● Wm1 is the necessary minimum tightening force [N] during operation.

● Wm2 is the necessary minimum tightening force [N] during gasket tightening.

● Since Wm1,Wm2 are calculated regardless of the type of fluid (gas, liquid),these values may cause the tightening force to be insufficient. To compensate this, Nichias set the minimum bolt load [N], Wm3.

※ Reference：JIS B 8265:2010 (Construction of pressure vessel - General principles)
                            Annex G Pressure Vessel Bolt Tightening Flange

※ Related FAQ「What is effective seating width of gasket and the diameter of gasket load reaction force?」

★ Gasket NAVI has content that can easily calculate the necessary tightening force and tightening torque.

     Please use together with  「Tightening torque calculator」.

※When the friction coefficient (μth) of the thread face and the friction coefficient (μw) of the nut bearing face are 0.15, K ≈ 0.20. μth and μw are considered to be around 0.15 when using lubricating oil applied to the surface of steel bolts and nuts with normal surface condition.

★ Related FAQ 「How can I calculate the minimum tightening force value of the gasket?」

★ Gasket NAVI has content that can easily calculate the necessary tightening force and tightening torque. Please use together with 「Tightening torque calculator」.

It is thought that the flange has a convexly bulging shape due to slight bending (flange rotation) from bolt tightening and internal pressure action. This is taken into account in determining the effective seating width of gasket and diameter of gasket load reaction force.

To determine effective seating width of gasket and diameter of gasket load reaction force

Basic width of the gasket (b0) will be calculated as a guideline.

The basic width of the gasket, b0,varies according to the shape of the gasket seat. Generally, taking b as half of the gasket contact width is common.

Please refer to JIS B 8265: 2010 (Construction of pressure vessel - General principles) Table G.3

  ● Effective width of gasket (b)

　  　　When b0≦6.35mm, b＝b0

　　  　When b0＞6.35mm, b＝2.52√b0

  ● Effective diameter of gasket (Ｇ)

　  　　When b0≦6.35mm, G = Mean diameter of the gasket contact face

　　  　When b0＞6.35mm, G = (Outer diameter of gasket contact face)－2b

　　　　　（However, exception is given for lap joint type flange）

★ Reference: JIS B 8265: 2010 (Construction of pressure vessel - General principles)
                       Appendix G Bolt tightening flange of pressure vessel

★ Related FAQ 「How can I calculate the minimum tightening force value of the gasket?」

For gasket such as jointing sheet and expanded graphite gasket, the structure of the pores inside these types of gasket is large and these pores need to be tightened to prevent leakage by gas penetration. If the pores inside the gasket are closed, this would eliminate the possibility of fluid passing through the gasket.

As gas-type fluid has smaller molecular size and lower viscosity as compared to water and oil type fluid, a stronger tightening force is needed to ensure the inside is tightly compressed. This is why higher seating stress is needed.

From a glance, the flange surface may look smooth but it actually has micro pores on the surface. Since the gasket is generally softer than the flange, gasket will usually deform according the flange surface after tightening. Thus, there is need to prevent leakage between the contact surfaces by ensuring the tightness between the flange surface and gasket. But once the flange is tightened, the gasket in between is plastically deformed and does not return to its’ original shape even after completely released from the clamping.

If this deformed gasket was used, the minute gaps that formed between the used gasket and flange may lead to leakage. Therefore, reuse of used gasket is not recommended.

◎･･･Recommended for use
△･･･If good gas sealability is desired, please use TOMBO No.1215-T.
×･･･Paste is not necessary (but still can be applied without problems)

<Precautions during paste selection>
When jointing sheet is used together with a solvent-based paste, the solvent will cause the rubber component in jointing sheet to swell and reduce the gasket strength. This may cause compressive failure during tightening. Also, silicone-based can easily promote sliding between gasket and flange and thus not recommended.

In expanded graphite gasket, usage of metal containing paste such as Never Seez in high temperatures will result in the paste to act as a catalyst and cause the reduction of graphite due to oxidation process. Please consult with us before use.

