Valve Notes Pressure Drop

Introduction: Valves Valv es are are inte integra grall comp compon onent ents s in pipi piping ng syst system ems s they they are are the the prim primar ary y meth method od of  controlling the flow, pressure and direction direction of the fluid. Valves may be required to operate contin continuou uously sly e.g. e.g. control control valves valves,, or they they may be operat operated ed intermi intermitte ttentl ntly y e.g. e.g. iso isolat lation ion valves, or they may be installed to operate rarely if ever e.g. safety valves. valves. A valve can can be an extremely simple, low cost item or it may be and extremely complicated, expensive item. In piping design the valves probably require more engineering effort than any other piping component.

Valve Classifications: Isolation/Stop valve: The The isol isolat atio ion n of the the downs downstr trea eam m syst system em from from the the upst upstrea ream m syst system em by use use of and and isolation /stop valve is a critically important function. The prime requirements of this valve are tight shut off when closed and minimum restriction to flow when opens.

Valves used for this function include gate valves, globe valves, ball valves, plug valves, butterfly valves, diaphragm valves and pinch valves

Regulation of flow: Many applications require the flow of the fluid be regulated (throttled) at some fixed or variable level between fully zero and maximum flow limits. This is achieved by introducing resist resistanc ance e to flow, or by bypass bypassing ing flow or by changing changing the direct direction ion of the flow. flow. An important feature for control valves is that the output variable (flow) is related to the input variable (valve (valve position). position). An ideal operating characteristic characteristic of a hand operated flow control valv valve e is that that the the flow flow is dire direct ctly ly prop propor orti tion onal al to the the posi positi tion on of the the hand handwh whee eell Valve Valve types for this functio function n includes globe, globe, needle, needle, ball, and butterfly butterfly.. Globe and needle needle valves are best suited for this duty but ball valves are also easily adapted to give reliable flow control.

Back flow prevention: In some circumstances it is important to prevent reversed fluid fluid flow. The type of valve for this this duty duty is a non-re non-retur turn-v n-valv alve e (NRV) (NRV) or check check valve. valve. The importa important nt criteria criteria when selecting these valves are tight shut off against reverse flow, low resistance to flow for forward forward flow, fast response. The valve can be operated operated to close by gravity, gravity, fluid flow, or spring. Two main valve types are available for this function lift check valves or swing check valves.

Pressure Regulation: In many applications, more generally associated with gases, there is a need to reduce the supply supply press to a set fixed value. It is also necessary necessary to maintain this reduced pressure pressure over a range of fluid fluid flow conditions conditions.. The pressure pressure regulator valve is engineered for this application. The valve is basically basically a globe valve biased biased open by an adjustable spring force with the feedback pressure tending to move the valve to a closed position such that at the set pressure the feedback pressure force just exceeds the spring force. The pressure regulator valve operates operates using the downstream downstream fluid pressure pressure as feedback. This This is mostly mostly taken from within within the valve valve (self (self acting acting). ). For more acc accura urate te control control a feedback connection can be taken from the downstream piping. The pressure regulation at low near zero flows is difficult and it is often necessary to include internal or external relief valve functions to ensure no high pressures in the downstream system.

Pressure Relief Valves- Safety valves: A very very impo importa rtant nt valv valve e for for safe safety ty is the the press pressur ure e relie relieff valv valve. e. This This valv valve e is used used in applications where excessive pressure in the system can cause damage or failure or can introduce a safety risk. Uncontrolled excessive pressures can can result in disastrous accidents accidents e.g. when potentially explosive explosive gases are being controlled. Relief valves are mainly spring loaded loa ded but they they can also be gravit gravity y operat operated ed and other more more specia specializ lized ed design designs s are available. The The burs bursti ting ng /rup /ruptu ture re disc disc must must be incl includ uded ed unde underr the the gene genera rall head headin ing g of safe safety ty valves. valves. This is is simply simply a disc which which ruptures ruptures when when a set pressure pressure is is exceeded. exceeded. The fluid fluid then escapes escapes through the ruptured disc. If the bursting bursting disc operates the system has be closed down and vented and the bursting disc is then replaced. Reli Relief ef valv valves es when when used used for for safe safety ty appl applic icat atio ions ns are are engin engineer eered ed in line line with with safe safety ty regulations and require regular inspections to confirm the settings and the operation. An important part of the relief valve installation is the routing of the the relieved fluid. fluid. This pipe route must be to a safe location and must be engineered such that it is always fully open.

Special and miscellaneous valves: There are a wide variety of special valves developed for specific industries.

