Lecture 5 Introduction to Reliefs

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    Safety in Process Plant

    Reliefs

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    Topics

    • Introduction to Reliefs (Chapter 9)

    • Relief Sizing (Chapter 10)

    Hazards Identification (Chapter 11)

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    Introduction to Relief

    Increase  of operating pressure beyond the safe

    level leads to in rupturing of process equipment,

    causing major release of toxic or flammable

    chemicals.

    To solve it:a) Prevent the pressure build up (Inherent safety)

    b) Better control

    c) Pressure relief

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    Relief design procedureRelief devices must be installed.

    Appropriate relief devicenature of the material relieved and the relief characteristics required

    Liquid, vapor, or two phases

    physical properties of the ejected material, and the relief is sized.Data Collection

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    Why do we need relief systems?

    • To protect personnel from the dangers of over

    pressurizing equipment.

    • To minimise chemical losses during pressure

    upsets

    • To prevent damage to equipment

    To prevent damage to adjoining property• To reduce insurance premiums

    • To comply with government regulations

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    Relief Concept

    Assumption:• Exothermic reaction

    • Cooling is lost

    Curve C, no relief v/v

    Curve A, relief v/v

    Curve B, 2 phase with relief v/v

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    Relief Definitions

    • Set pressure

    Pressure at which the relief device begins to activate

    • Maximum Allowable working pressure (MAWP):

    Also known as Design Pressure.

    Vessel Fails 4-5 times MAWP, vessel deformation started at twice the MAWP.

    • Operating pressure:Gauge pressure during normal service, usually 10% below MAWP

    • Accumulation:Pressure increase over the MAWP of a vessel during the relief process.Expressed as % of MAWP

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    Relief Definitions

    • Overpressure:The pressure increase in the vessel over the set pressure during the

    relieving process, expressed as a % of MAWP

    • Backpressure:

    The pressure at the outlet of the relief device during the relief process

    resulting from pressure in the discharge system.

    • Blowdown:

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    Relief Definitions

    • Maximum allowable accumulation pressure:The sum of the MAWP & the allowable accumulation

    • Relief system:Network components around a relief device, including:-

    pipe to relief, relief device, discharge pipe, knockout drum, scrubber,

    flare.

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    Guidelines for relief

    pressures. Adapted from

    API RP521, Guide for

    Pressure-Relieving and

    De-pressuring Systems,

    4th ed. (Washington, DC:

    American PetroleumInstitute 1997)

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     Location of Reliefs Must installed at any point identified as potentially hazardous. Upset conditions

    create pressure > MAWP

    All vessels need reliefs, including reactors, storage tanks, towers, and drums.

    Blocked-in sections of cool liquid-filled lines that are exposed to heat (such as the

    sun) or refrigeration need reliefs.

    Positive displacement pumps, compressors, and turbines need reliefs on the

    discharge side.

    Storage vessels need pressure and vacuum reliefs to protect against pumping in

    or out of a blocked-in vessel or against the generation of a vacuum bycondensation.

    Vessel steam jackets are often rated for low-pressure steam. Reliefs are installed

    in jackets to prevent excessive steam pressure due to operator error or regulator

    failure.

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    Example: Polymerisation reactor system

    Process:1. Pumping 100lb of initiator into reactor R-1

    2. Heating to the reaction Temp = 240˚F 

    3. Adding monomer for 3 hours

    4. Stripping residual monomer by means of a

    vacuum valve V -15

    5. Reaction is exothermic

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    Solution: Polymerisation reactor system

    Reactor (R-1): in general every process vessel needs a relief.

    PSV -1 (pressure safety valve 1)

    Positive displacement pump (P-1): will be overloaded,

    overheated, & damaged if they are dead-headed w/o PSV -2.

    Relief discharge is usually recycle back to the feed vessel.

    Heat exchanger (E-1): PSV -3 to prevent tubes rupture.

    Drum (D-1): all vessel need relief valves, PSV -4

    Reactor Coil: PSV -5 added to prevent pressure-ruptured.

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    Solution: Polymerisation reactor system

    With safety reliefs

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    Major types of Relief devices

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    Buckling-pin relief valve

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    Relief installation Practices

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    Relief installation Practices

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    Chatter

    It is the rapid opening and closing of a relief valve that cancause valve seat damage or the mechanical failure to the

    internals.

    It could be due to:• Excessive inlet pressure drop: can be prevented with large

    inlet pipe

    • High back pressure: can be prevented by increasing the size

    of the exit pipe• Oversize valves: can be prevented by adding different size

    valves.

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    Sample Check List

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