×10-13
Temperature Range -200ºC to 450ºC
Dimensions 16.50 OD x 13.750 ID maximum

How are they different?

CF Flanges can in different types as well.

  • Conflat® compatible
  • Genderless Mating
  • Rotatable or Non-rotatable
  • Tapped or clearance bolt holes
  • Material quality/composition certs available
  • Bakeable to 450C
  • Ideal for Ultra-high vacuum applications (10-13 torr)

We are working on getting an expert to discuss different types in the near future.

How is a CF Flange fastened?

According to the Kurt J. Lesker Company website, most CF Flanges only require 7-10 Ft. Lbs. to create a vacuum seal, and as a result a CF flange is usually tightened manually. And, the 16-1/2 Inch OD requires only 26 Lbs.

Does this shock you?

A supplier of hydraulic torque wrenches and impact tools telling you to use a hand wrench, ratchet, spanner or torque multiplier?

Well that is all you need to fasten a CF flange what can I say? The title of this post is “How To Correctly Tighten a CF Flange with a torque wrench” after all. Okay, I digress.

Now, you want to make certain the Cu gasket is evenly clamped. Doing so will allow you to create a leak-free seal.

As far as the sequence is concerned, let Dr. Faebian Bastiman explain,

For example a CF 40 has 6 slots, and should bolted in the sequence illustrated , but a CF 63 has 8 slots and should be joint by following the pattern depicted below:

How to Tigthen a CF 40 Flange - AMG Bolting SolutionsHow to Tigthen a CF 63 Flange - AMG Bolting Solutions

In his blog, Dr. Bastiman explains the math behind these bolting sequences for CF flanges. Other good sources are Mikell Seeley, Ph. D, and Greg’s Blog.

Thanks for checking out our post. Good luck tightening those flanges, and remember AMG Bolting Solutions is here to fasten everything correctly, safety, and efficiently, not just to sell bolting equipment.

If you have a question, please comment below, or ask Alan Gross across social media.