Maintenance of equipment and maintenance of plant is a delicate and severe issue for maintenance engineers and managers in the plant. Sometimes no one knows that how sudden breakdown of machines may occur. Moreover, there is no definite and exact cause of the problem in the beginning, unless and until the problem is rectified. Thus such issues causes for shutdown of the particular machines or process line. It may be pipeline, pumps, motors, generators,Fluid carrying systems, hydraulic systems or pneumatic lines of the plant. During this course of time, obviously, concerned engineer may have to face pressure from his management or managers. Even one minute breakdown of equipment may result in
to decrease in production rate and huge loss on overall performance and production rate target of the companies. Most of the industries consider and treat such issues very seriously and they always recruit skilled and experienced manpower to reduce the overall maintenance cost of the production. Company also provides suitable training to the engineers and managers enable them to get acquainted with the machinery.
So, no need to worry about your manager’s comments, if you are well trained or having technical ideas about root cause of the problem. Better to sit for a while and plan for the next steps. First understand the problems and start working on that. That’s all.
For instance, when it comes to sealing, the first thought that comes to mind is leakage. Leakage exists in all field (gas, vapor or liquid) systems to one degree or another. The amount of leakage that cab tolerated is a matter of choice. Acceptable leak rates can range from a slight drip, to bubble-tight, to mass spectrometer measurements, to diffusion of molecules through the base materials. Equipment users want trouble-free operation but it is not always practical to specify “zero” leak rates. Over specification in this area usually leads to increased costs, and sometimes impractical or unwieldy designs. Toxicity product or environmental contamination, combustibility, economics, and personnel considerations are factors, influencing the establishment of acceptable leak rates.
An increased awareness of leakage has come about through conscientious efforts to conserve fuel and energy. It has been estimated that leakage from hydraulic systems alone costs industry over thousands of dolor per day to provide make up. This cost becomes substantially greater when we combine the leakage encountered in other fluid systems, such as water, air, steam, refrigeration, and so forth. Leakage can also be hazardous.
Discharged fluids leaking onto the floors or walkways present a safety hazard to personnel as well as a potential fire hazard. And, of course, from an aesthetic point of view leakage is messy, unsightly and produces housekeeping problems. These and other problem area can be eliminated or at least minimized through proper leakage control. In addition to the savings in material and labor costs, other benefits that accrue from proper leakage co prevention of product spoilage. In general, the safe, continuous and proper operation of all fluid systems relies on the integrity of the systems and excessive leakage defeats this. Read more on.....
Industrial Health and Safety
Further when it comes to threaded systems, technological advancements in manufacturing have enabled a great many fluid carrying systems to be build with fewer components, thus reducing, the number of areas that might otherwise leak. This is the designer’s objective, but in some cases it is not practical, especially, accessibility for maintenance and repair is required, also, fluid systems such as water or steam are usually assembled permanently in their place of use. They require some type of connection to attach the various elements. These connections may be permanent or replaceable. Examples of permanent fittings using welded or brazed connections are shown in figure 1. The removable fittings, which will be discussed in this, are used more often because of their cost and convenience, they come in a variety of sizes, shape, materials, and performance capabilities. Of course, to be acceptable, they must be leak tight under the maximum surge pressure, vibration, shock or other fluid system abuses. Removable fitting end in either male or female threads. The main function of the threaded portion is to mechanically bond or attach fittings to other fittings or equipment housings, but they may also provide the necessary seal as we will see shortly.
There are three types of threads currently used on fittings. Two are tapered from the outside to the inside end of the fitting, and the third type is parallel or straight from one end to the other. The taper, shown in figure 2, provides a gradually increasing interference fit as the joint is tightened. A verity of common types of fittings using this taper is shown in sketches.
