Page 150 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

132 Major Process Equipment Maintenance and Repair

are of chrome, various ceramics, nickel-based alloys, or cobalt-based al-
loys. Desired features for the coatings include hardness, smoothness,
high bond strength, corrosion resistance, and low cost. No one coating
optimizes all of these features.
The ceramic coatings are harder than the metals but are brittle, porous,
and sometimes lower in bond strength. Porosity contributes to shorter
packing life. Mixing of hard particles such ‘as tungsten carbide into the
less-hard nickel or cobalt alloys has resulted in longer plunger life at the
expense of shorter packing life.

Drive-End Components

The drive end of a power pump is called a power end (see Figure 3-3).
Its function is to convert rotating motion from a driver to reciprocating
motion for the liquid end. The main component of the power end is the
power frame, which supports all other power end parts and, usually, the
liquid end. The second major item in the power end is the crankshaft
(sometimes, a camshaft). The function of the crankshaft in a power pump
is the same as a crankshaft in an internal-combustion engine, except that
the flow of energy is opposite.
The main bearings support the shaft in the power frame. The co~ect-
ing rod is driven by the throw of the crankshaft on one end, and drives a
crosshead on the other. The crosshead moves in pure reciprocating mo-
tion, the crankshaft in pure rotating motion. The connecting rod is the
link between the two.
Although similar in construction and motion to a piston in an internal-
combustion engine, the crosshead is fastened to a rod called an “exten-
sion,’’ “stub,” or “pony” rod. The other end of this rod is fastened to the
plunger or piston rod.
The function of the drive end (or steam end, or gas end) of a direct-
acting pump is to convert the differential pressure of the motive fluid to
reciprocating motion for the liquid end. The drive end is similar in con-
struction to the liquid end, containing a double-acting piston and valving.
The major difference is that the valve is mechanically actuated by a con-
trol system that senses the location of the drive piston, to cause the valve
to reverse the flow of the motive fluid when the drive piston reaches the
end of its stroke.
The main component of the drive end is the drive cylinder. This cylin-
der forms the major portion of the pressure boundary, and supports the
other drive-end parts. Unlike the power-pump’s power end, this cylinder
does not support the liquid end.