flange leakage.ppt

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An overview of flange leakage check in piping stress analysis

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Page 1: Flange Leakage.ppt

FLANGE LEAKAGE ANALYSISFLANGE LEAKAGE ANALYSIS

Page 2: Flange Leakage.ppt

TYPES OF FLANGES

FLANGEFLANGE

FLANGE LEAKAGE ANALYSIS METHODS

INTRODUCTION

Page 3: Flange Leakage.ppt

IntroductionIntroduction

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TypesTypes Of Of FlangesFlanges

WELDING NECK FLANGE

SLIP - ON FLANGE

LAP - JOINT FLANGE

BLIND FLANGE

SOCKET WELDED FLANGE

Back

THREADED FLANGE

Page 5: Flange Leakage.ppt

Slip - On FlangeSlip - On Flange

Back

They are typically used on low-pressure, low-hazard services such as fire water, cooling water and other services.Features:Lower costReduced accuracy required in cutting the pipe to lengthGreater ease of installation. Limitations:Strength under internal pressure is of the order of two-thirds that of WNRF.Life under fatigue is about one-third that of the WNRF. Hence, slip-on flanges are limited in sizes up to 2½” for 1500 #.The ASME Boiler Construction Code limits their use to the 4” size.

Page 6: Flange Leakage.ppt

Welding Neck Welding Neck FlangeFlange

Back

They are suitable for conditions where pressure as well as temperature are high. Normally used in petrochemical and refinery plants for all process service conditions.

Features:

Long tapered hub provides an important reinforcement for the flange from the standpoint of strength and resistance to dishing.The smooth transition from flange thickness to pipe wall thickness by the tapered hub is extremely beneficial under conditions of repeated bending, caused by line expansion or other variable forces. Thus this type of flange is preferred for very severe service condition.

Page 7: Flange Leakage.ppt

Lap - Joint Lap - Joint FlangeFlange

Back

Lap Joint Flanges are cost effective in expensive pipe such as stainless steel due to the fact that only the stub must match the pipe and the flange can be made of cheaper carbon steel material. Their pressure holding ability is better then that of SORF. The chief use of lap joint flanges in carbon or low alloy steel piping systems is:

• Services demanding frequent dismantling for inspection and cleaning.• Where the ability to swivel flanges and to align bolt holes simplifies the erection of large diameter or exceptionally stiff piping.

The fatigue life of the assembly is only one-tenth that of WNRF. Their use at points where severe bending stress occurs should be avoided.

Page 8: Flange Leakage.ppt

Socket Welded Socket Welded FlangeFlange

Back

Socket Welding Flanges were initially developed for use on small size high pressure piping. Their initial cost is about 10% greater than that of slip-on flanges. Their fatigue strength is 50% greater than slip-on flanges.

Page 9: Flange Leakage.ppt

ThreadedThreaded FlangeFlange

Back

Threaded flanges made of steel, are confined to special applications. Their chief merit lies in the fact that they can be assembled without welding. This explains their use in extremely high pressure services, where alloy steel is essential for strength and where the necessary post weld heat treatment is impractical.

Limitations

Threaded flanges are not suited for conditions involving temperature or bending stresses of any magnitude. Under cyclic conditions, leakage through the threads may occur in relatively few cycles. Seal welding is sometimes used to overcome this, but can not be considered as entirely satisfactory.

Page 10: Flange Leakage.ppt

Blind FlangeBlind FlangeBack

Blind flanges are used to blank off the ends of piping, valves, and pressure vessels openings. From the standpoint of internal pressure and bolt loading, blind flanges, particularly in the large sizes, are the most highly stressed.

Page 11: Flange Leakage.ppt

Normally, in spite of tight bolted connection between flanges, due to thermal growth of the piping / excessive deflection, bending moment will be created, which tries to open up the flange joint, causing the fluid leakage, which is hazardous. Hence, in refinery plants, the flange leakage analysis becomes mandatory for the following conditions.

• When nonstandard sizes of piping or flanges are specified.• When the application is critical; for example, Category M fluids• Where large bending moments exist at flanged joints.• As per project specification / guidelines.

Flange Leakage AnalysisFlange Leakage AnalysisBack

Page 12: Flange Leakage.ppt

Category M Fluid Service:

A fluid service in which the potential for personnel exposure is judged to be significant and in which a single exposure to a very small quantity of a toxic fluid, caused by leakage, can produce serious irreversible harm to persons on breathing or bodily contact.

Fluid ServiceFluid ServiceBack

Page 13: Flange Leakage.ppt

Flanged Joint Flanged Joint BehaviorBehavior

A typical flanged joint as shown in Figure 1 and consists of four inter-dependent elements; Bolts, Gasket, Flange ring, Taper hub. In different type of joints, these elements may change in shape but they retain their basic functions and perform in a similar way.

