A riser is a long pipe in a moving building
In a tall building the gas riser runs floor after floor inside a shaft, held at many points to a structure that is never perfectly still. If the pipe is fixed too rigidly, any movement turns into stress in the pipe, at its joints and at its supports. Stress analysis is the calculation that shows whether the pipe can take that movement safely for the life of the building.
What makes the pipe move
Temperature is the first cause. The steel riser warms and cools with the weather, sun on the shaft and the gas itself, and the PNGRB guidelines use a design basis of installation at 20 °C and operation at 65 °C. Wind makes tall towers sway, and the pipe is dragged sideways at every floor. Earthquake movement is checked as a separate load case. The building also settles over time, and the pipe carries its own weight and the gas pressure. As a simple example, steel grows by roughly 12 millionths of its length for each degree. Over a 45 °C swing, 120 m of pipe that is free to grow lengthens by about 65 mm, and 250 m lengthens by about 135 mm. If the pipe cannot absorb that, the stress goes into the pipe.
Why it matters more as buildings get taller
Longer runs collect more thermal growth, sway at the top of a tower is larger, and the load on each support adds up down the riser. A layout that works at 40 m can be overstressed at 120 m or more, so tall towers need a calculation, not a rule of thumb.
What the PNGRB T4S regulations require
The technical standards and safety standards for city gas distribution networks, notified by the PNGRB in 2008 and known as the T4S regulations, make ASME B31.8 the base design code and add to it where they are stricter. For above-ground steel piping they require thermal expansion and stresses to be evaluated, and where the gas temperature is below 65 °C the evaluation is done at a pipe temperature of 65 °C. Design calculations must show the pipe is sound and stable from installation through operation under the combined effect of pressure, temperature, bending and external loads, with seismic effects considered. A service line riser in a confined space such as a basement must be welded, kept at least 25 mm off the wall, supported at about 2 metre intervals and ventilated. These are general rules. They do not give a method for tall-building sway or a height limit, which is why the high-rise guidelines add them.
What the PNGRB high-rise guidelines require
The PNGRB Gas Supplies to Multi Occupancy Residential Buildings Guidelines, 2020 say the riser and lateral piping must be designed so that stresses from expansion, contraction, sway and settlement stay within acceptable limits, with wind, temperature and solar gain considered. For buildings taller than 40 metres they require a stress analysis. It must cover pressure, self-weight, settlement and seismic effect, and sway in the plus and minus X and Z directions, with a floor-by-floor sway table supplied for cross-checking. The design life is 40 years with one thermal cycle a day, which is about 15,000 cycles, with the expansion stress range reduced by a factor of about 0.8. Five load combinations are checked: sway in each direction, and one with seismic effect. The guidelines also ask for suitable mechanisms in the riser design to take up thermal movement.
What the analysis checks
The calculation checks pipe and joint stresses against the allowable limits in every load case, and the forces and movements at each support, clamp and guide. It shows whether expansion loops, bellows or sliding and guided supports are needed and where. It also compares the sway table with the displacements used in the model, so that the pipe design and the building data agree.
What can go wrong without it
Overstressed welds and fittings can leak. Supports can be overloaded or pulled out. Repeated daily heating and cooling can fatigue a joint over the years. The pipe is inside an occupied building, so a failure there has serious consequences, and fixing a fault after the walls are closed is difficult.
What you should receive
You should get a stress analysis report with the load cases and results, a support and flexibility layout for the installer, and the as-built drawings before commissioning. Ask the structural designer for the building's sway and seismic data early, because the analysis depends on it.
ERGNA Engineering is a registered vendor of Mahanagar Gas Limited for design, engineering and stress analysis of pipe natural gas networks in high-rise buildings. If you have a tower in planning, send us the height and the structural data and we will outline the scope. See our PNG riser stress analysis service.