Chemical Engineering September 2015 - 73

item
Pipe
Table 1. Turndown raTio of selecT equipmenT
Turndown ratio
Large, but depends on the definition of maximum and minimum flow
Storage containers
(Tank or vessels)
Centrifugal pump
Positive-displ. pump
Heat exchanger
Burner [1]
Very large; The maximum value is the total volume of the container, but the
minimum value could be dictated by a downstream component. For example,
a centrifugal pump may dictate a minimum volume to provide required NPSH
Typically: 3:1 to 5:1
Theoretically infinite
Small, depends on the type; for instance, less than 1.5:1
Depends on the type; for example:
Pressure jet type: ≈ 2:1
Twin fluid-atomizing type: >8:1
Table 2. uTiliTy surge conTainer To provide Td raTio
Surge container
residence time
Instrument air (IA)
Utility water (UW)
Utility steam (US)
Utility air (UA)
Cooling water (CW)
Cooling/heating glycol
item
Air receiver
Water tank
5-10 min. or higher depending on
whether it is connected to UA or not
Several hours
Utility steam cannot be stored for a long time without condensing; the options
for storing steam are the steam drum of a boiler, or if a conventional boiler is
not available, a vessel as an " external steam drum " could do the same task
No dedicated container; could " float " with IA
Cooling tower basin
Expansion drum
Depends on the size of the network
Depends on the size of the network
Table 3. Turndown raTio of selecT insTrumenTs
Turndown ratio
Flowmeter: orifice-type
Flowmeter: vortex-type
Flowmeter: Coriolis-type
Control valve
3:1 [2]
10:1 to 50:1 [2]
5:1 to 25:1 [2]
Depends on type and characteristics; generally 50:1, and less than 100:1
Table 4. arbiTrary values of flexibiliTy parameTers
Low flexibility
Equipment (TD ratio)
Instrument, control valves (rangeability)
< 1.2:1 to 2:1
≈ 4:1
by operations staff. During operation,
people expect the TD ratio to
answer the question in this scenario:
" My plant is running normally and
all parameters are normal. However,
occasionally, because of different
reasons (including shortage of
feed, reduced plant or unit capacity),
the flowrate falls. What is the
minimum value I can withstand
without compromising the quality of
the product? "
They basically interpret the TD ratio
so that the numerator is the " normal
level parameter " (and not the " high
level parameter " ). However, the difference
in the interpretation does not
generate a big difference in numerical
value of TD ratio, as the normal and
high level of parameters are often
not very far from each other. Due to
this potential confusion, the TD ratio
should be considered an approximate
parameter and not a precise
Medium flexibility
2:1 to 3:1
10:1 to 30:1
High flexibility
5:1 to 8:1
20:1 to 100:1
number. In general, the academic
definition of TD ratio generally uses
a high-to-low values set up, while in
the field, operators often define TD
ratio using normal-to-low values.
The TD ratio can be defined for
parameters other than flowrate, but
it generally refers to flowrate. One
reason for this is because flowrate
can be the most important parameter
of a plant, helping to define the
economy of the system. The other
reason is because the flowrate might
be influenced by constraints outside
of the plant (for instance, a lack
of stored feed), which the control
system cannot necessarily adjust
(thus making a reduction in flowrate
unavoidable).
While the TD ratio is not always a
requested parameter, and is often
not mentioned in project documents
for design purposes, operators are
usually looking for a TD ratio of least
ChemiCal engineering www.Chemengonline.Com september 2015
2:1 for a plant. The required TD
ratio could be as high as 3:1 or 4:1
for a plant.
Equipment flexibility
The TD ratio can also be determined
for a given piece of equipment, using
other values that are stated for the
component. For example, even when
a TD ratio is not explicitly stated for a
centrifugal pump, when the pump is
said to have a capacity of 100 m3/h
and a minimum flow of 30 m3/h, this
means that the centrifugal pump has
a TD ratio of 3:1.
The TD ratio of a reciprocating
pump could theoretically be defined
as infinite because it can work over a
very wide range of flows. However, in
practice, such a pump cannot handle
any flowrate that fails to fill the cylinder
of the pump in one stroke. Partial
filling of the cylinder may cause some
damage to mechanical components
of the pump over the long term. Thus
the minimum required flow is a function
of cylinder volume and stroke
speed of a specific pump.
The TD ratio for pipelines presents
a more complicated situation. With
piping systems, there are several different
ways to define the minimum
flow. For instance, it could be defined
as the minimum flow that does not
fall into the laminar flow regime. Or,
it could be considered as the minimum
flow that keeps a check valve
open (if a check valve is used).
For liquid flows in pipes, the minimum
flow is more commonly interpreted
as the minimum flow that
makes the pipe full, or the sealing
flowrate (that is, no partial flow), or a
flow threshold below which the fluid
will freeze in an outdoor pipe. If the
flow bears suspended solids, the
minimum flowrate could be defined
as that at which sedimentation of
suspended solids may occur.
Table 1 provides examples of
typical values and rules of thumb
regarding the TD ratio for various
types of process equipment. Note
that in Table 1, the TD ratio of storage
containers is relatively large. This
high TD helps to explain why large
containers are used for surge dampening
as part of a typical plant-wide
control system.
In some cases deciding on a required
TD ratio needs good judgment.
One example is chemicalinjection
packages. The TD ratio
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