NET 112:Nuclear Power Plant Components

NET 112:Nuclear Power Plant Components

Module 2: Valves

Valves

•Installed in pipelines to control flow of fluid

•Start, stop, or regulate a flow of liquid or gas through a plant system

–Throttle the flow (especially gases)

–Divert flow from one pipe to another

–Prevent backflow (check valve)

•Classification

–Function

•stop, throttle, control, etc.

–Construction

•gate, globe, butterfly, etc.

•Focus on construction

•Size determines its ability to regulate flow

–Smaller valve = finer control

–Very small (1/8 in) to very large (several ft)

•Must be built to withstand differentials in flow, temperature, and pressure

•Valve disc

–Part that controls flow of fluid through valve

–Most common difference in construction designation

–“Disc” might be totally different shape in one pump relative to another

Valve Components: Valve Body

•Largest part

•Attach valve to system or pipe

•Welded

–Inlet and outlet stubs are straight and smooth

•Flange mount (bolted)

•Threading

–Stubs threaded

–Low pressure

•Flow direction

–Straight-through

–Angled

Valve Components: Seating

•Immovable, internal part of the valve body

•Where disc closes on VB to stop flow

•Must be smooth and perfectly shaped

•Threaded, press-fit, welded, or directly cast into VB

–High temp/pressure: combo

•Materials

–Low temp/pressure: soft (bronze, teflon, etc)

–High temp/pressure: Stellite layer/face

•Tungsten carbide metal

•Resist cutting damage due to leakage of steam

Valve Components: Disc

•Part that closes against the seat to stop flow

•Open position: disc fully retracted from seat

•Closed position: pressed against seat

•Throttled position: partially open

•Very different from one valve to another

Valve Components: Stem and Bonnet

•Stem

–Connects to the handwheel or valve operator

–Transmits motion to the internal disc, opening and closing the valve

–Attachment

•Slip joint: disc slips over end of stem, held on by VB

•Threading

•Stem and disc made as one piece

•Pins or cotter keys

•Bonnet

–Top part of valve, holds other parts in place

–Attached to VB by bolting, threading, or welding

–Shape determined by type and shape of disc

•Must accommodate disc in open position

Valve Components: Stuffing Box

Valve Components: External Markings

•Bridge wall markings

–Indicates how internal parts are arranged

–Disc / seat type

–Flow direction shown by arrow

•Service markings

–Indicate service type and max allowable pressure for particular application

–"W“: Water service (low temp)

–"O“: Oil service (low temp)

–“G“: Gas service (low temp)

–Other designations

Gate Valves

•On or off service and flow isolation

•Straight, free flow

•Good for immediate shutoff

•Two types of discs: wedge and double disc

•Wedge-shaped disc

–Fits between near-vertical seats

–Tight shutoff when closed

–Directed by guide channels that insure proper seating alignment

–Sometimes split down the center

Gate Valves: Double Disc

•2 separate discs with spring

•No jamming or wedging

•Seated by system pressure, not stem torque

–Better for high-temp

–Won’t cause jamming

–Pressure pushes upstream disc off its seat

–Force transferred via spring to seat the other disc

–Higher pressure = tighter seat

Gate Valves

•Small valves generally have fixed seats

–More economical to replace valve than to repair seat

•Large valves

–Often welded in place

–Replaceable seats: less expensive than replacing entire valve.

•Rising gate valve stems

–Stem threaded through yoke bushing

–Non-rising handwheel: keyed to bushing

–Rising handwheel: keyed to stem

–Advantage: stem height indicates valve position

•Non-Rising gate valve stems

–Disc threaded to stem

–Handwheel keyed to stem

–Disc threads itself up stem as handwheel rotated

–Yoke bushing is unthreaded, so stem does not rise

–Space-saver

–Good for mechanical operation, since allowances for stem movement in and out are not necessary

–Indicator to show position of stem and disc

Gate Valves: Materials and Safety

•Low-pressure, low-temperature

–Bronze or brass

•Low-pressure steam and lubricating systems

–Cast iron

•High-pressure, high-temperature

–Special alloy metals

–Stainless steel to prevent corrosion

•Some may experience binding between seat and disc due to differing expansion and contraction rates

