Principles of Static Sealing
|The Function of a Gasket|
A gasket is used to create and retain a static seal between two stationary members of mechanical assemblies containing a wide variety of liquids or gases.
Gaskets are necessary and often vital to the satisfactory functioning of a broad range of industrial equipment. Hence, it is incumbent upon the gasket user to understand certain principles of sealing to select the correct gasket.
Gasket selection requires a knowledge of flange design, bolting data, dynamic forces and the behavior of gaskets when exposed to various temperatures, pressures and fluids.
Conditions that Occur in a Gasket Joint
In any dynamic mechanical assembly employing a gasket, a combination of forces act upon the gasketed joint. The net effect can be described, figuratively, as a battle between the gasket and the internal forces acting to penetrate or displace it.
The performance of any gasket is influenced by the following variables:
Flat Face Flange
Mating faces of both flanges are flat. Gaskets may be ring type, entirely inside of the bolts, or the full face type, covering essentially the entire face with holes punched for bolt clearance.
Raised Face Flange
Mating face is flat, but a portion inside of the bolt holes is raised 1/16" or 1/4". The gasket is usually a ring type, entirely within the bolts. Steel flanges that are to mate with cast-iron should have flat faces, to prevent breaking the cast-iron flange during bolt tightening.
Lapped Joint Flange
The facing is similar to the raised face type of flange. This connection is generally used when the process requires alloy piping, but the flanges may be made from a less exotic metal.
|Confined & Semi-Confined Gaskets|
Tongue & Groove Flange
The depth of the groove is generally equal to or less then the height of the tongue. The groove is not normally over 1/16" wider than the tongue. The joint must be pried open at disassembly. However, tongue & groove flanges do produce a high pressure on the gasket.
Ring Gasket Joint Flange
Both flanges have matching flat bottom grooves with the sides tapered from the vertical. The gasket is machined from solid metal and is either oval or octagonal in cross section.
Male - Female Flange
The depth of the recessed face is normally equal to or less than the height of the male or raised face, to prevent the possibility of flanges coming together when the gasket is compressed.
Male - Female Flange
This type of flange confines the gasket, usually an O-ring to the inside diameter of the flange.
Flat & Groove Flange
This is a metal-to-metal type of joint, with the gasket confined in a groove. The gasket is usually thicker than the depth of the groove.
|BASIC FLANGE DESIGN:|
While a detailed discussion of flange design is beyond the scope of this section, understanding basic flange features is essential to the selection of a gasket.
There are many recognized standard flange designs, such as those defined by ANSI, MSS, API and AWWA. ANSI Standard B16.5 currently is the most widely recognized and used. These standard flange designs are employed for piping in preference to custom designs, such as those based on the ASME Boiler and Pressure Vessel Code guidelines.
FLANGE FACING TYPES:
Examples of flange types are shown in the pictures above. The raised face design is most commonly used. The flat face design is normally used with flanges that are easily fractured or crushed. Male & female small, tongue & groove small, ring gasket joint and metal to metal designs are not suitable for most non-metallic gasket material.
Concentric Serrated Flange
Phonograph Finish Flange
Carbon Paper Print of Contact Pressure Distribution
Flange Surface FinishesWhile the majority of flange materials are metallic, non-metallic flanges, such as glass or reinforced plastic, are now being used.
Metallic flange surfaces may range from a rough casting to that produced by machine lapping, and each type of surface influences sealing effectiveness. Surface roughness is usually measured in micro-inches (micrometers) as an Arithmetic Average Roughness Height (AARH) or Root Mean Square (RMS). The former method is currently preferred.
Commonly used finishes for pipe flanges for non-metallic gaskets are the serrated-concentric and serrated-spiral (phonograph) finishes. Both finishes are usually made with tools of similar shape and the flange faces are cut to various depths, depending upon the metal. Both serrated finishes consist of a series of cuts, whose width also varies with the type of metal.
