Think Flange, Gasket, Bolt, Nut as pipe connecting system

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Flanges are used to connect large volumes of piping in chemical plants.

I thought of the flange as a gasket-bolt-nut system.

A flange is a flange, a gasket is a gasket, a bolt nut is a bolt nut. Other places are other places, we are home.

We tend to think in isolation like this.

For those who are familiar with mechanical knowledge, it is a matter of course, but we have summarized the contents that we want people who are ” beginners + α as a machine shop” to know like a machine shop at a chemical plant.

Just by knowing this, I think you will be at a fairly high level as a machine shop for chemical plants .

Think of the flange as a system!

Flange system

A system of pipe flanges and bolt nuts is shown below.

1 size down from flange (Flange)

This is an excellent system consisting of the following elements: piping, flanges, gaskets, bolts, and nuts (washers).

We will explain the important points as a flange system for chemical plants, focusing on materials and forces.


Bolts and nuts for piping in chemical plants drop one rank material from the flange.

The issue here is the pipe material and the flange/bolt/nut material.

Relationship between Flange and Piping

General combinations of flange materials and pipe materials are shown below.

high grade metalhigh grade metal

This means lowering the material of flanges and bolts and nuts by one rank or more than the piping material.

SGP piping

SGP piping material is the lowest grade .

Since the flange cannot be dropped from the pipe material, the lowest grade SS400 is used.

SUS304 piping

SUS304 piping material ranks very high in batch chemical plants .

The quantity is also very large.

The flange material is SS400, which ranks lower than the piping material.

This is because there is a stub end.

With stub ends, there is no reason to be particular about the flange material.

The lowest grade SS400 is fine.

high-grade metal piping

For high-grade metals , the pipe material and flange material should be the same.

This is because the pipe and flange are directly welded without using a stub end.

Relationship between bolt nut and flange

Introducing the relationship between bolt and nut materials and flange materials.

Set the bolt and nut material to be lower than the flange material.

See the table below.

Pipingflangebolt and nut
high grade metalhigh grade metalSUS316

The relationship between piping and flanges is the same as the above item.

Only the bolt nut item is added to the right end.

SGP piping

The flange material of SGP piping is SS400, so the bolt nut system is also SS400 .

Because it is the lowest grade, the idea is simple.

SUS304 piping

The flange material of SUS304 piping is SS400, so the bolt nut system is also SS400 .

high-grade metal piping

For high-grade metal piping, use SUS316 for the bolt-nut system.

SUS316 is the highest grade among general-purpose corrosion-resistant materials.

Since the piping and flanges are of high quality, the bolt and nut materials are kept at the highest grade of general-purpose materials, even if they are lower in rank.

The bolt and nut materials are made lower because the bolt and nut can be replaced .

When two materials of different grades come into contact, the material with the lower grade usually corrodes faster.

By reducing the material on the bolt/nut side, which is easier to replace, we intentionally select areas that will corrode and protect the piping and flange side from corrosion.

Selective anti-corrosion measure .

Some companies use SUS304 instead of SUS316.

This has to do with cost and procurement.

Relationship between bolt material and nut material

We will also introduce the relationship between bolt materials and nut materials.

See the diagram below.

high grade metalhigh grade metalSUS316SUS316L

SGP piping

For SGP piping, the material of the bolt and nut system is SS400 .

Instead of using different materials for bolts and nuts, use the same material.

SUS304 piping

Assuming loose flanges are used for SUS304 piping, the flanges will be SS400 .

If the flange is SS400, the bolts and nuts are also SS400.

high-grade metal piping

For high-grade metal piping, use SUS316L nuts and SUS316 bolts.

Nuts are ranked slightly higher than bolts .

In the case of stainless steel, if the bolt and nut are made of exactly the same material.“bite”will occur.

To prevent this, the materials of bolts and nuts should be changed even slightly.

When deciding whether to prioritize the material of the bolt or the nut, the material of the nut should be slightly higher .

This is a decision on whether nuts or bolts are cheaper .

When considering the combination of bolts and nuts, it is cheaper to let the nut side corrode and replace just the nut.

Bolt and Nut (Flange)

Washer material

If you have a stainless steel flange, the washer material will probably be zinc plated, one size smaller.

I think I’ll go down one size and use a regular iron nut.

That’s how I design it.

What about on-site?

I definitely don’t use them differently.

We do not check each piece to see if it is galvanized or not before installing flanges.

I think the standard is whether everything is galvanized or everything is iron.

Force applied to flange

When considering the behavior when a flange is tightened with bolts,Consider the flange side firstIt’s easier to understand.

This is because bolts can be intuitively considered based on their original function of “tightening a flange.”

