network of components around a relief device, including the pipe to the relief,
the relief device, discharge pipelines, knockout drum, scrubber, flare, or
other types of equipment that assist in the safe relief process.
system is most important system, device which provide safety factor from over
pressurization, (P> MAVVP) to defence against excessive pressure is to
install relief systems to relieve liquids or gases before excessive pressure
are developed. 2
absence of pressure relief approved only by pressure safety engineer. Safe
installation of pressure relief devices.
Figure 3.1 Relief method
The method used for the
safe installation of pressure relief devices is illustrated in Fig.
3.1. The first step in the
procedure is to specify where relief devices must be installed. Definitive guidelines
are available. Second, the appropriate relief device type must be selected.
The type depends mostly
on the nature of the material relieved and the relief characteristics required.
Third, scenarios are developed that describe the various ways in which a relief
can occur. The motivation is to determine the material mass flow rate through
the relief and the physical state of the material (liquid, vapour, or two
phases). Next, data are collected on the relief process, including physical
properties of the ejected material, and the relief is sized. Finally, the
worst-case scenario is selected and the final relief design is achieved.
of relief:- when system contain hazardous, flammable, toxic, oxygen, irritant,
radioactive material which is operated above 15psig pressure, when gas pressure
above 1 MPa ,liquid pressure above 10MPa ,system contain 75,000 ft-lb energy
than location of relief is most important factor. Location of this system such
that when this material is discharge it should not affect employees, equipment
of relief type: – Selection of relief depends on process condition, physical
and chemical properties of relieved fluid
scenarios: – A relief scenario is a description of different relief events, and
the worst case, is use for design relief. Size of reliefs is depends on type of
flow (single phase or two phase).
3.1 TYPES OF
spring-operated valves the adjustable spring tension offsets the inlet
pressure. The relief set pressure is usually specified at 10% above the normal
operating pressure. To avoid the possibility of an unauthorized person changing
this setting, the adjustable screw is covered with a threaded cap. There are
three subcategory types of spring-loaded pressure reliefs.
The relief valve is primarily for liquid service. The
relief valve (liquid only) begins to open at the set pressure. This valve
reaches full capacity when the pressure reaches 25% overpressure. The valve
closes as the pressure returns to the set pressure.
The safety relief valve is used for liquid and gas service.
Safety relief valves function as relief valves for liquids and as safety valves
The safety valve is for gas service. Safety valves
pop open when the pressure exceeds the set pressure. This is accomplished by
using a discharge nozzle that directs high-velocity material toward the valve
seat. After blowdown of the excess pressure, the valve reseats at approximately
4% below the set pressure; the valve has a 4% blowdown.
discs are specially designed to rupture at a specified relief set pressure.
They usually consist of a calibrated sheet of metal designed to rupture at a
well-specified pressure. They are used alone, in series, or in parallel to
spring-loaded relief devices.
problem with rupture discs is the flexing of the metal as process pressures
change. Flexing could lead to premature failure at pressures below the set
pressure. For this reason some rupture disc systems are designed to operate at
pressures well below the set pressure. Another problem with rupture disc
systems is that once they open, they remain open. This may lead to the complete
discharge of process material. It may also allow air to enter the process,
leading to a possible fire and/or explosion. In some accidents discs were
ruptured without the process operator being aware of the situation. 2
this problem, rupture discs are available with embedded wires that are cut when
the disc ruptures; this can activate an alarm in the control room to alert the operator.
Rupture discs are frequently installed in series to a spring-loaded relief. 2
1. To protect an expensive
spring-loaded device from a corrosive environment,
2. To give absolute isolation when
handling extremely toxic chemicals (spring-loaded reliefs may weep),
3. To give absolute isolation when
handling flammable gases,
4. To protect the relatively complex
parts of a spring-loaded device from reactive monomers that could cause
5. To relieve slurries that may plug
spring-loaded devices. When rupture discs are used before a spring-loaded
relief, a pressure gauge is installed between the two devices.
