WATER HAMMER

Water hammer is a series of pressure pulsations, of varying magnitude, above and below the normal pressure of the liquid in the pipe. The amplitude and periodicity depends on the extinguished velocity of the liquid, as well as the size, length and material of the pipeline. Shock results from these pulsations when any liquid, flowing with a certain velocity, is stopped in a short period of time. The pressure increase, when flow is stopped, is independent of the working pressure of the system. The surge pressure in any pipeline occurs when the total discharge is stopped in a period of time, equal to or less than the time required for the induced pressure wave to travel from the point of valve closure to the inlet end of the line and return. This time is:

Where:

t = Time for pressure wave to travel the length of the pipe and return (sec.)
L = Length of pipe line (m)
a = Velocity of pressure wave (m/sec)

When the liquid in the pipe is water, the velocity of the pressure wave “a” is determined by the following equation:

Where:
a = Velocity of pressure wave (m/sec).
4660= is the coefficient of the PPI formula (PPI Pex Design manual page 25)
0.304= is the coefficient for adjusting the value of “a” from ft/sec to m/sec
Kbulk = Bulk modulus of fluid (for example: 300,000 psi for water at 20ºC)
SDR = Pipe dimension ratio
E = Instantaneous (short term) modulus of elasticity (Psi) for the pipe material (obtained from Tensile tests).
For E values, please refer to table n°24 “Low surge pressures in Pexgol pipes”.

Where:
P = Surge pressure (Psi)
ρ = Fluid density (for example: 1 gr/cm³ for water at 20°C)
a = Velocity of pressure wave (m/sec)
V = Velocity of water stopped = line velocity (m/sec)
g = Acceleration caused by gravity (9.81 m/sec²)

Pressure caused by water hammer can be minimized by increasing closure times of valves to a value greater than 2L/a. For example:
When the closure time is 10 times 2L/a, the pressure surge can be 10%–20% of the surge caused by closure in a time equal to or less than 2L/a.
The value of the short-term modulus of elasticity E for Pexgol pipes is much lower than the value of E for steel pipes, concrete pipes or HDPE pipes. Since the velocity “a” of the pressure wave is related to the short-term modulus of elasticity E, the velocity “a” decreases when the value of E is lower.

In order to determine the resistance of the pipe material to the water hammer phenomenon, the total occurring pressure (surge pressure + working pressure) should be calculated and compared to the maximum allowable total occurring pressure in each pipe material.
The resistance of HDPE pipes depends on the nature of the water hammer. In case of recurring water hammer shock waves, HDPE pipes are limited to a maximum total occasional pressure of only 1.5 times the working pressure.
Because of the flexibility and resilience of Pexgol pipes, the surge pressures caused by the water hammer are much reduced. Furthermore, because of the cross-linked structure, the Pexgol pipe can withstand a total transient pressure (recurring or occasional surge pressure + working pressure) at least 2.5 times the design pressure in the relevant temperature.

The following comparison examples show the pressure surges caused by the water hammer for various pipes, which are considered for the same application. In all following examples:

• The line is horizontal; line length is 2,200 m.
• The flow rate is 150 cubic metre per hour, head losses are 5%.
• The line is designed for a pump pressure of 160 PSI.
• The fluid temperature: 20 °C.

The pipes calculated for this application are as follows:

1. Steel pipe 6” schedule 40, buried pipeline or above-ground installation.
2. PE 3408 6” DR 11, buried pipeline.
3. PEX 6” SDR 13.5, buried pipeline.
4. PE 4710 6” DR 13.5, buried pipeline.
5. PEX 6” SDR 11, above ground installation, ambient temperature 20°C and design temperature is 30°C.
6. PEX 6” SDR 9, above ground installation, ambient temperature 40°C and design temperature is 40°C.

Buried pipeline or above ground installation.

OD: 6.625”, w.t.: 0.28”, mm, d: 6.065”, V = 2.2 m/sec, E = 30×10^6

Surge pressure result: 417 Psi.

Total transient pressure: surge pressure (417 Psi) + pump pressure in the line (160 Psi) is 577 Psi.

Buried pipeline. Maximum allowable working pressure of the pipe is 160 psi at 20°C. Maximum allowable total transient

pressure: 240 psi.

OD: 6.625”, w.t.: 0.602”, d: 5.42”, V = 2.8 m/sec, E = 120.000 at 20°C

Surge pressure result: 120 Psi, total transient pressure: 160 + 120 = 280 Psi.

Total transient pressure: Exceeds the maximum allowable total transient pressure for this pipe material.

Buried pipeline. Maximum allowable working pressure of the pipe is 174 Psi at 20°C. Maximum allowable total transient pressure: 435 Psi.

OD: 6.63”, w.t.: 0.49”, d: 5.64”, V = 3 m/sec, E = 64.000 at 20°C

Surge pressure result: 62 Psi, total transient pressure: 160 + 62 = 222 Psi

Total transient pressure: Is much lower than the maximum allowable total transient pressure (435 Psi).

Buried pipeline. Maximum allowable working pressure of the pipe is 11 bar (160 psi) at 20°C. Allowable total pressure during recurring surge is 16.5 bar (240 Psi).

OD: 6.625”, w.t.: 0.491”, d: 5.64”, V = 2.6 m/sec, E = 120000 at 20°C

Surge pressure result: 99 Psi, total transient pressure: 160 + 99 = 259 Psi.

Total transient pressure: Exceeds the maximum allowable total transient pressure for this pipe material.

Above ground installation: ambient temperature is 20°C, design temperature is 30°C. Maximum allowable working pressure of the pipe is 13 bar (193 Psi) at 30°C. Maximum allowable total transient pressure: 30 bar.

OD: 6.63”, w.t.: 0.74”, d: 5.15”, V = 2.5 m/sec, E = 50000 psi at 30°C

Surge pressure result: 70 Psi, total transient pressure: 160+70 = 230 Psi

Total transient pressure: Is much lower than the maximum allowable total transient pressure (435 PSI).

Above ground installation: ambient temperature is 40°C, design temperature is 40°C. Maximum allowable working pressure of the pipe is 15 bar (214 Psi) at 40°C. Maximum allowable total transient pressure: 30 bar.

OD: 6.63”, w.t.: 0.74”, d: 5.15”, V = 2.7 m/sec, E = 37000 Psi at 40°C

Surge pressure result: 69 Psi, total transient pressure: 160 + 69 = 230 Psi

Total transient pressure: Is much lower than the maximum allowable total transient pressure (435 PSI).

1. The surge pressure caused by the water hammer in steel pipes is at least three times higher than the surge pressure in Pexgol pipes.
2. The surge pressure caused by the water hammer in HDPE pipes could sometimes be too high.
3. Pexgol pipes have a high margin for surge pressures in all temperature range and pipe SDR.