TS-136
A Study on Stress Condition of Auxiliary Boiler around Feed Water Inlet
Katsuya NAITO*, Hiromi SHIIHARA*
Abstract
A study. for estimating stress condition of the auxiliary boiler drum around the feed water inlet was carried out to seek probability of direct connection of the feed water inlet pipe with the boiler drum. The analysis for obtaining stress distribution on the boiler drum around the feed water inlet was carried out using a heat conduction formula of large thickness pipe model. The result of analysis shows that maximum stress is observed on the inner edge of the feed water inlet pipe and the stress decreases immediately as distance increases from the inner edge to boiler drum. Maximum stress less than 180 Mpa(N/mm2) is calculated on the inner edge of the feed water inlet pipe under the condition of boiler steam pressure of 2.3 Mpa and feed water temperature of 50℃. The result of this study shows that the feed water inlet pipe can connect directly with the boiler drum for a boiler of design steam pressure less than 2.0 Mpa.
Key Words: Thermal stress, turbulent flow, heat transfer coefficient, Prandtl's Formula. Von Karman's formula, heat conduction,
1. Introduction
Rules of Nippon Kaiji Kyokai (hereinafter to be referred as "Class NK") requires that the part of the boiler drum where feed water is introduced is to be provided with sleeves or other suitable devices so that an extreme thermal stress may not occur due to direct, contact of cold feed water with the drum. This requirement also applies to the desuperheater in the boiler drum, if installed, where the superheated steam pipes penetrate through the drum. Further, feed water discharge within the drum is to be so distributed that it may not impinge directly on the heating surfaces of the boiler at high temperature.
On the other hand, recently the construction and shape of the auxiliary boiler have been modified little by little. For example, formed end plate of some type's boilers has been exchanged for welded type's end plate with the boiler drum at the top corner, and formed Ogee Rings of the vertical boiler has been also exchanged for welded plate type.
A study for estimating stress condition of the boiler drum of the auxiliary boiler around feed water inlet was started in response to a requests from boiler manufacturers and ship owners that the feed water inlet pipe wants to be connected with the boiler drum directly. However this kind of estimation is very difficult because of difficult measurement of stress or strain on the part of the boiler drum where feed water is introduced under operating condition. And also residue stress and thermal stress must be estimated in addition to stress due to internal working pressure.
This study by theoretical analysis was carried out using thermal stress distribution model of large thickness pipe simplifying construction of the part of the boiler drum where feed water is introduced.
2. General construction of the part of the boiler drum where feed water is introduced