Figure 3.2 Major types
of relief devices 2
Buckling – Pin Reliefs
A buckling-pin relief is similar to a rupture
disc; that is, when the pressure buckles the pin, the valve opens fully. As
shown in Figure3..3 this is a relatively simple device. The major advantage of a
buckling-pin relief is that the pin buckles at a precise pressure, and the
major disadvantage of this device is that when the pin buckles, the valve opens
and stays open. 2
Figure 3.3 Diagram of
Buckling pin reliefs 2
The main valve of a pilot-operated relief valve is controlled
by a smaller pilot valve that is a spring-operated relief valve as shown in
Figure 3.4. When the pilot valve reaches the set pressure, it opens and
releases the pressure above the main valve. The large valve piston then opens
and exhausts the system fluid. The pilot and main valves reseat when the inlet
pressure drops below the set pressure. Pilot-operated relief valves are
commonly used when a large relieving area at high set pressures is required.
The set pressure of this type of valve can be very close to the operating
pressure. Pilot operated valves are frequently chosen when operating pressures
are within 5% of set pressures. The pilot valve exhausts either to the outlet
of the main valve or to the atmosphere. Pilot-operated relief valves are
commonly used in clean services. 2
Figure 3.4 Diagram of
Pilot operated reliefs 2
The major advantages and
disadvantages of the different types of reliefs are shown in table 3.1.
Table 3.1 Advantage and disadvantage of relief valves 2
Type of relief valve
Ø Very reliable
Ø Used in many services
Ø Reseats at pressure 4% below set
Ø Relief pressure affected by back
Spring operated (Balance bellows)
Ø Relief pressure not affected by
Ø Handles higher build up
Ø Protects springs from corrosion
Ø Bellows may ruptures
Ø Flow is function of back pressure
Ø May release flammable or toxic to
Ø No seal leakage
Ø Low cost and easy to replace
Ø Good for high volume release
Ø Less fouling or plugging
Ø Stay open after relief
Ø Burst pressure can’t be tested
Ø Required periodic replacement
Ø Sensitive to mechanical damage
Ø No fatigue problems
Ø Relief pressure are more accurate
than conventional devices
Ø Set pressure is not sensitive to
Ø Replacing pins is very easy and
Ø Elastomer seals limit temperature
to about 450 0F
Ø Initial cost is greater than for
Ø Relief pressure not affected by
Ø Can operated at pressure up to 98
% of set pressure
Ø Seals tightly even at pressure
approaching set pressure
Ø Main valve snaps fully open at low
Ø Limited to chemical and
temperature constrains of the seals
Ø Condensation and liquid
accumulation above the main piston may cause the problems
Ø Potential for back flow
3.2 RELIEF SCENARIOS
relief scenario is a description of one specific relief event. Usually each
relief has more than one relief event, and the worst-case scenario is the
scenario or event that requires the largest relief vent area. 2
of relief events are
1. A pump is dead-headed; the pump
relief is sized to handle the full pump capacity at its rated pressure.
2. The same pump relief is in a line
with a nitrogen regulator; the relief is sized to handle the nitrogen if the
3. The same pump is connected to a heat
exchanger with live steam; the relief is sized to handle steam injected into
the exchanger under uncontrolled conditions, for example, a steam regulator
is a list of scenarios for one specific relief. The relief vent area is
subsequently computed for each event (scenario) and the worst-case scenario is
the event requiring the largest relief vent area. The worst cases are a subset
of the overall developed scenarios for each relief. For each specific relief
all possible scenarios are identified and catalogued. This step of the relief
method is extremely important: The identification of the actual worst-case
scenario frequently has a more significant effect on the relief size than the accuracy
of relief sizing calculations. 2
Regardless of how carefully the relief is sized, specified, and
tested, a poor installation can result in completely unsatisfactory relief
performance. Some installation guidelines are illustrated in figure 3.5
Figure 3.5 Relief
installation practices. 2
As per ASME-American Standard Mechanical Engineering
B13-3, Chapter 6, High pressure piping system have 10 parts
design of high pressure component
service requirements for piping component
service requirements for piping joints
for piping component
assembly and erection
examination and testing
Applicability for piping designated by the owner as
being in high pressure fluid service. Its requirement are to be applied in full
to piping so designated. High pressure is considered to be pressure in excess of
that allowed by the ASME B16.5 PN 420 rating for the specific design
temperature and material group.
