Gasket principles | basic understand of sealing devices for begineer

gasket principle
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Gasket is very important knowledge for electromechanical engineers in chemical plants.

Just because a plumbing part is cheap doesn’t mean you can take it lightly.

It is extremely important in terms of preventing leakage of dangerous substances, including mechanical equipment and piping equipment.

There is no one-size-fits-all gasket, so in order to use it appropriately according to the usage conditions, it is important to understand the basic principles of sealing without neglecting gaskets with an easy idea.

I want to learn gaskets as soon as possible!

Gasket sealing principle

How are gaskets sealed?

Few factory engineers can answer this question immediately.

It is somehow tightened and somehow sealed.

This is probably the most common understanding.

As a mechanical and electrical engineer, I want to know that gasket seals are line seals , not face seals.

Gasket seals with wire

The gasket is wire sealed.

What does this actually mean?

First, let’s review the basics of gaskets.

Gasket and piping

The gasket is doughnut-shaped and is set in the pipe.

Many people have never thought about where the donut shape is sealed.

You loosely understand when the donut is sealed on all sides.

However, it is actually sealed as follows.

Gasket sealing part

It is not sealed over the entire gasket donut, but only partially.

This is a simple circle and seal, but it’s not a perfect circle.

It seals in an elliptical or circular shape that is partially distorted.

In any case, there is no sealing area as much as a face. line.

Neither side of the gasket is sealed on the same line.

The piping flanges and lines are sealed with gaskets for the following reasons.

  • Gasket surface not perfectly flat
  • Flange face not perfectly flat
  • Pipes and flanges not attached to flat surfaces

Flange groove

The flange face is often grooved.

The image is as follows.

Flange groove (Gasket)

Create a number of concentric grooves on the donut surface of the flange.

The gasket is sealed by contacting the convex part of the groove of this flange.

The reason for adding uneven grooves is to intentionally create a contact area.

The gasket is not sealed in a concentric line against this flange groove.

Strictly speaking, there are cases such as the following.

Not a perfect circle (Gasket)

The black part is the groove of the flange, and the blue part is the sealing part of the gasket.

Even if the flange groove is almost a perfect circle, the part that the gasket seals is part of it and is not a perfect circle.

I can’t really see what this looks like.

This drawing emphasizes the fact that the gasket seal is a line rather than a surface.

Flanges are very important in piping design.

Inclination of flange

It may not be possible to absorb the inclination of the flange surface without grooves .

If there is no groove on the flange surface and the flange surface is tilted, the gasket seal will look like the image below.

flange tilt gasket

The slanted flanges mean that there is a difference between narrow and wide distances between the flanges.

The gasket is sealed only on the narrow side of the gap, and the wide side is empty.

Leaks without sealing.

Effective sealing width of gasket

The gasket has a donut shape.

Let’s call the area determined by the inner and outer diameters of the donut the “gasket area.”

When the gasket is actually tightened, this “gasket area” itself does not provide an effective seal.

In reality, the seal works only in a smaller area.

Let’s call this the “effective seal width” .

Many other parts are “play” parts.

That’s why retightening it is effective.

Inclination of flange

Both the flanges and gaskets appear to be “flat” like in the picture here.

However, in reality, the flange always tilts.

This is because the flanges deform when tightened with bolts.

Roughly speaking, it goes like this.

Tilting Flange (Gasket)

The flange is subjected to bending stress towards the bolt and deforms.

The bolt side is narrow and the inside of the pipe is wide .

Of these, only the portion to which appropriate tightening force is applied is effective as a seal.

In many cases, the gasket part on the inner side of the pipe does not function as a seal originally.

That’s not to say that the inner side is meaningless.

The gasket part on the inner side has the effect of reducing liquid pooling.

Flange surface roughness

Besides the deformation of the flange, the roughness also affects the flange itself.

Distorted Flange (Gasket)

For metal flanges, there are standards such as 6.4 μm and 3.2 μm for Ra surface roughness in JIS.

Casting and forging are the same.

Glass linings and fluorine resin linings have rougher surfaces.

Rather, there is “undulation”.

Even if the flange is tightened and miraculously does not tilt and the gasket is perfectly flat, the flange itself is rough and the sealing width is narrow.

Roughness of gasket

If the flange is rough, the gasket will naturally be rough as well.

If it is an ordinary gasket, it is a level that you do not need to be conscious of.

As a physical concept, it is better to think that gaskets also have roughness.

Meaning of corrugated gasket

Gaskets should be flat, but there are types that are intentionally corrugated.

