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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 is accomplished with valves. Valves used in the machinery space piping systems, and constructed with threaded valve 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 or forged, and are made of carbon steel, low alloy steel, or stainless steel. Alloy steel valves are used in high pressure, high temperature systems. The disks and seats of these valves are usually surfaced with a chromium-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 bronze valves 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 of valves.

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

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

Valve designs vary greatly due to the demands of service. Some valves are combinations of the basic types mentioned, and others such as pressure reducing valves must be considered special valves. In general however we may consider stop valves to include globe valves, gate valves, piston valves, plug valves, needle valves, 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 should be noted that other valves may also have globe shaped bodies. The internal structure of the valve rather than the external shape is what distinguishes one valve from another.

In a globe-type stop valve, the disk is attached to the valve stem. The disk seats against a seating ring or a seating surface and thus shuts off the flow. When the disk is moved off the seating surface, the flow can pass through the valve. Globe valves can be used partially opened as well as fully opened or closed. The valve should always be installed with the inlet directed under the seat.

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

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

Globe valves are commonly used in steam, air, oil, and water piping.

 

Gate Valve

Gate valves are used in systems where straight flow with minimum restriction is desired, such as water lines. Firemain cut out valves are usually gate valves. Gate valves are also used in steam lines, particularly on newer ships. They are used in fuel systems, quick-closing fuel oil valves are usually gate valves. On tankers with manually operated tank valves, the most commonly used valve is the gate valve.

The part of a gate valve that serves the same purpose as the disk of a globe valve is called a gate. The gate is usually wedge shaped, but some gates are uniform thickness. When the gate is wide open the opening through the valve is the same size as the pipe in which the valve is installed. Therefore there is very little resistance to flow and very little pressure drop. Gate valves are not suitable for use as throttling valves, since the regulation of flow would be difficult and the flow against a partially opened gate can cause extensive damage to the valve.

The gate is connected to the valve stem. Turning the hand wheel positions the valve gate. Some gate valves have NON-RISING STEMS, the stem is threaded into the gate, so the gate travels up and down on the valve stem. Non-rising stem gate valve should be opened to the end of the last opening turn, then rotate the hand wheel in the closing direction by approximately 1/4 of a turn. Valves with RISING STEMS, both the stem and the gate move upward as the valve is opened. In some rising stem valves the stem projects above the hand wheel when the valve is opened. In other rising stem valves the stem does not project above the hand wheel and a pointer or gauge is required to indicate the position of the gate.

 

Butterfly Valve

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

The handle on properly installed butterfly valve must be parallel 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, notched positioning plate, and handle. The resilient seat is under compression when it is mounted in the valve body making a seal around the periphery of the disk, and both points where the stem passes through the seat. Packing is provided to form a positive seal around the stem if the seal formed by the seat should become damaged.

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

Butterfly valves are relative easy to maintain. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary. Because the seat is replaceable the valve does not require lapping, grinding , or machine work.

Butterfly valves are designed for a variety of systems, such as fresh 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 drain lines. 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 the inlet side is greater than that on the outlet side, and closes when the reverse is true. All such valves open and close automatically. Check valves are made with threaded, flanged, or union faces, with screwed or bolted caps, and for specific pressure ranges.

Swing Check

The disk of a swing check valve is raised as soon as the line pressure entering below the disk is of sufficient force. While the disk is raised continuous flow takes place. If the flow 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 swing check valve, except the valve disk moves in a up and down direction 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 both stop valves and check valves. These valves work like a lift check valve. However the valve stem is long enough so that when it is turned all the way down it holds the disk firmly against the seat preventing any flow. In this position the valve acts as a stop valve. When the stem is raised the disk can be opened by pressure on the inlet side. In this position the valve acts as a lift check valve to allow the flow of fluid in only one direction. The amount of the opening is controlled by the position of the valve stem, the amount of flow through the valve is thereby regulated.

 

Pressure Reducing Valves

Spring-Loaded Pressure Regulating Valve

Pressure reducing valves are automatic valves which are used to provide a steady pressure lower than the supply pressure. Pressure reducing valves can be set for any desired discharge pressure, within the limits of the design of the valve.

Various types of reducing valves are found aboard ship. The valve illustrated is a single-seated, direct-acting, spring-loaded diaphragm type. Control of water passing through this valve is effected by means of a pressure difference on opposite sides of the diaphragm. The diaphragm is secured to the stem. Reduced pressure from the valve outlet is led through an internal passage to a diaphragm chamber below the diaphragm. An adjusting spring acts on the upper side of the diaphragm. A leather cup washer or neoprene O-ring makes the seal between the valve inlet and the diaphragm chamber. This seal is located about halfway down the valve stem.

The amount of pressure applied to the underside of the diaphragm varies according to the discharge pressure. When the discharge pressure is greater than the spring pressure the diaphragm is forced up closing the valve or decreasing the amount of discharge. When the discharge pressure is less than the spring pressure the diaphragm and the valve stem are forced down 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 an adjusting screw. Turning the adjusting screw clockwise increases the pressure applied by the spring to the top of the diaphragm, increasing the discharge pressure. Turning the adjusting screw counterclockwise 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 steam systems.

