Valves

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Valves

Globe Gate Butterfly Check Stop Check Pressure Regulating Control Valves Traps and

Strainers Steam Pressure Reducing Valve Maintenance and Repair

Every piping system must have some means of controlling the amount and direction of flow. This isaccomplished with valves. Valves used in the machinery space piping systems, and constructed with threadedvalve stems, must be right-hand closing (clockwise).

Valves are usually made of bronze, brass, cast iron, malleable iron, or steel. Steel valves are either cast orforged, and are made of carbon steel, low alloy steel, or stainless steel. Alloy steel valves are used in highpressure, high temperature systems. The disks and seats of these valves are usually surfaced with achromium-cobalt alloy known as STELLITE. This material is extremely hard.

Bronze and brass valves are not used in high temperature systems or systems subject to high pressures,severe vibration and shock. Bronze valves are widely used in salt water systems. Seats and disks of bronzevalves used for salt water service are often made of Monel An alloy of nickel and copper and other metals(such as iron and/or manganese and/or aluminum) which is highly resistant to corrosion and erosion.

Many different types of valves are used to control the flow of liquids and gases. There are two main groups ofvalves.

STOP VALVES - Stop valves are used to shut off or, in some cases, partially shut off the flow of fluid. Stopvalves are controlled by the movement of the valve stem. Stop valves can be divided into four generalcategories: globe, gate, butterfly, and ball valves. Plug valves and needle valves may also be considered stopvalves.

CHECK VALVES - are used to permit the flow in only one direction. These valves are controlled by the flowitself.

Valve designs vary greatly due to the demands of service. Some valves are combinations of the basic typesmentioned, and others such as pressure reducing valves must be considered special valves. In generalhowever we may consider stop valves to include globe valves, gate valves, piston valves, plug valves, needlevalves, and butterfly valves. Check valves include swing-check and lift-check valves.

Excessive leakage and premature failure of valve packing is an indication of a scored valve stem.

Globe Valve

Globe valves are one of the commonest types of stop valves.Globe valves get their name from their globular shape. It shouldbe noted that other valves may also have globe shaped bodies.The internal structure of the valve rather than the externalshape is what distinguishes one valve from another.

In a globe-type stop valve, the disk is attached to the valvestem. The disk seats against a seating ring or a seating surfaceand thus shuts off the flow. When the disk is moved off theseating surface, the flow can pass through the valve. Globevalves can be used partially opened as well as fully opened orclosed. The valve should always be installed with the inletdirected under the seat.

Globe valves inlet and outlet openings are arranged variousways to suit different flows.

The cross-type globe valve has three openings, and frequentlyused in connection with bypass piping.

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Globe valves arecommonly used insteam, air, oil, and

water piping.

Gate Valve

Gate valves are used in systems where straight flow withminimum restriction is desired, such as water lines. Firemain cutout valves are usually gate valves. Gate valves are also used insteam lines, particularly on newer ships. They are used in fuelsystems, quick-closing fuel oil valves are usually gate valves. Ontankers with manually operated tank valves, the most commonlyused valve is the gate valve.

The part of a gate valve that serves the same purpose as thedisk of a globe valve is called a gate. The gate is usually wedgeshaped, but some gates are uniform thickness. When the gate iswide open the opening through the valve is the same size as thepipe in which the valve is installed. Therefore there is very littleresistance to flow and very little pressure drop. Gate valves arenot suitable for use as throttling valves, since the regulation offlow would be difficult and the flow against a partially openedgate can cause extensive damage to the valve.

The gate is connected to the valve stem. Turning the handwheel positions the valve gate. Some gate valves have NON-RISING STEMS, the stem is threaded into the gate, so the gatetravels up and down on the valve stem. Non-rising stem gatevalve should be opened to the end of the last opening turn, thenrotate the hand wheel in the closing direction by approximately1/4 of a turn. Valves with RISING STEMS, both the stem and thegate move upward as the valve is opened. In some rising stemvalves the stem projects above the hand wheel when the valveis opened. In other rising stem valves the stem does not projectabove the hand wheel and a pointer or gauge is required toindicate the position of the gate.

