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Water Hammer HSD 2009 - 1 JHYDRAULIC TRANSIENTS Any...

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Unformatted text preview: 1 JHYDRAULIC TRANSIENTS - Any unsteady state flow condition - Water hammer - velocity changes pressure changes — included in the analysis - comma-sebum e.‘ water - e‘astmr,‘ ‘5’ pp? - the larger the change andlor the faster the change the greater the transient pressure change. 2 JWater Hammer - t= : tune of closure - ‘lnstant‘ valve closure t: < ZUa were a = Nave $9930 ' I ’ \H = a V59 ‘1 . -- I - “Very stout“ valve closure in ’=' 20 Lra '- " ._\H = L wig t.) - ‘Slcw valve closure’ 20 Lr'a >t,_ :2 Lia 3 JControlting Transients - increase time of verve closure use caution filling pipes 1 increase pressure class of pipe limit pipe velocrty Reduce wave speed - we of pipe — inlect air a Pressure relief — vane —— surge tanks 1 j] HYDRAULIC TRANSIENTS « Any unsteady state flow condition o Water hammer ) l. _ — velocity changes pressure changes " — included in the analysis - compressbll'rty 0! water - elasticity of pipe - the larger the change andlor the faster the change the greats change. 2 jWater Hammer 0 It 2 time of closure - 'lnstant‘ valve closure .4 I I 9 x l‘ -: 4.1. - Ly), .- r- ,-' E. ' '"‘ c “.- ~ i ‘ 1G < 2Ua ’ x , ’1 where a = wave speed "- (7 1" 'j " 3' -"~- 1 ‘l I' v— r - “Very slow“ valve closure \_ a 3,. I? E a 1 l “ ‘r t: = 20 U3 g I - “3.. '1 AH = L we u - 'Slow valve closure' 20 Ua MC :2 Ua 3 JControlling Transients - increase time of valve closure - use caution filling pipes - increase pressure class of pipe Ii HYDIAUIIC HANDIOOK HYDRAL less than the time, required for the induced grueure wave to trevei c t tin f th from the point a! valve closure to the inlet end of the. line and the 323;.333; ch; return. Th1. time h, 3‘3th 3': water We: pressing! _ ~ velocity oi water stopp t =g. 39° 9" N CL” U R t investigations by the p. 1 pressure caused by an where: obtained by the correct . ' constants of the liquid t m tune. in seconds. for pressure wave to travel the length 0! bun; modulus of elastit the pipe and return. L =length, in feet, at the pipe line. I=veiocity. in feet per secoud. of pressure wave. ' ‘_ .glggl . . HIE!!! One form oi the iormula. developed to determine the velar“, '.iq-i of the pressure wave. is !=#!- 12 :E..=- = '- .fi- .‘ , ' :legg. v i + “E: i .332“ Where: a: velocity of pressure wave, fps. g = acceleration caused by gravity = 32.2 feet per sec. per sec. Iliii- w = weigh-t of one ctr. it. of water. lbs. d = inside dismeter of pipe. in. e = thickness oi pipe wall. in. k = bulk modulus of compressibility of water: approximately 300,000 psi. E=modulus of elasticity of pipe material. psi: for steel—- approximately 30,000,000. For cast iron— approximately 15,000,000. Maximum Water Hammer Pressure. The formula that eval- uates the maximum pressure caused by water hammer is: P! 0.433 a V g \Vhea'e: y = maximum pressure, psig. e = velocity oi pressure wave, fps. ‘5' = velocity of water stopped, fps. 3: acceleration caused by gravity .:.- 32.2 it. per sec. per sec. HG. 3. Maximum “,0 i163 = a constant used to convert feet of head to psi. on instantaneous closl M “VDIAUUC HANDBOOK Example : - What is the maximum pressure caused by water hammer in an ' 8-inch steel pipe has (0.322-inches wall thickness) transporting water at s steady velocity at 3 fps? Procedure in Using Chart: . . d __ insidedilofpipe. in. __ 7.98! _ Denim” u“ "m" 'e' _ wall thickness at pipe, in. _ 0.322 “ 2”“ Enter the chart at g = 24.8 and project upward to the intersec- tion with the line for steel pipe. Note that the value of the velocity oi the pressure wsve. .- -_.- 4225 fps. ' Project horizontally to the right, to an intersection with the 3 fps. velocity line and then down to the base line, where shock pres- sure of 170 psi is chi-auct‘. SPECIFIC GRAVITY AND HEAD The head develo by a centrifugal pump depends upon the peripheral velocity the impeller. It is expressed thus: 11' H=E Where ; H = Total Head at zero capacity developed by the pump in feet of liquid :1 = Velocity at periphery of impeller in feet per second Notice that the head developed by the pump is independent 0‘. the weight of the liquid pumped. Therefore in Fig. 4 the head if I_ muss» no. la .9 = 0.70 FIG. 4. Pressure—head relationship identical pumps handling liquids of diflering specific gravitios. HYD! in feet would he the with a specific gravi 3 s3. of 1.2 or a Rain reading on the get: peller diameter and The gauge readin Refer to Fig. 5. 1 at 50 psi. Because liquids each pump t ii the speed of all thr: have the largest dist: FIG. 5. Pl’fl'fiE; ~ handling liquid: r: Standard perform“ total head in feet 3.5! Water is the liquid :r. in feet developed by a gravity. if the head i: the desired head and curves without correr same as that of wate will apply-o.r11x_t_°_!i: liquids multiply the being pumped. J POWER, EFFICIE The Horse Powc ured from the follov. IOOK l pr 1 wave to travel ilet of the line and -e to travel the length oi '55“ re wave. 1 determine the velocity ‘h—u-III 32.2 feet per sec. per see. if i ; approximately ate-rial. psi: for steel— ist iron—-appr0ximately Fhe formula that eval- ater hammer is: 32.2 it. per sec. per sec. head to psi. HYDRAULIC FUNDAMEN1ALS 13 mm Computations of the preceding formulae permit the layout of the accompanying chart, Fig. 3. which discloses the maximum water hammer pressure for various pipe sizes, thickness. and the velocity of water stopped. This chart is for water only. but recent investigations by the petroleum industry. disclosed that the shock pressure caused by any relatively incompressible liquid can be obtained by the correct substitution of the formula -.-f the physical constants of the liquid: namely, those of weight p cu. ft. and boil modulus of elasticity. Ilaggflglmnlnlnl I arariyflnmlm ' gotten-2:2!!!lllfll ' ‘55::!=:;%:=%=E==!"' I i==='—:::==::§==§=t"' raineennes'" I. Eggleeegafieflagfll l Egggtggsggg-srsseglll II Elssssss-uiissseeii nun-m I III-Illi'lllll Ilillllfl=l=IIIIIIII= | llllll llllllllllllllll llllllllllllllll l|I||||| “in: II Ir .irie Diorr ., inches \Jall Thickness, Inches o:Velocily o! Frets-are Ware -FI. Set. FIG. 3. Minimum shock pressure caused by wore! hamme' {based on instantaneous closure of valves}. ...
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