This document is no longer a CPR. This document implements the requirements of Corporate Procedure ESH100.2.PS.1, Control Pressure Safety Hazards.

IMPORTANT NOTICE: A printed copy of this document may not be the document currently in effect. The official version is the online version located on the Sandia Restricted Network (SRN).

3. PRESSURE SAFETY PRACTICES

Subject Matter Experts: Shane Page, and David Paoletta
Contributor: Pressure Safety Committee
MN471000, Issue S
Revision Date:March 31, 2008; Replaces Document Dated: October 8, 2007
Administrative Changes: June 8, 2010, May 26, 2011, and January 19, 2012

• OVERVIEW
• Objectives
• Policy
• MINIMIZING RISKS AND EXPOSURE
• Introduction
• Minimizing Risks and Exposure to Pressure Hazards
• The Hazard of Liquids Under Pressure
• KEY RELATIONSHIPS IN PROPERLY DESIGNING A PRESSURE SYSTEM
  • List of Key Factors
• SELECTING THE FACTOR OF SAFETY
• SELECTING PROPER MATERIALS AND FABRICATION PROCESSES
• Responsibility
• Important Factors Most Often Overlooked
• CONDUCTING ADEQUATE OVERPRESSURE TESTS
• DESIGNING AND SELECTING PRESSURE VESSELS
• CONSIDERATIONS FOR SECONDARY CONTAINMENT AND PROTECTIVE SHIELDS
• Types of Protective Measures
• When to Use Shields
• Examples of When to Use Shields
• Design Consideration
• AVAILABLE CONSULTING SERVICES

OVERVIEW

Objectives

The objectives of good pressure system design are to:

• Operate safely, which is Sandia's primary goal.
• Attain the intended system objectives.

Policy

A pressurized operation is justified in manned areas at SNL only if the system is demonstrated to be safe.

Unmanned operation is mandatory if safety cannot be demonstrated. Even for unmanned operation, the responsible line manager must assess the consequences of damage to facilities and equipment.

The Pressure Safety Committee (PSC) will make determinations of interpretation or adequacy should any doubt exist that the intent of this Program has been met.

MINIMIZING RISKS AND EXPOSURE

Introduction

Minimizing Risks and Exposure to Pressure Hazards

Requirements

Members of the Workforce shall use one or more of the following measures to minimize risks and exposure to pressure system hazards:

• Identify all hazards and consequences. Methodically identify all hazards. (Perform a Preliminary Hazard Assessment.) Consider especially how failure may occur, and the possible consequences.
• Go "remote." The safest locations for pressure systems are remote from people. Consider these factors:
  • How often are people near pressure systems?
  • How long do people remain near pressure systems?
  • What is the potential for much greater system damage at high pressure?
• Minimize pressure and total volume. Stored energy available for release in case of sudden rupture of a pressure vessel is proportional to total volume, pressure, and compressibility of the fluid. Examine carefully all alternatives to placing high-energy systems in densely populated, high-dollar-value areas.

  Caution: Do not use volume or pressure greater than required.

• Use recognized standards. Standards are available for design of a system that derives from analytical procedures and experience. (See the Bibliography)
• Design conservatively. When in doubt, use conservative judgment.

  Caution: Do not rely too heavily on what you think is an "inherent" safety factor. Sometimes the supposed factor is not present.

• Use material with a predictably safe failure mode. "Brittle" materials sometimes fail unpredictably. For such materials it may be pointless to increase the factor of safety.

  Caution: In manned areas never use a brittle material for a pressure system unless the system is properly shielded.

• Demonstrate structural integrity by overpressure test. Place pressure vessels in service only after they pass appropriate overpressure tests. Conduct the overpressure tests at a level exceeding the Maximum Allowable Working Pressure (MAWP), as shown in Figure 3-1.
• Operate within the original design intent. Do not exceed MAWP. Do not change working fluids or service environments without taking into consideration possible harmful effects.
• Provide backup protection. When required, install suitable pressure relief devices at appropriate locations in pressure systems to ensure that the pressure level will stay within predetermined safe limits in spite of possible equipment malfunctions or operational errors.

  Note:  Redundancy in relief devices is recommended for the more hazardous systems.