TYPICAL TAPER FITTINGS
STRAIGHT THREAD AND TAPER THREAD ASSEMBLIES
The straight thread and American Standard Taper Pipe Thread (NPT) are assembled with a sealant to provide a satisfactory pressure tight joint. As indicated in figure 4, the straight thread assembly has open channels or leak paths that must be sealed. The NPT assemblies’ produces more metal to metal contact along the thread flanks but still leaves a spiral leak path at the thread crest and root. The dry seal standard taper pipe thread (NPTF), also shown in figure 4, was adopted to eliminate the use of a sealant. To accomplish this some modification of thread form, greater accuracy in manufacturing and more analytical gauging is required. The roots of the both external and internal threads are truncated slightly more than the crests, i.e. roots have wider flats than the crests, so that metal to metal contact occurs, at the crests and roots coincident with or prior to flank contact. Thus as the Dry seal thread is assembled, the roots of the threads crush the sharper crests of the mating threads. The result is a leak tight connection without spiral voids. Due to the crushing and wear that occur with disassembly and reassembly, tapered threads are less effective on re use than straight threads. The method of overcoming this disadvantage is to use a straight thread and a lock nut with an embedded seal or insert, The lock nut is threaded on to the male fitting and the fitting is then threaded in to the female component only far enough to ensure a mechanical connection. After positioning the fittings, the lock nut is tightened down on to the fitting until it makes a secure contact. In this way, the straight threads provide the mechanical attachment and insure re usability while the embedded insert seals against fluid leakage.
PIPE THREAD LOCKNUTThe O-Ring seal is another way of providing the leak tightness while the thread furnishes the mechanical attachment only. Two types of straight thread O Ring fittings are available. One type (non-adjustable) is used for adapters, plugs, connectors, etc., and utilizes an O-ring mounted in a groove about the threaded portion of the fitting, Figure6 The other type, which permits angular adjustments of these fittings, utilizes a backup washer and lock nut.
NON ADJUSTABLE & ADJUSTABLE STRAIGHT THREADS
Tube fittings are another form of connection. Figure 8 shows a typical 2-piece and 3-piece flared assembly. They derive their seal from the swaged portion of the tube. The female tubing nut, once engaged and tightened, compressed the flared tubing against the mating cone of the male fitting. The 3 piece fitting adds an extra element that provides better structural integrity and is less affected by vibration.
If all of the fittings discussed were 100% efficient, industry would not have the expense noted earlier in providing make up. The facts remain that fittings leak. Some fittings control leakage better than others but usually at higher cost. The most crucial factor involved with the selection, installation, and maintenance of fittings is the “human factor”. Designers will strive to cut costs by using less expensive fittings that are borderline for a particular condition. The assembler often fails to follow recommended installations practices and the maintenance worker will re use the old fittings rather than buy new ones.
TYPICAL FLARED FITTINGNPT fittings (ref. figure 4) were shown to have spiral leak paths at the thread crest and root after installation. Straight threads had similar gaps but much more pronounced and extended along the thread flanks. Sealants are required on fittings or port connections that utilize these thread forms. Although the Dry seal (NPTF) thread eliminated these spiral gaps, galling may occur during assembly. The galling is produced when the threads are crushed and deformed, creating small scratch- like leak paths. A lubricating sealant is recommended to reduce the associated friction and prevent galling of the threads. Also, extremely high stress points are developed at the threads crests and roots on the Dry seal fittings. Over tightening may create cracks in the mating fitting or housing because of these stress points. Initially, the crack might not be visible but could show up later as fatigue failures or fluid seepage, as the cracks begin to grow.
The fittings shown in fig 5, 6 and 7 all used plastic or rubber inserts to form their seal. For these fittings to properly seal, the insert or O- Ring must flow in to all surface imperfections. Under tightening of the fittings does not allow proper deformation of the insert to occur and conversely, over tightening could crack or extrude the seal from its mating surfaces. The compatibility of the insert with the environment is also important because some plastics and rubbers degrade when in contact with certain fluids. Temperature extremes will also cause some materials to become brittle and crack over a period of time.
The tube fittings shown in figure 8, are also subjected to the same human factors as the others. The proper flaring of the tube could be cited as the single most important factor. A flare that is too short reduces the clamping area and the wall thickness may be decreased when clamped. A flare that is too long may stick or jam on the fitting thread when assembling. Examples of incorrect flares are shown in figure 9. The correct length and diameter should provide a flare which extends beyond the maximum I.D. of fitting sleeve but not beyond the O.D. of the sleeve.
