Presentation on Valves

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    PRESENTATION

    ON

    VALVES

    OBJECTIVES

    Explain the various ways in which valves control fluid flow in piping systems.

    Identify gate, globe, needle, ball, butterfly, plug, and check valves,

    Selection Criteria

    Fertilizer Plant Experience.

    A valve is a device that regulates the flow of substances (either gases, fluidized solids, slurries, or liquids) by opening,

    closing, or partially obstructing various passageways.

    Body :The majority of the valve consists of the valve body, including most of the exterior. The valve body is the vessel or

    casing that holds the fluid going through inside the valve. Valve bodies are most commonly made of various metals or

    plastics.

    Ports : The body has two or more openings, often called the ports, through which the fluid can enter or leave the valve.

    Discs and rotors : Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc.

    Although valve discs of some kinds of valves are traditionally disc-shaped, discs can come in various shapes. Although the

    valve body remains stationary within the fluid system, the disc in the valve is movable so it can control flow.

    Seat : The valve seat is the interior surface in the body which contacts or could contact the disc to form a seal which should

    be leak-tight, particularly when the valve is shut. If the disc moves linearly as the valve is controlled, the disc comes into

    contact with the seat when the valve is shut.

    Stem : The stem is a rod or similar piece spanning the inside and the outside of the valve, transmitting motion to control the

    internal disc or rotor from outside the valve. Inside the valve, the rod is joined to or contacts the disc/rotor. Outside the

    valve the stem is attached to a handle or another controlling device.

    Bonnet :A bonnet basically acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve

    body.

    Spring : Many valves have a spring for spring-loading, to normally shift the disc into some position by default but allow

    control to reposition the disc. Relief valves commonly use a spring to keep the valve shut, but allow excessive pressure to

    force the valve open against the spring-loading,

    Flow thru a valve

    Type of Valves

    Ball valve, which is good for on/off control.

    Butterfly valve , particularly in large pipes.

    Check valve or Non-return valve, allows the fluid to pass in one direction only. Check valves are designed to restrict the

    flow to one direction. If the flow reverses direction, the check valve closes.

    Diaphragm valve , a sanitary valve predominantly used in the pharmaceutical industry

    A flow control valve maintains a constant flow rate through the valve.

    Gate valve , mainly for on/off control.

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    Globe valve , which is good for regulating flow.

    Hydraulic valve (diaphragm valve).

    Needle valve for gently releasing high pressures.

    Pilot valves regulate flow or pressure to other valves.

    Plug valve , for on/off control.

    A pressure reducing valve (PRV), also called pressure regulator, reduces pressure to a preset level downstream of the

    valve.

    Relief valves are used to regulate the operating pressure of incompressible flow.

    Safety valves are used to release excess pressure in gases or compressible fluids.

    Equal Percentage : equal increments of valve travel produce an equal percentage in flow change

    Equal Percentage (most commonly used valve control)

    a) Used in processes where large changes in pressure drop are expected

    b) Used in processes where a small percentage of the total pressure drop is permitted by the valve

    c) Used in temperature and pressure control loops

    Linear: valve travel is directly proportional to the valve stoke

    Linear

    a. Used in liquid level or flow loops

    b. Used in systems where the pressure drop across the valve is expected to remain fairly constant (i.e. steady state systems)

    Quick opening : large increase in flow with a small change in valve stroke

    Quick Opening

    a. Used for frequent on-off service

    b. Used for processes where instantly large flow is needed (i.e. safety systems or cooling water systems)

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    TYPE OF VALVES

    Gate Valves

    Best Suited Control: Quick Opening

    Recommended Uses :

    1. Fully open/closed, non-throttling

    2. Infrequent operation

    Applications: Oil, gas, air, slurries, heavy liquids, steam, non condensing gases, and corrosive liquids

    Advantages :

    1 . High capaci ty

    2 . T ight s hu to ff

    3 . L ow cos t

    4. Li tt le resis tance to f low

    Disadvantages:

    1 . Poor con tr ol

    2. Cavitate at low pressure drops

    3. Cannot be used for throt t ling

    Best Suited Control: Linear and Equal percentage

    Recommended Uses:

    1. Throttling service/flow regulation

    2. Frequent operation

    Applications: Liquids, vapors, gases, corrosive substances, slurries

    Advantages :

    1. Efficient throttling

    2. Accurate flow control

    3. Available in multiple ports

    Disadvantages :

