ela971 unit 1 - pressure protection system concepts

Copyright ©American Institute of Chemical Engineers 2017. All rights reserved.

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SAChE® Certificate Program

Level 2, Course 12: Hazards and Risk – Introduction to Pressure Protection

Unit 1 – Pressure Protection System Concepts

Narration:

[No narration]


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Getting Started

Narration:

If this is your first time taking a SAChE course, please take a few minutes to explore the interface.

This slide will explain how to use the controls to navigate through the course. All of the units in

the course use the same interface.

This interface has four main features that you should be aware of:

• Here is the left navigation bar. It contains a list of the slides as well as the narrative

transcript. At any point in the course, if you would like to revisit any content, click the

slide title to jump back.

• You may also use the Previous button on the bottom of the player. To advance forward,

use the Next button.

• The Search feature allows you to search for content using any word in the current unit.

• On the top menu bar you will find the Help, Abbreviations, Glossary, Resources and Exit

options. The resources included in this course include any unit-specific attachment as


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well as a printable copy of the unit slides and narrative. Use the Exit tab to leave this

unit at any time.

Click the arrows if you want to learn more about the interface features. Click ‘Next’ when you’re

ready to continue.


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About This Training Program

Narration:

Welcome to the American Institute of Chemical Engineers’ online Process Safety training

program. This course is the second of four Level 2 courses designed to introduce you to hazards

and risks when working with hazardous materials and energies.

In this course, Hazards and Risk: Introduction to Pressure Protection, we will introduce you to

pressure protection systems. It is divided into four units:

• Unit 1 – Pressure Protection System Concepts;

• Unit 2 – Scenarios Requiring Pressure Protection;

• Unit 3 – Types of Pressure Protection Equipment; and

• Unit 4 – Designing Emergency Relief and Managing Discharge.

Each unit takes about 30 to 45 minutes to complete. At the end of each unit, you will be

presented with a quiz. You must pass the quiz in order to have the unit marked as complete, so

be sure to pay close attention to the content and answer all the review questions along the way.

After completing all of the units in the course, you will take a final exam. You must pass the

exam to have the course marked as completed.


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Objectives

Narration:

This is the first of four units in the “Hazards and Risks: Introduction to Pressure Protection”

course. In this unit, we’ll explore why pressure protection is needed and define some common

pressure relief design terminology. By the end of this unit, titled “Pressure Protection System

Concepts,” you will be able to:

• State why pressure protection is essential for process equipment; and

• Define some of the terms used when designing pressure relief equipment.


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SECTION 1: Why Pressure Protection is Needed

Narration:

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Why is Pressure Protection Needed?

Narration:

When a process vessel (or other equipment) is subject to excessive pressure or an overpressure,

there is a danger that this pressure will exceed the vessel’s ability to contain it.

In this incident, the rupture disc was unable to relieve the pressure increase fast enough and the

3” thick-walled reactor exploded. Four people died, 32 people were injured and the blast

shattered windows and walls of nearby businesses. Although several process safety system

weaknesses contributed to this incident, a properly designed relief system could have helped

reduce the likelihood and the consequences of this catastrophe.

We need to understand the hazards and risks of our processes, and when pressures could

exceed safe equipment design, we need to properly design pressure protection systems to help

prevent catastrophic equipment failures.


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Why is Pressure Protection Needed? (continued)

Narration:

Large atmospheric storage tanks, for example, must be protected from potential vacuum

scenarios, otherwise processing conditions – such as pumping out its contents – could pull a

vacuum inside and cause a catastrophic collapse of a tank not designed to handle a vacuum. The

image shown here is of a tank that imploded when a piece of plastic covered the tank’s

atmospheric vent.

Vessels have also been sucked in when the hot steam used to clean the insides of the vessel cool

too fast, resulting in a vacuum when the steam condenses in a closed system. If the vessel is not

vacuum-rated, the atmospheric pressure will crush the vessel that is under the vacuum.

As noted for scenarios with the potential for overpressure, we need to understand the hazards

and risks of our processes when there is the potential for a vacuum as well. We need to properly

design pressure protection systems to help prevent damage to the equipment, whether it could

be overpressurized or be placed under vacuum.


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Pressure Protection Guidance

Narration:

Industry and regulators get guidance when designing equipment with safe pressure protection

systems from major technical organizations, including:

• The American Petroleum Institute (API);

• The American Society of Mechanical Engineers (ASME);

• The National Fire Protection Association (NFPA); and

• The American Institute of Chemical Engineers (AIChE).


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ASME Boiler and Pressure Vessel Code (BPVC)

Narration:

Note that some of the pressure allowances when designing relief systems presented in this

course are from Section VIII of the ASME Boiler and Pressure Vessel Code (or BPVC). Although

codes, standards, and internal company guidelines may differ slightly, the terminology is the

same.

ASME has prepared a brochure outlining the various sections in the code. You can download a

copy of the brochure by clicking the book icon. Your organization may have a copy of Section VIII

of the BPVC for you to reference.


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Pressure Protection Codes and Standards

Narration:

Pressure vessel design codes and standards, often referred to in regulations, usually include the

following requirements:

• All pressure vessels subject to excess pressure or an overpressure must be protected by

a pressure relieving device or system.

• Multiple equipment subject to common overpressure scenarios may be protected by a

single relief device or system, provided there is:

• A clear, unobstructed path to and from the device or system; and

• The device or system is designed to relieve the maximum flow which could potentially

occur if all the equipment is overpressurized at the same time.

• At least one pressure relief device must be set at or below the equipment with the

lowest Maximum Allowable Working Pressure, or MAWP.


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Some Pressure Protection Standards

Narration:

As shown in this table, some useful standards have been developed by ASME, API and NFPA.

