OTC-31337-MS
Safer Design by Tube Rupture Analysis
Copyright 2022, Offshore Technology Conference DOI 10.4043/31337-MS
This paper was prepared for presentation at the Offshore Technology Conference Asia held in Kuala Lumpur, Malaysia, 22 - 25 March 2022.
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Abstract
When a tube ruptures in a shell and tube heat exchanger, the effect of liquid hammering may induce very
high transient pressure on shell side due to the leaked mass from tube side travelling to shell side. This
article describes a novel technical approach to adequately translate the volume displacement effect by the
leaked mass from tube side onto the shell side holdup volume in the unit. The transient pressure from the
liquid hammering effect is then accurately predicted by a first principle simulator, and proper mitigation
measures may be identified to meet safety requirement while minimizing capital cost.
While assuming tube side pressure at tube sheet location remains constant, the mass flow rate profile
through the ruptured tube as function of downstream (shell side) local pressure is determined according
to industry standards and/or project standards. This profile is then transformed to volumetric flow rate
profile displacing shell side hold up volume as function of time in milliseconds time scale. The resulting
volumetric profile is then applied to a first principle simulator to predict the transient pressure as a result
of liquid hammering effect. The mitigation measure, if any, may be at the same time tested and refined by
the simulator.
The constraints imposed by the project are iteratively evaluated, and adjusted if necessary, to achieve the
best reconciliation among factors of capital cost, safety requirement and project schedule etc.
In this article, a compressor discharge after cooler of double shells, with one stacked on top of another,
is used for the discussion. Furthermore, the scope of the model extends to include the surrounding piping,
and include any considerable lead line length to the relief device. The details of the exchanger geometry,
including internal components such as the baffles, bundle type, nozzle etc. are modeled with adequate
resolution. The pressure wave propagation along the path of shell side flow in milliseconds time scale are
simulated and the localized peak pressures are reported.
The high peak pressure necessitates a mitigation measure to be implemented, while maintaining the
proposed shell side design pressure to stay for this particular unit. Note that this type of study, for safety
concerns, it could result in elevated shell side design pressure, even after considering mitigation measure,
leading to major changes to associated supply and return piping, resulting in cost and schedule delays.
The technical approach illustrated in this article describes the work flow to transform the mapping of
mass flow rate as a function of pressure to volumetric flow rate as a function of time in milliseconds time
scale, a technique considered as the first time to be introduced into the practice. The approach increases the
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Makarand Kulkarni and Tongyuan Song, McDermott International, Ltd
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fidelity of the study greatly, resulting in reduced capital cost as much as possible, while largely mitigating
safety concerns.
The approach also affords us to test multiple configurations of pipe size, pipe routing, relief device
response, and shell layouts iteratively in a relatively short period of time to optimize the design.
Introduction
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Pressure relief devices are often mandatory on shell and tube heat exchangers, commonly seen in process
industries [1]. For a tube rupture event in a shell and tube heat exchanger unit, a dynamic liquid hammering
transient analysis may be required according to codes [2]. When the failure takes place, high pressure tubeside fluid rushes into the low pressure shell-side, increasing the shell pressure. If a pressure relief device is in
place and its set point reached, it will open and vent shell-side fluid to the flare, therefore reducing the peak
pressure exerted on the shell. In an offshore setting, both plot space and equipment weight are confined.
Liquid hammering transient study is more critical in the effort to select the most economical approach [3].
Reference [4] provides a comprehensive review of the background and past efforts on the subject of tube
rupture study in a shell and tube heat exchanger.
The criteria of whether a tube rupture transient study is required or not may vary from project to project,
from one company to another and from one geographical region to another, although all of them may be
traced back to some industry specific standard documents, which this paper does not discuss in detail.
Similarly, for the notion of maximum allowed pressure for tube rupture purpose, the definition varies from
project to project and from one company to another. This paper does not discuss in detail but only assumes
there is a predefined maximum allowed pressure for tube rupture.
Assume the project determines a tube rupture transient study is required for a shell and tube heat
exchanger, the first subject is to qualify the rupture rate as a function of operating parameters of both sides.
The ultimate source of equations to use for calculating the rupture rates are mostly traced back to standard
documents like API 521. During actual practice, project teams often customize the calculation formula
to better fit in the specific project requirements. This paper therefore focuses only on the most important
parameters to consider when determining rupture rate, the pressures.
