C-11 GASKETS

TRAINING MANUAL- PIPING

GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 1

CONTENTS

Page

0.0 Cover Sheet 1

1.0 Introduction 2 – 3

2.0 Gasket Types 3

3.0 Gasket Selection 3 – 5

4.0 Standards 5

5.0 Figure 6

Applicable Revision: R1Prepared: NNG

Date: 15.01.2001

Checked: AKB

Date:

Approved: RUD

Date:First Edition: R0Prepared: DNL

Date: 14.09.2000

Checked: AKB

Date: 20.09.2000

Approved: RUD

Date: 31.10.2000File Name: C- 11 Server: PUNE:KUMUS 207 VKO: KUMUS 209

Directory: PUNE: Refer \ Pi \ Training Manual VKO: Refer \ Training Manual


GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 2

1.0 INTRODUCTION:

Leak-proof metal-to-metal surfaces in which gaskets are not used are difficult to fabricateeven by use of very accurate machined surfaces. Irregularities in clearances of only a fewmillionths of an inch will permit the escape of a fluid under pressure. The function of a gasketis to interpose a semi-plastic material between the flange facings, the material which, throughdeformation under load seals the minute surface irregularities to prevent leakage of the fluid.The amount of flow of the gasket material that is required to produce a tight seal is dependentupon the roughness of the surface. The amount of force that must be applied to the gasket tocause the gasket to flow and seal the surface irregularities is known as the “yield” or “seating”force. This force is usually expressed as a unit stress in pounds per square inch and isindependent of the pressure in the vessel. Thus, this yield stress represents the minimumload that must be applied to the gasket to seat it even though very low pressure are used inthe vessel. Usually the gasket is seated by tightening the bolt load on the flanges prior to theapplication of the internal pressure in the vessel.

Upon the application of the internal pressure in the vessel, an end force tends to separate theflanges and to decrease the unit stress on the gasket. Figure 1.0 shows the three majorforces acting on the gasket.

Leakage will occur under pressure if the hydrostatic end force is sufficiently great and thedifference between hydrostatic end force and the bolt-load force reduces the gasket loadbelow a critical value. Also, it may be possible with too low a contact pressure on the gasketfor the gasket to be blown out by the internal pressure. The ratio of the gasket stress, whenthe vessel is under pressure, to the internal pressure is termed the “gasket factor”. Thegasket factor is a property of the gasket material and the construction and is independent ofthe internal pressure over a wide range of pressures. In selecting the proper gasket for anexisting closure, one of the first steps should involve the determination of the total amount offorce necessary to make the gasket yield and to maintain a tight seal under operatingconditions.

Appendix-2, Table 2.5.1 of ASME Section VIII Div.1 shows sectional views of some commontypes of gaskets and lists the gasket factor 'm' and the minimum design seating stress, 'y' foreach type of gasket.

There is a considerable possible choice of gasket material in may applications. The decisionas to which gasket material is to be selected is often based upon the required gasket width. Ifthe gasket is made too narrow, the unit pressure on it may be excessive. If the gasket ismade too wide, the bolt load will be unnecessarily increased. A relationship for making apreliminary estimate of the proportions of the gasket may be derived as followed :

(Gasket seating force) - (Hydrostatic pressure force) = (Residual Gasket force )

The residual gasket force can not be less than that required to prevent leakage of the internalfluid under operating pressure. Therefore

(do2 - di

2) y - (do2 ) p = (do

2 - di2 ) pm

where,

y = yield stress, pound per square inchm = gasket factorp = internal pressure, pounds per square inchdo = outside diameter of gasket, inchesdi = inside diameter of gasket, inches

π4

π4

π4


GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 3

Above equation may be re-written as follows :

=

In the case where it is desirable to retain the gasket material selected and to retain the gasketwidth, a gasket seating stress greater than y may be used with certain reservations. If theseating stress greatly exceeds y, the gasket may be crushed, or a ductile, unrestrained gasketbe squeezed out between the flange faces. In general the use of seating stresses exceedingy should be limited to solid-metal gaskets in tongue-and-grove joints.

2.0 GASKET TYPES:

Gaskets can be defined into three main categories as follows:

• Non-metallic Gaskets:

Usually composite sheet materials are used with flat face Flanges and low pressure classapplications. Non-metallic gaskets are manufactured non-asbestos material orCompressed Asbestos Fibre (CAF). Non-asbestos types include arimid fibre, glass fibre,elastomer, Teflon (PTFE) and flexible graphite gaskets. Full face gasket types aresuitable for use with flat-face (FF) flanges and flat-ring gasket types are suitable for usewith raised face (RF) flanges.

• Semi-metallic Gaskets:

Semi-metallic gaskets are composites of metal and non-metallic materials. The metal isintended to offer strength and resiliency while the non-metallic portion of a gasketprovides conformability and sealability Commonly used semi-metallic gaskets are spiralwound, metal jacketed, Cam profile and a variety of metal-reinforced graphite gaskets.Semi metallic gaskets are designed for the widest range of operating conditions oftemperature and pressure. Semi-metallic gaskets are used on raised face, male-and-female and tongue-and –groove flanges.

• Metallic Gaskets:

Metallic gaskets are fabricated from one or a combination of metal to the desired shapeand size. Common metallic gaskets are ring-joint gaskets and lens rings. They aresuitable for high-pressure and temperature applications and require high bolt load to seal.

