Valves & piping

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Module 11 PIPINGS and VALVES

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Course Contents

1. Pipe2. Piping standard - Nominal pipe diameter - Schedule number - Piping standards code - Tubes - Fittings and other piping auxiliaries3. Method of joining sections - Threaded - Bell and spigot - Flanged - Welded connection - Fitting4. Types of flanged joints5. Expansion joints6. Blinds7. Spaces

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Course Contents

8. Valves - Types of valves9. Valves - Gate valve - Globe valve - Diaphragm valve - Plug valve - Ball valve - Butterfly valve - Needle valve - Check valve - Safety valve10. Function of valve11. Basic control function of valves

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Course Contents

10. Application of Valve types11. Valve ratings12. Valve operation and maintenance13. Do’s and don’t for better valve service14. Pressure relieving devices - How high pressure develops - Types of pressure relieving devices15. Definition of pressure relief terms

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Pipes

• Piping systems are the arteries and veins of a petrochemical plant just as they are in the human body. In a petrochemical, piping

systems handle all liquids, solids, gas or vapor.• Material is frequently stored and handled in the fluid state. Most fluid cannot be handled in open channels but required closed ducts. • In ancient these ducts were hollowed logs and later they were made

of section of wood or of pottery.• Development of iron brought about the manufacture of cast iron and

wrought iron pipes.• Any structural material now available is used for pipe in application

where its peculiar advantages are most valuable.

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Piping Standards

Nominal pipe diameterPipe sizes are based on the approximate diameter and are reported asnominal pipe sizes. Although the wall thickness varies depending onthe schedule number, the outside diameter of any pipe having a givennominal size is constant and independent of the schedule number. Thispermits the use of standard fittings and treading tools on pipes ofdifferent schedule numbers

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Piping Standards

Schedule number• Pipes were originally classified on the basis of wall thickness

as standard (extra strong, and double extra strong).• Because of modern industrial demands for more exact spec,

pipes are now specified according to wall thickness by a standard formula for schedule number designated by the American Standards Association.

• Schedule number is defined by ASS as: = 1000 Ps/Ss

where Ps = safe working pressure Ss = safe working fiber stress

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Piping StandardsSchedule number• Ten schedule numbers are in use at present.• These are 10, 20, 40, 60, 80, 100, 120, 140, 160• For pipe diameters up to 10 in, schedule 40 corresponds to the

former “standard” pipe and schedule 80 corresponds to the former “extra strong” pipe.

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Piping Standards

Schedule numberHow they came up with the formula?Bursting pressure of a thin walled cylinder may be estimated fromthe following equation: Pb = 2STtm/Dm where: Pb = bursting pressure

ST = tensile strength

tm = minimum wall thickeness

Dm = mean diameter

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Piping Standards

Schedule numberA safe working pressure Ps can be evaluated from equation if the

tensile strength is replaced by a safe working fiber stress Ss

Ps = 2Sstm/Dm

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Piping Standards Piping standard codes

• The American National Standards Institute (ANSI) and the American Petroleum Institute (API) have established dimensional standards for most widely used piping components

• ANSI B31 - List of those standards can be found in the ANSI B31 code section - Section also lists specifications for pipe and fitting materials and testing methods of the American Society for Testing and Materials (ASTM) - American Welding Society (AWS) specification - Standard of the manufacturers Standardization Society of the Valve and Fitting Industry (MSS)

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Piping Standards

Piping standard code (con’t)

The design of piping system applied to this project is listed asASME B31.3• ASME stands for American Society of Mechanical Engineer• ASME 31.3 is actually a section of ANSI B31 • ASME (ANSI) 31.3 is a Standard Number and designation is

Chemical Plant and Petroleum Refinery Piping• ASME (ANSI) 31.1 scope and application: - For all piping within the property limits of the facilities engaged in the processing or handling of chemical, petroleum or

related product unless specifically excluded by the code• Information on latest issue can be obtain for ASME. 345 East

47th st. New York NY 10017

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Piping Standard

TubingCopper tubing, brass tubing are used extensively in industrialoperations. Other metals, such as nicklel and stainless steel, are alsoavailable in the form of tubing.Although pipe specifications are based on standard nominal sizes,tubing specs are based on the actual outside diameter with adesignated wall thickness.Conventional system, such as the Birmingham wire gauge (BWG) areused to indicate the wall thickness.

