Valves General- Piping

Valves Gate valves • The gate valve is probably the most widely used valve in plant operations, particularly in the larger sizes of piping installations and those of the most severe service. • It consists of a main body and a stem which raises or lowers a “gate” across the fluid flow. In high pressure installations the gates must be of heavy construction and are sometimes difficult to open. Some are motor operated and some have a small by-pass line for equalizing the pressure on both sides of the gate before opening. • The rising stem gate valve is used in both water and process piping. It requires more overall space for installation, but is used to an advantage in handling corrosive streams that tend to damage stem threads in other type valves.

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Valves Gate valves (cont) • The non-rising stem gate valve is used extensively in water piping. Its chief disadvantage is the fact that the degree of opening cannot be readily determined. • A gate valve should be used only in service where it can be kept in a fully opened position. In partially opened positions the gate will vibrate and mar the seating surfaces, preventing complete closure. • Wrenches and levers should not be used to exert extra pressure on valve stems when the gate has become completely seated in closing. This practice will result in galling and marring of the seat and disc, making complete closure impossible.

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Valves Gate valves – used to minimize pressure drop in the open position and to stop flow rather than to regulate it.

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Valves Globe valves • The essential feature of this type valve is a globular body with an internal, horizontal partition having a circular opening in which is inserted a ring or seat. The globe or disc seats on this ring to effect shut-off. • The valve opening is so arranged that flow through it must make two 90-degree changes of direction. This results in a relatively high pressure drop, or resistance to flow. • The globe valve is generally used in small sized piping for throttling or control. They are used principally in steam or air service where throttling and a positive shut-off is desirable. • Installation should be such that flow is up through the seat ring and against the bottom of the disc or glove. This will prevent opening difficulties caused by the accumulation of debris above the disc. This should be avoided. 41

Valves Globe valves (cont) • With this type of valve it is important that the valve is installed with flow passing through it in the correct or required manner. Normally the flow enters the “S” shaped passage underneath the valve plug. When the plug lifts, the liquid flows up past the plug and through the outlet. This method of installation keeps the pressure from the packing gland when the valve is in the closed position. • Occasionally the flow is reversed and the inlet pressure flow enters on top of the seat. This is normally done when it is wished to have the pressure assist or ensure the closing of the valve. With oils of a certain type, this is bad practice as, if the oil has a tendency to congeal, gum will form on the valve stem and prevent the valve plug and stem from rising. 42

Valves Globe valves - offer ease in throttling

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Valves Diaphragm valves

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Valve Diaphragm valves

Diaphragm valves – limited to 50psi, excellent for fluid containing suspended solids 45

Valves Plug valves • The plug valve, or cock, is a conical plug within a body. Its chief advantage is in its tight shut off. A one quarter (90 degree) turn of the plug changes it from a fully closed to a wide open position • Since the plug tends to stick when used in high temperature and high pressure service, some types of these valves have been provided with a means of lubrication through the stem of the cock. • A rotary life plug valve is non lubricated. In operation, as the stem is turned, the plug lifts slightly from its seat, rotates to the opposite position then lowers back into place. This operation can be performed in about one-fourth to one-half a turn in the smaller sizes. Large valves require more rotation of the stem. • The quick opening principle of plug valves makes them impractical as a means of controlling rate of flow.

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Valves

Plug valve – for throttling service, little likelihood of leakage when closed.

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Valves Ball valves • Ball valves are adaptations of plug valves using ball-shaped discs instead of conical plugs located within the body. A hole through the center of this disc provides the straight-through flow which is characteristic of ball valves. Ball valves are used to shut off or permit full flow of fluid through the valve. • Ball valves have all the advantages of plug valves in terms of excellent flow characteristics - quick opening, straight-through flow, minimum turbulence, tight closure and compactness - plus the additional advantage of not requiring any lubrication or sealant. They achieve tight closure by establishing a controlled squeeze of the ball against the sear rings which are made of plastic material such as teflon.

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Valve Ball valves

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Valves Butterfly valves • The butterfly valve derives its name from the wing-like action of the disc which operates at right angles to the flow. Ordinarily, they are not intended for tight shut off, but resilient seats have broadened their possibilities and they are now frequently advertised as having “bubble tight” closure.

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Valves Butterfly valve

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Valves Needle valves • Needle valves are designed to give very fine control of flow in small diameter piping systems. They get their name from their sharppointed conical disc and matching seat. The stem threads are fine so that more turns of the hand wheel are required to increase or decrease the opening through the seat.

