Summary/How to Use the Guidebook

1.2.1 General

This section provides the reader with a “road map” to the information presented in this guide and to facilitate easy access to it. The Table of Contents provides a fairly

descriptive title for each section. Section 2 provides the nomenclature and glossary of terms that are common in the industry and used throughout the text. Aspects of component construction common to several different types of valves and actuators are discussed in Section 2. Figures are used extensively to illustrate the different types of valves and specific component details and features.

1 Numbers in brackets denote technical references given in Section 22.

1.2.2 Valve Functions

Section 3 provides the basic valve functions and the features necessary to perform these functions. These functions generally fall into one of the following four categories:

Isolation. The valve is used to isolate portions of a system, an entire system from other systems, or a given piece of equipment (such as a heat exchanger) within a system. To achieve isolation, the valve is typically closed and is expected to exhibit a very low seat leakage.

Modulating/Throttling. In performing a modulating function, the position of the valve closure element (gate, plug, disc, or ball) is varied between the fully open and the fully closed positions. The position of the closure element is controlled by an actuator that is an integral part of the valve or is attached to the valve stem. The position of the valve closure element is automatically controlled by a feedback signal to the actuator to achieve a desired condition (for example, flow rate, fluid level, temperature, pressure) within the system. Modulating valves are used where automatic, repeatable, and accurate control of a system fluid parameter is required.

A throttling function is similar to the modulating function except that the position of the valve closure element is manually controlled either locally or remotely (using a power source to the actuator). The valve closure element is positioned at a fixed percentage of valve opening to satisfy a specific system flow requirement. The valve then provides a constant hydraulic resistance to achieve a fixed pressure drop at a given system flow rate. When the system flow requirement changes, the valve is

manually repositioned to provide the necessary hydraulic resistance and pressure drop.

In this guide, the discussions of air-operated valves and solenoid valves are kept to a minimum because these valves are discussed in great detail in References 1.2 and 1.7 respectively.

Check (Non-Return). Check valves are located in a hydraulic system to ensure that the process medium flows in one direction only. A common application for check valves is at the discharge of multiple pumps in parallel that provide flow and pressure head to a common manifold. In the event that one of the pumps ceases to produce flow and pressure head, a check valve located in its discharge line prevents a flow reversal through the non-operating pump caused by the pressure head produced by the

operating pump(s). Another typical application is at system interfaces where the intent is to allow flow in one direction only from one system into another. Check valves are not normally considered isolation valves because they may exhibit higher leakage rates than usually required for isolation applications.

In this volume of the guide, the discussion of check valve application, use and

maintenance is kept to a minimum because these subjects are discussed in great detail in References 1.20 and 1.21. Volume 2 of this guide provides detailed guidance for check valve repair.

Pressure-Relief. Pressure-relief valves are used to protect piping systems and components from overpressurization by dissipating excess system pressure to a pressure suppression system or to the atmosphere. Pressure relief is performed in a number of ways including:

• The valve opens automatically to discharge system media when pressure at valve inlet (acting directly on valve disc) exceeds a predetermined level. No external power source is needed.

• A pilot valve opens automatically when pressure at the inlet of the pilot valve exceeds a predetermined level. The opening of the pilot valve subsequently opens the main valve. Alternatively, the pilot valve may be opened at any inlet pressure by the application of an external power source.

• The valve opens when the actuator power source receives a signal that the valve inlet pressure exceeds a predetermined level.

• The valve opens when the actuator’s power source receives a signal that other system conditions or events have occurred that will cause a pressure rise to occur (for example, power failure to a pump or the sub-normal pressure preceding a pressure surge or water hammer).

In this guide, the discussions of pressure relief valves are eliminated because these valves are discussed in great detail in Reference 1.4.

1.2.3 Specific Valve Types by Function

Sections 4 through 12 provide information on specific types of valves commonly used to perform isolation and modulating/throttling functions. The specific types addressed are gate, globe, butterfly, ball, plug, and diaphragm valves. The information provided focuses on a number of areas pertinent to the application of each specific valve type.

These are as follows:

Introduction and Application. Performance features and capabilities of the specific valve type are discussed with respect to the stated function, together with other application considerations. For example, for flow isolation, fully open gate valves offer minimal flow resistances and pressure drops (thus reducing pumping costs). However, gate valves require a relatively long stem travel to open and close. Therefore, stroke times

for gate valves are relatively longer than for globe valves, which could adversely affect the system performance. On the other hand, globe valves, while satisfying stroke time requirements, introduce high flow resistances and pressure drops, which may be unacceptable in some applications.

Design. Using a valve cross-sectional drawing, the design features of the specific valve type are discussed.

The effect of different variants of the valve type (for example, solid wedge versus flexible wedge gate valves) on valve performance is noted. The advantages and disadvantages of the variants are discussed.

Installation Practices. The proximity of other components (pumps, piping connections, etc.) may affect valve performance. Installation configuration, direction of flow, forces, and moments applied to the valve by the connecting pipe, orientation to vertical, and accumulation of debris/biological growth inside the valve are typical installation considerations. These are discussed as they apply to each specific valve type and function, and an assessment is provided where a particular sensitivity to any of these exists. General guidelines for valve installation are given in Section 19.

Operation Practices and Precautions. Methods to improve the functional reliability of valves through correct operational practices are discussed. Practices that may adversely affect the performance of valves are presented. Such practices include applying

excessive actuator loading thrust to reduce seat leakage and using of valves for other than the intended function (for example, long-term throttling with a gate valve).