A rubber diaphragm is installed in the middle of the dome. The bottom of the diaphragm is separated from the bottom half of the dome by a fixed steel plate. The area immediately above the diaphragm communicates with the upper part of the dome through holes in shrouding. The upper half of the dome carries a level of water (condensate) for sealing. The lower half of the dome carries a level of glycerine for sealing. The area above the glycerine is charged with air, which exerts a downward pressure on the glycerine and forces some of it to go up the tube toward the diaphragm. This pressure causes the diaphragm and the stem to move upward, opening the valve.

From the outlet connection an actuating line leads back to the upper part of the dome. Steam at the reduced pressure is allowed to exert a force on the top of the water seal. This force is transmitted through the water and tends to move the diaphragm downward.

When the pressure of the steam from the actuating line exceeds the loading air pressure in the lower half of the dome the diaphragm moves downward to close the valve. The closing of the valve reduces the pressure 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 the dome. However the valve itself has weight and is equipped with a light spring to close it, its necessary to introduce slightly more are pressure. High pressure valves about 10 Psi additional air pressure is required. If air is added when the valve is cold slightly less air pressure will be needed as the pressure will increase as the valve is warmed up.

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

 

Spring Loaded Internal Pilot Steam Pressure Reducing Valve

The spring-loaded, steam pressure, reducing valve shown uses an auxiliary valve also called a pilot valve, to control the main valve. The auxiliary valve controls the flow of the high pressure steam to the main piston through the high pressure port.

The Auxiliary valve opens when the adjusting spring tension, acting on the diaphragm overcomes the control port pressure. It closes when control port pressure exceeds the tension of the adjusting spring.

The main valve is opened when the auxiliary valve opens, allowing the steam from the high pressure port to the top of main piston opening the main valve. The main is closed by the main valve spring and high pressure steam acting on the under side of seating disk.

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

Turning the pressure adjusting stud clockwise will increase the spring tension on the diaphragm increasing the outlet pressure. Turning the pressure adjusting stud counter-clockwise decreases the spring tension on the 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-temperature regulator valve shown is the same spring loaded internal pilot steam pressure reducing valve shown above, with a temperature regulating device installed. The valve regulates pressure as well as corresponding fluid outlet temperature and can be characterized by the term proportional plus reset control.

Turning the pressure adjusting stud clockwise (as viewed from the top) and the temperature adjusting ring counterclockwise will result in a higher outlet pressure, with a higher controlled fluid outlet temperature.

Like the spring loaded internal pilot steam pressure reducing valve shown above the vertical grooves machined on the main valve provide for quieter valve operation.

 

Control Valves

Hydraulic Control Valve

Hydraulic control valves are used in many shipboard systems where access to valves is limited or a remote operation is required. This type of valve may be operated from one or more remote stations by a hydraulic control system. There are many configurations of globe, gate, and butterfly hydraulic control valves available for use on shipboard systems.

The valve shown is piston operated globe valve. It is normally held in the closed position by spring tension. When hydraulic pressure is admitted to the underside of the piston the force overcomes the spring tension causing the valve to open.

When hydraulic pressure is released from under the piston the spring acts to force the hydraulic fluid out of the cylinder and back to the remote control station, closing the valve.

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

 

Air Operated Diaphragm Control Valve

 

Double seated, pneumatically controlled, regulating valves exhibit good balancing characteristics essential for low-sensitivity applications because high pressure enters between the seats and creates equal, but opposing forces.

 Air operated diaphragm control valve and the control pilot valves used in controlling them (see below) are available in many different configurations.

The air operated diaphragm control valve shown is direct acting. On direct acting valves the air from a control source is applied to the top of the diaphragm. Reverse acting valves, the air is applied to the under side of the diaphragm.

This valve is downward seating, air pressure applied to the top of the diaphragm with sufficient pressure it over come the opposed spring tension will move the stem downward. This tends to close the valve. The force exerted on the spring is equal to the air pressure multiplied by the area of the diaphragm.

The valve stem is sealed with a packing gland, care must be taken when adjusting, binding will cause erratic operation. Screwing down on sleeve adjusting nut lessens the spring tension causing the valve, to close at a lower loading pressure. The spring force must be within the operating range of the pilot output loading pressure.

 

Air Operated Control Pilot

Pilot controls are used are used in conjunction with air operated diaphragm control valves, (see above) providing operating or loading pressure.

They are available in many different configurations. Air operated pilot control valves may be directing acting or reverse acting. Directing acting valves have there control pressure applied to the top of the diaphragm while reverse acting valves have there control pressure applied to the underside of the diaphragm. This control pressure is usually supplied from the discharge side of the diaphragm control valve.

 

VALVE MAINTENANCE

Traps and Strainers

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

Bucket Type Steam Trap

The bucket type steam trap is suitable for pressures up to 150 Psi. Operation of these traps is regulated by the condensate level in the trap body. The bucket, being buoyant, floats as condensate enters the trap body. The valve is connected to the bucket and closes as the bucket rises. As condensate Continues to flow into the valve body the valve remains closed until the bucket is filled. When the bucket is filled it sinks and opens the valve. The valve will remain open until sufficient condensate is blown out to allow the bucket to float, which closes the valve and starts the cycle again.

