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# Solar Gain calculations

## Solar Gain calculations

(OP)
i am trying to determine if overpressure protection is required in a line due to solar gain.
in other words if solar gain will generate high enough pressure in a line that overpressure protection will be required to comply with code requirements.

anyone has experience with this type of analysis?

### RE: Solar Gain calculations

I assume you mean thermal expansion due to the temperature change from sun exposure?

If so, a blocked liquid full line would need relief due to thermal expansion. If that's your only credible relief scenario, API 521 has guidance that a small (0.75 x 1") relief valve may be used without calculations for general applications. If you have large bore piping and/or significant length of piping, you may want to run the calculations (equations included in API 521).

I don't know of specific calculation for heat transfer from sun exposure, but to simplify it we've made a conservative assumption for the temperature change based on location and gone from there. The relief rate required for most thermal expansion cases is so small most small relief valves are acceptable.

### RE: Solar Gain calculations

For an isolated section of liquid-filled pipe, there's no need to calculate the solar heat input or the resulting pressure rise. The relevant questions to consider are: (1) can the segment of piping be completely isolated, (2) is the segment of pipe truly liquid-filled, and (3) is the liquid temperature low enough such that atmospheric exposure (including solar radiation for uninsulated pipe) will cause it to rise. If those three conditions are all true, then the pipe is going to be overpressured and experience loss of containment (usually from a blown gasket). Severe overpressure will occur from just a small increase in temperature.

When those three conditions are true, you need to either install a thermal expansion valve, or do something to ensure that the pipe is never completely isolated. As RVAmeche stated, all you need to do in most cases is install a 3/4 x 1 PSV. PSV sizing calculations are not needed unless the pipe segments is a very long one.

In almost all cases where you're assessing the need for a thermal PSV due to atm heating, calculating the resulting pressure rise is an academic calculation that has no practical value. If a system is isolated and liquid-full, all that matters is whether or not the liquid temperature can rise.

### RE: Solar Gain calculations

In the propane industry, a propane supplier must install on their bulk storage tanks hydrostatic relief valves on liquid lines that can be isolated by two remote shut off valves so as to relieve pressure increase as a result of an increase in ambient temperature or from solar gain.

### RE: Solar Gain calculations

If the line minimum normal operating temp is less than the solar heated metalwall temp in summer( which varies with location latitude, and is approx 55degC in the tropics, 65degC in the Middle East), the line is liquid full and can get blocked in, then a thermal RV would most likely be required.

The API overpressure guideline dont say how to calculate this load. I've done such calcs from first principles for inplant lines and resulting relief flows are very small. A 3/4x1 or a 1D2 RV is more than adequate in most cases. To calculate this load, (a) determine the surface area of exposed pipe for the section blocked in (b) estimate the heat input to to the blocked in section from solar radiation (c) estimate the rate of volumetric expansion due to temp rise - rate of change of density coincident with the time at which pressure has reached thermal RV set pressure.

Solar radiation is approx 900-1000W/m2, max for a blackbody with α=1, but only 50% of this is in the infra red and capable of heating.

### RE: Solar Gain calculations

There is an FAQ here https://www.eng-tips.com/faqs.cfm?fid=1339

Generally you can reckon on somewhere between 3-5 bar per deg C for a closed volume

There is sometimes a ROT about not providing reliefs for certain volumes of liquid between valves to prevent installing thousands of TRVs on a plant.

Don 1980 and George have it right. Unless your liquid starts at a high temperature or very low pressure compared to piping rating then just design some in.

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