Pressure increase in solar heating system

[detbert]

Hi, maybe someone can help me out.

I am looking at a solar heating system,(small diameter pipe - OD 16 mm).

We had a failure of the pipe (regular operation conditions 8 bar at 60 deg. C). Looks like the pressure was much higher and had a ductile failure.

How do one calculate the pressure increase in this closed system caused by increased temperature (appr. 10 - 20 deg. C).

Thanks....

 

[pkelly54]

The pressure increase is a function of the expansion of the liquid and the pressure required to compress it.

From the steam tables compare the change in specific volume of saturated liquid for the two temperatures in question.

From the table of the compressibility of water one can obtain the force required to compress water at various temperatures and pressures.

Here the nebulous part; as the system pressure increases, the pipe will deform elastically. This deformation will increase the containment volume tending to lower the final system pressure to less than that of an inelastic pipe system. To calculate the actual increase in pressure one must interate the change in system volume due to the pressure increase with the increase in pressure due to the change in volume caused by a temperature increase.

The entire problem becomes simplified with the use of thermal relief valves. These are low capacity relief valves intended to protect a system from overpressureization when they are "bottled up".

 

[butelja]

Another possible solution if a thermal relief valve is undesireable is to add an accumulator. This is a small pressure vessel partially pressurized with air and has an elastomer membrane to prevent air/water mixing. As the water expands, it goes into the accumulator and compresses the air. You will still have a pressure rise, but it will be much less than the system without an accumulator.