pam_basic principles water hammer

7/30/2019 PAM_basic Principles Water Hammer

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Gate valves Euro 20

Gate valves Euro 20

Gate valves Euro 20

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Equipmentfor dams and

reservoirs

Equipment for pressure systems

and stations

Connecting and repair pieces

Control Valves

Operating accessories

Operating accessories

WATER HAMMER


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Wave velocity: a =1

(

1

+

D

)

Ee

Pressure increased and decreased valves:

H = a

V

(Allievi) (1)

g

H =

2 L V

(Michaud)(2)

gt

where:

a: propagation rate (m/s)

: density of the liquid (1 000 kg/m3

for water)

: The bulk modulus of the volume of liquid (water: 2,05 109 N/m2)

E: modulus of elasticity of the pipe material (ductile iron: 1,7 1011

N/m2)

D: internal diameter (m)

e: pipe thickness (m)

V:

absolute value of the difference between the velocities in the normal

system before and after water hammer (m/s)

H:absolute value of the difference in maximum pressure (m.c.a.)

L: length of main (m)

t: effective time of closing (s)

g: acceleration due to gravity (9,81 m/s2)

CONSEQUENCESPressure surges can, in critical cases, rupture certain pipes which do not

present sufficient safety factors (plastic pipes). Pressure drops can create

pockets of cavitation, prejudicial to pipes, equipment and valves, as well

as cause collapse (steel or plastic pipes).

SIMPLIFIED CALCULATION

In practice, the wave velocity of water in ductile iron pipes is of the order

of 1200 m/s. Formula (1) takes into account a rapid change in the velocity

of flow:

( t < 2L a )

Formula (2) takes into account a linear variation in velocity of flow in

function of time (the function of the law of closing a valve, for example):

( t > 2L a )

The pressure varies from H with respect to the normal static pressure.

This is the maximum value for the instant closing of a valve, for example.

These simplified formulas give the maximum estimate of water hammer

Pgina 2 de 4Saint Gobain, Pressures - Maximum Permissible - Water Hammer

02-10-2011http://www.saint-gobain-canalizacao.com.br/manual/ariete.asp?lng=eng


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and should be used with caution. They suppose that the main is not

equipped with protection devices and that the head loss is negligible. On

the other hand, they do not consider limiting factors, such as pumps

acting as turbines or the pressure of steam saturated in pressure drops.

ExamplesMain DN 200, K9, length 1 000 m, pumping at 1,5 m/s: a = 1200 m/s

case n 1: the sudden stopping of a pump (negligible head loss, no anti-water hammer devices)

H = [(1200 1,5) 9,81] = 183m (or little more than

1,8 MPa)

case n 2: the sudden closing of a valve (effective time three seconds):H = [(2 1000 1,5) (9,81 3)] = 102m (or little more

than 1,0 MPa)

FULL CALCULATIONSThe graphical method of Bergeron permits the accurate calculation of

pressure increases and decreases based on time, at all points of a main

subjected to water hammer.

Today there are computer programs adapted to solve these complex

problems.

PREVENTIONThe means of protection necessary to limit water hammer to a permissible

level are different and adaptable to each individual case. They act either

by moderating the change in the velocity of the water or by limiting the

pressure increase in relation to the decrease.

The project designer should calculate the size of the pressure increase or

decrease created by the water hammer and decide according to the

pipeline profile, the type of protection to adopt:

inertia wheel on pump,

relief valve*,

wave anticipation valve *,

pump control valve*,

compensatory water column,

one way feeding tank -TAU,

hydropneumatic tank - RHO.

* See Control Valves.ConsiderationsNote, on the other hand, that ductile iron pipes have a significant safety

margin:

portugus espaol

Pgina 3 de 4Saint Gobain, Pressures - Maximum Permissible - Water Hammer

02-10-2011http://www.saint-gobain-canalizacao.com.br/manual/ariete.asp?lng=eng


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pressure surges: the safety margin of the pipes permits a 20% increase inthe permissible working pressures for transitory pressure surges,

pressure drops: the joint guarantees leaktightness, even in the case of apartial vacuum in the main.

pam_basic principles water hammer

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