Chapter 3 Vessels - Chapter Notes, Chemical Engineering Chemical Engineering Notes | EduRev

 Page 1


3
VESSELS
3.1 INTRODUCTION
This chapter presents potential failure mechanisms for vessels and suggests
design alternatives for reducing the risks associated with such failures. The
types of vessels covered in this chapter include:
• In-process vessels (surge drums, accumulators, separators, etc.)
• Pressurized tanks (spheres, bullets)
• Atmospheric, fixed roof storage tanks (cone/dome roof)
• Atmospheric, floating roof storage tanks
Reactors are a unique subset of vessels, in that they are specifically
designed to contain chemical reactions. Because reactors have unique failure
scenarios specifically attributable to the reaction (e.g., reactant accumulation),
a complete chapter (Chapter 4) is devoted to this important class of equip-
ment. However, many of the generic vessel failure modes discussed in this
chapter, such as corrosion related failures or autopolymerization may also
apply to reactors.
3.2 PAST INCIDENTS
"Those who cannot remember the past are condemned to repeat it" (Santay-
ana 1905). Important lessons can be learned from prior mistakes. Several case
histories of incidents involving vessel failures are provided to reinforce the
need for the safe design and operating practices presented in this chapter.
3.2. / Storage Tank Autopolymerization Incident
Plant operating problems had resulted in the production of a tank (approxi-
mately 32,000 Ib) of glacial acrylic acid (GAA) which did not meet specifica-
Page 2


3
VESSELS
3.1 INTRODUCTION
This chapter presents potential failure mechanisms for vessels and suggests
design alternatives for reducing the risks associated with such failures. The
types of vessels covered in this chapter include:
• In-process vessels (surge drums, accumulators, separators, etc.)
• Pressurized tanks (spheres, bullets)
• Atmospheric, fixed roof storage tanks (cone/dome roof)
• Atmospheric, floating roof storage tanks
Reactors are a unique subset of vessels, in that they are specifically
designed to contain chemical reactions. Because reactors have unique failure
scenarios specifically attributable to the reaction (e.g., reactant accumulation),
a complete chapter (Chapter 4) is devoted to this important class of equip-
ment. However, many of the generic vessel failure modes discussed in this
chapter, such as corrosion related failures or autopolymerization may also
apply to reactors.
3.2 PAST INCIDENTS
"Those who cannot remember the past are condemned to repeat it" (Santay-
ana 1905). Important lessons can be learned from prior mistakes. Several case
histories of incidents involving vessel failures are provided to reinforce the
need for the safe design and operating practices presented in this chapter.
3.2. / Storage Tank Autopolymerization Incident
Plant operating problems had resulted in the production of a tank (approxi-
mately 32,000 Ib) of glacial acrylic acid (GAA) which did not meet specifica-
tions due to high water content. The material was held in storage until it was
loaded into a tank wagon, where it was to be kept until the GAA could be
reworked. The operator's logbook specified that warm water (25
0
C maxi-
mum) was to be used to keep the GAA from freezing (freezing point = 13
0
C).
The outside temperature was 5-1O
0
C at the time. A standard steam-water
mixing station was used to supply the warm water to the tank wagon coils.
Water flow was maintained to the tank wagon, but no measuring devices were
available for observing actual temperature or flow rate. The steam-water
mixing station operation was monitored and adjusted by observing that warm
water was running out of the coil outlet (noting vapor evolving from water in
the cold weather). It was not clear after the incident whether the tank wagon
dome lid was open, or just loosened to allow "breathing" during the hold
period.
Approximately l5
l
/2 hours after the tank wagon was filled, vapors started
blowing out the loosened tank wagon lid and accumulating in the vicinity of the
tank wagon. The steam-water mixer was shut off and approximately six minutes
later the tank wagon exploded. The blast effect from the explosion destroyed an
adjacent loading rack/pipe rack, and damaged other plant structures.