Power boilers and low pressure tanks are subject to different rules, documented in ASME I, API

650 and API 620.


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Recommendations for Maximum Relieving Pressure

Narration:

This illustration by Crowl and Tipler provides recommendations for sizing pressure relief devices.

As shown in this illustration, the typical maximum allowable operating pressure is set at 90% of

the Maximum Allowable Working Pressure. When processes do not operate as close to the

equipment’s MAWP, the emergency relief device’s set point may be set closer to the operating

pressure to provide additional equipment protection by relieving at a pressure even lower than

the maximum allowable set pressure, providing an additional safety factor for protecting the

vessel.


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SECTION 2: Some Pressure Relief Design Terminology

Narration:

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Some Pressure Relief Design Terminology

Narration:

In this section we will define some terms commonly used when designing pressure relief

systems. Your understanding of these terms will be important as you progress through the

remaining units in this course.


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Equipment Maximum Allowable Working Pressure (MAWP)

Narration:

Maximum Allowable Working Pressure is a mechanical design term which specifies the

maximum allowable pressure at the top of a vessel at a designated temperature. It is often

referred to as the “design pressure,” but they are not always the same. We’ll define design

pressure on the next slide.

The MAWP is the pressure on the vessel’s nameplate which is fastened to the vessel. The MAWP

is the pressure which the vessel can withstand.


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Equipment Design Pressure

Narration:

Design Pressure is a process design term which specifies the minimum pressure to which the

vessel must be designed; that is, it is the pressure which the vessel must withstand to perform

its intended function.

The MAWP must be greater or equal to design pressure. In practice, the two are often the same,

but again that is not always the case.

Note that if the temperature of the vessel exceeds its design temperature, its operating

pressure must be reduced. The reason why will be described later in this course.


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Relief Device Set Pressure

Narration:

The relief device “Set Pressure” is the pressure at which the relief device operates.

In this case, the rupture disc will rupture once the vessel in the pressure exceeds the rupture

disc’s pressure rating. When a relief valve is used, the set pressure is the pressure when the

relief valve starts to open. The different types of relief devices will be described later in this

course in Unit 3 – Types of Pressure Protection Equipment.

If the relief device fails to relieve the pressure quickly enough, the vessel could catastrophically

fail. Depending on the type of service, a typical relief device set pressure may be set at 90% of

the equipment’s Maximum Allowable Working Pressure (MAWP).


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Relief Device Design “Overpressure”

Narration:

When designing a relief device, the “overpressure” is defined as the pressure increase over the

relief device’s set pressure. This is usually expressed as a percentage of the set pressure.


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Relief Device Design “Accumulation”

Narration:

When designing a pressure protection system, the "accumulation” is defined as the pressure

increase over the MAWP that occurs after a relief device has opened. This is usually expressed

as a percentage of MAWP.


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Process Hazards Analysis (PHA) Scenario “Overpressure”

Narration:

When determining potential high pressure scenarios in a process hazards analysis, the potential

excess pressure vessels or piping may experience during the process upset is defined as the

system’s “overpressure.” The system “overpressure” is the maximum potential pressure that

could occur on the processing equipment and piping during the unexpected process upset that

is over the vessel’s or piping’s Maximum Available Working Pressure. This is usually expressed as

a percentage of the vessel’s or piping’s MAWP.

When designing a pressure protection system, this is also known as the “accumulation.” Thus,

when the relief device is set at MAWP, the vessel “overpressure” and “accumulation” have the

same value.


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Guidelines for Relief Pressures

Narration:

This interactive chart will enable you to view the differences between four terms we have

covered in this section.

Drag the slider on the left side of the scale to see pressure vessel requirements as a percent of

maximum allowable working pressure.

Drag the slider on the right side of the scale to see typical characteristics of safety relief valves

as a percent of maximum allowable working pressure.

Click the book icon if you would like to open a printable copy of an annotated version of the

chart.


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Left 90 (Slide Layer)


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Right 100 (Slide Layer)


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SECTION 3: Overpressure Case Study

Narration:

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Overpressure Case Study

Narration:

When sufficient pressure relief is not provided, the effects of an excess pressure incident can

sometimes be devastating. Let’s examine a case study.

A laboratory technician decided to field test his bench-scale process. He took advantage of an

available, idle, 1135 liter reactor out on the plant floor.

Unfortunately, the technician did not receive clearance for his experiment. He did not follow a

“management of change” (MOC) procedure to ensure that the hazards and risks associated with

this change were properly reviewed and approved.

During the first scale-up run, a violent runaway reaction ensued, rapidly raising the pressure in

the reactor, overwhelming the relieving capability of the reactor’s relief device. Ultimately, the

excess pressure inside the reactor caused its violent, catastrophic physical failure.


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Overpressure Case Study (continued)

Narration:

The reactor, located on the first level, split in two; the lower reactor portion was propelled into

the building’s ground floor and the mixer on the top was propelled upwards and became

embedded in the concrete ceiling above the reactor. The building sustained major physical

damage.

Fortunately, no one was injured, as the event happened during a lunch break and the building

was nearly empty.

Thus, reactor vessels with insufficiently designed relief systems may catastrophically fail when

an uncontrolled or not understood runaway reaction occurs.


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Unit 1 Summary

Narration:

We’ve reached the end of the first unit in the Hazards and Risks: Introduction to Pressure

Protection course. Having completed this unit titled “Pressure Protection System Concepts,” you

should now be able to:

• State why pressure protection is essential for process equipment; and

• Define some of the terms used when designing pressure relief equipment.

In Unit 2, we’ll explore scenarios requiring pressure protection. But first, please take the quiz for

Unit 1 beginning on the next slide.

ela971 unit 1 - pressure protection system concepts

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