MATERIALS VALVE BODY MATERIALS The choice of material is usually based upon requirements for pressure, temperature, resistance to corrosion, abrasion and method of manufacture. A material which is resistant to abrasion may have little resistance to corrosion for a particular fluid. Some fluids necessitate the use of special, costly materials or materials which pose manufacturing difficulties in order to prevent severe corrosion damage to the valve. The majority of process flu- ids do not impose excessive requirements with regard to pressure, temperature and corrosion, which is why cast iron and cast carbon steel are the most common materials used. When specifying materials it is always advisable to quote a well-known standard designation. In the tables below are reported the common materials used for valve body. The UNS and ASTM designations are quoted for each material. CARBON AND STAINLESS STEEL Group Material Generic Name Major chemical constituents UNS spec. Forging spec. (ASTM) Casting spec. (ASTM) Carbon steel CS C-Mn-Fe K03504 A105/A105N A216 WCB Low tempera- ture carbon steel LTCS C-Mn-Fe K03011 A350 LF2 A352 LCA A352 LCB A352 LCC Stainless steel Austenitic St. St. 300 series 316 13Cr-12Ni- 2Mo S31600 A182 F316 A351 CF8M 316L 13Cr-12Ni- 2Mo S31603 A182 F316L A351 CF3M Duplex 2205 22Cr-5Ni-3Mo-N S31803 A182 F51 A890-J92205 Super Duplex 2507 25Cr-7Ni-4Mo-N S32750 A182 F53 A351-CD4MCu A890 5A Super Austenitic 6Mo 20Cr-18Ni-6Mo S31254 A182 F44 A182-F44 A351- CK3MCuN SPECIAL AND EXOTIC MATERIALS Group Material Generic Name Major chemical constituents UNS spec. Forging spec. (ASTM) Casting spec. (ASTM) Nickel-Iron Alloy Incoloy 800 33Ni-42F e 21 Cr N08800 B564-N08800 - Incoloy 825 42Ni-21.5Cr-3Mo -2.3Cu N08825 B564-N08825 A494-CU5MCuC Nickel Nickel 99/95Ni N02200 B160-N02200 A94-Cz-100 Nickel-Copper Monel 400 67Ni-30Cu N04400 B564-N04400 A94-M35-1 Monel 500 N05500 B564-N05500 - Nickel Superalloys Inconel 600 72Ni-15Cr-8Fe N06600 B564-N06600 A94-CY40 Inconel 625 60Ni-22Cr-9Mo- .5Cb N06625 B564-N06625 A94-CW-6MC Hastelloy C-276 54Ni-15Cr-16Mo N10276 B564-N10276 A94-CW-2M Titanium Titanium 98Ti R50400 B381-GR2 B367-C2 Ti6-4 90Ti 4÷6Al R56400 B381-GR5 -
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MATERIALS
VALVE BODY MATERIALS
The choice of material is usually based upon requirements for pressure, temperature, resistance to corrosion, abrasion and method of manufacture. A material which is resistant to abrasion may have little resistance to corrosion for a particular fluid. Some fluids necessitate the use of special, costly materials or materials which pose manufacturing difficulties in order to prevent severe corrosion damage to the valve. The majority of process flu- ids do not impose excessive requirements with regard to pressure, temperature and corrosion, which is why cast iron and cast carbon steel are the most common materials used. When specifying materials it is always advisable to quote a well-known standard designation. In the tables below are reported the common materials used for valve body. The UNS and ASTM designations are quoted for each material.
When selecting materials, always keep within the temperature limitations specified and make sure that the material combinations are suitable. Because the material for the valve trim is often selected with regard to its resistance to abrasion or cavitation, it is essential that the material combination is compatible, i.e. they resist wear and galling in combination. The process fluid may, or may not, possess lubricating qualities which can assist with both wear and galling. Lube oil or ethylene glycol will assist some combinations whereas steam or paraffin will have little effect. 304 and 316 stainless steel can be surface treated to increase the hardness to about 1100 Hv. The treatment does not impair the corrosion resistance. The ability to improve the hardness dramatically means both materials can be used as trim and self-mated without the associated risks of galling. Soft, non-metallic materials are used for valve seats or seat inserts when it is necessary to have a completely leak-free valve. Choice of the correct material requires detailed knowledge of the process fluid and conditions.
26
TRIM MATERIALS
Group Material Generic Name Major chemical con-
stituents UNS spec.
Forging spec.
(ASTM)
Carbon Steel CS + E.N.P. C-Mn-Fe + E.N.P. Electroless Nickel Plating 75 µ to ASTM B 733
Low Temperature Carbon Steel LTCS + E.N.P. C-Mn-Fe + E.N.P. Electroless Nickel Plating 75 µ to ASTM B 733
Nickel-Iron Alloy Incoloy 800 33Ni-42F e 21 Cr N08800 B564-N08800
Alloy steel 4140 1Cr0,25Mo0,40C0,81Mn UNS G41400 AISI 4140
MATERIALS
SEAL MATERIALS
The choice of seal material is critical to the ultimate success of any sealing system. The "valve normal" operating conditions are usually well-established and can be used for seal selection. However, operation through process upsets is often forgotten; how much do pressure and temperature change? How quickly and for how long? If a sealing system is intended to survive a short upset excursion and remain effective, the conditions must be evaluated during initial selection. Other non-operational conditions may have a dramatic effect on seal life :
1st cleaning with steam 2nd cleaning with chemicals 3rd sterilizing
The valve seal does not differentiate between operational and non-operational conditions. It is exposed to temperature, pressure, fluid and spindle/stem movement, or the lack of movement. The correct material selection can only be made after the valve "life style" has been thoroughly examined. Failure to consider any aspect of "life style" can result in unscheduled stops for maintenance, loss of production and high costs for spare parts. Standard seal material options as follows:
GENERAL
GENERAL ( SEALING )
Generic
Name DESIGNATION
SERVICE TEM-
PERATURE APPLICATION
Graphite
Graphite can be produced as plain fibre and braided to form a self-lubricating high temperature packing, up to 340 °C for general applications. In an inert environment, ambient air excluded, temperatures up to 500 °C are possible. Graphite can be braided with PTFE fibres and lubricated for medium pressure universal applications up to 260°C Pressures up to 400 barg over the full pH range are possible. Typical applications include high pressure steam valves.
Metallic
Spiral wound
Stainless steel 316 with graphite filler
Ring Joint
NSSL can provide RJ in any material designation
MATERIALS
ELASTOMERS
28
- Designations as per ISO 1629 / ASTM D 1418
ELASTOMER ( SEALING )
Generic
Name DESIGNATION
SERVICE TEM-
PERATURE APPLICATION
Nitrile rubber
N.B.R.