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Butterfly Valve

Butterfly valves are light weight, and take up less space thanglobe and gate valves. They are easy to overhaul, and arequick acting. Although the design and construction ofbutterfly valves vary some what, a butterfly-type disk andsome means of sealing are common to all butterfly valves.

The handle on properly installed butterfly valve must beparallel to the flow when in the fully open position.

The butterfly valve illustrated here consists of a body,resilient seat, butterfly-type disk, stem, packing, notchedpositioning plate, and handle. The resilient seat is undercompression when it is mounted in the valve body making aseal around the periphery of the disk, and both points wherethe stem passes through the seat. Packing is provided to forma positive seal around the stem if the seal formed by the seatshould become damaged.

To close the valve it is only necessary to turn the handle aquarter turn. The resilient seat exerts positive pressureagainst the disk, assuring a tight seal.

Butterfly valves are relative easy to maintain. The resilientseat is held in place by mechanical means, and neitherbonding nor cementing is necessary. Because the seat isreplaceable the valve does not require lapping, grinding , ormachine work.

Butterfly valves are designed for a variety of systems, such asfresh water, salt water, fuel oil, and lube oil.

Check Valves

Check valves are designed to permit flow through a line in one direction only. A good example would be drainlines. Care must be taken to install this type of valve is properly installed. Most check valves have an arrow,or the word inlet cast on the valve body to indicate the direction of flow.

The port in a check valve may be closed by a disk, ball, or plunger. The valve opens when the pressure on theinlet side is greater than that on the outlet side, and closes when the reverse is true. All such valves openand close automatically. Check valves are made with threaded, flanged, or union faces, with screwed orbolted caps, and for specific pressure ranges.

Swing Check

The disk of a swing check valve is raised as soon as theline pressure entering below the disk is of sufficient force.

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While the disk is raised continuous flow takes place. If theflow is reversed or back pressure builds up this opposing

pressure forces the disk to seat, stopping the flow.

Lift Check

The operation of a lift check valve is similar to that of a swingcheck valve, except the valve disk moves in a up and downdirection instead of through and ark.

Stop Check Valves

Most valves may be classified as either stop valves or check valves. Stop check valves function as bothstop valves and check valves. These valves work like a lift check valve. However the valve stem is longenough so that when it is turned all the way down it holds the disk firmly against the seat preventing anyflow. In this position the valve acts as a stop valve. When the stem is raised the disk can be opened bypressure on the inlet side. In this position the valve acts as a lift check valve to allow the flow of fluid inonly one direction. The amount of the opening is controlled by the position of the valve stem, the amountof flow through the valve is thereby regulated.

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Pressure Reducing Valves

Spring-Loaded Pressure Regulating Valve

Pressure reducing valves are automatic valves whichare used to provide a steady pressure lower than thesupply pressure. Pressure reducing valves can be setfor any desired discharge pressure, within the limits ofthe design of the valve.

Various types of reducing valves are found aboardship. The valve illustrated is a single-seated, direct-acting, spring-loaded diaphragm type. Control ofwater passing through this valve is effected by meansof a pressure difference on opposite sides of thediaphragm. The diaphragm is secured to the stem.Reduced pressure from the valve outlet is led throughan internal passage to a diaphragm chamber belowthe diaphragm. An adjusting spring acts on the upperside of the diaphragm. A leather cup washer orneoprene O-ring makes the seal between the valveinlet and the diaphragm chamber. This seal is locatedabout halfway down the valve stem.

The amount of pressure applied to the underside ofthe diaphragm varies according to the dischargepressure. When the discharge pressure is greater thanthe spring pressure the diaphragm is forced up closingthe valve or decreasing the amount of discharge.When the discharge pressure is less than the springpressure the diaphragm and the valve stem are forceddown opening the valve wider and increasing the

amount of discharge.