For most shell and tube exchangers, it is a good conservative assumption that the tube side pressure
holds unchanged after a tube rupture failure, thus the credit of a reduced rupture rate due to reduced tube
side pressure is not usually taken - Care should be taken to avoid over-design if this is not true. This paper
discusses the case where the tube side pressure can be considered constant.
The heat and material balances cases reflect the plant capacity. Projects often have multiple heat and
material balance cases to consider, typically with one being the design case and the rest rating cases. It
is a project requirement to determine one governing case, usually the most conservative, for tube rupture
studies. This requirement is often traced back to commercial contract terms.
For a given tube side fluid thermodynamic properties, there might be a different set of equations to
calculate rupture rate, depending on whether the tube side fluid is in vapor phase, flashing liquid, or sub
cooled liquid etc.
For actual project practice, these set of equations determining rupture rates are often implemented in
Excel spreadsheets, and/or in a commercial flowsheet simulator like Aspen HYSYS or UniSim. Note, that
if using a commercial simulator, the fidelity of the simulation will have to be validated against the source
equations. The benefits of using a commercial simulator is that multiple rupture rates as a function of
changing shell side pressure may be easily acquired and refreshed during iteration of the project execution.
After volumetric rupture rate profile against shell side pressure is acquired, it will be imposed to the
liquid hammering transient model, constructed in a commercial liquid hammering simulation software tool.
There are a few mature enough liquid hammering software tools on the market, this paper does not
go into details of evaluating them. As a guideline to select, the tool should be based upon first principle
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equations of mass and momentum balance, that is, based upon the Joukowsky equation for instantaneous
liquid hammering and otherwise rigorous mass and momentum balance simulation to account for pressure
peaks and wave propagation.
It should be noted that as the procedure of imposing volumetric rate profile onto the transient model
implies, the temperature effect of mixing tube side fluid with shell side fluid is ignored. On the other hand,
the volume expanding effect of tube side fluid entering shell side space, however, is rigorously accounted
for, with the level of detail considering local shell pressure affecting actual volumetric rupture rate at shell
side local pressure. Due to the nature of rapid pressure surge phenomenon, the time window to observe
pressure profile during tube rupture is usually in milliseconds, not usually exceeding a second, the ignorance
of temperature effect is considered valid.
The volumetric shell side rupture rate profile against shell side local pressure, a curve in mathematical
form, is then represented in the format understood by the selected liquid hammering simulation tool. The
concrete form may vary from one tool to another, and from one modeler to another, but the essence is that
the rate will change over time during tube rupture transient event, since the local pressure at the shell side
changes, and it changes according to the pre-defined profile afore-mentioned.
Carrying on to the next aspect of the engineering study practice, the liquid hammering transient model
is then constructed in a commercial software tool specialized just for that. Most of the simulation tools
support only one-dimensional with respect to space. The shell side space should then be discretized along
shell side fluid flow direction, capturing and mapping the net volume the fluid would be able to fill up at
each discretized unit with an equivalent diameter, by subtracting the volumes taken away by tubes, baffles,
supports and any other type of shell internals from the gross volume.
The model extent (scope) depends on the project philosophy for liquid hammering transient study. A
typical setup takes into account of the shell side inlet line and outlet line piping to absorb part of the extra
flow traffic from the ruptured tube. The scope therefore needs to include the inlet line piping and outlet
piping besides the shell itself, up to a location, often at the main headers, where pressures may be assumed
constant as boundary condition. Alternatively, length of piping at either side should be included to ensure
that the travel time of the pressure wave is at least 30 milliseconds on this piping. The pressure boundary
conditions may then be imposed at the far end of the both inlet piping and outlet piping.
The model should then be validated against the selected heat and material balance, or the selected case
on the unit's datasheets, with respect to the flow in volume, the operating pressure, temperature and any
other thermodynamic parameters of interest etc.
Carrying on to the next aspect of the engineering study practice, plot space and total weight are two of
the most important engineering parameters to optimize, especially for offshore projects, where space may
be particularly confined. See picture below that shows the plot plan limits that leads to layout restrictions
on the relief devices and heat exchangers.
These layout restrictions usually come from considerations of personnel accessibility, safety, fire-fighting
along with other factors. It often implies that the rupture devices may not be installed on shell or even in
close vicinity of the shell, although preferable.
