Chemical Engineering November 2010 - 48

Cover Story
100
∆6%
50
4
12
Control signal, mA
20
∆1 mA
Figure 7. Rotary PD pump flow is proportional
to speed, which is proportional
to control signal
velop infinite pressure until something
in the system releases it - normally a
pressure relief valve that is integral to
the pump (illustrated at the far right
side of Figure 6). The PD pump-performance
curve illustrated in Figure 8
shows that a 100-psi change in system
pressure results in only a 10-gal/min
change in flowrate (whereas a 23-ft or
10-psi change in system head caused
a 150-gal/min change in flow on the
centrifugal curve, Figure 2).
A rotary PD pump's performance
curve is theoretically vertical, but the
minimal slope shown (Figure 8) is due
to slip through the pump, between the
internal clearances of the rotating elements,
from the discharge (high pressure)
side to the inlet (low pressure)
side. As viscosity increases, the slip
becomes negligible and the slope approaches
that theoretical vertical line.
Many rotary PD pump systems are
operated without a flowmeter at all,
since speed is proportional to flow, especially
at higher viscosities.
Figure 5 illustrates why a flowmeter
is needed if the pump is to be operated
at very low flowrates, regardless
of viscosity. The curve (solid line)
is cut off at the far left side, because
slip becomes a larger percentage of
the flowrate at very slow speeds. The
pump can still be controlled to the far
left side of the curve (dashed line) beyond
the cutoff point with the use of
a flowmeter, since the controller will
adjust the speed as needed to compensate
for slip, to maintain the desired
flowrate all the way down to 1%
of rated capacity, providing true 100:1
turndown capability.
240
200
160
120
80
40
Gear pump, QS4124A,
on 20 cSt, S.G.=
1 liquid, 300 gal/min
62 psi (144 ft of head)
at 340 rpm, 13.7 BHP
100
75
∆100
psi
50
25
Figure 8. This
rotary PD pumpperformance
curve
shows that
a 100-psi change
in system pressure
results in a
10-gal/min change
in flowrate
100
200
∆10 gal/min 400
Flow, gal/min
Process control comparison
Three things should be evident. First
is that process control methods for
centrifugal and rotary PD pumps
are completely different, and the
process designer must understand
those differences. Second is that the
allowable control range for rotary PD
pumps comprises the entire length of
the curve, while centrifugal pumps'
ranges should be limited to an area
near the BEP to maintain reliability.
And third is that rotary PD pump systems
are inherently easier to control
because capacity is directly proportional
to speed, regardless of changes
in system head.
The general public would suppose
that process plants have computerized
recipes that automatically mix
this liquid with that one, and the end
product is consistent every time. In
fact, there is incredible variation in
both chemical and biological processes.
Feedstocks change daily based on cost
and availability. Product specifications
change depending on customer
requirements, and processes are continually
modified as product demands
change from day to day. The systems
employed must be capable of adapting
to all of these variations to ensure
product quality and minimize waste.
The process designer should always
consider the best type of pump for the
process. This review of controllability
suggests that centrifugal pumps
are better suited to simple batch processes
and tank transfer applications
with limited variability than to continuous
processes, where two liquid
streams must be blended in constant
32 ChemiCal engineering www.Che.Com november 2010
proportions within restrictive control
limits. Rotary PD pumps, in contrast,
are equally suited to both batch and
continuous processes, as well as those
with large variation due to changing
feedstocks and recipes.
Pump selection for scaleup
Nowhere is a broad range of control
more important than in the development
of new processes, whether chemical
or biological, for fuels, pharmaceuticals
or in other chemical process
industries (CPI). New processes are
generally developed in a laboratory,
scaled up to a pilot plant to test the
commercial viability, then scaled further,
either to " demonstration scale "
or to full commercial scale production.
There are always unforeseen problems
and variables, because few technologies
are completely scalable directly
from laboratory to commercial process.
Selecting scalable pumps helps to
eliminate those unforeseen variables,
simplifies calculations and allows for
testing extreme process variations.
To effectively manage risk, each
stage of process development from
laboratory to pilot to commercial
scale should strive to use process
equipment that is both scalable (that
is, nearly identical in operation from
very small to very large scale), and
versatile (capable of handling a wide
range of materials, flowrates, pressures
and temperatures).
Some pumps are great for commercial
scale work, but don't scale
down very well to laboratory or pilot
processes. Centrifugal pumps offer
flowrates to thousands of gallons per
Pump speed, %
Head, ft
Pressure. psi
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Chemical Engineering November 2010

Table of Contents for the Digital Edition of Chemical Engineering November 2010
