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Training Package TP 05/05. Prevention of Over - Pressurization. Asia Industrial Gases Association 298 Tiong Bahru Road, #20-01 Central Plaza, Singapore 168730 Internet: http// 1. Prevention of Over - Pressurization. Disclaimer

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Training Package

TP 05/05

Prevention of Over - Pressurization

Asia Industrial Gases Association

298 Tiong Bahru Road, #20-01 Central Plaza, Singapore 168730

Internet: http//


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Prevention of Over - Pressurization


All publications of AIGA or bearing AIGA’s name contain information, including Codes of Practice, safety procedures and other technical information that were obtained from sources believed by AIGA to be reliable and/ or based on technical information and experience currently available from members of AIGA and others at the date of the publication. As such, we do not make any representation or warranty nor accept any liability as to the accuracy, completeness or correctness of the information contained in these publications.

While AIGA recommends that its members refer to or use its publications, such reference to or use thereof by its members or third parties is purely voluntary and not binding.

AIGA or its members make no guarantee of the results and assume no liability or responsibility in connection with the reference to or use of information or suggestions contained in AIGA’s publications.

AIGA has no control whatsoever as regards, performance or non performance, misinterpretation, proper or improper use of any information or suggestions contained in AIGA’s publications by any person or entity (including AIGA members) and AIGA expressly disclaims any liability in connection thereto.

AIGA’s publications are subject to periodic review and users are cautioned to obtain the latest edition.

 AIGA 2005 - AIGA grants permission to reproduce this publication provided the Association is acknowledged as the source

Asia Industrial Gases Association 298 Tiong Bahru Road, #20-01 Central Plaza, Singapore 168730

Internet: http//

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What is Over-Pressurization?

  • The increase of pressure inside a piece of equipment beyond its ability to hold that pressure.

    • Vessels and tanks

    • Cylinders and bottles

    • Compressors and pumps

    • Piping and tubing

    • Hoses and flex joints

    • Instruments and valves

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Imagine - Blowing Up a Balloon

This could hurt. Now imagine if you were near a vessel, pipe or cylinder when it ruptured!

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Over-Pressurization Hazards

  • Ruptured process vessels, cylinders, piping

  • Whipping hoses

  • Flying debris and shrapnel

  • Serious injury

  • Death

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Hazardous result

An ammonia cylinder that failed due to overfilling & hydraulic-king

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Over-Pressurization Kills

  • Incident 1: Liquid trapped in the ball of a ball valve expanded, causing the valve to come apart. Worker dies when contents of CO2 storage tank released in his direction.

  • Incident 2: A chlorine cylinder ruptures during fill procedure, killing worker in the same room.

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Over-Pressurization Kills

  • Over-pressurization can occur at your location too!

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Causes of Over-Pressurization

  • Faulty process control

  • Process upsets

  • Trapped liquefied gases

    • In a pipe or hose between two closed valves

    • In a ball or gate valve not designed to relieve internal pressure

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Causes of Over-Pressurization

  • Poor equipment and process design

  • Uncontrolled modifications

  • Human ignorance

  • Human error

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Faulty Process Control

  • Improperly calibrated control devices (temperature, pressure, flow, level…)

  • Malfunctioning control sensing device such as:

    • Isolation valves closed

    • Inlet ports blocked

    • Devices damaged

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Faulty Process Control (cont’d)

  • Malfunctioning control valves

    • Sticking control valves, solenoids

    • Closed or throttled isolation or bypass valves

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Process Upsets

  • Sudden flow rate, temperature or pressure changes

  • Process operation outside of established operating limits

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Trapped Liquefied Gases

  • Expand as they warm and vaporize

  • Pressure rises can exceed most process valves, piping and hoses pressure ratings

  • If no vapour in the trapped space, hydraulic loading can occur - leads to extremely high pressures in a very short time

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Liquefied Gases

  • If allowed to reach ambient (room) temperatures:

  • Liquid carbon dioxide could reach 1,100 psi!*

  • Liquid gases like helium, hydrogen, nitrogen, oxygen and argon could reach 22,000 to 50,000 psi!*

    * If the container could hold the pressure.

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Liquefied Gases (cont’d)

  • Do not over-fill a cylinder or tank

    • Maintain proper vapour space

  • Do not trap liquid gases

    • Must be able to relieve pressure

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Liquefied Gas (cont’d)

  • Expand 700 to 800 times in volume as they suddenly change (“flash”) from a liquid state into a gas.

  • Results in a tremendous force similar to that released in an explosion.

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Trapped Liquid With vapour

As temperature rises, a given mass of liquid can require more room (its density changes)...

Vapour can be compressed. So it makes room for the expanding liquid. Pressure rises gradually with temperature. Eventually, most liquefied gases will develop pressures that will rupture the container.