When implementing the transient model, the lead line piping to the relief device(s) should be obviously
captured. Consider a typical wave speed of 1.2m/ms, it will take about 10 milliseconds for pressure wave
to travel from inlet of lead line to the inlet of the relief device, for a typical 12 m long lead line piping.
This lead time causes considerable delays for the relief device to act on, and reduces the effectivity of
the relief configuration. The lead line piping diameter also factors in, potentially increasing the plot space
requirements and adding on total weight.
For tube rupture events, if a relief device is found necessary, its response time in terms of opening stroke
time after reaching set pressure should be accounted for. This response time usually ranges from 5 ms to
20 ms for rupture discs and rupture pins. However, it is highly recommended to contact vendor to get the
quotes together with the rating datasheets. Conversely, as a conservative assumption, for a similar physical
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mechanism, when a tube ruptures, its opening stroke time may take the lower bound of that for a typical
rupture device, say 5 ms, to become fully ruptured.
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The rupture devices are calibrated with vendor provided performance datasheets such that for any given
inlet and back pressure, the flow would match between model and vendor datasheets.
Past projects experiences have demonstrated that for the same exchanger, the worst peak pressures are
observed at tube rupture locations of the tube sheet and the U bend.
For a double-decked unit, tube rupture cases should include tube sheet and U bend for both of the shells,
therefore 4 cases to simulate. The pressure profiles against time at sampled locations along shell side flow
direction may then be generated. The pressure profiles against distance at sampled time and the maximum
pressure profile against distance may also be generated.
The heat exchanger unit discussed in this paper is a cooler with two shells stacked one on top of the
other. The cooling medium is closed circuit circulating cooling water (initial fill /make-up from potable
water quality), supply and return temperature at 95°F and 120 °F respectively with operating inlet pressure
of about 42 psig. The process side condition selected for the study is natural gas production stream with
maximum operating pressure at 750 psig at this location. The rupture rate for this case is consequently
determined as 32,000 lb/hr.
The unit is specified as shell and tube (S&T) type exchangers (TEMA-type AEU). The shell inner
diameter is 1.18 m and is 6 m long. The tube number is 1130 with diameter of 19 mm.
Shown below is the stack plan:
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•
•
•
•
To determine the maximum incidental pressure on shell side (as well as cooling water inlet/outlet
piping) during the tube rupture
To determine the required relief device (rupture disc/buckling pin) capacity to maintain the
incidental pressure below the maximum allowed.
To provide process data at relieving condition to size relief device (i.e. rupture disc /buckling pin).
To recommend location and number of relief device (rupture disc /buckling pin) inlet nozzle(s) on
the heat exchanger shell.
The rating of the shell side with the associated piping may also be revised as outcome of this study. The
maximum allowed pressure for tube rupture study of 350 psig is then determined with the rating as the basis.
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Cooling water enters bottom shell from below near tube sheet side, traveling through tube towards Ubend side and then exits bottom shell via a 14" flange connection entering upper shell at U-bend side. Water
eventually exits top shell from top near tube sheet.
Tube may rupture at any location within the shells, leaking process gas into shell side therefore
pressurizing shells quickly, causing water hammer effect to shell and water line piping.
There are two relief lines planned originally, connecting to the two nozzles as illustrated above. As an
outcome of this study, only the top relief route is required. The one at the bottom is eliminated.
The cooling water inlet piping of 31m (from the 40" supply header to the unit) of 10" in diameter is
included in the model for the study. The cooling water outlet piping of 30m (from the unit to the 40" return
header) of 10" diameter is also included in the model for the study. The lead line piping of 12 m of 12" in
diameter from nozzle to the relief device is also considered in the model for the study. All elevation changes
are considered according to plant ISO drawings.
Tube rupture studies are typically consequential studies. They are performed to determine the sizing and
rating basis for the relief devices, the associated relief line piping, the rating of the shells as well as the
cooling water supply and return line piping. Specifically:
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Methodology and Assumptions
The step of qualifying the tube rupture rate is the same regardless the phase type of tube side fluid. With tube
side pressure considered as constant as conservative assumption during characterization, the volumetric
rupture rate at shell side is shown below as function of local shell side pressure:
Result and Discussion
To recap briefly, the cooler has two shells stacked one on top of the other. The cooling system is a closed
circuit circulating cooling water (initial fill /make-up from potable water quality), with supply and return
temperatures at 75 °F and 100 °F respectively, and with operating inlet pressure of about 35 psig (3.5 bar).
