Chemical Engineering July 2012 - 45

Solids Processing
Environmental Manager
Overpressure Protection: Consider
Low Temperature Effects in Design
Understanding the
inherent limitations of
current overpressureprotection
analyses is
key to developing a more
robust heuristic
Aubry Shackelford
Inglenook Engineering
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Brian Pack
BP North America
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I
n designing and sizing reliefdevice
and effluent-handling
systems, one commonly overlooked
aspect of the performance
is examining the potential for
low temperatures that can cause the
components of the system to reach
temperatures below their respective,
minimum-design metal temperatures
(MDMT), which may result in
brittle fracture with subsequent loss
of containment. This article points
out limitations of the typical overpressure-protection-analysis
philosophy,
discusses common sources of
low temperatures for further investigation,
and addresses possible design
remedies for MDMT concerns.
The primary objectives of a process
engineering evaluation of an effluent
handling system (such as a flare
system) include ensuring that operation
of the pressure relief devices discharging
into the collection system
(flare headers, for example) is not adversely
affected; and that the effluent
handling equipment are properly designed
to perform safely. The results of
an overpressure-protection design are
the primary input for this engineering
evaluation; however, there are several
potential gaps in the ability of these
data to identify situations in which
the MDMT may be exceeded.
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Mach
FIGURE 1. Temperature drop relative to stagnation as a result of lowing
Current-practice limitations
Common practices for pressure relief
and effluent handling are found
in numerous references [1-5]. The
processes for estimating a discharge
temperature and performing the outlet
pressure-drop calculations in the
pressure-relief-device discharge piping
are limited in their ability to accurately
predict flowing temperatures
for many situations.
First, the discharge calculations
are quite often only performed for the
controlling contingency for which the
pressure relief device was sized, which
does not necessarily represent the
most likely cause of overpressure or
the cause resulting in the lowest discharge
temperatures.
Second, the outlet pressure-drop
calculations for individual pressure
relief valves consider the outlet discharge
piping and potentially exclude
the remaining, downstream piping
system. This practice can result in a
temperature discontinuity between
the calculated discharge temperature
for the individual relief device and
that calculated for the same section
of piping considering the entire downstream
piping system using an effluent-handling
hydraulic model.
Third, the temperature estimates are
typically made for isothermal pressuredrop
equations and do not account for
effects like retrograde condensation.
Fourth, some simplifications of the
calculations that are used for the purposes
of estimating the outlet pressure
drop do not represent flashing
effects (for example, highly subcooled
flashing liquids are often choked at
the bubblepoint; therefore, the sizing
of the valve may assume the backpressure
is at the bubblepoint).
Finally, the temperature estimates
tend to be based on either relieving
temperatures or isenthalpic flashes
from relief conditions, which do not
account for kinetic energy effects.
These effects can be substantial if the
developed velocity in the outlet piping
is high and can be compounded when
there are multiple relief devices discharging
simultaneously into a collection
system, or when large diameter
differences exist between the tail-pipe
and the main effluent header.
Temperature drop. Figure 1 shows
the temperature drop from the stagnaCHEMICAL
ENGINEERING WWW.CHE.COM JULY 2012 45
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k=1.1
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Temperature drop, °F
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Chemical Engineering July 2012

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