3.0 GASKET SELECTION:

i. The gasket material selected should be one which is not adversely affected physically orchemically by the service conditions.

ii. The two types of gaskets most commonly known are ring gaskets and full face gaskets.The latter as the name implies, covers the entire flange face and are pierced by the boltholes. They are intended for use with flat face flanges. Ring gaskets extend to the insideof the flange bolt holes and consequently are self centering. They are usually used withraised face or lap joint flanges but may also be used with flat-faced flanges.

dodi

y - p m y - p ( m + 1)


GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 4

iii. Flat-ring gaskets are widely used wherever service condition permits because of the easewith which they may be cut from flat sheet and installed. They are commonly fabricatedfrom such materials as rubber, paper, cloth, asbestos, plastics, copper, lead, aluminum,nickel, monel, and soft iron. The gaskets are usually made in thickness from 1/64 to 1/8in. Paper, cloth and rubber gaskets are not recommended for use above 250° F.Asbestos-composition gaskets may be used up to 650° F or slightly higher, ferrous andnickel-alloy metal gaskets may be used up to the maximum temperature rating of theflanges.

iv. Upon initial compression a gasket will flow both axially and radially. The axial flow isrequired to fill depressions in the flange facing and prevent leakage. Radial flow serves nouseful purpose unless the gasket is confined.

Where a flange joint is heated, a greater gasket pressure is produced due to thedifference between the flange body and the bolts. This greater pressure coupled with theusual softening of the gasket material at elevated temperatures causes additional axialand radial gasket flow. To compensate for this, the flange bolts are usually re-tightened asecond or third time after the joint is heated to the normal operating temperature.

A thick gasket will flow radially to a far greater extent than a thin gasket. Some thingaskets show practically no radial flow at extremely high unit pressures. Consequently, forhigh temperatures a thin gasket has the advantage of maintaining a permanent thicknesswhile a thick gasket will continue to flow radially and may leak, in time, due to the resultingreduced gasket pressure. However in attempting utmost utilization of thin gasketadvantage, one may find that gasket selected has insufficient thickness to seal theirregularities, in the commercial flange faces.

The spiral wound asbestos-metallic gasket combines the advantages of both the thickand thin gasket. Although a relatively thick gasket (most common types are 0.175” thick)its spirally laminated construction confines the asbestos filler between axially flexiblemetal layers. This eliminates the radial flow characteristics of a thick gasket and providesthe resiliency to adjust to vary service conditions. Spiral wound gaskets are available withdifferent filler materials such as Teflon, grafoil etc. to suit fluid compatibility. Spiral woundgaskets used with raised face flanges usually have an inner metal ring and an outercentering ring.

iv. Laminated gaskets are fabricated with a metal jacket and a soft filler, usually of asbestos.Such gaskets can be used up to temperatures of about 750° F to 850° F and require lessbolt load to seat and keep tight than solid metal flat ring gaskets.

v. Serrated metal gaskets are fabricated of solid metal and have concentric groovesmachined into the faces. This greatly reduces the contact area on initially tighteningthereby reducing the bolt load. As the gasket is deformed, the contact surface areaincreases. Serrated gaskets are useful where soft gaskets or laminated gaskets areunsatisfactory and bolt load is excessive with a flat-ring metal gasket. Smooth-finishedflange faces should be used with serrated gaskets.

vi. Corrugated gaskets with asbestos filling are similar to laminated gaskets except that thesurface is rigid with concentric rings as with the case of serrated gaskets. Corrugatedgaskets require less seating force than laminated or serrated gaskets and are extensivelyused in low-pressure liquid and gas service. Corrugated metal gaskets without asbestosmay be used to higher temperature than those with asbestos filling.

vii. Two standard types of ring-joint gaskets are available for high-pressure service. One typehas an oval cross section, and the other has an octagonal cross section. These rings arefabricated of solid metal, usually soft iron, soft steel, monel, 4-6% chrome, and stainlesssteels. The alloy-steel rings should be heat treated to soften them.


GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 5

viii. It is recommended that only Group No. 1 (Refer Appendix-I) or ring joint gasket be usedfor class 150 flanged joints. When the ring joint or spiral wound gasket is selected, it isrecommended that line flanges be of the welding neck type.

ix. If low strength bolts are used, gaskets are listed under group ‘Ia’ (Refer appendix-I) shallbe used.

4.0 STANDARDS:

Following standards are usually adopted for specifying gaskets.

• ASME B16.21 Non-metallic flat gaskets for pipe flanges.• ASME B16.20 Metallic Gaskets for steel pipe flanges, Ring Joint, Spiral Wound

and Jacketed• IS2712 Specification for compressed Asbestos fibre jointing.• BS 3381 Sprial Wound Gaskets to suit BS 1560 Flanges

IS 2712 covers six grades of Compressed Asbestos Fibre (CAF) sheets, classified based onapplication as follows:

GRADE APPLICATION

w/1 Water, steam & for some chemicals - High Service conditionsw/2 Water, steam & for some chemicals - Medium Service conditionsw/3 Water, steam & for some chemicals - Low Service conditionso/1 oils - High Service conditionso/2 oils - Medium and nominal service conditionsa/1 acids - highly corrosive

Usually grades W/1, O/1, and a/1 are used in nominal thickness of 1.5mm for raised faceflanges and 3mm thick for flat face flanges.

R1


GASKETS

Uhde India Limited

DOC No. : 29040-PI-UFR-0011

Rev. : R1

Page : 6

5.0 FIGURE:

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C-11 GASKETS