Common Problems. For each valve type, a section is devoted to provide a concise list of the common valve problems and malfunctions. Wherever possible, suggested corrective and preventive actions are given. Detailed repair procedures are given in Volume 2 [1.1].

Maintenance. General discussions of maintenance methods and practices for specific valves are provided. The focus is on areas that are considered critical to achieve

satisfactory valve performance. General discussions of other valve maintenance issues including programmatic consideration, troubleshooting, corrective action, maintenance requirements, and diagnostic equipment are given in Sections 17 through 20.

1.2.4 Actuator Types

Section 13 provides a general introduction to the different types of valve actuators.

Section 14 is dedicated to manual actuators. For other types of actuators, the reader is referred to other EPRI documents [1.2, 1.4, 1.7, and 1.22 through 1.26].

1.2.5 General Design Requirements for Valves and Actuators

Deficient performance and valve failures result from the use of valves under operating conditions for which they were not intended. A complete knowledge of all of the conditions to which the valve will be subjected is extremely valuable in avoiding problems. This includes system start-up, shutdown, and anticipated transient conditions. All verified pertinent valve data should be recorded and filed for future reference. Section 15 provides detailed discussion of general design requirements that need to be defined and applied to valves during the original or replacement

procurement cycle.

1.2.6 Valve Pressure Boundary and Structural Integrity

The valve is an integral part of the system pressure boundary and must be designed so that the integrity of the system is maintained. Section 16 discusses pressure boundary and structural integrity requirements including:

• Applicable codes and standards

• Pressure temperature ratings

• Materials and material compatibility

• Pressure boundary materials and their proper selection

• Corrosion allowance

• Valve end connections

• Pipeline loads and vibrations

• Leakage, and shop hydrostatic testing

• Structural integrity and valve operability

1.2.7 Valve Maintenance and Inspection Programs

In the last few years, there has been ever-increasing pressure on the electric power industry to improve plant efficiency, shorten plant outages, and cut costs. Under this environment, valve maintenance groups are required to improve the efficiency of valve repairs and reliability. Section 17 discusses the different factors that affect valve

maintenance and have direct impact on valve reliability and plant availability.

Recommendations to enhance maintenance programs and procedures are also included in Section 17.

1.2.8 Troubleshooting and Recommended Corrective Actions

One of the most important responsibilities of plant maintenance and operation

personnel is to quickly identify valve problems and determine the necessary corrective actions. In many cases, the root cause is simple but not obvious. Section 18 provides guidance on troubleshooting and recommended corrective actions for gate, globe, butterfly, ball, plug, and diaphragm valves. The use of checklists can improve the quality and the effectiveness of the maintenance activities and are recommended in this guide (see Section 18 for sample checklists).

1.2.9 Installation, Testing, and Maintenance Requirements

Valve installation, testing, and maintenance must meet certain code and regulatory requirements. For nuclear power plants, these requirements are more stringent than in any other application. Section 19 provides a detailed discussion of these requirements and identifies the governing codes that should be reviewed for additional information.

1.2.10 Diagnostic Equipment and Methods

Recent advances in computers and measurement equipment coupled with innovative solutions for measurement problems resulted in a surge in valve diagnostic equipment and methods. Section 20 provides a summary of the state of the art of valve and

actuator diagnostic equipment and methods. It is expected that these advances will continue and new equipment will be developed while existing equipment will be further refined. Thus, the reader is encouraged to continue to obtain new information from diagnostic equipment vendors and service companies that develop and maintain the equipment. However, the information provided in Section 20 can be used as a starting point to identify the specific plant needs.

1.2.11 Valve Selection Chart

Section 21 provides information on using the Valve Selection Chart shown in Figure 21-1. The chart is in the form of an algorithm and is provided for use as a wall chart. It provides a structured path of the mental process of selecting a new valve or evaluating an existing valve. Caution should be exercised in using the chart because it is not a

“go/no-go” device, but rather one that suggests options to be evaluated and points to the direction of needed additional investigation. Some of the options shown may not always be available to the user. Decisions such as the type of valve end connections, valve body/bonnet material, etc., may be mandated by overall system considerations.

Several typical valve applications are presented in the text to assist the reader in the use of the Valve Selection Chart.

1.2.12 References and Bibliography

As mentioned above, the vast amount of information/documents developed over the last few years makes it difficult for plant personnel to locate the applicable documents for a particular need. In this guide, a listing of the key references, codes, and standards are provided to enable the reader to locate additional documents for further study. In Section 22, the references and bibliography are listed according to their categories in six different groups. Proprietary documents (available only to certain program

participants) are included in separate sections and clearly identified. Most of these references provide additional references for specific information such as valve test reports and friction coefficient data.

1.2.13 Appendices

Appendices are provided to broaden the scope of knowledge presented in the text.

References in the text are made to specific appendices where additional information is given on the subject being discussed.

Section 23 provides a brief discussion of recent advances in the valve and actuators technology along with latest regulatory requirements. Section 23 also provides a brief summary of some key EPRI/NMAC documents that are believed to be of particular interest to the reader.

Section 24 provides a brief discussion of control valve sizing methods based on the Instrument Society of America (ISA) approach. Several examples are provided to

further clarify the methods used and to understand their limitations. It should be noted that several computer programs have been developed by valve manufacturers and others to perform control valve sizing calculations. Evaluation and discussion of these computer programs are outside the scope of this guide. It is recommended, however, that the reader seek information about such software from the developing

organizations.

Section 25 provides valve procurement specifications. Suggested data sheets for use by the purchaser and bidder/seller are included for convenience.

Finally, complete reading of the entire guide, including the appendices, should provide the reader with an overall view of the current state of the art of valve and actuator technology.

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