    1. High pressure drop

    2. More expensive than other valves

    Best Suited Control: Quick opening, linear

    Recommended Uses:

    1. Fully open/closed, limited-throttling

    2. Higher temperature fluids

    Applications: Most liquids, high temperatures, slurries

    Advantages:

    1. Low cost

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    2. High capacity

    3. Low leakage and maint.

    4. Tight sealing with low torque

    Disadvantages :

    1. Poor throttling characteristics

    2. Prone to cavitation

    Best Suited Control :Linear, Equal percentage

    Recommended Uses :

    1. Fully open/closed or throttling services

    2. Frequent operation

    Applications: Liquids, gases, slurries, liquids with suspended solids

    Advantages :

    1. Low cost and maint.

    2. High capacity

    3. Good flow control

    Disadvantages :

    1. High torque required for control

    2. Prone to cavitation at lower flows

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    TYPE OF CHECK VALVES

    VALVES- SELECTION CRITERIA

    Control valves

    Sizing : Control valves shall be sized in compliance with standard ISA S75.01 using calculation methods given in the

    Masoneilan Handbook for Control Valve sizing. In general the Cv selected shall equal at least the maximum Cv calculated multiplied by 1.3.

    Butterfly valves shall be sized for a maximum opening angle of 60 with the exception of those provided with a

    characterised disc, which shall be sized for a maximum opening angle of 90.

    Normally the valves exclusively envisaged for shut-off service shall be line size. Alternatively, they could be sized as

    control valves.

    The fluid velocity at the valve outlet flange shall not exceed 6 m/sec for liquids whereas the velocity of gas or vapour shall

    not normally exceed 0.3 Mach under operating conditions.

    In order to reduce the fluid velocity below such limits use can be made of widely-sized valves equipped with reduced trims.

    Fluid velocity values could exceed the above-mentioned limits in the case of valves provided with a labyrinth plug, cage

    trim or angle body.

    Choice of type

    Without establishing rigid rules it is generally possible to assume that the following types of valves are used:

    Globe valves for most common applications

    Butterfly valves if low pressure drops and high flow capacities are required

    Characterised-ball valves (e.g. V-ball type) for fluids which entrain solids or which easily crystallise

    Angle valves when high pressure drops on gas or erosive fluids

    Three-way valves if the flow must be split into two fluid streams

    Saunders valves for corrosive or dirty and muddy fluids.

    Choice of body

    Valves with bodies having nominal dimensions of 1 1/4", 2 1/2" and 5 shall not be used.

    In the case of lines with a diameter of up to 1" the valve size shall normally equal that of the line.

    In the case of lines with a diameter larger than 1 the valve size shall not be less than 1. -

    The body material shall be chosen in compliance with materials required in the line specification and, in any case, shall be

    suitable for the process fluid.

    Valves shall generally have flanged connections as per rating envisaged in the line specification with the exception of

    valves with a nominal diameter smaller than or equal to 1 1/2 which shall have a minimum rating of 300 ANSI.

    Some types of valves such as characterised ball valves, butterfly valves or valves with an eccentric rotating plug shall be of

    the wafer type, i.e. suitable to be installed between pipeline flanges.

    Self-actuating valves could have a nominal diameter smaller than 1 irrespective of the pipe diameter.

    Trim selection

    Control characteristic

    In the case of globe valves or of valves equipped with an eccentric rotating plug the trim characteristic shall generally be

    chosen on the basis of the following criteria:

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    quick opening for valves exclusively envisaged for shut-off service

    equal percentage if the valve pressure drop at the maximum flow rate is less than about one third of the overall system

    pressure drop (valve + line)

    linear in all other cases.

    In the case of butterfly valves, of valves with characterised ball (e.g. V-ball) and of angle valves the characteristic willdepend on the type of valve as well as on the supplier selected.

    Trim for use under severe conditions

    In the presence of liquids which, because of particular operating conditions, can give rise to cavitation phenomena, use

    shall be made of special trims (e.g. multi-drops cage trim) so as to avoid that similar phenomena may take place or, at least,

    to reduce their effect.

    The maximum acceptable noise level permanently produced by control valves (measured at one metre downstream from

    the valve and one metre from the pipe) shall not exceed 85 dB (A).

    In order to ensure that this limit is not exceeded when high pressure drops and large gas flow-rates are involve. use shall be

    made of special trims (e.g. labyrinth plugs) or to suitably designed valves (e.g. multi-step angle valves).

