Challenging drainage pipe cavitation modelling

Cavitation is when small cavities are created in a liquid which occurs when the liquid is subjected to extreme changes in pressure. This then creates a great amount of force which causes significant amounts of damage. The velocity at which these are induced varies based on pressure (atmospheric/system) and temperature.

Cavitation analysis can be applied to fluid flows where low pressures may occur and high velocities may be experienced,
such as:

  • reservoir spill ways
  • flows in pipes
  • flows through partially open control structures or valves; and,
  • other applications where high velocities and low pressures are possible.

Challenge

Our Energy and Water Technology Optimisation Services (EWTOPS) team were asked to review the design for a challenging drainage pipe and identify any potential problems.

The design showed the drain collecting runoff water from an area of moorland at an elevation of 165m AOD and descending to 93m AOD, the pipe length was 190m. The descent can be split into two portions by gradient, the first and longest has an average gradient of 1 in 2, the second 1 in 8.

Solution

The starting point for this analysis was:

  • an inflow hydrograph showing a peak flow rate of 0.6m3/s
  • the pipe invert levels, based on LiDAR data for the slope
  • a uniform plastic pipe with 400mm internal diameter.

We were asked to determine the:

  • flow depth within the pipe
  • velocity – comparing multiple techniques for calculating velocity
  • risk of hydraulic jump occurring within the pipe
  • risk of cavitation occurring within the pipe
  • risk of scour occurring within the pipe.

Determining the risk of cavitation required the following parameters:

  • local absolute pressure (a function of altitude and possibly the local network pressure for a closed or pipe full analysis)
  • vapor pressure (a function of temperature and liquid being modelled)
  • kinetic energy per volume of the fluid (a function of the fluid density and velocity).

Benefit

Having calculated the above parameters, and finding the velocity to be 12 m/s in the partially full pipe, we determined there was a risk of cavitation. We then calculated the velocity that would see the risk of inducing cavitation reduced to an acceptable level, which was less than or equal to 10.4 m/s.

Following receipt of this analysis, the client and design team decided to change the route of the pipe to reduce the gradient.

Want to know more?

If you have any concerns about cavitation, or would simply like to find out more, please contact Andy Collier. You can also find out more on our EWTOPS webpage.



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