Chemical Engineering August 2018 - 19

Parker Hannifin
formed on the piping
leading into and out
of the device. Regulations
specify that the
inlet pressure drop to a
safety-relief valve must
be less than 3% of the
set pressure and, for
conventional valves,
the
outlet
pressure
FIGURE 2. Parker Hannifin's Cartridge Safety Relief Valves, designed
to offer the highest level of protection while maintaining easy serviceability,
allow for a simple and easy upgrade path to the new cartridgebased
design. The cartridge-based design allows for replacement
while the housing remains in the system
* Desired operating pressure. Will
a spring-loaded valve work or will
a pilot-operated relief valve be
required?
* Environmental considerations. Is
industry standard seat leakage acceptable?
*
Inlet and discharge piping design.
Does the design of the inlet piping
create excessive line loss that can
affect the operation of the valve?
Does the discharge piping system
create excessive backpressure
during a relief event that will affect
operation of the valve and de-rate
its relief capacity?
* Required capacity. Does the required
capacity necessitate more
than one safety-relief device to
safely protect the process?
Ken Kurko, director, process
safety services with Fauske & Associates
(Burr Ridge, Ill.; www.
fauske.com), continues to explain
the importance of these considerations.
" The first step for our clients
is deciding what overpressure scenario
they want to protect the vessel
against in order to determine
how large the device needs to be,
as well as how much flow area is required
to vent the vapor, gas or liquid
that is going to be relieved from
the vessel. Once the proper size of
the device is selected, there should
be pressure drop evaluations perdrop
from the valve
must be 10% less than
the set pressure. "
Other considerations
when dealing with the
CPI, says Gabe Wood,
manager, thermal hazards
testing and consulting
with Fauske, include
what material is
being vented to the environment,
because the
venting of many materials,
as well as the amount vented,
must be reported to environmental
agencies. " Often, processors want
to use valves because they reclose,
which means less material is vented,
but in potential chemical runaway
situations, a rupture disk may be
the better choice because it opens
to remove as much reactive material
as is possible from a process vessel.
For reasons such as this, a process
engineer must balance environmental
issues with safety decisions. "
Also, runaway reactions are more
likely to create two-phase flow,
meaning a liquid and a gas may
flow through the relief device at the
same time, notes Benjamin Doup,
senior nuclear and chemical engineer
with Fauske. " In these potential
situations, the critical velocity of the
two-phase flow may necessitate an
increase in the size of the relief device
to prevent the vessel from being
over pressurized. "
Chemical applications are among
the most demanding relief scenarios
because they often include exothermic
or runaway reactions, so it is
important to select the proper size
and design. Fortunately, there are
multiple industry resources to assist
in the design and selection of safety
relief systems. The most widely used
resources, according to Owens, are
American Petroleum Institute's (API;
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Chemical Engineering August 2018

Table of Contents for the Digital Edition of Chemical Engineering August 2018
