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Surge pressure and water hammer |
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Surge pressure and water hammer refer to various
degrees of the same phenomenon. When water moves through pipe,
kinetic energy is created in relation to the mass of water and
the velocity at which it is moving. When the flow is stopped,
the kinetic energy exerts itself in the form of a momentary increase
in pressure above the normal static pressure. This is called
a surge pressure or pressure rise. The intensity of this surge
depends on the speed of closure of the valve which stops the
flow in addition to the factor mentioned above. |
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Since this surge varies with these several
factors. It may be advisable to calculate actual surge pressure
for certain condition, but actual conditions such as unusual
length of line or extremely fast closing valves may make further
derating of flows advisable in same cases. |
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The
most commonly used formula for calculating pressure rise is: |
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P = Pressure rise (psi) above static pressure |
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V = Velocity of flow (ft per second) |
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L = Length of pipe (ft) on pressure side
of valve |
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T = Closing time of the valve (second) |
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Surge pressures at a given velocity are, therefore
dependent on both the length of pipe and the closing time of
the valve. |
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For an example of these effects, the following
calculations are shown for: |
- 1" schedule 80 PVC pipe
- Velocity of flow at 8.90 ft./sec (20 GPM)
- Static pressure 100 PSI
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With 100' of pipe to the valve and the
valve closure time of approximately 10 seconds: |
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P = .070 |
8.90 x 100 |
= .70 |
890 |
= .070 x 89= 6.23 psi of pressure
rise + 100 psi of static pressure = 106.23 psi |
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10 |
10 |
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Same conditions; 100' of pipe, valve closer
time of approximately 1 second |
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P = .070 |
8.90 x 100 |
= .70 |
890 |
= .070 x 890 = 62.3 psi of pressure
rise + 100 psi of static pressure = 162.3 psi |
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1 |
1 |
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Same conditions; 1000' of pipe, valve closer
time of approximately 10 seconds |
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P = .070 |
8.90 x 1000 |
= .70 |
8900 |
= .070 x 890 = 62.3 psi of pressure
rise + 100 psi of static pressure = 162.3 psi |
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10 |
10 |
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Same conditions; 1000' of pipe, valve closer
time of approximately 1 second |
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P = .070 |
8.90 x 1000 |
= .70 |
8900 |
= .070 x 8900 = 623 psi of pressure
rise + 100 psi of static pressure = 723 psi |
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1 |
1 |
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Severe damaging water
hammer |
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If length of line to valve cannot be changed,
water hammer must be reduced to or below the safe working pressure
of 1" schedule 80 PVC pipe by increasing pipe size for the
required flow to reduce velocity, by slowing the closure time
of the valve or both. |
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Use extreme caution where long
main line and fast closing valves are used. |