Page 14: Flange Leakage.ppt

Bolts are used to assemble / disassemble a flanged joint. They are also required to hold the joint together under pressure and to pre-stress the gasket sufficiently to enable it to function as a seal.

All bolts behave like a heavy spring. As you turn down the nut against the flange, the bolt stretches and the flange and gasket compress.

Bolts stretch according to Hooke’s law:

BoltsBolts

Back

AsELbFpLb

Lb = Change in length of bolt, (in)

Fp = Applied tensile load, (lb)

Lb = Effective length of bolt length in which

tensile stress is applied (in)E = Young’s Modulus of elasticity, psi As = Tensile stress area of bolts, in²

Page 15: Flange Leakage.ppt

GasketGasketGasket is introduced between the flanges to prevent the contained fluid from leaking. It is usually made from a softer medium and is thereby capable of adapting to the shape of the flange surfaces, making intimate contact. Sealing can thus be achieved at a lower pre-stress and more economically than would be required with two metal flange faces being brought together without a gasket. Gaskets are also convenient because they are relatively cheap and easy to replace and should require minimal rework when in service. Tightening of the bolts with correct pre-stressing of the gasket is vital to the successful performance of a joint.

Back

In high-temperature services, the flanges will heat up at a faster rate than the bolts. This results in a higher thermal expansion of the flanges with respect to the bolts, increasing the bolt load and gasket stress. The gasket will then deform under the higher applied load during this cycle. Most gasket will deform permanently and will not rebound when the cycle goes away. During the cooling cycle, the bolt load will decrease and hence loss of gasket stress. As gasket stress decreases leak rate increases.

Page 16: Flange Leakage.ppt

Typical Gasket Behavior

Page 17: Flange Leakage.ppt

Figure illustrates some of the more common typical gasket characteristics. On first loading, as the bolts are tightened up, the gasket usually follows a non-linear and non-recoverable path. During this initial phase (O-A) the gasket is forced to conform to the flange faces, filling the irregularities present on any surface. The point at which the gasket provides the minimum effective seal is known as the gasket seating stress (y).The region marked A-B-C is the useful sealing range of the gasket. For an effective seal, the joint should be assembled to some stress value between the gasket seating stress (y) at point A and the crushing limit of the gasket at point C. The seating stress is given in the code. The crushing limit is usually be obtained from gasket manufacturers. When the gasket is compressed beyond its crushing limit, some form of breakdown usually occurs in such a manner that joint sealing is adversely affected.If the gasket is tightened to some value between A and C and then the gasket is unloaded (by internal pressure or bolt loosening), it will follow a path something like `B-B'. When the gasket is reloaded it will follow a path close to the decompression line. When the loading again reaches point B, the gasket then continues to follow the initial loading curve A-C as though it had never been unloaded. At some point during the unloading of the gasket, it reaches a point at which it can no longer reliably perform its sealing functions. This minimum gasket sealing stress (or pressure) is dependent on the gasket type and the internal pressure. It is usually calculated from the product of the gasket factor m and the maximum internal pressure in order to ensure that the gasket pressure always exceeds the internal pressure.

Typical Gasket Behavior

Page 18: Flange Leakage.ppt

Bolt Load And Gasket Reaction

Description of the equilibrium :

Balance of the assembly axial forces

Mechanical model

Page 19: Flange Leakage.ppt

When a flange is bolted up and is not under internal pressure, the bolt load is balanced by the Gasket Reaction.

To secure a tight joint, it is necessary to seat the gasket properly by applying a minimum load in the cold condition . This load is a function of the gasket material and the effective gasket area to be seated. This is known as minimum gasket seating stress “y”

As internal pressure is applied, the bolt load is balanced by the sum of the gasket reaction, pressure load on flange face and hydrostatic end load below.

w w

G

HG

Where W : Bolt LoadHG : Gasket LoadH : Hydrostatic End force

w w

H

HG = W-H

Bolt Load And Gasket Reaction

Page 20: Flange Leakage.ppt

The compressive load on the gasket is reduced as the internal pressure increases. Leakage will occur when the gasket pressure reduces to some gasket minimum sealing pressure (Pgm). Theoretically a joint will seal provided the gasket pressure remains greater than the internal pressure. but in practice it is found that in order to have some margin of safety against leakage, it is necessary to keep the gasket pressure above the internal pressure P, by some factor ‘m’

i.e. Pgm >= m*P

where “m” is the gasket factor which is a function of gasket material. The Code equation defines this term as the ratio of residual gasket load (Original load - Internal fluid pressure) to fluid pressure at leak.

Bolt Load And Gasket Reaction

Page 21: Flange Leakage.ppt

The bolt loads used in calculating the required cross-sectional area of bolts shall be determined as follows.

