Page 238 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

21 0 Applied Process Design for Chemical and Petrochemical Plants

ate often should be placed in service on a regular sched-
ule just to be certain they are in working order.
If solids are carried in the fluid, this can present a diffi-
cult problem if they are not properly flushed from the
pump on shutdown. Some spare or second pumps are
selected for 100 percent spare; others are selected so that
each of two pumps operate in parallel on 50 percent of the
flow, with each being capable of handling 67 to 75 percent
of total load if one pump should fall off the line. This then
only reduces production by about 25 percent for a short
period, and is acceptable in many situations. These pumps
are usually somewhat smaller than the full size spares.
When it is necessary to plan several pumps in parallel,
the pump manufacturer must be advised, and care must
be taken in arranging suction piping for the pumps, oth-
erwise each may not carry its share of the flow.
There are many flow conditions, and pumps should be
selected to operate as efficiently as possible over the
widest range of capacity.
If the flow is expected to vary during the system opera-
tion, the high and low GPM (and corresponding heads)
should be given to allow proper evaluation.

Fluid Conditions

The manufacturer must be told the conditions of the
liquid, percent suspended solids, physical properties, cor-
rosive nature and maximum and minimum temperature
ranges. For extremely hot liquids, special hot pumps must
be used, and temperature effects taken into account.
P.O. I: 1
System Conditions
Figure 3-61. Centrifugal pump specifications.
The manufacturer must know if the suction side of the
pump is associated with vacuum equipment, or is to lift
gestions for materials of construction for pump parts in the liquid. This can make a difference as to the type of
the services indicated. The effect of impurities, tempera- impeller suction opening he provides. If the system oper-
ture, analysis variations and many other properties make ates intermittently it should be noted. A piping diagram is
it important to obtain specific corrosion service data in often helpful in obtaining full benefit of the manufactur-
the specific fluid being pumped. Sometimes this is not er's special knowledge.
possible, and generalized corrosion tables and experience
of other users must be relied on as the best information Type of Pump
for the materials selection.
If there is a preference as to horizontal or vertical split
casing, it should be stated. Also the suction and discharge
Number of Pumping Units connections should be stated as to top or end, or special,
together with the preference as to flanged (rating) or
A single pump is the cheapest first-cost installation. screwed. Small pumps are commonly furnished with
However, if downtime has any value such as in lost pro- screwed connections unless otherwise specified.
duction, in hazards created in rest of process, etc., then a
stand-by duplicate unit should be considered. A spare or Type of Driver
stand-by can be installed adjacent to the operating unit,
and switched into service on very short notice, provided it Pumps are usually driven by electric motors, steam or
is properly maintained. Spare pumps which do not oper- gas turbine or gas (or gasoline) engines either direct or