If supposed I exceeded the maximum spacing between two sprinklers which is 15 ft, what remedy should I do
You have a number of options depending on the type of sprinkler. Assuming that the original sprinklers are standard coverage, you could take them out and install extended coverage sprinklers that can handle the larger area. This might require more water flow and/or a higher pressure from the water supply, so new hydraulic calculations would need to be conducted.
A second option would be to install a third sprinkler between the two that are too far apart. For example, if the sprinklers are 16 ft apart, you could put a third sprinkler exactly in the middle (causing an 8 ft spacing). This would change some of the fundamental assumptions about the number of sprinklers that would open in a fire, which would mean that the hydraulic calculations would need to be completely re-done.
A third option would be to move all of the sprinklers so that they comply with the spacing rules. Without seeing the space, it's hard to say whether this option is possible. This may or may not require new hydraulic calculations to be performed depending on just how much each sprinkler needs to be moved and what was calculated in the first place.
I think the answer depends on the whether this was a new or existing installation and whether it is in the most remote areas of the sprinkler system for the hydraulic demand. Most sprinklers not in the remote areas will exceed the water density factor for the hazard classification involved. From my experience on surveying existing sprinkler systems exceeding the distances may be subject to some reasonable judgement based on the hazard classification and the hydraulic requirements of the system and the scope or how many of the sprinklers were installed that way in the system. If the situation was prevalent in an existing system, I would contact a fire protection engineer for an engineering judgement that the system as installed in their opinion adequately suppress a fire.
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