Water )الطرق المائي( Hammer -3 rd Class Dr. Sataa A. F. Al-Bayati (08-09)

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1 بسم هللا الرحمن الرحيم Water )الطرق المائي( Hammer -3 rd Class Dr. Sataa A. F. Al-Bayati (08-09) Water hammer is the momentary inrease in ressure, whih ours in a water system when there is a sudden hange of diretion or veloity of the water. When a raidly losed valve suddenly stos water flowing in a ieline, ressure energy is transferred to the valve and ie wall. Pressure waves travel bakward until enountering the next solid obstale, then forward, then bak again, until dissiated by frition losses.. The ressure wave s veloity is equal to the seed of the sound in water 1000m/s. Water hammer an our in any liquid, so water hammer an be alled hydrauli or fluid transient. Fig.(1) Tank, ie, & valve Fig.(2) Illustration of Water hammer. 1

2 Causes of water hammer are: 1. Pum startu an ause the raid ollase of a void sae that exists downstream from a starting um. This generates high ressures. 2. Pum ower failure an reate a raid hange in flow, whih auses a ressure usurge on the sution side and a ressure downsurge on the disharge side. The downsurge is usually the major roblem. The ressure on the disharge side reahes vaor ressure, resulting in vaor olumn searation. 3. Valve oening and losing is fundamental to safe ieline oeration. Closing a valve at the downstream end of a ieline reates a ressure wave that moves toward the reservoir. Closing a valve in less time than it takes for the ressure surge to travel to the end of the ieline and bak is alled sudden valve losure. Sudden valve losure will hange veloity quikly and an result in a ressure surge. The ressure surge resulting from a sudden valve oening is usually not as exessive. 4. Imroer oeration or inororation of surge rotetion devies an do more harm than good. An examle is oversizing the surge relief valve or imroerly seleting the vauum breaker-air relief valve. Another examle is to try to use some means of reventing water hammer when it may not be a roblem. )سرعت موجت الضغط في االنابيب( Pies Veloity of Pressure Wave in Raid hanges in flow make ressure transmission in waves. v g ---(1) v v = bulk modulus of elastiity of fluid = 300,000si ( kn/m 2 ) for water = elerity = veloity of ressure wave of fluid = 4720fs (1440m/s) for water During water hammer the ie exanded, so ombined modulus is, D v 1 t v 1 v 1 D t = modulus of elastiity of the ie material, D = ie diameter, 2

3 t = ie wall thikness. The veloity of ressure wave in elasti ie, g ( ) g v D v 1 ( ) t D v 1 t g 1 v D t ---- (2) steel = 30,000,000si ast iron = 15,000,000si onrete = 3,000,000si Instantaneous Closure: Critial time, T = 2L/ --- (3) L = ie length Instantaneous water hammer t < T < t slow losure t = losure time. Instantaneous losure (Partial or omlete losure), Δ = ρ Δv ----(4) Δ = hange in ressure, Δv = hange in flow veloity. Slow losure (artial or omlete losure) (Δ) s = 2L Δ/( t ) ---(5) xamle: Water flows at 10fs in a 400ft long steel ie of 8-in diameter with in thik walls. Calulate the ritial time & maximum ressure rise theoretially aused by instantaneously losing the end valve a) omletely b) artially, reduing the veloity to 6fs. 3

4 Solution: D v 1 t = 4110ft/s Critial time, T = 2L/ = 2 400/4110 = 0.195s Comlete losure, Δ = ρ Δv = (62.4/32.2)(4110)(10 0) = 79,600sf = 553si Partial losure, Δ = ρ Δv = (62.4/32.2)(4110)(10 6) = 31,800sf. 5 7 )الحماية من الطرق المائي( Hammer Protetion from Water 1. Valves Automatially-ontrolled سيطرة آليت( valves )صماماث : Water hammer often damages entrifugal ums when eletrial ower fails. In this situation, the best form of revention is to have automatially-ontrolled valves, whih lose slowly. These valves do the job without eletriity or batteries, but the diretion of the flow ontrols them. Closing the valve slowly an moderate the rise in the ressure when the downsurge wave (resulting from the valve losing) returns from the reservoir. Pressure relief valves: ntrained air or temerature hanges of the water an be ontrolled by ressure relief valves, whih are set to oen with exess ressure in the line and then losed when ressure dros. Relief valves are ommonly used in um stations to ontrol ressure surges and to rotet the um station. These valves an be an effetive method 4

5 of ontrolling transients. However, they must be roerly sized and seleted to erform the task for whih they are intended without roduing side effets. Fig,(3) Relief valve oens when line ressure exeeds reset loading on the sring. Vauum relief valves: If ressure may dro at high oints, an air and vauum relief valve should be used. All downhill runs where ressure may fall very low should be roteted with vauum relief valves. Vauum breaker-air release valves, if roerly sized and seleted, an be the least exensive means of roteting a iing system. A vauum breaker valve should be large enough to admit suffiient quantities of air during a downsurge so that the ressure in the ieline does not dro too low. However, it should not be so large that it ontains an unneessarily large volume of air, beause this air will have to be vented slowly, inreasing the downtime of the system. The sizing of air release valves is, as mentioned, ritial. 2. Surge Tank In long ielines, surge an be relieved with a tank of water diretly onneted to the ieline alled a surge tank. When surge is enountered, the tank will at to relieve the ressure, and an store exess liquid, giving the flow alternative storage better than that rovided by exansion of the ie wall and omression of the fluid. Surge tanks an serve for both ositive and negative ressure flutuations. These surge tanks an also be designed to suly fluid to the system during a downsurge, thereby reventing or minimizing vaor olumn searation. However, surge tanks may be an exensive surge ontrol devie. 5

6 3. Air Chamber Air hambers are installed in areas where water hammer is enountered frequently. Shaed like thin, uside-down bottles with a small orifie onnetion to the ie, they are air-filled. The air omresses to absorb the shok, roteting the um and iing. Air vessel installation Ditionary:,(حصادف) enountering,(موجاث الضغط) waves Pressure,(مؤقج) momentary,(أندفاع علوي) usurge,(أنهيار) ollase,(حشخج) dissiated,(عقبت) obstale downsurge سفلي),(أندفاع searation,(أنفصال) fundamental,(أساسي) sudden,(حفريغ) relief,(أذى) harm,(عالي) exessive,غلق فجائي= valve losure 6

7 instantaneous Closure آني),(غلق,(الخغيراث) transients task,(هدف) downhill,(حرك ب) installed,(حقلباث) flutuations,(يحدد), enountered (أنحدار) Referene: Franzini, J. B., & Finnemore,.J., 1997, Fluid Mehanis with ngineering Aliations 9 th d. H.W: M D = (no.) = 8.5in H.W: M t = (no.) = 0.26in H.W: M D = (no.) = 10.5in t = (no.) = 0.26in D = (no.) = 8.5in t = (no.) = 0.27in 7