 

Ball Type Steam Trap

 

This trap works much in the same way as the bucket trap. Condensate and steam enter the body of the trap, and the condensate collects at the bottom. As the condensate level rises, the ball float rises until it is raised enough to open the outlet valve of the trap. When the outlet valve opens, the condensate flows out of the trap into the drain system, and the float level drops, shutting off the valve until the condensate level rises again.

 

Bimetallic Steam Trap

Bimetallic steam traps of the type shown are used in many ships to drain condensate from main steam lines, auxiliary steam lines, and other steam components. The main 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 valve open. When steam enters the trap body, the bimetallic element expands unequally because of the different response to the temperature of the two metals; the bimetallic element deflects upward at its free end, thus moving the valve stem upward and closing the valve. As the steam cools and condenses, the bimetallic element moves downward, toward the horizontal position, thus opening the valve and allowing some condensate to flow out 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 the ball of the check valve, the valve now opens wide and allows a full capacity flow of condensate.

 

 

Thermostatic Type Steam Trap

The thermostatic type steam trap is often called a bellows type steam trap. this type of steam trap has fewer moving parts than mechanical steam traps and is more compact. The bellows type trap is used only for pressures up to 100 Psi. Operation of this trap is controlled by expansion of vapor from volatile liquid A liquid that changes readily from liquid to a vapor which is enclosed in a bellows type element. Steam enters the trap body, heating the volatile liquid in the sealed bellows and  causes expansion of the bellows. The valve is attached to the bellows and closes when the bellows expands. The valve remains closed trapping the steam in the trap body. Condensation of the steam cools the bellows and causes it to contract opening the valve and drains the condensate.

 

Lavatory Traps

 

In parts A and B, the P-type lavatory trap is illustrated, with and without clean out plug. Part C shows the S-type trap specified for shipboard use. This latter type, equipped with clean-out plug. These fittings are made from brass, and are usually chrome platted.

 

 Basket Strainer

Strainers are located in all piping systems to prevent the passage of foreign mater. They must be installed so the flow will be through the strainer element. The bilge strainer shown is an example of a basket strainer. In some locations duplex strainers are used so that the flow of fluid through the system need not be interrupted when one element is removed for cleaning.

 

Valve Maintenance

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

Whenever you are going to install a valve, be sure you know the function the valve is going to perform—that is, whether it must start flow, stop flow, regulate flow, regulate pressure, or prevent back-flow. Inspect the valve 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 type of end connection with which it is fitted. Operating characteristics and the material are factors that affect the length and kind of service that a valve will give; end connections indicate whether or not a particular valve is 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 is acceptable, but avoid the inverted position (stem pointing downward). If the valve is installed with the stem pointing downward, sediment will collect in the bonnet and score the stem. Also, in a line that is subject to freezing 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 which method will be best for the operation, protection, maintenance, and repair of the machinery served by the system), you should use caution. The question of what would happen if the disk became detached from the stem is a major consideration in determining whether pressure should be above the disk or below it. If you are required to install a globe valve, be SURE to check the blueprints for the system to see which way the valve must be installed. Very serious casualties can result if a valve is installed with pressure above the disk when it should 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, or a 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 disk surface. When doing this work, you should observe the following precautions:

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

 

Spotting-In Valves

The method used to visually determine whether the seat and the disk of a valve make good contact with each other is called spotting-in. To spot-in a valve seat, you first apply a thin coating of prussian blue evenly over the 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 the disk makes contact.

The illustration shows the appearance of a correct seat when it is spotted-in; it also shows the appearance of various kinds of imperfect seats.

After you have noted the condition of the seat surface, wipe all the prussian blue off the disk face surface. Apply a thin, even coat of prussian blue to the contact face of the seat, place the disk on the valve seat again, and rotate the disk one-quarter turn. Examine the resulting blue ring on the valve disk. The ring should be unbroken and of uniform width. If the blue ring is broken in any way, the disk is 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 seat and disk is called lapping-in. Lapping-in should not be confused with refacing processes in which lathes, valve reseating 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 disk into the valve and rotate the disk back and forth about one-quarter turn; shift the disk-seat relationship from time to time so the disk will be moved gradually, in increments, through several rotations. During the lapping process, 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 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 disk are first spotted-in after they have been machined, the seat contact will be very narrow and will be located close to the bore. Lapping-in, using finer and finer compounds as the work progresses, causes the seat contact to become broader. The contact area should be a perfect ring covering about one-third of the seating surface.

Be careful to avoid over-lapping a valve seat or disk. Over-lapping will produce a groove in the seating surface of the disk; it will also round off the straight, angular surface of the disk. Machining is the only process by which 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 by tightening up on the packing. You must be careful, however, to avoid excessive thread engagement of the packing gland studs (if used) and to avoid tightening old, hardened packing, which will cause the valve to seize. Subsequent operation of such a valve may score or bend the stem. Packing a badly scored valve stem will 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 be used in repacking a particular valve will depend on the valve size, application, and type. Packing materials will be discussed in more detail later in this chapter.