Maximum pressure and wave travel time calculations are mobiledevicefriendly as of February 26, 2015
Maximum Pressure Change due to Sudden Valve Closure using Joukowski Equation
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Pressure Wave Travel Time Calculator (free)
Copy and paste the celerity from the above calculation into the
calculation below to compute wave travel time (may need to use keyboard controlc and controlv
instead of rightclicking):
Units in rapid valve closure water hammer calculators:
cm=centimeter, ft=foot, g=gram, G=Giga, k=kilo, kg=kilogram, lb=pound, m=meter, M=Mega, mm=millimeter, N=Newton, Pa=Pascal, psf=pound per square foot, psi=pound per square inch, s=second.
INTRODUCTION TO RAPID VALVE CLOSURE CALCULATIONS
Liquid initially flows at a constant velocity through a pipe. A
downstream valve closes instantaneously, and the liquid slams against the closed valve causing a pressure
spike ΔP.

If a valve is closed faster than the wave travel time, then it is considered an
instantaneous valve closure.
t_{w} = 2L/c 
EQUATIONS FOR PRESSURE DUE TO RAPID VALVE CLOSURE AND FOR WAVE TRAVEL TIME
The instantaneous valve closure calculation predicts the maximum increase in pressure that
will occur due to a sudden valve closure. The valve closure time is considered to be
instantaneous if the valve closes faster than (or equal to) the time required for a
pressure wave to travel two pipe lengths (i.e. the time for the wave to travel upstream
from the valve, reflect off the upstream boundary and return to the valve). The pressure
predicted by the instantaneous valve closure calculation provides the
engineer with the expected maximum pressure increase. The calculation can also be used in
reverse  to compute the pipe velocity  if a maximum pressure rise is input. For the
"Click to Calculate" button to function, the pressure calculation requires registration, but the wave travel time calculation does not.
Joukowski Equation
Onedimensional momentum conservation for frictionless flow is used to derive the
Joukowski equation. The equation was developed for a liquid flowing steadily through a
pipe and then instantly the velocity drops to zero due to a sudden valve closure. The
equation assumes that liquid compression and pipe friction are negligible. Though the
Joukowski equation's primary applicability is for a liquid velocity that drops to zero
upon contacting a closed valve, the equation is valid for any instantaneous drop in
velocity, not necessarily a drop to zero velocity. The Joukowski equation is seen with and
without a negative sign on the right hand side depending on whether the pressure wave is
traveling upstream or downstream. In either case, the pressure increase is a positive
number. The Joukowski equation is (Wylie, 1993, p. 4; Chaudhry, 1987, p. 8; Hwang and
Houghtalen, 1996, p. 118):
ΔP = ρ c ΔV
The equation for wave speed, c, is based on mass conservation and allows the pipe wall
material to expand (Wylie, 1993, p. 6; Chaudhry, 1987, p. 34; Hwang and Houghtalen, 1996,
p. 115):
The ΔP equation was derived for liquid upstream of the valve and does not include
effects downstream of the valve. The D/(wE_{p}) portion was derived using a
thinwalled pipe approximation.
Wave Travel Time Equation
Instantaneous valve closure is defined to occur if the valve is closed faster than the wave travel time.
The wave travel time is (Hwang and Houghtalen, 1996, p. 119):
t_{w} = 2L/c
The wave travel time, t_{w}, is the time for a pressure
wave to propagate from the valve, upstream to the reservoir, and back down to the valve.
If a valve takes longer than t_{w} to close, then our other water
hammer calculation should be used, where the user can enter the valve closure time.
VARIABLES IN PRESSURE AND WAVE EQUATIONS
Dimensions: F=Force, L=Length, M=Mass, T=Time
c = Celerity (wave speed) [L/T].
D = Inside diameter of pipe [L].
E = Composite elastic modulus [F/L^{2}].
E_{f} = Elastic modulus of fluid [F/L^{2}].
E_{p} = Elastic modulus of pipe material [F/L^{2}].
L = Pipe length [L].
t_{w} = Wave travel time [T].
w = Pipe wall thickness [L].
ΔP = Maximum pipe pressure increase due to sudden valve closure [F/L^{2}].
ΔV = Change in velocity [L/T].
ρ = Fluid density [M/L^{3}].
FLUID PROPERTIES, PIPE PROPERTIES, and PIPE WALL THICKNESS
Fluid Properties
Fluid density, viscosity, and elastic modulus provided by the dropdown menus in
the calculation have been compiled from the closed conduit pipe flow references shown on
our literature web page.
Pipe Properties
The pipe material elastic moduli built into our calculation have been compiled from the references shown below.
ERROR MESSAGES GIVEN BY PRESSURE AND WAVE TRAVEL TIME CALCULATIONS
Pressure calculation
"Need Density > 0". "Need E_{f} > 0". "Need E_{p}
> 0". "Need Diameter > 0". "Need Thickness > 0".
Density, elastic modulus of fluid and pipe, diameter, and wall thickness must be entered
as positive numbers.
"Need ΔV > 0". "Need ΔP > 0". If ΔV or
ΔP is selected as an input, it must be positive.
Wave travel time calculation
"Need Length>0". "Need Celerity>0". Length and celerity
(wave speed) must be entered as positive numbers.
REFERENCES
Chaudhry, M. Hanif. 1987. Applied Hydraulic Transients. Van Nostrand Reinhold Co. 2ed.
Hwang, Ned H .C. and Robert J. Houghtalen. 1996. Fundamentals of Hydraulic Engineering
Systems. Prentice Hall, Inc. 3ed.
LMNO Engineering, Research, and Software. 2009. Newsletter comparing water hammer
calculations.
Mays, Larry W. 1999. Hydraulic Design Handbook. McGrawHill.
Wylie, E. Benjamin and Victor L. Streeter. 1993. Fluid Transients in Systems.
PrenticeHall, Inc.
© 20092015 LMNO Engineering, Research, and
Software, Ltd. All rights reserved.
Please contact us for consulting or other questions.
LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd. Athens, Ohio 45701 USA Phone and
fax: (740) 5921890
LMNO@LMNOeng.com http://www.LMNOeng.com
August 25, 2015: Made text fields show 8 significant figures rather than 16. Calculation still uses double precision internally.

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