Chemical Engineering September 2015 - 75

Utility consumer branch
Sub-header
Utility
generation unit
Main header
Low turndown ratio
High turndown ratio
Figure 3. Shown here is a map of turndown ratio for a typical utility network.The pipes closer to the utility
generation system (main header) need less turndown ratio compared to sub-headers and branches
generate centrifugal force, so any
reduced flow will reduce the centrifugal
force, which may reduce
the effectiveness of the system
* Equipment containing loose, porous
media may show a lower TD
ratio in liquid service, and the TD
ratio may be lower when the porous
media is comprised of larger
solid particle sizes. Examples
include sand filtration systems,
catalyst contactors and related
systems
* Despite a common misconception,
perforated-pipe flow distributors
do not necessarily have limited
TD ratios [3]
As noted, the utility network should
have a relatively large TD ratio. Fortunately,
utility networks consist mainly
of pipes in different sizes, which have
inherently large TD ratios. If control
valves are needed on the network,
their lower TD ratios may generate
bottlenecks. In such situations, it
may be necessary to install parallel
control valves with split control, because
of the required large TD ratio.
Using parallel equipment. Instead
of using a component with a capacity
of 100 m3/h, this technique is essential
to use an arrangement that
employs two parallel components,
each with the capacity of 50 m3/h.
By doing so, a TD ratio of at least
2:1 can often be provided. It should
be noted that the equipment by itself
may have some inherent TD-ratio capability,
which may have to be added
to the provided 2:1 TD ratio.
For example, instead of using one
shell-and-tube heat exchanger with
the capacity of 100 m3/h, three heat
exchangers - each with the capacity
of 33 m3/h -can be used
to achieve a TD ratio of at least 3:1.
The TD ratio may actually be higher
because each shell-and-tube heat
exchanger has an inherent TD ratio
too, even though it is very small. This
technique has additional benefits.
The parallel arrangement provides
higher availability for the system,
because the failure of two or three
parallel equipment components is
less likely than the potential for failure
when the system relies on a single
equipment component.
Using two control valves in parallel
in a single control loop (through a
" split range " control) is also another
example of this technique in the area
of instrumentation.
However, there are some disadvantages
associated with this technique.
In particular, capital cost and
operating cost considerations may
rule against it.
Providing recirculation pipe. Implementing
a recirculation pipe from
the equipment outlet to its inlet is
a widely used method to increase
the TD ratio of the system. In many
cases, a pump and definitely a control
system, are needed to implement
this technique. As long as you
can afford an extra pump and control
system on the recirculation pipe, this
technique can be used. The recirculation
pipe needs a control system,
otherwise all flow goes through the
recirculation pipe back to the inlet of
the unit of interest (Figure 4).
One example of this technique is
using a minimum-flow line for a centrifugal
pump. A centrifugal pump with
a capacity of 100 m3/h and a minimum-flow
line of 30 m3/h (thus, with a
TD ratio of 1:3) can be equipped with
a minimum-flow line with an appropriate
control system to increase its TD
ratio. If the minimum-flow line and the
ChemiCal engineering www.Chemengonline.Com september 2015
control system are designed to handle
a maximum flowrate of 30 m3/h, it
means the TD ratio of the pump can
theoretically be increased to infinite,
by zeroing the minimum flow.
Another example is a vertical
falling-film evaporator. This type of
evaporator has a vertical tube bundle
that is similar to the ones found in a
shell-and-tube heat exchanger. The
tube-side flow is two-phase flow. The
liquid flows down by gravity, and the
vapor (of the same liquid) is pushed
down by liquid drag. The flow inside
the tubes is an " annular regime, "
meaning the liquid covers the internal
perimeter of tubes and the vapor
is in the center of the tubes.
In the case of low flow, there is a
chance of " dry patches " forming on
the tube's internal surface. Because
of this, vertical, falling-film evaporators
are typically equipped with recirculation
pipes to provide a minimum
practical TD ratio (Figure 5).
However, this method cannot be
applied for all equipment. For example
it is not a good technique to
increase the TD ratio of a furnace or
fired heater, because recirculation
of fluid around a furnace may increase
the furnace coil temperature
and cause burning out if the firing
system doesn't have sufficient TD
ratio. Table 4 provides some rules of
thumb to gauge the flexibiliy of different
elements of a process plant.
Resistance against surge
While TD ratio refers to the static behavior
of a plant, there are two additional
parameters (resistance against
surge, and speed of recovery from
upset) that refer to its dynamic
behavior. However, there is less
emphasis on dynamic theories,
and only practical aspects of
dynamic behavior.
A process upset could result from
a surge. Surge can arbitrarily be defined
as the deviation of a parameter
(such as flowrate) beyond its normal
level. The final value of the parameter
may or may not be in a band
between high level and low level and
the change often occurs quickly.
When a parameter moves quickly,
an upset could happen. The surge/
upset could be defined for each parameter
including flowrate, temperature,
pressure and even composition.
A surge in the composition is often
called a slug. Level surge is generally
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Chemical Engineering September 2015

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