0° to + 80°[C]
Also known as acrylonitrile rubber, nitrile content can vary from18% to 50%. Higher nitrile content rubbers are suitable for petrol and petroleum products. An oil-resistant synthetic rubber produced from a copolymer of acrylonitrile and butadiene. Its main applications are in fuel hoses, gaskets, rollers, and other products in which oil resistance is required.
Hydrogenated Nitrile
H.N.B.R.
-40° to + 150°[C]
HNBR has also been widely employed in industrial sealing for oil field exploration and processing, as well as rolls for steel and paper mills. HNBR fulfill “Explosive decompression Application” requirements.
Viton®
F.K.M (F.P.M.)
-40° to + 150°[C]
Compounds with excellent resistance to petroleum products, acids, alkalies and many solvents . Viton is generally compatible with hydrocarbon compatible with Ketones such as ace- tone and organic such as acetic acid. O-rings made of Viton are typically color coded as black, but new gaskets, seals and O- rings should be green FKM or black FKM, but with a green mark on the outer edge. The name is a registered trademark of DuPont Performance Elastomers L.L.C.
Chemraz® Karlez®
F.F.K.M.
-20° to + 220°[C]
Fluoro-elastomers are more expensive than neoprene or nitrile rubber elastomers partly because they provide additional heat and chemical resistance.
MATERIALS
THERMOPLASTICS
THERMOPLASTIC ( SEALING )
NSSL DESIGNATION SERVICE
TEMPERATURE APPLICATION
P.T.F.E
Polytetrafluoroethylene
-100° to + 180÷220°[C]
Attacked by molten alkaline metals, such as sodium, and some halogenated compounds. Cannot be moulded, with the exception of some of its very similar alloys. Seals are machined from solid. its unsurpassed lubricating qualities assist with dry operation. PTFE has poor resilience but excel- lent chemical resistance. The resilience problem can be overcome by applying a PTFE coating to a more elastic core, such as graphite, natural or synthetic rubber, silicone rubber or Viton®.
-100° to + 180÷250°[C] Reinforced PTFE seats are made with glass filled PTFE (20%). They are harder than virgin PTFE. White in colour with green or blue speckles.
R.P.T.F.E. / carbon graphite
Polytetrafl uoroethylene carbon-graphite fi lled
-100° to + 180÷250°[C] Reinforced PTFE with 20% carbon and 5% Graphite. These seats are black in colour
POM (Delrin®)
Polyoxymethylene acetal resin
-57° to + 85°[C]
Delrin® bridges the gap between metals and ordinary plastics with a unique combination of creep resistance, strength, stiffness, hardness, dimensional stability, toughness, fatigue resistance, solvent and fuel resistance, abrasion resistance, low wear and low friction. No oxygen service.
Nylon 12G Polyamide -50° to + 120°[C] Limited in temperature service, Nylon G12 is suggested for high pressure instead of PTFE
Devlon®
Polyamide
-100° to + 150°[C]
At high temperature material with good abrasion resistance and low coefficient of friction. Good resistance to hydrocarbon products. Similar to Nylon 12G but with a wide range of temperature.
Vespel® Sp21
15% Graphite Filled Polyimide
-100° to + 340°[C]
Low wear at high bearing, low coefficient of friction, long term thermal stability, outstanding performance with or without lubrication, good strength.
PEEK
Polyetheretherketone
-100° to + 260°[C]
PEEK is a semi-crystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained to high temperatures. The Young’s modulus is 3.9 GPa and its tensile strength 90 to 100 MPa. PEEK is used to fabricate items used in demanding applications, including bearings piston parts, valves and pumps.
PCTFE (Kel-F®) Polyclorotrifluoroethilene -250° to + 150°[C] Recommended for cryogenic services.
UHMWPE
Ultra High Molecular Weight Polyethylene
-200° to + 80°[C]
Outstanding abrasion resistance, superior impact resistance, non-sticking and self-lubricating properties and excellent mechanical properties, even in cryogenic conditions
- Designations as per ISO 1043.1 / ASTM D 1600
MATERIALS
ABOUT CORROSION
The name NACE (originally known as the National Association of Corrosion Engineers) has become synonymous with the requirement to combat Sulphide Stress Cracking, SSC, even though NACE produces more information on all forms of corrosion. The NACE Standard MR-01-75 is applied to equipment which is exposed to hydrogen sulphide. In very broad terms, sulphide stress cracking is a problem for materials which have been hardened and tempered to increase strength. The main thrust of the NACE requirements is to limit material hardness. In oilfield applications, fluids are divided into two major groups:
1. sweet 2. sour
Sweet fluids contain no hydrogen sulphide. Sour fluids contain hydrogen sulphide, usually as a trace element. The percentage of hydrogen sulphide and the partial pressure are important factors in the probability of SSC. NACE excludes the use of cast iron for pressure containment as well as the use of free-machining variety of steels. For carbon steels, the hardness is restricted to 22 HRc, which is approximately 238 BHN. Welded
components must be stress relieved to ensure the heat-affected-zone (HAZ), meets the hardness limit. Alternatively, the carbon content can be restricted to less than 0.18% which effectively removes the gardening possibility. With carbon steels, compliance with NACE does not create problems. Major problems are present with alloy steels. Consider bolting : AISI 4140, ASTM A193 B7 is the most popular bolt/stud material. NACE limits the hardness to 22 HRc when the normal hardness would be 24 to 32 HRC. The reduction in hardness produces a reduction in strength; the ultimate strength will be reduced from 850/1000 MPa to about 770 MPa maximum. This means a bolted connection will only have approximately 90% of the strength. Equipment working close to the pressure limit may require alternative, better materials. The austenitic stainless steels cannot be work hardened to increase the strength. Again, this can be a major problem for bolting and similar bar components. SSC is gen- erally a problem experienced by materials in tension. Valve seats, pressed into the body, will be in compression and should not be susceptible to SSC. When NACE requirements are applied to a valve the design must be checked, not just the materials. Changing from B7 studs to B7M studs may create new problems while eliminating the SSC issue.
A list of some popular materials with their respective hardness limitations can be found in Table 13.2 of NACE.
Typical example of Intergranular Cracking due to Stress Corrosion in correspondence of the grain boundary
MATERIALS
ABOUT CORROSION
Causes of premature fracture influenced by the corrosion of a structural component.
