Thenetwork of components around a relief device, including the pipe to the relief,the relief device, discharge pipelines, knockout drum, scrubber, flare, orother types of equipment that assist in the safe relief process.Reliefsystem is most important system, device which provide safety factor from overpressurization, (P> MAVVP) to defence against excessive pressure is toinstall relief systems to relieve liquids or gases before excessive pressureare developed. 2Theabsence of pressure relief approved only by pressure safety engineer. Safeinstallation of pressure relief devices.Figure 3.
1 Relief method2 The method used for thesafe installation of pressure relief devices is illustrated in Fig.3.1. The first step in theprocedure is to specify where relief devices must be installed. Definitive guidelinesare available. Second, the appropriate relief device type must be selected.
The type depends mostlyon the nature of the material relieved and the relief characteristics required.Third, scenarios are developed that describe the various ways in which a reliefcan occur. The motivation is to determine the material mass flow rate throughthe relief and the physical state of the material (liquid, vapour, or twophases). Next, data are collected on the relief process, including physicalproperties of the ejected material, and the relief is sized. Finally, theworst-case scenario is selected and the final relief design is achieved.2Locationof relief:- when system contain hazardous, flammable, toxic, oxygen, irritant,radioactive material which is operated above 15psig pressure, when gas pressureabove 1 MPa ,liquid pressure above 10MPa ,system contain 75,000 ft-lb energythan location of relief is most important factor. Location of this system suchthat when this material is discharge it should not affect employees, equipmentetc.
2Selectionof relief type: – Selection of relief depends on process condition, physicaland chemical properties of relieved fluidReliefscenarios: – A relief scenario is a description of different relief events, andthe worst case, is use for design relief. Size of reliefs is depends on type offlow (single phase or two phase). 3.1 TYPES OFRELIEF DEVICES 1. Spring operated Onspring-operated valves the adjustable spring tension offsets the inletpressure. The relief set pressure is usually specified at 10% above the normaloperating pressure. To avoid the possibility of an unauthorized person changingthis setting, the adjustable screw is covered with a threaded cap.
There arethree subcategory types of spring-loaded pressure reliefs.2 1. The relief valve is primarily for liquid service. Therelief valve (liquid only) begins to open at the set pressure.
This valvereaches full capacity when the pressure reaches 25% overpressure. The valvecloses as the pressure returns to the set pressure. 2. The safety relief valve is used for liquid and gas service.
Safety relief valves function as relief valves for liquids and as safety valvesfor gases.3. The safety valve is for gas service. Safety valvespop open when the pressure exceeds the set pressure. This is accomplished byusing a discharge nozzle that directs high-velocity material toward the valveseat. After blowdown of the excess pressure, the valve reseats at approximately4% below the set pressure; the valve has a 4% blowdown.
2. Rupture discs Rupturediscs are specially designed to rupture at a specified relief set pressure.They usually consist of a calibrated sheet of metal designed to rupture at awell-specified pressure. They are used alone, in series, or in parallel tospring-loaded relief devices. An importantproblem with rupture discs is the flexing of the metal as process pressureschange. Flexing could lead to premature failure at pressures below the setpressure. For this reason some rupture disc systems are designed to operate atpressures well below the set pressure.
Another problem with rupture discsystems is that once they open, they remain open. This may lead to the completedischarge of process material. It may also allow air to enter the process,leading to a possible fire and/or explosion.
In some accidents discs wereruptured without the process operator being aware of the situation. 2 To preventthis problem, rupture discs are available with embedded wires that are cut whenthe 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. 21. To protect an expensivespring-loaded device from a corrosive environment,2.
To give absolute isolation whenhandling extremely toxic chemicals (spring-loaded reliefs may weep), 3. To give absolute isolation whenhandling flammable gases, 4. To protect the relatively complexparts of a spring-loaded device from reactive monomers that could causeplugging, 5. To relieve slurries that may plugspring-loaded devices.
