THE HAZARDS OF THERMAL EXPANSION
J. C. Ramirez, R. A. Ogle, A. R. Carpenter, and D. R. Morrison
Exponent Failure Analysis Associates
1011 Warrenville Road, Suite 215
Lisle, IL 60532
Phone (630) 274-3225
ABSTRACT
We examine the sometimes overlooked hazards posed by thermal expansion of
trapped liquids in piping systems. Trapped liquids can generate large pressure increases
with only modest temperature changes. This phenomenon is well understood by the
process safety community. However, it is also a phenomenon that is easily overlooked or
dismissed as being of small consequence.
This paper discusses a simple model for estimating the overpressure hazard from
thermal expansion and illustrates its use with some typical liquids encountered in heat
transfer systems. The control of these overpressure hazards is discussed in terms of
applicable safety guidelines and standards. We then illustrate the hazard of heating
trapped liquids with two case studies; one was a heating system and the other was a
refrigeration system. We show that the basic cause of these catastrophic failures could
have been avoided by following applicable safety standards in conjunction with a process
hazard analysis during the design phase. Our findings underscore two important design
guidelines: 1) install a pressure relief device wherever liquid can be trapped and
subjected to heating; and 2) verify that each component of the piping system that can be
isolated from pressure relief satisfies the design pressure.
1. Introduction
The heating of trapped liquids can lead to the catastrophic failure of piping or
process equipment. Although the hazard is well-known, it can be easily overlooked in the
absence of a systematic process hazard analysis. It is a relevant hazard scenario for
process systems in which the system temperature is different from the ambient
temperature or where ambient temperature changes can have significant effects. Thus it
can be an especially significant hazard in process systems involving either heating or
refrigeration. A loss of containment of process fluid caused by thermal expansion
overpressure may seem like a de minimis risk when compared to hazard scenarios like
fire exposure or runaway chemical reactions. But the loss of containment of a process
fluid can lead to a catastrophic loss if it is a high hazard material. Examples of this
situation include a combustible liquid heated near or above its flashpoint, a liquid heated
above its normal boiling point, or a volatile toxic liquid. These catastrophic incidents can
cause flash fires, explosions, and toxic releases with all the related casualties and
property damages.
This paper is divided into five sections. First we present a thermodynamic model
for evaluating the potential magnitude of the overpressure hazard posed by heating a
trapped process liquid. Next we describe safeguards for controlling the overpressure
hazard and identify several design standards and guidelines that document good
engineering practices for implementing these safeguards. This is followed by a
presentation of two case studies of explosions caused by heating trapped liquids. Finally,
we offer some lessons learned that may assist the engineer in performing a process hazard
analysis on systems that involve the heating of process liquids.
2. Thermal Expansion Hazards
Liquids contained within process vessels and piping will normally expand when
heated. The expansion will damage pipes and vessels if the pipe or vessel is filled
completely with fluid and the liquid is blocked in. Therefore, trapped liquid subjected to
heating is an example of hazardous energy storage. This hazard is specifically indicated
in the safety literature as the potential for excess pressure to develop due to thermal
expansion or vaporization of trapped liquid [1,2].
The thermal expansion of liquids is a phenomenon with which we are all exposed
due to its presence in everyday life. For instance, the mercury thermometer’s operation is
based on this principle. Similarly, a common recommendation for automobile drivers to
help save fuel is not to overfill the tank, because this might cause the fuel to overflow
later due to thermal expansion [3]. The potential for expansion of liquid propane, for
example in 20-pound gas grill cylinders, has also resulted in the legal requirement for
overfill protection devices to limit the likelihood for cylinder releases. The phenomenon
of thermal expansion applies to fluids that are liquids at normal conditions as well as
liquefied gases.