Two basic corrosion reactions, anodic and cathodic, dominate the SCC process in conjunction with mechanical stress. The chemical composition of the environment, including pH and the presence of hydrogen recombination poisons that affect the cathode reaction product, and the composition and metallurgical condition of the metal determine which of the two partial corrosion reactions is dominant. Anodic SCC (active path corrosion) involves the dissolution of metal during the initiation and propagation of cracks. Cathodic SCC (embrittlement by corrosion product hydrogen) involves the deposition of hydrogen at cathodic sites on the metal surface or on the walls of a fissure or crack and its subsequent absorption into the metal lattice.
MATERIALS
ABOUT CORROSION
Stress-corrosion cracking (SCC) is a term used to describe service failures in engineering materials that occur by slow environmentally induced crack propagation. The observed crack propagation is the result of the combined and synergistic interaction of mechanical stress and corrosion reactions. This is a simple definition of a complex
subject, and like most simplifications, it fails to identify the boundaries of the subject. As a result, before this problem can be dis- cussed in detail, one must clearly define the type of loading involved, the types of materials involved, the types of environments that cause this type of crack propagation, and the nature of the interactions that result in this phenomenon. The term stress-corrosion cracking is frequently used to describe any type of environmentally induced or assisted crack propagation.
Effect of strain rate on SCC and hydrogen-induced
32 cracking
MATERIAL ENVIROMENT
Carbon steel Hot nitrate, hydroxide, and carbonate/bicarbonate solutions
High-strength steels Aqueous electrolytes, particularly when containing H2S
Zirconium alloys Aqueous Cl- solutions; organic liquids; I2 at 350 °C (660 °F)
In general, SCC is observed in alloy-environment combinations that result in the formation of a film on the metal surface. These films may be passivating layers, tarnish films, or dealloyed layers. In many cases, these films re- duce the rate of general or uniform corrosion, making the alloy desirable for resistance to uniform corrosion in the environment. As a result, SCC is of greatest concern in the corrosion-resistant alloys exposed to aggressive aqueous environments.
MATERIALS
ABOUT CORROSION
Schematic of a typical time to failure as a function of initially applied stress for smooth sample stress corrosion cracking tests
Schematic diagram of typical crack propagation rate as a function of crack tip stress intensity behavior illustrating the regions of stage 1, 2, and 3 crack propagation as well as identifying the plateau velocity and the threshold stress intensity.
Crack Initiation at Surface Discontinuities. Stress-corrosion cracking frequently initiates at preexisting or
corrosion-induced surface features. These features may include grooves, laps, or burrs resulting from fabrication processes. Examples of such features are shown in Fig. 1; these were produced during grinding in the
preparation of a joint for welding. The feature shown in Fig. (a) is a lap, which subsequently recrystallized during welding and could now act as a crevice at which deleterious cations concentrate. The highly sensitized recrystallized material could also more readily become the site of crack initiation by intergranular corrosion. A cold-worked layer and surface burrs, shown in Fig. (b), can also assist crack initiation.
Metallographic examination: optical micrographs showing defects on the inner surface of type 304 stainless steel pipe near weld root (a) and near through crack (b).
❶
DEFINITIONS
Annealing
Heating steel to, and holding at a suitable temperature, followed by relatively slow cooling. The purpose of annealing may be to remove stresses, to soften the steel, to improve machining, to improve cold working properties, to obtain a desired structure. The annealing process usually involves allowing the steel to cool slowly in the furnace. Anti-static design
A design which provides electrical continuity for the moving valve components and across the valve body to the connected piping. ANSI
The American National Standards Institute, the National Standards Authority of the USA. API
The American Petroleum Institute ASME
American Society of Mechanical Engineers ASTM
American Society for Testing and Materials Atmospheric pressure
Atmospheric pressure is standardised, technically, at 101.325 kPa (absolute), equivalent to 14.696 psi( a). Remember! This is the pressure at sea level. Equipment working at altitude will not experience the same atmospheric pressure. Also the local atmospheric pressure changes with the weather. Local values are
34 likely to vary from 94 to 108 kPa at sea level. Austempering
Quenching from a temperature above the transformation range to a temperature above the upper limit of martensite formation, and holding at this temperature until the austenite is completely transformed to the desired intermediate structure, for the purpose of conferring certain mechanical properties. Austenite
A face centred cubic crystal structure found in chrome-nickel stainless steels. Austenitic stainless steel
Steels containing high percentages of certain alloying elements such as manganese and nickel which are austenitic at room temperature and cannot be hardened by normal heat-treatment but do work harden. They are also non-magnetic in the annealed condition and very corrosion resistant. Chromium content usually falls between 16 and 26% and nickel content less than 35%. Typical examples of austenitic steels include the 18/8 stainless steels and 14 % manganese steel. Work gardening or welding may induce some magnetism. Back pressure
The pressure at the safety valve outlet. This can be expressed as a percentage of the fluid inlet pressure or as a specific pressure. Back pressure is very important and has a significant effect on valve performance. Blowdown valve
A valve which permits pressurised fluid to be released to the environment. Bolted gland
A gland which is attached to the bonnet by two or more bolts or studs. Because of the difficulty of maintaining exact alignment on adjustable versions some form of spherical bearing on the packing follower is recommended.
Bonnet
The component attached to the valve body through which the stem or spindle passes and has locations for mounting the actuator. The bonnet includes the packing box which is equivalent to the stuffing box on a pump or compressor. The bonnet can be threaded, flanged, union, clamped, breech-lock or welded to the body. Special valves can have the bonnet retained by shear rings. Braided packing
Strands or filaments, all running in the same longitudinal direction, that are twisted together to form square or rectangular sections. Various types of braiding are possible to produce harder, stiffer or softer, more pliable, packing. The braiding can be applied over a soft or hard central core. Reinforced corners can be incorporated. The strands can be coated with solid or fluid lubricant to extend operating conditions. Metallic strands can be included for reinforcement and to improve heat transfer. Braided packing can be extruded to compress the section and control the finished size. Braided packing is generally supplied as a continuous length, 5 to 20 m, depending upon the cross section. Sealing rings are produced by cutting the required length from the coil. Breakaway force
The force required on a linear motion sealing element to open the valve against maximum differential pressure. Breakaway torque
The torque required on a rotary motion sealing element to open the valve against maximum differential pressure. Bright drawing
The process of drawing hot rolled steel through a die to impart close dimensional tolerances, a bright, scale free surface, and improved mechanical properties. The product is termed bright. Built-up back pressure
The pressure experienced at the safety valve outlet when passing 100% flow, due to friction losses in attached pipework and any static back pressure. Butt fusion weld
A thermal welding technique used with some non-metallic piping systems. Materials such as polypropylene can be welded by using electric heaters. Machines can join 1200 mm diameter pipe. Butt weld joint
A welded joint between two components usually of the same diameter and with identical weld preparations; for example, a connection between two pipes or a pipe and a fitting. Welded pipe can be manufactured with a butt weld joint. A pipe or a fitting can be butt welded to a flat surface without a weld preparation but this form is not acceptable to many purchasers. Butt weld pipe joints can be the highest integrity connection, if proved by suitable NDE, or NDT, such as 100% radiography. Butt fusion weld
A thermal welding technique used with some non-metallic piping systems. Materials such as polypropylene can be welded by using electric heaters. Machines can join 1200 mm diameter pipe. C
The chemical symbol for the element carbon; alloyed with iron to produce cast iron and steel. Carbon increases the strength of iron and steel and can increase the hardness.
