Chemical Engineering September 2015 - 77

the impact of increasing the inlet stream.
The first line of defense against a surge is provided by
the control system or control valves. However, control
valves alone cannot totally eliminate a surge, but will
only stop a surge from impacting a downstream system.
Ultimately, the surge needs to be handled, but by
other methods.
There are basically two surge-management methods
that can be implemented for each piece of equipment
or group of equipment in a plant:
* Boxing-in a surge in a specific equipment component
or series of equipmen
* Transferring the surge to an external or auxiliary
system
Understanding the applicability of each of these techniques
requires some knowledge about the inherent
dynamic characteristics of the systems from a process
control viewpoint. The three dynamic features of each
equipment or unit are resistance, capacitance and inertia
(dead time) [4]. A brief qualitative explanation of
these three features is presented next.
If a system is more dominantly a " resistance " type,
this system will be able to prevent the surge from transferring
to downstream equipment. A piece of pipe is
one example of a resistance-type element. A pipe
could inherently stop the surge if it is narrow enough.
However, because a pipe's main function is to transfer
fluid, the designer generally sizes the pipe based on
its duty (transferring fluid) and then, if needed, a control
valve is placed on the pipe to stop a potential surge.
The capability of a system to dampen the surge depends
on the " capacitance characteristics " of the system.
The higher the capacitance characteristic, the
more it is able to dampen a surge.
Here, a capacitance-type element refers to whatever
element that can be used to temporarily store excess
mass (such as liquid volume or gas pressure) or energy
(such as thermal or chemical energy).
For instance, large-volume equipment generally have
a higher capacitance feature. Implementing a surge
tank, equalization tank, surge drum (or even pond) is
one means of providing a system with sufficient capacity
to dampen the surge.
Another example of using a high-capacitance system
is when transferring a surge to heat-exchange media.
Utility heat exchangers use streams such as cooling
water, steam, and other media, to transfer the heat
to or from process streams. These utility streams are
also able to absorb a temperature surge in the system.
The capacitance feature of a utility network can be provided
in part by pipes in the network (the pipes function
mainly as resistance elements but they have some
capacitance too), and also their surge tank, as
discussed above.
A system is called robust against upset when it can
tolerate a large surge (as defined for each process parameter)
and no upset occurs, thereby allowing the
process to proceed smoothly.
If an upset cannot be tolerated, one solution is to
implement a rate-of-change control loop in the system.
The following llist provides some general rules of thumb
on the capability of a system to handle surges:
1.Generally speaking, equipment with larger volume
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Chemical Engineering September 2015

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