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Valves Needle Valves – offer flow adjustment on small lines

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Valves Check valves • Check valves are automatic in operation. They prevent flow in one direction, but allow it in the other. These valves should be carefully checked for direction of flow in lines. • Swing check valves are unsuitable for use in vertical lines. Note importance of correct installation of a check valve in relation to the stream flow. Usually the valve body has features that indicate correct direction. • Sometimes an arrow indicates the direction of flow permitted through the valve. Do not rely upon a check valve to give full shut-off against a back flow. Only on special occasions, [authorized] will the check valve be depended upon to prevent back flow of liquid or other hazardous material when opening lines into other systems or to the atmosphere. Always use a block valve when possible. 54

Valves Safety valves • Safety valves are installed in strategic locations to provide pressure relief for process equipment subjected to excessive pressure. • A safety valve consists of six basic parts; spring, spindle or stem, adjusting screw, disc, nozzle or seat, and body. Safety, or relief, valves are intended for emergency protection only and should not be used as an operating valve. • Frequent “popping” of a relief valve contributes to their failure, therefore, they should set at 25% above working pressure. For example, relief valves on the discharge of reciprocating pumps will relieve with each pump stroke if the set pressure is too low. Continuous pounding of the disc upon the seat causes damage and resultant leakage of the valve.

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Valves Safety valves (cont) • Other causes of valve leakage are spring failure and debris on seating surfaces. Extreme caution should be exercised in tampering with relief valves while a unit is in operation. The proper place for reseating a leaking valve is in the shop. Pressure should be reset on the test rack. • When pressure builds up in a vessel or line, pressure is also exerted on the plug. The plug is lifted from its seat and the excess pressure in immediately released. At the same time, the spring tends to push back the plug on to its seat. When the spring tension is once again greater than the vessel pressure then the plug will again reseat.

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Pressure Relieving Devices Conventional Relief Valve

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Functions Of Valves •

Piping system, in order to effectively perform the function of transporting fluids from the point to another, need devices to control or regulate the flow of fluids. Although devices are available – which by suitable instrumentation automatically regulate or control the flow of fluids in response to flow, level, pressure, temperature, or other control signals – only manually operated devices will be discussed in this section. Check valves, relief valves, steam traps, drain valves, etc although not manually operated are also discussed in this section. • Manually operated valves are devices which are opened or closed by turning a hand wheel or moving, rotating or otherwise manipulating a hand lever or wrench. Gate, globe, plug, ball, diaphragm and butterfly valves are in this category.

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Functions Of Valves (cont) • When valves are large size, difficult to operate due to flow conditions, located in inaccessible positions, require rapid opening and closing, or need to be operated remotely from a central control area, mechanical or power actuated operators are frequently installed on these valves. Gear operators, chain wheels, or electric motor operators are some of the mechanical or power actuated operators most often used. • Each type of valve is designated for a specific purpose to meet and/or satisfy the following basic control functions desired in a piping system.

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Basic Control Functions Of Valves

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Starting and stopping Flow To start or stop the fluid flow is the function for which valves are most generally used. Gate, plug, ball, butterfly and diaphragm valves effectively perform this function with very little pressure drop through the valve. • Regulating and throttling Flow Regulating or throttling flow is most efficiently performed with globe, angle, or needle valves. These valves are seldom used in size above 8 inches because of the difficulty opening and closing the larger valves against pressure. Butterfly and diaphragm valves are also effective as regulating or throttling valves at limited fluid flow characteristics. 60

Basic Control Function Of Valves (cont) • Preventing backflow Check valves perform the function of checking or preventing reversal of flow in piping systems. Flow keeps these valves open if the flow is in the right direction, while gravity and reversal of flow closes them automatically. Check valves are available in two basic types - swing and lift checks. • Relieving Pressure Relief and safety valves are installed on equipment such as boilers, vessels, drums, piping systems, etc. which can be seriously damaged if subject to pressures in excess of the equipment design. They are usually spring loaded and automatically open to release pressure which exceeds the limit for which the valve was set.

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Application Of Valve Types

Gate valves • The Principal characteristic of a gate valve is the fact that the flow through the valve is stopped by sliding a relatively flat surface, called a gate or disc, across the flow path of the fluid and started by moving the disc into a chamber in the valve bonnet out of the flow path. Gate valves are most effective fully closed or fully opened. When fully opened the fluid flows through the gate valve in a straight line with very little pressure drop.