DEFINITIONS
Carbo-nitriding
A case-hardening process in which steel components are heated in an atmosphere containing both carbon and nitrogen. Carbon steel
A steel whose properties are determined primarily by the amount of carbon present. Apart from iron and carbon, manganese up to 1.5% may be present as well as residual amounts of alloying elements such as nickel, chromium, molybdenum, etc. It is when one or more alloying elements are added in sufficient amount that it is classed as an alloy steel. Case-hardening
The process of hardening the surface of steel whilst leaving the interior unchanged. Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually up to a maximum of 0.2%. Components subject to this process, particularly in the case of alloy steels, have a hard, wear- resistant surface with a tough core. Cast iron
A definition can be applied that cast iron is an alloy of iron and carbon in which the carbon is in excess of the amount that can be retained in solid solution in austenite at the eutectic temperature. Carbon is usually present in the range of 1.8% to 4.5%. In addition, silicon, manganese, sulfur and phosphorus are contained in varying amounts. Various types of cast iron are covered by many of the National Standard authorities and include grey, malleable and white irons. Elements such as nickel, chromium, molybdenum, vanadium can be added to produce alloy cast irons. CE
Carbon equivalent, an assessment made of carbon and carbon manganese steels used in a sour environment (H2S present), according to the percentages of the constituents: CE = C + Mn/6 + (Cr + Mo + V )/5 + (Ni + Cu)/15 Change-over valve
A valve with three connections which allows one circuit to be connected to one of a possible two circuits. The circuit not connected is isolated. Valves of this type are frequently built into equipment such as duplex filters. The change-over valve allows one filter element to be used while the other is cleaned or re- placed. In the case of duplex filters, two change-over valves are ganged together. An inlet change-over valve is ganged with an outlet change-over valve so that two filters work with one inlet and outlet process connection. Very special change-over valves are available to be used with safety relief valves. One process connection may be connected to one of two safety relief valves (SRVs). The valve ensures that at least one SRV is in circuit at all times. Both SRV cannot be isolated simultaneously. Charpy test
A test to measure the impact properties of steel. A prepared test piece, usually notched, is broken by a swinging pendulum. The energy consumed in breaking the test piece is measured in Joules. The more brittle the steel the lower the impact strength. CEN and ISO provide test methods. Chokes
A manual valve used for continuous throttling applications; a heavy duty needle valve. An American term used mainly in connection with crude oil production.
Co
Chemical symbol for Cobalt. It may be present in steels as an impurity added with nickel. It is added specifically to tool steels to improve hot strength. Cobalt becomes highly radioactive when irradiated; specifications for nuclear applications restrict the cobalt concentration to very low levels. Closed-loop control system
A control system in which the measurement of process variables corrects, if necessary, the position of a control element. Con- sider a householder sitting in the living-room of a house with gas/water central heating. The living-room is fitted with a room thermostat for temperature control. The householder goes to the kitchen leaving the living-room door open. Cold air from the hall enters the living-room reducing the temperature. The thermostat detects the fall in temperature and sends a signal to the boiler to maintain the water temperature. If the radiator was fitted with a thermostatic control valve, the sensor would detect the fall in temperature. The thermostatic valve would open and allow more hot water to flow through the radiator. The hot water temperature would fall prompting the boiler to turn on or increase the fuel flow. Cold working
Altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading. It is carried out below the re-crystallization point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered. The cold rolling and cold drawing of steel significantly improves surface finish. Control valve
A valve which opens and closes in response to an external signal and is powered by an external source. Similar to a regulator but requires an external supply of electricity, compressed air or pressurized liquid. The control signal may be electric analogue, electric digital or fluid analogue. Controlled atmosphere
A gas or mixture of gases in which steel is heated to produce or maintain a specific surface condition. Controlled atmosphere furnaces are widely used in the heat treatment of steel as scaling and decarburization of components is minimized by this process. In the case of steel this refers to a component that has been case-hardened where the centre is softer than the hard surface
layer or case. It can also be applied to the central part of a rolled rimming steel. Controlled bore pipe
Pipe which is specially manufactured to close tolerances. Ordinary pipes may have a 12.5% tolerance on the wall thickness which leads to considerable variation in the bore size. Controlled bore pipes use strict manufacturing controls to ensure the bore size varies from 1.5% or 3.2 mm in the larger sizes. Cr
The chemical symbol for the element chromium, sometimes abbreviated to "chrome". Chrome is added in small quantities to steel, less than 3%, to improve the effectiveness of heat treatment and hardening and to increase the strength. When the content is increased to between 10 and 30%, the steel acquires stain resistance or becomes stainless, i.e. less likely to oxidize.
DEFINITIONS
Creep
The gradual permanent change in dimensions of solid materials when exposed to long term tensile stress. Non-metallic materials tend to creep much faster than metallic materials. High temperature is not necessary but it will accelerate the effect. Cu
The chemical symbol for the element copper, which is usually present in steels as a trace impurity. It is added to some chrome- nickel alloys to impart precipitation hardening properties. It is added to some stainless steels to improve the strength and corrosion resistance at the expense of elasticity, and possibly castability. Damper
A hydraulic device which can be fitted to power-actuated valves to eliminate spindle/stem vibration and/or control actuation speed. Dashpot
A device which relies on fluid friction to produce a force which opposes the motion. This can be a piston in a cylinder or a disc in a close-fitting housing. The opposing force increases as velocity increases. Data sheet
A formatted sheet or sheets which fully describes the operating conditions the valve will experience. Operating conditions are not limited to the internal process fluid. External environmental factors can play a decisive part in the valve selection and the
36 design of auxiliary equipment Dead band
The range through which the controlled variable can reverse and change in magnitude without initiating a regulator response. Duplex stainless steels
A high strength, corrosion resistant group of stainless steels, iron
-chrome-nickel alloys, where the crystal structure is a mixture of body-centred cubic ferrite and face-centred cubic austenite. The balance between the two crystal groups is usually about 50-50 but heat treatment can change the proportions. The ferrite con- tent is usually controlled to avoid cracking or lack of toughness or susceptibility to stress corrosion cracking but a minimum percentage is required to ensure the strength Ductility
The property of materials which permits it to be reduced in cross-sectional area, while experiencing tensile stress, without fracture. In a tensile test, ductile metals show considerable elongation eventually failing by necking, with consequent rapid in- crease in local stresses. Dye penetrant inspection
Colloquially known as "dye-pen", a method for detecting surface porosity or cracks in metal. The part to be inspected is thoroughly cleaned and coated with a dye which penetrates any surface flaws that may be present. The surface is wiped clean and coated with a white powder. The powder absorbs the dye held in the defects indicating their location Elongation
A measure of the ductility of a material; how much a material can stretch before it fractures Ferrite
A body-centred cubic crystal structure found in chromium alloys.