Instrumented Piping: Instrumented piping within the
scope of high pressure piping involves, sealed fluid filled tubing system, with
instruments as temperature orb pressure responsibility devices, control piping
for air, hydraulically.
capacity of the pressure relieving services shall be sufficient to prevent the
pressure from rising more than 10%above the pipe pressure at the operating
temperature during the reliving condition for the single reliving device or
more than 16% above the design pressure more than one device is provide except
as a provided in 3 below.
protection must include one relief device set at or below the design pressure
at the operating temperature for the reliving condition with one device set to
operate a pressure greater than 105% of the device pressure provided 3 below.
pressure reliving device provide for protection against over pressure due to
fire or other unexpected source of external heat shall be set to operate
pressure not greater than 110% of the design pressure of the piping system and
shall be capable the liming the maximum pressure during relief no more than
121% of the design pressure.
The allowable stress is provided in appendix Table K-1 which
is equal to two third of the material yield strength. For solution heat treated
austenite SS and certain nickel alloys with similar stress-strain behaviour,
the minimum of two-thirds the specified minimum yield strength and 90 percent
of the yield strength at temp is used. Similar to base code, this is because
the material has significant strength beyond the nominal 0.2 percent offset
As in the base code, materials
may be used above the maximum temperature foe which allowable stresses are
provided. The temperature must be below that at which creep properties would
govern the allowable stress that would be determined using the base Code
allowable stress criteria.
materials may be used provide the conform to published specification covering
chemistry, physical and mechanical properties. Method and process of
manufacture, heat treatment and quality controls and otherwise meet requirement
of chapter IX.
The impact test requirements are a
very important part of the materials requirements in chapter IX for the high
pressure piping. Essentially all high pressure piping materials and welds must
be impact-tested to determine that they have enough notch toughness for any
temperature condition at which stresses exceed 41 MPa (6 psi).transverse
specimens are required, unless the component size or shape does not permit
cutting transverse specimens.in that case, longitudinal specimens may be used.
However the required impact energy absorption is higher.
For the materials, at least one set
of impact tests per lot is required.
For the impact tests on welds,
significantly more testing is required than in the base code.
For test on welds, separate tests
are not required for each lot of material. Test specimens for the welds and
heat affected ones are required.
The minimum permissible temperature
for a material is the minimum temperature at which an impact test that
satisfies the code requirements was performed. The only exception to this is
the 41-MPa exemption, but that exemption may only be used down to -46°C.Impact
testing, regardless of stress, is required for use at temperatures below that
Normally in process industry safety is provide by relief devices, valves
which is mechanical method. High integrity pressure protecting system is
instrumented system which protects the system from overpressure.
to the complexity, there is increased pressure from community and regulatory
authorities to reduce venting and combustion of gases. It is now unacceptable
to flare large volumes of gas. The need to balance safety requirements and
environmental requirements has resulted in increased focus on using an
alternative approach to pressure protection.
521 and Code Case 2211 of ASME Section VIII, Division 1 and 2, provide an
alternative to pressure relief devices. Which is the use of an instrumented
system to protect against overpressure. The safety instrument system (SIS).it
is design by international standard IEC 61511.For overpressure protection
results in high SIS integrity; therefore, these system is known as High
Integrity Pressure Protection Systems (HIPPS) or High Integrity Protection
Shutdowns (HIPS).High integrity pressure protecting
system is when pressure reliving devices are impractical typical cases are. 6
reactions so fast the pressure propagation rate could result in loss of containment
prior to the relief device opening. Examples are “hot spots,” decompositions,
and internal detonation, fires.
reactions so fast the lowest possible relieving rate yields impractically large
reactions occurring at uncontrollable rates, resulting in a very high
propagation rate for the process pressure. (The pressure propagation rate for
these reactions is often poorly understood)
polymerization, or deposition formed during normal operation, which have
historically partially or completely blocked pressure relief devices;
process chemicals relieved into lateral headers with polymerization and thus
plugging, rendering the relief device useless;
venting, where actual vent rate is difficult to predict;
relief device installation creates additional hazards, due to its vent
overpressure protection can be provided by a SIS in lieu of a pressure relief
device under the following conditions. 6
The vessel is not
exclusively in air, water, or steam service.
