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Codes Cod es & Standar ds Static An Static An al y sis A Brief Brief Descri Descriptio ption n of S way Brace, Strut Strut and Spring Hanger Snubber (Dynamic (Dy namic Restraints) for pipe supporting for process industries 23rd June 2014
An u p
piping stress
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Whenev er unplanned dy nam namic ic events occur, dynamic dynamic restraints carry carr y the respons ibi ibili lity ty of protecting th e piping and ot her components from damage.
Undesirablee abrupt movement Undesirabl movement of the co mponen ts in the sys tem can be caused by: Press Pr ess ure sh ocks from valve operation/ PSV Wat er hammer hammer Boiler Boil er even ts Pipe breakage Wind load Mechanical vibrations vibrations transmi transmitted tted fr from om pumps, pumps, com compress press ors, turbines or other process equipments. Seismic events Fluid Fl uid dist urbances Explosions etc.
Dynamic Dynam ic restraints restraints a re specially specially des igned to ab sorb s udden incr increase ease in load load from the pipe an d trans fer into into t he s tructure and to dampen any oppo sing os cil cillati lation on between the p ipe and the st ructure. These rest rai raints nts are not intended to carry the weight weight
of pipe work and s hould no t impede th e function of the s uppo rts. Dynamic restraints are required to be v ery stiff, to hav e high load capacity and to minimize free movement between pipe and structure. The main sup ports t hat make up the dy namic restraints for process piping are1.
Sway Braces
2.
Hydraulic and Mech anical Snubb ers
3.
Rigid Struts
4.
Clamps
5.
Welding Clevis etc.
In the following paragraphs we will discus s in brief about Sway Braces, Rigid Struts and Snubbers. Sway Braces: Sway braces can be defined as sp ring loaded units mounted o n pipe work which are used to limit the s waying or vibration induced by external forces (vibration force) by ap plying an op pos ing force on th e pipe. They are d ouble acting v ariable spring units which can handle both tens ile and compress ive loads. It is commonly us ed to allow unrestrained thermal movements while “tuning” the system dynamically to eliminate vibration. It could be pre-loaded in the cold or installed pos ition, so th at after thermal pipe movement (growth ) it reaches the neutral pos ition and th e load on t he s ys tem in the operating (OPE) cond ition is n egligible (almos t zero). The cons truction is fairly simple, the unit has two piston plates: one on either s ide of the h elical coil compress ion sp ring connected by a single piston rod.
Fig. 1: Schematic Representation of Sway Brace construction.
If a tensile load is app lied, the to p pisto n plate is pu lled do wn causing t he s pring to compress & if a compress ive load is applied the thrust nut/rod coupling pushes the bottom piston plate to push up which causes the spring to compress. Therefore in both o f the situation the sp ring gets compress ed but du e to des ign (see cut away section abov e) the unit is capable of hand ling b oth compress ive & tens ile movements /forces. The spring is pre-compressed (usually a full inch =25 mm) providing an initial force (preload) that instantaneously opposes vibration. Wh enever any movement from the sway brace neu tral position o ccurs it is opp osed by a load equal to th e pre-load plus t ravel from the neutral pos ition times the s way brace s pring con st ant. To explain it further, if the piping load on t he s way brace is less than pre load the n there will not b e any line movement. If the load is equ al to preload the n the line will be on t he verge of movement, but then also the line will not move. If the load is more than the preload the line will deflect causing the spring to compress further. The deflection of the s pring / pipe in this cas e will be as given in equation 1. Pipe deflection= (piping load – Pre load) / sp ring rate
Eqn 1
So there is no pipe movement if the load is less than t he preload and with load in excess of preload th e deflection is a s g iven in Eqn 1. When sway brace with a preload P is installed in a pipe there is n o force exerted by t he s way brace on th e pipe. But for the pipe to h ave an y movement in either direction along t he line of sway brace ins tallation it will experience a reactive force eq ual to P plus travel from neutral position times the s way brace spring cons tant. It is des ired to have n o force on th e pipe during normal operation o f the pipe. So s way brace are normally attac hed d uring n ormal operation or adjusted to th e nut ral positon during normal operating condition. When maximum allowed travel (us ually 3-in. / 75 mm in either direction) is reach ed th e s way brace locks prev enting additional movement and act as a rigid restraint. The preload for LISEGA s way braces can be adjusted as per requirement at site. But for C&P or others t he u nit is s hipped after adjusting required preload. The effect of sway brace on the piping sy stem is to increase the K value in the equation
Mx2(t) +Cx(t)
+Kx(t)=F(t) This in turn will raise t he nat ural frequencies of the vibratory modes & thu s n ormally reduce th e respo nse of the pipe to dynamic loads & vibrations .
