Safety Relief valve

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    RELIEF SYSTEMS IN PROCESS PLANTS

    ARE THEY NEEDED?

    RELIEF SYSTEM

    SAFETY INTERLOCK

    SYSTEM

    ALARM SYSTEM

    BASIC PROCESS

    CONTROL SYSTEM

    Could

    these all

    fail due

    to a

    common

    fault?

    Hint: Yes!!

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    RELIEF SYSTEMS IN PROCESS PLANTS

    RELIEF SYSTEM

    SAFETY INTERLOCK

    SYSTEM

    ALARM SYSTEM

    BASIC PROCESS

    CONTROL SYSTEM

    Location

    Equipment selection

    Documenting on

    drawings

    Maximum capacity

    Whats in thistopic?

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    Copyrights by CCPS/American Institute of Chemical Engineers and copied with the permission of AIChE

    WE SHOULD ALSO PROTECT AGAINST

    EXCESSIVE VACUUM

    The following example uses buckling pins

    overpressure

    underpressure

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    WE MUST SAFELY PROCESS OR DISPLOSE

    OF MATERIAL FROM RELIEF SYSTEM!

    Fromrelief

    To environment Vent steam, air

    Holding for later processing Waste water treating

    Recycle to process Fuel gas, fuel oil, solvent

    Recover part to process

    Immediate neutralization Flare, toxic materials

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    SIZING RELIEF SYSTEMS

    $ DETERMINE THE RELIEF FLOW (maximum

    possible)

    $ DETERMINE THE SET PRESSURE

    - based on process needs and equipment materials andconstruction

    $ SELECT THE RELIEF TYPE- based on advantages/disadvantages of both

    $ CALCULATE THE NEEDED AREA FOR FLOW

    $ SELECT THE COMMERCIAL DEVICE FROMVENDOR=S SPECIFICATIONS

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    DETERMINING FLOWS FOR SIMPLE SITUATIONS

    Fin Fout = ?

    QFout = ?

    Tsource T

    Relief FlowSource

    Fout = ?

    Liquid or vaporfilled system

    Two-phasesystem withheat input

    Liquid systemwith no phasechange andwith heattransfer

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    DETERMINING FLOWS FOR SIMPLE SITUATIONS

    Fin Fout = ?

    Q Fout = ?

    Tsource T

    Relief FlowSource

    Fout = ?

    Liquid or vaporfilled system

    Two-phasesystem withheat input

    Filled Liquid systemwith no phasechange andwith heattransfer

    Fin

    Q(Vg-Vl)/Hvap

    Q/CpWhat is the mechanism for heat transfer? Radiation or

    convection?

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    DETERMINE THE MAXIMUM FLOW FOR THE

    OVERHEAD VAPOR

    Under what

    conditions is the

    vapor at its

    maximum?F

    Z

    q

    R

    D

    B

    LC

    LC

    PC

    A

    XD

    AXB

    V

    Relief

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    DETERMINE THE MAXIMUM FLOW FOR THE

    RELIEF SYSTEM

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    DETERMINING THE AREA FOR DIAPHRAGMS IN

    VAPOR SERVICE$

    Basic equation for flow through an orifice venting toatmosphere

    $ Rearrange to solve forarea without correction

    factors

    What special factor

    is important in the

    diaphragm relieving to

    a recovery system?

    Pg2AC=F

    c0

    To flare

    Liquid

    recovery

    K.O.drum

    Burstdiaphragm

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    DETERMINING THE AREAS FOR VALVES AND

    DIAPHRAGMS IN VAPOR OR GAS SERVICE

    $ Often, the flow is sonic or Achoked@ - in this situation, theflow depends on the upstream pressure but not the

    downstream pressure. This occurs when Ps > 0.5Pb ; this is

    rough estimate, see references.

    $ The defining equation is

    With K* depending on T, heat capacity ratio, mol. weight, back

    pressure, and compressibility

    Question: How do we know if the flow is choked?

    Does this mean that an increase in back pressure has no effect on flow?

    PAK=Fchoked*

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    DETERMINING THE AREAS FOR VALVES AND

    DIAPHRAGMS

    FOR EXOTHERMIC REACTORS

    CAUTION!

    $ Two-phase venting has proved difficult to model reliably

    - DIERS, (Design Inst. for Emergency Relief Systems) formed by

    companies and AIChE- goals are to determine sizing methods, verify

    experimentally, and prepare program

    $ Experience has shown that

    - Two-phase flow occurs often- Two-phase flow during runaway

    reactions requires a much larger area than predicted by one-phase

    methods

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    DETERMINING THE AREAS FOR VALVES AND

    DIAPHRAGMS

    FOR EXOTHERMIC REACTORS

    CAUTION!

    $ A standard experimental apparatus is used to collect adiabatic data onrunaway reactions

    - small scale so little hazard - gives results for sizing relief devices (T and

    P vs time; flow regime - foamy or non-foamy; and viscosity - laminar or

    turbulent regime)

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    SAFETY RELIEF SYSTEMS,

    WHAT HAVE WE LEARNED?

    These are self-actuated, no external power is required

    Two major devices are relief valve and rupture disk

    Relief is required for all closed volumes

    We must divert material to a adequate handling

    system

    We are now ready to gain experience in designing and

    evaluating safety relief systems.

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    RELIEF SYSTEM LEARNING TASKS

    1. Review the CCPS WWW material

    (on course site in restricted area)

    Lesson 1. Relief Required

    Lesson 2. Runaway Reactions

    Lesson 3. Safeguards (explosions)

    2. Review this lesson and solutions.

    3. Read Perrys Handbook, seventh Edition,

    p. 26-26 to 26-38, 26-73 to 26-74

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