The pressure wave generated by a flow-control operation propagates with speed a, reaching the other end of the pipeline in a time interval equal to L/a seconds. The same time interval is necessary for the reflected wave to travel back to its origin, for a total of 2 L/a seconds. The quantity 2 L/a is termed the characteristic time for the pipeline. It is used to classify the relative speed of a maneuver that causes a hydraulic transient.

If a flow-control operation produces a velocity change in a time interval less than or equal to a pipeline's characteristic time, the operation is considered "rapid." Flow-control operations that occur over an interval longer than the characteristic time are designated "gradual" or "slow." The classifications and associated nomenclature are summarized in the following table for different operation time, T _m .

Table 14-4: Classification of Flow Control Operations Based on System Characteristic Time

Time of Maneuver	Operation Classification
	Instantaneous
	Rapid
	Gradual
	Slow

The characteristic time is significant in transient flow analysis because it dictates which method is applicable for evaluating a particular flow-control operation in a given system. The rigid model provides accurate results only for surge transients generated by slow flow-control operations that do not cause significant liquid compression or pipe deformation. Instantaneous, rapid, and gradual changes must be analyzed with the elastic model. Bentley HAMMER V8i uses the elastic model by default to ensure an accurate solution, regardless of the system's characteristic time.