Chemical Engineering November 2012 - 56

Engineering Practice
rates are chosen in a way that ensures
the injection system covers all of the
possible operating scenaros.
Qmax
is considered to be 1.5 to 2
times the normal flowrate. From the
other side, chemical-injection systems
are one of the few process systems
that are expected to be fully accurate
even during the startup. Because of
this, Qmin is usually taken as 25% of
the normal flowrate.
When there is more than one usage
for a given chemical in a plant, there is
always the chance of offloading to the
wrong tank, or connecting the wrong
chemical tote tank to the injection
system. This can be prevented by the
use of suitable signage or by using
purposefully incompatible hardware
that can prevent any unintended
chemical flow.
3. Correct preparation. In most
cases, chemicals are transported to
the process plant in a concentrated solution
or powder form. Therefore, the
most popular chemical-preparation
system involves a dilution system.
Two issues should be considered in
this regard: the first is whether the
viscosity of a chemical is suitable for
the selected pump, and the second is
whether the diluted chemical has experienced
enough aging time.
The issue of sufficient aging, for
both dilution and solution formation,
is generally applicable to polymerbased
chemicals. Polymers (with their
chain of atoms) are usually " cramped "
in concentrated form. To be most effective,
a chemical polymer must be in its
" un-cramped " molecular form.
Thus, a specific amount of time
should be given for the diluted polymer
to settle slowly into its un-cramped
form. Severe agitation cannot accelerate
this process, and may be detrimental
for polymers, especially those with
long chains (such as flocculants). This
goes against the general practice that
usually involves severe mixing for
better chemical mixing. Therefore, the
mixing of polymeric chemicals should
be done more gently, with the impeller
tip speed limited to avoid polymer
chain breakage.
When there is a need for two-stage
dilution (that is, aging, followed by dilution),
the system could be designed
as a two-container system, using
batch operations or a vessel-in-vessel
system with continuous overflow.
4. Choosing the appropriate injection
system. The injection pump capacity
is usually calculated by multiplying
Qmax by the maximum dosage.
The idea is to choose a system that can
inject the chemical at a specific flowrate
with sufficient accuracy across
a wide range of potential destination
flowrates. This means that the injection
system should have enough precision
and good accuracy. A good injecting
pump can satisfy the precision
requirements, while an appropriate
control system guarantees quick pacing
of the destination flow, which dictates
the accuracy of the system.
Usually, injection systems possess a
capacity of less than 5 m3/h (but sometimes
up to 10 m3/h). The required accuracy
is usually better than 1-2%.
The only process parameter that a
chemical injection system needs to
monitor and follow accurately is the
flowrate of the injected stream. To satisfy
the required precision, a positivedisplacement
pump (PD) is often used.
PD pumps with high accuracy are
called metering or dosing pumps. The
most popular types of dosing pumps
are piston, plunger and diaphragm
pumps. A gravity-flow pump or centrifugal
pump usually cannot meet the
required accuracy for chemical-injection
systems.
When setting up injection piping,
a dedicated pipe should be used for
each injection pump. Using a shared
injection pump for more than one injection
point is not a good practice, as
it may result in variable flowrates of
different chemical streams to different
destinations. This goes against
the main objective of an injection system,
which is to deliver specific (and
accurate) flowrates at the desired
injection point.
When using PD pumps, it is critical
to use a pressure safety valve (PSV) at
the discharge side of the pump. Usually
a back-pressure regulator is placed at
the discharge side of the pump to ensure
that the injection system delivers
an accurate specified flowrate in all
cases, even when there is fluctuation
in destination pressure.
If the system does not have a backpressure
regulator, the occurrence
50 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2012
of instantaneous, very-low pressure
at the injection point could cause an
unknown flowrate at the destination.
The use of a back-pressure regulator
is especially critical if the discharge
pipe has a backward U shape in its
route to the destination point (for example,
if the injection pipe goes over
the pipe rack).
To satisfy the required precision of
the system, the injection pump should
be able to accommodate almost every
flowrate below its capacity. For example,
if the design capacity of an
injection pump is 1,000 L/h, it can
be expected that this pump can provide
an accurate flowrate from 1,000
L/h down to possibly 10 L/h. This can
be done by adding a variable-speed
device (VSD) to the electric motor of
the pump or stroke-adjustment system.
The stroke-adjustment system
can be operated either manually or
via a servo-mechanism. Variablefrequency
devices (VFDs) are one
popular type of VSD.
To satisfy the accuracy requirements
of the chemical-injection system, users
need to make sure that the injection
system follows the process flowrate
over its entire turndown ratio very
quickly. To ensure such swift responsiveness,
the injection system needs a
good control system. A combined feedback/feedforward
control system is the
ideal choice for complicated cases such
as acid-injection situations. The feedforward
portion of the control system
is a ratio control system that forces the
chemical flowrate to follow the destination
flowrate. The feedback portion
is a control loop that is governed by a
property in the resultant stream. The
control system can order the VFD or
automatic stroke-adjustment mechanism
(or both in a split-range control
system).
For less complicated cases, a simple
feedback control would be sufficient.
For the purpose of pump sizing, the
decision on the capacity of the dosing
pump is critical. To make sure that
the dosing pump can handle all the
situations, Qmax
times maximum
dosage is typically used to define the
most appropriate capacity for the
dosing pump.
5. Determining the right dosage.
The reported dosage of a specific chem
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Chemical Engineering November 2012

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