The process side condition selected for the study is with maximum operating pressure at 830 psig (58 bar)
at this location.
On an offshore setting, the lead line length is inevitable. The longer the line, the more compromised is
the effectivity of the relief setup.
The transient tube rupture studies are very valuable in the EPC business since the outcome has major
impact on the economics of exchanger design. The tube rupture study is regularly required to provide the
adequate protection for the exchanger with high pressure on the tube side and comparative low pressure on
the shell side. The option of raising the shell thickness and MAWP to match tube side MAWP is usually
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For the relief device of 6" normal diameter, its performance is calibrated against the vendor provided
performance datasheet, passing 8584 GPM with inlet pressure of 152 psig. The relief device set pressure is
168 psig. The stoke time used in the simulation is 10 milliseconds. Note that it is assumed the tube would
rupture in 5 milliseconds.
When tube ruptures, process side (tube side) fluid enters cooling medium side. The equivalent volumetric
rate of cooling medium is assumed in place of the process side fluid.
For each case, the simulation is run steadily for the first 5 milliseconds (ms), then a tube rupture event
is introduced within a shell nearby the U-bend. Note a rupture at the tube sheet produce similar result. But
it is not documented in this paper for brevity.
Furthermore, only one tube is assumed to rupture and it is assumed to be a full-bore rupture with both
ends passing fluid. The tube-side fluid enters the shell-side assuming no temperature mixing effect. Note
the volumetric rupture flow between tube side fluid and shell fluid is setup to be conserved exactly.
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economically unfeasible. It would have also additionally required major changes to associated supply and
return piping, resulting in cost and schedule delays.
The pressure profiles along time and flow path are of primary interests as the pressure peaks need to be
contained under maximum allowed working pressure to ensure metal mechanical integrity.
For the cooler examined in this study, the pressure sampled at upper shell by tube sheet is as follows:
As seen, the maximum pressure could be at a location on the surrounding piping, rather than on the shell
itself. The peak pressure value could be a few folds higher than the normal operating pressure.
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The pressure sampled at cooling water supply line, 20 meter from main header:
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The pressure profile along flow path may also be plotted. For example, the maximum pressure profile
along flow path (distance), two shells in series, including connection piping in between:
The placement of a relief device (or devices) alleviate the hammering effect. It is interested in seeing the
relief device's relief rate profile along time:
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Maximum pressure profile along distance, cooling water supply line, from main header to unit:
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Conclusion
The transient tube rupture study answers the important questions of how high the peak pressure would
be and what is the required response of the protective device like a rupture pin. The technical study has
effectively eliminated the project schedule delay and helped the project to retain most of the original design
parameters with least system modifications.
The engineering study practice described in this paper adequately predicts the pressure shock waves
taking place in milliseconds time scale after a tube rupture. The practice provides rigorous sizing and rating
basis for shell, the associated piping and the relief device itself.
During the course of the study, the primary investigation team needs to work closely with members in
other discipline teams working on the same project in an iterative way to improve the design. By doing
so, the project has saved tremendous time, effort, capital cost, and ensured execution schedule is on time
and within budget.
The paper has discussed the major aspects of the practice, covered characterization of tube rupture
rate, rate mapping onto liquid hammering transient model. Model scope identification, model construction,
boundary setup consideration, relief device calibration are based upon project philosophy. The practice
discussed in this paper is considered generic practice and may be applied directly on various tube side and
shell side fluid types.
References
1.
American Petroleum Institute. API Standard 521: Pressure-relieving and Depressuring Systems,
6th ed., Washington, DC: American Petroleum Institute; 2016.
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Note that the work flow goes through a few iterations between the study team and the relevant teams
in other discipline of the same project, and even relief device vendors, making justifications and possible
revisions of the design parameters such as design pressure, piping length and diameter, and nevertheless,
the placement and sizing of the relief devices.
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2.
3.
4.
Feng J, Aggarwal A, Dasgupta S, Shariat H. Using dynamic analysis to reduce weight of offshore
installations. 2009.
Ahmed Harhara and M.M. Faruque Hasan, Dynamic modeling of heat exchanger tube rupture,
2020.
Cassata J, Feng Z, Dasgupta S, Samways R. Prevent overpressure failures on heat exchangers.
Hydrocarbon Process. 1998;77 (11):123–8.
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