The force required to restrain th e pipe work can be ca lculated as follows: If the pipework is v ibrating with frequ ency f Hz at a maximum displacement (half amplitude) of x mm then, in simple harmonic motion, the restoring force exerted by the pipework at maximum displacement (kgf) = 4π2 f2 m x/1000 g. W here m is t he equivalent mass of the pipework in kg. It is likely that a Sway Brace having a preload greater than this value willfully restrain the pipe at the support location, while a Sway Brace for which this value is greater than the preload, but less than the maximum load will have a significant effect. Manufacturers no rmally recommend a s pecific size of sway b race for a pipe n ominal diameter. If the exact rest raining force required to control the piping vibration is known beforehand then a more spe cific sway brace se lection is p oss ible. The energy neces sary t o con trol the piping sy stem is proportional to th e mass, amplitude of movement an d th e external force which is caus ing the vibration. From this relation the exact rest raining force required t o co ntrol the p iping vibration can be calculated and an app ropriate s way brace size can be s elected. Sway braces need to b e installed in operating condition. However, it can be inst alled in cold con dition. But for that case when the p lant starts operating, the pipe may hav e thermal movements. This may cause the s pring in the sway brace to compress by an amount equal to the thermal movement/displacement. A t th is po int the sway brace will be exerting a force equal to th e pre-load + movement X s pring co nst ant. The load need to be released by d oing “Neutral adjustment”. This can be ach ieved by rotating t he Rod cou pling sh own abo ve in a direction su ch that the pisto n plate ge ts released & res ts agains t the end plate. In this situation the sway b race will not e xert any force on t he pipe. During s hut d own, as t he pipe cools & gets in to the cold position, the sway brace will exert a force on the pipe as the spring will get compressed. To summarize, Sustained loads on sway brace = Pre-Load + Hot Deflection * Spring Rate In OPE case the displacement allows thermal expansion and the sway assumes neutral position exerting zero or negligible load on the p ipe. i. e, Operating cas e restraint loads on s way brace =~ 0.0 (does n ot rest rain th ermal expans ion)
Main A pplication:
Sway Braces are mainly us ed to reduce pipe vibration amplitude and at the s ame time does not increase t he expans ion stres s in operating cas e. It prevents the pipe from vibrating at its reson ant frequency. Typical examples of using t he s way braces are in the pipe line feeding the flare s tack in a refinery. When gases at very high p ressu res are pas sed in the pipe line in the flare stack, it tends to vibrate & the sway brace will try & limit the vibrations . Every time the vibrating force has to act as oppos ite to the sway brace preload+ the s tiffness multiplied by distance moved from neutral position. When th e line
movement exceeds the s way brace becomes rigid and act as a rigid gu ide in that d irection. The s pring s tiffness and p reload is fixed dep ending on pipe size. However for special applications manufacturer can chang e thos e values as per requirement.
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Struts: When we need t o limit the disp lacement which do es not effect in increase of t hermal stress in operating condition or when the disturbed displacement is at an axis normal to the thermal displacement it is preferable and less expensive to use a rigid strut o r strut. Rigid struts are selected to su it the force that they will resist and the s pace available to fit them. The anchor point to the s tructu re is the most s imple to s elect s ince it is only dep ende nt on t he s ize of the rigid strut. The p ipe attac hment is depend ant on both p ipe size and s trut size but it is also influenced by th e orientation of the s trut relative to the pipe arrangement. The strut is often more difficult to specify because it may be resisting forces in the three primary axes, x, y and z. It is therefore neces sary to use so me s imple trigono metry to res olve the given forces into axial force acting o n the strut and to calculate the actual length of the s trut between th e fixing point and the pipe attac hment. Because th e s trut is held between two pinned con nections its ability to resist compressive force is greater the s horter the s trut is. A long s trut will have a lower safe working load in compress ion than a s hort st rut. However its length does not affect the tens ile load capacity of the st rut.
The s trut is th erefore se lected by c ons idering the direction and magnitude o f the axial force and if compress ive forces are acting, the length be tween the fixing pins of th e conn ections. After the the s trut s ize is s elected, the welding clevis will automatically s pecified to s uit the s trut size. The pipe attach ment is s elected now by c ons idering the pipe s ize, the s trut size and the connection requirements between the s trut and the clamp. It is es sent ial that th e strut can attach to the clamp without obs truction and any t hermal movements are able to occur without the strut interfering on the clamp. Therefore it is very important t o con sider the t ransition o f the as sembly du ring all expected displacements .
Main A pplication: Rigid Struts are us ed in Turbine and Compress or connected lines near the no zzle connections to take the advantag e of very less friction. Otherwise s truts can b e used as a s ubs titute for guide sup ports where structure is not available for us ing standard guides.
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Snubbers:
The us e of sn ubbers (Also called s hock abs orber) is p referred in thermally op erating p iping s yst ems. In a d ynamic event, snu bbers ins tantaneo usly form a practically rigid restraint bet ween the protected componen t and the s tructure. Resu lting dynamic energy can at once be abs orbed and harmlessly trans ferred while the operational displacements due to thermal expans ion and contraction must not enco unter any noticeable resistance. Through the s pecial function of the shock
absorbers, thermal displacements during normal operation remain unhindered (offers very little resistance to pipe movement). When however a sudd en impact load acts upon snu bber internal braking device engage, thus controlling the movement of pipe. Snubb er is s aid to be “lock up” a nd in this c ond ition the s nub ber acts as a rigid restraint. Wh en th e load has diss ipated, the s nubber u nlocks and a gain allows g radual movement of the pipe. Depending on internal mechanism of working s nubb ers are of two types : 1.