The decision to utilize
overpressure protection of a vessel by system design is the responsibility of
the user. The manufacturer is responsible only for verifying that the user has
specified overpressure protection by system design, and for listing Code Case
2211 on the Data Report.
The user must ensure the
MAWP of the vessel is higher than the highest pressure that can reasonably be
achieved by the system.
qualitative risk analysis of the proposed system must be made addressing:
credible overpressure scenarios, demonstrating the proposed system is
independent of the potential causes for overpressure; is as reliable as the
pressure relief device it replaces; and is capable of completely mitigating the
HIGH INTEGRITY PRESSURE PROTECTING
The effective design of a pressure vessel and piping system
entails that all components operate safely and according to their design
objectives under all the conditions, including upset conditions. The upset
conditions such as fire, blocked outlet, control valve failure and others can
lead to excessively high pressure in the system.
When the pressure due to such an incident exceeds the
design pressure of the system, it can cause the rupture of the vessel or piping
which in turn can cause severe hazard for human life, plant assets or the
environment. Therefore, it is essential to protect the system from the effects
There are two ways of protecting the system from
overpressure. One, by using mechanical devices such as a pressure safety valve
(PSV). Other, by using a safety instrumented system such as high integrity
pressure protection system (HIPPS). An HIPPS protects the pressure vessel and
piping systems by removing the source of overpressure when pressure in the
system reaches a pre-set value. This value must be less than or equal to the
design pressure of the system.
system has main three component 6
process variables (PV) commonly measured in HIPPS are pressure, temperature and
flow. Traditionally, these variables were monitored using discrete switches as
the input sensor to the safety instrumented systems. Switches worked well for
three reasons: 1) Most trip conditions are discrete events, i.e., a high
pressure, high temperature, or low flow; 2) Relay systems and early
programmable logic controllers (PLCs) processed discrete signal much easier
than analog signals; and 3) Switches were usually less expensive than analog transmitters.
Process sensors detect the parameters which is input.
independence of the safety logic reduces the probability that a loss of the
basic process control system hardware will result in the loss of HIPPS
functioning. From a software standpoint, independence also reduces the
possibility that inadvertent changes to the HIPPS safety functionality could
occur during modification of basic process control functions.Logic solver
processes the input from the sensors to an output to the final element.
The majority of HIPPS utilize dual devices in
a 1oo2 configuration. The final elements are typically either 1) relays in the
motor control circuit for shutdown of motor operated valves, compressors, or
pumps or 2) fail safe valves opened or closed using solenoids in the instrument
both PSV and HIPPS serve the purpose of providing protection against
overpressure, they function entirely different. A PSV is a mechanical device
whereas HIPPS is an instrumented system. The advantages and disadvantages of
one over the other are underpinned by the fact that the PSV provides
overpressure protection by releasing containments from the system into
atmosphere (often after flaring) whereas HIPPS does the same by shutting the
source of the overpressure.
5.1 Comparison between HIPPS and PSV 6
is safety instrumented system
is a mechanical safety device
provides protection against overpressure by closing source of overpressure.
provides protection against overpressure by releasing the excess fluids from
of HIPPS leads to shutdown of the system
of PSV leads to discharge of contents from the system without shutting it down
does not require any disposal system
may require a disposal system like flare system for disposing the discharge
contents from the system
cannot be applied for every overpressure scenario. Like it cannot be applied
for fire, thermal expansion
in general can be applied for each and every commonly known overpressure