The serrated-spiral finish is spirally cut, with a continuous spiral groove extending from the bore of the flange to the outer perimeter (see pictures above). The serrated-concentric finish has definite "hills and valleys", each endless.
"Smooth" finishes which appear to have no surface irregularities are also quite common. When microscopically viewed, the face presents a different picture. The "smooth" finish has wavy contours and slight surface irregularities, which cannot be sealed by naked face-to-face flange contact, and a gasket is an essential requirement.
There are general rules applicable to surface finishes:
Bolting & Flange ForcesIn most flanges, the distribution of force around the gasket is usually not the same at all points. For example, two large diameter bolts could supply the same force as 12 smaller diameter bolts, but the distribution of force would be poor. Therefore, to equalize as near as possible the distribution of the load on a gasket, the greater number of properly spaced bolts should be used.
As bolts are torqued, the result is a compressive load on the flange and gasket; however, the gasket area surrounding the bolts undergoes greater compression while the lowest compression on the gasket occurs mid-way between the bolts due to flange bowing (see picture above: "Contact Pressure").
A basic question to be considered in any gasket application is whether there is enough bolting force to create a final, enduring seal. An initial step is the design of adequate bolt load on the gasket as it is being installed, to insure flange / gasket conformity. This load, as already mentioned, is the minimum seating stress. Its numerical value ("y" factor) is dependent on the gasket material itself, as well as its thickness and contour. Soft materials, such as cellulose fiber or a low durometer rubber, do not require nearly the stress that harder materials do, such as compressed materials. To assure an adequate seal at operating pressure, an additional preload on the gasket may be required. This preload, the maintenance or "m" factor, also is dependent on the gasket material. Thus, in designing a joint, both the "y" and "m" factors should be obtained from the gasket manufacturer.
|Practical assembling of a gasketed flange requires tightening of a series of bolts to affect the required compressive load. Ideally, the amount of gasket compression used is to assure that minimum gasket factors and seating stresses are achieved.|
Usually, the bolts must be torqued to assure that the minimum initial assembly stress (S) is achieved. The calculation for S takes into account the hydrostatic end thrust (H), the gasket factor "m" and the minimum seating stress "y".
|S = y + mI + H/A|
|Where: I = internal pressure|
A = area of gasket under compression
H = hydrostatic end thrust
a = area enclosed by mean dimension of the gasket
|Since: H = I x a,|
|Then: S = y + mI + Ia/A|
|After the minimum assembly stress, S, is calculated and the proper bolt spacing is established, the minimum torque per bolt, t, can be calculated:|
|t = S x A x D x 0.2/N|
|Where 0.2 = factor for loss due to friction|
A = gasket area
D = bolt diameter
N = number of bolts
|Each bolt can only be torqued to a maximum limit. Exceeding this limit would stretch the bolt beyond its yield point (the point at which it is permanently deformed), or possibly strip the threads beyond usefulness.|
Care must also be exercised so that over-torquing does not distort the flanges so that they are out of parallel. Listed in the table above are Torque Data, which show the limits to which alloy steel bolts and studs may be subjected for those grades shown by manufacturers with a yield point above 70,000 PSI.
To determine the flange load on the gasket, the following formula applies:
|Approximate flange load (gasket seating stress) =|
N x T/0.2 x D x A
|Where: N = number of bolts|
T = maximum torque (in.-lbs.)
D = diameter of bolt (in.)
A = area of gasket (in2.)
0.2 = factor for friction
|It must be cautioned that flange load calculations are only approximate. Dry bolts require greater torquing effort than lubricated bolts, since there is an exceptional amount of friction to overcome. Heavier flanges, higher yield bolts, etc., are all significant in arriving at an accurate flange load value. Use the formula only as a guide.|
It must also be re-stated that flanges are specifically designed for certain pressure and temperature applications and that obtaining the initial seal is only one part of the gasketing story. There other conditions to consider before a selection is made.
|The sequence in which bolts are tightened has a substantial bearing upon the distribution of the contact area stress. Improper bolting may move the flange out of parallel. A gasket will usually compensate for a small amount of distortion of this type, but serious difficulties may be encountered if close paralleling is not maintained.|
Correct bolting procedures are shown in the picture above. Following the recommended procedure, these rules should be observed:
Note: It is important for proper sealing that the flanges are clean and free of any serious defects.
|CREATING THE INITIAL SEAL|
A seal is effected by compressing and forcing the gasket material into the imperfection of the joint. A tight, unbroken barrier occurs, leaving no interface pathway for escape of the confined fluid.