•Excessive closing forces make worse

•Attention has to be paid to valve closing torque in hot systems which will later be cooled down

Gate Valves: Pressure Locking & Thermal Binding

•Gate valves often fail due to disc binding

•Pressure Locking

–Valve disc locks closed due to high pressure water trapped in the bonnet cavity

•Thermal Binding

–Valve disc locks closed due to differential thermal contraction

–VB and seat cool faster than disc

–Seats pinch the disc

•Recovery from thermal binding

–Heat the VB to expand seat

•Excessive closing force

–Exacerbates thermal binding

–Causes:

•Tight manual closing

•Excessive air pressure on the valve manual operator

•Mis-adjusted or defective motor operator torque switches (motor over-tightened valve)

•To recover from bonnet pressurization

–Remove any valve insulation

–Allow valve to cool

–*IF* there is in approved methodology, sometimes ok to loosen the packing to depressurize the bonnet

Globe Valves

•Isolation, start-stop, or regulation of flow

•Called control valve when used in regulation

–Different shape/arrangement of disc and seats

•Tight seal and good throttling performance

•Seat, disc, and body type differ according to design and function of valve

•4 basic types of disc and seat arrangements

–Ball-shaped disc

–Composition disc

–Plug-type disc

–Needle point disc

•Body types: many

–Example: angled globe valve

•Change direction of flow without added joints

•More joints = more potential leakage

•Ball-type (globe-type) disc

–Seats against tapered, flat-surfaced seat

–Used fully open or shut

–Moderate throttling

–Low-pressure, low temp

•Composition disc

–Renewable

–Variety of materials

–Seating surface often formed by a rubber "O" ring or washer.

•Plug-type disc

–Conical

–Wide seating area

–Good for throttling

–All pressures and temps

•Needle point valve

–Modified plug disc design

–Diameter of the opening very narrow

–Disc descends well below opening and into orifice formed by the seat

–Very close or delicate regulation of flow

Gate and Globe Valves: Backseating

•Seating arrangement that provides seal between the stem and bonnet

•Especially for steam service

•Stem has upper seating area similar to a globe valve disc

•Valve fully open: back seat on stem seats with bonnet seat

•Prevents pressure from building against packing

•Prevents leakage into the upper part of the valve

•Prior to backseating, verify that the valve actually has a backseat

•Never power a motor operated valve onto its backseat

Plug and Ball Valves

•Disc does not rise from seat

•Disc rotates in place

•When valve actuated, disc makes ¼ turn

•Stops flow quickly

•Low pressure: good seal

•High pressure: Seal quality not as good as globe

•Often used in lubricating systems

•Good for slurry

•Single port

–Suction from one source

•Dual-port

–Suction can be shifted from one source to another without loss of flow

•Seats often coated with plastic self-lubricating material

Butterfly Valves

•High-volume, low-pressure, low-temp systems

•Thin – save space

•Flat, circular disc turns through 90 deg range between fully open and fully closed

•Disc is same diameter as piping

–Smoother flow

–Less pressure drop

•Operating lever in-line with piping: valve open

•Perpendicular to piping: valve closed

•Rubber seat provides tight seal when disc closed

•Can be used for throttling, but rare because performs poorly as throttle

•Valve must have some kind of retention mechanism so it is not forced open or closed by pressure

–Ratchet plate for hand-operated

–Stepper or servo motor for automatic

Diaphragm Valves

•Disc and seat made of flexible material

–Seat: rubber, neoprene, or soft plastic

–Good flow control

–Good sealing

–Prevent abrasion and corrosion of valve components

•VB, handwheel, stem, and bonnet like other valves

•A compressor is connected to stem

•Stem lowered: compressor pushes diaphragm into seat to stop flow

•Good seal

•Good for slurry

•Packing not necessary

Diaphragm Valves: Weir and Straight-Through

•Straight-through diaphragm valve

–No obstructions where diaphragm seats

–Valve bore same piping

–Prevents losses (and losses = pressure drop)

