Engineering Training Modules for Nuclear Plant Engineers. Relief and Safety Valves

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1 WARNING: Please read the License Agreement on the back cover before removing the Wrapping Material Engineering Technical Training Modules for Nuclear Plant Engineers Mechanical Series: Module #11 Relief and Safety Valves

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3 Engineering Training Modules for Nuclear Plant Engineers Mechanical Series: Module #11 Relief and Safety Valves Technical Report, October 1999 EPRI Project Manager(s) T. Eckert M. Hooker

4 EPRI 3412 Hillview Avenue, Palo Alto, California PO Box 10412, Palo Alto, California USA

5 DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI This is an EPRI Technical Update report. A Technical Update report is intended as an informal report of continuing research, a meeting, or a topical study. It is not a final EPRI technical report. ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA Toll free number: , press 2, or internally x5379; voice: ; fax: Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright 2005 Electric Power Research Institute, Inc. All rights reserved.

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7 TABLE OF CONTENTS 1.0 SCOPE AND PURPOSE SUGGESTED SKILLS AND KNOWLEDGE OBJECTIVES NOMENCLATURE PRINCIPLES AND PROPERTIES Operation of Relief Valves Safety Valves Safety Relief Vavles Power Operated Relief Valves Valve Performance Requirements Relieving Valve Sizing Relieving Valve Design Considerations Valve/System Interactions Pressure Relieving Valve Testing Requirements NUCLEAR CONSIDERATIONS UTILITY EXAMPLE EXERCISES AND SOLUTIONS

8 8.0 SOURCE DOCUMENTATION INDUSTRY OPERATING EXPERIENCE PROFICIENCY MEASURES

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10 1.0 SCOPE AND PURPOSE Pressure relieving valves (relief, safety and safety relief valves) are used throughout the nuclear power generating industry to provide overpressure protection of pressurized systems. These valves normally operate automatically, on sensing the increased system pressure, but in many models, can be operated manually as well. This module provides an overview of the operation of relief and safety valves in nuclear power generating plants and the associated maintenance and testing requirements. 2

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12 2.0 SUGGESTED SKILLS AND KNOWLEDGE It is suggested that the student understand the fundamentals of fluid mechanics and hydraulics. 4

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14 3.0 OBJECTIVES The objective of this training module is to familiarize the student with the following principles, properties and applications. At the conclusion of this module, the student will describe: Function and Application of Relief and Safety Valves Materials Used in the Valve Construction ASME XI Test Requirements Valve Sizing Simplified analysis to determine the forces to open and close relief valves 6

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16 4.0 NOMENCLATURE 4.1 Symbols and Abbreviations A = cross sectional area of the safety valve nozzle throat Cv = valve flow coefficient g = acceleration of gravity Kb = capacity correction factor due to back pressure, used in the sizing equations of safety relief valve for gas or vapor service KD = coefficient of discharge of pressure relief valves, expressing the ratio of the actual measured relieving capacity to the theoretical relieving capacity, based on the nominal flow area F o = Opening Force Ds = Sealing Diameter DK = Piston Diameter P1 = Static Inlet Fluid Pressure F o = Initial Closing Force H = Compression displacement of the main spring (equal to the disc lift) k = Spring Constant Dstem = Stem Diameter Pz = Pressure above piston (backpressure) w = rated flow saturated steam a = actual discharge area through the valve developed lift P = (set pressure x 1.03) plus atmospheric pressure K = coefficient of discharge = 0.9 KD 4.2 Definitions Maximum Allowable working Pressure: Maximum Allowable Working Pressure (MAWP) is the highest or lowest pressure a vessel is expected to be exposed to during various operating 8

17 conditions. The vessel may not be operated outside these set conditions. Consequently, this is the highest or lowest pressure at which the primary pressure relieving valve is set to open. Set Pressure: Set pressure is the inlet pressure at which the pressure relieving valve starts to discharge under service conditions. Set pressure is normally measured in pounds per square inch gauge. Accumulation: Pressure increase over the MAWP of the vessel during discharge through the valve. It is expressed as a percentage (eg. 10% accumulation). Consequently, it is the increase in pressure above MAWP that occurs. Overpressure: Pressure increase or decrease beyond the set pressure of the primary relieving device. Overpressure is the same as accumulation when the relieving device is set to open at the MAWP of the vessel. Blowdown: Blowdown is the difference between the set pressure ( popping pressure) and the resetting pressure of a pressure relieving valve. This pressure is commonly expressed as a percentage of the set pressure such as 5% Blowdown. Another way of describing blowdown is to say that it is the difference between set pressure of the valve and system pressure when the valve recloses. Sequential Lift Series: Sequential lift series applies when there is more than one pressure relieving device in the system. It is an important concept to understand since ASME codes dictate the percentage of operating pressure at which the first relieving device must open (e.g. 105%, 107%, etc.) and the required capacity which all the relieving devices must have when open. Simmer: Simmer is leakage between the seat and disc just prior to the valve opening. Nozzle: The nozzle performs three functions: Acts as the valve seat Directs the flow under the valve disc Controls the rate that fluid is allowed to escape from the system. Adjusting Rings: Adjusting rings regulate blowdown and simmer. Rings vary from one manufacturer to another; some have upper and lower rings while many have only a lower ring. 9