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Piping Standard

Fittings and other piping auxiliaries• Fittings, flanges, valves, flow meters, steam traps and many other auxiliaries are often rated on the basis of the safe operating

pressure as 25 psi – low pressure 125 psi - standard 250 psi – extra heavy 300 to 10,000 - hydraulic

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Method of Joining Sections

Method of joining sections The methods of joining sections are generally similar for all materials. The principal methods involve are 1. Threaded 2. Bell and spigot 3. Flanged 4. Welded connections 5. Fitting

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Threaded Joints

• Threaded pipe is most commonly encountered iin industry because practically all small sizes of pipe are joined by thismethod, whether all small sizes of pipe are joined by thismethod, whether fabricated of steel, wrought iron, cast iron, brass, or plastic.fabricated of steel, wrought iron, cast iron, brass, or plastic.

• This system is simple because the outside diameters of the pipe are This system is simple because the outside diameters of the pipe are kept constant with a tolerance of 1/64 in. oversize and 1/32 in. kept constant with a tolerance of 1/64 in. oversize and 1/32 in. undersize, and the inside diameters of fittings are kept within the undersize, and the inside diameters of fittings are kept within the same limits, regardless of materials.same limits, regardless of materials.

• The tolerance for the wall thickness of the different materials varies The tolerance for the wall thickness of the different materials varies but is usually 12.5 per cent.but is usually 12.5 per cent.

• Pipe larger than 12in. Is rarely threaded, and the outside diameter Pipe larger than 12in. Is rarely threaded, and the outside diameter corresponds to the nominal pipe size.corresponds to the nominal pipe size.

• Standard lengths of pipe are from 16 to 22 feet. Standard lengths of pipe are from 16 to 22 feet.

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Bell and Spigot Joints

Bell and spigot connected pipe handle more fluid than any other connection since it is the most practical type of joint in large pipe sizes when pipes are made of materials other than steel. These joints are usually calked with oakum and lead but the mechanical joint is becoming more popular because of the tighter joint, simplicity of installation, greater latitude of angular displacement and expansion. These joints may be “locked” with a groove in the spigot which prevents pulling apart the joint, “roll on” with a rubber gasket tightened with a bolted ring or “screwed gland”, with a ring gland drawn up against the gasket when screwed into threads in the bell.

Materials for pipe joined in this manner are usually cast iron, clay, or concrete, although glass, plastic and cement asbestos are sometimes employed.

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Welded Connections

• The modern trend for pipe in sizes above 2 in. is toward more welded connections. No threading, calking, or bolts is needed, and no gaskets are required when the system is fused into an unknown line of material.

• Pipe ends needs no treatment other than scarfing (beveling) and very few fittings are required if the welder shapes the necessary pieces from pipe sections.

• Fittings and valves are of steel and are of two types, butt weld and socket weld. The butt weld fittings are of the same dimension as the pipe, and the socket weld fittings have enlarged ends similar to threaded fittings, but the pipe slips into place and is fillet welded.

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Fittings

Threaded joint fittings• Couplings – join successive straight length of pipes with no change

in direction or size.• Reducing couplings – used when size is to be reduced or enlarged.• Elbows – used when direction is to changed (90 and 45 deg elbows

are available) • Reducing elbows – used when both size and direction are changed.• Nipples – because of the mechanical difficulty of cutting sharp

thread, nipples are made in factory in a series of standard lengths from about 4 pipe diameter in length to close nipples, whose threads merge from each end of the section.

• Tees and crosses – used if more than 2 branches of piping are to be connected at the same point.

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Fittings

Threaded joint fittings (con’t)• Unions – since most piping must be broken at interval for

maintenance and since standard pipe threads are right hand, thus making it impossible to use right hand fittings exclusively in connecting pipe from one fixed point to another fixed point, a union serves as a connector. The two halves of the union may be tightened to the pipe section independently, and the final connection made by tightening the bonnet of the union.

• Reducing bushing – used if size reduction is desired at a tapped connection.

• Street elbow – used if a simultaneous change in direction and connection to a tapped outlet is required. (male thread at one end female thread on the other).

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Fittings

Threaded joint fittings (con’t)• Cap – used to close end of a pipe• Plug/capped nipple – used to close an opening in a piece of

equipment.

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Fittings

Welded connections Fittings are of steel and are of 2 types, butt weld and socket weld.

