Expansion Joints How to Select and Maintain Them

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    -FIGURE I-This Texas refinery uses both slip-type expansion joints and doubleoffset U-bends to absorb pipe line expansion and contraction. See next page for othertypes of expansion joinh.

    Expansion JointsHow to Select and Maintain Them

    K. S. RobertsYarnaii-Waring CompanyPhiledelphia

    LARGE NUMBERS of pipe linesconveying steam, hot liquids andother fluids for refining and petrochemical processing must be fittedwith some means to absorb expansion

    I f you are confused about all the types ofexpansion joints, here's a method of selecting and maintaining the right one for the job.

    caused by a rise in the temperatureof the pipe. There are also instanceswhere means for absorbing pipe contraction must be provided, if the lineis installed at a temperature higherthan the minimum experienced inoperation. Positive control of pipe expansion and contraction is absolutelynecessary if dangerously large or de-

    structive forces at the anchors andother restraints are to be avoided.

    Three proven means for absorbingpipe expansion and contraction arepopular in refineries and petrochemical plants today. These are: ( 1) expansion bends, ( 2) corrugated- orbellows-type expansion joints, and(3) slip-type expansion joints. This

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    Expansion Joints .- . . _...__. ___ .__._ ---.FIGURE 3-Simplified sketch show-ing a hinge-type corrugated expansion joint suitable for refinery

    FIGURE 4-Single-end anddouble-end gland-packedslip-type expansion jointsuse conentional asbestosrubber and duck ring

    FIGURE 2-Simplifiedsketch showing a typicalcorrugated-type expansion.joint suitable for refinerypiping systems.

    article discusses the relative merits ofeach, and the important factors inapplication to modem piping systemsin refining and petrochemical plants.Expansion Bends-These are made

    in several different shapes, dependingon the amount of space available, sizeof the pipe, expansion or contractionto be absorbed, an d the temperatureand pressure of the fluid flowingthrough the pipe. In general, themore intricate the bend configuration,the greater the expansion that can beabsorbed for a given stress in the pipe.

    The common types of expansionbends used today are either fabricatedor built-up. Fabricated bends may besimple U, double-offset U (Figure 1) ,circle, or single offset U. Creasedbends, though once used to some extent, have become less popular inrecent years. Built-up bends are madein a number of configurations fromwelding elbows.

    Expansion bends require no routinemaintenance if they are uninsulated.I f the bend is insulated or fitted witha drip trap, or both, routine inspection of the insulation and trap isrequired.

    All expansion bends, whether fabricated or not, require more installationspace than either corrugated or sliptype expansion joints (Figure 1). Thisis often an important considerationin crowded refinery areas, bu t canusually be overlooked on tank farmsand other locations where a largeamount of space is available. Expansion bends are usually mounted vertically (Figure 1), or horizontally,depending on the amount of spaceavailable.

    piping systems.packing.

    The first cost of a fabricated expansion bend may exceed the firstcost of a bellows or slip-type expansion joint. A built-up expansion bendusually costs less than a fabricatedbend or either type of expansionjoint, unless a large amount of laboris required to build the bend.Corrugated Joints-These jointsconsist of one or more coiTUgations ina metal suitable for the temperature

    and pressure in the pipe (Figure 2) .Typical materials used for corrugatedjoints include copper, stainless steel,or Inconel in the bellows and castiron or cast-steel end flanges, or steelwelding nipples.

    The corrugated joint, when welldesigned and constructed, has advantages where the installation isinaccessible and maintenance of thejoint is difficult or impossible. Corrugated joints do not require maintenance in the form of repacking orlubrication. However, the selection ofa corrugated joint must take intocareful consideration the extent andfrequency of the cycles (degree andnumber of expansions and contractions) of operation anticipated in theTtfe of the installation. Otherwise, thejoint may fail prematurely due tooverstressing the corrugations orbellows.

    There are other advantages of thecorrugated type. When made of stainless steel they can be used at temperatures beyond the range of slipjoints; the latter are limited packing and lubrication considerations.Stainless steel corrugated joints areexcellent for handling corrosive fluids

    or fluids which must not be subject tocontamination.In very large sizes (over 30 inches)corrugated joints are easier to fabricate, transport, and install than slipjoints. They are often used in largeducts and manifolds, particularly forhigh-temperature exhaust systems and

    other refinery applications. They arealso useful when the piping is subjectto lateral and angular misalignment,as well as axial motion, although hereagain it is important to select andinstall the joint properly so that thecorrugations will not be overstressed.