• Use proven hardware. The lack of information about the manufacturer's design, test, fabrication, and quality control procedures often makes obtaining reliable hardware the most difficult requirement. If the quality of any pressure hardware received is questionable, perform a thorough inspection and evaluation as necessary.

  The Just-in-Time (JIT) contract imposes quality program requirements on pressure hardware suppliers for Sandia

• Use protective shields. Secondary containment structures, barricades and/or shielding are required in these circumstances:
  • Material has an unpredictable failure mode
  • Quality of critical hardware is unknown
  • All conventional criteria have not been met.

    Example: Too low a factor of safety

• Use tiedowns. Tying down tubes, hoses, and piping at frequent intervals can prevent serious injury. Flexible lines that fail under pressure oftentimes will thrash about unless restrained.

The Hazard of Liquids Under Pressure

Requirements

KEY RELATIONSHIPS IN PROPERLY DESIGNING A PRESSURE SYSTEM

Guidance

Members of the Workforce should review Figure 3-1 for the key relationships in the proper design of a pressure system.

Figure 3-1. Key Relationships

List of Key Factors

Requirements

Members of the Workforce shall ensure that the following key factors are included in the design of a pressure system.

• Relief Devices. When a pressure relief device is required, it should not actuate at operating pressure. The setting of the relief device must not exceed the MAWP of the protected component. (See Chapter 8 for periodic inspection and test requirements.)
• Operating Pressure. The MAWP becomes the design pressure for the system. The highest operating pressure is usually equal to 0.85 MAWP.
• Overpressure Test. Overpressure-test the vessel before placing it into service. Overpressure-test should be 1.3 x MAWP (based on design factor of safety as indicated below).

SELECTING THE FACTOR OF SAFETY

Requirements

Managers shall ensure that pressure systems have the following:

• A minimum factor of safety of 3.5, based on maximum principal stress and ultimate material strength, is required for ASME Section VIII, Division 1 type vessels. Vessels or components that do not fall within the jurisdiction of the ASME or other Codes (ANSI, API, etc.), because of shape, size, operational characteristics, etc., must be analyzed to the extent necessary to show that, by design, the vessel or component meets or exceeds the minimum Code standards prior to use in a manned area.
• An adequate factor of safety must exist and be documented for pressure components to be used in a manned area. Examples of components that do not normally have a factor of safety of 3.5 but that fall under the jurisdiction of other standards, and are therefore acceptable, are:
  • Department of Transportation (DOT) cylinders (see Title DOT 49 CFR).
  • Weapon systems (see SAND90-8220, Pinto Criteria, or DG 10210 Reservoir Design and Certification).

Note: Some systems or components may not have a factor of safety of 3.5 because their designed operational parameters place them beyond the scope of the existing Codes:

• Optimum design sophistication and material/processes selection combined with quantitative nondestructive evaluation (NDE) requirements, or extensive service experience and testing, are necessary before justifying less than a factor of safety of 3.5.

  Note: Shielding of components and/or operators may be appropriate in these instances, and is required when any doubt exists as to the adequacy of the design safety margin.

SELECTING PROPER MATERIALS AND FABRICATION PROCESSES

Note:  Pressure vessel design criteria, including the ASME Code, may not properly address toughness and defect considerations. Some pressure vessel failures are directly attributable to this oversight. It is, of course, very important to also satisfy material strength and chemical compatibility requirements.

Responsibility

Requirements

Important Factors Most Often Overlooked

Guidance

Members of the Workforce should be aware that:

• The majority of pressure vessel failures can be traced directly to using materials and processes that are not tolerant enough of defects (i.e., they lack toughness). For low-temperature applications, consult the Materials Application Engineering and Design Support contact for the proper material selection.
• Several frequently used pressure vessel steels exhibit a significant decrease in toughness at low temperature. This fact makes the use of these materials questionable. With appropriate specifications, this problem can be eliminated. Many pressure vessel steel specifications have, as a supplementary callout, toughness-vs.-temperature requirements.
• Selecting proper materials and fabrication processes according to the ASME Code is satisfactory if, and only if, the appropriate toughness-vs.-temperature requirements are specified. (See ASME Code Section VIII, Division 1, Paragraph UCS-68-68.)

Caution: Do not procure a pressure vessel without dealing with the toughness issue.

