Page 141 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring Ecuum Pumps 123

The liquid cylinder is the major pressure-retaining part of the liquid
end, and forms the major portion of the pumping chamber. It usually
contains or supports all other liquid-end components.
A piston (“a,” Figure 3-5) is a flat cylindrical disk, mounted on a rod,
and usually contains some type of sealing rings. A plunger (“b,” Figure
3-5) is a smooth rod and, in its normal configuration, can only be single-
acting. With a piston, the sealing elements move. With a plunger, they
are stationary. A piston must seal against a cylinder or liner inside the
pump. A plunger must seal only in the stuffing box, and touches only
packing and possibly stuffing box bushings.
A piston pump is normally equipped with a replaceable liner (sleeve)
that absorbs the wear from the piston rings. Because a plunger contacts
only stuffing box components, plunger pumps do not require liners.
Sealing between the pumping chamber and atmosphere is accom-
plished in a stuffing box or packing box (“c,” Figure 3-5). The stuffing
box contains rings of packing that conform to and seal against ?he stuff-
ing box ID and the rod.
If a lubricant, sealing liquid or flushing liquid is injected into the center
of the packing, a lantern ring or seal cage is required. This ring provides
an annular space between the packing rings so that the injected fluid can
freely flow to the rod surface.
The valves in a reciprocating pump are opened by the liquid differen-
tial pressure, and allow flow in only one direction. They have a variety
of shapes, including spheres, hemispheres, disk, and bevel seats (Figure
3-6).

Packing Maintenance

The biggest maintenance problem on most reciprocating pumps is
packing. Although the life of standard packing in a power pump is about
2,500 hr, some installations with special stuffing box arrangements have
experienced a life of more than 18,000 hr, at discharge pressures of up to
4,000 psig.
Short packing life can result from any of the following conditions:

1. Improper packing for the application.
2. Insufficient lubrication.
3. Misalignment of plunger (or rod) with stuffing box.
4. Worn plunger, rod, stuffing box bore or stuffing box bushings.
5. Packing gland too tight or too loose.
6. High speed or high pressure.
7. High or low temperature of pumpage.
8. Excessive friction (too much packing in box).