  • Corrugation on the surface itself … Metal jacket
  • Corrugated sheet metal is attached to the core    … PTFE-coated gasket
  • Stacking multiple gaskets   … Spiral gasket

The idea of ​​metal jacket is the original idea of ​​waveform.

Due to the corrugated shape, the contact area of ​​the gasket is small, but high tightening surface pressure can be obtained even with a low tightening force.

In batch-type chemical plants, corrugated sheet metal is attached to the coreFluororesin coated gasketThere will be a lot of turns.

Since the flange undulations cannot be absorbed by the PTFE gasket or the core gasket, corrugated sheet metal is inserted into the core to ensure a good seal.

It is used to ensure the tightening surface pressure even when the bolt tightening force cannot be increased, such as glass lining.

Spiral gaskets deviate slightly from the definition of corrugated sheets.

However, when pressure is applied to the gasket, it deforms into a shape similar to that of a corrugated sheet.

The idea is the same as the other two in terms of “minimizing the sealing area” for the purpose of “sealing at high pressure”.

Gasket factor m and minimum design tightening pressure y

A flange system consists of flanges, bolts, and gaskets .

Here, gaskets have somewhat special properties.

So it’s easier to ignore the gaskets first and focus on the flange bolts.

A gasket would be fine after that.

Please also refer to the following articles that focus on flanges and bolts.

As a facility engineer, it is better to have enough knowledge to understand the meaning and interpretation of the gasket coefficient and minimum design tightening pressure.

Minimum design tightening pressure y

Which of the two, the gasket factor and the minimum design tightening pressure, is more famous is obviously the gasket factor .

It’s an easy-to-understand name.

Many plant engineers use the term “gasket factor” to express that the gasket is not a simple spring model.

“What is the gasket coefficient for that gasket?”

At the field level, most people who ask these questions are just casually listening and don’t really take the implications seriously.

The minimum design tightening pressure comes before the gasket factor.

This is clear from the definition.

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

The minimum design tightening pressure y becomes an issue when a gasket is sandwiched between flanges and fixed with bolts and nuts .

After that, we will discuss the gasket factor when pressure is applied inside the pipe for operation .

Minimum design tightening pressure → Gasket factor

This is the order.

The minimum design tightening pressure varies considerably depending on the type of gasket.

We will discuss this later.

It doesn’t matter, but the reason why they understand in order of gasket factor → minimum design tightening pressure is that JIS B 8265 writes in this order.

I think the reason is simple.

Gasket factor m

The gasket coefficient can be thought of as the force applied to the gasket during operation / the internal pressure of the pipe.

Looking only at the gasket coefficient and minimum design tightening pressure, if the flange is thoroughly thickened, the gasket can withstand any increase in internal pressure of the pipe.

I make this mistake.

This is properly defined by the concept of “allowable tightening pressure” .

Example of joint sheet

Let’s take gaskets used in batch chemical plants as an example.

Take the joint sheet gasket Tombo No.1120 t3 from NICHIAS Corporation as an example.

Minimum design tightening pressure y11.0N/ mm2
Gasket factor m2.0
Allowable tightening pressure147.1N/ mm2

Since the minimum design tightening pressure is 11.0 N/mm 2 , when fixing with 4 bolts, 11.0 / 4 = 2.75 N/mm 2 of stress must be applied to each bolt.

The gasket coefficient is 2.0, so if the internal pressure of the pipe is 1.0N/mm 2 (=1MPa), the tightening force applied to the gasket will be 2.0N/mm 2 .

If the initial bolt is tightened at 11.0 N/mm 2 and an internal pressure of 1.0 N/mm 2 is applied, the force applied to the bolt will be the pipe internal pressure + the tightening force applied to the gasket .

Generally, it will be larger than the initial bolt tightening force (11.0 N/mm 2 in this case).

Since it does not hold in all cases,

  • Initial bolt tightening force
  • Bolt tightening force applied during use

The procedure of JIS B 8265 is to compare the magnitude relationship between the two.

Even if the flange thickness is infinitely increased and the bolt is huge, if the allowable tightening pressure of 147.1N/mm 2 is exceeded, this gasket will be destroyed.

Since the gasket coefficient m is 2.0, the internal pressure of the pipe is 147.1 / 2 =73.6N/mm 2

to destroy it.

That said, this gasket is often used at JIS 10k (1 MPa) and is not used at 73.6 N/mm2 (73.6 MPa) pressure…


Compare gasket factors and minimum design tightening pressures for gaskets used in batch chemical plants.