CONDUCTING ADEQUATE OVERPRESSURE TESTS

Note: The purpose of the overpressure test and nondestructive evaluation is to assure that the pressure vessel is free from critical defects or critical design flaws. The critical defect size depends on the applied stresses, flaw geometry, and material toughness.

See Chapter 6, "Testing and Evaluating Pressure Systems," for a complete discussion of overpressure testing.

Requirements

Managers shall verify that an overpressure test has been performed before placing a pressure vessel in service.

DESIGNING AND SELECTING PRESSURE VESSELS

Guidance

Managers may choose one of two design alternatives when obtaining pressure vessels:

• An ASME Code design.
• A non-Code design.

Once a design is chosen, the user must then choose whether the equipment is to be:

• Bought commercially, or
• Fabricated in-house.

CONSIDERATIONS FOR SECONDARY CONTAINMENT AND PROTECTIVE SHIELDS

Note:  A protective shield is designed as a barrier between a pressurized component and personnel or facilities to protect against the effects of rupture or leakage. In general, pressure systems should be designed, built, and tested to allow use of the system without auxiliary shielding. However, some systems require special shields to ensure a safe working environment.

Requirements

• Projectiles.
• Overpressure.
• Toxic effects.
• Fire.
• High-energy jets resulting from leakage.

Types of Protective Measures

Guidance

Members of the Workforce should be aware that the most effective protective measure against the hazards of pressure systems is keeping a safe distance between the pressurized component and themselves or facilities that require protection.

In addition to distance, there are three conventional types of protective shields:

1. A rated cell can be designed to:
   • Protect against both penetration and external overpressure.
   • Avoid damage from toxicity.
2. The secondary container, if properly designed and used, is capable of:
   • Containing the explosive force, preventing external damage and escape of toxic materials.
   • Preventing fire.
3. A barricade, which is designed to provide a barrier only against penetration. This is the least effective shield.

When To Use Shields

Requirements

• Marginal.
• Impractical.
• Impossible.

Examples of When to Use Shields

Guidance

• The material used in the pressure system is known or suspected to be subject to brittle fracture.
• Factors of safety are less than recommended.
• No proof test has been conducted.
• An item of hardware is questionable.
• The experiment is an intentional burst test.
• The validity of supporting calculations is questionable.
• System complexity or unusual operating circumstances warrant added protection.

Design Consideration

Note:  What constitutes a protective shield is a difficult question. For example, a 1/4-in. steel plate around a small glass vacuum bell jar is overdesign. On the other hand, nothing but distance or a specially designed bunker can protect against sudden release of energy levels up to 74 X 106 ft-lb (equivalent to 50 lb of TNT).

Requirements

Guidance

Pressure Installers should consider the following design points:

• Brittle materials can pose extreme hazards, depending on factors such as a location near personnel and facilities, energy level, and mode of operation. Protective shields must be adequate to stop fragments and to control overpressure.
• No brittle materials, but some question of design detail, raises the concern of how to stop flying fragments or how to protect against hazards, such as whipping lines, component response, and ruptured gage faces. In this case the hazard itself suggests the type of shielding.
• Hydraulic lines contain liquids under pressure that can be forced through small orifices such as ruptures, holes, or leaking connections, at velocities great enough to penetrate the human body. Except in extreme cases, lightweight metal structures can provide the required protection.
• Vacuum vessels. The failure of a vacuum vessel reduces local pressure below atmospheric pressure and can generate debris.

AVAILABLE CONSULTING SERVICES

Guidance

• The organizational PA, who should be the first point of contact for advice and assistance. Advise the PA early of your plans for new pressure systems.
• Applied Mechanics groups, which furnish analysis and design support for new pressure vessels ranging from simple to complex.
• The Materials Organization, which assists in selecting materials and fabricating processes for pressure vessels.
• The NDE group, which provides nondestructive techniques for evaluating pressure systems.
• Experimental Mechanics, which conducts proof tests and monitors pressure vessel response.
• The Tritium Research Laboratory Safety Advisory Committee at Livermore for approval of experiments to be conducted in the TRL.
• Safety Engineering.
• The Pressure Drawing Review Program Coordinator.

Shane Page, srpage@sandia.gov
Al Bendure, aobendu@sandia.gov

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