When using an operating rule to gradually close a valve element (FCV, TCV, etc) in Bentley HAMMER, the time history graph of flow and head is constant before dropping suddenly at the time corresponding to 100% closure?Why do I not see a gradual change in head/flow during a transient, when a valve operating rule is configured as such?
In order to see the proper results, you need to enter a minor loss or headloss coefficient for the valve. For example, if the operating rule is placed on a flow control valve (FCV) and the calculated status is Inactive during the initial conditions time step used for the transient analysis, then it will need to use the minor loss coefficient to determine headloss during valve closure. If the calculated status of the valve in Inactive and the minor loss coefficient is zero, there will be no headloss during the closure until the valve fully closed.Background informationWhen an FCV is active, it is producing a head loss to reduce the amount of flow. HAMMER will take the value for the head loss needed to make the flow reduction and use it to compute a discharge coefficient. The discharge coefficient is necessary to define the relationship between head loss and discharge as the valve closes.If the valve setting is inactive, or if the calculated setting is inactive, the headloss-discharge relationship is determined by the minor loss coefficient. The program converts the minor loss coefficient to a discharge coefficient for use in the transient analysis. The loss coefficient represents the losses when the valve is fully open. A discharge coefficient will be computed based on this and the relative area of the valve, which is defined in the valve characteristics table.
Basically the coefficient stays the same for all closure percentages, but since the area is decreasing, the velocity increases, which is used in the headloss calculation. So, you get a varying amount of headloss, which should be reflected in the transient results viewer.
Example closure with headloss coefficient omitted:
Example closure with headloss coefficients included:
Modeling Reference - Valves of Various Type
Modeling an Initially Partially Closed Valve