The butt weld fittings are of the same dimension as the pipe, and the socket weld fittings have enlarged ends similar to threaded to threaded fittings, but the pipe slips into place and is fillet welded.welded.

Fittings are similar to equivalent threaded type. Fittings are similar to equivalent threaded type.

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Fittings

Bell and spigot connections Fittings Fittings [of the same materials as the pipe] are similar in type and

function to those for threaded pipe.

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Fittings

Compression fittings• Compression fittings are widely used for small sized tubing at both

low and high pressure. These are convenient and efficient, particularly if the connection is to be broken.

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Flanged Joints

Flanged joints are the most common method for joining pipe in sizes 2 inches and above. The use of flanges allows for making the piping up in sections that are easy to handle and also allows access for cleaning, draining, etc.

1. Socket welding flanges 2. Slip on welding flanges 3. Threaded flanges 4. Welding neck flanges 5. Lap joint flanges

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Flanged Joints

Socket welding flanges• Socket welding flanges are widely used for moderate service,

particularly in the smaller sizes, because of the ease of fit p and alignment. Although usually welded at the flange hub only, the pipe end may also be welded without having to reface the flange. The pipe end weld can be ground to provide a smooth bore.

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Flanged Joints

Slip-on Welding Flanges• Slip-on flanges are popular for normal service conditions because of

the ease of fit up and alignment and the greater tolerance permissible in cutting the pipe to length. Recommended fabrication practice is to wed at both the flange hub and the pipe end.

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Flanged Joints

Threaded Flanges • Threaded flanges are widely used because no welding equipment is

required for assembly and both the pipe and flange can be completely salvaged upon dismantling. Accurately cut, clean, tapered pipe threads dimensioned to ANSI A2. 1 Pipe Threads, assure strong, tight joints.

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Flanged Joints

Welding Neck Flanges• Welding neck flanges are designed to be butt welded to the pipe.

The long tampered hub reinforces the flange, permits stress-relieving, magnafluxing or x-raying the weld, when required, and removes the flange face from the heat affected zone. These advantages make welding neck flanges particularly suitable for severe service involving high pressure, extreme temperatures, or hazardous fluids.

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Flanged Joints

Lap Joint Flanges• Lap joint flanges, while requiring a separate end connector, provide

a joint in which the product does not come in contact with the flange. In addition, the ability of the flange to rotate simplifies assemble and alignment of bolting on systems requiring frequent dismantling.

• Screwed flanges must be seal welded after the screwed joint is made up tight. Threading of pipe above 2” is difficult and this type is not used too often.

• Slip-on flanges are widely used because of their low cost, ease of fit-up and alignment, however, they are not too good for service where the temperature and pressure fluctuate. In some services corrosion of the internal weld may be a problem.

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Flanged Joints

Lap Joint Flanges (con’t)• Welding neck flanges are probably the best all around type of

flange. They are the strongest of all those shown. There are no flow problems associated with the assembly and the butt joint is the best method to insure a sound weld.

• Lap joint flanges are good for a combination of alloy pipe and carbon steel flanges. This combination reduces the cost and usually reduces the delivery time compared to waiting for solid alloy flanges.

• The gaskets used with flange joints must always be softer than the flange material because one or the other must be deformed in order to make a tight joint.

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Flanged Joints

Lap Joint Flanges (con’t)• The elastomers include rubber, nylon, plastic, etc. These

materials have a disadvantage in that they can cold flow when squeezed - - even at room temperature. Thus, it is difficult to maintain a tight joint. These materials also have a tendency to flow out ant must be reinforced with cloth or wire mesh if any significant pressure is involved. Some elastomers are good for no more than 100oF. At this temperature that will soften and some will actually begin to melt. Some elastomers are soluble in oil.

• The spiral wound metal gasket with asbestos filler requires special flange finishing. Tightening of joints using these gaskets normally requires special attention and will take more time to properly tighten.

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Expansion Joints

Expansion Joints• All metals expand with increasing temperature. Steel pipe is no

exception to this rule. Expansion must be considered to accommodate the change in length in the change from atmospheric temperature to operating temperature. Expansion joints are used to absorb the effect of the increased length with temperature. These can be bends or corrugated expansion joints. The attached piping must be properly anchored and guided to direct the expansion to utilize the expansion to utilize the expansion joint most effectively.

