Surge pressure
Surge pressures, or pressure surges, occur in a pipe transporting fluid as a result of a change in the flow velocity, e. g. if a valve is closed or opened too rapidly. If a valve is closed rapidly, the energy of the fluid moving forward in the pipe leads to a sudden rise in pressure due to compression upstream of the valve.
Negative pressure will develop immediately downstream of the valve as a result of the fluid being still in motion, causing a temporary separation of the liquid column followed by a reverse flow back towards the valve if the pressure gradient is high enough. This may lead to the destruction of both the valve and the pipe.
The highest surge pressures often occur when the maximum number of pumps in operation at a pumping station stop as a result of a power failure. As the change in pressure and velocity is not limited to the point of disturbance, it continues up- and downstream a pressure wave propagation velocity.
The boundary conditions determine the degree to which waves are reflected at locations of non-steady flow (e. g. pipe branches, valves,, changes in cross-sections, tanks) resulting in a phase or amplitude reversal.
The condition observed at a defined location and time is determined by the superposition of all waves arriving at this defined location at the defined time. The pressure fluctuations and resultant maximum pressures cause extremely high loads for the system.
Although the pressure level cannot fall under that of vapour pressure the minimum pressure may reach that level. If this situation persists for a prolonged period of time, a cavitation zone will develop leading to the separation of the liquid column into two columns with resultant flow separation. Following their change in velocity direction, the liquid columns will reverse and often collide with high velocity differentials, causing a new pressure surge (see Water hammer, Sudden collapse of vapour-filled cavities).
This pressure surge is often significantly greater than the original surge.
Maximum permissible negative pressures depending on piping material and layout
- Max. 0.2 bar for plastic and fibre-glass reinforced plastic pipes
- Approx. 0.4 bar abs. (corresponding to a negative pressure of 0.6 bar) for welded steel pipes (depending on the wall thickness) for connection to steel or cast pipes via sockets
The minimum pressure should never fall below the atmospheric pressure at any point in the system which means that this system pressure must also be maintained following a power supply failure. The systems most at risk from pressure surges are low- and medium-pressure systems and not high-pressure systems.
According to Joukowsky, the following applies to the maximum change in pressure:
Δp = ρ · a · Δv
Δp Change in pressure in bar
ρ Density of water in kg/m3
a Pressure wave propagation velocity in m/s
Δv Change in flow velocity in m/s
Für die maximale Änderung der Druckhöhe gilt:
ΔHmax Maximum change in pressure head in m
g Acceleration due to gravity in m/s2
v0 Undisturbed velocity in m/s
The maximum change in pressure head (ΔHmax) may only develop if this applies to the time (ts) during which the full change in velocity (v0) takes place:
tr Reflection time
L Distance between the nearest reflection point and the point of disturbance
a Pressure wave propagation velocity
According to Joukowsky, the pressure change is superimposed both positively and negatively by the steady-state pressure upstream of the point of disturbance. This may lead to negative pressures in the piping.
If the time (ts) for the change in velocity is longer than the reflection time (tr), then the change in velocity within the reflection time must be used to calculate the maximum change in pressure head (ΔH) at v0.
The relationship between the switching and reflection time implicitly expresses that the longer the switching time (ts) is in relation to the system’s reflection time (tr), the smaller the changes in pressure (Δp) become. The ratio should be around 5 to 1 in order to keep the consequences of a pressure surge within permissible limits.
A differentiation is made between two cases; the measures taken to limit the maximum pressure will have to become effective the second step only:
ts is given and tr must be reduced in accordance with the ratio above by:
- Keeping the piping as short as possible
- Providing intermediate reflection points for long pipes (e.g. provision of surge tanks or drums at the highest point, stand pipes at intermediate and highest points, venting the pipe at the highest points, provision of a one-way surge tank)
tr is given and ts must be increased in accordance with the ratio above by:
- Selecting appropriate valve closure characteristics
- Extending the pump's run-down time after it is switched off (e.g. via flywheel mass)
- Providing further liquid from a tank (e. g. accumulator, stand pipe, one-way surge tank)
- Discharging suddenly accumulated masses of fluid (e.g. via additional outlets, safety valves, bypass)
According to Joukowsky's equation, the change in pressure is low if the changes in the velocity of the fluid flow (Δv) and pressure wave propagation (a) remain low. This applies to light fluids, low flow velocities and low sound velocities (e. g. via large-diameter piping).
If the problems resulting from surge pressure have not already been considered at the planning stage, the possibilities of influencing these parameters are often very low or involve considerable effort.
Impermissible high or low pressures can often only be avoided by employing appropriate surge control systems. The selection and sizing of required surge control systems are so complex that computerised methods are needed to calculate them (see also technical instruction leaflet W 303 Dynamic Changes in Water Supply Systems published by the German Association of the Gas and Water Sector [German abbreviation: DVGW])