    Levels in excess of 85 dB (A) can be accepted in the case of noise of limited duration in accordance with provisions

    envisaged in OSHA regulations.

    Construction and materials

    With the exception of Saunders valves, the trim of valves shall be of AISI 316 unless the fluid and operating conditions

    require a more suitable material.

    The plug, seat and stem guide posts shall be fully satellite coated in the following cases:

    when the process fluid is a liquid with suspended solids

    in the presence of liquids which consistently vaporise in the valve body

    if the pressure drop across the valve (at the maximum flow rate) exceeds or is equal to 10 bar

    if the operating temperature exceeds 280 C.

    The use of special materials recommended by the control valve manufacturer could be considered Instead of satellite

    -coating.

    Bonnet

    Bonnet shall generally be of standard execution. However, it shall be supplied as follows:

    extended if the fluid operating temperature is comprised between -40 and 0 C

    finned if the operating temperature is above 230 C

    extra-long if the operating temperature is below -40 C

    Packing

    The packing shall be of:

    Teflon if the fluid operating temperature is below 230 C

    Graphite-asbestos if the operating temperature is above 230 C.

    The use of special packing materials may be considered according to the control valve manufacturer recommendation.

    Double packing will be used on toxic services ( wet H2S, etc.).

    Trim tightness

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    The control valve seat leakage class shall be established on the basis of ANSI B 16.104.

    If the allowed seat leakage does not exceed class II double seat valves can be used.

    Valve tightness corresponding to class III, IV or V will be achieved via a metal to metal single-seat trim.

    If the seat leakage required is of class VI provision shall be made for a soft insert in the seat as long as the operating

    temperature is less than about 200 C and the pressure drop, with the valve closed, does not exceed 15 bar. ESD valves shall be Class V metal to metal as minimum.

    Special requirements could be met on the basis of past experience of qualified suppliers.

    Stream flow action

    In the case of valves for shut-off service only the process fluid action shall coincide with that of the valve when control air

    pressure fails.

    In case of valves used for fluid throttling the following criteria shall generally apply:

    angle valves: fluid action tending to close

    valves with eccentric rotating plug: the fluid action shall coincide with that of the valve in the case of control air outages

    other types of valves (globe, ball, etc.): fluid action tending to open. However, special considerations could impose a different fluid action

    Hand-wheel and bypass assembly

    Control valves shall be either supplied with a manual hand-wheel or envisaged with a bypass assembly. Shut-off valves

    shall be provided without bypass; hand-wheel shall be provided only if demanded by operation needs.

    Control valves sized 3" or less shall be provided with both isolating and bypass valves. Valves with body dimensions

    exceeding 3 shall normally be supplied with the hand-wheel.

    Control valves exceeding 3 and installed on essential services (e.g. main lines for fuel, cooling water, etc.) or on corrosive,

    erosive or other special fluids shall be supplied with both isolating and bypass valves if so envisaged in the process

    diagrams.

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    Valves for emergency

    shutdown service (ESD)

    Valves controlled by the emergency shutdown system (ESD) and used either for isolating or for depressurising the plant

    under emergency conditions shall be supplied in accordance with process requirements. These valves shall be fitted with

    actuators designed in such a way that, in the event of control air failure, they can drive the plant toward safe conditions

    even when the envisaged maximum pressure drop, caused by the process fluids, is applied to the valve ports.

    If there is no return spring (double acting actuators) provision shall be made for air accumulators with a storage capacity

    suitable for the execution of 2 strokes.

    Each ESD valve shall be equipped with one proximity type limit switch for remote indication of the trip position.

    ESD valves shall be provided with automatic pilots or solenoid valves for their remote control and with local facilities for

    manual reset.

    Solenoid valves in hazardous area will be explosion proof.

    Cavitation

    If the speed through the valv e is high enough, the pressu re in the liquid may drop to a level whe re the fluid

    may start bubble or flash. The pressure recovers sufficiently and the bubbles collapse upon themselves.Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely

    destructive and may wear out the trim and body parts of the valve in short time.

    Cavitation can be avoided by using more than one control valve or more convenient - a multistage control

    valve. vena contracta is much lower for a single stage valve than a multi stage valve. Depending on the

    pressure drop and the temperature of the fluid its possible to avoid cavitation conditions using more than

    one stage in a valve

    Safety-Relief valve sizing

    With the exception of safety valves for liquid thermal expansion, safety valves shall be sized in compliance with formulae

    and calculation methods given in API RP 520 and in accordance with local codes keeping into account all possible reasons

    of intervention.