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Trapped Liquid Without vapour

As temperature rises, a given mass of liquid can require more room (its density changes)...

Liquid cannot be compressed, so as it gets bigger, it has to go somewhere! It quickly develops extremely high pressure.

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With and Without vapour

The pressure of liquid without vapour rises quickly.

Liquid and vapour pressure rises gradually, but still reaches higher pressures than what typical equipment can contain.

Pressure (psig)

Heat Input

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Ball Valve Hazard Potential

(Side View)


Valve Body


(Higher Pressure)


(Lower Pressure)


Trapped Liquid

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Hazardous Result

(Side View)

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Valve Design for Internal Relief

Alternatively, ball valves have special self-relieving seats

Ball valves have

1/8” hole in ball


(Higher Pressure)


(Lower Pressure)

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Gate Valve

Notch cut in gate

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Faulty System Design

  • Equipment not designed for pressure or temperature extremes

  • Missing or isolated pressure relief devices

  • Relief devices that are too small

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Faulty System Design (cont’d)

  • Relief devices not set properly

  • Unanticipated modes of operation

  • Stressed piping, tubing, connectors

  • Relying solely on humans to control pressure

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Case Study: Faulty System Design

  • A gas heater vessel ruptured at a CO2 plant in 1998

    • A dryer switching valve failed in open position.

    • High pressure process gas (800 psig) then entered the lower-pressure regeneration gas system.

    • The regeneration gas heater safety relief valve was not big enough to handle process gas flow rates.

    • The heater vessel ruptured.

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Human Ignorance & Error

  • Filling a low pressure cylinder on a high pressure filling manifold

  • Replacing relief devices with devices of a different design or pressure setting

  • Replacing a section of pipe, a valve or other process component with one that has a lower pressure rating than the original

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Human Ignorance & Error(cont’d)

  • Compromising the integrity of a process component (dents, crimping, unchecked corrosion)

  • Not following established procedures

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Case Study: Human Ignorance & Error

  • Whipping hose during leak test:

    • Employee connected a 250 psig rated air hose to a high pressure nitrogen source.

    • He pressurized hose and manifold to 350 psig.

    • He discovered manifold leak, vented pressure and repaired leak.

    • Blow gun was attached to hose and employee claims that pressure was set to 200 psig.

    • Hose fitting failed, hose whipped and shredded a portion of employee’s coveralls.

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Protecting Ourselves

  • Knowledgeable Employees

    • Proper training

  • Properly Designed Processes

    • Equipment that is good for the pressure

    • Control system designs that consider all scenarios

    • Valves with internal relief capability (for liquefied gases)

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Protecting Ourselves (cont’d)

  • Pressure Relief Devices

    • Relief valves, rupture disks

    • Dual relief valves with diverter valves

  • Proper Operating Procedures

  • Proper Maintenance Procedures

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Proper Maintenance

  • Relief Valves Calibration

    • Bench tested and reset every 5 years

    • Relief devices missing their wire seal must be removed from service and tested

    • Recalibration must be by a qualified person

  • Malfunctioning Relief Devices

    • Plugged or restricted inlets or outlets

    • Rust and dirt or ice in spring mechanism

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Proper Maintenance (cont’d)

  • Process Controls and Sensing Devices Calibration

    • Gauges and thermometers

    • Metering devices

    • Solenoids

    • Temperature and pressure limit controls

    • Mercoids

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Proper Maintenance (cont’d)

  • Ensure Replacement Parts are “Replacement-In-Kind”

    • Pipe, fittings and components with same pressure rating

    • Valves with same pressure relief capabilities

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Management of Change

  • Follow established review procedures

  • Consider all modes of operations

  • For each process, ask “How can we have high pressure?”

  • Ensure proper pressure relief strategy for every process component

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High Pressure Valve

Pressure Relief Device

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Portable Liquid Container

Rupture Disk

Pressure Relief Device

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Pressure Relief Device

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Dual Storage Tank Safety Valves

Pressure Relief Device

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High Pressure Line Safety

Pressure Relief


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Solenoid Shut-off Valve

Pressure Relief Device

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Pressure / Temperature Switch

Temperature Sensor

Pressure Inlet to Sensor

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Mercoid Switch

Mercoid Switch

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High Pressure Shut-Off Switches

Pressure Shut-off Switches

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Review Questions

  • Give examples of processes or pieces of equipment at your location which can present an over-pressurization hazard?

  • What are the causes of over-pressurization?

  • Liquefied gases can expand up to __________ times in volume from liquid to gas.

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Review Questions

  • Which is the most dangerous hazard, trapped liquid with or without vapour? Why?

  • What kinds of things can we do to protect from over-pressurization hazards?