Do you know Flange Difference between RF and FF

RF/FF Flange
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There are two types of piping flange shapes: RF and FF.

Even if you don’t know it, you’ll be able to do something about it, but if you call yourself a plant mechanical engineer, it’s a must-know knowledge.

I have never worked with a clear distinction between RF and FF. I don’t really care.

However, not knowing the difference and not knowing the difference are two different things.

It would be better to know in order to acquire knowledge on one level and step up as an engineer.

Flange and pipe connection

First, we will focus on the connection relationship between the flange and the pipe.

There are three main types.

connection flange

Roughly speaking, each feature is as follows.

  • socket weld simple
  • butt weld strength
  • loose flange cost

socket weld

Socket welding has a simple structure with a normal disc flange shape.

It is very often used in batch chemical plants.

It’s as simple as that.

butt weld

A butt weld is a flange with a “neck”.

Commonly used in petrochemicals.

The reason is “strength is high“.

  • Fewer welds compared to socket welds
  • Weldable like piping

Socket welds are cumbersome and time consuming because of the internal welds.

loose flange

A loose flange welds a stub end “collar” to the pipe.

The flange itself is not welded and has a loose positional relationship.

Loose flanges are used together with stub ends.

Advantages of loose flanges

  • low cost
  • Easy to adjust piping

One of the advantages of loose flanges is that they can use less material than stub ends or pipes.

Cost can be reduced by reducing the volume of stainless steel.

Relationship between stainless steel piping and flanges

  • Stainless steel for fixed flanges
  • Iron for loose flanges

In addition, by using a loose flange, it is possible to easily adjust the angle of the pipe and flange, so it is possible to absorb manufacturing errors in the pipe.

Flange shape

Next, we will explain the shape of the flange.

There are four types, but they can be broadly categorized into two.

  • FF/RF standard
  • MF/TG high pressure

Under the low-pressure environment of the batch-type chemical plant class, there is no problem if these two can be used properly.

Raised face / full face (FF / RF)

It is expressed as flat seat and full seat.

You might also say RF and FF flanges .


Batch-type chemical plants basically do not use them differently.

If it’s steel, it’s a flat face, and if it’s a stainless steel, it’s a loose flange, so you’ll inevitably seal it with a flat face.

The loose flange itself is a flat face, but it is treated as a flat face because it has a stub end.

Inset type / Groove type (MF / TG)

Finally, we will introduce the shape relationship between the flange and the sealing material.

sealing material

The leftmost shape is the most standard shape.

A simple disk gasket is sandwiched between a simple disk flange.

This is standard.

The shape in the middle is called the “inset type”. Also called MF.

M means Male, F means Female.

By reducing the contact surface of the gasket, it is possible to secure the tightening pressure even with a small force.

It is used in places where high airtightness is desired, such as high-pressure piping.

The shape on the right is called a “groove”. Also called TG.

Use when you want to seal more strongly than MF.

When it comes to gaskets in this area, spiral wound gaskets are sometimes used.

Batch-type chemical plants hardly use TG, let alone MF.

Relationship between flange and gasket

When considering the flange RF and FF, Relationship between flange and gasket.

It is easy to understand when considering

To minimize gasket variety, batch chemical plants use the same size gasket regardless of flange RF and FF.

Let’s take a look at the relationship between flange differences and gaskets.

①RF (flat seat)

RF (Flat seat) has a convex part on the flange surface as shown in the figure below.

RF flange

For simplicity, the gasket and flange faces are the same size.

RF flange contact surface = gasket

I think. As an RF shape, a pattern like this one is rare. in fact,

RF flange face > Gasket

will be the relationship.

②Loose flange

Let’s take a look at the relationship between loose flanges and gaskets.

loose flange

In some cases, the inner diameter of the flange side is properly flat compared to the RF flange.

in this case,

Flange face of loose flange > Gasket

However, as mentioned above, it is within the margin of error.

③ FF (full seat)

Let’s take a look at the structure of FF (full seat).

FF flange

This is, of course, related to:

Flange surface of FF > Gasket

The difference between the RF type (including the loose flange) and the FF type is the contact surface of the gasket, but unless the gasket dimensions are changed, the tightening pressure applied to the gasket will not change if the bolt is tightened with the same tightening force.

Proper selection of gaskets is essential for the intended and proper operation of RF flanges.

Difference between FF and RF of flange

There are two general-purpose flanges, FF and RF.

I doubt it.

It’s simple, but batch chemical plants don’t care about the difference between FF and RF.

I have been doing this job for over 10 years and have never been conscious of any difference.

As long as the shape is different, the properties are different, but the difference is insignificant.

The differences are explained below.

Different tightening surface pressure

The tightening surface pressure is different between FF and RF flanges.

different contact area

The difference between RF and FF is the contact area of ​​the gasket.

