Dispatched for a fire alarm activation; north riser flow switch. Upon our arrival, access was gained thru a locked gate and then entry was made into Plant #1 by fire personnel utilizing the keys from the Knox Box. Upon investigation of the alarm, a sprinkler system line valve was noted to be broken with water flowing from the sprinkler system. A nearby valve was closed in order to discontinue water flow to the broken valve. The audible alarm is then silenced at the control panel, but the alarm system cannot be reset. Dispatch made contact with a key holder for the facility and advise the key holder of the situation. The key holder advised dispatch that they would be enroute to the scene with an ETA of approximately 45 minutes. We re-secured the building and the gate. The keys are returned and secured in the Knox Box. All units cleared the scene and returned to quarters.