troubleshooting pzv installations

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Engineering EncyclopediaSaudi Aramco DeskTop Standards

Troubleshooting PZV Installations

Engineering Encyclopedia Instrumentation


Saudi Aramco DeskTop Standards

Content Page

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE CHATTERING.......... 1

PZV Chattering and Its Effects .................................................................................. 1

Causes of Chattering and Their Corrective Actions ........................................ 1

Inadequate Piping Support ............................................................................. 2

Excessive Inlet Pressure Drop........................................................................ 2

Oversize Excessive Backpressure................................................................... 2

Incorrect Valve Selection............................................................................... 2

Excessive Flow Velocity ................................................................................ 3

Unstable Process............................................................................................ 3

Incorrect Set Pressure.................................................................................... 3

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING LEAKAGE .................................................................................................... 4

PZV Leakage and Its Effects ..................................................................................... 4

Causes of PZV Leakage and Their Corrective Actions............................................... 4

Inadequate Piping Support ............................................................................. 5

Oversize ........................................................................................................ 5

Excessive Backpressure ................................................................................. 5



Improper Metallurgy...................................................................................... 5

Incorrect Set Pressure.................................................................................... 6

Insufficient Operating Pressure Gap ............................................................... 6

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE BELLOWS........................................................................................... 7

Bellows Failure and its Effects................................................................................... 7

Cause of Bellows Failure and Their Corrective Actions.............................................. 7

Excessive Backpressure ................................................................................. 8






Improper Metallurgy...................................................................................... 8

Valve Chattering............................................................................................ 8

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE SPRING ................................................................................................ 9

PZV Spring Failure and its Effects............................................................................. 9

Causes of PZV Spring Failure and Their Corrective Actions ...................................... 9

Incorrect Valve Selection..............................................................................10

Excessive Flow Velocity ...............................................................................10

Improper Metallurgy.....................................................................................10

Environmental Factors ..................................................................................10

Chattering.....................................................................................................10

Incorrect Spring Design ................................................................................11

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE PILOT CONTROL...............................................................................12

Pilot Control Failure and its Effects ..........................................................................12

Causes of Pilot Controls and Their Corrective Actions..............................................12

Spring Failure ...............................................................................................13

Dirty System Media ......................................................................................13

Excessive Process Temperature ....................................................................13

Excessive Pressure Drop in Sensing Line ......................................................13

Unstable Process...........................................................................................13

WORK AID 1: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE CHATTERING.........14

WORK AID 2: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING LEAKAGE ...................................................................................................15

WORK AID 3: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING SETPOINT DRIFT.......................................................................................16




WORK AID 4: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE BELLOWS...........................................................................................17

WORK AID 5: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE SPRING ...............................................................................................18

WORK AID 6: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE PILOT CONTROL...............................................................................19

GLOSSARY........................................................................................................................20



Saudi Aramco DeskTop Standards 1

DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE CHATTERING

This module will describe the corrective actions from problems associated with PZV installationsincluding:

• relief valve chattering problems

• relief valve leakage problems

• relief valve failures involving setpoint drift

• relief valve failure involving the bellows

• relief valve failure involving the spring

• relief valve failure involving pilot control

Chattering, as discussed in Module 2, is the rapid oscillation of a PZV. The term "chattering"refers to the distinctive sound created by the rapid popping and closing of the valve.

It is important to understand that damage to one part of a PZV can cause damage to other partsin the valve. Chattering can be caused by a bent valve stem in a PZV, and conversely, chatteringcan cause a valve stem to bend. Therefore, PZVs are subjected to periodic testing and inspection(T&I). These testing procedures (including disassembly) as will be discussed in Module 6.Improper maintenance can lead to chattering.

PZV Chattering and Its Effects

At a minimum, chattering will damage the PZV disk and seat. As discussed in Module 4,chattering will also cause bellows failure. Chattering can cause valve stems to bend, and/or springmisalignment which can cause stem problems. Chattering induces vibrational stress throughoutthe PZV.

Causes of Chattering and Their Corrective Actions

The following is a list of common causes of chattering and the actions to be taken to correct theproblems:

• Inlet restrictions to the PZV - Inspect all piping to the PZV.




• Outlet restrictions from the PZV - Inspect all PZV discharge piping.

• Oversized PZV - Addition of a second smaller PZV (staggered setting) may berequired.

• Faulty Maintenance - Adjust blowdown and/or nozzle rings.

• Faulty Maintenance - Correct spring misalignment.

• Faulty Maintenance - Correct bent stem or defective disk retainer.

• Faulty Maintenance - Correct scratched or dirty disk and/or seat.

• Faulty Maintenance - Check disk or seat for flatness (lapped at an angle).

• Leaking PZV - Perform required PZV maintenance.

Inadequate Piping Support

Improper piping design and/or structural support, as covered earlier in PCI-110, is theresponsibility of piping design engineers. Improper piping support leads to gross failures of PZVs.

Excessive Inlet Pressure Drop

Excessive inlet pressure drop may be caused by build-up of material in the PZV feed line whenpressure drop design requirements (3% of set pressure) are followed in the piping design. Pipingmust be inspected and flushed if necessary. If flushing does not adequately address the problem,the piping should be replaced.

Oversize Excessive Backpressure

Oversize piping does not lead to backpressure. However, excessive and variable superimposedbackpressure can cause chattering. This is a piping problem within the header, and/or disposalsystem, and beyond the scope of PCI-110.

Incorrect Valve Selection

Chattering may or may not be caused by incorrect valve selection. Incorrect PZV selection hasbeen discussed in other modules.




Excessive Flow Velocity

Excessive flow velocity can cause chattering and is a piping sizing and design problem if the PZVitself has been sized and selected correctly.

Unstable Process

Unstable process is an operations problem and a frequent cause of PZV failure. However,correction usually leads to major unit operations re-design, and is beyond the scope of PCI-110.These issues were covered in Modules 1 and 2 of PCI 110.

Incorrect Set Pressure

When the set pressure for PZV is set too low, chatter may result. However, this would be a rareoccurrence. Refer to previous discussions of incorrect PZV set pressure throughout PCI-110.




DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING LEAKAGE

Most leaking PZVs require maintenance, and are corrected by maintenance. Sometimes, asdiscussed in previous modules, an oversized PZV does not reseat due to a boundary layer effectwhen the valve reseats. Oversized PZVs will usually chatter.

PZV Leakage and Its Effects

Leakage from PZVs discharges valuable process fluid into the disposal system. Over time, aleaking PZV will seal closed due to polymer, salts, and/or corrosive buildup at the leakage point.These contaminations can cause the PZV to remain sealed in an emergency overpressurecondition and lead to equipment failure. T&I maintenance schedules are designed to prevent thisproblem.

Most leaking PZVs result in chattering which destroys the disk and seat. It is difficult todetermine if chattering caused leakage or leakage caused chattering.

Most leakage (not the result of chattering) is caused by dirt and contamination between the diskand seat when the valve reseats after a discharge. Periodic maintenance will correct theseconditions. However, if the leakage results in chattering, extensive maintenance may be required(stem replacement as opposed to simply cleaning the disk and seat).

Causes of PZV Leakage and Their Corrective Actions

The following is a list of common causes for PZV leakage and possible corrective actions:

• Contamination between the PZV disk and seat - Correct by performing requiredmaintenance, including cleaning disk and seat.

• Corrosion of Seat and/or Disk - Correct by performing maintenance. Corrosioncauses should be investigated by Operations and Inspection engineering (this will bediscussed further in PCI 110 Module 6).

• Oversized PZV - Addition of a second (staggered setting) smaller PZV may berequired.

• Faulty Maintenance - Correct improper adjustment of blowdown and/or nozzle rings.

• Faulty Maintenance - Correct spring misalignment.




• Faulty Maintenance - Correct bent stem or defective disk retainer.

• Faulty Maintenance - Correct scratched or dirty disk and/or seat.

• Faulty Maintenance - Check disk or seat for flatness (lapped at an angle).

• Chattering PZV - Perform required PZV maintenance.

Inadequate Piping Support

Improper piping design and/or structural support, as discussed previously throughout PCI-110, isthe responsibility of piping design engineers. Improper piping support leads to gross failures ofPZVs.

Oversize

Oversizing of PZVs, and oversizing in relation to leakage has been reviewed in PCI 110 Modules1 - 3.

Excessive Backpressure

Excessive backpressure does not cause a PZV to leak.


PZV sizing and selection has been adequately reviewed in prior modules.


Excessive flow velocity can lead to extensive PZV damage due to vibrational stresses. It is not adirect cause of PZV leakage, although leakage may result from damaged valve parts as asecondary effect.

Improper Metallurgy

Material of construction has been covered extensively in other modules. Corrosion will causeleakage and worse in a PZV.




Incorrect Set Pressure

Incorrect set pressure alone does not cause PZV leakage. If a PZV is otherwise maintainedproperly it will not discharge at the correct pressure required by Saudi Aramco standards andASME codes if is set incorrectly, but it will discharge and reseat, if otherwise set correctly(Blowdown ring, etc.).

Insufficient Operating Pressure Gap

An incorrect (including insufficient) operating pressure gap is a maintenance problem and cancause leakage (but more likely chattering) in a PZV. Maintenance procedures relative to PCI-110will be discussed in Module 6.




DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE BELLOWS

Bellows are required in all PZVs in corrosive service. This Saudi Aramco requirement (SAES-J-600) makes the bellows the vulnerable point of the PZV. It is important to note that a balancingpiston is also required in corrosive service (refer to Auxiliary Piston, in Form 8020-611-ENG).

The major cause of bellows failure is corrosion, as reviewed in Module 2. Chattering is the nextworse cause of bellows failure. Excessive back pressure will cause bellows failure, but underSaudi Aramco standards relating to maximum allowable vent header pressure, this cause shouldnever occur.

Bellows Failure and its Effects

Bellows failure may cause secondary corrosion on the balancing piston and in the stem guide andbonnet area, in which case the PZV will fail in an unpredictable way. T&I maintenance schedulesare designed to preclude advanced bellows corrosion. However, bellows corrosion begins withpin-hole leakage. Gross failure of the kinds illustrated in the examples cited in Module 4 onlyoccur after prolonged exposure to adverse process conditions.

Chattering causes component problems throughout the PZV. However, since the bellows is themost fragile component in a PZV it might fail first. A combination of high back pressure (not inexcess of rated bellows back pressure), PZV chattering, and corrosion will cause the type ofcatastrophic failure discussed in Module 4.

The effects of bellows failure are not predictable. These effects range from total PZV failure toonly bellows failure.

Cause of Bellows Failure and Their Corrective Actions

Some common causes of bellows failure and possible corrective actions are listed below:

• Corrosion - Correct by performing required maintenance, consult manufacturer andreplace bellows.

• Chattering - Solve the chattering problem. Always replace the bellows when a PZVshows evidence of chattering.

• Faulty Maintenance - Replace the bellows to correct pin-holes in bellows that werenot detected during T&I.




• Faulty Maintenance - Replace the bellows that were not replaced on a PZV known tohave experienced chatter.

• Faulty Maintenance - Eliminate the corrosion in piston area.

Excessive Backpressure

Excessive backpressure will cause bellows failure as reviewed and discussed in PCI-110 Modules2-4.


Incorrect PZV selection does not cause bellows failure. Bellows requirements, and failurediscussions, presuppose a bellows PZV is specified.


Excessive flow velocity can lead to extensive PZV damage due to vibrational stresses. It is not adirect cause of bellows failure, although bellows failure may result from damaged valve parts as asecondary effect.

Improper Metallurgy

Improper metallurgy is probably the most common 'root-cause' of bellows failure. It does notnecessarily result from a PZV specified incorrectly. Most often this is due to incorrect processdata regarding corrosive effects of process fluids.

Valve Chattering

Extensive PZV chattering will always cause bellows failure, as noted above.




DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE SPRING

Spring malfunctions will result in erratic PZV set point behavior. As reviewed in Module 2,spring misalignment, corroded springs and/or spring washers, and incorrect springs (based onPZV manufacturer requirements) will cause a PZV to fail to open within the required set pressuretolerance range.

Most PZV manufacturers stamp a serial (part) number on PZV springs. Only approvedreplacements based on that number are certified by the manufacturer Oftentimes, springs aresalvaged from retired (de-commissioned PZVs), and these springs do not meet manufacturerstandards. This is a universal problem among PZV users.

Spare PZV springs must be stored according to PZV manufacturer specifications, as noted intheir installation, testing and maintenance manual.

Springs must be protected from corrosive materials by proper venting, and replaced on a timelybases when subjected to corrosive materials (i.e., vented to a 'sour gas' header).

PZV Spring Failure and its Effects

Spring failure results in either:

• a PZV opening below set point and releasing valuable process fluid to a disposalarea

• a PZV not opening at the MAWP of the PZV and endangering the protectedequipment

• PZV chattering, and/or PZV leakage (PZV not closing after blowdown)

Causes of PZV Spring Failure and Their Corrective Actions

Excessive flow velocity can lead to extensive PZV damage due to vibrational stresses. It is not adirect cause of PZV leakage, although leakage may result from damaged valve parts as asecondary effect. Common causes of PZV spring failure and corrective actions are listed below:

• Corrosion - Correct by performance required maintenance, investigate cause ofcorrosion.




• Chattering - Solve the chattering problem. Always check spring/stem alignmentwhen PZV shows evidence of chattering.

• Faulty Maintenance - Replace the wrong spring/washer set that was installed in thePZV during T&I maintenance (stamped no. not checked).

• Faulty Maintenance - Replace the spring/washer set that was installed improperly(spring not aligned properly on PZV stem).

• Faulty Maintenance - Replace the spring that was too old per manufacturerreplacement guidelines, and/or not stored properly in accordance with manualinstructions.