It is the gasket surface only that does the sealing. The body of the gasket essentially provides the elastic, resilient properties. It is true that there are gasketing materials which may exhibit porosity, i.e., leakage, through the body of the gasket under certain conditions. A good seal is often obtained, particularly for elastomeric gasketing materials, through "swell" of the gasket in contact with the fluid it is sealing. A certain amount of swell is desirable, provided the swell reaches a stage of equilibrium and is not a degradation process signaling the breakdown of the gasket material.
CONSIDERATIONS TO MINIMIZE TORQUE LOSS
One of the first considerations for minimizing torque loss is choice of gasketing material. Some materials have inherently less creep relaxation than others and are less subject to torque loss. For most materials the factors listed below also influence torque retention.
LUBRICATING, BOLTING & WASHERS
Further, minimizing of torque loss may be obtained through refinement of hardware holding the flange together. As a rule, the higher the initial gasket load the less torque loss occurs. To obtain a higher torque load, the following alternatives can be considered:
|RETAINING THE INITIAL SEAL|
Obtaining a good gasket seal on the first go around doesn't necessarily insure against gasket failure. The function of a gasket is to seal and retain its seal in operation. Replacing a "leaker" in a relatively inaccessible gasket location may result in considerable expense and downtime. Even proper flange design, adequate bolting and sound installation practice do not always insure against the subtle changes that take place in a gasketing joint once in service. Changes in addition to creep relaxation of the gasketing material itself may adversely affect the seal. Due to fatigue, heat, mechanical vibration (the total dynamic process affecting the flange unit), or chemical attack the gasket material may undergo physical changes that may reduce its resilience that the total bolt load holding the gasket in place may drop below the force required to maintain a seal. In such cases leakage occurs.
The effects of temperature lead to the classification of materials as to torque retention as shown in the table below. Under ambient conditions, most gasket materials will not show any significant torque loss. As temperature rises above 200 degrees Fahrenheit, torque loss becomes a serious consideration and requires a gasket material which will be minimally affected. If the gasket material is suitable for the temperature, re-torquing at room temperature may compensate for torque loss. Studies have shown that most torque loss occurs within 24 hours at temperatures above 200º F. However, re-torquing adjustments are not always feasible or desirable, and the gasket user should select the gasket material with the most suitable torque retention characteristics. As shown in the table below, flexible graphite is the most temperature resistant gasket material, with compressed asbestos the best of elastomeric bonded materials under extreme conditions. This statement should not be interpreted to mean that similar performances for all types and grades of compressed asbestos can be expected. The stress relaxation qualities even among this one class of materials is variable. Only tests and the experience of the gasket manufacturer or user will determine the most desirable.
|Rank in Order of|
|Flexible Graphite||5400º F *||1|
|Compressed Asbestos||650 / 1000º F||2|
|Compressed Non-Asbestos||600 / 750º F||3|
|Asbestos Beater Addition||450 / 650º F||4|
|Non-Asbestos Beater Addition||350 / 750º F||3-4|
|Expanded PTFE||500º F||5|
|Vegetable Fiber||250º F||6|
|Rubber (Chloroprene / SBR)||212 / 250º F||7|
|Cork Composition||250º F||9|
|* = Non-oxidizing environment|
1. Non-asbestos compressed and beater addition exhibit a wider range of torque retention values than asbestos counterparts.
2. For specific service temperatures, consult the manufacturer.
Numerically rated from 1 to 10, indicating comparative torque retention, i.e., "1" is best, "10" is worst.
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