An issue with Palo Alto's water-pressure system set off at least 24 false fire-alarm calls in rapid succession throughout the city in late January, forcing dispatchers and emergency crews to scramble.

Palo Alto Utilities and San Francisco Public Utilities Commission (SFPUC) puzzled over why the pressure from the SFPUC's water system dropped and possibly triggered a sudden pressure rise as Palo Alto's system attempted to compensate, thus setting off the alarms.

The succession of rapid alerts began rolling in on Jan. 26 starting 1:47 p.m. and continued for about 30 minutes, according to police scanner calls and the online emergency alert app PulsePoint. Calls continued to come in from throughout the city in one- to two-minute intervals.

Firefighters who were quickly dispatched to each location found there was no evidence of fire, Deputy Fire Chief Kevin McNally said. The alerts spanned the city, predominantly from El Camino Real and east to the bay, he said.

The false alarms put a high stress on the city's response system as fire crews dispatched to location after location.

"It annihilated dispatch," McNally said.

But the dispatchers were good about triaging, he added. They first routed crews and emergency medical services to known emergencies such as medical incidents and crashes. They also prioritized alarm alerts where there were potential signs of fire, such as reported smoke, he said. As crews were freed up, the department checked each alarm location, resetting the systems. The sprinklers were not activated and there was no water damage to businesses, he added.

Such false alarms have happened in the past when there was a water pressure drop, but it has not been of this magnitude, McNally said.

When the pressure drops from outside of the Palo Alto system such as from the SFPUC network, from which the city receives its water, Palo Alto Utilities' system automatically boosts its pressurized system. The sudden rise can cause "water hammer," a kind of high-pressure shock wave through a piping system when moving water is forced to change direction or abruptly stops. The phenomenon can set off the sprinkler alarms, he said. False alarms can also be triggered from within Palo Alto when someone hits a water line, disrupting the water pressure, he added.

Palo Alto Utilities said the problem originated outside of its system and referred all questions to the SFPUC.

Joseph Sweiss, SFPUC press secretary, said on Monday that the agency has been coordinating with Palo Alto on the city's project to replace piping and make improvements in its water system pipeline vault downstream of the SFPUC's California Avenue vault. The work is related to capping connections to operations that are no longer in use. But the coordination included a shutdown from 6 p.m. on Jan. 26 to 6 a.m. on Jan. 27, hours after the incident.

"Both utilities are working to understand the early afternoon pressure changes in the Palo Alto system given the actual shutdown schedule occurred late at night on Thursday," he said.