The amount of pressure applied by the spring to the top of the diaphragm can be varied by turning anadjusting screw. Turning the adjusting screw clockwise increases the pressure applied by the spring to thetop of the diaphragm, increasing the discharge pressure. Turning the adjusting screw counterclockwise

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decreases the amount of pressure to the top of the diaphragm, decreasing the discharge pressure.

Pneumatic Pressure Controlled Reducing Valve

The pneumatic pressure controlled (or gas-loaded)reducing valve is used to reduce pressure in steamsystems.

A rubber diaphragm is installed in the middle of thedome. The bottom of the diaphragm is separatedfrom the bottom half of the dome by a fixed steelplate. The area immediately above the diaphragmcommunicates with the upper part of the domethrough holes in shrouding. The upper half of thedome carries a level of water (condensate) forsealing. The lower half of the dome carries a level ofglycerine for sealing. The area above the glycerineis charged with air, which exerts a downwardpressure on the glycerine and forces some of it togo up the tube toward the diaphragm. This pressurecauses the diaphragm and the stem to moveupward, opening the valve.

From the outlet connection an actuating line leadsback to the upper part of the dome. Steam at thereduced pressure is allowed to exert a force on thetop of the water seal. This force is transmittedthrough the water and tends to move the diaphragmdownward.

When the pressure of the steam from the actuating line exceeds the loading air pressure in the lower halfof the dome the diaphragm moves downward to close the valve. The closing of the valve reduces thepressure of the steam on the discharge side of the valve.

Theoretically the valve should deliver a pressure of steam equal to the air charge in the lower half of thedome. However the valve itself has weight and is equipped with a light spring to close it, its necessary tointroduce slightly more are pressure. High pressure valves about 10 Psi additional air pressure is required. Ifair is added when the valve is cold slightly less air pressure will be needed as the pressure will increase asthe valve is warmed up.

High pressure valves have cooling fins extending outside the dome from the center flange. The fins allowtransmission of heat from the upper half of the dome. This keeps the heat from passing to the lower halfwhere it would cause an excessive rise in air pressure.

Spring Loaded Internal Pilot Steam Pressure Reducing Valve

The spring-loaded, steampressure, reducing valveshown uses an auxiliaryvalve also called a pilotvalve, to control themain valve. The auxiliaryvalve controls the flow ofthe high pressure steamto the main pistonthrough the highpressure port.

The Auxiliary valve openswhen the adjustingspring tension, acting onthe diaphragm overcomes

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the control portpressure. It closes whencontrol port pressureexceeds the tension ofthe adjusting spring.

The main valve is openedwhen the auxiliary valveopens, allowing thesteam from the highpressure port to the topof main piston openingthe main valve. The mainis closed by the mainvalve spring and highpressure steam acting onthe under side of seatingdisk.

The main piston is larger than the main valve seat to allow control action to be accomplished with arelatively small amount of high pressure steam. The vertical grooves machined on the main valve provide forquieter valve operation.

Turning the pressure adjusting stud clockwise will increase the spring tension on the diaphragm increasingthe outlet pressure. Turning the pressure adjusting stud counter-clockwise decreases the spring tension onthe diaphragm, less pressure is required to close the auxiliary valve therefor the outlet pressure is lowered.Valves should be warmed-up and drained before they are adjusted.

Dual Pressure-Temperature Regulator

The dual pressure-

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temperature regulator valveshown is the same springloaded internal pilot steampressure reducing valveshown above, with atemperature regulatingdevice installed. The valveregulates pressure as well ascorresponding fluid outlettemperature and can becharacterized by the termproportional plus resetcontrol.

Turning the pressureadjusting stud clockwise (asviewed from the top) andthe temperature adjustingring counterclockwise willresult in a higher outletpressure, with a highercontrolled fluid outlettemperature.