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Application Of Valve Types Globe valves • Globe valves are named after the globular shape of their body. They are most suitable for throttling fluid flows because of their type of seating arrangements, however, they can also be used in fully shut off and fully open service • Globe valves seating is parallel to the line of flow with all contact between seat and disc ending when flow begins. • The fluid flow through a globe valve follows a changing course which causes resistance to flow and considerable pressure drop. However, in order to control the fluid flow we must sacrifice some pressure drop.

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Application Of Valve Types Angle valves • Angle valves are similar to the design of globe valve but have less resistance which results in less pressure drop. • The angle valve effectively utilizes globe valve seating principle while providing for a 90 degree turn in piping. It is less resisting to flow than the globe valve it displaces. Requires fewer joints; saves make-up time and labor.

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Application Of Valve Types Needle valves • Needle valves are designed to give very fine control of flow in small diameter piping systems. • Generally used in chemical injections to process and delivery lines

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Application Of Valve Types Plug valves • The plug valve is often referred to as the oldest member of the valve family, however, this is only true of the non-lubricated type which may be considered a modification of the simple cock. • The non-lubricated plug valve is an evolution of the simple plug cock. All the advantages of the simple plug cock are maintained but the tendency of the plug to seize or bind has been overcome by changes in the design and construction of the plug valve. This has been accomplished by the use of various plastics or other materials with low friction values for contact or sealing surfaces. As a result of these changes non-lubricated plug valves are now available for a much greater range of service conditions. • Teflon (tetrafluoroethylene) is one of the principle materials with low friction value being used today for contact surfaces. 66

Application Of Valve Types Plug valves (cont’) Teflon, however, has its own problems in terms of cold flow and temperature limitations. • The lubricated plug valve with lubricant or sealant injected into the valve under pressure to maintain a leak-tight seal and to permit ease of movement of the plug inside the valve utilizes the lubricant as a hydraulic jack to raise the plug slightly to reduce the friction during operation of the plug valve.

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Application Of Valve Types Ball valves • Ball valves are adaptations of plug valves using ball-shaped discs instead of conical plugs located within the body. A hole through the center of this disc provides the straight-through flow which is characteristic of ball valves. Ball valves are used to shut off or permit full flow of fluid through the valve. • Ball valves have all the advantages of plug valves in terms of excellent flow characteristics - quick opening, straight-through flow, minimum turbulence, tight closure and compactness - plus the additional advantage of not requiring any lubrication or sealant. They achieve tight closure by establishing a controlled squeeze of the ball against the sear rings which are made of plastic material such as teflon.

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Application Of Valve Types Ball valves (cont’) • Because of the sealing of ball valves is accomplished with a ball shaped disc seating against plastic materials, the temperatures for which these valves can be used are limited.

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Application Of Valve Types Butterfly valves • They are widely used in the municipal field for the control of water flow. • Butterfly valves are excellent for handling large flow of gases and liquid at relatively low pressure. • The butterfly valve in full open position has relatively low pressure drop and minimum turbulence since the disc position is parallel to the fluid flow. Some of the advantages of butterfly valves are little head room required, light in weight, durability, low initial cost and ease of operation. However, they are limited in pressure and temperature applications.

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Application Of Valve Types Check valves • The principal function of a check valve is to automatically prevent reversal of the direction of the fluid flow. Pressure of the fluid flowing through the check valve keeps it open – reversal of the flow closes it. Most manufacturers plainly mark check valves for direction of flow. The design of check valves are relatively simple and are available in three basically different types - swing check, lift check, and ball check.

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Application Of Valve Types Check Valves • Used to prevent reversal of flow 1. Swing check valve - Normal design is for use only in horizontal lines 2. Lift check valves - Vertical lift check valve – for installation in vertical line, where the flow is normally upward - Globe check valve – for use in horizontal lines - Angle check valve – used for installation where a vertical line with upward flow turn horizontal 3. Tilting disk check valve - May be installed both horizontally and vertically. Less pressure drop at low velocity but greater at high velocity. Arrests slamming. 72

Application Of Valve Types Swing check valve • Swing check valves have a disc, hinged at the top, which seats against a machined seat in the tilted bridge wall opening. The disc swings freely in an arc from the fully closed position to one providing unobstructed flow. The fluid flows through the body in a straight line. The straight line flow results in low pressure drop because of the minimum resistance to flow.

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Application Of Valve Types Check Valves

Swing check valve

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