    Whenever possible and particularly when very high flow rates requiring the use of several spring loaded safety valves are

    involved or when the operating pressure is very close to the protected vessel design pressure, use shall be made of pilot-

    operated safety valves.

    Dimensions of safety valves for liquid thermal expansion shall generally be 3/4 x 1" NPT.

    Set pressure, over pressure, spring set pressure

    The set pressure, which is defined as the valve opening pressure measured at the valve inlet, shall not exceed the design

    pressure of the protected equipment except in cases where the flow rate calculated is discharged by more than one safety

    device (safety valve or bursting disc). In the latter case, at least one safety device shall open at a pressure not exceeding the

    lowest design pressure among all the protected vessels; the other device/s could open at a higher pressure without

    exceeding 105% of the previously mentioned lowest design pressure.

    The overpressure, which is defined as the increase in pressure above the set pressure required for the valve to discharge the

    full flow rate - expressed as a percentage of the set pressure - shall not be:

    lower than 15% for valves which discharge liquids

    higher than 10% for valves which discharge gas and vapour for any cause of intervention except for fire, in which case

    a set pressure not exceeding 21% shall be permitted.

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    The sum of the set pressure and overpressure shall not exceed 110% of the lowest design pressure of the protected

    equipment.

    The spring set pressure (setting carried out on a test bench and under atmospheric back pressure) shall equal the set

    pressure if the valve discharges directly into the atmosphere or is fitted with balancing bellows; in all other cases it shall

    equal the set pressure less the maximum back pressure (with the valve closed) except for pilot operated valves.

    Construction, materials, accessories

    The body material shall be chosen in compliance with what envisaged in the line specifications and shall be suitable for the

    process fluid.

    Internals shall be of stainless steel unless the fluid and process conditions require a more suitable material.

    Whenever possible, the same material shall be used for the balancing bellows.

    The spring material shall be chosen according to the product discharge temperature.

    The following shall generally be used:

    AISI 304 or AISI 316 for discharge temperatures between -240 C and -60 C

    phosphatised carbon steel for discharge temperatures between -60 C and +230 C

    phosphatised tungsten steel for discharge temperatures between +230 C and 538 C

    for temperatures outside the above ranges the material shall be agreed upon with the manufacturer.

    Connections shall generally be flanged; the rating of the inlet connection shall be chosen according to the trip conditions

    and that of the outlet connection shall normally be rated ANSI 150.

    Valves sized 3/4" x 1" shall generally be provided with threaded connections ANSI B 1.20.1 NPT.

    Provision shall normally be made for balancing and sealing bellows in the following cases:

    when the process fluid contains lethal or toxic substances

    when the variable back pressure exceeds the safety valve set pressure by 10%

    when the imposed back pressure (i.e. with the valve closed) is higher than the atmospheric pressure and the difference

    between the set pressure and normal operating pressure would, without the bellows, be less than 10% of the set

    pressure.

    Alternatively, pilot operated valves can be used.

    A lifting lever shall be provided when the process fluid is steam or air where required by the governing code.

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    Actuator, positioner and accessories

    Actuator

    All control valves without positioner shall generally be provided with a pneumatic actuator with a spring range of 0.2 1

    bar.

    Other spring ranges, in particular for shut-off valves, could be used either to meet operating requirements or to limit actualvalve overall dimensions.

    Positioner

    With the exception of the shut-off valves, to be provided without positioner, control valves shall be supplied with a

    pneumatic positioner in the following cases:

    when the valve is employed in split-range service

    when the valve nominal diameter is larger than 1

    when the bonnet is not standard

    when the actuator signal differs from 0.2-1 bar

    under laminar motion conditions (very viscous fluids, etc)

    of consistent amounts of flashing liquids

    when the valve pressure drop., at the maximum operating flow rate, exceeds 5 bar for single seated valves or 10 bar for

    double seated valves.

    The positioner shall be supplied with a bypass and a set of pressure gauges. No bypass will be provided for valves in

    split-range service or when the input and output signal to from the positioner are different.

    (This presentat ion was given by Mr. V.N. Rai at Internat ional Valves Seminar in New Delhi on 13th

    December 2006)