Page 114 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

98 Applied Process Design for Chemical and Petrochemical Plants

The pressure that can develop from the shock wave can
be destructive to the containing system hardware, partic-
ularly in long pipe. Examples of conditions that can devel-
Tonk op water hammer are:

125 feet 1. start, stop, or an abrupt change in a pump's speed
Straight pipe 90° Elbow 2. power failure
(Welding Type) 3. rapid closing of a valve (usually a control valve,
which can slam shut in one or two seconds)

__ e" Gate Valve The magnitude of this shock wave can be expressed
Full Open [19, 20]:
Suction
Line
Centrifugal (2-69)
Pump
Figure 2-27. Example 2-5, pipe system for pipe sizing calculations.
For water:

Total pressure drop from face of discharge flange on a,.= 4660/(l + Ki,, Br)ll ft/sec (2-70)
2,
pump to nozzle connection on tank:
where h,..h = maximum pressure developed by hydraulic
shock, ft of water
�p = (125 + 67.5) [(0.720)/100] + 0 v,.. = reduction in velocity, ft/sec (actual flowing veloc-
�p = 1.386 psi ity, ft/ sec)
�p = 1.386 psi (2.31 feet/ psi) = 3.20 feet water
g = gravitational constant, 32.2 ft/sec
Ki,, = ratio of elastic modulus of water to that of the
Note that a somewhat more accurate result may be pipe material (See list below)
obtained by following the detailed loss coefficients given Br = ratio of pipe diameter (I.D.) to wall thickness
in Figures 2-12 through 2-16. However, most preliminary a; = velocity of propagation of elastic vibration in the
engineering design calculations for this type of water sys- discharge pipe, fl/sec
tem do not warrant the extra detail.
Some typical Ki,s values for water/metal are [19]:
Flow of Water from Open-End Horizontal Pipe

The equation of Brooke [36] is useful in estimating Metal Ki...
water or similar fluids flow from the end of open pipes: Copper 0.017
Steel 0.010
GPM = 1.04 a (1) (2-68) Brass O.Ql7
Wrought iron 0.012
Malleable cast iron 0.012
where GPM = flow rate, gallons per minute Aluminum 0.030
a = internal cross-sectional area for flow in pipe, sq in.
I = horizontal distance from pipe opening to point
where flow stream has fallen one ft, in. The time interval t 5, required for the pressure wave to
travel back and forth in the pipe is:
Water Hammer [19]
t,. = 2 L/a,., sec (2-71)
Water hammer is an important problem that occurs in
some liquid control systems. It is defined as hydraulic L = length of pipe, ft (not equivalent ft)
shock that occurs when a non-viscous liquid flowing in a
pipe experiences a sudden change in velocity, such as the
fast closing of a valve. The kinetic energy of the moving When the actual abrupt closing of a device to stop the
mass of liquid upon sudden stoppage or abrupt change of flow has a time shorter than ts, then the maximum pres-
direction is transformed into pressure energy, thereby sure, hwh, will be exerted on the closed device and line.
causing an abrupt pressure rise in the system, often result- Note that the value of, hwh• is added to the existing static
ing in severe mechanical damage [53]. pressure in the system.