    In recent years corrugated expansion joints have been used increasingly as hinge, universal and gimbaljoints. These are specialized applications of the bellows principle whichhave definite advantages for certaintypes of installations. One of the mostuseful is the hinge joint, Figure 3,which, when circumstances permit,can be used to absorb expansion without the need for anchors.Slip-Type Joints-In spite of theversatility of the corrugated joint outlined above, the slip joint (Figure 4)has certain advantages which make itthe choice for many refinery andpetrochemical installations. Whenwell designed it is a rugged, dependable, and economical means of absorbing expansion and contraction. Itis particularly useful in steam andhot-water transmission lines wherethe expansions and contractions arefrequent and of large magnitude. Slipjoints contain no highly-stressed flexing element subject to failure after afinite number of cycles.

    Slip joints require a nominal89

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    --- _______ _______ ---------Expansion Joints . . . -

    _...,._____zzo'---- . . f . - . - --

    8il )--:=FIGURE 9-Typical refinery piping system using slip joints to absorb tile expansion and contraction.

    constructed and installed will needno attention for the first few weeks.However, it is quite normal for thepacking to require some attentionafter a month or so in service. In aconventional gland-packed joint thisconsists of tightening the gland nutsevenly until signs of leakage disappear.

    In the gun-packed type, the addition of only a few plugs of plasticpacking (depending on size of joint)is necessary. Once this initial adjustment period has passed a regularmaintenance schedule should be established, the frequency of which willdepend on the operating conditions(steam pressure and temperature}and the type of joint used.

    Lubrication-Good maintenance ofexpansion joints is primarily a matterof keeping the packing tight to prevent leakage and lubricated to keepsleeve friction low. Though not recommended, most joints will operatein a dry, unlubricated condition, bu tthe stresses in the pipe, fittings, andanchors will be considerably reducedby a nominal amount of the correctlubricant. Furthermore, lubricationhelps retain the sealing qualities ofthe packing, in the conventional aswell as the gun-packed type.

    In the better designs, alemite-typefittings are provided on the body ofthe expansion joint, through whichlubricant can be added to the stuffingbox with a lever-operated gun. Fittings are usually the hydraulic orbutton-head type and have built-incheck valves to prevent blow backfrom the pressure in the stuffing-box.

    In the average case, gun-packedjoints on continuous service need lubrication no more than two or three

    times a year. One shot (stroke ofpump-gun lever) in each fitting willusually suffice to maintain optimumpacking quality. Only in rare cases ofrelatively high pressure and temperature should it be necessary to increasethis rate. Excessive lubrication isshown by oil being extruded betweenthe sleeve an d gland. Always use thelubricant recommended by the manufacturer of the joint.

    Packing-Packing requirements ofconventionally packed expansion

    Meetthe

    Author

    K. S. ROBERTS received hisB.S. degree in engineering fromHartford College, Hartford, Pa.in 1942. His professional experience includes being a field engineer with the U. S. Coast andGeodetic Survey, WashingtonD. C. from 1947 to 1948; anda cost engineer with GeneralShea-Morrison, Inc., contractorsfor the Hungry Horse Dam inMontana from 1948-49. Robertsis presently a mechanical engineer for Yarnall-Waring Co.,Philadelphia, Pa., where hisduties include the design andapplication of expansion jointsand other products.

    ------------------joints will vary according to a varietyof factors such as service pressure,and temperature, traverse, frequencyof expansions and contractions, design of joint, and alignment of installation.Conventionally packed joints arekept tight by taking up on the glandnuts from time to time. Eventually,of course, steam must be shut off andthe stuff'mg box overhauled; i.e., thegland pulled back, old packing removed, and fresh packing installed.Packing rings should be carefullymeasured and cut to fit the stuff'mgbox snugly. Rings should be installedwith ends staggered so as to minimizethe possibility of leakage from onering to the next. It is most importantthat the gland be tightened evenly sothat cocking is eliminated (no binding on the sleeve) and packing pressure is uniform around the sleevecircumference.

    Th e great advantage of the gunpacked expansion joint is that it doesnot require overhauling. The cost andinconvenience of shutting down thesystem is completely eliminated. Leaksare sealed simply by forcing smallplugs of a specially prepared plasticpacking into the stuffing box by turning a threaded plunger with a shortwrench. This feature is shown inFigure 6.

    In normal service, packing needsto be inserted in gun-packed jointsonly two to four times a year. Experience varies considerably, dependingon service conditions, and some installations have required no morethan one packing addition per year.The amount of packing inserted varies from one or two plugs to severalplugs per packing gun, the number ofguns pe r joint varying from one to12, depending on the size of the joint.As a rule it is wise to establish aregular schedule of packing and lubrication maintenance, the natureand frequency of which can best bedetermined by experience in the particular installation.Refinery and petrochemical pipingsystems provide many opportunitiesfor wise use of all the available meansto absorb pipe expansion. Th e ultimate choice of the method to be usedlies with the designer of the pipingsystem. The pointers outlined in thisarticle should he useful to all designers, operators, and supervisors whomust choose, use an d maintain devices for absorbing pipe line expansions and contractions. # #

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