18 Adjusting ring(s) affect simmer and blowdown since a change in position changes/alters the effective area that fluid acts upon. Blowdown Adjustment: If the valve has both upper and lower adjusting rings, altering the position of the upper ring changes the size of the huddling chamber giving a greater/lesser percentage of blowdown when the valve opens. If one lower adjusting ring is used, repositioning it adjusts the nozzling effect on the fluid. It then increases or reduces the amount of fluid focused on the disc assembly and the length of time the fluid is held in the nozzle and disk area. This increases or reduces the amount of upward force and results in a greater or lesser percentage of blowdown. Huddling Chamber: A ring shaped pressure chamber located beyond the valve seat diameter. It gives safety and safety-relief valves a popping action by providing additional surface area once the valve initially comes off its seat. 10

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20 5.0 PRINCIPLES AND PROPERTIES Pressure relieving valves play an important role in the protection of station systems, components and personnel. In order to do this, these valves must perform two important functions. Pressure relieving valves must: Relieve overpressure conditions Control system volume Pressure relieving valves are used in virtually all piping systems throughout the station. These include systems that are common to nuclear generating stations such as: Primary Coolant Component Cooling Water Main and Auxiliary Feedwater Service Water Demineralized Water Makeup Filtered Water Station and Instrument Air Main and Auxiliary Steam 5.1 Operation of Relief Valves Relief valves are pressure relieving devices actuated by the static pressure upstream of the device. They open in proportion to the increase in pressure over the valve set pressure and are used primarily in liquid service. Relief valves function in the following manner. First, an initial downward force is applied by compressing a spring with an adjusting screw. When system pressure increases, upward force on the disc is developed. As long as downward force applied by spring load is greater than upward force created by the fluid acting on the disc area, the valve will remain closed. Also, valve opening will be proportional to overpressure since the disc area that fluid is acting upon remains the same. 12

21 Relief valves do not need a large opening to relieve over pressure as do safety valves since the fluid being discharged does not expand. System pressure drops rapidly once valve discharge begins. Therefore, slow opening is acceptable in liquid service applications. As the inlet pressure reaches the valve set pressure, the disc lifts a minimal amount and small steady stream of fluid begins to flow. When the pressure begins to decrease, the flow and the lift decrease until the valve closes at a pressure at or below the set point. Relief valves are normally designed with a closed bonnet. The bonnet is usually vented to the discharge side of the valve to avoid adverse effects on the performance of the valve. The set pressure of the relief valve is determined by the force of the spring acting on the disc and the back pressure on the valve. The amount of the backpressure effect is directly proportional to the area exposed to the backpressure. Disk Figure 1: Relief Valve 13

22 5.2 Safety Valves Safety valves are pressure relieving devices actuated by the inlet static pressure and are characterized by rapid opening or popping action. Safety valves may be spring loaded or pilot actuated. In the spring loaded design, the force of the system pressure on the disc is opposed by a main spring. In the pilot actuated design, the system pressure acts on a small pilot valve, which opens at the set point of the valve and creates a pressure imbalance in the main valve, causing the main disc to open. Both types of valves actuate automatically when the fluid pressure in the protected system reaches a predetermined set pressure. Safety valves are used on steam, gas and vapor services. Since the fluid contained is compressed, a larger valve opening is required to achieve a given pressure drop in the system. The "popping" characteristic results since the fluid being discharged is acting on a larger disc area when the valve opens and suddenly expanding to a larger volume. Safety valves have essentially two disc areas for fluid to act upon. The "initial" area corresponds to the inside diameter of the nozzle. The "final" area is larger, therefore, more upward force is created as the valve begins to discharge fluid. They are designed to mitigate pressure rise in the system to below a defined design value. Subsequent to the pressure transient, the safety valve reseats and is prepared to provide pressure relief again, if required. Many safety valves vents the spring housing to the discharge side of the valve. The operational characteristics are directly affected by changes in the back pressure on the valve. A balanced safety valveallow incorporates a bellows assembly to minimize the effects of back Test levers for pressure on its operational characteristics. manually lifting pressure relief for test Safety valvespurposes are usedorforpm's. compressible fluid service when large volumes are required to be exhausted for pressure relief. D Nozzle Ring Huddlin g Nozzl 14 Figure 2: Safety Valve