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Blinds

Blinds are a piping item used extensively in the plant. A blind is used to provide positive isolation in between piping flanges and thus insure that any leakage that takes place will be to the atmosphere and not through the pipe.• Normally, blinds are used to prepare vessels or tanks for entry

and to isolate equipment and piping in order to provide complete safety in welding.

• Blinds are also used for hydrostatic testing but in this case they must be of special design in order to handle the test pressures that will be involved.

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Spacers

Spacers are required when there is a blind of appreciable thickness to be used. It is ordinarily impossible to spring the flanges far enough apart to install a blind of thickness beyond ½ in.• Under the above conditions a spacer is required to take up the

space occupied by the blind when the equipment is returned to operating condition.

• Sometimes the blind and the spacer are attached to each other and in this case it is known as figure 8 or spectacle blind.

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Valves

• Valves are used for the control of volume and pressure of fluids moving through piping or in enclosed vessels. They may be operated automatically or by hand. Type of construction is as varied as the operations within the plant; each is designed for the service to be performed.

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Valves

Types of valves1. Gate valves2. Globe valves3. Diaphragm valves4. Plug valves5. Ball valves6. Butterfly valves7. Needle valves8. Check valves 9. Safety valves

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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.

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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.

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Valves

Globe valves - offer ease in throttling

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Valves

Diaphragm valves

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Valve

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

Diaphragm valves

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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 sharp-pointed 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.

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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

• Starting and stopping FlowTo 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 FlowRegulating 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.

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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 PressureRelief 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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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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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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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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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.

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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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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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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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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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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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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.

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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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Tilting disk check valve

Check valves

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Lift Check Valve• The lift check valve has a disc equipped with a short guide, usually

above and below, which moves vertically in integral guides in the cap and bridge wall. Fluid flow moving through the body in a frequent changing course keeps the valve open.

• When flow stops gravity or reverse of flow direction automatically closes the valve by forcing the disc against the seat. Lift check valves provide a tighter seal against back flow than can be achieve with the swing check valve.

• Lift check valves can only be used in horizontal pipe lines and are generally used in conjunction with globe and angle valves.

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Lift check valve (vertical)

Check valves

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Lift check valve (angle)

Check valves

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Ball Check Valve• Ball check valves are similar to lift check valves except that a ball is

used in place of the lift disc for closure purposes. The ball is pushed away from the seat during fluid flow and closes rapidly when flow stops or is reversed. This valve has extensive use in piping systems moving hot oils and can only be used in vertical lines.

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Stop-Check or Non-Return Valve• A special and more complicated check valve called the stop-check

or non-return valve is essential to the safe operation of a boiler. Their design must conform to the ASME Boiler Construction Code for Non-Return Stop valves. The valves are intended to perform four important functions in boiler steam piping:1. Act as an automatic non-return valve preventing backflow of steam from the connected main steam header into the boiler in the event of failure of that boiler.

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• Stop-Check or Non-Return Valve (cont’)2. Assist in cutting out a boiler, when ceasing to fire that boiler. In this case, the disc automatically closes and prevents steam header pressure from entering the boiler. The valves are equipped with a hand wheel which permits closing the valves under pressure or, if already closed automatically, permits holding the disc in the closed position.

3. Assist in bringing a boiler into service after shutdown. This operation requires considerable care when performed manually but is accomplished automatically by a stop-check valve without pressure fluctuations or disturbance of the water level.

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Stop-Check or Non-Return Valve (cont’)4. Act as a “safety first” valve by preventing back flow of steam from the header into a boiler, shut down for inspection or repairs, should an attendant accidentally open the valve.

Since the valves are equipped with hand wheels it is important to note that there is no mechanical connection between the disc and steam. When the stem is raised by the hand wheel, only the boiler pressure can lift the disc.

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• Water drain valvesCompressed air system, constantly accumulate water and oil. Such liquids in air lines are a hazard and a hindrance to effective use of the air system for process uses, tools, machinery, air cylinders, etc.

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Safety valves• Applications of safety valves will be discussed separately later in

this module under pressure relieving devices.

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ActuatorsAn actuator, [a transducer according to definition for instrumentationfinal control element]) translates the control signal from one form orlevel of energy or power to another, e.g. from a pneumatic signal into amechanical action which is used to manipulate a process variable.