If the gasket is properly inserted, the contact area of ​​the gasket does not change between RF and FF.

It is not always possible to insert the gasket so neatly.

When the gasket is slightly misaligned, RF may reduce the contact area of ​​the gasket.

There is a condition “when the gasket is slightly misaligned”, but the probability is quite high.

The probability is high, but the impact is very low.

This is because the range of displacement of the gasket is limited.

The gasket can only be displaced between the gasket outline and the bolt hole.

Careful workers carefully set gaskets when installing each pipe.

Some operators of chemical plants adhere to this rule.

Younger people tend to be more sloppy.

Still no leaks.

Ideal gasket insertion state

First, consider the ideal gasket insertion state.

Take a look at the diagram below.





Red part is the gasket .

The flange is in contact with the front surface of the gasket for both FF and RF.

If this state can be maintained, there is no difference between FF and RF.

The point here is to maintain this state.

The gasket may be misaligned when inserted.

It is quite difficult for the gasket to be perfectly aligned with the center of the flange surface.

Take a look at the diagram below.


Thus, it is possible for the flange and gasket to be installed out of alignment.

When this happens, shear forces are applied to the gasket.

Why Gaskets Slip

I will explain why the gasket is misaligned with the flange surface.

Take a look at the diagram below.

be depressed

The places where the gasket is likely to slip are:

  • horizontal pipe flange
  • Flanges for vertical piping, the two flanges are not centered
  • High-place piping flange

I feel that it is easy to slip in places that are difficult to work, but surprisingly it is easy to slip with horizontal piping.

This is not a height issue.

To install gaskets in horizontal piping, follow these steps:

  1. Temporarily set 3 bolts
  2. Insert the gasket from the remaining one space
  3. Temporarily set the remaining one bolt
  4. Lift the position of the gasket with a thin iron plate or a flathead screwdriver.
  5. Tighten the 4 bolts

A lazy worker skips step 4.

“It’s okay if it’s a little off.”

The number of workers who think this way is increasing, especially young workers.

Considering cost-effectiveness dry, this is correct.

Even if the gasket is misaligned, it’s certainly not a big problem.

However, with this way of thinking, we do not realize that if we neglect each task, an accident will occur somewhere .

Approximately 2mm deviation at 50A

How much is the gasket gap between FF and RF?

Let’s do a simple calculation.

There is a risk that the gasket will deviate by about 2mm between 50A FF and RF.

Since the RF flange differs depending on the manufacturer, please treat it as a reference.

This is because the distance between the outer circumference of the RF flange and the inner diameter of the bolt hole is 2 to 3 mm on one side.

Since the gasket contact surface of the 50A flange is about 10 to 15 mm on one side, a gasket misalignment of 2 to 3 mm will result in a misalignment of about 20% in the contact area.

If the contact area is reduced by about 20%, the gasket tightening pressure will also change by 20%, so it is quite possible that the gasket will break.

Bolt tightening force is the same

The bolt tightening force is the same whether the flange is RF or FF.

The operator does not change the force applied to the bolt after making a calm judgment by looking at the flange surface.

RF has greater tightening surface pressure

If the flange is RF, the tightening surface pressure may be higher than that of FF.

Let’s clean up the logic mentioned above.

  • Contact area may be higher for RF
  • Bolt tightening force is the same

Tightening surface pressure = bolt tightening force / contact area

Because of this relationship, the logic is that the smaller the contact area, the higher the tightening surface pressure.

In the case of a batch-type chemical plant, it is only “possible” .

Low internal pressure in piping

The internal pressures handled in batch chemical plants are very low.

Even if the flange shape is different and the tightening surface pressure is different , it is OK as long as the pipe internal pressure < tightening surface pressure is maintained.

More than 90% of batch chemical plants use JIS 10k flanges.

Since it is JIS10k, it is handled at 1,000kPa or less.

The internal pressure of the piping does not exceed 500kPa.

Discontinuity on RF flange

There is a discontinuity in the RF flange.

Take a look at the diagram below.


This does not exist on FF flanges.

Stress concentration occurs to some extent at discontinuous parts .

Uneven stress is applied.

This effect is subtly seen on the gasket side.

Difference between FF and RF of gasket

We introduced that there are two types of flange shapes: FF (full face) and RF (raised face) .

Same with this, gaskets also have a difference between FF and RF.

FF gasket

Since FF gaskets have bolt holes, the following uses are possible.

  • Prevent gaskets from being drawn in on vacuum lines
  • Used instead of a blocking plate
  • Almost no misalignment between piping and gasket

In “vacuum piping” , the PTFE-coated part of the PTFE-coated gasket, in particular, can be pulled into the vacuum pump side.

Even PTFE coating can be handled by sewing or welding the outer circumference.

Even with solid PTFE gaskets, if there is a risk of entrainment, the bolt holes can be used as a “bracing” against entrainment.

