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ECE 4353
Chemical Process Safety
Relief Concepts, Systems and
Scenarios
(Lecture 12)
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LEARNING OBJECTIVES
Explain the government-industry’s responsibility for
health and safety
Evaluate the nature of hazards posed by materials
which are flammable, toxic and reactive
Identify and quantify common industrial methods to
control hazards.
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Lecture 12
12.1 Pressure Relief Systems
12.2 Overpressurisation
12.3 Location of reliefs
12.4 Definitions and Code Requirements
12.5 Types of Relief Devices
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Layers of Protection
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What are
Pressure Relief systems
for?
12.1 Pressure Relief Systems
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12.1 Pressure Relief Systems
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Pressure relief systems are needed for the following
reasons:
Protect personnel from the dangers of overpressurizing
equipment.
Prevent damage to equipment.
Prevent damage to adjoining property.
Minimise chemical losses during pressure upsets.
Reduce insurance premiums.
12.1 Pressure Relief Systems
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Pressure relief systems prevent pressurization above
a system’s design pressure; for venting during an unusual
or emergency situation; and for normal depressurization
during a shutdown.
12.1 Pressure relief systems
When venting non-combustible gases including steam, air and nitrogen may vent into the
atmosphere. When venting flammable or toxic compounds, an elaborate systems for the disposal of vented gases may be required.
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Causes of overpressurization according to API include:
• Blocked discharge
• Failure of control valve
• External fire
• Utility failure : Cooling water failure, Power failure or
Instrument air failure
• Heat-exchanger-tube failure
12.2 Overpressurization
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Blocked discharge Closure of block valve at outlet exposing the vessel to a pressure that may
exceed the maximum allowable working pressure (MAWP).
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Blocked discharge Closure of block valve at outlet exposing the vessel to a pressure that may
exceed the maximum allowable working pressure (MAWP).
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Failure of Control valve Failure of control valve due to controls malfunction or valve stuck close or fail
close may lead to overpressure in process vessel.
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Failure of Control valve
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External fire If the vessel has some liquid inventory, liquid inside will absorb the heat through
the vessel wall when it is exposed to an external fire. If the liquid inventory is
low or if the content is a gas, the heat from external fire is not dissipated by
vaporizing liquid; resulting in overpressure in the vessel.
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External fire
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Thermal External heat input can cause liquid temperature, and hence volume, to rise.
Eg. A vessel or pipeline containing liquid is heated by direct solar radiation.
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Heat Exchanger tube failure Complete tube rupture, in which a large quantity of high-pressure fluid (from
tube) will flow to the low-pressure side of the exchanger (to shell) results in
shell overpressure.
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Runaway Chemical Reactions A runaway reaction is a chemical reaction over which
control has been lost. It continues to accelerate in
reaction speed until it either runs out of reactants or the
vessel containing it overpressures and loses containment.
During runaway reactions, which tend to accelerate with
rising temperature, internal pressure of a vessel or
pipeline can rise rapidly.
Basically, pressure relief valves may not provide adequate
protection at all because of their relatively slow response
time. In such a situation, vapor depressuring systems,
rupture discs and emergency vents are preferred.
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To specify the location of reliefs requires a review of every
unit operation in:
- the process and
- every process operating step
Anticipate the potential problems which may result in
increased pressures.
Pressure relief devices are installed at every point where
upset conditions create pressures that may exceed MAWP.
12.3 Location of Reliefs
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What happens with
loss of cooling,
heating or
agitation?
What happens if the process is contaminated or has a mischarge of a catalyst or monomer?
What happens if
the unit operation
is engulfed in a
fire?
What happens if the operator makes an error?
What conditions cause
runaway reactions, and how
are relief systems designed to
handle the discharge as a
result of runaway reactions.
What is the consequence of closing valves on vessels or in lines which are filled with liquids and exposed to heat or refrigeration?
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GUIDELINES FOR SPECIFYING RELIEF POSITIONS
Vessel steam jackets are often rated for low pressure steam. Reliefs are installed in jackets to prevent excessive steam pressures due to operator error or regulator failure.
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General Code requirements include:
ASME Boiler & Pressure Vessel Codes
ASME B31.3 / Petroleum Refinery Piping
ASME B16.5 / Flanges & Flanged Fittings
When to install PSV? • All pressure vessels subject to overpressure shall be
protected by a pressure relieving device.
• Liquid filled vessels or piping subject to thermal
expansion must be protected by a thermal relief device.
12.4 Code Requirements
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sIngle PSV on multiple vessels? • Multiple vessels may be protected by a single relief device
provided there is a clear, unobstructed path to the device
Multiple PSVs on a vessel? • At least one pressure relief device must be set at or
below the MAWP
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12.5 Types of relief devices
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Rupture discs
Spring Operated PSV (Conventional and Balanced
Bellows)
Pilot Operated PSV
<video on Direct acting and Pilot operated)
12.5 Types of Relief Devices
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RUPTURE DISCS
Rupture discs are designed to rupture at a specified relief set pressure.
Once they open, they remain open.
Rupture discs are available in much larger sizes than spring operated
relief valves.
Rupture discs cost less than spring loaded PSVs.
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RUPTURE DISCS are installed in series to a spring loaded
relief to:
1. Protect an expensive spring loaded PSV from a corrosive
environment
2. To give positive isolation when handling extremely toxic
chemicals
3. To give positive isolation when handling flammable
gases
4. To prevent plugging of PSVs
An indicator (tell tale gauge) is placed between the rupture
disc and the PSV to indicate when the disc ruptures.
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CONVENTIONAL TYPE PSV
A conventional pressure relief valve is a spring loaded pressure relief
valve whose operational characteristics are
directly affected by changes in the back pressure.
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CONVENTIONAL TYPE
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BALANCED BELLOWS PSV
A balanced pressure relief valve is a spring loaded pressure
relief valve that incorporates a bellows or other means
for minimizing the effect of back pressure on the
operational characteristics of the valve
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Balanced bellows PSV
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PILOT PSV
A pilot operated pressure relief valve is a pressure relief
valve in which the major relieving device or main valve is
combined with and controlled by a self actuated auxiliary
pressure relief valve (pilot).
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PILOT TYPE PSV
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Pressure relief valve: A pressure relief device
designed to open and relieve excess pressure and to
reclose and prevent the further flow of fluid after normal
conditions have been restored.
a. A relief valve is a spring loaded pressure relief valve used
primarily with incompressible fluids.(liquids)
b. A safety valve is a spring loaded pressure relief valve
characterized by rapid opening or pop action. A safety valve
is normally used with compressible fluids.(gas)
c. A safety relief valve is a spring loaded pressure relief
valve that may be used as either a safety or relief valve
depending on the application.