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6. A thrust support for the spring; either a collar keyed to the shaft or a step in the
shaft. CHEMICAL ENGINEERING
The assembled seal is fitted into a gland housing (stuffing box) and held in place by a
retaining ring (gland plate).
Mechanical seals are classified as inside or outside, depending on whether, the primary
(rotating ring) is located inside the housing; running in the fluid, or, outside. Outside seals
are easier to maintain, but inside seals are more commonly used in the process industries,
as it is easier to lubricate and flush this type.

Double seals
Where it is necessary to prevent any leakage of fluid to the atmosphere, a double
mechanical seal is used. The space between the two seals is flushed with a harmless
fluid, compatible with the process fluid, and provides a buffer between the two seals.

Seal-less pumps (canned pumps)
Pumps that have no seal on the shaft between the pump and the drive motor are available.
They are used for severe duties, where it is essential that there is no leakage into the
process fluid, or the environment.
The drive motor and pump are enclosed in a single casing and the stator windings
and armature are protected by metal cans; they are usually referred to as canned pumps.
The motor runs in the process fluid. The use of canned pumps to control environmental
pollution is discussed by Webster (1979).

5.5. MECHANICAL DESIGN OF PIPING SYSTEMS
5.5.1. Wall thickness: pipe schedule

The pipe wall thickness is selected to resist the internal pressure, with an allowance
for corrosion. Processes pipes can normally be considered as thin cylinders; only high-
pressure pipes, such as high-pressure steam lines, are likely to be classified as thick
cylinders and must be given special consideration (see Chapter 13).
The British Standard 5500 gives the following formula for pipe thickness:
Pd
t D 5.8
20 d C P
where P D internal pressure, bar,
d D pipe od, mm,
2
d D design stress at working temperature, N/mm .
Pipes are often specified by a schedule number (based on the thin cylinder formula).
The schedule number is defined by:
P s ð 1000
Schedule number D 5.9
s