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Valve Ratings

Most valve manufacturers comply with MSS Standard Marking System SP-25 in the identification of their valves. The following symbols are generally used:

S - SteamO - OilG - GasW - WaterL - Liquid

General purpose valves may show two service ratings. One is a steam rating, based on a specific pressure/temperature condition. The second rating is for cold service.

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Valves Ratings

• Steam RatingsSteam ratings are used as a basis for determining the suitability of a material for a given application. For lower temperatures the safe working pressure of a material is usually greater that the steam rating.

• Cold RatingsMost valves have two service ratings. In addition to the steam rating, explained above, cold service ratings are usually designated by the mark WOG, which stands for cold water, oil, or gas, non-shock.

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Valve Ratings

• Steel Ratings are DifferentTemperature and pressure are not always the only factors to be considered, however. Frequently, steel materials are used for their structural ability to meet unusually severe conditions beyond the range for which brass or iron are recommended, such as shock, vibration, line stresses, fire hazard etc.

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Valve Ratings

• Steel ratings are different (con’t)Cast and forged steel valves and fittings bear a mark such as 150, 300, 600 etc. These figures denote the maximum pressure at a certain maximum temperature for which an item is suited. A certain 600-pound steel valve, for example, may be suitable for 600-pound pressure at temperatures up to 850oF. But if the temperature exceeds that point, up to 1000oF., let’s say, the valve is not recommended for pressures over 170 pounds. This important effect of temperature makes it imperative to know both pressure and temperature conditions of a service, and to consult the manufacturer’s service recommendation tables.

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Valve Operation And Maintenance

Variations in Stem Operation of Gate, Globe and Angle Valves1. Rising stem with outside screw and yoke. In this construction the stem screw remains outside the valve body whether valve is opened or closed. Stem threads are not subjected to the effects of fluids in the line - such as caused by corrosion, erosion, sediment, etc. This construction also permits convenient lubrication of stem threads.

The rising stem shows at a glance the position of the disc. Adequate headroom must be provided for the rising stem when the valve is opened, and the stem should be protected against damage when raised.

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Valve Operation And Maintenance

(con’t)2. Rising stem with inside screw. This is the simplest and most common stem construction for gate, globe, and angle valves in the smaller sizes. The position of the stem indicates the position of the disc. The stem should be protected against damage when the valve is open.

3. Non-rising stem with inside screw. Stem does not rise when disc is raised, but merely turns with handwheel. Ideal where headroom is limited. Since stem merely turns when operated, wear on packing is minimized.

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Do’s And Don’t For Better Valve Service

1. Don’t expose valves to damaging blows. Valves can’t be abused and still operate efficiently. A bent stem not only cripples valves, but may cause a shutdown that results in costly delay and repair.

2. Don’t overlook leaks – big and small. A leak in a valve often can be remedied simply and in a hurry, if caught in time. Stem leaks normally can be fixed by slightly tightening the packing nut or gland.

Stuffing box leaks usually can be stopped by merely “pulling” up the packing nut. On bolted glands, care must be taken to tighten bolts evenly… as severely coking the gland will bind the stem. If the stuffing box must be repacked, this repair should be scheduled for qualified maintenance mechanics.

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(con’t)2. Bonnet and flange leaks can be caused by bolts loosening under service strain, If tightening the joints doesn’t stop the leak, the gasket may be damaged and this repair should also be scheduled for qualified maintenance mechanics.

3. Don’t spare the oil can. Wear on stem packing is due mainly to the rising and turning motion of the valve stem, combined with deteriorating effects of service conditions. A few drops of oil on the stem, now and then, help to reduce friction - - and wear, Don’t forget to lubricate exposed stem threads.

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(con’t)

4. Don’t operate gate valves continuously in a “cracked open” position because the valve seats will be severely damaged. Damage to valve seats will cause valve to leak when closed.

5. Do not use large pipe wrenches to close valves.

6. Rising stem valves should be backed off slightly to relieve tension on the stem.

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Pressure-Relieving Devices

How high pressure develops

The possibilities for development of excess pressure exist in nearlyevery process plant.Excess pressure can develop from: 1. explosion 2. chemical reaction 3. reciprocating pumps or compressors 4. process upsets 5. external fire around equipment

• In addition to the possible injury to personnel, the loss of equipment can be serious and an economic setback

• Most countries have laws specifying the minimum attention required in the applications of pressure-relieving equipment in process and steam power plants.