From the thermodynamics of fluids in single-phase regions, pressure,
temperature, T are independent and we can think of the specific volume,
,
function of these two,
and
as being a
. The differential of such a function is
1
Two thermodynamic properties related to the partial derivatives appearing in the
differential are the coefficient of thermal expansion
1
2
and the isothermal compressibility coefficient
1
The unit for
3
is the reciprocal of that for temperature and the unit for
is the
reciprocal of that for pressure. The volume expansivity is an indication of the change in
volume that occurs when the temperature changes while pressure remains constant. The
isothermal compressibility is an indication of the change in volume that takes place when
pressure changes while temperature remains constant [4].
For the liquid phase, the constant temperature lines on a
constant pressure lines on a
/
and
/
diagram and the
diagram are very steep and closely spaced, thus both
and hence both
and
are small. This characteristic behavior
of liquids – far from the critical point – leads to the idealization of the incompressible
fluid, for which both
and
are zero. For real liquids,
temperature and pressure, so for small changes in
and
are weak functions of
and T little error is introduced if we
regard them as constant [5].
Note that, for constant volume (
= 0, as in a rigid container or pipe) equations
(1) through (3) can be combined and integrated to yield:
∆
∆
4
where ∆ is the pressure increase induced by the temperature increase ∆ . Thus,
Equation (4) can be used to estimate the pressure increase with temperature of a liquid
when trapped inside a fixed volume, rigid container. This calculation is only an estimate
since, at small∆ , real piping and pressure vessels will undergo elastic deformation
which will relieve some of the overpressure caused by thermal expansion. As ∆
increases, however, the overpressure can easily reach the burst pressure of the
component. The burst pressure for an individual vessel or component can be estimated
using various methods, which won’t be covered here. Calculations and tests are described
in the pressure vessel literature and the ASME pressure vessel design code (e.g., see
Table 2).
Compressibility and expansion coefficients are tabulated for some fluids, but most
, ,
often these parameters must be calculated from thermodynamic state
Figure 1 is a schematic representation of a typical pressure (
– enthalpy
diagrams.
diagram
focusing on the region left of saturation dome, the compressed (or subcooled) liquid
region.
P Compressed liquid 2 3 1 Saturated liquid‐vapor mixture h Fig 1. Graphical estimation of isothermal compressibility and thermal expansion
coefficients
Under the assumption that
and
are approximately constant, the partial derivatives
in equations (2) and (3) can be estimated with finite differences. Note that in Figure 1,
states 1 and 2 are at the same temperature, and states 1 and 3 are at the same pressure.
Therefore:
1
and
5
1
6
where the subscript refers to the state at which the thermodynamic property is evaluated.
Table 1 summarizes the thermal expansion and isothermal compressibility coefficients
for several typical process fluids or refrigerants.
Table 1. Isothermal compressibility and thermal expansion coefficients for select liquids
∆ range,
∆ range,
°C
bar gauge
Water
20 to 80
0‐2000
Fuel oil
27 to 44
28 to 165
Silicone oil
25 to 150
0 to 55
‐40 to ‐18
Propane
Liquid
, °C-1
, bar-1
/ , bar/°C
Reference
12.4
6
N/A
9.3
7
10
1.5 · 10
6.7
8
28 to 69
4.0 · 10
4 · 10
100
9
‐183 to 27
10 to 30
1.56 · 10
6.18 · 10
2.5
10
R-134a
‐60 to 0
3 to 40
2 · 10
3.09 · 10
6.6
9
Ammonia
0 to 60
26 to 50
2.47 · 10
3.7 · 10
6.8
9
Carbon
dioxide
4.5 · 10
N/A
3.64 · 10
.
Figure 2 shows the pressure increase as a function of the temperature increase in a
rigid container for the 7 liquids considered. The slope of each line is the ratio / .