Let’s break it down into parts like we learned in high school physics.

Focus only on the flange surface.

When no internal pressure is applied

No internal pressure (Flange)

When the flange surface is closed with bolts, the flange receives the tightening force from the bolts.

To counter this force, the gasket shrinks and the flange receives the gasket’s reaction force.

Force received from bolt = Force received from gasket

This relationship holds true.

When internal pressure is applied

Let’s think about driving with the bolts tightened.

The internal pressure of the pipe will be added.

Internal pressure is added (Flange)

The flange that receives the internal pressure of the pipe acts against the bolt tightening force.

Force received from bolt = Force received from gasket + Piping internal pressure

Assuming that the force received from the bolt does not change, the force received from the gasket will decrease by the amount of the pipe’s internal pressure.

Lower force received from the gasket means less tightening force applied to the gasket.

This means that the gasket is more likely to leak .

Gasket coefficient m and minimum design tightening pressure y are talking about the world around here.

  •  The force applied to the gasket to determine the bolt load when the gasket coefficient m is used
  • Minimum tightening pressure yThe   force applied to the gasket to determine the bolt load when tightening the gasket

This information is enough for your reference.

When using spiral gaskets in batch chemical plants, the minimum tightening pressure y becomes a problem .

force on bolt

Once the force applied to the flange side is known, the bolt side is easy.

It is the “reverse” of the force exerted by the bolt on the flange .

Force applied to bolt (Flange)

As shown in the diagram above, the bolt is subjected to an elongating force .

There is a problem of over-tightening the bolt and breaking it, but it means that the bolt cannot withstand the stretching force and breaks .

It looks like you are tightening the bolt by tightening the nut manually, but what is actually tightened is the flange, and the bolt is stretched.

This part is a little confusing, but also interesting.

SS400 tensile strength

The SS400 is very easy to use when doing simple physics calculations.

It is an extremely versatile material.

I often use this for my calculations.

safety ratio

Tensile strength can be thought of as the value at which a material will break if a stress greater than this value is applied.

The key is the limit value.

When designing, we do not use this value directly.

This is because even if the strength is higher or lower than this value, it will break depending on the usage environment.

A margin rate is provided for versatility.

I calculate this as 4 .

100N/ mm2 for SS400

The easiest to understand is the best.

Note that this article also assumes 100N/mm 2 for SS400 in the calculations. (The basis for the calculation is not provided.)

Thermal stress and tensile strength

Calculation of thermal stress itself can be found on various sites.

Suppose that the metal is heated from room temperature to 800 degrees by welding with a Young’s modulus of 2.05*10 5 N/mm 2 and a coefficient of linear expansion of 12.5*10 -6 (1/K) for SS400.

Now let’s calculate the thermal stress that works when the metal is completely fixed.

2.05* 105 (N/mm2 ) *12.5* 10-6 (1/K)*800K=2,050N/mm2 > 100N/ mm2

Considering the allowable stress of SS400 including the safety factor at 100N/mm 2 , the thermal stress is 20 times that!

Completely split!

Of course, it is not possible to completely fix the metal during welding, and it escapes as strain.

I would like you to recognize that it is amazing as an order.

You may think that the thermal stress is high because the temperature rises to 800°C, but even if the temperature rises to 1/10 of 80°C, it is 205N/mm 2 , so it still exceeds the allowable stress of 100N/mm 2 .

80°C can occur when the equipment is heated.

Bolt tightening force

When tightening metal with bolts, determine the tightening torque of the bolt within a range that will not damage the bolt.

This is the main premise

Stress applied to the bolt = Bolt tightening force / Bolt cross-sectional area

, and the cross-sectional area of ​​the metal that is tightened with the bolt is larger than the bolt, so

Stress applied to metal tightened by bolt < Stress applied to bolt

It becomes.

Force applied to the metal tightened by a bolt = Force applied to the bolt


stress = force / cross-sectional area

It’s a logic that works.

Considering that the material of the bolt is also the same as SS400, we can get a clear answer that the stress applied to the metal fastened with the bolt is less than the allowable stress of SS400.

Bolt and nut specifications

Bolts and nuts come in many specifications and options.

It is described within the scope of use in batch chemical plants.

Machine bolt (half thread) and stud bolt (full thread)

Machine bolts are frequently used in chemical plants.

Machine bolts are also called “half-thread” bolts.

The opposite of “half thread” is “full thread.” Also called stud bolt.

Half-threads are less strong than full-threads, but they are also cheaper.

“Half-screw” has a discontinuous part in the shaft of the screw, so its strength is weakened.

“Half-thread” requires less thread processing, so the cost is low.