A combination of local overheating (hot surface) and local inhibitor defi-
ciency was considered the most probable mechanism for initiation of polym-
erization. Contamination may have contributed to the violence of the
polymerization once it was initiated. Water and iron were the two main candi-
dates in contamination considerations. Screening experiments showed that
water can reduce GAA stability at temperatures > 10O
0
C, and that soluble iron
in the 1-100 ppm range can also reduce stability. See item 10 in Table 3 for
potential design solutions.
Ed. Note: This example illustrates the hazard of using temporary facilities for
the storage of hazardous materials. Such facilities are often not subject to the same
scrutiny as permanent facilities.
3.2.2 Storage Tank Stratification Incident
Acetic anhydride is used as an acetylating agent for many compounds. When it
reacts with a hydroxyl group, acetic acid is formed as a byproduct. Pure acetic
anhydride will react energetically with water to form acetic acid. In typical ace-
tylation reactions, an excess of anhydride is used to drive the reaction to com-
pletion. This excess is then reacted in the receiver tank with water to convert
the excess anhydride to acid. The acid is then refined and remanufactured into
anhydride. This operation can be performed safely, since die presence of acetic
acid makes water and acetic anhydride miscible, and therefore the rate of reac-
tion can be controlled by the rate of water addition.
Page 3


3
VESSELS
3.1 INTRODUCTION
This chapter presents potential failure mechanisms for vessels and suggests
design alternatives for reducing the risks associated with such failures. The
types of vessels covered in this chapter include:
• In-process vessels (surge drums, accumulators, separators, etc.)
• Pressurized tanks (spheres, bullets)
• Atmospheric, fixed roof storage tanks (cone/dome roof)
• Atmospheric, floating roof storage tanks
Reactors are a unique subset of vessels, in that they are specifically
designed to contain chemical reactions. Because reactors have unique failure
scenarios specifically attributable to the reaction (e.g., reactant accumulation),
a complete chapter (Chapter 4) is devoted to this important class of equip-
ment. However, many of the generic vessel failure modes discussed in this
chapter, such as corrosion related failures or autopolymerization may also
apply to reactors.
3.2 PAST INCIDENTS
"Those who cannot remember the past are condemned to repeat it" (Santay-
ana 1905). Important lessons can be learned from prior mistakes. Several case
histories of incidents involving vessel failures are provided to reinforce the
need for the safe design and operating practices presented in this chapter.
3.2. / Storage Tank Autopolymerization Incident
Plant operating problems had resulted in the production of a tank (approxi-
mately 32,000 Ib) of glacial acrylic acid (GAA) which did not meet specifica-
tions due to high water content. The material was held in storage until it was
loaded into a tank wagon, where it was to be kept until the GAA could be
reworked. The operator's logbook specified that warm water (25
0
C maxi-
mum) was to be used to keep the GAA from freezing (freezing point = 13
0
C).
The outside temperature was 5-1O
0
C at the time. A standard steam-water
mixing station was used to supply the warm water to the tank wagon coils.
Water flow was maintained to the tank wagon, but no measuring devices were
available for observing actual temperature or flow rate. The steam-water
mixing station operation was monitored and adjusted by observing that warm
water was running out of the coil outlet (noting vapor evolving from water in
the cold weather). It was not clear after the incident whether the tank wagon
dome lid was open, or just loosened to allow "breathing" during the hold
period.
Approximately l5
l
/2 hours after the tank wagon was filled, vapors started
blowing out the loosened tank wagon lid and accumulating in the vicinity of the
tank wagon. The steam-water mixer was shut off and approximately six minutes
later the tank wagon exploded. The blast effect from the explosion destroyed an
adjacent loading rack/pipe rack, and damaged other plant structures.
A combination of local overheating (hot surface) and local inhibitor defi-
ciency was considered the most probable mechanism for initiation of polym-
erization. Contamination may have contributed to the violence of the
polymerization once it was initiated. Water and iron were the two main candi-
dates in contamination considerations. Screening experiments showed that
water can reduce GAA stability at temperatures > 10O
0
C, and that soluble iron
in the 1-100 ppm range can also reduce stability. See item 10 in Table 3 for
potential design solutions.