Fatigue
The effect on materials of repeated cycles of stress. The dangerous feature of fatigue failure is that there is not necessarily an obvious warning, a crack forms without appreciable deformation of structure making it difficult to detect the presence of growing cracks. Fractures usually start from small surface nicks or scratches or abrupt changes in shape which cause Iocalized stress concentrations. Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or the overall operating environment. Ferritic stainless steel
A term usually applied to a group of stain resistant steels with a chromium content in the range of 12 to 30% and whose structure consists largely of ferrite. Alloys with chromium content at the bottom of the range will not be "stainless" in the generally accepted sense of the term. Such steels possess good ductility and are easily worked but do not respond to any hardening or tempering processes and have poor high temperature proper- ties. Fire-safe
A valve which is suitable for installations where the risk of being engulfed by fire cannot be ignored. Valves are tested to a specification and qualified, see Chapter 15. Most valves which qualify are ball and butterfly. Linear valves are often said to suffer from stem thermal problems but some wedge gate valves have qualified. Most wafer and lug valves are generally not suitable. Flanged connection
A pipe joint which uses two discs held together by bolts. Many flange designs are available for high and low pressure. Flanged connections are usually the preferred method of joining pipes for 3" nb, DN 80 and larger. Flow coefficient Cv, Kv
A factor, determined experimentally, which indicates the flow capacity of incompressible fluid during non-choked, non-flashing flow of a valve with a unit differential pressure. Two flow coefficients are in common use; the American Cv and the metric Kv. Equations 1.1 and 1.2 relate to liquid flow. Liquid flow coefficients are based on turbulent flow conditions for a water-like liquid. Equ 1.1 Cv= Q sqr sg/ where: Cv = US flow coefficient (USgpm/psi) Q = liquid flow (USgpm) sg= liquid specific gravity (non-dimensional) Ap= differential pressure (Ib/in 2) Flow-to-close, FTC The direction of flow which tends to close the valve. In single- seat valves, it is the direction in which fluid flows over the sealing element and then out through the seat. Flow-to-open, FTO A linear motion valve fitted in a system so that the fluid flows through the seat and passed the plug to the outlet. The hydraulic process forces tend to open the valve. Fluid
The term fluid is a generalization. It describes a group of "states'' liquids, vapors, gases and plasmas. The term could be used to describe mixtures of these states and also mixtures with solids. Fluid is a vague term and should be used with great caution. It is not, as many people seem to think, synonymous with liquid.
DEFINITIONS
Forging
A process of working metal to a finished shape by hammering or pressing and is primarily a "hot" operation. It is applied to the production of shapes either impossible or too costly to make by other methods or needing properties not obtainable by casting, such as inherent integrity. Categories of forgings include hammer, press, drop or stamping. Full bore
A valve which has the same flow area as the attached pipework. This concept is not as simple as it seems. Pipework is produced in discrete sizes and a valve will be matched to a particular wall thickness with a similar pressure/temperature rating. Always check actual sizes. Refer to Reduced-port for an indication of problems. Gas
A state of matter, achieved by heating beyond the superheated vapour phase to a temperature higher than the critical temperature, in which the molecules move randomly to fill the volume available. A gas cannot be converted to liquid by simply increasing the pressure. The volume of a gas changes easily to accommodate changes in the confining volume. The pressure and temperature of an ideal or perfect gas can be calculated by using the characteristic equation and Boyle's and Charles' Laws. Ideal gas properties can be converted to real gas properties by including compressibility factors in the characteristic equation. Gases can exist in mixtures, air, and can be dissolved in liquids, like fresh water or seawater. Compressors are designed to handle gases. Special compressors, like refrigeration compressors, are de- signed to handle gases and vapours, sometimes with trace quantities of liquid. Gauge pressure
A pressure measurement relative to the local atmospheric pressure. Normal pressure gauges indicate this relative pressure. Units are usually suffixed with "g", but this common practice is frowned upon by the Standards Authorities. Gland
The flange or special nut which retains and/or compresses the packing in the packing box. GMAW
Gas Metal-Arc Welding, another name for MIG welding, Metal Inert-Gas welding. GOST-R
Federal Agency on Technical Regulating and Metrology, the National Standards Authority of Russia. Grey iron
Grey cast iron, also known as "flake iron", is the normal cast iron used for many applications and is available in many grades (strengths). GTAW
Gas Tungsten-Arc Welding, another name for TIG welding, Tungsten Inert-Gas welding. Guide bushing
A replaceable bushing, considered as a stationary trim component, within a valve to align the moving element with the seat. Hard facing
A method of increasing the wear resistance of a metal by the deposition of a hard protective coating. Alloys such as Stellite or Colmonoy, or a metallic carbide or a ceramic oxide are most often used for the coating. Increased corrosion resistance is an additional benefit.