Fluid Properties and operating conditions: The properties of the fluid be controlled have a major impact on the design and materials of  construction of the valve. valve. The piping industry, over the years, had developed a wide range of valve valve design designs s and materia materiall to handle handle virtu virtuall ally y all of the fluid fluids s being being handled handled.. The selection of the valve should take into account fluid viscosity, temperature, density and flow rate. rate. The valve valve must must be suitable suitable to withst withstand and resultin resulting g corros corrosion ion and erosion erosion and if  neces ecessa sary ry the the valv valve e may have have to be desi design gn for no inte intern rna al hold old up of flui fluids ds.. Important considerations include for absolute internal and external leak tightness when handling toxic or explosive fluids. fluids. There regulations also include include for the need for a fire safe

valve to maintain its internal and external integrity when the valve surrounded by flames from a fire. Valve Connections: There are a number of methods of connecting valves into the piping systems are as follows: • •

•

•

•

•

Flanges: The valve is provided with suitable rated flanges. Wafer: The valve is provided with suitable sealing faces and is trapped between line flanges. Butt Welded: The valve is provided with butt weld end and welded into the piping system using high integrity joints. Socket Welded: Socket welds allow and welded into the piping system using fillet welds. Screwed Ends: Ends can be provided with female or male screwed ends. The threads can be taper or parallel Compression Fittings: Ends can be provided with compression fittings

Valve Containment: An important requirement in valve design is to minimize the leakage of fluids into the surro surroun undi ding ng envi environ ronme ment nt.. This This is very very impo import rtan antt in the the nucl nuclea earr indu indust stry ry and and when when transferring transferring toxic or flammabl flammable e fluids. fluids. The possibl possible e leakage leakage points points on valves valves are listed listed below • • • • •

The end connections with the piping.The spindle gland seals -allowing axial and rotary motion. For top entry valves the sealed top closure joint For three piece ball valves - the two split joints Valve drain connections and vent connections

The best option for minimizing risk of leakage from the pipe connections is to use butt welded joints which can can be verified by non-destructive-testing non-destructive-testing (NDT). This option obviously eliminates the valve types which have to be remo emoved for maintenance. nce. The options for for eliminating risk risk of gland gland leakage are to use bellows sealed valves. valves. The risk can also also be reduced by incorporat incorporating ing dual seals seals with a test point point between. between. Pinch valves valves and diaphragm valves do not include gland sealing and are therefore not at risk of gland leakage. Flow Factors: It is clear clear that that differ different ent siz sized ed valves valves have differe different nt flow flow rate rate capac capaciti ities es and it is very very important to be able to assess the flow through a valve for a fluid at certain conditions. The flow characteristics for an on-off valve are fixed and can be evaluated directly using the relevant flow factor. The flow through flow control valves, relief valves, pressure reducing valves and check valves depend on the operating condition for the valve and require a more detailed evaluation. The most general method of identifying the flow capacity of a valve is the C v Factor. The C v factor based on American imperial units and is defined as follows: C

v

= The flow of water through a valve at 60 oF in US gallon/minute at a pressure drop of 1 lb/in2

The metric flow factor (K v) is used throughout outside of America and is defined as follows: K

v

= the flow of water through a valve at 20 oC in m3 /hr with a pressure drop of (1 bar)

The conversion between the two factors is K

v

= 0.865 C

v

Also kv is defined as follows: k

v

= the flow of water through a valve at 20 oC in litres/min with a pressure drop of (1 bar)

The conversion between k

v

and C

v

is k

v

= 14.42 C

v

Liquid Flow: To establish the flow (Q) in litres /m at a differential pressure (Dp) in bar for a liquid with a specific gravity relative to water (γw).

Note: This relationship only applies for liquids similar to water at reasonable flows (subsonic). Gas Flow: For gases and supersonic flows more complicated formulae are required.

The formulae below must be used only for rule of thumb estimates. estimates. The assumption assumption that the critical Dp is at P 1 /2 does not hold hold for all valves. valves. For accurate accurate flow flow calculat calculations ions the valve manufacturer’s data sheets must be used.

For gases flowing at sub-sonic velocities the following relationship holds.

T1 = Inlet gas temperature deg K = Deg C + 273 γa =Gas specific gravity relative to air q

n

= gas flow at normal conditions

P1 = Inlet Pressure (absolute) For gases flowing at super-sonic velocities the following relationship holds.

Gas Reference conditions Norm Normal al cond condit itio ions ns:: P=10 P=1013. 13.25m 25mb b and T=273.15K Standard Standard conditions conditions:: P=1013.25mb P=1013.25mb and T=288.75K.