Ed. Note: This example illustrates the hazard of using temporary facilities for
the storage of hazardous materials. Such facilities are often not subject to the same
scrutiny as permanent facilities.
3.2.2 Storage Tank Stratification Incident
Acetic anhydride is used as an acetylating agent for many compounds. When it
reacts with a hydroxyl group, acetic acid is formed as a byproduct. Pure acetic
anhydride will react energetically with water to form acetic acid. In typical ace-
tylation reactions, an excess of anhydride is used to drive the reaction to com-
pletion. This excess is then reacted in the receiver tank with water to convert
the excess anhydride to acid. The acid is then refined and remanufactured into
anhydride. This operation can be performed safely, since die presence of acetic
acid makes water and acetic anhydride miscible, and therefore the rate of reac-
tion can be controlled by the rate of water addition.
In this case, the acetylation reaction did not proceed as designed, due to an
inadvertent omission of the strong mineral acid catalyst needed to initiate the
reaction at low temperatures (-1O
0
F). Thus, the receiver tank did not contain a
mixture of acetic anhydride and acetic acid, but only very cold, pure anhy-
dride. The operator in charge of the water addition did not realize the change
in composition, and additionally failed to turn the tank agitator on prior to
beginning the water addition. After several minutes of water addition, he real-
ized his mistake with the agitator, and hit the start button. Immediately, the
water, which had layered out on top of the cold anhydride, mixed and reacted
violently. This caused a partial vaporization in the tank, and eruption through
an open manway, resulting in fatal burning of the operator.
Had the agitator been turned on prior to beginning the water addition,
the reaction rate would have again been controlled by the water addition rate.
In this case, the water was added at near-stoichiometric concentrations virtu-
ally instantaneously, resulting in an uncontrolled exotherm.
3.2.3 Botch Pharmaceutical Reactor Accident
While two operators were charging fiber drums containing a penicillin
powder into a reactor containing a mixture of acetone and methanol, an explo-
sion occurred at the reactor manhole. The two operators were blown back by
the force of the explosion, and were covered with solvent-wet powder.
The incident was initiated by the ignition of solvent vapors, which
resulted in a dust explosion of the dry powder. The solvent liquid mixture in
the reactor did not ignite. Tests on the polyethylene liner inside the fiber
drums, which had been grounded at the time of the incident, showed that they
were of the non-conducting type. The most probable cause of the ignition was
an electrostatic discharge from the polyethylene liner during reactor charging.
After this accident, the company instituted the following procedures
(Drogaris 1993):
• Requiring nitrogen inerting when pouring dry solids into flammable
solvents
• Adding dry powder to the reactor by means of grounded metal scoops,
where possible, rather than by pouring in directly from drums with
polyethylene liners
• Using only conductive polyethylene liners
• Using a closed charging system rather than pouring dry powders into
flammable solvents directly via an open manhole
• Performing an electrostatic hazard review of the whole plant and all the
processes whenever powders and flammable solvents are used
Page 4


3
VESSELS
3.1 INTRODUCTION
This chapter presents potential failure mechanisms for vessels and suggests
design alternatives for reducing the risks associated with such failures. The
types of vessels covered in this chapter include:
• In-process vessels (surge drums, accumulators, separators, etc.)
• Pressurized tanks (spheres, bullets)
• Atmospheric, fixed roof storage tanks (cone/dome roof)
• Atmospheric, floating roof storage tanks
Reactors are a unique subset of vessels, in that they are specifically
designed to contain chemical reactions. Because reactors have unique failure
scenarios specifically attributable to the reaction (e.g., reactant accumulation),
a complete chapter (Chapter 4) is devoted to this important class of equip-
ment. However, many of the generic vessel failure modes discussed in this
chapter, such as corrosion related failures or autopolymerization may also
apply to reactors.
3.2 PAST INCIDENTS
"Those who cannot remember the past are condemned to repeat it" (Santay-
ana 1905). Important lessons can be learned from prior mistakes. Several case
histories of incidents involving vessel failures are provided to reinforce the
need for the safe design and operating practices presented in this chapter.