Pressure increase, ΔP (bar)
700
600
500
Water
400
Fuel Oil
300
Silicone Oil, R‐134a, NH3
200
Liquid CO2
100
Propane
0
0
10
20
30
40
50
Temperature increase, ΔT (ºC)
Fig 2. Pressure increase in a rigid container vs liquid temperature increase
3. Thermal Expansion Safeguards
The hazard presented by the heating of a trapped liquid is overpressure. If the
overpressure generated by thermal expansion exceeds the burst pressure of its container,
then a release of the pressurized liquid will occur. The significance of a loss of
containment accident depends on the magnitude of its consequence. If the process fluid is
a high hazard material, then appropriate safeguards should be implemented. A high
hazard material as considered here poses a significant hazard to health, safety, and
surrounding equipment if suddenly released due to vessel failure. Depending upon the
mode of failure for the vessel (e.g., leak versus catastrophic/fragmenting failure), the
nature of the fluid (e.g., flammable, above its boiling point, and/or toxic), and the
surroundings (e.g., inside an occupied building or adjacent to other high hazard process
units), the consequences of overpressure-induced vessel failure can be extreme.
The discipline of system safety suggests that hazard control is best considered in
terms of a hierarchy of safeguards. In order of decreasing effectiveness, the system safety
hierarchy is
•
Eliminate the hazard
•
Use passive engineering controls to manage the hazard
•
Use active engineering controls to manage the hazard
•
Manage the hazard with warnings and procedures
Depending on the severity of the consequences, it may be best to manage an overpressure
hazard with multiple safeguards.
There are several design standards and guidelines that contain requirements for
overpressure protection. These documents tend to emphasize two strategies for hazard
control, pressure containment (passive engineering control) and pressure relief (active
engineering control). In essence, the recommended practice is this: if a liquid can be
trapped and heated in a section of piping or within a container, then the designer must
either provide a means for pressure relief or he must design that component to withstand
the greatest pressure that can be developed. Table 2 is a partial listing of references for
recommended practices.
Table 2. Recommended practices for overpressure protection from thermal expansion
Component
Design Standard or Guideline
Pressure vessels
ASME Boiler and Pressure Vessel Code
Process piping
ASME Code for Pressure Piping B31.3 Process Piping
Refrigeration piping
ASME Code for Pressure Piping B31.5 Refrigeration Piping
Refrigeration systems
ASHRAE 15 Safety Standard for Refrigeration Systems
CCPS Guidelines for Pressure Relief and Effluent Handling
Pressure relief design requirements
Systems
API 521 Guide for Pressure-Relieving and Depressuring
Systems
Propane systems
NFPA 58 Standard for the Storage and Handling of Liquefied
Petroleum Gases
IIAR 2 Equipment, Design, and Installation of Ammonia
Ammonia refrigeration systems
Mechanical Refrigerating Systems
IIAR 116 Avoiding Component Failure in Industrial
Refrigeration Systems Caused by Abnormal Pressure or Shock
CGA G-6 Carbon Dioxide
Carbon dioxide systems
CGA G-6.1 Standard for Insulated Liquid Carbon Dioxide
Systems at Consumer Sites
4. Case Studies
In this section we present synopses two case studies that have resulted in serious
injuries and/or significant business interruption. The first case involved an industrial oven
(heating system) which used a heat transfer oil and the second case involved a custommade freeze dryer for a pharmaceutical application in which nitrogen vapor cooled a
stream of silicone oil in a heat exchanger (refrigeration system).
4.1 Case Study #1: Industrial Oven
An industrial oven for a food processing application used a heat transfer oil (HTO) as
the heat source. The oven was comprised of several contiguous zones and each zone was
connected to a plant-wide HTO network via supply and return pipes. Figure 3 shows a
schematic representation of this system.
expansion tank Main pump and heater to/from other user
to/from other user
to/from other user
5 zone oven
booster pump Fig. 3. Schematic representation of plant-wide heat transfer oil system
A pump circulates the thermal fluid through a natural gas fired heater and on to the
rest of the thermal fluid system. The heated fluid is distributed across the plant to several
users, one of which is the 5-zone oven. After circulating through the oven and other
users, the thermal fluid is returned to the heaters, passing through an expansion tank
along the way.