<When conducting tightness test>
Usage of paste for jointing sheet and fluororesin gasket in tightness test is recommended, even if the fluid tested is liquids.

The amount of compression of a jointing sheet under appropriate seating stress should be about 10%. Therefore for a gasket nominal thickness of 1.5 mm, the amount of compression required will be 0.15 mm.

As there is a deviation of deflection and the plane of the flange, such crucial management is difficult. Therefore rather than judging from compression amount, a torque management system is more suitable.

・ For gaskets other than sheet gasket (e.g. VORTEX etc.), the compression thickness is larger than sheet gasket so it is more suitable to use compression as judgement.

The minimum tightening torque is the “lower limit” of tightening torque necessary to be adopted to ensure no leakage from occurring. However, in the actual tightening process, it is difficult to apply torque accurately and uniformly. Therefore, it is encouraged to tighten with the recommended torque (minimum tightening torque multiplied by the safety factor).

The concept of this safety factor varies depending on the different usage. In general,  it is recommended to:

　・Use 1.2 times for sheet gasket (joint sheet etc.)

　・Use 1.3 to 1.5 times for semi-metallic gasket (spiral wound gasket, etc.)

This is sometimes referred as the recommended tightening torque.

※ However, please be careful not to exceed allowable tightening torque.

※ Please determine the compatibility of bolt strength against tightening force with your own judgement.

By selecting the bolt material via the "Tightening Torque Calculator" of Gasket NAVI, the value of design bolt stress and the minimum yield point of the bolt can also be displayed in the calculation result. Please feel free to use it.

※Source ： JIS Z 8126:1999（Vacuum Technology-Vocabulary）. Unit：mmHg, Torr are unit conversion.

There are two types of representation systems used in pressure measurement : Gauge pressure which uses atmospheric pressure (1 atm) as its zero point and absolute pressure which uses absolute vacuum as its zero point.

In general, pressure used at positive pressure (pressurized) in piping, equipment, and etc. is expressed by gauge pressure (denoted by ‘G’ after the unit). The pressure used at negative pressure (depressurized) in vacuum equipment and etc.  is expressed by absolute pressure (denoted by ‘a’ or ‘abs’ after the unit)※. Other than the unit [Pa], [mmHg][Torr] are also commonly used.

※However, negative pressure can also be expressed by gauge pressure.

●F.V.（Full Vacuum）

There is no problem as long as the air (atmospheric) with differential pressure up to 0.1 MPa can be sealed. In a similar way of selecting a gasket under pressurized condition, in addition to considering the corrosion resistance to internal fluids, heat resistance and stress relaxation characteristics, gasket with excellent air sealing property should be selected.

The gasket selection based on Japan Petroleum Institute Standard, JPI-7S-81：2005 “Basic Standard of Gaskets for Piping” is covered until 0.7kPa abs.

●Vacuum Equipment

Rubber O ring (TOMBO No. 2670), metal O-ring (TOMBO No. 9200 series) and etc. are used when high degree of vacuum is desired.

When increasing the degree of vacuum, the rubber O ring is affected by gas permeation while the metal O-ring is affected by the surface roughness of the flange and gasket.

Furthermore, when using a vent hole type metal O-ring (TOMBO No. 9200 V), it is necessary to open the hole at the outer diameter side for vacuum applications.

Example of gasket selection for negative pressure application.

● Rubber Sheet Gasket

Since the gasket is easily pulled into the bore, hence despite the flange is a flat seat flange (raised face, RF), it is still necessary to take countermeasures such as making the shape of the gasket into full face (FF) type.

●PTFE Envelope Gasket

Since the outer skin is slippery and structurally easy to be pulled into the bore, it is necessary to take measures such as making a full-face shaped outer skin or apply welding or sewing to the outer circumference of skin.

●Jointing Sheet Gasket and Expanded Graphite Gasket

Since air is more probable to penetrate into the gasket, supplementary material such as gasket paste and impermeability treatment might be needed.

●Fluoropolymer Gasket

There is no special countermeasure required. Since pure PTFE gasket exhibits high creep deformation, it is suitable to be used for Tongue and Groove (T&G) flange seating.

Case ①

Case ②

Case ③