Expansion Bends

Three proven means for absorbing pipe expansion and contraction are (1) expansion bends, (2) corrugated or bellows-type expansion joints and (3) slip-type expansion joints. 

The common types of expansion bends are either fabricated or built-up. Fabricated bends may be of simple-17, double-offset-77 (Fig. 109), circle, or single-offset-77 shapes. Creased bends, though once used to some extent, have become less popular in recent years. Built-up bends are made in a number of configurations from welding elbows. 

All expansion bends, whether fabricated or not, require more installation space than either corrugated or slip-type expansion joints. This is often an important consideration in crowded refinery or petrochemical plant areas but can usually be overlooked on tank farms and other locations where a large amount of space is available. 

The first cost of a fabricated expansion bend may exceed the first cost of a bellows or slip-type expansion joint. A built-up expansion bend usually costs less than a fabricated bend or either type of expansion joint, unless a great deal of labor is required to build the bend. 

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Thermal Expansion in Flare Header - Sliding-type expansion joints may be used in flare headers as an alternative to piping expansion loops, if required to achieve a reduction in pressure drop or where expansion bends may result in liquid surging, subject to the following conditions ... 

Cavitation in pipe systems is possible wherever there are changes in section or flow direction such as expansions, bends and branches. However, serious erosion problems are normally only associated with components within which flow is severely constricted and consequently accelerated. If pumps are excluded then, in most systems, this situation applies to devices used... 

A device that discharges a mixture of coal and warm air to the burner. Expansion bends ... 

A convenient way to correlate data for excess pressure drop when there are contractions, expansions, bends and turns, etc., when the flow is turbulent is to write them as ... 

Except for some specialized applications of the corrugated type, expansion joints and bends require good pipe alignment to perform satisfactorily. Guides serve two important purposes. When installed near an expansion joint they prevent cocking of the sleeve in a slip-type joint or distortion and possible buckling of a corrugated-type joint or an expansion bend. 

In-service inspection and monitoring is based on the requirements of ASME section XI, Div 3. For the main vessel, in addition to ASME requirements of continuous monitoring, ultrasonic examination is planned to be carried out through the main vessel - safety vessel interspace (300 mm nominal gap). A periscope is provided for visual examination of reactor internals. Eddy current inspection is under development for the SG tubes. SG tube size and expansion bend design takes into account this inspection requirement. Ultrasonic examination is planned for the dissimilar joints of the roof slab - main vessel and SG transition joint. The subject of ISI for other reactor components important to safety is under study. For the safety related reactor assembly components, which are non-inspectable, an additional factor of safety in design is envisaged. 

Hydraulic tests to assess the flow distribution and measure vibration of tube bundle are carried out on a 60 degree sector model of SG (fig 13a). The velocity distribution measured in the inlet plenum was found matching with prediction of the 3D hydraulic calculations (fig 13b). Vibration measurements are carried out in the straight spans and expansion bend regions. Although the vibration level is found higher in the inlet span where cross flow takes place across the tube bundle, it is within the acceptable values based on structural mechanics analysis (fig 13c). 

The intermediate shell-and-tube sodium/sodium heat exchangers which have tubes with expansion bends have been operated for 13 years jvithout faults or troubles (Fig. 2.30). Preparations are currently being made to remove one of the IHXs from the reactor for inspection to determine the residual life before the specified service life of 20 years expires. 

Intermediate heat exchanger (IHIQ This is a vertical shell and tube counter-flow heat exchanger with no free sodium level. The tubes are straight with no expansion bends and the tube-to-tubesheet joint is rolled and welded. Thermal shields are provided to protect the bottom tubesheet from thermal transients. Antivibration belts surround the tube bundle to prevent vibration. A sleeve-type valve is used to isolate the IHX primary side when it is non-operational. 

Risers passing through decks should have adequately liberal expansion bends to absorb the part of the stresses imposed by shock. 

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