23 5.3 Safety Relief Valves Safety relief valves are pressure relieving devices actuated by the inlet static pressure and characterized by rapid opening or popping action, or by opening in proportion to the increase in pressure over the opening pressure, depending on application. Safety relief valves can be used for either liquid or compressible fluid service. The primary difference between a safety relief valve and a safety valve is that the safety relief valve has a fluid tight bonnet, allowing it to be used for liquid service. Similar to the safety valve, safety relief valves may be spring loaded or pilot actuated. A variation of the safety relief valve is the balanced safety relief valve. In this design, the uncompensated area of the disc is isolated from the backpressure by a bellows, which is vented to the atmosphere. D Nozzl Nozzle 15 Figure 3: Safety Relief Valve

24 5.4 Power Operated Relief Valves Power operated relief valves (PORVs) are pressure relieving devices which require an external power supply for actuation. These valves are typically controlled by an electrical signal resulting from high system pressure or manually from the control room. The electrical signal initiates the relief action by activating the valve actuator, either electrically or pneumatically. The primary function of PORVs is to inhibit pressure increases due to anticipated operational transients, and minimize the probability of safety or safety relief valve actuation by mitigating pressure rise. PORVs are commonly used in steam and primary side applications in nuclear power plants. 5.5 Valve Performance Requirements In order to establish performance requirements and then to properly size a valve, it is necessary to determine the fluid parameters being contained by the valve and consider other interactions that could affect valve operations. Fluid parameters are important factors in choosing the best valve for the application. These parameters will influence the choice of the valve type and size, the valve body, seat, and disc material. 16

25 5.5.1 Flow Media Flow media chemistry will determine the type of material used for the valve body and other wetted parts. A highly corrosive media will require careful consideration of the valve type and the materials used. At nuclear power plants, the potential for leakage from a valve of radioactive fluid or a combustible gas such as hydrogen requires special consideration in valve selection and application Pressure/Temperature System design pressure and temperature are based on the most severe conditions expected during system operation. These design conditions are used to determine the primary valve pressure rating. The adjusted pressure/temperature rating tables given in ANSI B16.34 and B16.5 list the maximum allowable working pressures for a given temperature. The maximum operating temperature may be limited to temperature lower than that specified in the pressure/temperature rating table if elastomers are used for the seats and seals. This restriction is described in the applicable code Fluid Density, Specific Volume, and Specific Gravity Density or specific gravity of the fluid must be considered if the pressure drop is calculated using the Cv coefficient (Cv is defined based on flow of water at 60oF). The density of the fluid changes with temperature, but very little with pressure unless very high pressures are being considered. The densities of gasses and vapor, however, are greatly affected by pressure changes Radiation Elastomers and synthetics are commonly used in valve construction for seats, seals, liners, and sleeves. They have a lower resistance to radiation than metals. Therefore, if elastomers or synthetics are considered for use in valves which will be exposed to long term radiation exposure, the total integrated dose (TID) must be specified to establish the valve design basis. TID, temperature and flow media chemistry, will determine the type of elastomer is best suited for the application and its service life Fluid Transients Dynamic fluid effects such as water hammer, flashing, and cavitation, and thermal shock initiated from operation of either the valve or the system must be considered Water hammer is the dynamic effect caused by rapid acceleration, deceleration, or flow reversal of a mass of liquid. Severe water hammer may be cause by: Steam pocket collapse in a pipe upon injection of steam into subcooled water. Steam and subcooled water interactions in horizontal pipes. 17

26 Subcooled water drawn into a collapsing steam-filled vertical pipe. Hot water entering a lower pressure line resulting in a steam bubble flash and subsequent collapse. Steam-propelled water slug flow. Rapid valve operation Water column separation and rejoining. Normal transients that should be accommodated in the valve design, include: Pump start with inadvertently voided discharge lines. Rapid valve opening, closing, or instability. Water entrainment in steam lines caused by steam line control or safety valves opening with condensed steam in the line. Cavitation is a phenomenon that usually occurs in systems where liquid velocities are high, and pressures are near the saturation pressures of the liquid in the system. When the velocity of the liquid increases at sudden changes of pipe cross section (at sharp bends, at throttle valves, or in other similar situations), the localized liquid pressure drops below the vapor pressure of the liquid, and the liquid will flash. The flowing stream now consists of liquid plus pockets of vapor. As the liquid flows back to regions of higher pressure, the pockets of vapor collapse. It is the collapse of the vapor pockets which causes the damage. Although mild cavitation is of little concern, severe cavitation can destroy valves and piping and must be avoided. Flashing occurs when a liquid is reduced in pressure below its vapor pressure. Flashing may occur in a valve when the liquid passes through a restriction and then expands again. When this condition exists, vapor bubbles form and the result in two-phase flow. Thermal shock results from the rapid heating or cooling of metals. It is usually the result of rapid realignment of systems or portions of systems, disturbance of thermal stratification, or operation of pressure relief devices. In nuclear power plants it may not be possible to eliminate thermal transients during certain plant conditions due to operational sequences which require safety systems to operate rapidly to mitigate or prevent more serious conditions. Severe thermal transients can affect valve operability by distorting the working parts, causing binding and sometimes incomplete stroking. 5.6 Relieving Valve Sizing In selection and sizing of a relief valve, it is necessary to determine all potential sources of overpressure and then calculate the fluid removal rate under emergency conditions to maintain the pressure within allowable limits. The ASME B&PV Code provides rules for the design, manufacture, capacity certification, selection, and application of pressure devices used to protect fired and unfired pressure vessels. This section specifically deals with the requirements of overpressure protection in Section III. 18

