Why does using an air valve sometimes result in higher maximum transient pressures than without it?
This can occur sometimes, if the air pocket that enters the air valve is expelled too quickly. This can result in the adjacent water columns colliding at a high velocity, which causes a high pressure transient upsurge. Care must be taken to select an appropriate air valve type and size, so as not to cause worse transients than if no valve had been used. For example, a smaller outflow orifice may be necessary, to cushion air outflow.