HAZ
Heat affected zone; the parent material immediately next to a weld; the material which could have been "heat-treated" by the welding process. Welding can change the local hardness and/or modify the grain structure of the parent material. Hazardous fluids
These are fluids, which by nature of their chemical properties, create a potential for human injury, damage to property, to the environment or a combination of these. There is no European or International agreement on which fluids should be classed as hazardous. The UK Health and Safety Executive has a document, EH40, regarding the safety of personnel. In America there is a useful document produced by the National Fire Protection Agency, NFPA 325M. Another good source of information is the requirements for the transportation by road of dangerous goods, commonly known as ADR. A similar agreement applies to rail transportation, RID. These documents are published in the local language, not just English, French and German, and cover over 20 countries. For flammable fluids useful data can be found in IEC 79-12 which identifies gases and vapours for potentially explosive atmospheres for electrical equipment. Dust clouds can create a potential hazard and these should be checked rather than assuming dust to be safe. High pressure change-over valve
A type of regulator which automatically selects between two feed systems depending upon the downstream pressure. For rapidly filling vessels a low pressure high flow supply, from a centrifugal pump, can be used. As the vessel pressure approaches the pump closed valve head, the change-over valve changes supplies to a low flow high pressure source, possibly from a positive displacement pump. HRc
Rockwell hardness on Scale "C" H2S trim
A valve with all contact materials in compliance with NACE MR-01-75, latest edition. Hydraulic lock
A situation usually encountered in gas systems when liquid has become trapped in a confined volume which should only contain gas, and prevents the movement of components. Inclusions
Usually non-metallic particles contained in metal. In steel they may consist of simple or complex oxides, sulphides, silicates and sometimes nitrides of iron, manganese, silicon, aluminium and other elements. In general they are detrimental to the mechanical properties but much depends on the number, their size, shape and distribution. Intercrystalline corrosion
Chromium-nickel austenitic stainless steels are prone to this form of corrosion when they are welded and subsequently in contact with certain types of corrosive media. When the metal is heated to within a temperature range of 450 to 800~ precipitation of the chromium carbides takes place at the grain boundaries in the area of the weld and these areas no longer have the protection of the chromium on the peripheries of the grains. This type of corrosion is also known as Weld Decay and Inter- granular Corrosion. The most common way to avoid the problem is to select a grade of steel that is very low in carbon i.e. 0.03% or less, or one that is stabilised with niobium or titanium.
DEFINITIONS
Isolating valve
A valve which is intended to be wide open or closed, with no flow regulation functions. Izod Impact test
A test specimen, usually of square cross-section is notched and held between a pair of jaws, to be broken by a swinging or falling weight. When the pendulum of the Izod testing machine is re- leased it swings with a downward movement and when it reaches the vertical the hammer makes contact with the specimen which is broken by the force of the blow. The hammer continues its upward motion but the energy absorbed in breaking the test piece reduces its momentum. A graduated scale enables a reading to be taken of the energy used to fracture the test piece. To obtain a representative result the average of three tests is used and to ensure that the results conform to those of the steel specification the test specimens should be machined and polished accurately. Charpy testing is now more popular. Knoop hardness test
A micro hardness test in which a pyramid shaped diamond is pressed into the surface. Lamination
A problem which can occur with rolled raw materials, like steel plate. The bulk material is not homogeneous and can separate into layers. Lantern ring
A spacer/bush used in packing boxes to allow the injection of 38 lubricant, the collection of leakage and the application of a
water seal, to the spindle/stem. Lantern rings are usually metallic but plastic or ceramic can be used. A short spacer with material removed from the central portion to provide storage or collec- tion volumes in the bore and around the od. Internal and exter- nal volumes are connected by a series of holes or slots. Leakage rates
For substances hazardous to the environment the allowable leakage rates are controlled by local, national or country group- ing legislation. There is no complete list of hazardous substances but a list entitled Red List Chemicals or NFPA 325, is available for guidance. Local inspectors evaluate the site and issue an operat- ing licence which specifies maximum emissions. More than one licence may be necessary for a site. For substances hazardous to personnel, but not to the environ- ment, the allowable leakage rate is dependent upon local condi- tions; it is the concentration in the air which is important. Totally enclosed processes, with little or no air movement, will achieve high concentrations with low seal leakage. Open processes, with very large air volumes and continuous air movement, will be able to tolerate larger leakage rates without creating dangerous concentrations. Leakage will generally be dispersed downwind. Lubricator
A connection on the packing box to facilitate packing lubrication. Martensitic stainless steel
An alloy of iron, carbon and chromium which can be hardened by heat treatment, are magnetic but are not as corrosion resis- tant as other grades of stainless steel. The chrome content is usually between 10 and 18% with a carbon content over 1%. Carbon allows the material to be very hard and provides a degree
MIG welding
Metal arc Inert-Gas welding is an electric arc welding process where the arc is struck with the filler metal. The weld site is pro- tected from atmospheric contamination by an inert gas shield, argon, helium or carbon dioxide being the most popular. Mo
Molybdenum is alloyed with high strength steel to improve the high temperature physical properties. It is added to stainless steels to increase resistance to pitting corrosion. "Moly" has a favourable influence on the welding qualities. NACE
National Association of Corrosion Engineers (of America). Al- though the NACE organisation provides guidance on all types of corrosion its name has become synonymous with sour service applications where hydrogen sulphide may be a problem. Nb
The element niobium (used to be known as Columbium) is added to some steels to stabilise carbon, in a similar way to titanium, but has the added benefit of improving elevated temperature strength. nb
Nominal bore, the nominal size of a valve or pipe in inches. Nominal bore pipework has a fixed outside diameter and vari- able wall thickness to accommodate different pressure ratings, up to 12". Pipe sizes over 12" are designated by the outside di- ameter. In low pressure pipe the bore size may be larger than the nominal bore. In high pressure pipe the bore size can be considerably smaller than the nominal bore and the actual diameter must be checked when calculating flow velocities. The wall thickness of pipe is subject to manufacturing tolerances and +12.5% is fairly typical. The pipe bore is therefore variable to some extent. The pipe wall thickness available for stressing and corrosion is also variable. DN is the metric equivalent of nb. NDE
All those forms of testing that do not result in permanent dam- age or deformation to the part being tested. Typical examples include magnetic crack detection, ultrasonic inspection, X-ray inspection and gamma radiography. Some types of hardness testing do not leave any damaging impressions. Nipple
A short length of pipe, usually of small diameter, 2" nb, DN50, or smaller. Non-rising stem
Used on linear motion valves with inside screws. The nut is built into the sealing element which moves due to rotation only of the stem. Non-rotating tip
The disc, plug or needle of a linear valve is attached to the stem but does not necessarily rotate. It is a good way to reduce wear when tightening the tip against the seat. Normalising
A heat treatment process that has the object of relieving internal stresses, refining the grain size and improving the mechanical properties. The steel is heated to 800 to 900~ according to analysis, held at temperature to allow a full soak and cooled in still air.
