pressure in boiler

slm_2

Hey guys

If there is 50 gallons of water in the system and that water expands 5% when heated that’s 2.5 more gallons added to the system. How much will that 2.5 gallons increase the system pressure and where is that pressure added? Does the suction and discharge both increase? Then what happens at the expansion tank the pressure can’t change right? The way I see it and I know I’m wrong is that when the water is heated the extra water should go into the expansion tank and the system pressure should not increase. But it does increase and I can’t seem to figure out why. Maybe I am over thinking this any help will be much appreciated.

Thanks

Don_4

Answer is comparitively simple. The water can only expand into the tank on a closed system. The air cushion in the tank is at the same presure as the water in the system. Heated water expands into the tank which compresses the water and increases the presure. The water must increase in presure to expand.

Fred Harwood

Expansion

With one modification: water does not compress. The pressure of the air in the expansion tank, compressed by the expanding water, increases. The increase shows up all throughout the system. As the water cools, air pressure in the expansion tank presses water back into the system, and looses pressure as it expands.

The size of the expansion tank matters. It must be able to receive and give back the change in water volume without its air pressure rising above the boiler's pressure relief valve rating. Check with your supplier to choose the right tank.

slm_2

one more question

so if a boiler is filled with a static fill pressure of 12 psi and has a pump with 13 ft of head pressure(about 7 psi)
there would be 12 psi on the suction side and 19 psi on the discharge side and 12 psi at the expansion tank(pumping away). say the expanding water adds 2 more psi. that would make the suction pressure 14 psi and the discharge pressure 21 psi does the pressure at the expansion tank also go to 14 psi or does it stay at 12 psi?

thanks for the help

Steamhead

It would rise to 14 PSI

since that would be the static fill pressure after the water has expanded.

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