The temperature of each zone is controlled by independent flow control valves for
each zone. It is possible to operate the oven with each zone operating at different
temperatures, or while some zones are off. Manual isolation valves were installed on
both the upstream and downstream side of the control valves to allow for maintenance of
the valves. Additionally, the facility installed manual isolation valves at the supply and
return of each oven zone, as well as a drain valve to drain the HTO from individual
zones.
One the day prior to the incident, zones 2 and 3 were manually isolated from the oil
network by a contractor to repair a minor leak. The oven was started the following day,
and the pump was activated. However, the manual isolation valves were not reopened
following the previous day’s maintenance activities. The operator was unaware that the
piping for zones 2 and 3 contained trapped liquid. The temperature in three of the five
zones increased as he expected, but temperatures in the isolated zones rose at a much
slower rate than normal.
The operator determined that the zones must still be isolated and dispatched a
separate operator to open the isolation valves for the two zones. Immediately after the
isolation valves were opened, the primary operator observed a pool of HTO flowing out
from one of the previously isolated zones. While the fluid was trapped, the heat from the
adjacent zones heated the HTO resulting in thermal expansion of the oil which caused a
rupture in the internal piping. The fresh, hot oil spilled out from the rupture and
eventually ignited resulting in an explosion and fire which caused minor injuries to
facility personnel, structural damage to the facility and a significant interruption to
operations. During the accident investigation it was determined that the ruptured piping
component had a lower burst pressure than the surrounding portions of the piping system.
The weakest point in the piping system happened to be in the oven, thus the high
consequence of the release.
4.2 Case Study #2: Cryogenic Refrigeration System
Lyophilization, also referred to as freeze-drying, is a process of removing moisture
from a pharmaceutical product by first freezing it and then by subjecting it to a vacuum.
This process promotes the direct phase change from solid ice to water vapor without a
transition through a liquid phase - a process known as sublimation. In a conventional
drying process, a product is heated at atmospheric pressure, requiring much higher
temperatures, to promote this phase change. Lyophilization benefits pharmaceuticals by
increasing product shelf life without heating which can have a deleterious impact on
product quality and/or performance.
The subject freeze-dryer (Figure 4) was composed of a process chamber, a
condensing unit, vacuum pumps, and a utility skid. The utility skid cooled silicone oil
that was used as a heat transfer fluid with liquid nitrogen. The cold silicone oil was
circulated through a heat exchanger in the process chamber to chill the product. The
silicone oil was also used to cool the condenser. After the product was chilled, the
vacuum pumps would lower the pressure in the process chamber causing water to
sublime as vapor from the product. This vapor was then deposited on plates in the
condenser. Electric immersion heaters were also installed in the silicone oil flow circuit
for additional temperature control.
Figure 4. Major components of subject freeze dryer One the day if the incident, a power outage occurred on the freeze-dryer’s control
system power supply. As a result of the loss of power a number of key sensors and
control elements were unavailable to the programmable logic controller. In response, the
control system managed a chain of events that led to the freeze-up of the oil-nitrogen heat
exchanger and the call for heat in the electric heater. Figure 5 shows a schematic
representation of the shelf cooling system.
shelves
circulation pump and check valve (typ. 2) silicone oil
nitrogen heat exchanger electric heater
Figure 5. Schematic representation of silicone oil circuit The silicone oil inside the heat exchanger froze, forming a solid plug. There was
liquid silicone oil trapped between the frozen oil in the heat exchanger and the check
valves. The control system initiated a call for heat which resulted in the thermal
expansion of the trapped liquid oil. This resulted in the catastrophic rupture of the
housing for the electric heater. The ejected oil ignited causing a flash fire and serious
injury to an operator. Subsequent to the accident it was discovered that the housing for
the electric heater had been defectively fabricated, causing it to rupture at a pressure
significantly less than the burst pressure of the piping system.
5. Recommendations
Based on observations from these case studies and from similar accident
investigations, we offer some lessons learned that may assist the engineer in performing a
process hazard analysis on systems that involve the heating of process liquids.