3.2. / Storage Tank Autopolymerization Incident
Plant operating problems had resulted in the production of a tank (approxi-
mately 32,000 Ib) of glacial acrylic acid (GAA) which did not meet specifica-
tions due to high water content. The material was held in storage until it was
loaded into a tank wagon, where it was to be kept until the GAA could be
reworked. The operator's logbook specified that warm water (25
0
C maxi-
mum) was to be used to keep the GAA from freezing (freezing point = 13
0
C).
The outside temperature was 5-1O
0
C at the time. A standard steam-water
mixing station was used to supply the warm water to the tank wagon coils.
Water flow was maintained to the tank wagon, but no measuring devices were
available for observing actual temperature or flow rate. The steam-water
mixing station operation was monitored and adjusted by observing that warm
water was running out of the coil outlet (noting vapor evolving from water in
the cold weather). It was not clear after the incident whether the tank wagon
dome lid was open, or just loosened to allow "breathing" during the hold
period.
Approximately l5
l
/2 hours after the tank wagon was filled, vapors started
blowing out the loosened tank wagon lid and accumulating in the vicinity of the
tank wagon. The steam-water mixer was shut off and approximately six minutes
later the tank wagon exploded. The blast effect from the explosion destroyed an
adjacent loading rack/pipe rack, and damaged other plant structures.
A combination of local overheating (hot surface) and local inhibitor defi-
ciency was considered the most probable mechanism for initiation of polym-
erization. Contamination may have contributed to the violence of the
polymerization once it was initiated. Water and iron were the two main candi-
dates in contamination considerations. Screening experiments showed that
water can reduce GAA stability at temperatures > 10O
0
C, and that soluble iron
in the 1-100 ppm range can also reduce stability. See item 10 in Table 3 for
potential design solutions.
Ed. Note: This example illustrates the hazard of using temporary facilities for
the storage of hazardous materials. Such facilities are often not subject to the same
scrutiny as permanent facilities.
3.2.2 Storage Tank Stratification Incident
Acetic anhydride is used as an acetylating agent for many compounds. When it
reacts with a hydroxyl group, acetic acid is formed as a byproduct. Pure acetic
anhydride will react energetically with water to form acetic acid. In typical ace-
tylation reactions, an excess of anhydride is used to drive the reaction to com-
pletion. This excess is then reacted in the receiver tank with water to convert
the excess anhydride to acid. The acid is then refined and remanufactured into
anhydride. This operation can be performed safely, since die presence of acetic
acid makes water and acetic anhydride miscible, and therefore the rate of reac-
tion can be controlled by the rate of water addition.
In this case, the acetylation reaction did not proceed as designed, due to an
inadvertent omission of the strong mineral acid catalyst needed to initiate the
reaction at low temperatures (-1O
0
F). Thus, the receiver tank did not contain a
mixture of acetic anhydride and acetic acid, but only very cold, pure anhy-
dride. The operator in charge of the water addition did not realize the change
in composition, and additionally failed to turn the tank agitator on prior to
beginning the water addition. After several minutes of water addition, he real-
ized his mistake with the agitator, and hit the start button. Immediately, the
water, which had layered out on top of the cold anhydride, mixed and reacted
violently. This caused a partial vaporization in the tank, and eruption through
an open manway, resulting in fatal burning of the operator.
Had the agitator been turned on prior to beginning the water addition,
the reaction rate would have again been controlled by the water addition rate.
In this case, the water was added at near-stoichiometric concentrations virtu-
ally instantaneously, resulting in an uncontrolled exotherm.
3.2.3 Botch Pharmaceutical Reactor Accident
While two operators were charging fiber drums containing a penicillin
powder into a reactor containing a mixture of acetone and methanol, an explo-
sion occurred at the reactor manhole. The two operators were blown back by
the force of the explosion, and were covered with solvent-wet powder.
The incident was initiated by the ignition of solvent vapors, which
resulted in a dust explosion of the dry powder. The solvent liquid mixture in
the reactor did not ignite. Tests on the polyethylene liner inside the fiber
drums, which had been grounded at the time of the incident, showed that they
were of the non-conducting type. The most probable cause of the ignition was
an electrostatic discharge from the polyethylene liner during reactor charging.
