A riser is built to last as long as the building
A gas riser is not replaced every few years. It is expected to serve for the working life of the tower, so the stress analysis looks at what the pipe will go through over decades, not only on the first day.
The figures in the guidelines
The PNGRB Gas Supplies to Multi Occupancy Residential Buildings Guidelines, 2020 take a design life of 40 years and about 15,000 cycles of temperature change over that life. The guidelines also set an installation temperature of 20 °C and a design temperature of 65 °C.
Why cycles matter
A pipe that is stressed once and then left alone can carry a high stress. A pipe that is stressed and relaxed thousands of times can crack at a much lower stress. This is fatigue. The cycles come from the pipe warming and cooling, and also from the building moving in wind and from settlement.
What the analysis does with the numbers
The analysis works out the stress range, which is the difference between the highest and lowest stress the pipe sees, and compares it with an allowable range that depends on the number of cycles. The more cycles, the lower the allowable range.
What this means for design
A layout that looks fine for a single heating cycle can fail the allowable range over a lifetime of cycles. That is why expansion loops, bellows, guides and supports are sized by calculation and not copied from another tower.
Keep the design basis on record
The design life, cycle count and temperatures used should be written in the stress analysis report, so that anyone who reviews or modifies the riser later knows what it was designed for.
To have a riser checked against its design life, send us the building details. See our PNG riser stress analysis service.