Like the spring loadedinternal pilot steam pressurereducing valve shown abovethe vertical groovesmachined on the main valveprovide for quieter valveoperation.

Control Valves

Hydraulic Control Valve

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Hydraulic control valves are used in many shipboardsystems where access to valves is limited or aremote operation is required. This type of valve maybe operated from one or more remote stations by ahydraulic control system. There are manyconfigurations of globe, gate, and butterfly hydrauliccontrol valves available for use on shipboardsystems.

The valve shown is piston operated globe valve. It isnormally held in the closed position by spring tension.When hydraulic pressure is admitted to the undersideof the piston the force overcomes the spring tensioncausing the valve to open.

When hydraulic pressure is released from under thepiston the spring acts to force the hydraulic fluid outof the cylinder and back to the remote controlstation, closing the valve.

A rachet lever is fitted to the valve to permitemergency opening of the valve by hand.

Air Operated Diaphragm Control Valve

Double seated, pneumatically controlled, regulatingvalves exhibit good balancing characteristicsessential for low-sensitivity applications becausehigh pressure enters between the seats andcreates equal, but opposing forces.

Air operated diaphragm control valve and thecontrol pilot valves used in controlling them (seebelow) are available in many differentconfigurations.

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The air operated diaphragm control valve shown isdirect acting. On direct acting valves the air froma control source is applied to the top of thediaphragm. Reverse acting valves, the air isapplied to the under side of the diaphragm.

This valve is downward seating, air pressureapplied to the top of the diaphragm with sufficientpressure it over come the opposed spring tensionwill move the stem downward. This tends to closethe valve. The force exerted on the spring is equalto the air pressure multiplied by the area of thediaphragm.

The valve stem is sealed with a packing gland,care must be taken when adjusting, binding willcause erratic operation. Screwing down on sleeveadjusting nut lessens the spring tension causingthe valve, to close at a lower loading pressure.The spring force must be within the operatingrange of the pilot output loading pressure.

Air Operated Control Pilot

Pilot controls are used are used in conjunction with airoperated diaphragm control valves, (see above) providingoperating or loading pressure.

They are available in many different configurations. Airoperated pilot control valves may be directing acting orreverse acting. Directing acting valves have there controlpressure applied to the top of the diaphragm while reverseacting valves have there control pressure applied to theunderside of the diaphragm. This control pressure is usuallysupplied from the discharge side of the diaphragm controlvalve.

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VALVE MAINTENANCE

Traps and Strainers

Traps are used to remove various undesirable materials from piping systems. In air lines a trap is installed toremove water which is usually present. In steam lines traps are installed to remove condensate. Some typesof steam traps are suitable for low pressure and others for high pressure. All steam traps consist of a valveand some device or arrangement which will cause the valve to open and close, as necessary to draincondensate from the lines without allowing steam to escape. The three types of steam traps most commonlyused are mechanical, thermostatic, flash, and impulse.

Bucket Type Steam Trap

The bucket type steam trap is suitable for pressures up to150 Psi. Operation of these traps is regulated by thecondensate level in the trap body. The bucket, beingbuoyant, floats as condensate enters the trap body. Thevalve is connected to the bucket and closes as the bucketrises. As condensate Continues to flow into the valve bodythe valve remains closed until the bucket is filled. When thebucket is filled it sinks and opens the valve. The valve willremain open until sufficient condensate is blown out to allowthe bucket to float, which closes the valve and starts thecycle again.

Ball Type Steam Trap

This trap works much in the same way as the buckettrap. Condensate and steam enter the body of the trap,and the condensate collects at the bottom. As thecondensate level rises, the ball float rises until it israised enough to open the outlet valve of the trap.When the outlet valve opens, the condensate flows outof the trap into the drain system, and the float leveldrops, shutting off the valve until the condensate levelrises again.

Bimetallic Steam Trap

Bimetallic steam traps of the type shown are used in

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many ships to drain condensate from main steam lines,auxiliary steam lines, and other steam components. Themain working parts of this steam trap are a segmented

bimetallic element and a ball-type check valve.