(1) Wm2 = y*π*G*b

=gasket seating stress * surface area(2*pi*r*L)

(2) Wm1 = P*π/4*G² + 2*m*P*π*G*b

The force applied from bolts (outside) should be sufficient enough for

a) for providing enough gasket seating stress

b) overcoming the internal pressure

The first term on the right side represents the hydrostatic pressure load acting on the effective gasket diameter, and the second term gasket reaction. The effective gasket contact width becomes 2b because of the appearance of the factors mP in place of y.

Wm2 = Required bolt load for gasket seating.Wm1 = Required bolt load for operating conditions.b = Effective gasket seating width.G = Diameter at location of gasket load reaction.y = Gasket or joint -contact surface unit seating load.m = Gasket factor.P = Design Pressure.

“y” factor is the minimum gasket seating stress that is required to seat the gasket to prevent leaks in the joint as the system is pressurized. It is the flange pressure to compress the gasket enough to eliminate pores.

“m” is the gasket factor. The Code equation defines this term as the ratio of residual gasket load (Original load - Internal fluid pressure) to fluid pressure at leak.

Bolt Load And Gasket Reaction

Page 22: Flange Leakage.ppt

In the second equation, the first term on the right side represents the hydrostatic pressure load acting on the effective gasket diameter, and the second term gasket reaction. The effective gasket contact width becomes 2b because of the appearance of the factors mP in place of y.

Bolt Area:

If Sb denotes the allowable stress at the operating temperature of the bolts and Sa the allowable bolt stress at atmospheric temperature, then the minimum total bolt area Am required is obtained as follows.Am = Wm1/Sb or Wm2 /Sa,

Whichever is greater.

Bolt Load:

Under operating condition, bolt load (W) is:W = Wm1

For gasket seating,W = AbSa

where Sa is Bolt allowable stress at ambient temperature

Bolt Load And Gasket Reaction

Page 23: Flange Leakage.ppt

Flange Loading

Page 24: Flange Leakage.ppt

Flange Loading

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The total flange moment under operating conditions is,

HG is the gasket minimum sealing load as given in the second part of the equation for Wm1 which is considered to be located at the gasket effective diameter i.e. at a distance hG from the PCD.HD represents Hydrostatic end force on the inside area of the flangeHT is the difference between total hydrostatic end force and hydrostatic end force on inside area of flange i.e H-HD where H is the total hydrostatic end force (π/4*G²*P)

Flange Loading

The total flange moment for gasket seating is

20

GCWM

GGTTDD hHhHhHM 0

Page 26: Flange Leakage.ppt

Flange Stresses

Longitudinal Hub Stress

Radial Flange Stress

Tangential Flange Stress

BLg

fMSH 2

1

0

BLt

MteSR 2

0133.1

RT ZSBt

YMS

20

For Notations, Please refer ASME Sec VIII, Div 1, Appendix 2, para 2-3

Flange Leakage Calculation

Page 27: Flange Leakage.ppt

These are the longitudinal hub stress SH, radial flange stress SR and the tangential flange stress ST, which are limited by,

where, SF is the Allowable stress for the flange at the operating temperature.For Gasket Seating, use corresponding M0 for calculating the stresses and compare with Allowable stress at ambient temperature.From these it can be seen that since the allowable design stress is usually about 2/3 of the material yield, then this allows the hub to be stressed up to the material yield point, allowing yielding in the hub during hydrotest. The flange stress limits are set to a level which should keep the main flange bodies elastic under all conditions, providing the joint is not over tightened during bolting-up. The latter two stress limits are the application of a Tresca type criterion to the bi-directional stresses at the interface between the flange and hub.

Flange Allowable Stresses

fTH S

SS

2

fH SS 5.1 fR SS fT SS

fRH S

SS

2

Page 28: Flange Leakage.ppt

The equivalent pressure method combines the effect of external load with design pressure.In 50's, the equivalent pressure method was devised at Kellogg and has been adopted in some ASME sections and is frequently used in industry.

In the Equivalent Pressure Method, we compare the total pressure on the flange with the Test pressure given in the flange standard.

Sg = F / (π * G * b) + M / (π /4 * G² * b)

Sg = Peq * (π / 4) * G² / (π * G * b)

Where:

Sg = Stress on gasketF = Applied pipe forceG = Gasket diameterb = Gasket widthPeq = Equivalent Pressure due to external Moment & Force

[Applied Force / Area of gasket;Applied moment / Second moment of gasket area

Pressure * Internal area / Area of gasket]

Equivalent Pressure Equivalent Pressure MethodMethod

Page 29: Flange Leakage.ppt

So, to determine what equivalent pressure would cause the same stress as the applied piping loads, set them equal to each other:

F / (π * G * b) + M / (π /4 * G² * b) = Peq * (π /4) * G² / (π* G * b)

Simplifying:

4F / (π * G²) + 16M / (π * G3) = Peq

PTotal = Peq + P

This is known as Pressure Equivalent Method.

Equivalent Pressure Equivalent Pressure MethodMethod