DEFINITIONS
Normally closed
An isolating or control valve which requires control pressures above atmospheric to open. If the actuating system fails and the control pressure is reduced to atmospheric, the valve will close. A control valve can also be called direct-acting. Normally open
An isolating or control valve which requires control pressures above atmospheric to close. If the actuating system fails and the control pressure is reduced to atmospheric, the valve will open wide. A control valve may also be called reverse-acting. NPS
Nominal pipe size (see nb) NPT
National Pipe Thread (of America), is a tapered pipe thread which seals the fluid. Thread tape or sealant is applied to the thread during assembly. The basic 60° thread form is cut on a taper of 0.75" per foot on diameter. The standard provides sizes up to 24" nb, but in practice 6" is about the largest used in proc- ess applications, Many users will not accept threaded fittings over 3". Obturator
The moving sealing element in a valve. Oxidation
The most common form of chemical reaction which is the com- bining of oxygen with various elements and compounds. The corrosion of metals is a form of oxidation, rust on iron and steel for example is iron oxide. Refined metals can be considered as unstable compounds; the natural stable state is their original mineral ores. Packing box
The part of the bonnet which contains the seal or packing to reduce leakage along the valve stem. Packing lubricator
An optional fitting on the bonnet to allow the packing to be lu- bricated while working. Packing tape
Packing boxes can be sealed using thin tape which is rolled to fill the box. Usually only PTFE and graphite is available in this form. Panel mounting
A facility supplied on smaller valves which enables the valve to be mounted with the connections behind the panel. Usually applied to valves with screwed glands, extra nuts are provided to clamp the panel. Pipe
A conduit for carrying fluid which is designated by its nominal bore, nb, or its nominal diameter, DN. Pipes below 14" nb have unusual outside diameters i.e. 6.625", 10.75", etc. Pipes 14" and above use the od as the size. Pipe with an appropriate wall thick- ness can be threaded with standard pipe threads. PN
Pressure nominal, a European designation, in bar-g, of the maxi- mum working pressure at ambient temperature. Pressure- tem- perature rating tables or graphs must be reviewed for the actual material at specific operating temperatures. Metallic materials are usually suitable up to 65° without de-rating. Some non-metallic materials may need de-rating over 20°C.
Positioner
A control valve actuator used for accurate control applications. The positioner supplies air to the diaphragm or piston actuator to adjust the valve in response to control signals. The positioner is coupled to the valve/actuator stem and monitors the valve position to ensure the valve does adjust in response to the con- trol requirement. The feedback of valve stem position allows very precise valve control. Positive Material Identification, PMI
Certain critical components may have the material of construc- tion identified beyond any doubt. A piece of the component can be removed and given to a witnessing inspector who carries the sample to a laboratory and watches the analysis. Alternatively, the component is analysed by a portable instrument which con- firms the chemical composition. PRE
Pitting resistance equivalent, calculated from a formula, depend- ing on the material considered :
PREN= Cr + 1.5 x (Mo + W + Nb) for stainless steel PREN= 3.3 x Mo + 16 x N2 for duplex and super duplex steel PREN= Cr -0.8 x Cu + 1.5 (Mo+ W) for nickel alloy weld wire Precipitation-hardening steels
There are relatively few precipitation-hardening steels compared to other alloy categories. A group of iron-chrome-nickel alloys with relatively small nickel proportions. In the annual condition, alloys can be austenitic or martensitic. Heat treatment hardens the martensitic crystal structure. Pressure containing part
A valve part which is in physical contact with the process fluid or any actuating fluid. These parts may be subject to corrosion or erosion by the fluid. Parts include: valve body, bonnet, stem and spindle. Pressure retaining part
A valve part which is stressed by internal fluid pressure due to its function of holding one or more pressure containing parts in position. Pressure retaining parts are not in direct physical con- tact with the process or actuating fluids and are not subject to corrosion or erosion by them. Pressure retaining parts may be subject to corrosion or erosion by atmospheric pollution. Parts include: bonnet fasteners, glands, gland fasteners, stems, spin- dles and stem nuts. Pressure-temperature ratings
Metallic and non-metallic materials used for valve manufacture usually weaken and become more elastic as temperature in- creases. Some materials become brittle as temperature de- creases. These factors are well known and have been studied extensively by classification societies and standards authorities. Allowable stress values have been correlated with working temperature by pressure vessel standards and codes. The most popular Standards are EN 13445, ASME VIII and ANSI B 16.34. BS 5500 has been replaced by PD 5500 and is popular outside Europe. These Standards use different formulae to calcu- late the stresses arising from fluid pressure; the allowable stress values are not interchangeable. Pressure-temperature ratings are usually qualified as non-shock, may be time limited or have specified full-cycle pressure limitations.
DEFINITIONS
RAL
The German organisation, Reichsausschuss Lieferbedingungen, was instigated to standardise precise technical terms to allow rationalisation of purchasing (long before the EU Single Market Project). Now the RAL prefix is used for colour shades in accor- dance with international standards such as BS 381C. Reduced-port
Always described as reduced-bore, a valve in which the seat area or the ports leading to the seat are smaller than the nominal pipe area in which it is fitted. In globe valves the ports may be smaller and the seat may be larger than the nominal pipe area. Gate valves may have 50% flow area, globe valves even less. The increased velocity in the port or seat can result in increate ero- sion, corrosion and noise. Accelerated material loss is not limited to solids handling. Increased liquid velocities can damage the protective oxide layer resulting in unexpected corrosion. Some valves do not have circular ports. Sudden changes in area can create violent turbulence leading to greatly increased erosion and corrosion Rising stem
On linear motion valves the stem rises through the yoke when the valve is opened. RJT
Ring Joint Type; a hygienic union pipe connector. Not to be con- fused with RTJ. Rockwell hardness testing
40 A method for testing the hardness of metals by determining the depth of penetration of a steel ball or a diamond sphero-conical indentor. The value is read from a dial and is an arbitrary num- ber related to the depth of penetration. For testing hard steels, a sphero-conical diamond is used with a 150 kg load, the result is read from the black scale on the dial and is prefixed with the letter C. A hardened tool steel would typically give a reading of 62 HRc. For softer metals Scale B is used with a 1/16" diameter steel ball and a standard load of 100 kg. SAW
Submerged Arc Welded, a type of electric welding process used on thick wall pipe and tube. The submerged arc process pro- duces a very high quality, clean weld. Powdered flux is poured into a dam which is clamped either side of the seam. The end of the welding electrode is covered by the flux and air is effectively excluded. The powdered flux melts to protect the molten filler metal. Both flux and filler metal solidify on cooling. Unused pow- dered flux is collected for re-use. When cold, the flux forms a solid slag which peels off the weld metal. Typical pipe sizes would be in the range of 400 mm diameter and larger with wall thickness of 6 to 32 mm. For corrosive applications the filler metal should be the same chemical composition as the parent metal or slightly more noble. For critical applications clarification will be worthwhile. SCC
Standards Council of Canada, the National Standards Authority of Canada. Screwed bonnet
A bonnet which is threaded and screwed directly into the valve body. Screwed gland
A gland which screws into a female thread in the bonnet. It can be either adjustable or non-adjustable.