Understand the limitations of the pressure relief system
A process system typically consists of a network of vessels and piping. Pressure relief
devices are incorporated into the design with specific hazard scenarios in mind. It is
essential that the design basis for pressure relief is thoroughly understood and
documented. As process changes are implemented over the lifetime of the process
system, the impact of the change on the pressure relief system must be considered (via
the management of change program). For example, most indirect heating systems
circulate heat transfer oil through an expansion tank. Pressure relief for the entire flow
circuit is often provided only at the expansion tank [12]. Care must be taken to ensure
that any process modification does not present the opportunity to trap liquid anywhere in
the flow circuit.
Guard against blockages in the flow path
Flow blockages come in all shapes and sizes. Once a process liquid has become
isolated from the pressure relief system, it becomes a potential victim of uncontrolled
thermal expansion. Manual block valves, control valves that fail closed, and check valves
are obvious forms of flow blockage in a piping circuit. However, other blockages may
not be so obvious. For example, in refrigeration systems with secondary coolants it may
be possible to freeze a heat exchanger to the extent that it completely blocks the flow of
the secondary coolant. The potential for non-obvious hazard scenarios underscores the
importance of conducting a systematic process hazards analysis.
Know/verify the design pressure of each piping component
Just like a chain will fail at its weakest link, so too in any given piping circuit, the
component with the lowest design pressure will be the first component to fail under
overpressure. The lowest design pressure for a component will usually dictate the design
pressure for the system. The setpoint for the pressure relief system should be based on
protecting the component with the lowest design pressure.
Beware of defectively fabricated piping components
Defectively fabricated piping components may have failure pressures far less than
expected, presenting the possibility of premature failure under abnormal operating
conditions. A mechanical integrity program is an essential safeguard for overpressure
protection, and one of its objectives should be to verify that the process piping and
vessels have been fabricated in accordance with the design documents.
Conclusions
The heating of a trapped liquid is a well-recognized hazard scenario, but it is
sometimes overlooked. This seemingly minor source of overpressure can actually result
in catastrophic accident. The two primary strategies for protecting against this
overpressure hazard are pressure containment and pressure relief. There are numerous
design standards and guidelines that document good engineering practices for
implementing these strategies. Recommendations were presented to assist in the process
hazard analysis effort.
References
1. Center for Chemical Process Safety, Guidelines for Hazard Evaluation
Procedures, Second Edition with Worked Examples; New York: AIChE, 1992.
2. Crowl, D., and Louvar, J. Chemical Process Safety; Fundamentals with
Applications, 2nd edition. Englewood Cliffs, NJ: Prentice Hall, 2001.
3. Cengel, Y. and Boles. Thermodynamics (An Engineering Approach), 6th edition.
McGraw-Hill, 2006
4. Moran, M. and Shaprio, H. Fundamentals of Engineering Thermodynamics, 6th
edition, Hoboken, NJ: Wiley, 2008
5. Smith, J., Van Ness, H., and Abbott, M. Introduction to Chemical Engineering
Thermodynamics, 5th edition. New York: McGraw-Hill, 1996.
6. Dean, J. (Ed.), Lange’s Handbook of Chemistry, 11th edition. New York:
McGraw-Hill, 1973
7. 1960 Crane’s Catalog, p. 14
8. Clearco, Compressibility of Clearco Pure Silicone 20CST, and Product Information
Sheet, www.clearcoproducts.com.
9. ASHRAE, 1997 ASHRAE Handbook: Fundamentals. Atlanta: ASHRAE, 1997
10. ASTM, Standard Specification for Propane Thermophysical Property Tables
D4362-08, West Cronshohocken, PA, 2009.
11. Oetinger, Jim, Prevent Fires in Thermal Fluid. www.paratherm.com
12. Singh, J. Heat Transfer Fluids and Systems for Process and Energy Applications.
Boca Raton: CRC Press, 1985.