Page 231 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Pumping of Liquids 201

brake horsepower at the point of maximum efficiency for This condition would require a brake horsepower from
any particular impeller diameter [7]. the pump curve between 7.5 and 10, that is, about 9.25
BHP for the pump's input shaft (for water calculates at
BHP (max.) = 1.18 (BHP at max. efficiency point) (3-19) 9.03 BHP), estimating the spread between 7.5 and l 0.
Thus a 10 hp (next standard size motor) would be
Unless specifically identified otherwise, the BHP values required, and this would satisfy the original condition and
read from a manufacturers performance curve represent the the second condition for water. It would still be satisfacto-
power only for handling a fluid of viscosity about the same ry for any fluid with a specific gravity < I.O, but if pump-
as water and a specific gravity the same as water, i.e., SpGr = ing a liquid of 1.28 SpGr ( ethyl chloride, for example),
1.0. To obtain actual horsepower for liquids of specific then (1), the original BHP would need to be 1.28(5.75) =
gravity other than 1.0, the curve values must be multiplied 7.36 BHP, and (2), the second condition would require
by the gravity referenced to water. Viscosity corrections 1.28(9.25) = 11.84 BHP (calculates 11.56). Whereas, a 10-
are discussed in another section. Good design must allow hp motor would be non-overloading for the water pump-
for variations in these physical properties. ing case, it would require a 15-hp (next standard above a
10 hp) motor direct drive to satisfy the ethyl chloride case
Driver Horsepoioer under the 160 GPM condition.
If you do not select a non-overloading motor, and vari-
The driver horsepower must be greater than the calcu- ations in head and/ or flow occur, the motor could over-
lated ( or value read from curves) input BHP to the shaft heat and stop operating. Study the pump-capacity curve
of the pump. The mechanical losses in the coupling, V- shape to recognize the possible variations.
belt, gear-box, or other drive plus the losses in the driver
must be accounted for in order that the driver rated Important note: Any specific pump impeller operating in
power output will be sufficient Lo handle the pump. a physical (mechanical) system will only perform along its
Best practice suggests the application of a non-over- operating characteristic curve. If there is a change in the
loading driver to the pump. Thus a motor rated equal to system flow characteristics (rate or friction resistance or
or greater than the maximum required BHP of the pump, pressure head), the performance will be defined by the
assuming no other power losses, would be non-overload- new conditions and the pump performance will "slide"
ing over the entire pumping range of the impeller. It is along its fixed curve. Thus, the designer cannot arbitrari-
important to examine the pump characteristic curve and ly pick a point and expect the pump to 'Jump" to that
follow the changes in power requirements before select- point. Refer to Figure 3-36A. Using a 6-inch impeller
ing a driver. curve, for example, the designer cannot make this pump
For example, referring to Figure 3-36A, if your pump operate at a point of 100 GPM and 150 feet head. This
were selected with a 6-inch diameter impeller for a rated would require about a 6X-inch diameter impeller. The 6-
normal pumping of 100 GPivl, the pump would put out inch curve will only put out 138 feet (approx.) at the
about 138 feet of head of any fluid ( neglecting viscosity intersection of 100 GPM and the 6-inch curve.
effects for the moment). The intersection of the 100-GPM A driver selected to just handle the power require-
vertical line with the 6-inch performance curve would ments of the design point ( other than maximum) is usu-
indicate that 5.75 brake horsepower (hp) would be ally a poor approach to economy. Of course, there are
required/or water (between 5 hp and 7.5 hp). Therefore, applications where the control system takes care of the
to be non-overloading (that is, the motor driver will not possibilities of power overload.
overheat or lose power) at this condition would require a
7.5 horsepower motor (if no other losses occur between
driver and pump), because there is no standard motor for Affinity Laws
direct connected service between the standard 5 and 7.5 The affinity laws relate the performance of a known
hp. Now, if you know or project that you may need at pump along its characteristic curve to a new performance
some time to pump 160 GPM of any fluid with this pump curve when the speed is changed. This would represent
at 160 feet head, then (l) this pump could not be used the same "family" of pump curves. As an example, see Fig-
because it will not physically take an impeller larger than ures 3-36A, B, and C.
6.5-inch-diameter. However, recognizing this, (2) if you
change the external physical piping, valves, etc., and l. For change in speed with a geometrically similar
reduce the head Lo fit the 6.5-inch impeller curve, at 160 family of fixed impeller design, diameter and efficiency,
GP�·I, you could handle 152 feet head ( estimated from the following conditions and characteristics change simul-
the curve for a 6.5-inch impeller). taneously [25]: