RELIEF VALVESRELIEF VALVESININ

PARALLELPARALLEL

James R. Lawrence Sr.

Mary Kay O'Connor Process Safety Center

International SymposiumBEYOND REGULATORY COMPLIANCEMAKING SAFETY SECOND NATURE

October 26-28, 2010

Why use a Relief Valve Why use a Relief Valve (PRV)(PRV)

It is Required by CodeIt is Required by Code Protect our PersonnelProtect our Personnel Prevent loss of Capital Prevent loss of Capital

InvestmentInvestment Conserve the ProductConserve the Product Minimize DowntimeMinimize Downtime Protect the EnvironmentProtect the Environment

PRV Defined (official PRV Defined (official version)version)

A pressure relief device designed to actuate on inlet static pressure and reclose after normal conditions have been restored. relief valve: characterized by gradual opening relief valve: characterized by gradual opening

that is generally proportional to the increase in that is generally proportional to the increase in pressure. It is normally used for incompressible pressure. It is normally used for incompressible fluids.fluids.

safety relief valve: characterized by rapid safety relief valve: characterized by rapid opening or by gradual opening that is generally opening or by gradual opening that is generally proportional to the increase in pressure. It can be proportional to the increase in pressure. It can be used for compressible or incompressible fluids.used for compressible or incompressible fluids.

safety valve: characterized by rapid opening and safety valve: characterized by rapid opening and normally used to relieve compressible fluids.normally used to relieve compressible fluids.

ASME PTC-25

PRV Defined (real world)PRV Defined (real world) A PRV is nothing more than a self-contained A PRV is nothing more than a self-contained

back pressure regulator. A PRV is highly back pressure regulator. A PRV is highly reliable but poorly designed Control Valve.reliable but poorly designed Control Valve.

Like a regulator, a PRV can be direct acting Like a regulator, a PRV can be direct acting or pilot typeor pilot type

PRV trim characteristic is “quick opening” PRV trim characteristic is “quick opening” PRV trim is not as robust (or accurate) as CV PRV trim is not as robust (or accurate) as CV

trim. A CV is designed to open and a PRV is trim. A CV is designed to open and a PRV is designed to stay closed (open infrequently).designed to stay closed (open infrequently).

Remember – an open PRV Remember – an open PRV maintainsmaintains a a pressure pressure overover MAWP. It does not reduce MAWP. It does not reduce pressure.pressure.

Pressure TerminologyPressure Terminology

Operating Operating PressurePressure

MAWPMAWP Design PressureDesign Pressure Set PressureSet Pressure Back PressureBack Pressure Re-seat PressureRe-seat Pressure Over PressureOver Pressure BlowdownBlowdown

Pre

ssur

e R

ange

100%

0%

Over

PressureMAWP &

Design Pressure

Max. Set

PressureSet @ 100%

Re-Seat

Pressure

OperatingPressure

Blowdown

BackPressure

Code & StandardsCode & Standards

ASME for Boilers & Pressure VesselsASME for Boilers & Pressure Vessels UG-125(c)(1) allows for 16% (or 4 psi) UG-125(c)(1) allows for 16% (or 4 psi)

over MAWP when using multiple over MAWP when using multiple devices.devices.

UG-134(a) Single device cannot be UG-134(a) Single device cannot be set over MAWP. When required set over MAWP. When required capacity is provided in more than one capacity is provided in more than one device, only one need be set at or device, only one need be set at or below MAWP. 2below MAWP. 2ndnd device cannot be device cannot be higher than 105% of MAWP.higher than 105% of MAWP.

Chatter is BadChatter is Bad Chatter is the rapid opening and Chatter is the rapid opening and

closing of a PRV. It can happen in closing of a PRV. It can happen in any service with any PRV valve any service with any PRV valve type.type.

Chatter damages the PRV and can Chatter damages the PRV and can damage the connecting Pipe.damage the connecting Pipe.

Chatter reduces the PRV’s capacityChatter reduces the PRV’s capacity!!

Chatter can be violent enough to Chatter can be violent enough to cause catastrophic failure releasing cause catastrophic failure releasing process fluid to the atmosphere.process fluid to the atmosphere.

Chatter - The CauseChatter - The Cause

Excessive pressure drop at the inletExcessive pressure drop at the inlet Changes in the back pressure (built-up or Changes in the back pressure (built-up or

from another valve or related source)from another valve or related source) AnyAny combination of the above combination of the above Operational ResonanceOperational Resonance

An oversized PRV is a primary culprit.An oversized PRV is a primary culprit.

Chatter is caused by varying Chatter is caused by varying differential pressure across the PRV differential pressure across the PRV resulting from:resulting from:

EngineeringEngineering The basis must be determinedThe basis must be determined Inlet and outlet piping must be consideredInlet and outlet piping must be considered Inlet piping pressure drop and backpressure must Inlet piping pressure drop and backpressure must

be estimated/calculatedbe estimated/calculated A relief valve type is selectedA relief valve type is selected A sizing calculation is madeA sizing calculation is made Relief and flare headers must be evaluated if Relief and flare headers must be evaluated if

applicableapplicable Piping reactions must be considered and proper Piping reactions must be considered and proper

support providedsupport provided Consideration must be given to noise both with Consideration must be given to noise both with

regard to personnel protection and to the regard to personnel protection and to the potential for piping damagepotential for piping damage

Project IssuesProject Issues

PRV sizing and selection should be PRV sizing and selection should be an iterative process.an iterative process.

Cost and Schedule considerations Cost and Schedule considerations often leave PRVs oversized.often leave PRVs oversized.

PRV SizingPRV Sizing

The basis must be determined.The basis must be determined. Are there multiple cases with widely varying Are there multiple cases with widely varying

flow rates?flow rates? What is the design effect on the relief What is the design effect on the relief

header?header? What is the relief header effect on the PRV?What is the relief header effect on the PRV? CalculateCalculate Select PRV orifice with “no safety factor” Select PRV orifice with “no safety factor”

appliedapplied Confirm relief load and reactions are still Confirm relief load and reactions are still

within acceptable limits.within acceptable limits.

Sizing Rules of ThumbSizing Rules of Thumb

Maximum Selection based on Required Maximum Selection based on Required OrificeOrifice

140% absolute maximum140% absolute maximum 125% ideal max for vapor service125% ideal max for vapor service 110% ideal max for liquid service110% ideal max for liquid service

Minimum Minimum 25-30% Relief flow against Capacity for 25-30% Relief flow against Capacity for

spring operated devices. (if there is a spring operated devices. (if there is a case <30%, the installation is a candidate case <30%, the installation is a candidate for dual valves per UG-125 & 134)for dual valves per UG-125 & 134)

Piping & Instrumentation Piping & Instrumentation DiagramsDiagrams

Base Line document for a FacilityBase Line document for a Facility Defines the Facility more succinctly Defines the Facility more succinctly

than any other documentthan any other document P&ID must always represent the P&ID must always represent the

FacilityFacility Management of Change is requiredManagement of Change is required P&ID’s rule – full stop.P&ID’s rule – full stop.

The Problem at HandThe Problem at Hand

When there is an installation that When there is an installation that provides for a provides for a sparespare relief valve that relief valve that is supposed to be is supposed to be normally closednormally closed to to the process and that spare valve is the process and that spare valve is then opened to the process along then opened to the process along with the primary valve, facility with the primary valve, facility safety is compromised. There are a safety is compromised. There are a number of reasons why this is true.number of reasons why this is true.

1 - Interaction1 - Interaction

This misapplication of relief valves This misapplication of relief valves gives new definition to the term gives new definition to the term “Chatter”. We can now add valve “Chatter”. We can now add valve Interaction to the previous definition.Interaction to the previous definition.

It should be readily apparent what the It should be readily apparent what the additional interactions can do to a pair additional interactions can do to a pair of fully sized relief valves open to the of fully sized relief valves open to the process.process.

2 - Relief System Piping 2 - Relief System Piping DesignDesign

Transient increase in relief load Transient increase in relief load translates to increased moment in translates to increased moment in the pipe worksthe pipe works

Transient increase Sound Pressure Transient increase Sound Pressure Level (SPL)Level (SPL)

Transient increase in fluid velocityTransient increase in fluid velocity

3 - Flare Loading3 - Flare Loading

Flare system and Flare Tip design Flare system and Flare Tip design can be exceeded.can be exceeded.

Spurious opening of a pilot operated Spurious opening of a pilot operated valve. A pilot operated PRV is a “Fail valve. A pilot operated PRV is a “Fail Open” device. By opening both PRVs Open” device. By opening both PRVs to the process, the MTBF is cut in to the process, the MTBF is cut in half.half.

4 - Safety Issues & 4 - Safety Issues & AssumptionsAssumptions

Fundamental assumption is incorrect: Fundamental assumption is incorrect: it is it is not safernot safer!!

Layer of Protection Analysis: Apparently Layer of Protection Analysis: Apparently LOPA credit is given for opening the 2LOPA credit is given for opening the 2ndnd PRV to the process. This is bad practice PRV to the process. This is bad practice and gives a false legitimacy to the and gives a false legitimacy to the practice.practice.

If set points are staggered and one valve If set points are staggered and one valve comes out of service for maintenance, it comes out of service for maintenance, it could leave the valve set at 105% in could leave the valve set at 105% in service – an unacceptable practice.service – an unacceptable practice.

SummarySummaryPotential IssuesPotential Issues

The facility line-up at the time of an accident or The facility line-up at the time of an accident or event does not match the authority approved event does not match the authority approved P&ID.P&ID.

Relief valve pairs may chatter on and off thus Relief valve pairs may chatter on and off thus decreasing capacity possibly below the amount decreasing capacity possibly below the amount required to maintain MAWPrequired to maintain MAWP

Relief valve damageRelief valve damage Damage to piping and/or pipe supports. Note Damage to piping and/or pipe supports. Note

that the phenomenon of stress is cumulative. that the phenomenon of stress is cumulative. Flare or relief system overload with resultant Flare or relief system overload with resultant

overpressure.overpressure. Relief into the process area due to damaged Relief into the process area due to damaged

headers or venting at unexpected locations.headers or venting at unexpected locations.

RecommendationsRecommendations If a pair of relief valves is to remain open to the process, If a pair of relief valves is to remain open to the process,

ensure the installation is applied correctly per Code, and ensure the installation is applied correctly per Code, and ensure the provided capacity does not exceed 140% of the ensure the provided capacity does not exceed 140% of the required capacity.required capacity.

Make engineering design contractors aware of the desired Make engineering design contractors aware of the desired practice at the start of a facility design so engineering is done practice at the start of a facility design so engineering is done correctly and work does not have to be repeated.correctly and work does not have to be repeated.

Ensure operations personnel understand the sanctity of the Ensure operations personnel understand the sanctity of the P&ID, the procedures required to make revisions, and P&ID, the procedures required to make revisions, and management of change.management of change.

If a set-point is changed, the valve nameplate and data sheet If a set-point is changed, the valve nameplate and data sheet must be changed accordingly. must be changed accordingly.

If a facility has been operating not in accordance with If a facility has been operating not in accordance with approved design and there have been incidents of dual PSV approved design and there have been incidents of dual PSV lift, inspect the relief system piping and supports for damage.lift, inspect the relief system piping and supports for damage.

Never provide parallel valves open to the process Never provide parallel valves open to the process when both are fully rated for the maximum load.when both are fully rated for the maximum load.