All data are publicly available data from Nichias Corporation.

kindsremarksGasket factor mMinimum design tightening pressure y
joint sheet0.8mm3.5044.8
PTFE coated gasketType A3.5014.7
Fluoroplastic gasket1.5mm3.2022.5

A lot can be learned from these data.


For the same internal pressure, the thicker the gasket, the smaller the tightening pressure .

You can think of it that the thicker the gasket, the better the cushioning.

With the spring model, the tightening pressure is the same regardless of thickness, but that is not the case with gaskets.

Like the gasket factor, the minimum design tightening pressure also depends on the gasket thickness.

If you want to seal with a weak force, the rule of thumb is to increase the thickness of the gasket .

This topic comes up with glass lining equipment and fluororesin lining equipment in batch chemical plants.

PTFE coated gasket

PTFE-coated gaskets can be considered softer than fluororesin gaskets .

The manufacturer describes it as a “easy-to-fit” gasket.

Even from the definition of the minimum design tightening pressure, it is more appropriate to say that it is “easy to get used to”, but the site is the site .

You can think of it as a soft gasket w

PTFE deforms more and more when force is applied .

Therefore, even if a fluororesin gasket is tightened, the gasket will deform and fit in, but with a PTFE-coated gasket, the core will deform before the PTFE deforms, resulting in a seal.

That’s what it means.

The reason why this gasket is used for glass lining equipment and fluorine resin lining equipment in a batch chemical plant is because

This is because the minimum design tightening pressure is prioritized .

A high gasket coefficient may not be a problem.

They are not used under high pressure conditions in batch chemical plants, and most of them are used under normal pressure or reduced pressure.

Blind flange vs full gasket

A blind flange is a flange attached to the end of a pipe.

The idea of ​​this blind flange can be divided into two patterns depending on whether or not a full-face gasket is used.

Blind flange and full gasket (Gasket)

The left side is when blind flanges and normal gaskets are used, and the right side is when full gaskets are used.

This time, the left side is called a blind flange and the right side is called a full gasket, and the advantages and disadvantages are compared.

It doesn’t matter, but the full gasket is likely to be confused with the flat face and flat face of the flange.


Full-face gaskets are often more cost effective.

Gaskets have become quite expensive recently, but they are cheaper than flanges.

For SUS304 pipingflangegasket
blind flangeSUS304PTFE-based
full face gasketSS400PTFE-based

As you can see, it is possible to change the flange to SS400 for full gaskets.

This has great cost benefits.

If the material is SUS304 or higher, the effect will be even greater.


In terms of delivery time , full gaskets are generally advantageous.

It is not the case that PTFE gaskets have fast delivery times.

I can’t say that the supply stability of the manufacturer is high, so my anxiety remains.

Still, it will be more stable than blind flanges such as SUS304.


Sealability is better on the blind flange side.

Many people think that there is not much difference between the two.

As for how gaskets are sealed in the first place, they are sealed by receiving compression force .

It’s so basic that most people don’t realize it.

Ordinary gaskets naturally receive compressive force, so you don’t have to be conscious of it.

However, if it is a full gasket, it will look like the image below.

Sealability (Gasket)

The blue part is not tightened with a flange, so it is not compressed.

This has some effect on the seal.

PTFE looks versatile, but it has its weaknesses. It is gas permeable .

Hydrochloric acid gas is well known.

In the case of a full-face gasket, gas easily permeates from the part where the tightening force is not applied.

Transparent (Gasket)

Corrosive gas may corrode the blind flange of SS400 and cause leakage.

In general, it is said that gas permeability is lower when compressive force is applied to PTFE.

By applying compression, the gaps in the PTFE are crushed, so the image is that gas permeability is low.

negative pressure

A blind flange is more advantageous on the negative pressure side in terms of pressure conditions .

The positive pressure side is mostly determined by the flange thickness and gasket thickness, so there shouldn’t be much difference.

On the negative pressure side, on the other hand, full gaskets are disadvantageous.

Under negative pressure, a force acts to draw the gasket into the pipe.

A full-face gasket is disadvantageous because it has a larger area that receives negative pressure.

It also works in the direction of worsening the sealing performance.


created by Rinker
¥6,768 (2024/03/01 09:38:46時点 Amazon調べ-詳細)

Related article


I explained the gaskets used in chemical plant machinery.

Gasket sealing principle, effective sealing width, gasket factor, minimum tightening pressure

As an application, we also touch on the sealing performance of full-surface gaskets.

Although it is not conspicuous as a piping component, it is very important from the design point of view.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.) *I will read all the comments and answer them seriously.