The major failure mode for distillation equipment is uplift. This applies to packed beds, trays, collectors, distributors, and all other types of tower internals. Making equipment stronger allows them to ride through more upsets. The most damage prone equipment types are trays, collectors, and pan-orifice distributors. Trays present the greatest obstruction to flow of a pressure front up the tower. All are constructed in the same general fashion.
Trays, collectors, and pan-orifice distributors are built as a collection of panels. Integral trusses are used because they are cheapest. The edge of a tray panel is bent under to form a truss. The panels overlap, the unbent edge of one nesting over the truss at the bent edge of the adjacent panel. Manways are simple flat sections with no integral truss.
Figure 4 shows the standard way of joining tray sections. A bolt is run through the top of the integral truss of one panel. A washer on the bolt holds down the adjacent tray section. This is called a friction fit (the washer is a friction washer). The advantage of this is that it is easy to manufacture and quick to install. Tray fit does not need to be exact, the overlap hides a multitude of sins including varied tray panel sizes and out-of-round towers. Installation is also cheap. Just thread the bolt with the friction washer through and bolt it in. Figure 4 shows both a standard and an extended overlap design. The extended overlap is intended to prevent the tray panels from coming apart as easily. It helps, but not very much. Severe services require surer measures.
Trays join the tower at the tray ring. Clamps attached to the tray with bolts press the tray deck to the tray support ring by holding the underside of the tray support ring. Figure 5 is the underside of standard tray at the tray ring showing the clamps. The main advantages are ease, speed, and low installation cost. The integral truss, the key element of a tray’s strength, does not extend to be connected to the tray support ring.
None of these methods include strength as an advantage. That’s
because it’s not. The tray panel-to-tray panel joints and
the tray support ring clamps are the weakest part of the tray.
Making trays stronger requires changing the standard assembly
method. The simple steps that strengthen a tray include shear
clips and through bolting. Making trays thicker and trusses wider
and deeper help, but the shear clip and through bolting add the
most strength to the tray. They are both cheap and the required
parts are easy to fabricate in the field. Their greatest cost
lies in installation. Through bolting in particular can add significantly
(up to 50%) to the installation cost in some cases. Every plant
has to make its own decision on what to do balancing costs against
benefits.
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