When rupture discs are used before a spring-loadedrelief, a pressure gauge is installed between the two devices. Figure 3.2 Major typesof relief devices 2 3. Buckling – Pin ReliefsA buckling-pin relief is similar to a rupturedisc; that is, when the pressure buckles the pin, the valve opens fully. Asshown in Figure3..3 this is a relatively simple device. The major advantage of abuckling-pin relief is that the pin buckles at a precise pressure, and themajor disadvantage of this device is that when the pin buckles, the valve opensand stays open.
2 Figure 3.3 Diagram ofBuckling pin reliefs 2 4. Pilot OperatedReliefsThe main valve of a pilot-operated relief valve is controlledby a smaller pilot valve that is a spring-operated relief valve as shown inFigure 3.4. When the pilot valve reaches the set pressure, it opens andreleases the pressure above the main valve. The large valve piston then opensand exhausts the system fluid. The pilot and main valves reseat when the inletpressure drops below the set pressure. Pilot-operated relief valves arecommonly 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 operatingpressure. Pilot operated valves are frequently chosen when operating pressuresare within 5% of set pressures. The pilot valve exhausts either to the outletof the main valve or to the atmosphere. Pilot-operated relief valves arecommonly used in clean services. 2Figure 3.4 Diagram ofPilot operated reliefs 2The major advantages anddisadvantages 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 Advantages Disadvantage Spring Operated Ø Very reliable Ø Used in many services Ø Reseats at pressure 4% below set pressure Ø Relief pressure affected by back pressure. Spring operated (Balance bellows) Ø Relief pressure not affected by back pressure. Ø Handles higher build up backpressure Ø Protects springs from corrosion Ø Bellows may ruptures Ø Flow is function of back pressure Ø May release flammable or toxic to atmosphere Rupture disk 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 Buckling Pin Ø No fatigue problems Ø Relief pressure are more accurate than conventional devices Ø Set pressure is not sensitive to operating temperature Ø Replacing pins is very easy and not expensive Ø Elastomer seals limit temperature to about 450 0F Ø Initial cost is greater than for rupture disk Pilot operated Ø Relief pressure not affected by back pressure 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 overpressure 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 Arelief scenario is a description of one specific relief event. Usually eachrelief has more than one relief event, and the worst-case scenario is thescenario or event that requires the largest relief vent area. 2Examplesof relief events are1.
A pump is dead-headed; the pumprelief is sized to handle the full pump capacity at its rated pressure.2. The same pump relief is in a linewith a nitrogen regulator; the relief is sized to handle the nitrogen if theregulator fails.3. The same pump is connected to a heatexchanger with live steam; the relief is sized to handle steam injected intothe exchanger under uncontrolled conditions, for example, a steam regulatorfailureThisis a list of scenarios for one specific relief. The relief vent area issubsequently computed for each event (scenario) and the worst-case scenario isthe event requiring the largest relief vent area. The worst cases are a subsetof the overall developed scenarios for each relief. For each specific reliefall possible scenarios are identified and catalogued.
This step of the reliefmethod is extremely important: The identification of the actual worst-casescenario frequently has a more significant effect on the relief size than the accuracyof relief sizing calculations. 2 3.3 RELIEFINSTALLATION Regardless of how carefully the relief is sized, specified, andtested, a poor installation can result in completely unsatisfactory reliefperformance. Some installation guidelines are illustrated in figure 3.
5Figure 3.5 Reliefinstallation practices. 2 HighPressure PipingAs per ASME-American Standard Mechanical EngineeringB13-3, Chapter 6, High pressure piping system have 10 parts1. Conditionand Criteria2.
Pressuredesign of high pressure component3. Fluidservice requirements for piping component4. Fluidservice requirements for piping joints5. Flexibilityand support6. System7. Materials8. Standardfor piping component9.
Fabrication,assembly and erection10. Inspection,examination and testingApplicability for piping designated by the owner asbeing in high pressure fluid service. Its requirement are to be applied in fullto piping so designated. High pressure is considered to be pressure in excess ofthat allowed by the ASME B16.5 PN 420 rating for the specific designtemperature and material group.3.1 SystemInstrumented Piping: Instrumented piping within thescope of high pressure piping involves, sealed fluid filled tubing system, withinstruments as temperature orb pressure responsibility devices, control pipingfor air, hydraulically.
