Chemical Engineering April 2013 - 58

GE/Consolidated and Allied Valve
Engineering Practice
[2]. The mechanical equilibrium assumption
is valid for flashing flows
[2]. The thermodynamic equilibrium
assumption is valid for nozzles with
a length longer than 10 cm [4]. Most
standard relief valves have a nozzle
that is slightly longer than this [11].
Pros and cons
Advantages of this Method. The Direct
Integration Method is not bound
by the same constraints as many
other models or methods. Using this
approach, the same method can be
used whether the flow is choked or not
choked, flashing or not flashing, single
or two-phase, close or far from the
critical point, subcooled or supercritical.
The only assumptions required
for the Direct Integration Method are
that flow through the relief valve is
isentropic, homogeneous, and in thermodynamic
and mechanical equilibrium,
although it is possible to adjust
the method to account for mechanical
non-equilibrium or slip [6].
Although most other methods give
unsatisfactory results near the thermodynamic
critical point, the Direct Integration
Method continues to function
properly [12]. Additionally, many other
concerns that come up when using
relief-valve model equations, such as
determining the heat capacity ratio or
isentropic expansion coefficients, are
no longer relevant since they are inherent
to the simulator itself [3].
Downsides to this Method. The Direct
Integration Method can produce
References
1. American Petroleum Inst., " Sizing, Selection,
and Installation of Pressure-Relieving Devices
in Refineries, " ANSI/API RP 520, 8th
Ed., Part 1: Sizing and Selection, Washington,
D.C., Dec. 2008.
2. Darby, R., Size safety-relief valves for any
conditions, Chem. Eng., pp. 42-50, Sept.
2005.
3. Kim, J.S., H J. Dunsheath and N.R. Singh,
Proper relief-valve sizing requires equation
mastery, Hydrocarbon Proc., pp. 77-80, Dec.
2011.
4. Huff, J., Flow through emergency relief devices
and reaction forces, J. Loss Prev. Process
Ind., Vol. 3, pp. 43-49, 1990.
5. Bird, R.B., and others, " Transport Phenomena, "
pp. 481, John Wiley, New York, 1960.
6. Darby, R., On two-phase frozen and flashing
flows in safety relief valves: Recommended
calculation method and the proper use of the
discharge coefficient, J. Loss Prev. Process
Ind., Vol. 17, pp. 255-259, 2004.
FIGURE 2. The Direct Integration
Method is not only easy to use, but
provides more accurate results when
sizing pressure relief valves, since this
approach does not rely on a potentially
sensitive equation of state model
overly conservative results in a couple
of circumstances, which can lead
to under-prediction of the mass flux
and selection of an oversized valve.
This appears to be an issue only when
the fluid is in two-phase frozen flow
(no flashing), or the relief valve has
a short nozzle and there is flashing
flow [2].
This potential limitation can be
compensated for in both situations by
applying a slip factor. However, at this
time, there is insufficient literature
available to provide accurate guidance
on the value of a slip factor. The accuracy
of the calculation is also limited
by the accuracy of the physical property
data in the simulator.
7. AspenTech, Aspen HYSYS Customization
Guide, Version 7.2, July 2010.
8. Leung, J.C., The Omega Method for Discharge
Rate Evaluation, in " International
Symposium on Runaway Reactions and
Pressure Relief Design, " G.A. Melhem and
H.G. Fisher, Eds., pp. 367-393, AIChE., New
York, N.Y., 1995.
9. Center for Chemical Process Safety, " Guidelines
for Pressure Relief and Effluent Handling
Systems, " AIChE, New York, N.Y., 1998.
10. Diener, R., and J. Schmidt, Sizing of throttling
device for gas/liquid two-phase flow,
Part 1: Safety valves, Process Safety Prog.,
Vol. 23, No. 4, pp. 335-344, 2004.
11. Fisher, H.G., and others, " Emergency Relief
System Design Using DIERS Technology -
The Design Institute for Emergency Relief
Systems (DIERS) Project Manual, " pp. 91,
Wiley-AIChE, 1992.
12. Schmidt, J., and S. Egan, Case studies of
sizing pressure relief valves for two-phase
flow, Chem. Eng. Technol., Vol. 32, No. 2, pp.
263-272, 2009.
56 CHEMICAL ENGINEERING WWW.CHE.COM APRIL 2013
Closing remarks
Using a spreadsheet to import data
from a simulator and to calculate the
summation over a range of pressures
is extremely easy and straightforward.
One simply needs to simulate
the relieving stream and perform
a flash operation at each pressure
and capture the required data. Not
only is the Numerical Integration
Method much simpler than the alternatives
for two-phase flow, but it
is also more accurate, since it does
not rely on a potentially sensitive
equation-of-state model. There is no
need for a model because physical
property data are generated for each
data point directly from simulation.
In addition, the Numerical Integration
Method can be used for singlephase
flow and choked or not-choked
conditions. This versatility and ease
of calculation makes Numerical Integration
the obvious choice for any
relief valve calculation where physical
property data are available in a
process simulator.
■
Edited by Suzanne Shelley
Authors
Mark Siegal (Email: msiegal2
@gmail.com), was, until recently,
a process engineer at
Valdes Engineering Company
where he was responsible for
process design, process modeling,
and emergency relief
system design. He holds a
B.S.Ch.E. from the University
of Illinois at Urbana-Champaign.
Silvan
Larson is a principal
process engineer at Valdes
Engineering Company (100
W 22nd St., Suite 185, Lombard,
IL 60148; Phone: 630792-1886;
Email: slarson@
valdeseng.com),where he is
responsible for process design
and emergency-relief-system
design. He has more than 30
years of experience in manufacturing
and process design
engineering in the chemicals and petroleum refining
industries. He holds a B.S.Ch.E. from University
of WisconsinMadison and is a registered
professional engineer in Ill.
William A. Freivald is the
manager of process engineering
at Valdes Engineering
Company (Phone: 630-7921886;
Email: wfreivald@
valdeseng.com). He has more
than 17 years of international
process design experience in
specialty chemicals, gas processing
and refining. He holds
a B.S.Ch.E. from Northwestern
University and is a registered
professional engineer in Illinois.
http://www.gmail.com http://WWW.CHE.COM
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