Problem

When attempting to compute a transient simulation in HAMMER, the calculation fails and the following User Notification appears:

"Initial flow is zero, which would result in a discharge coefficient equal to 0, which is not allowed."

Solution

This situation can occur with PRVs, PSVs and FCVs, if the initial status of the valve is set to Active, but the flow through the valve is zero. HAMMER cannot compute the model in this case because it needs to represent the initial position as a discharge coefficient (Cv), and when the valve status is active it obtains that discharge coefficient from the calculated results from the initial conditions. In a case where the initial flow is zero, the discharge coefficient cannot be computed (see the "Discharge Coefficient Setting (Calculated) field in the Results section of the properties) and hence the error message.

To resolve this, you can set the initial status of the PRV, PSV or FCV to Inactive, which will tell HAMMER to derive the discharge coefficient based on the fully open position. That fully open position is defined by way of the "Valve Coefficient Type", where you can enter either a minor loss coefficient or discharge coefficient.

This situation may also indicate a problem with the model configuration - if you expected positive flow value through the valve, check to make sure the pipes are physically connected, the orientation of any check valves is correct, and that appropriate demands have been entered. 

See Also

Modeling Reference - Valve Closure

Using Modulating PRVs