In my last article on stress analysis of tank piping I have described the effect of tank bulging. Click here to refresh yourself on the effect of tank bulging. In this article I will describe about the effect of tank settlement on stress analysis of piping system connected to large tanks.
Why settlement occurs for tanks but not for other equipments:
Equipment diameter is small (up to 3m). Therefore it is possible to design its foundation with large raft (say 10 m), to minimize or have insignificant settlement.
Whereas tank diameters are generally large, of the order of 10 m to 60 m. Due to this it is impractical to design its foundation with raft, which would be much bigger than this. Many times it has ring foundation with soil compacted within this concrete ring.
How much settlement to be considered:
Amount of settlement depends on the location of tank. The amount of settlement is normally mentioned soil investigation report or geo-technological investigation report.
IN CASE OF SAND:
Majority of the total settlement occurs during hydro test of tank (before piping is connected). This is generally permanent.
… Typically 60%
For balance 40% of settlement, this occurs after piping connection, piping needs to be designed properly with settlement effect.
IN CASE OF CLAY: Progressive settlement.
The settlement is more at the centre of tank, and typically 50% at the edge of tank.
Since our nozzles and tank roof are connected / supported on shell, that is on outer edge of tank, we need to consider the settlement at outer edge of tank.
Following Data to be obtained from civil for each tank (for each project)
- Total long term settlement.
- Settlement that will occur during construction and hydro test of tank.
- Recovery (if any) following construction and hydro test of tank.
- Further settlement, after hydro test of tank, (at the edge of the tank).
Sample Data from civil for each tank for a typical project is shown in Fig. 1 for understanding.
- It contains each Tank number.
- Settlement at Centre of Tank.
- Settlement at Edge of Tank.
Then out of total settlement at Edge, 40% of total settlement is what we consider in piping stress analysis.
Pipe routing guidelines (Fig. 2) to minimize effect of tank settlement:
To reduce effect of tank settlement on piping:
- First support shall be kept sufficiently away from the tank nozzle.
- Large dia. piping combined with large tank settlement may call for use of spring support.
However use of spring support shall be avoided because accidental draining of line will cause excessive upward force on piping and tank nozzle.
So, if spring support is used
- WNC (Weight with No Content) load case shall be mandatory for liquid lines. In fact for all liquid lines with spring support, (whether it is connected to tank or any other line), WNC run shall be mandatory. In case of tank, all lines connected to tank will be carrying liquid only.
- Spring setting should be adjusted in such a way, that nozzle load is within limit in normal operating case, as well as in WNC case.
- That is set spring support for load lower than what is required. This will increase nozzle load in normal operating case, but will reduce load in WNC case.
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