Engineers' Guide to Pressure Equipment - Index of

32
Engineers’ Guide to Pressure Equipment
which this simplistic approach is no longer permissible. Typically, one code
specifies
Maximum branch bore = minimum (Di/2, 500 mm) if Di ≤ 1500 mm,
or
= minimum (Di/3, 1000 mm) if Di > 1500 mm
A general rule
As a ‘rule of thumb’ a single opening, 75 mm diameter or less in plate 6 mm
thick or greater, does not require compensation and short pipes of this size
are sometimes welded straight into a vessel shell.
3.2.7 Inspection openings
Various vessel openings are required in vessels to allow internal
examinations as may be required by, for example, The Pressure System
Safety Regulations (see Chapter 12). The size and disposition of the
openings depend upon the duty and size of the vessel. In a small vessel a
single handhole or a flanged-in inspection opening may be adequate,
whereas large vessels require elliptical access manholes, often with
reinforcement/seating rings. Alternatively, heads may be flanged inwards
(reverse knuckle) to provide a seating surface (see Fig. 3.9).
The opening is sealed usually by an internal door, a gasket and one or two
bridges (or dogs), and studs. The door is elliptical to permit its removal if
necessary, for remachining of a damaged gasket seating surface. The minor
axis of an elliptical opening in a cylindrical shell generally lies parallel to
the longitudinal axis of the shell. The studs provide the initial sealing force,
i.e. the initial seating pressure on the gasket face before the fluid is
pressurized. When the fluid pressure later rises, the door tends to be selfsealing
as the pressure load on the door increases the gasket contact
pressure. The load on the studs then decreases. However, some vessel codes
specify that the door must withstand simultaneous bending by both fluid
pressure and maximum possible stud (or bolt) tightening.
A typical flat door calculation thickness, t, is determined from the generic
form of
(C1. fluid pressure. door area + C2. bolt stress . bolt area)/t2 ≤ S;
where C1, C2 are constants.
Some vessel doors are equipped with a locating spigot to aid their
engagement when closing. If the door is heavy then provision must be made
for supporting it during opening or closing, any such support must not
interfere with even take-up of the gasket, nor must it hinder easy access to
the vessel.