When feed water flows in feed water inlet pipe, slope of temperature is formed between inner edge of feed water inlet pipe and boiler drum because of temperature difference between feed water and boiler water which is saturated temperature corresponding to steam pressure. As stress distribution in boiler drum around feed water inlet is formed by this temperature slope, it can be understood that a location in boiler drum having low stress is corresponding to a location having temperature near by saturated boiler water temperature.
It is supposed that maximum stress is observed at the inner edge of a hole when a pressure is loaded to the inner edge of a hole in very large disc. It can be recognized that the stress may decrease as length from the edge the hole increases.
To seek a location in boiler drum where stress is enough low as compared with maximum stress or allowable stress of the boilerplate, stress on a large disc is calculated by following formulas when a pressure is loaded to the inner edge of a hole of the large disc;
σθ: Circumferential stress
σr: Radial stress
r1: Inner radius
r2: Outer radius
r: A certain radius
P: Pressure loaded to inner edge of a hole in a large disc.
Fig. 2 shows the result of the above stress calculation, it is understood that maximum stress is measured at the inner edge of a hole and the stress decreases immediately as the length from the edge increases. Moreover, it may be recognized that the stress decreases enough low after a point located in r = 3 r1 from the center of the hole.
So it may be decided that the outer radius can be three times the inner radius. This point of outer radius. In large thickness pipe corresponds to a point on the boiler drum where the temperature is near by the saturated boiler water temperature.
Fig. 2 Stress distribution on a large disc having a hole when a pressure is loaded to inner edge of the hole.
5. Result of analysis for thermal stress distribution
Thermal stress distribution was calculated using obtained temperature distribution through calculation of heat transfer coefficient between feed water and inner edge of feed water inlet pipe. The heat transfer coefficient between feed water and inner edge of feed water inlet pipe; α was calculated using two formulas said above;
(1) In a case of calculation by Prandtl's formula
α was obtained in three case of pipe diameter of 25A, 32A, 40A (sch 80) as follows;
α = 7895 in a case of pipe diameter; 25A
α = 4791 in a case of pipe diameter; 32A
α = 3616 in a case of pipe diameter; 40A
Using above heat transfer coefficient, α, temperature at inner edge of feed water inlet pipe was calculated under a condition of feed water temperature of 50℃ and some outer edge temperatures corresponding to saturated boiler water pressure from 0.63 Mpa to 2.3 Mpa. Table. 1, Table. 2 and Table. 3 show temperatures at inner edge of feed water inlet pipe and temperatures at outer edge of large thickness pipe corresponding to temperatures at a point located in r = 3 r1 from the center of the pipe. In this study circumferential stress is only used for stress estimation because of highest stress value.
Table. 1 Temperatures at the inner edge, Ti with temperature at the outer edge, T0 in case of pipe diameter; 25A (sch 80)
