purpose of isolating valves and relief valves in fire main line and what requirements to be need to fitted?

In hydrants, it necessary to have these pipelines appropriate isolated, section wise, so that in event of fire, certain section of the pipeline are protected and the whole pipeline will not damage incase let say there is a fire in machinery space and main fire pumps driven by main power positioned in engine room cannot be approached to put into service, so emergency pump needs to be started.

If there is no valve to keep the machinery space isolated from the emergency fire pump supply line.

Relief valve are fitted to the fire lines in order to reduce to relieve excess pressure that may be created on the discharge side of the fire pumps.

Requirement :

• Isolating valves for separating the section of the fire-line within the machinery space containing the main fire pump or pumps, from the rest of the fire-main, must be fitted;
• These isolating valves must be fitted in a place that is easily accessible and tenable position, outside the machinery spaces;
• The fire main shall be so arranged that when the isolating valves are shut, all the hydrants on the ship, except the ones in the machinery space(as referred to in the two previous bullets), can be supplied with water, by another fire pump or an emergency fire pump:
• The emergency fire pump, its sea-water inlet, and the suction and delivery pipes and the isolating valves, must be located outside the machinery space. If this arrangement is not feasible to be made, the sea chest may be fitted in the machinery space, if the valve is remotely controlled from a position in the same compartment as the emergency fire pump, and, the suction pipe is as short as practicable;
• Short lengths of suction or discharge pipes may be allowed to penetrate the machinery space, provided these are enclosed in a substantial steel casing, or, are insulated to "A-60" Class standard. The pipes must have substantial wall thickness, but in no case less than 11 mm, and must be welded, except for the flanged connection to the sea-inlet valve.
• A valve must be fitted to serve each fire hydrant, so that any fire hose may be removed while the fire pumps are in operation;
• In tankers, isolating valves must be fitted in the fire main at the poop front, in a protected position and on the tank deck at intervals of not more than 40 meters, to preserve the integrity of the fire main system, in the case of a fire or explosion;
• Relief valves must be provided in conjunction with the fire pumps, if the pumps are capable of developing a pressure which is higher than the design pressure of the water service pipes, hydrants and hoses. The positioning of these valves shall be such, and, they be so adjusted that they will prevent excessive water pressure, in any part of the fire main system.

*Source : eLearning by dg