Problem

How does HAMMER calculate the pressure spike "surge" (increase in hydraulic grade) resulting from air pocket collapse? In some cases, it is quite severe.

Solution

HAMMER uses the equations from the Method of Characteristics (MoC) to calculate changes in hydraulic grade and flow at each time step and pipe section point (increment along pipeline length). It relies on Nodes such as air valves, pumps, turbines, etc to have their own ways of being expressed as equations, then interacts with the changes resulting from those node elements.

In other words, at a node element, the equations of the MoC will be combined with node equations (such as air valve air flow equation), momentum equation and continuity equations. When these combined equations are solved by using numerical methods, head, flow and other node results (such as air volume) are found.

Boundary conditions are used to build node equations. For example, constant atmospheric pressure can be a boundary condition and it is used in gas equation to calculate the air flow to and from air valve.

So for an air valve, the air flow rate equations (mentioned in the air valve theory section of the help) are the ones that describe that node element. The MoC then solves the change in head and flow resulting from changes to the node. In other words, the air valve's air flow rate on air pocket collapse determine how fast the adjacent water columns are coming together. When the air is fully expelled, the MoC calculates the corresponding change in head and flow resulting in this, leading to the results that you see. This is similar to the pressure increase on the upstream side of a valve, resutling from the sudden stopped of the water column.