Overpressure Protection:1. Thecapacity of the pressure relieving services shall be sufficient to prevent thepressure from rising more than 10%above the pipe pressure at the operatingtemperature during the reliving condition for the single reliving device ormore than 16% above the design pressure more than one device is provide exceptas a provided in 3 below.2. Systemprotection must include one relief device set at or below the design pressureat the operating temperature for the reliving condition with one device set tooperate a pressure greater than 105% of the device pressure provided 3 below.
3. Supplementarypressure reliving device provide for protection against over pressure due tofire or other unexpected source of external heat shall be set to operatepressure not greater than 110% of the design pressure of the piping system andshall be capable the liming the maximum pressure during relief no more than121% of the design pressure. 3.2 MaterialThe allowable stress is provided in appendix Table K-1 whichis equal to two third of the material yield strength. For solution heat treatedaustenite SS and certain nickel alloys with similar stress-strain behaviour,the minimum of two-thirds the specified minimum yield strength and 90 percentof the yield strength at temp is used. Similar to base code, this is becausethe material has significant strength beyond the nominal 0.
2 percent offsetyield stress.As in the base code, materialsmay be used above the maximum temperature foe which allowable stresses areprovided. The temperature must be below that at which creep properties wouldgovern the allowable stress that would be determined using the base Codeallowable stress criteria.
Unlistedmaterials may be used provide the conform to published specification coveringchemistry, physical and mechanical properties. Method and process ofmanufacture, heat treatment and quality controls and otherwise meet requirementof chapter IX.The impact test requirements are avery important part of the materials requirements in chapter IX for the highpressure piping. Essentially all high pressure piping materials and welds mustbe impact-tested to determine that they have enough notch toughness for anytemperature condition at which stresses exceed 41 MPa (6 psi).transversespecimens are required, unless the component size or shape does not permitcutting transverse specimens.in that case, longitudinal specimens may be used.
However the required impact energy absorption is higher.For the materials, at least one setof 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 testsare not required for each lot of material.
Test specimens for the welds andheat affected ones are required. The minimum permissible temperaturefor a material is the minimum temperature at which an impact test thatsatisfies the code requirements was performed. The only exception to this isthe 41-MPa exemption, but that exemption may only be used down to -46°C.Impacttesting, regardless of stress, is required for use at temperatures below thattemperature. Normally in process industry safety is provide by relief devices, valveswhich is mechanical method. High integrity pressure protecting system isinstrumented system which protects the system from overpressure.Addingto the complexity, there is increased pressure from community and regulatoryauthorities to reduce venting and combustion of gases. It is now unacceptableto flare large volumes of gas.
The need to balance safety requirements andenvironmental requirements has resulted in increased focus on using analternative approach to pressure protection.API521 and Code Case 2211 of ASME Section VIII, Division 1 and 2, provide analternative to pressure relief devices. Which is the use of an instrumentedsystem to protect against overpressure. The safety instrument system (SIS).itis design by international standard IEC 61511.For overpressure protectionresults in high SIS integrity; therefore, these system is known as HighIntegrity Pressure Protection Systems (HIPPS) or High Integrity ProtectionShutdowns (HIPS).High integrity pressure protectingsystem is when pressure reliving devices are impractical typical cases are. 61.
Chemicalreactions so fast the pressure propagation rate could result in loss of containmentprior to the relief device opening. Examples are “hot spots,” decompositions,and internal detonation, fires.2. Chemicalreactions so fast the lowest possible relieving rate yields impractically largevent areas;3. Exothermicreactions occurring at uncontrollable rates, resulting in a very highpropagation rate for the process pressure. (The pressure propagation rate forthese reactions is often poorly understood)4. Plugging,polymerization, or deposition formed during normal operation, which havehistorically partially or completely blocked pressure relief devices; 5. Reactiveprocess chemicals relieved into lateral headers with polymerization and thusplugging, rendering the relief device useless; 6.