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Types of pressure relieving devices1. Relief valves2. Safety valves3. Safety-relief valve4. Conventional and balance valves5. Frangible disk (rupture disk)

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Relief Valve• A relief valve is an automatic pressure-relieving device actuated by

the static pressure upstream of the valve, and which opens further with increase in pressure over the set pressure

• Opening of valve is proportional to the increase in pressure over the opening pressure

• Used primarily for liquid services• Rated capacity is usually attained at 25 percent over pressure

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Storage Tank and High Pressure Relief Valve

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Bellows Relief Valve

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Emergency Relief Vent

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Safety Valves • An automatic pressure relieving device actuated by the static

pressure upstream of the valve, and characterized by rapid full opening or pop action upon opening.

• It is used for steam, gas or vapor service.• Rated capacity is reached at 3, 10 or 20 percent overpressure,

depending upon applicable code.

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Safety-relief valve• Safety relief valve is an automatic pressure relieving device

actuated by the static pressure upstream of the valve and characterized by an adjustment to allow either a “pop” or a “non pop” action and a nozzle type entrance

• Suitable for use as either a safety or relief valve, depending on application.

• Safety, relief and safety relief valves are installed in strategic locations to provide pressure relief for process equipment subjected to excessive pressures. These valves consist of six basic parts: spring, stem, adjusting screw, disc, nozzle or seat, and body.

• Rated capacity is reached at 3 or 10 percent overpressure, depending upon code and/or process conditions. Pressure relieving valves are intended for emergency protection only and should not be used as an operation valve. Frequent “popping” of a relief or safety valve contributes to their failure and are therefore set at 25% - 50% above the working pressure.

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Safety-relief valve (con’t)• 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.

• Other causes of valve leakage are spring failure and debris on seating surfaces.

• Used on steam, gas, vapor and liquid (with adjustments) and is probably the most general type of valve in petrochemical and chemical plants

Do not tamper with relief valves while a unit is in operation. The proper place for reseating a leaking valve is in the shop with pressure reset on the test rack.

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Safety relieve valve

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Conventional and balance safety valve• Conventional safety valves operate satisfactorily only when there is

relatively constant back pressure. Changes in back pressure can seriously affect its operational pressure and flow capacity

• Balance valves operate satisfactorily under varying back pressure as this has little influence on performance

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Frangible disk (rupture disk)• A rupture disk is a thin diaphragm (metal, plastic, non metallic) held

between flanges and designed to burst at a predetermined pressure.

• Each bursting requires the installation of a new disk• Used in corrosive service, and for required bursting pressure not

easily accommodated by the conventional valve.• Applicable to steam, gas vapor and liquid system

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Definition of Pressure Relief Terms

1. Set pressure• Is the inlet pressure at which the safety or relief valve is adjusted

to open• This pressure is set regardless of any back pressure on the

discharge of the valve

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2. Overpressure• Pressure increase over the set pressure of the primary relieving

device is over pressure• It is the same as accumulation when the relieving device is set at

the maximum allowable working pressure of the vessel

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3. Accumulation• Pressure increase over the maximum allowable working pressure

of the vessel during discharge through the safety or relief valve, expressed as a percent of that pressure.

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4. Maximum allowable working pressure• The maximum allowable working pressure of an unfired pressure

vessel is that pressure determined by code requirements, the metal material of construction and its operating temperature, above which the vessel may not be operated

• For a given metal temperature, this pressure is the highest pressure at which the safety device may be set to open.

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5. Operating pressure• Pressure (gage), to which the vessel is subjected in service• A processing vessel is usually designed for a maximum allowable

working pressure, which will provide a suitable margin above the operating pressure to prevent undesirable operation of the relief device

• Margin is approximately 10 % higher, or 25 psi – which ever is higher.

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6. Blowdown• The reduction in flowing pressure below the set point required for

a device to close • Is the difference between the set pressure and the reseating

pressure of a safety or relief valve• Expressed as percent of the set pressure or psi

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7. Back pressure• Pressure developed on the discharge side of safety valves is

back pressure• This pressure may be generated by the flowing fluid as it passes

through the relief discharge piping, or it may be an established pressure as a part of a discharge vent system into which the valve is discharging

• It may be combination of these two.

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Vacuum Breaker

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Electric Solenoid

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Thermal Valve

Valves & piping

Engineering

Transcript of Valves & piping