A “blocking plate” is a type that does not have a through hole on the inner circumference like this one.

RF type gaskets can also be manufactured for the type that does not have an inner circumference.

The advantage of using an FF gasket is that it is easy to see that the FF gasket is inserted from the outer surface of the pipe.

It is very important for operation to confirm whether or not the blocking plate is installed.

In order to facilitate this confirmation, it is conceivable to use the merits of FF type gaskets.

It is not impossible to set the blocking plate with the RF type, but with the FF type, misalignment between the pipe and the gasket is less likely to occur.

There are many cases where the FF type is used for the blocking plate in order to increase the reliability of the sealing performance.

RF gasket

FF gaskets come with bolt holes.

Conversely, RF gaskets do not have bolt holes.

What is the impact with/without bolt holes?

The advantages of RF gaskets include:

  • low cost
  • Easy to remove
  • Can be coated with PTFE

“Low cost” is natural because the RF gasket has a smaller area.

“Easy to remove” means that the gasket can be removed by removing 3 of the 4 bolts.

Since the FF gasket has bolt holes, it cannot be removed unless all four bolts are removed.

“Possibility of PTFE coating” is a characteristic of batch-type chemical plants.

If you are an industry that can handle only solid PTFE gaskets, you don’t need to worry here.

Demand for PTFE-coated gaskets is high in batch-type chemical plants with many glass linings and fluorine resin linings.

The demand for RF gaskets is also high.

It is difficult to make bolt holes with PTFE coating.

  • Create a PTFE-coated gasket with an area equivalent to the FF gasket.
  • drill a bolt hole
  • Cover the bolt holes with PTFE .

This process will come.

Comparison of gaskets and flanges

Let’s organize a comparison of RF and FF for gaskets and flanges.

typeRF gasketFF gasket
RF flangegeneral purposedon’t usually use
FF flangegeneral purposefor blocking

Since RF gaskets are common, they are often used regardless of whether the flange is FF or RF.

On the other hand, I have not seen many FF gaskets against RF flanges.

There are many scenes where the FF gasket is inserted into the FF flange.

I don’t think there is a problem with batch-type chemical plants even if FF gaskets are attached to RF flanges.

FF gasket is special

FF gaskets are special, and RF gaskets are common. we call FF gaskets “full-face gaskets”

It is positioned as a special gasket.

Is the lining flange RF?

The lining is for example glass or PTFE.

The contact surface of the flange is equivalent to RF

The contact surface of the flange after lining is equivalent to RF.

Would it be more correct to say that there is no need for lining beyond the bolt holes in the flange?

If the entire flange surface is lined, gaskets and bolts must also be considered for the entire surface.

The gasket is made of solid PTFE, and the bolt is made of highly corrosion-resistant metal.

Lining equipment is not special in a batch system, but rather a general one.

Flange base material is FF

In the first place, RF flanges are slightly more expensive than FF flanges.

For example, SUS304 JIS10k 20A is about 900 yen for FF, but about 990 yen for RF.

About 10% higher unit price.

This is big.

At this point, there is no need to choose in a batch system.

If the idea is to use a stub end + loose flange for the stainless steel pipe flange, then the flange should also be a cheap FF without any problems in terms of specifications.

Even if you stand in the position of lining, this is a matter of course.

This is because the RF part of the flange gets in the way when processing the lining end face.

If the flange is RF, processing is required to align the RF of the flange with the end face of the lining.

It will take one more step.

Full gasket on stainless steel blind flange

There are two types: a donut-shaped gasket is used for stainless steel blind flanges , and a full gasket is used for iron blind flanges.

This means that the full gasket is not universal.

Consider the context of a full face gasket on a steel blind flange.

RF costs less

If the entire blind flange is made of stainless steel, it is naturally expensive.

That’s why I thought about lining stainless steel blind flanges.

It’s a very basic way of thinking.

RF has a small tightening torque

RF has a smaller contact area than FF, so even a small tightening torque can compress the gasket.

In that sense, RF has an advantage over FF.

Considering this, I come to the conclusion that the only way to make a stainless steel blind flange is to lining a steel blind flange with stainless steel on the RF.

Somehow, I think that thinking about principles and principles means something different from just saying, “Blind flanges made of stainless steel have a lining!”

plumbing bible

If you want to learn more about pipe flanges, the piping bible is a good place to start.

Let’s start with the Plant Piping Pocketbook.

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

The diagrams are clearly drawn for each part, so you can see the differences in the RF-FF this time.

This book is easy to use the moment you understand the difference.

Related article

Related information


This is a summary of the pipe flange shapes used in batch chemical plants.

After explaining the connections between flanges and pipes and the types of flanges, the RF and FF of flanges are summarized.

RF/FF gaskets, linings, blind flanges, etc. are also listed as related information.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)

*We will read all the comments and reply seriously.