27 5.6.1 Overpressure Protection The principal design basis for pressure vessels is the safe containment of design pressure. Protection against overpressure is a very important aspect of pressure vessel design. Proper selection, use, location, and maintenance of relief valves are also essential to protect personnel and equipment as well as to comply with codes and laws Determining Required Relieving Capacity The design of the proper pressure relief valve must take into consideration all of the following upset conditions for the individual equipment if such an upset can occur: Blocked discharge Fire exposure Tube ruptures Control valve failure Thermal expansion Utility or auxiliary service failure Each upset condition must be carefully evaluated to determine the worst case condition that will dictate the relieving capacity. The first step in selecting a pressure relief valve for a vessel or system is to determine the required relieving capacity. This requires that all sources of fluid flow and energy into the system be considered and evaluated. Sources of energy might be heat, fire, chemical reaction, etc. Sources of fluid flow could include pumps, compressors, stuck open reducing valves, malfunctioning control valves or inadvertent valve operation, failure of internal heat exchanger tube, or a combination of these types of events. All of these events must be taken into account to determine the worse case overpressure condition. ASME B&PV Code Section III, Article N(X)-7000 provides a comprehensive set of rules for overpressure protection of nuclear plant components. The rules require an analysis of all conditions that could cause overpressure and an analysis of pressure transient conditions. For each plant or installation, this analysis is captured in a document known as the Overpressure Protection Report Determining Set Pressure Section III limits the overpressure to 10% or 3 psi, whichever is greater, above design pressure for most systems of any component within the systems pressure retaining boundary. After the relieving capacity and allowable overpressure have been established, the next step is to determine the set pressure of the safety or relief valve. The rule for setting 19

28 safety and relief valves is that at least one device be set to open at or below the design pressure of the system or vessel being protected. When more than one safety or relief valve is used, the additional valves may be set at slightly higher pressures. This allowable increase in set pressure over MAWP varies throughout the Code depending on the application and circumstances. In all cases, the valves must open fully and relieve their rated capacity at a pressure equal to, or below the system allowable overpressure. Section III requires at least one pressure relieving valve be set at or below the design pressure. Additional devices may be set at slightly higher pressures, so long as the maximum overpressure does not exceed 10% of the design pressure when all relieving valves are discharging. Section VIII requires one safety or relief valve be set at or below MAWP. If additional safety and relief valves are used, they may be set up to 5% above MAWP. The section further allows as supplemental valve added to a vessel to protect against hazard due to fire to be set up to 10% above MAWP Determining Blowdown A pressure relieving valve will generally not reseat until the inlet pressure is reduced to below the set pressure. The difference between actual opening pressure and reseating pressure is called blowdown. Section III allows 5-7% blowdown for safety valves unless a different value is allowed and specified in the valve design specification. For other types of valves, blowdown is specified in the design specification. Section VIII has no blowdown requirements for valves as they are shipped from the manufacturer. 20

