Page 340 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 305

Other Types of Chemical Pumps

With continuous development of structural composite materials, pump
manufacturers are offering various types of nonmetallic pumps in the
ANSI envelope. ANSI specifications do not encompass nonmetallic
pumps and there are no national standards for pressure, temperature, or
limitations. These pumps can be horizontal cradle pumps, self-priming
pumps, or submersible sump pumps. The design of these pumps will be
discussed later in this chapter,

Sealless Pumps

Another type of chemical pump is referred to as a sealless pump, but
more properly should be called a vapor tight or leakproof pump. These
pumps have no mechanical seals; therefore, there are minimum risks due
to seal failure or pump liquid being exposed to the atmosphere. Some
manufacturers offer pumps up to 500 horsepower; however, the majority
of vapor tight pumps are offered below 10 horsepower. Some companies
manufacture the casing dimensional envelopes to be the same as the
B73.1.
The two popular drives are magnetic drive and canned motor pumps
(Figures 15-13 and 15-14). In both cases the normal limiting factors are
the size of particles that can be pumped through their mechanism. The
length of life of these types of designs depends on the type of bearings
that are used. Excessive wear or seizure of the bearing results in down-
time of the whole unit. Surveys of failures show that most are caused by
flashing of the bearing lubricant rather than to the presence of particles.
Bearings are sleeve journal and flat plate thrust or conical combination
for journal and thrust. Monitors indicating bearing wear can be supplied
to prevent catastrophic failures. The magnetic drive usually uses perma-
nent magnets and can operate at higher temperature limits before cooling
is required. The magnetic drive allows the use of a standard type motor to
drive the magnets.
The canned motor pump has the advantage of being one complete unit
for the pump and motor; thus, it is shorter than the magnetic drive. The
outside liner of a canned motor pump is reinforced by the stator of the
motor resulting in high allowable pressures. Maximum viscosity for this
type of pump is 150 centipoise. This value can be much less depending
on the size, torque, and speed of the equipment.
Initial capital expenditure of sealless pumps is higher than standard
horizontal pumps with mechanical seals; however, manufacturers of this
type of equipment suggest that this can be amortized in a short period
because mechanical seal maintenance cost will be eliminated.