•Weir diaphragm valve

–Raised seat

–Allows stiffer / stronger diaphragm material

–Reduces flow area: Large pressure drop

•Throttling

–Good but not best

–Relatively low pressure drops features

•Well suited for chemicals or harmful gases

Diaphragm Valves: Hermavalves

•Type/brand of diaphragm valve commonly used in US nuclear plants

•500 or more per plant

•Venting and draining applications where zero leakage to the atmosphere is specified

•Common Hermavalve failures:

–Improper installation

–Over-torquing caused damage to yoke bushing and seat

Check Valves

•Permits flow in one direction only

•Prevent backflow

•Cannot be used as isolation valves. Close only if:

•Flow is stopped

•System reverses direction of flow

•Most CVs have replaceable seats, discs, and access caps

•Made of bronze, cast iron, or steel

•Special: stainless steel, nickel, or other corrosion-resistant material

•3 basic types: swing, lift, and stop check

Check Valves: Swing-Type

•Disc attached to pivoting arm or hinge pin

•Flow through system keeps disc open

•If flow stopped or reversed, disc will close immediately

•VB equipped with access cap for disc maintenance

Check Valves: Lift-Type

•Disc not pivoted

•Globe-type disc raised when system pressure > weight of disc

•Disc guides prevent disc jamming

•Good for fluctuating flow

–Steam, air, and gases

–Don’t damage as easily as swing check valves

Check Valves: Lift-Type: Ball

•Ball check is variation of lift check

•Disc = ball

•Used on gauge glass isolation valves

Check Valves: Stop-Check

•Resembles globe valve

•Can be used for isolation

•Stem down: disc seated, valve closed

•Stem raised: operates like lift-type check valve

•Responds to system pressure

Check Valves: Failures

•Can lead to

–Water hammer

–Overpressurization of low pressure systems

–Steam binding of emergency feedwater systems

–Extensive damage to other components

•Major causes

–Misapplication

•Oversized

•Wrong type

–Inadequate preventive maintenance

•San Onofre 1, San Diego: Feedwater system damaged and disabled by water hammer due to backflow through 5 failed CVs

•Turkey Point 3 and 4, near Miami: Steam leaked by the normally closed steam isolation valve, causing stop/check valve disc flutter. Excessive wear and eventually in high-cycle fatigue failure.

•Farley 1, Dothan, AL: EFW system backflow due to a pump discharge CV failing to reseat

•Majority of CV failure events involve the EFW system

•How are operators alerted?

–Check discharge temperature of EFW system

•> 150°F  problem

–Listening to the valve

•Rattling/tapping = loose parts or fluttering

Cryogenic Valves

•Cryogenics: storage and handling of substances such as liquid nitrogen or oxygen at very low temperatures (< -200C)

•Special problems for design characteristics of valve

–Cold causes many metals and other mat’ls to become brittle rapidly

–Heat gain from atmosphere and other piping

•Expensive: auxiliary equipment to keep cold

•Pipes can be heavily insulated, but valves must be accessible

•Cryogenic valve made to withstand extremely cold temps with minimum heat gain

•Disc and seat similar to globe valve

•Different stem and packing

•Stem extends through a long, hollow operating tube

•Vacuum jacket acts as an insulator around hollow tube and VB

•Thermal foam at top of tube

•Valve packed at top ("warm“) end of the valve with chevron packing or an "O" ring to prevent leakage

Cryogenic Valves: Brittle Failure

•Constructed from less-brittle materials like bronze and austenitic stainless steel

•Carbon steel (most common material valves) too brittle at extremely low temps

•Liquefaction of gases is more efficient at high pressure in addition to low temp

•If pipes or valves were to become brittle under such pressure, they would burst

Piston Balance Valves

•Single-disc valve: pressure differential (P) exerted entirely on seat

•Force due to P must be overcome by valve actuator

•High-pressure  large valve actuator

•Piston balance valve

–Force due to high pressure is counterbalanced by another disc

–Force is equalized open or closed, giving smooth operation

Relief Valves

•Protect system against overpressure

•Primarily used with pressurized liquids

•Not used with steam or gas

–Too slow

–Steam cutting of disc and seat

•2 basic types of relief valves:

–Self-actuated

–Pilot-actuated

Relief Valves: Self-Actuated

•Simpler of the two

•Normally: disc seated by preset

spring force

•If pressure increases above

established setpoint

–Overcomes the spring force

–Lifts disc off seat

–Allows flow through the valve until

pressure falls below setpoint again

•Often used downstream of PDPs

•Discharge to open drain or holding tank

•Setpoint controlled by adjusting the spring with an adjusting bolt

•Valve may be locked shut with a gagging cap-and-screw device may be used to lock the stem

Relief Valves: Pilot-Actuated

•Separate sensing device that reacts to excessive pressure

•Allows detection in one part of system while valve protects another area

•Pilot section

–Detects and directs pressure to hydraulic actuator in main section

•Main section

–Hydraulically-operated globe valve

–Reverse-seated: pressure keeps valve tightly shut

–Helical compression spring also holds disc shut

–Stem extends through seat port to actuating piston in VB

•Pilot and relief valves may be:

–Both in one unit, or…

–Two totally separate units, interconnected by piping

•Relief pressure settings adjusted by setting stem position

•When system is safe and pilot deactivates, spring force and system pressure on the reverse-seated globe work together to close the main valve

•Can also be gagged shut with gagging bolt, but ONLY the pilot valve or pilot section since the main valve is reverse-seated

•Main valve may also open if it experiences backpressure

Safety Valves

•Safety valves are classified by:

–How they open

–# of adjusting rings they contain

•Our focus: One-Ring Huddling Chamber Safety Valve (HCSV)

•Parts:

–VB (casing)

–Inlet and outlet

–Disc (feather) and seat

–Spring

–Disc guide

–Spindle

–Spring washers

–Spring adjusting screw

Traps

•In handling steam, condensation must be dealt with

•Large amounts of water in steam pipes could

–cause water hammer

–damage turbine blades

–damage other internal components

–reduce efficiency

•Traps used to remove undesirable moisture in steam lines (steam traps) and compressed gas lines (moisture traps)

•Automatically remove condensate without discharging steam or gas

•Steam traps also remove non-condensable gases (like air)

Mechanical Traps

•Large capacity

•Low pressures and narrow temperature range

•Drain regulator (aka float trap)

–Inlet on top

–Condensate floods chamber, causing float to rise

–Connecting rod lifts, and opens valve to discharge condensate

•Inverted bucket trap

–Normally, bucket floats on pocket of steam

–When trap full of water, no more room for steam

–Bucket sinks to bottom of trap reservoir

–Bucket pulls open discharge valve

–Water expelled due to pressure from steam line

–Steam displaces water in the bucket

–Bucket floats back up and closes discharge valve

–Vent hole allows air & steam to escape bucket so it can refill with water

–Water never completely discharged, so steam can’t blow through

Mechanical Traps: Special Considerations

•Trap should be installed to operate only under conditions for which it is designed

•Mechanically operated traps should be level.

•Upstream steam strainers are used to prevent rust, grit, or scale from entering trap

•Trap should be located at lower level than unit to be drained

•Traps should be checked to make sure they are operating correctly by feeling the trap body and discharge line

–Warm = proper operation

–Hot = steam blow-through (waste)

–Cold = no drainage

Impulse (aka Orifice) Traps

•Impulse, flash, orifice, or venturi traps

–Steam or condensate (or both) enters trap through strainer

–Control piston has pinhole to exit called orifice or venturi

–Piston normally down so valve plug is seated

–Orifice allows pressure to drop in control cylinder above the piston

–When pressure reaches <=85% of inlet, piston forced up higher pressure below flange

–Piston opens valve to release condensate

–Combination of hot flow into inlet and pressure drop causes some condensate to flash to steam, choking the orifice

–Pressure rises in control cylinder and piston drops to seat valve

Thermostatically Actuated Traps: Bimetallic

•Consist of valve and bimetallic element

•B.E. anchored at one end

•Free-floating valve stem and disc attached to other end

•B.E. bends in response to temperature change to open and close the valve

•Opening temperature determined by clearance between disc and seat when the trap is cold

–Can be adjusted by a nut on the stem

Thermostatically Actuated Traps: Bellows

•Used mainly for draining heating systems and auxiliary machinery

•Generally limited to pressures <100 psi