29 5.6.5 Selecting Pressure Relieving Valves When the required relieving capacity has been established and the allowable set pressure and overpressure determined, the pressure relieving devices can then be selected. The choice of device is limited to those permitted by the particular code section covering the service in question. The most widely accepted pressure relieving valve by the various Sections of the Code is the spring-loaded pressure relieving valve, either balanced or conventional. Traditionally, spring-loaded valves for boiler or steam application have been called safety valves, while spring-loaded valves for liquid service have been called relief valves, and multi-purpose spring-loaded valves that are used for steam, other compressible fluids, or liquid, have been called safety-relief valves. A spring-loaded relief valve is permitted by all sections of the Code as an overpressure protection device. Operational and construction requirements, however, vary among the sections. A type of auxiliary device used on spring-loaded and pilot valves is the auxiliary actuating device that assists the valve to open and then allows the valve to reseat normally. When the valve is called upon to open, the assist device is actuated, opening the pressure relieving valve by mechanical means. When a safe pressure is restored, the assist device is deactivated and the valve closes normally. If the assist device should fail to operate, there is no interference with the normal operation of the valve. A second type of pressure relieving valve is the pilot-operated valve. This is a valve in which the major relieving device or main valve is combined with and controlled by a self-actuated auxiliary pressure relieving valve, or pilot. The main valve consists of a nozzle and a disc similar to the conventional spring-loaded except that the disc is held in place by system pressure. When the system pressure rises above the set pressure of the valve, the pilot senses that pressure and vents the pressure above the disc allowing the main valve to open. Pilot-operated pressure relieving valves have the advantage that their operation is less influenced by fluid conditions at the valve inlet; they generally can be tested in situ, and a high seating load is maintained up to the opening point of the valve. The valve has more operating parts, however, and because of the typically small passages in the pilot, the cleanliness of the fluid on which this type of valve operates could become a concern. Sections III and VIII permit pilot-operated pressure relieving valves. Another type of pressure relieving valve is the power-actuated valve. These valves depend upon an external energy source provided by electrical, pneumatic, or hydraulic systems and generally operate n response to signals from pressure or temperature-sensing devices. This type of pressure relieving valve offers the benefits of a wide variety of control systems but has the disadvantage of relying on an external source of power that may fail under emergency conditions. Power-actuated relieving valves are often furnished on drum type boilers as a convenience to the operators, even though for that type of boiler Section I permits no credit for their relieving capacity. Power-actuated 21

30 pressure relieving valves are permitted by Sections I and III, provided they are used in addition to self-actuated pressure relieving valves in Section I service and provided that redundant controls and energy sources are included for Section III service Relieving Valve Operational Characteristics The following explains the operational characteristics, Code definitions, and how they are applied to relieving valve designs Set Pressure and Lift The relieving valve set pressure is the pressure where the valve disc has measurable movement in the opening direction to (lift) due to an inlet pressure. The valve lift begins when the inlet fluid pressure has increased to the point where the upward fluid force begins to exceed the downward force on the disc. In a spring-loaded, selfactuated pressure relieving valve, the set pressure or opening characteristics will vary depending upon the service fluid. For compressible fluid such as air, gas and steam, the valve will open with a popping action. For a non-compressible fluid, such as water, opening will occur with minimal disc lift, causing the valve to discharge a small, steady stream of liquid that increases with the inlet pressure. Depending upon the design of the control passage, the disc lift may increase substantially at a pressure value above the opening pressure with an equally substantial increase n flow rate and then increase further to the rated lift at the desired overpressure. The ASME Codes, Section I and VIII and Subsections of ASME Section III NB, NC, ND, and NE and valve design specifications require that pressure relieving valves open within certain set pressure tolerance limits depending upon valve service requirements and set pressure value. Ambient temperatures can affect the valve's set pressure profile. Large ambient temperature transients may cause the valve to open outside expected lift set pressure tolerance. The valve setpoint should be tested at the opening ambient conditions of the valve. However, if the valve is tested at room ambient conditions, a correction factor should be applied. Other variables that may affect the actual set pressure of the valve include: The physical condition of the valve and its parts including the condition of the valve seat The maintenance practices related to the valve. The testing practices related to the valve. The physical environment of the installed condition of the valve (such as ambient and fluid temperature, vibration, back pressure on conventional nonbalance valves, etc.) 22

31 Reseat Pressure and Blowdown Pressure relieving valves, after opening, normally close at a pressure that is below its setpoint and/or actual opening pressure and above the system's normal operating pressure. Closing pressure is a value of inlet static pressure at which the disc reestablishes contact with the seat. The actual value of closing or reseat pressure is used with the actual valve opening pressure to calculate blowdown. Blowdown can be expressed in pressure units or percent and is calculated as follows: Eq. 1 actual set pressure actual closing pressure Percent ( Blowdown ) = x100 set pressure Blowdown( pressure units ) = actual set pressure actual closing pressure Eq. 2 The blowdown requirements and or reseating requirements for pressure relieving valves vary depending upon the valve set pressure value and the valve service conditions. Consequently, the user should understand these requirements. Factors that can affect valve reseat pressure and blowdown are: Improper setting of the valve control ring Changes in fluid and fluid temperature Improper testing practices Improper installation of the valve Overpressure and Capacity Overpressure is a pressure increase at the valve inlet that exceeds the set pressure of a pressure relieving valve is usually expressed as a percent of set pressure. That is if a pressure relieving valve opened at 1000 psig to relieved fluid at 10 percent overpressure, the inlet fluid pressure at the valve inlet would be 1100 psig. The relieving capacity of pressure relieving valves is determined and rated at an overpressure permitted by the applicable code or regulation to which the valve is rated capacity certified. Codes and standards credit each valve design with a portion of the actual measured capacity. In nuclear power plants, pressure relieving valves are usually designed and manufactured to ASME Codes Sections I, III, and VIII. These Codes specify the overpressure at which the valve design is capacity rated and provides formulas for establishing the portion of the actual measured flow that is used as its rated capacity. It is this capacity (rated relieving or nameplate marked capacity) that is used as a basis for the selection and application of a pressure relieving valve for overpressure protection of a vessel or system. 23

