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Technical Document No. TP-V300 Effective May 1997 Crosby Pressure Relief Valve Engineering Handbook Crosby Valve Inc. An FMC Corporation subsidiary COV/CON.PM69/22/97, 756 AM1 Table of Contents Notes COV/CON.PM69/22/97, 756 AM2 *United States Customary System Warning The ination contained in this handbook is for inational purposes only. See also Crosbys computer sizing program, CROSBY-SIZE. The actual selection of valves and valve products is dependent upon numerous factors and should be made only after consultation with applicable Crosby personnel. Crosby assumes no responsibility for the actual selection of such products and hereby expressly disclaims liability for any and all claims and damages which may result from the use or application of this ination or from any consultation with Crosby personnel. CROSBY Pressure Relief Valve ENGINEERING HANDBOOK CONTENTS Chapter 1Introduction to Crosby Engineering Handbook Chapter 2Fundamentals of Pressure Relief Valve Design Chapter 3Terminology Chapter 4Codes and Standards - Summary Chapter 5Valve Sizing and Selection - U.S.C.S.* Units Chapter 6Valve Sizing and Selection - Metric Units Chapter 7Engineering Support Ination AppendixASME Section VIII, Division 1, 1992 Edition rpts Other InationOrdering Ination Pressure Relief Valve Specification Sheet COV/CON.PM69/22/97, 757 AM3 click on chapter for quick access The Crosby Pressure Relief Valve Engineering Hand- book contains important technical ination relating to pressure relief valves. The primary purpose of a pressure relief valve is protec- tion of life and property by venting fluid from an overpressurized vessel. Ination contained in this handbook applies to the overpressure protection of pressure vessels, lines and systems. Reference is made to the ASME Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels. The ination in this handbook is NOT to be used for the application of overpressure protection to power boilers and nuclear power plant components which are addressed in the ASME Boiler and Pressure Vessel Code, Section I, Power Boilers, and Section III, Nuclear Power Plant Components, respectively. Proper sizing, selection, manufacture, assembly, test, installation and maintenance of a pressure relief valve are all critical to obtaining maximum protection. This handbook has been designed to provide a service to Crosby’s customers by presenting reference data and technical recommendations based on our many years of experience in sizing, selecting, testing, installing and operating pressure relief valves. Sufficient data is supplied so to properly size and select Crosby pressure relief valves for specific applications. Ination cov- ering terminology, standards, codes, basic design, siz- ing and selection ination, including examples, are presented in an easy to use at. Some of the material in this handbook is reprinted or excerpted from publications developed by associations or committees in which Crosby has participated. The ination contained in the manual is offered as a guide. Those who use the ination are reminded of the limitations of such a publication and that there is no substitute for qualified engineering analysis. Crosby pressure relief valves are manufactured in ac- cordance with a controlled Quality Assurance Program which meets or exceeds ASME Code Quality Control Program requirements. Capacities are certified by the National Board of Boiler and Pressure Vessel Inspec- tors. These features are assured by the presence of an ASME Code Symbol Stamp and the letters NB on each valve nameplate. Crosbys valves are designed, manu- factured and tested in accordance with a quality man- agement system approved to the International Stan- dard Organizations ISO 9000 Quality Standard Series requirements. With proper sizing and selection, the user can thus be assured that Crosby products are of the highest quality and technical standards in the world of pressure relief technology. When in doubt as to the proper application of any particular data, the user is advised to contact the near- est Crosby Regional Office or Representative. Crosby has a large staff of highly trained people strategically located throughout the world who should be contacted when a question arises. Refer to Crosbys Worldwide Directory for an up-to-date contact listing. Crosbys Computer Aided Valve Sizing Program - “CROSBY-SIZE“ Crosby has designed a computer sizing program, CROSBY-SIZE, to provide maximum service to our cus- tomers by presenting recommendations based on Crosbys many years of experience. Use of this program allows an accurate determination of such parameters as orifice size, maximum flow and predicted sound level. The program is a powerful tool, yet easy to use. Its many features include quick and accurate calculations, user selected units, selection of valve size and style, valve data storage, printed reports, specification sheets and dimensional drawings. Crosby Engineering Handbook Technical Publication No. TP-V300 Chapter I Introduction 1 - 1 CHAPONE.PM69/22/97, 741 AM1 HOME 1 - 2 Crosby Engineering Handbook Chapter 1 Introduction Program control via pop-up windows, function keys, extensive on-line help facilities, easy to read atted screens, immediate flagging of errors, easy editing of displayed s and other features combine to make the program easy to understand and operate. It is assumed that the user of CROSBY-SIZE has a basic understanding of relief valve sizing calculations. The user is responsible for correct determination of service conditions and the suitability of this program for a specific application. CROSBY-SIZE and Crosbys Engineering Handbook are useful tools in sizing pressure relief valves. Should additional clarification be required, contact Crosby. CHAPONE.PM69/22/97, 741 AM2 Introduction A pressure relief valve is a safety device designed to protect a pressurized vessel or system during an over- pressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pres- sure or maximum allowable working pressure MAWP. Since pressure relief valves are safety devices, there are many Codes and Standards written to control their design and application. The purpose of this discussion is to familiarize you with the various parameters involved in the design of a pressure relief valve and provide a brief introduction to some of the Codes and Standards which govern the design and use of pressure relief valves. Excerpts of various applicable Codes and Standards are included in other sections of this handbook. Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure relief valve must be capable of operating at all times, espe- cially during a period of power failure when system controls are nonfunctional. The sole source of power for the pressure relief valve, therefore, is the process fluid. Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure relief valve may be the only device remaining to prevent a catastrophic failure. Since reliability is directly related to the complexity of the device, it is important that the design of the pressure relief valve be as simple as possible. The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a specified over- pressure, and close when the system pressure has returned to a safe level. Pressure relief valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive Crosby Engineering Handbook Technical Publication No. TP-V300 Chapter 2 Design Fundamentals Crosby Style JOS Spring Loaded Pressure Relief Valve Figure F2-1 2 - 1 media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases. These design parameters lead to the wide array of Crosby products available in the market today and provide the challenge for future prod- uct development. Spring Loaded Design The basic spring loaded pressure relief valve has been developed to meet the need for a simple, reliable, system actuated device to provide overpressure protection. Fig- ure F2-1 shows the construction of a spring loaded pressure relief valve. The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disc held against the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed, and a body/bonnet to contain the operating elements. The spring load is adjustable to vary the pressure at which the valve will open. CHAPTWO.PM69/22/97, 746 AM1 HOME Crosby Engineering Handbook 2 - 2 Chapter 2 Design Fundamentals The design of the control or huddling chamber involves a series of design tradeoffs. If the design maximizes lift effort then blowdown will be long. If the design objective is to minimize blowdown, then the lift effort will be diminished. Many pressure relief valves are, therefore, equipped with a nozzle ring which can be adjusted to vary the geometry of the control chamber to meet a particular system operating requirement Figures F2-2 and F2-3. Liquid Trim Designs For liquid applications, Crosby offers a unique, patented liquid trim design designated as Style JLT-JOS or JLT- JBS. See Figure F2-4 showing liquid trim available in metal or soft seated valves. These designs provide stable non-chattering valve perance and high capacity at 10 overpressure. Figure F2-2 is a simple sketch showing the disc held in the closed position by the spring. When system pressure reaches the desired opening pressure, the force of pressure acting over Area A1 equals the force of the spring, and the disc will lift and allow fluid to flow out through the valve. When pressure in the system returns to a safe level, the valve will return to the closed position. When a pressure relief valve begins to lift, the spring force increases. Thus system pressure must increase if lift is to continue. For this reason pressure relief valves are allowed an overpressure allowance to reach full lift. This allowable overpressure is generally 10 for valves on unfired systems. This margin is relatively small and some means must be provided to assist in the lift effort. Trim Areas Diagram Figure F2-2 Most pressure relief valves, therefore, have a secondary control chamber or huddling chamber to enhance lift. A typical configuration is shown in Figure F2-3. As the disc begins to lift, fluid enters the control chamber exposing a larger area A2 of the disc Figure F2-2 to system pressure. This causes an incremental change in force which overcompensates for the increase in spring force and causes the valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift. These effects combine to allow the valve to achieve maximum lift and maximum flow within the allowable overpressure limits. Because of the larger disc area A2 Figure F2-2 exposed to system pressure after the valve achieves lift, the valve will not close until system pressure has been reduced to some level below the set pressure. The design of the control chamber determines where the closing point will occur. The difference between the set pressure and the closing point pressure is called blowdown and is usually ex- pressed as a percentage of set pressure. Crosby Style JOS Pressure Relief Valve Trim Figure F2-3 Metal Seat O-Ring Soft Seat Crosby Styles JLT-JOS and JLT-JBS Figure F2-4 CHAPTWO.PM69/22/97, 746 AM2 Crosby Engineering Handbook 2 - 3 Chapter 2 Design Fundamentals Materials of Construction Compatibility with the process fluid is achieved by care- ful selection of materials of construction. Materials must be chosen with sufficient strength to withstand the pres- sure and temperature of the system fluid. Materials must also resist chemical attack by the process fluid and the local environment to ensure valve function is not im- paired over long periods of exposure. Bearing proper- ties are carefully uated for parts with guiding sur- faces. The ability to achieve a fine finish on the seating surfaces of the disc and nozzle is required for tight shut off. Rates of expansion caused by temperature of mating parts is another design factor. Back Pressure Considerations Pressure relief valves on clean non-toxic, non-corrosive systems may be vented directly to atmosphere. Pres- sure relief valves on corrosive, toxic or valuable recover- able fluids are vented into closed systems. Valves that vent to the atmosphere, either directly or through short vent stacks, are not subjected to elevated back pressure conditions. For valves installed in a closed system, or when a long vent pipe is used, there is a possibility of developing high back pressure. The back pressure on a pressure relief valve must always be uated and its effect on valve perance and relieving capacity must be considered. A review of the force balance on the disc Figure F2-2 on page 2-2 shows that the force of fluid pressure acting on the inlet side of the disc will be balanced by the force of the spring plus whatever pressure exists on the outlet side of the valve. If pressure in the valve outlet varies while the valve is closed, the valve set pressure will change. If back pressure varies while the valve is open and flowing, valve lift and flow rate through the valve can be affected. Care must be taken in the design and application of pressure relief valves to compensate for these variations. Conventional Valves Back pressure which may occur in the downstream system while the valve is closed is called superimposed back pressure. This back pressure may be a result of the valve outlet being connected to a normally pressurized system or may be caused by other pressure relief valves venting into a common header. Compensation for su- perimposed back pressure which is constant may be provided by reducing the spring force. Under this condi- tion the force of the spring plus back pressure acting on the disc would equal the force of the inlet set pressure acting to open the disc. It must be remembered, how- ever, that the value of the set pressure will vary directly with any change in back pressure. Balanced Bellows Valves and Balanced Piston Valves When superimposed back pressure is variable, a bal- anced bellows or balanced piston design is recom- mended. Typical balanced bellows and piston style valves are shown in Figure F2-5. The bellows or piston is designed with an effective pressure area equal to the seat area of th