Hydraulic Snubbers and
2.
Mech anical Snubbers
1.
Hydraulic Snubbers:
Similar to an automobile shock arrestor the hydraulic snubber is built around a cylinder containing hydraulic fluid with a piston (See Fig. 2) that d isplaces the fluid from one e nd of th e cylinder to the o ther. Displacement of fluid results from the movement of the pipe causing the piston to displace within the cylinder resulting in high press ure in one end of the cylinder and a relatively low press ure in the oth er. The velocity of the piston will dictate the actual difference in pres sure. The fluid pass es t hrough a s pring-loaded valve, the spring being us ed to h old the valve open. If the differential press ure across the valve exceeds the effective press ure exerted by the spring, the valve will close. This caus es th e sn ubber to beco me almos t rigid and furth er movement or displacement is su bs tant ially prevent ed. The hydraulic snubb er is generally us ed when th e axis of restraint is in the direction of expansion/ cont raction o f the pipe. The s nubber is therefore required to extend/ retract with the normal operation o f the pipe work. The s nubber h as low resistance to displacement/ movement at very low velocities. The resistance to normal thermal movements (pipe velocity less than 1 mm/Sec and with amplitude of vibration less than 3 mm) is less than 2% of the rated load of the snubber.
Fig. 2: Hydraulic Snub bers.
1.
Mechanical Snubbers:
Whilst having th e s ame application as the hy draulic s nubber, retardation of th e pipe is due to cent rifugal braking within the snu bber. A s plit flywheel is rotated at h igh velocity which caus es the s teel balls to be forced radially out wards. The flywheel is forced apart by the s teel balls causing braking plates to co me tog ether thus retarding the axial movement/displacement o f the snubber. Rotation of the flywheel is generated by the linear displacement of the main rod acting on a ball-screw or similar device. Mechanical snubb ers (See Fig. 3) are us ed in cas es of applications where human access is restricted, for instance d ue to high radiation atmosphere in t he n uclear plant or due to h igh elevation point where no scaffold is available & maintenance work is not eas y to do. No maintenance s ervice is required for mechanical snubbers & are designed t o generate t he required
resistance force instantly on reaching th reshold acceleration, to restrain a d isplacement of piping caus ed by an earthquake or other dyn amic events & resume its free movement as soo n as the d ynamic displacement is s uppress ed while developing very little (a negligible level of) frictional resistance force during the slow thermal displacement mode of piping.
Selection of Snub bers:
The s nubber is influenced by th e s ame factors that the rigid s trut is, the magnitude and d irection of axial force, but it is also neces sary to con sider the thermal disp lacement the s nubb er have to undergo. Again it is n ecess ary to us e trigonometry for calculating the force and the length of the s nubber alongwith the actual displacement ap plied to the s nubb er. Displacements in the p rimary axes cannot be co mbined s imply to d etermine the s nubber movement/displacement; it is nece ss ary to calculate the overall length of the s nubber in the various installed and operating conditions in order to d etermine the neede d s troke. After calculating th e actual s troke it is go od en gineering p ractice to t ake a margin of excess travel at each end of the des ign travel. So, Always select a snu bber that is capab le of allowing greater displacement t han is theo retically required.
Orientation of the s nubb er is a lso important for both hydraulic and mechanical types. Acc ess to either lubrication po ints or inspection po ints is n ormally required and must be cons idered du ring th e des ign and inst allation of the rest raint. It may also be required to a llow in-situ tes ting of the s nub ber for validating its funct ionality and s o acces s may be a permanent requirement.
Fig. 3: Mechanical Snubbers. For selecting proper Snubber, determine the minimum required stroke by taking the anticipated design movement and adding an allowance for excess travel. This allowance should normally be at least 20% of the anticipated design movement. Then select a snu bber where the cy linder s troke is greater than o r equal to t he minimum required stroke and the ap plied loadings in tens ion and compress ion are less than the allowable maximum loadings in tens ion and compress ion for the s ize and length of snu bber as sho wn in the ca talogue. For intermediate lengths, allowable compress ive loadings may b e determined by interpolation. The length of th e s nubbe r must be s uch t hat th e maximum angulations are not exceeded. To calculate the required closed centres for the s nubber, us e the following formulae: Close d Centres = Ins talled Centres — X Where X = (Stroke – Design Movement in Extens ion) / 2
or
X = (Stroke + Design Movement in Compression) / 2 This method will resu lt in the s pare travel being distributed evenly o n either s ide of the d esign movement.
Main A pplication: Snubbers are normally us ed for reducing the damaging effects of Earthquake events .
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This article has 3 comments
Bruce Hammerson Saturday 24 August 2013, 6:15 am The blog is informative and provides the much needed information about the mechanics of Snubbers. Thanks for sharing. Regards Bruce Hammerson Hydraulic Hammers
leninbabu2020 Monday 2 September 2013, 8:30 pm Superb. i didn't find this much useful information before..thank you
Anup Kumar Dey Tuesday 3 September 2013, 4:53 am Thank you..
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