After this accident, the company instituted the following procedures
(Drogaris 1993):
• Requiring nitrogen inerting when pouring dry solids into flammable
solvents
• Adding dry powder to the reactor by means of grounded metal scoops,
where possible, rather than by pouring in directly from drums with
polyethylene liners
• Using only conductive polyethylene liners
• Using a closed charging system rather than pouring dry powders into
flammable solvents directly via an open manhole
• Performing an electrostatic hazard review of the whole plant and all the
processes whenever powders and flammable solvents are used
Ed. Note: Even though this incident involved a reactor, it applies as well to any
vessel, open-manhole, charging operation. Most likely the liners were loose and the
operators not grounded. If fixed liners were in place and the operators grounded, the
accident might not have occurred.
3.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
The information on equipment failure scenarios and associated design
solutions is introduced in table format in this chapter and followed in each
subsequent equipment chapter. The organization of the tables is the same in
each chapter. The table headings used are described below.
• Operational Deviation—generic operational parameter deviation such
as overpressure. Analogous to HAZOP parameter deviation.
• Failure Scenario—specific failure mechanism/cause for specified
generic parameter deviation (e.g., overpressure due to upstream control
system failure).
• Potential Design Solution—potential design solutions that could be
considered to reduce the risk of the failure scenario. For the reasons
given in Chapter 2, the design solutions are grouped into the following
three categories: inherently safer/passive, active and procedural.
Vessel failure scenarios, along with associated design solutions, are pre-
sented in Table 3. Design solutions are provided for each scenario, although
some scenarios do not have practical design solutions for all categories. Opera-
tional deviations marked with (T) are discussed in further detail in the chapter
text.
3.4 DISCUSSION
3.4. / Use of Potential Design Solutions Table
It should be recognized that the design solutions presented are possible
approaches for reducing the risk of the associated failure scenario. The authors
of this book could not anticipate all the possible applications nor conditions
that may pertain to a specific design situation. Also, the design solutions are
not necessarily equivalent in terms of benefit in reducing the risk of the stated
hazard scenario. Therefore, it is intended that the table be used in conjunction
with the design basis selection methodology presented in Chapter 2 to arrive
at the optimal design solution for a given application. Furthermore, some
solutions are not applicable to all classes of vessels. (For example, designing
Page 5


3
VESSELS
3.1 INTRODUCTION
This chapter presents potential failure mechanisms for vessels and suggests
design alternatives for reducing the risks associated with such failures. The
types of vessels covered in this chapter include:
• In-process vessels (surge drums, accumulators, separators, etc.)
• Pressurized tanks (spheres, bullets)
• Atmospheric, fixed roof storage tanks (cone/dome roof)
• Atmospheric, floating roof storage tanks
Reactors are a unique subset of vessels, in that they are specifically
designed to contain chemical reactions. Because reactors have unique failure
scenarios specifically attributable to the reaction (e.g., reactant accumulation),
a complete chapter (Chapter 4) is devoted to this important class of equip-
ment. However, many of the generic vessel failure modes discussed in this
chapter, such as corrosion related failures or autopolymerization may also
apply to reactors.
3.2 PAST INCIDENTS
"Those who cannot remember the past are condemned to repeat it" (Santay-
ana 1905). Important lessons can be learned from prior mistakes. Several case
histories of incidents involving vessel failures are provided to reinforce the
need for the safe design and operating practices presented in this chapter.
3.2. / Storage Tank Autopolymerization Incident
Plant operating problems had resulted in the production of a tank (approxi-
mately 32,000 Ib) of glacial acrylic acid (GAA) which did not meet specifica-
tions due to high water content. The material was held in storage until it was
loaded into a tank wagon, where it was to be kept until the GAA could be
reworked. The operator's logbook specified that warm water (25
0
C maxi-
mum) was to be used to keep the GAA from freezing (freezing point = 13
0
C).
The outside temperature was 5-1O
0
C at the time. A standard steam-water
mixing station was used to supply the warm water to the tank wagon coils.