32 To obtain a capacity certification such as ASME, the manufacturer submits the information on the valve design advising the ASME of the Code section and fluid to be used. The valve design is then performance and capacity tested to the requirements of this Code and in accordance with safety and relief valve performance and test code ASME OM-1/ANSI PTC-25. After tests are successfully completed on that fluid, a coefficient of discharge is established. This coefficient may be derated depending upon the Code requirements. It is this value that is used with the ASME formula to calculate the rated capacity. The following is a typical formula for an ASME Section III Class 1 full-lift safety valve on steam service (below 1500 psig). The formula is used to determine the valve's rated capacity after being certified (see ASME Section III, SubSection NB, Article NB-7000). w = 51.45aPK Eq. 3 Note: The Code or standard under which the valve is manufactured must be used to determine the rated capacity Chatter and Flutter Pressure relieving valves are designed to operate without chatter and flutter. Chatter is defined as the rapid reciprocating motion of the movable parts where the disc contacts the seat. Flutter is the same as chatter, but the disc does not contact the seat. Chatter or flutter during valve operation can cause damage to the valve internals and can occur on opening or closing. Chatter or flutter can be system or mechanically caused. System caused malfunctions are usually a result of poor inlet piping design or installation of the valve in a poor location. A pressure relieving valve on opening will cause a rapid pressure decay at the valve inlet. Since the valve is a force-balanced device, the pressure decays rapidly below the valve's normal closing pressure, causing the disc lift to be reduced and the disc to move to the closed position. Because system pressure has not been reduced to a level that will permit complete disc closure, increased pressure at the valve inlet will cause the valve to reopen. When this occurs, increased pressure at the valve inlet will cause the valve to reopen. When this occurs, the inlet piping to the valve should be reviewed for pressure losses and restrictions. 24

33 Mechanically caused malfunctions vary from: The incorrect relieving valve selection for the fluid and service condition, i.e. liquid valve for gas service or, conventional valve rather than a balanced valve for a backpressure condition. Poor valve maintenance practices that result in improper: Valve set pressure setting Setting of control/adjusting rings Valve assembly that could cause mechanical interference with the moving parts. Fv Pa Pa Pa Stem Disc Dk Ds mg Pa Pa P1 D Nozzle Figure 4: Valve Opening and Closing Forces The opening force of a relieving valve at zero lift is the force working upon the moving parts in the opening direction due to the fluid pressure (P1) and the ambient pressure (Pa) as follows: Eq. 4 Fo = π 2 π Ds P1 + ( DK2 Ds2 ) Pa

34 The initial closing force of a relieving valve at zero lift is the force working upon the moving parts in the closing direction caused by the main spring force (Fv), the mass of the other moving parts (mg) and the ambient pressure (Pa), as follows: Eq. 5 Fo = Fv + π 2 Dk Pa + mg 4 At zero lift the opening force equals the initial closing force, i.e. F o = F o. Fv = π 2 Ds ( P1 Pa ) mg 4 Eq. 6 Fv is approximately equal to the force required to open the safety and relief valve. Once the initial closing force is overcome by the increased fluid pressure in the system, the disc will lift and relieve the system pressure. The subsequent closing force can be defined as: π 2 F = Fo + hk + ( DK2 Dstem )( Pz Pa ) 4 Eq. 7 The second term in the above equation is not constant since the backpressure is liftdependent. 5.7 Relieving Valve Design Considerations The seat of the valve is the fixed, pressure containing portion of a valve which comes in contact with the closure part, i.e. disc, of the valve. The seat can be all metal construction or may incorporate soft conforming seat inserts, such as elastomers or plastics to make a tighter seal or to reduce the required load to seat. To design a leak tight valve, there must be sufficient force between the seat and the disc to elastically or plastically deform the mating surfaces until the leak path is blocked. In addition to the basic design of the seat itself, other factors which directly affect seating and operability are distortions that can occur at the disc/seat interface due to pressure, thermal gradients, and mechanical loads transmitted to the valve body by the adjacent piping. For 26