Line pressure acting on the check valve keeps the valveopen. When steam enters the trap body, the bimetallicelement expands unequally because of the differentresponse to the temperature of the two metals; thebimetallic element deflects upward at its free end, thusmoving the valve stem upward and closing the valve. Asthe steam cools and condenses, the bimetallic elementmoves downward, toward the horizontal position, thusopening the valve and allowing some condensate to flowout through the valve. As the flow of condensate begins,an unbalance of line pressure across the valve is created;since the line pressure is greater on the upper side of theball of the check valve, the valve now opens wide andallows a full capacity flow of condensate.

Thermostatic Type Steam Trap

The thermostatic type steam trap is oftencalled a bellows type steam trap. this type ofsteam trap has fewer moving parts thanmechanical steam traps and is more compact.The bellows type trap is used only for pressuresup to 100 Psi. Operation of this trap iscontrolled by expansion of vapor from volatileliquid A liquid that changes readily from liquid toa vapor which is enclosed in a bellows typeelement. Steam enters the trap body, heatingthe volatile liquid in the sealed bellows and causes expansion of the bellows. The valve isattached to the bellows and closes when thebellows expands. The valve remains closedtrapping the steam in the trap body.Condensation of the steam cools the bellowsand causes it to contract opening the valve anddrains the condensate.

Lavatory Traps

In parts A and B, the P-type lavatory trap is illustrated, with

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and without clean out plug. Part C shows the S-type trapspecified for shipboard use. This latter type, equipped withclean-out plug. These fittings are made from brass, and areusually chrome platted.

Basket Strainer

Strainers are located in all piping systems to prevent thepassage of foreign mater. They must be installed so the flowwill be through the strainer element. The bilge strainer shownis an example of a basket strainer. In some locations duplexstrainers are used so that the flow of fluid through the systemneed not be interrupted when one element is removed forcleaning.

Valve Maintenance

Preventive maintenance is the best way to extend the life of valves and fittings. When making repairs on moresophisticated valve types, use the available manufacturers technical manuals. As soon as you observe a leak,determine the cause, and then apply the proper corrective maintenance. Maintenance may be as simple astightening a packing nut or gland. A leaking flange joint may need only to have the bolts tightened or to havea new gasket or O-ring inserted. Dirt and scale, if allowed to collect, will cause leakage. Loose hangers permitsections of a line to sag, and the weight of the pipe and the fluid in these sagging sections may strain jointsto the point of leakage.

Whenever you are going to install a valve, be sure you know the function the valve is going to performthatis, whether it must start flow, stop flow, regulate flow, regulate pressure, or prevent back-flow. Inspect thevalve body for the information that is stamped upon it by the manufacturer: type of system (oil, water, gas),operating pressure, direction of flow, and other information.

You should also know the operating characteristics of the valve, the metal from which it is made, and the typeof end connection with which it is fitted. Operating characteristics and the material are factors that affect thelength and kind of service that a valve will give; end connections indicate whether or not a particular valve is

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suited to the installation.

When you install valves, ensure they are readily accessible and allow enough headroom for full operation.Install valves with stems pointing upward if possible. A stem position between straight up and horizontal isacceptable, but avoid the inverted position (stem pointing downward). If the valve is installed with the stempointing downward, sediment will collect in the bonnet and score the stem. Also, in a line that is subject tofreezing temperatures, liquid that is trapped in the valve bonnet may freeze and rupture it.

Since you can install a globe valve with pressure either above the disk or below the disk (depending on whichmethod will be best for the operation, protection, maintenance, and repair of the machinery served by thesystem), you should use caution. The question of what would happen if the disk became detached from thestem is a major consideration in determining whether pressure should be above the disk or below it. If you arerequired to install a globe valve, be SURE to check the blueprints for the system to see which way the valvemust be installed. Very serious casualties can result if a valve is installed with pressure above the disk when itshould be below the disk, or below the disk when it should be above.