Water flow was maintained to the tank wagon, but no measuring devices were
available for observing actual temperature or flow rate. The steam-water
mixing station operation was monitored and adjusted by observing that warm
water was running out of the coil outlet (noting vapor evolving from water in
the cold weather). It was not clear after the incident whether the tank wagon
dome lid was open, or just loosened to allow "breathing" during the hold
period.
Approximately l5
l
/2 hours after the tank wagon was filled, vapors started
blowing out the loosened tank wagon lid and accumulating in the vicinity of the
tank wagon. The steam-water mixer was shut off and approximately six minutes
later the tank wagon exploded. The blast effect from the explosion destroyed an
adjacent loading rack/pipe rack, and damaged other plant structures.
A combination of local overheating (hot surface) and local inhibitor defi-
ciency was considered the most probable mechanism for initiation of polym-
erization. Contamination may have contributed to the violence of the
polymerization once it was initiated. Water and iron were the two main candi-
dates in contamination considerations. Screening experiments showed that
water can reduce GAA stability at temperatures > 10O
0
C, and that soluble iron
in the 1-100 ppm range can also reduce stability. See item 10 in Table 3 for
potential design solutions.
Ed. Note: This example illustrates the hazard of using temporary facilities for
the storage of hazardous materials. Such facilities are often not subject to the same
scrutiny as permanent facilities.
3.2.2 Storage Tank Stratification Incident
Acetic anhydride is used as an acetylating agent for many compounds. When it
reacts with a hydroxyl group, acetic acid is formed as a byproduct. Pure acetic
anhydride will react energetically with water to form acetic acid. In typical ace-
tylation reactions, an excess of anhydride is used to drive the reaction to com-
pletion. This excess is then reacted in the receiver tank with water to convert
the excess anhydride to acid. The acid is then refined and remanufactured into
anhydride. This operation can be performed safely, since die presence of acetic
acid makes water and acetic anhydride miscible, and therefore the rate of reac-
tion can be controlled by the rate of water addition.
In this case, the acetylation reaction did not proceed as designed, due to an
inadvertent omission of the strong mineral acid catalyst needed to initiate the
reaction at low temperatures (-1O
0
F). Thus, the receiver tank did not contain a
mixture of acetic anhydride and acetic acid, but only very cold, pure anhy-
dride. The operator in charge of the water addition did not realize the change
in composition, and additionally failed to turn the tank agitator on prior to
beginning the water addition. After several minutes of water addition, he real-
ized his mistake with the agitator, and hit the start button. Immediately, the
water, which had layered out on top of the cold anhydride, mixed and reacted
violently. This caused a partial vaporization in the tank, and eruption through
an open manway, resulting in fatal burning of the operator.
Had the agitator been turned on prior to beginning the water addition,
the reaction rate would have again been controlled by the water addition rate.
In this case, the water was added at near-stoichiometric concentrations virtu-
ally instantaneously, resulting in an uncontrolled exotherm.
3.2.3 Botch Pharmaceutical Reactor Accident
While two operators were charging fiber drums containing a penicillin
powder into a reactor containing a mixture of acetone and methanol, an explo-
sion occurred at the reactor manhole. The two operators were blown back by
the force of the explosion, and were covered with solvent-wet powder.
The incident was initiated by the ignition of solvent vapors, which
resulted in a dust explosion of the dry powder. The solvent liquid mixture in
the reactor did not ignite. Tests on the polyethylene liner inside the fiber
drums, which had been grounded at the time of the incident, showed that they
were of the non-conducting type. The most probable cause of the ignition was
an electrostatic discharge from the polyethylene liner during reactor charging.
After this accident, the company instituted the following procedures
(Drogaris 1993):
• Requiring nitrogen inerting when pouring dry solids into flammable
solvents
• Adding dry powder to the reactor by means of grounded metal scoops,
where possible, rather than by pouring in directly from drums with
polyethylene liners
• Using only conductive polyethylene liners
• Using a closed charging system rather than pouring dry powders into
flammable solvents directly via an open manhole
• Performing an electrostatic hazard review of the whole plant and all the
processes whenever powders and flammable solvents are used
Ed. Note: Even though this incident involved a reactor, it applies as well to any
vessel, open-manhole, charging operation. Most likely the liners were loose and the
operators not grounded. If fixed liners were in place and the operators grounded, the
accident might not have occurred.