35 example, applied bending forces on a valve body can cause the seat plane to tilt and distort, which can result in leakage. To avoid leakage or binding problems caused by line loads, valves should not be located at points of large line loads. Also, the section modulus of the valve body should be significantly greater than the pipe to keep the stresses and distortions within acceptable limits. When using metal to metal seating, the high compressive stresses required to produce surface conformance between the two seating surfaces are achieved by making narrow line to line contact between the disc and the seat. Narrow line to line contact should provide for a certain minimum width in order to establish a tight seal and prevent indentation type of damage caused by the plug on the seat. Additionally, the seat should have enough base width to provide adequate backup cross section capable of supporting the high compressive stress at the disc seat interface without yielding the base material. The required degree of seat tightness and accompanying stem thrust should be reasonable selected. Specifying high seat tightness increases the size and cost of the actuator needed to develop the higher loads. An alternative to using high contact forces is the use of "superfinished" mating surfaces. However a superfinish can degrade quickly in applications where fluid contaminants are present which can get trapped between the mating surfaces during opening and closing action. Another common method used to accomplish a tight seat is to lap the disc and seat during assembly. Developing high compressive stress to achieve good seating should be weighed against potential damage due to galling or gross surface yielding. Cobalt 60 has been identified as the principal isotope responsible for out of core radiation contamination problems plaguing the nuclear power industry. Cobalt 60 is an activation product of natural cobalt which is found in cobalt based alloys, such as Stellite. Stellite is used mostly for hardfacing the seats and discs in valves. Natural cobalt in these alloys is pure Cobalt 59, having a thermal neutron absorption of 34 barns and, if it is in a pathway to the reactor vessel, will pass through the core, be exposed to thermal neutron flux, and be activated to Cobalt 60, having a half life of 5.25 years and emitting 1.3 mev gamma rays. These small particles accumulate in the piping system in crevices and cracks where the flow velocity is small and in stagnant pockets or "crud traps" which are inherent in the design of some valve bodies. The strength of these radioactive sources thus grows with time and becomes a major hindrance to access for maintenance work. 27

36 Recently, there have been efforts to develop low cobalt or cobalt free alloys to replace cobalt base alloys to reduce the exposure of service personnel to radiation due to Cobalt 60. Several EPRI sponsored efforts have been conducted to evaluate the release of cobalt from PWR valves and from valve repair. 28

37 5.8 Valve/System Interactions Various interactions between the valve and the system may have an effect on the pressure boundary of the valve. These types of interactions include pipeline end load, system leakage, and piping vibration Pipeline End Loads Since valves in major industrial piping, and in fossil and nuclear power plant piping, are usually installed using welded ends, and sometimes flanged ends, in lower pressure systems, these connections must be designed to adequately transmit all piping loads while maintaining pressure integrity. The appropriate industrial or ASME codes have the requirements to satisfy these conditions, however, the effect of piping loads must also be considered on the operability of the valve itself. The adequacy of the pressure boundary integrity of the valve and nozzles is normally assured by verifying that the section modulus of the valve, in the approximate area of the intersection of the body and bonnet, is greater than the section modulus of the piping. The Code requires that, as a minimum, the modulus of the valve be at least 10 percent greater than that of the piping. In general practice, the modulus of the valve should be significantly greater than that of the piping, in order to assure operability of the valve. Thus, the piping which is analyzed for loading adequacy will be assumed to fail first. For a nuclear safety related active valve, a specific test is normally done on a prototype valve by imposing loading on the valve, including internal pressure loads and nozzle loads (either directly or indirectly), and operating the valve Leakage System leakage should always be evaluated in consideration of whether the fluid is hazardous or corrosive. Flammable fluid leaks could pose a fire hazard. Boric acid leaks are of particular concern in a PWR plant because of the rapidity with which boric acid can corrode carbon steel over which it may trickle from a stem leak of a stainless steel valve. All reactor water in a PWR plant contains boric acid. Leakage which collects or dribbles on a warm surface will become concentrated as the water evaporates. Concentration may increase to the point at which the boric acid precipitates as crystals on the warm surface. Corrosion will continue underneath the crystals so long as moisture, even in the form of high humidity, is present. 29

38 Leakage or radioactive fluids always presents a hazard which must be considered. If the fluid is highly radioactive, packless valves are generally used. 30

39 5.8.3 Vibration It is prudent to consider that all valves in the plant will be subject to vibration. Vibration may be transmitted to the valve through piping connected to rotating equipment, or it may result from hydrodynamic causes in the valve itself or in adjacent piping. By itself, vibration of such small amplitudes is not a problem requiring correction, but it could cause loosening of attachments and often complete separation. Screwed connections of any kind require positive locking to prevent unscrewing or complete separation of the mating pieces. A positive locking device is one which does not depend for its functioning on friction in any way. Thus, a split washer is not a positive locking device; a castellated nut with a split pin is a positive locking device. Taper pins are not positive locking devices. Small beads of weld metal intended to secure a pin can crack from vibration or thermal cycling. Upset threads depend on friction. Many examples exist of such failures leading to valve damage or worse. This is especially the case when the loose fastener is not observable because the valve is inaccessible or the fastener is inside the valve body, or inside the housing of a valve operator. Vibration considerations must also include checking that the vibration frequency does not match the resonant frequency of the piping. 5.9 Pressure Relieving Valve Testing Requirements The requirements for nuclear safety and relief valve performance are listed in the ASME B&PV Code Section III Article NB-7000 for Class 1 valves, NC-7000 for Class II, and ND7000 for Class III valves. Requirements are set forth for valve set pressures, blowdown, and overpressure capacity. The summary below is based on the 1994 ASME Code Winter Addenda issued February It is important that the Code and Addenda to which the valve was manufactured, as shown on the valve nameplate and Code data report, be used for performance requirements. Set Pressure (Note 1) Manufacturer s Tolerance Set Pressure Blowdown Max. (Note 1) Overpressure for Rated Capacity 0 70 psi psi psi Over 1000 psi ±2 psi ±3% ±10 psi ±1% 95% Rated Capacity (Nameplate) is at 3% or 2 psi above set pressure whichever is greater 31