Multi-phaseventing, where actual vent rate is difficult to predict; 7. Pressurerelief device installation creates additional hazards, due to its ventlocation. Theoverpressure protection can be provided by a SIS in lieu of a pressure reliefdevice under the following conditions. 61.
The vessel is notexclusively in air, water, or steam service. 2. The decision to utilizeoverpressure protection of a vessel by system design is the responsibility ofthe user. The manufacturer is responsible only for verifying that the user hasspecified overpressure protection by system design, and for listing Code Case2211 on the Data Report.
3. The user must ensure theMAWP of the vessel is higher than the highest pressure that can reasonably beachieved by the system. 4.
Aantitative orqualitative risk analysis of the proposed system must be made addressing:credible overpressure scenarios, demonstrating the proposed system isindependent of the potential causes for overpressure; is as reliable as thepressure relief device it replaces; and is capable of completely mitigating theoverpressure event.HIGH INTEGRITY PRESSURE PROTECTINGSYSTEM (HIPPS)The effective design of a pressure vessel and piping systementails that all components operate safely and according to their designobjectives under all the conditions, including upset conditions. The upsetconditions such as fire, blocked outlet, control valve failure and others canlead to excessively high pressure in the system.When the pressure due to such an incident exceeds thedesign pressure of the system, it can cause the rupture of the vessel or pipingwhich in turn can cause severe hazard for human life, plant assets or theenvironment. Therefore, it is essential to protect the system from the effectsof over-pressure.6There are two ways of protecting the system fromoverpressure.
One, by using mechanical devices such as a pressure safety valve(PSV). Other, by using a safety instrumented system such as high integritypressure protection system (HIPPS). An HIPPS protects the pressure vessel andpiping systems by removing the source of overpressure when pressure in thesystem reaches a pre-set value. This value must be less than or equal to thedesign pressure of the system. Thesystem has main three component 61. Process sensors:Theprocess variables (PV) commonly measured in HIPPS are pressure, temperature andflow.
Traditionally, these variables were monitored using discrete switches asthe input sensor to the safety instrumented systems. Switches worked well forthree reasons: 1) Most trip conditions are discrete events, i.e.
, a highpressure, high temperature, or low flow; 2) Relay systems and earlyprogrammable logic controllers (PLCs) processed discrete signal much easierthan analog signals; and 3) Switches were usually less expensive than analog transmitters.Process sensors detect the parameters which is input.2. Logical solver:Adequateindependence of the safety logic reduces the probability that a loss of thebasic process control system hardware will result in the loss of HIPPSfunctioning. From a software standpoint, independence also reduces thepossibility that inadvertent changes to the HIPPS safety functionality couldoccur during modification of basic process control functions.Logic solverprocesses the input from the sensors to an output to the final element.
3. Final element: The majority of HIPPS utilize dual devices ina 1oo2 configuration. The final elements are typically either 1) relays in themotor control circuit for shutdown of motor operated valves, compressors, orpumps or 2) fail safe valves opened or closed using solenoids in the instrumentair supply.
Whileboth PSV and HIPPS serve the purpose of providing protection againstoverpressure, they function entirely different. A PSV is a mechanical devicewhereas HIPPS is an instrumented system. The advantages and disadvantages ofone over the other are underpinned by the fact that the PSV providesoverpressure protection by releasing containments from the system intoatmosphere (often after flaring) whereas HIPPS does the same by shutting thesource of the overpressure. Table5.1 Comparison between HIPPS and PSV 6 HIPPS PSV HIPPS is safety instrumented system PSV is a mechanical safety device HIPPS provides protection against overpressure by closing source of overpressure. PSV provides protection against overpressure by releasing the excess fluids from the system Activation of HIPPS leads to shutdown of the system Activation of PSV leads to discharge of contents from the system without shutting it down HIPPS does not require any disposal system PSV may require a disposal system like flare system for disposing the discharge contents from the system HIPPS cannot be applied for every overpressure scenario.
Like it cannot be applied for fire, thermal expansion PSV in general can be applied for each and every commonly known overpressure system scenario