Incorrect PZV selection does not apply to failures of the relief valve spring. Spring requirementsand failure discussions, presuppose a correct spring is supplied by the PZV manufacturer. (Notethat ISS does not require spring range specification. The PZV manufacturer is responsible forproviding the correct spring range.)


Excessive flow velocity can lead to extensive PZV damage due to vibrational stresses. It is not adirect cause of spring failure, although spring failure may result from damaged valve parts as asecondary effect.

Improper Metallurgy

Spring failure due to improper metallurgy may be due to incorrect process data regardingcorrosive effects from vent header fluids rather than from a PZV that was specified incorrectly.

Environmental Factors

If the corrosive effects of environment were not understood or reviewed during projectengineering, corrosion of springs vented to the atmosphere may result.

Chattering

Extensive PZV chattering will cause spring/washer and/or stem misalignment




Incorrect Spring Design

Since the spring is provided by the PZV valve manufacturer, incorrect spring design is unlikely.Incorrect spring replacement during maintenance is a more common cause of spring failure.




DETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVE FAILUREINVOLVING THE PILOT CONTROL

Pilot operators of PZVs were discussed in PCI-110 Module 2. Recall that the pilot operatordriving a main valve shares many common characteristics with a spring loaded PZV. The pilotvalve is a regulator and contains many precision components which are sensitive to contaminationfrom dirty process fluids.

As reviewed in Module 2, pilot operated PZVs are not permitted in dirty or corrosive servicewithout special approval (SAES-J-600).

Corrective actions for pilot operated PZV failure are essentially the same as those for other PZVs.Recall that the pilot valve and the main valve consist of a set of valves that comprise one PZV.

Pilot Control Failure and its Effects

Pilot valve failure has the same effect as spring failure in PZVs discussed above. The pilot-operated PZV is unreliable, and might allow protected equipment to be damaged as the result ofover pressure, and/or cause the loss of valuable process fluids if the valve opens below the correctset pressure.

Causes of Pilot Controls and Their Corrective Actions

Some common causes of pilot control failures and possible corrective action is listed below:

• Corrosion - Perform required maintenance and investigate cause of corrosion.

• Chattering - Solve the chattering problem. Investigate blockage of pilot sensing lineand blockage of filter in sensing lined. Also, always check spring/stem alignment inpilot valve.

• Faulty Maintenance - Replace the improper replacement pilot. Pilot valves arereplaced per Model/Serial No. designations per manufacturer requirements.

• Faulty Maintenance - Correct the pilot/main valve set pressure that was not checkedwith both valves on the test stand together.

• Faulty Maintenance - Replace the pilot valve that was repaired rather than replacedper Manufacturer requirements.




• Installation - Correct the sense line to pilot valve not on separate process connection(refer to Module 2).

Spring Failure

Spring failure results in either:

• a PZV opening below set point and releasing valuable process fluid to a disposalarea

• a PZV not opening at the MAWP of the PZV and endangering the protectedequipment

• PZV chattering, and/or PZV leakage (PZV not closing after blowdown)

Sprint failure in pilot-operated valves has the same effects as spring failure in spring-loaded PZVs.

Dirty System Media

Dirty system media is the major cause of pilot operated PZV failures. Follow manufacturer'srequirements regarding installation and maintenance.

Excessive Process Temperature

Excessive process temperature will cause unspecified problems with all PZVs. Pilot operators aremore sensitive to temperature than other PZVs. This is a sizing and selection, and pipinginstallation concern. More than likely, it cannot be discerned from T&I and maintenance reviews.

Excessive Pressure Drop in Sensing Line

Excessive pressure drop in sensing line causes pilot, and consequently main valves, to open at thewrong set pressure.

Unstable Process

Since pilot operated PZVs are used when precision protection is required, pilot operated PZVsare more prone to pop as the result of process transients. Process stability is managed by processcontrol technology, PZVs are provided to protect process equipment from over pressure.Unstable processes must be addressed by other engineering considerations.

During process overpressure transients, valuable fluids are lost to PZV disposal systemsregardless of the type of PZV employed when the valve pops.




WORK AID 1: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVECHATTERING




WORK AID 2: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVEFAILURE INVOLVING LEAKAGE




WORK AID 3: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVEFAILURE INVOLVING SETPOINT DRIFT




WORK AID 4: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVEFAILURE INVOLVING THE BELLOWS




WORK AID 5: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVEFAILURE INVOLVING THE SPRING




WORK AID 6: RESOURCES AND TROUBLESHOOTING GUIDELINES FORDETERMINING CORRECTIVE ACTIONS FOR RELIEF VALVEFAILURE INVOLVING THE PILOT CONTROL




GLOSSARY

staggered setting

troubleshooting pzv installations

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