Se
The element Selenium, added to some steels to improve Machining. Seal weld
A threaded pipe connection, usually a taper thread but could be applied to a parallel thread, which is made dry without sealants. A fillet weld is added at the external joint face to prevent any leakage. This type of construction is only acceptable when the process fluid is not corrosive. The pipe threads, which provide the strength for the joint, are not protected and are susceptible to crevice corrosion or galvanic corrosion if the metals are sig- nificantly different. Seamless
Pipe or tube which does not have a welded seam. Some rolled plate and strip products have no seam because the edges are joined by pressure welding when the metal is hot. Seat
The pressure-retaining contact seal between the stationary and moving parts of the valve. A valve may have more than one seat. Seat leakage
The leakage through the seat when the valve is closed. Leakage is standardised in many cases please refer to applicable specifi- cations. Self-energised packing
Packing which uses the fluid pressure to create the sealing forces to prevent leakage. Higher pressures result in higher sealing forces. Shutdown valve, SDV
An isolating valve, part of the ESD system, which closes to isolate equipment or a process during the shutdown sequence.
Smooth finish
A flange facing finish which is smooth but not polished. The fin- ish would normally lie between 1.6 and 3.2 µm. No visible tool marks should be discernible. Socket fusion weld
A thermal welding technique used with some non-metallic pip- ing systems. Materials such as polypropylene can be welded by using electric heaters. Socket weld fittings
Fittings for connecting pipes, which use a single fillet weld around the od of the pipe/tube for mechanical attachment and sealing. The pipe/tube is inserted into the fitting, then welded. It should only be used with safe fluids. The weld cannot be X-rayed to verify integrity. Soft-seat
A soft insert of plastic, elastomer or other readily deformable material, in the valve body or the moving element which de- forms on contact with the other sealing surface to provide low or zero seat leakage. Solution annealing
A heat treatment to allow precipitated constituents to return in to the solution. Quenching "freezes" all the constituents in the required state. Sour
A description of the environment or the process fluid; hydrogen sulphide, H2S, is present.
DEFINITIONS
Specification
A document issued as part of a contractual agreement for equip- ment. Most specifications refer to standards and amend or ex- tend requirements to suit the particular application. Some speci- fications are "vague" in the sense that requirements are aspira- tions and cannot be verified. Before writing a specification, au- thors could be usefully guided by consulting Standard writing rules, like BS 0 (Standards, specifications for structure, drafting and presentation.). A specification which is vague or ambiguous will be interpreted to the suppliers" advantage. A specification which is capable of interpretation, is of little value to purchaser. SSC
Sulphide stress cracking; a type of corrosion mechanism which attacks components made of high strength materials which de- rive their strength from heat treatment to relatively high hard- ness levels. Sulphide stress cracking is usually associated with hydrogen sulphide in connection with crude oil and natural gas production. Stainless steel
An alloy of iron and chromium which is usually, at least, 50% iron and up to 30% chromium. Carbon is usually present from very low concentrations, 0.03% to 1.0% or more. Stainless steel can be divided into five groups: martensitic, ferritic, austenitic, du- plex and precipitation-hardening. Standard
A specification, issued by a National Standards Authority, a gov- ernment sponsored organisation, which is characterised by strict editorial control and verifiable requirements. Standards should consist of clauses which mainly use "shall" to indicate a "requirement" which must be met. Compliance with standards must be verifiable, otherwise the effort is wasted. "Should" is used to describe recommendations; "may" indicates a course of action which is acceptable. Clauses which require an agreement should be kept to an absolute minimum. Steel
An alloy of iron and carbon with small quantities of silicon, man- ganese, moly or nickel. Phosphorous and sulphur are usually present, in quite small quantities, less than 0.05%, as impurities which impair the physical and chemical properties. If the carbon content is between 0.4 and 1.2% approximately the steel can be hardened by heat treatment although steels with lower carbon contents can be heat treated, inadvertently, during welding. Stem
The shaft which reciprocates in linear valves and actuators. Stem connector
A clamp, usually in two halves, to connect the valve stem to the actuator stem. Stem leakage
Leakage through the stern (or spindle) seal when the valve is open or closed. Some valve designs seal the packing box when open; others when closed. Rotary valves generally are unable to seal the packing box. Stock finish
The standard surface finish applied to flange facings by the flange manufacturer. Stock finish is a shallow spiral groove ma- chined as one continuous cut across the face. The angled crests created apply a strong grip to the gasket to resist blow-outs. Flanges up to 12" have a 1/16" radius groove cut with a feed rate of 1/32" per revolution. Larger flanges use a 1/8" radius tool and 3/64" per revolution.
Stress relieving
A heat treatment including heating and soaking at a suitable temperature e.g. 600 to 650°C followed by cooling at an appro- priate rate in order to reduce internal stresses without substan- tially modifying the steel's structure. This treatment may be used to relieve stresses induced by machining, quenching, welding or cold working. TIG welding
Tungsten arc Inert-Gas welding is an electric arc welding process, where the arc is struck with a dedicated tungsten electrode across to the workpiece. A separate filler rod or wire is fed into the weld pool which is protected by an inert gas shield. Toughness
The ability of a metal to rapidly distribute within itself both the stress and strain caused by a suddenly applied load, or more simply expressed, the ability of a material to withstand shock loading, It is the opposite of "brittleness" which carries the impli- cation of sudden failure. A brittle material has little resistance to failure once the elastic limit has been reached. Trim
The internal components of a valve in contact with the process fluid. Ultrasonic
Sound above the range of human hearing; typically frequencies greater than 20 kHz. These high frequency sounds can be de- tected by piezo-electric crystals. Vickers hardness test
This test, more commonly known as the Vickers test, finds greater use in the laboratory than the workshop. It employs a pyramid shaped diamond with an included angle of 136 °which is impressed into the specimen using loads of 5 to 120 kg making a small square impression. This test is used for finished or polished components because the impression can be very small. The dia- mond pyramid hardness number is obtained from a calculation based on measuring the diagonals of the impressions in the material. Test results are usually quoted as VPN, Vickers Pyramid Number, or just HV. W
The chemical symbol for tungsten, which used to be called Wolf- ram. Tungsten can greatly improve the hot properties of tool steels. It is added to some chrome-nickel alloys to aid carbide formation. Welded bonnet
For applications requiring very high integrity the bonnet can be welded to the body. Care must be taken to ensure the weld can be inspected adequately to verify the soundness. Weldolet
A specially-designed, mass-produced, forged fitting for making tees in pipe runs. It produces a better result, from stressing and fluid flow criteria, than butting a branch directly to the run pipe. The trade name has passed into common usage to represent fittings of this style produced by other manufacturers. Versions are available for connections on bends, branches at an angle and adding screwed connections. WPS