For batch chemical plants where operating pressure is low and cost is an issue, a “half-thread” machine hatch is the only choice.

On the other hand, in areas with high pressure, “full thread” stud bolts should be an option.

Metric/Inch and Coarse/Fine Thread

Metric coarse threads are often used in chemical plants.

Because this is common.

The option different from “coarse thread” is “fine thread”

A “fine thread” has a small thread pitch. Looking at the appearance, it looks like a fine pitch.

“Fine threads” have low strength, are expensive to manufacture, and are prone to seizing.

Instead, it has fine tightening accuracy and can be tightened with a small torque.

What are the specifications like this? If it is a batch type chemical plant level, there is no question.

Choose “coarse thread”.

There are two units of length, “meter” and “inch”, but in Japan it’s normal to use “meter” .

Therefore, the only option is metric coarse thread .

Screw Accuracy “ABC” / Finishing Degree “Fine Medium Rough”

Manufacturing accuracy and finishing level are specified for screws.

This is related to the story of “fit.”

In Japan, accuracy has been classified in the past into grades 1, 2, and 3.

Nowadays, they are also expressed as “A class,” “B class,” and “C class.”

This corresponds to the classification of “fitting accuracy”.

“Grade 2” is categorized as a “fit” accuracy of 6H for female threads and 6g for male threads.

“Class 2” and “B class” are often used in chemical plants .

Because it’s the most common.

In a chemical plant, you don’t really care about accuracy, at least not with bolts.

The degree of finishing is also classified into “fine,” “medium,” and “coarse.”

As you can probably guess, “medium” finishing is used in batch chemical plants.

Strength classification 4.8

There are 10 strength classifications: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9.

Chemical plants use “4.8” .

Because it’s common.

By the way, “4.8” means that the nominal tensile strength is 4×100=400N/mm2 and the nominal yield point is 400×0.8=320N/mm2.

If you are at the engineer level of a chemical plant, there is no need to know deeply.

All you need to know is that there are options to increase the strength of bolts.

Even if the specification is to increase the strength, it is normal to specify the material before considering the strength .

80% nut and 100% nut

It is normal to use an 80% nut .

An 80% nut means that the thickness of the nut is 80% of the bolt diameter.

Another option for the 80% nut is the 100% nut.

There is also a type called 60% nut.

The thicker the nut, the stronger it is.

In chemical plants, strength is not a concern, so an 80% nut is sufficient.

Electro-galvanizing (unichrome plating) and hot-dip galvanizing (dipping)

Bolts have the option of being plated or not.

Zinc is used for plating.

There is a difference between electrogalvanizing and hot-dip galvanizing .

Electrogalvanizing involves attaching bolts to a galvanizing solution and applying zinc to the bolts through electrolysis.

Electrogalvanizing is a chemical bonding method, so it has high precision, and the thin film makes it easy to conduct electricity.

Chemical plants prefer electrogalvanizing .

Because it conducts electricity.

The most common type is unichrome plating, which has a silvery luster .

If special plating is used, it will be colored gold or black.

Hot-dip galvanizing is also called “dobutsu”.

This is a method of physically attaching the bolts to the galvanizing solution.

It has a thick film and is highly corrosive.

However, bolts are commonly used for sacrificial corrosion in chemical plants.

The advantages of hot-dip galvanizing actually become disadvantages.

Hot-dip galvanizing is used when increasing the corrosion resistance of the duct itself without considering sacrificial corrosion, such as in electrical instrumentation ducts .

bolt length

The bolt length is basically so that 2 to 3 threads protrude from the nut.

However, there are cases where more threads are protruding, and conversely, there are cases where they are insufficient.

It is difficult to set a uniform bolt length because there are many options for pipe flanges in chemical plants, such as diameter, flange thickness, stub end thickness, gasket thickness, and the presence or absence of washers.

This phenomenon occurs because the necessary bolt lengths are not prepared at the installation site and the surplus bolts are used.

It is easy to check the bolt length in detail, but I would like you to understand that there are unavoidable parts in construction.


Flanges are very important when considering piping in chemical plants.

There are surprisingly few people who view flanges as a system, and just knowing this article will be useful in many ways in terms of operation.

Of course, the following books are also very useful.

created by Rinker
¥3,300 (2024/02/29 22:05:21時点 Amazon調べ-詳細)

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I explained the flange, gasket, bolt and nut system of a chemical plant.

The minimum required materials, strength, and specification options are described.

Among these, the range that can be memorized and used in practice is limited, such as materials, and the rest of the information is for reference only.

If there is such a system, it should be possible to refer to it if necessary.

Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*We will read all comments received and respond seriously.