Calculation of the overpressure peak
(fluid hammer) for oleodynamic pipes.

1. Legend :
2. 1 - Time for intervention of the valve in closing.
3. 2 - Max positive pressure peak at the end of the line.
4. 3 - Work pressure of the system (static pressure).

The impulsive overpressures said "water hammer" manifest themselves when the fluid is accelerated or braked sharply.

Depending on the time for intervention of the valve, on the fluid and on the pipe characteristics, a pressure wave diffuses itself in the same pipe, causing extra stresses for all the components.

The hydraulic systems, both automatics and manuals, are continuosly subject to this phenomen which should be considered in design phase to avoid the break or usury of the interested components.

Data input

Fluid data.

Fluid type selection

Hydraulic oil H-LP 32 Hydraulic oil H-LP 46 Hydraulic oil H-LP 68 Water

Fluid density at 20°C

kg/l

Elastic modulus

GPa

Pipes data.

Pipe dimension

Chosen pipe dimensions

Øe (mm)

Øi (mm)

Selection pipe material

Hydraulic pipe St 37.4 Hydraulic pipe St 52.4 INOX pipe Aisi 304-316 Copper pipe

Ultimate strength Rm

N/mm²

Elastic modulus E

N/mm²

Notes of parameters considered with the safety factor:

The global safety factor 3.7 considers:

• Basic safety factor (1.5) on ultimate strength Rm.
• Fatigue coefficient for impulsive load (1 000 000 cycles).
• Coefficient for dimensional error of 5%.
• No decay coefficient (corrosion or wear).

The global safety factor 4.2 considers:

• Basic safety factor (1.5) on ultimate strength Rm.
• Fatigue coefficient for impulsive load (2 000 000 cycles).
• Coefficient for dimensional error of 10%.
• No decay coefficient (corrosion or wear).

Global safety factor

x

Pipe length

m

System characteristics.

Time for intervention of the valves:

s

Typical values for solenoid valves CETOP 3-5.

Typical values for solenoid valves CETOP 7-8-10.

Fluid velocity (v):

m/s

Typical velocity values used in hydraulic (oleodynamic) systems.

Max system pressure:

kg/cm²

Setting pressure of the valve of max pressure.

Max overpressure.
In this graph we can see the trend of the peak overpressure (Delta P) related to the time of intervention of the valve (v m/s), below the phase time (Tf) the overpressure doesn't increase.
The phase time is related to the shock wave velocity that is influenced by the temperature of the pipe and of the fluid (their elastic modules change), in this calculation the temperature is considered of 20°C.

Peak pressure.
The peak pressure is the max work pressure of the system summed to the peak overpressure calculated.

Reduction of the peak.
For some kind of circuits, such as for the intake manifolds, is possible to reduce the peaks adding some dumper springs for dissipate the stored energy.