FLANGE LEAKAGE ANALYSIS
FLANGE
INTRODUCTION TYPES OF FLANGES FLANGE LEAKAGE ANALYSIS METHODS
Introduction
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Types Of Flanges WELDING NECK FLANGE SLIP - ON FLANGE LAP - JOINT FLANGE SOCKET WELDED FLANGE THREADED FLANGE BLIND FLANGE
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Slip - On Flange
They are typically used on low-pressure, low-hazard services such as fire water, cooling water and other services. Features: Lower cost Reduced accuracy required in cutting the pipe to length Greater 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.
Welding Neck Flange
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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.
Lap - Joint Flange
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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.
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Socket Welded Flange
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.
Threaded Flange
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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.
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Blind Flange
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.
Flange Leakage Analysis
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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.
Fluid Service 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.
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Flanged Joint Behavior A typical flanged joint as shown in Figure 1 and consists of four interdependent 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.
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Bolts 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:
FpLb Lb EAs Lb Fp Lb
= = =
E As
= =
Change in length of bolt, (in) Applied tensile load, (lb) Effective length of bolt length in which tensile stress is applied (in) Young‟s Modulus of elasticity, psi Tensile stress area of bolts, in²
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Gasket Gasket 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. 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.
Typical Gasket Behavior
Typical Gasket Behavior 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.
Bolt Load And Gasket Reaction
Mechanical model Description of the equilibrium :
Balance of the assembly axial forces
Bolt Load And Gasket Reaction 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” HG
HG = W-H G
H Where W : Bolt Load HG : Gasket Load H : Hydrostatic End force
w
w
w
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
Bolt Load And Gasket Reaction 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 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 Wm1 b
= Required bolt load for gasket seating. = Required bolt load for operating conditions. = Effective gasket seating width.
Bolt Load And Gasket Reaction 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
Flange Loading
Flange Loading
Flange Loading The total flange moment under operating conditions is,
M 0 H D hD H T hT H G hG The total flange moment for gasket seating is
M0 W
C G 2
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 flange HT 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 Stresses Longitudinal Hub Stress
SH
fM 0 Lg 12 B
Radial Flange Stress
SR
1.33te 1M 0 Lt 2 B
Tangential Flange Stress
ST
YM 0 ZS R 2 t B
For Notations, Please refer ASME Sec VIII, Div 1, Appendix 2, para 2-3
Flange Leakage Calculation
Flange Allowable Stresses These are the longitudinal hub stress SH, radial flange stress SR and the tangential flange stress ST, which are limited by,
S H 1.5S f
SH SR Sf 2
SR S f
ST S f
S H ST Sf 2
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.
Equivalent Pressure Method 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. [Applied Force / Area of gasket; Sg = F / (π * G * b) + M / (π /4 * G² * b) Applied moment / Second moment of gasket area Sg
=
Peq * (π / 4) * G² / (π * G * b)
= = = = =
Stress on gasket Applied pipe force Gasket diameter Gasket width Equivalent Pressure due to external Moment & Force
Pressure * Internal area / Area of gasket]
Where: Sg F G b Peq
Equivalent Pressure Method 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.