Valves that have been in constant service for a long time will eventually require gland tightening, repacking, ora complete overhaul of all parts. If you know that a valve is not doing the job for which it was intended,dismantle the valve and inspect all parts. You must repair or replace all defective parts.

The repair of globe valves (other than routine renewal of packing) is limited to refinishing the seat and/or disksurface. When doing this work, you should observe the following precautions:

When refinishing the valve seat, do not remove more material than is necessary. You can finish valvesthat do not have replaceable valve seats only a limited number of times.

Before doing any repair to the seat and disk of a globe valve, check the valve disk to make certain it issecured rigidly to and is square on the valve stem. Also, check to be sure that the stem is straight. Ifthe stem is not straight, the valve disk cannot seat properly,

Carefully inspect the valve seat and valve disk for evidence of wear, for cuts on the seating area, andfor improper fit of the disk to the seat. Even if the disk and seat appear to be in good condition, youshould perform a spot-in check to find out whether they actually are in good condition.

Standard checkoff diagram for performing a routine inspection and minor maintenance of a valve.

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Spotting-In Valves

The method used to visually determine whether the seat and the disk of a valve make good contact with eachother is called spotting-in. To spot-in a valve seat, you first apply a thin coating of prussian blue evenly overthe entire machined face surface of the disk. Insert the disk into the valve and rotate it one-quarter turn,using a light downward pressure. The prussian blue will adhere to the valve seat at those points where thedisk makes contact.

The illustration shows the appearance of a correct seat when it isspotted-in; it also shows the appearance of various kinds ofimperfect seats.

After you have noted the condition of the seat surface, wipe all theprussian blue off the disk face surface. Apply a thin, even coat ofprussian blue to the contact face of the seat, place the disk on thevalve seat again, and rotate the disk one-quarter turn. Examine theresulting blue ring on the valve disk. The ring should be unbrokenand of uniform width. If the blue ring is broken in any way, the diskis not making proper contact with the seat.

Lapping-In Valves

The manual process used to remove small irregularities by grinding together the contact surfaces of the seatand disk is called lapping-in. Lapping-in should not be confused with refacing processes in which lathes, valvereseating machines, or power grinders are used to re-condition the seating surfaces.

To lap-in a valve, first apply a light coating of lapping compound to the face of the disk. Then insert the diskinto the valve and rotate the disk back and forth about one-quarter turn; shift the disk-seat relationship fromtime to time so the disk will be moved gradually, in increments, through several rotations. During the lappingprocess, the lapping compound will gradually be displaced from between the seat and disk surfaces; therefore,you must stop every minute or so to replenish the compound. When you do this, wipe both the seat and the

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disk clean before applying the new compound to the disk face.

Lapping-in is also used to follow up all machining work on valve seats or disks. When the valve seat and diskare first spotted-in after they have been machined, the seat contact will be very narrow and will be locatedclose to the bore. Lapping-in, using finer and finer compounds as the work progresses, causes the seatcontact to become broader. The contact area should be a perfect ring covering about one-third of the seatingsurface.

Be careful to avoid over-lapping a valve seat or disk. Over-lapping will produce a groove in the seating surfaceof the disk; it will also round off the straight, angular surface of the disk. Machining is the only process bywhich over-grinding can be corrected.

Repacking Valves

If the stem and packing of a valve are in good condition, you can normally stop packing gland leaks bytightening up on the packing. You must be careful, however, to avoid excessive thread engagement of thepacking gland studs (if used) and to avoid tightening old, hardened packing, which will cause the valve toseize. Subsequent operation of such a valve may score or bend the stem. Packing a badly scored valve stemwill cause leaking and premature failure of the packing.

Coils, rings, and corrugated ribbon are the common forms of packing used in valves. The form of packing to beused in repacking a particular valve will depend on the valve size, application, and type. Packing materials willbe discussed in more detail later in this chapter.

Valves

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