Waste Heat Boiler

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Design of Waste Heat Boiler

1.00 Introduction:
It is a High Pressure & High Temperature exchanger to transfer heat from synthesis gas of Ammonia and generate high pressure steam from boiler feed water.

Such exchangers are being designed as per International Codes viz. ASME Section VIII, Div-2 an American code or AD Markbleter a German code. Since it is generating steam on shell side, the shell is also to be designed as per IBR (Indian Boiler Regulation) code.

In India and other foreign countries the Waste Heat Boiler of original design has failed at many places. To overcome the design deficiencies now a day Waste Heat Boilers with new improved design are being developed by few renowned suppliers. Before we present the design of old and New WHB in detail we will first understand the pressure vessel design.

2.00 Pressure Vessel Design
2.01 Basic Design References:
2.01.01 Operating Pressure
The pressure at the top of a pressure vessel at which it normally operates. It shall not exceed the design pressure and it is usually kept at a suitable level below the setting of the pressure relieving devices to prevent their frequent opening. 2.01.02 Design Pressure.

The pressure used in the design of a vessel for the purpose of determining the minimum permissible thickness or physical characteristic of the different parts of the vessel. When applicable, static head shall be added to the design pressure to determine the thickness of any specific part of the vessel. Static loads occurring during operation due to liquids/filling materials shall also be taken into consideration. The minimum design pressure must be 10% or 1.05 bar higher than the normal operating pressure, whichever is higher. This is also called “maximum allowable operating pressure”. 2.01.03 Test Pressure.

The test pressure is the pressure to be applied during testing and shall be based on design pressure and design code. 2.01.04 Operating Temperature.
The operating temperature is the temperature required for or occurring during performance of any given process step. 2.01.05 Design Temperature.
The design temperature shall be at least equal to the operating temperature plus 15 deg C, if not otherwise specified. This is also called maximum allowable temperature. For equipment operating at lower temperature the design temperature shall be 10 deg C lower than operating temperature, if not otherwise specified. The figure so obtained shall further be rounded of to next higher/ lower (for low temperature service) whole multiple of 5. This is also called as minimum operating temperature. When determining design temperature, temperature that may occur during normal startup or shutdown shall also be taken into consideration.

2.01.06 Seismic Forces.
For equipment having length to diameter ratio greater than or equal to 7 the coefficient alpha-h shall be actually calculated as per IS-1893 For equipment having this ratio less than 7 the coefficient alpha-h = 0.1.

There are two methods to calculate coefficient alpha-h
1) Seismic coefficient Method.

=1.0 x 1.5 x 0.04
= 0.06
2) Response Spectrum Method.

=1.0 x 1.5 x 0.20 x 0.28
=0.084

2.01.06 Wind Forces.

2.02 Stresses in Pressure Vessel.
2.02.01 Design Philosophy.
In general pressure vessel codes gives formulas for thickness and stress of basic components, it is up to designer to select appropriate analytical procedures for determining stress due to other loadings. The designer must select the most probable combination of simultaneous loads for an economical and safe design. The maximum general primary membrane stress must be less than allowable stresses outlined in material selection. The maximum primary membrane stress plus the primary bending stress may not exceed 1.5 times the allowable stress of the material selection.

It is general practice when doing more detailed stress analysis to apply higher allowable stresses. In effect, the detail evaluation...
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