3.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
The information on equipment failure scenarios and associated design
solutions is introduced in table format in this chapter and followed in each
subsequent equipment chapter. The organization of the tables is the same in
each chapter. The table headings used are described below.
• Operational Deviation—generic operational parameter deviation such
as overpressure. Analogous to HAZOP parameter deviation.
• Failure Scenario—specific failure mechanism/cause for specified
generic parameter deviation (e.g., overpressure due to upstream control
system failure).
• Potential Design Solution—potential design solutions that could be
considered to reduce the risk of the failure scenario. For the reasons
given in Chapter 2, the design solutions are grouped into the following
three categories: inherently safer/passive, active and procedural.
Vessel failure scenarios, along with associated design solutions, are pre-
sented in Table 3. Design solutions are provided for each scenario, although
some scenarios do not have practical design solutions for all categories. Opera-
tional deviations marked with (T) are discussed in further detail in the chapter
text.
3.4 DISCUSSION
3.4. / Use of Potential Design Solutions Table
It should be recognized that the design solutions presented are possible
approaches for reducing the risk of the associated failure scenario. The authors
of this book could not anticipate all the possible applications nor conditions
that may pertain to a specific design situation. Also, the design solutions are
not necessarily equivalent in terms of benefit in reducing the risk of the stated
hazard scenario. Therefore, it is intended that the table be used in conjunction
with the design basis selection methodology presented in Chapter 2 to arrive
at the optimal design solution for a given application. Furthermore, some
solutions are not applicable to all classes of vessels. (For example, designing
for maximum expected deflagration pressure is not practical for tanks designed
to API Std 650 (1988) but should be considered for some pressure vessels.)
Use of the design solutions presented in Table 3 should be combined with
sound engineering judgment and consideration of all relevant factors. For
example, let us assume that it is decided that a nitrogen blanketing system will
be installed on an atmospheric storage tank to reduce the risk of internal explo-
sion. Typically nitrogen supply pressures are significantly higher than the
design pressure of a storage tank designed to API Std 650 (1988). Conse-
quently the total system design also needs to address the hazard of overpres-
sure due to uncontrolled opening of a high pressure utility system.
This example illustrates an important aspect of the intended use of the
equipment failure tables. The design and installation of safety systems, espe-
cially active systems, can also introduce potential hazards that were not origi-
nally present. Therefore, it is necessary to use the table in the context of the
total design concept to insure that all hazards have been considered. As shown
in the example, this may involve combining several scenario design solutions
to arrive at a final acceptable design. Consequently, the table should be con-
sulted at various stages of the design to reaffirm that all failure mechanisms are
considered.
Utilizing several design solutions for the same scenario is also possible and
often desirable. Again referring to the design of a flammable liquid storage
tank, employing ignition source controls (e.g., non-splash filling, grounding)
as well as vapor space inerting may be desirable based on the consequences of
catastrophic tank failure.
In addition to providing the required degree of reliability for any one fail-
ure scenario, multiple safeguards may be the optimum approach to process
deviations caused by very different failure scenarios. For example, suppose a
vessel can be overpressured by deflagration in the vapor space in one scenario
and by runaway reaction in another scenario. The deflagration event may be
characterized by a high pressure rise rate but a modest pressure rise ratio. The
reaction runaway may be characterized by a very high pressure rise ratio but a
modest reaction rate early in the runaway. With this disparity in the scenarios,
the optimum safeguard design might be pressure containment for the defla-
gration and emergency pressure relief for the runaway reaction. In this situa-
tion, these safeguards are not redundant.
3.4.2 Special Considerations
The tables contain numerous design solutions derived from a variety of
sources and actual situations. Many of the solutions are readily understood. In
some instances, additional explanation is warranted to fully appreciate the