Piping Problem

...then a quick response is needed to this question:

What is the ambient temperature when the water gets to 48degC?

Ambient temperature is also 48C . the water is tap water.... scorching heat of sun worm it....

Then insulation won't make any difference. There. Answered.

If the ambient is 48degC and there is no scope for cooling by providing a sink at a lower temperature, then cooling won't take place. Consider refrigeration equipment, though how humans are able to survive for any length of time at 48degC must be addressed on health and safety grounds.

You could use a cooling tower to cool the water to the local wet-bulb temperature, and then always "waste" some at the far end of the pipe, so that cool water keeps entering from the tower. ...Or a refrigeration process, as PW mentions.

Insulation does not work unless the cold face has some where for the conducted heat to escape.. Insulation is a conductor. it slows the conductivity but does not stop it. You can have piece of pipe insulated while doing PWHT and be able to lay your bare hand on it without being burned but if you lay a wood ladder on it, it will soon be starting to burn. If you insulate a piece of steel and put in a furnace it reach the temperature of another identical non insulated piece of steel. Adding a reflector will help for a time. so even though the water source is cool if it sits in the sun it will reach at least ambient temperature and because items in the sun may often far exceed ambient temperature.

Consider having a porous crock at the use station. Evaporation will cool the water inside. Make sure the storage tank at the supply is protected from the sun and heat.