40 Table 1: CLASS I SAFETY VALVE PERFORMANCE TOLERANCES (ASME Section III, Article NB-7000) Note1: A greater set pressure tolerance and blowdown is permitted but must be specified in the valve design specification. Gagging refers the installation of a threaded rod or similar device into the bonnet of a pressure relieving valve in order to manually hold the valve closed. This holds the spindle down and prevents the valve from opening. Valves are gagged when: Adjusting blowdown with the valve installed in an operating system Hydrostatically testing a system above the relieving valve's set pressure During other special testing procedures. Valves are normally gagged by: Inserting a threaded screw through the bonnet of the valve, or By installing a clamping device that resembles a push puller on open bonnet valves. CAUTION: Valve gags must be removed after adjustment or testing. Failure to do so will leave the valve inoperable and remove all overpressure protection for the system or component that is provided by the valve. Operating experience has shown that relieving valve gags have been left in place after testing and adjustment. One such case resulted in the rupture of a tank in a confined space where replacement was difficult Test Frequencies The requirements for testing frequencies and types are set forth in the ASME B&PV Code Section XI Subsection IWV 3510, ASME/ANSI Operation and Maintenance of Nuclear Power Plants Part 1 (OM 1), and the ANSI/ASME Performance Test Codes (PTC) Prior to the winter 1985 Addenda of Section XI IWV 3510, Section XI required that all safety and relief valves be tested according to the Performance Test Code Although PTC 25.3 (1988) is a Code for certification of pressure relieving valves, it does provide some guidelines, definitions, and requirements for calibration of instruments, 32

41 training of test personnel, etc., PTC 25.3 also has procedures for carrying out several types of tests on different process fluids, and there are sections devoted to capacity calculations for various test fluids. The winter 1985 Addenda of Section XI of the ASME Code requires that safety and relief valves be tested to ANSI/ASME OM 1. The 1994 edition of the ASME OM 1 Code, Appendix I, a mandatory appendix, is the result of years of Code work for inservice testing of pressure relieving valves in light water reactor power plants. The general requirements sections of OM 1, Appendix I defines the scope, terminology, and the responsibilities of plant personnel. Also included are the requirements for testing frequency which requires all Class 1 valves of each type and manufacturer to be tested at least once every five years. All Class 2 valves are to be tested once every ten years. OM 1, Appendix I provides explicit instructions as to the types of testing required for as found and as left conditions and the sequence in which the testing shall be accomplished. The test scope for an individual valve depends on its ASME component class, its function in the plant and design of the valve Test Parameters There are four basic parameter associated with safety valve operation that must be tested. These are set pressure, blowdown, capacity, and seat tightness or leakage. However, inservice testing of pressure relieving valves according to OM 1, Appendix I, does not require blowdown or capacity testing Set Pressure The set pressure of a pressure relieving valve on a steam or a compressible fluid is the pressure at which the valve will pop. This parameter is directly affected by the amount of compressive force exerted on the valve seat by the valve spring. The control for how much force the spring exerts is accomplished by raising or lowering the adjusting bolt or compression screw. There are several acceptable methods for testing for relieving valve set pressure. The test media for set pressure testing will vary with the valve service. For steam valves, saturated steam or other compressible fluid with correlating data shall be used. For valves on a compressible fluid other than steam, air or nitrogen shall be used. For liquid service valves, liquid shall be used. The accumulator used shall have a volume 33

42 and pressure source sufficient to determine the valve set pressure. The valve lift may be restricted if a free falling disc at full lift would damage the valve. An assist device is allowable for set pressure testing of compressible fluid valves only. A requirement for set pressure testing is that the temperature of the valve body must be stabilized and known prior to testing. Additionally, the ambient temperature of the operating environment shall be simulated during testing, or a correlation factor used. The Code allows the control rings to be adjusted to ensure valve action. However, when testing for set pressure, the control rings cannot be altered between openings. Consideration for superimposed backpressure must be known and compensated for in valve set pressure (if required). Currently the minimum elapsed time between set pressure openings is 10 minutes. However, the ASME OM 1 Code Committee is looking into reducing this time interval to 5 minutes. 34