Page 229 - 4637

15 SLUSH PUMPS

The duplex double-acting steam pump, which was used
exclusively in drilling for almost 40 years, is no longer in common
use and therefore will not be discussed at this time.
The power-driven duplex double-action slush pump, which
followed, made use of the technology that had been developed in
the fluid end of the steam pump. There is no essential difference in
the basic design of cylinders, liners, valves, pistons or rods.
An important difference, however, in a crank-driven pump,
is the operation cycle of the two pistons which are 90° out of phase
rather than 180° as in the steam pump, which produces –
essentially a uniform piston velocity throughout the entire stroke.
Even with harmonic motion of the pistons some surges in
discharge volumes would result.
Since, in crank-type pumps, harmonic motion is not
achieved, these surges are further increased by two factors. First,
piston movement on the outer half of the working stroke is faster
than on the first half of than stroke. Second, the piston rods reduce
displacement of the inner end of the cylinder. This is especially
significant when small liners are in use.
This shortcoming can be alleviated to a considerable degree
by use of large air chambers, or precharged pulsation dampeners,
on the pump discharge manifold. Figure 15.1 is cross-section view
of a typical duplex, double-acting, power-driven slush pump.
Figure 15.1 left is a view of fluid end of a slush pump.
The single-acting triplex slush pump has gained prominence
in recent years. Triplex plunger pumps have been used extensively
in industry for many years. Their basic advantages are: (1) more
even discharge rates, so less pressure pulsation, (2) substitution of
liners and half pistons for plungers and packing, (3) charging of
the suction pressure.
The conventional triplex plunger pump did not prove
practical as a slush pump, but a modified version has proved

228