lecture 12 relief

ECE 4353

Chemical Process Safety

Relief Concepts, Systems and

Scenarios

(Lecture 12)

Last Updated:1 December 2015

© LMS SEGi education group 2

LEARNING OBJECTIVES

Explain the government-industry’s responsibility for

health and safety

Evaluate the nature of hazards posed by materials

which are flammable, toxic and reactive

Identify and quantify common industrial methods to

control hazards.



Lecture 12

12.1 Pressure Relief Systems

12.2 Overpressurisation

12.3 Location of reliefs

12.4 Definitions and Code Requirements

12.5 Types of Relief Devices



Last Updated:1 December 2015 4

Layers of Protection




What are

Pressure Relief systems

for?





Pressure relief systems are needed for the following

reasons:

Protect personnel from the dangers of overpressurizing

equipment.

Prevent damage to equipment.

Prevent damage to adjoining property.

Minimise chemical losses during pressure upsets.

Reduce insurance premiums.





Pressure relief systems prevent pressurization above

a system’s design pressure; for venting during an unusual

or emergency situation; and for normal depressurization

during a shutdown.

12.1 Pressure relief systems

When venting non-combustible gases including steam, air and nitrogen may vent into the

atmosphere. When venting flammable or toxic compounds, an elaborate systems for the disposal of vented gases may be required.




Causes of overpressurization according to API include:

• Blocked discharge

• Failure of control valve

• External fire

• Utility failure : Cooling water failure, Power failure or

Instrument air failure

• Heat-exchanger-tube failure

12.2 Overpressurization




Blocked discharge Closure of block valve at outlet exposing the vessel to a pressure that may

exceed the maximum allowable working pressure (MAWP).




Failure of Control valve Failure of control valve due to controls malfunction or valve stuck close or fail

close may lead to overpressure in process vessel.




Failure of Control valve




External fire If the vessel has some liquid inventory, liquid inside will absorb the heat through

the vessel wall when it is exposed to an external fire. If the liquid inventory is

low or if the content is a gas, the heat from external fire is not dissipated by

vaporizing liquid; resulting in overpressure in the vessel.




External fire




Thermal External heat input can cause liquid temperature, and hence volume, to rise.

Eg. A vessel or pipeline containing liquid is heated by direct solar radiation.




Heat Exchanger tube failure Complete tube rupture, in which a large quantity of high-pressure fluid (from

tube) will flow to the low-pressure side of the exchanger (to shell) results in

shell overpressure.




Runaway Chemical Reactions A runaway reaction is a chemical reaction over which

control has been lost. It continues to accelerate in

reaction speed until it either runs out of reactants or the

vessel containing it overpressures and loses containment.

During runaway reactions, which tend to accelerate with

rising temperature, internal pressure of a vessel or

pipeline can rise rapidly.

Basically, pressure relief valves may not provide adequate

protection at all because of their relatively slow response

time. In such a situation, vapor depressuring systems,

rupture discs and emergency vents are preferred.




To specify the location of reliefs requires a review of every

unit operation in:

- the process and

- every process operating step

Anticipate the potential problems which may result in

increased pressures.

Pressure relief devices are installed at every point where

upset conditions create pressures that may exceed MAWP.

12.3 Location of Reliefs




What happens with

loss of cooling,

heating or

agitation?

What happens if the process is contaminated or has a mischarge of a catalyst or monomer?

What happens if

the unit operation

is engulfed in a

fire?

What happens if the operator makes an error?

What conditions cause

runaway reactions, and how

are relief systems designed to

handle the discharge as a

result of runaway reactions.

What is the consequence of closing valves on vessels or in lines which are filled with liquids and exposed to heat or refrigeration?




GUIDELINES FOR SPECIFYING RELIEF POSITIONS

Vessel steam jackets are often rated for low pressure steam. Reliefs are installed in jackets to prevent excessive steam pressures due to operator error or regulator failure.




General Code requirements include:

ASME Boiler & Pressure Vessel Codes

ASME B31.3 / Petroleum Refinery Piping

ASME B16.5 / Flanges & Flanged Fittings

When to install PSV? • All pressure vessels subject to overpressure shall be

protected by a pressure relieving device.

• Liquid filled vessels or piping subject to thermal

expansion must be protected by a thermal relief device.

12.4 Code Requirements




sIngle PSV on multiple vessels? • Multiple vessels may be protected by a single relief device

provided there is a clear, unobstructed path to the device

Multiple PSVs on a vessel? • At least one pressure relief device must be set at or

below the MAWP




12.5 Types of relief devices




Rupture discs

Spring Operated PSV (Conventional and Balanced

Bellows)

Pilot Operated PSV

<video on Direct acting and Pilot operated)

12.5 Types of Relief Devices




RUPTURE DISCS

Rupture discs are designed to rupture at a specified relief set pressure.

Once they open, they remain open.

Rupture discs are available in much larger sizes than spring operated

relief valves.

Rupture discs cost less than spring loaded PSVs.




RUPTURE DISCS are installed in series to a spring loaded

relief to:

1. Protect an expensive spring loaded PSV from a corrosive

environment

2. To give positive isolation when handling extremely toxic

chemicals

3. To give positive isolation when handling flammable

gases

4. To prevent plugging of PSVs

An indicator (tell tale gauge) is placed between the rupture

disc and the PSV to indicate when the disc ruptures.




CONVENTIONAL TYPE PSV

A conventional pressure relief valve is a spring loaded pressure relief

valve whose operational characteristics are

directly affected by changes in the back pressure.




CONVENTIONAL TYPE




BALANCED BELLOWS PSV

A balanced pressure relief valve is a spring loaded pressure

relief valve that incorporates a bellows or other means

for minimizing the effect of back pressure on the

operational characteristics of the valve




Balanced bellows PSV




PILOT PSV

A pilot operated pressure relief valve is a pressure relief

valve in which the major relieving device or main valve is

combined with and controlled by a self actuated auxiliary

pressure relief valve (pilot).




PILOT TYPE PSV




Pressure relief valve: A pressure relief device

designed to open and relieve excess pressure and to

reclose and prevent the further flow of fluid after normal

conditions have been restored.

a. A relief valve is a spring loaded pressure relief valve used

primarily with incompressible fluids.(liquids)

b. A safety valve is a spring loaded pressure relief valve

characterized by rapid opening or pop action. A safety valve

is normally used with compressible fluids.(gas)

c. A safety relief valve is a spring loaded pressure relief

valve that may be used as either a safety or relief valve

depending on the application.

lecture 12 relief

Documents

lms segi education group

pressure relief systems

systems design pressure

pressure upsets

elaborate systems

power failure

utility failure

tube failure