CORRECT & INCORRECT FLARE FITTINGSIn summary, fittings rely on intimate contact to form their seal. The threaded portion of a fitting has three functions:
1. To form the seal as was shown with the Dry seal thread.
2. Provide the Mechanical attachment to other fittings or equipment ports.
3. Provide the force needed to compress the sealing element, i.e. O- Ring, Plastic insert, flared tubing, etc,
Each function requires that the proper tightening torque be observed and that movements of the fittings in a loosening direction should not occur. Such movements cause either some or all of the contact pressure to be lost depending upon the nature of the seal. For example, Dry seal threads and flared tubing having very little compatibility and small movements in the loosening direction caused immediate loss of the original metal to metal contact. The O-Ring or plastic seal, on the other hand, can accommodate some movement which is proportional to the elasticity of the elastomeric material used. Vibration loosening of fittings has always been a major concern in controlling leakage. Even though fittings are considered to be static seals, they are subjected to a variety of dynamic forces that are discussed in the next section on gasket assemblies.
FLANGE SYSTEMS STATIC SEALS
All types’ fluid seals perform the same basic function and
that is keeping the process fluid (gas, liquid and vapor) where it belongs.
They accomplish this by forming an impervious barrier against fluid transfer
between two mating surfaces. Traditionally seals have been categorized as
either static or dynamic. The primary distinction between them involves the
degree of movement with respect to the mating surfaces. For example, dynamics
seals are used to retain fluids or throttle leakage between a sliding or a
rotating part and a stationary one where as static seals prevent fluid loss
between two stationary surfaces. This generic classification of seals is
somewhat misleading and implies that static joints are completely rigid.
Granted there are no gross movements between the mating parts, but movements
are present because of several agents that act alone or in combination with one
another. The four most important agents are temperature, fluid pressure, fluid
velocity and system vibration. These agents produce adverse affects on the seal
in ways that are not always fully understood. The following discussion will
attempt to develop an understanding of these agents.
TYPICAL FLANGE SYSTEM
TEMPERATURE EFFECT
Fluctuations in temperature of a gasket-ed assembly create a
variety of changes, all of which can affect sealing properties of that
assembly. High thermal stresses can be developed within the gasket-ed joint
because of the various sizes, shapes and materials of the elements involved,
i.e. bolts, flanges and gaskets. Stresses from thermal load occur when the
components are subjected to temperature gradients or to uniform temperatures
where the components have different coefficients of thermal expansion. These
thermal loads are high enough to be of engineering concern. The thermal
stresses can produce abrading, crushing, extrusion or loss of bold load. A
thermal change of 370 degree F. a change in bolt length by the same amount as
an 80,000 psi prestress. This means that for each 10F difference in temperature
between the bolt and the flange bolted to a steel flange expands radial, 0.013
inches more than the steel for every 10” of flange diameter and for in
change temperature.
PRESSURE AND VELOCITY EFFECTS
Fluid pressure and velocity surges can also be present in
the system. Surges of this type usually result in vibration. For example, if a
valve is suddenly closed or an obstruction blocks the flow of fluid, a pressure
wave is generated by the kinetic energy of the fluid. This pressure waves
travels at the speed of the sound for the fluid through the downstream line
until the wave is reflected back to the point of origin. If the system has
separate branches, the wave is reflected separately through each branch. This
phenomenon is repeated with shock waves over lapping each other until the
original kinetic energy is absorbed by friction. In a water system the shock
waves are referred to as water hammer because of noise and vibration that
usually accompanies the waves.
EXTERNAL VIBRATION
External vibrations also stress a gasket joint in a manner
similar to the internal vibrations caused by shock waves. An out of balance
pump can vibrate the connected pipe rather severely, causing premature joint
failure. Likewise, joints made on cars, airplanes and boats will be subjected
to all of the vibration and twisting motions of that vehicle.
In summary, we can say that static seals are not truly
static, but are a throbbing, squirming and shaking system composed of several
elements. These elements are the flanges, bolts, and gaskets or seal and
sometimes the attachment of the pipe to a shaking vehicle frame. The design,
material section and maintenance of any gasket-ed joints must include consideration
of all the agents acting on the system. We shall see that, very often the
system is a compromise of choices. For instance, the material that will seal
rough flanges may not be the material that will withstand hydraulic Hammering.
Sometimes this compromise means changing some of the elements in order to get
an effective joint seal. For ease of analysis, however, we will study each one
of the important elements of a gasketed system individually and not attempt to
pull them together until the end of this discussion.
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