VALVE DESCRIPTIONS: Gate Valves:

Gate valves are generally used in the the process industry industry for on-off on-off service. The design is not suitable for throttling duty because the sealing surfaces can easily suffer from wire drawing (erosion) when low flows are being maintained against high differential pressures and the design give very poor flow control characteristics.

The gate valve can be manufactured in a wide range of sizes from 5mm to above 2000mm dia. The designs designs are proven and well well tested. tested. There is a tendency tendency to to move move to butterfly butterfly valves as a lower cost option.

The valve can be based on a solid wedge, a wedge which can adjust to suit the seal faces, or a parallel faced based on two discs which slide between parallel sealing faces with a mechanism form forcing the discs discs out on the last part of the spindle travel. The valve can be based on a simple rising spindle design or a fixed spindle which screws into the gate.

There are a large number of gate valve variations including slide valves, knife valves, penstock valves, sluice valves, and venturi valves.

Fixed spindle Gate Valve Rising Spindle Gate Valve



Globe Valves: The globe valve includes an orifice set into the body through through which the the fluid flows. flows. A disc located on the end of the spindle is engineered to move in and out along the axis of the orifice. When the disc is moved to sit in the orifice orifice the flow path is shut-off. shut-off. The flow path is progressively increased as the disc is moved away from the orifice.

The surface of the orifice (seat) is generally generally engineering engineering as a replaceabl replaceable e item made from erosion resistant material with a polished surface finish. finish. The disc can be fitted with a soft seat seat if a tight tight shut-o shut-off ff is requir required. ed. For flow control control duties duties the disc disc is supplied supplied with an engineered shape often with a contoured skirt.

For manually operated valves the spindle screwed so that rotation of the handle moves the disc in and out. For actuated control valves the spindle spindle is moved in and out using a linear actuator which can be pneumatic, hydraulic or electric.

The fluid flow path through globe valves is such that there is normally a high fluid head loss through through the valve. The inline inline body design has has the highest highest head loss; the the angle pattern pattern body design has has a lower head loss. loss. There are certain certain designs of globe valves valves which have been engineered to have low head loss characteristics. (Ref Dynamic Controls cartridge valves). valves). Globe valves valves are supplied supplied in sizes from 3mm bore bore through 400mm and and can be used, size size limiting limiting at pressure pressures s up to 450 barg. barg. Depending Depending on the sealing sealing systems systems the o valves can be used at temperatures the 600 C.

Small Size Screwed Globe Valve Needle Valves: The needle needle valve valve is used used specif specifica ically lly for acc accura uratel tely y control controllin ling g the flow flow of fluids fluids at low flows. The valve is is basically a globe valve valve without the disc. It is generally used provided in small sizes of up to 20mm bore.

Needle Valve Plug Valves: The plug valve valve is the oldest oldest of the valves valves.. Plug Plug valves valves have been in use for over 2000 years. This valve has been in continuous development over recent years. The plug valve is basically basically and on-off valve based on a plug with a rectangular rectangular hole through which the fluid fluid flows. The plug is either tapered or cylindrical and is located in the valve valve body and and can be rotated through a quarter turn to line the hole up with the pipe when open or across the pipe when closed.

The plug can be adapted for multi-port use allows the valve to be used for diverting flow. The valve can be engineered with a lubricated plug which uses the lubricant to enable convenient operation over a wide range of pressures. The lubrication lubrication film also provides a seal. The unlubricated design includes seals in the plug and requires plastic bearing systems. The valve can include a cage between the plug and the body which includes the bearing sealing sealing systems systems and allow allow convenient convenient maintenance. maintenance. These valves valves have been specially specially develo developed ped for use in indust industrie ries s requir requiring ing high high perform performanc ance e operat operation ion under under arduous arduous conditions and allowing remote maintenance e.g. the nuclear industry. The valve is a full bore and has virtually no internal cavities.

Lubricated Plug Valve Ball Valves: The ball valve valve is basically basically a plug valve with a spherical plug and a round hole. Over recent years the materials of construction of the ball valve have been developed such that the ball valve valve is becoming becoming the most popular popular valve valve for most most process process applicati applications. ons. There are two primary options for the ball valve design • •

Floating Ball Design- This is low cost option for the lower duties Trunnion Ball Design- This is a more costly option for the higher duties

The ball valve is generally provided as a reduced bore design allowing a smaller body but still with relatively low head loss compared to most other valve options e.g. 25nb valve has a 20mm reduced bore,. The full bore option has a larger body but provides zero restriction to flow. The valve valve can be supplies supplies as a multi-port multi-port design design for flow diverting diverting but only only with the reduced bore option.

The engineering of the ball valve has to include for fitting and removing the ball and seat

system. Ideally this has to be engineered to enable enable the valve to to be maintained inline. One method of achieving this is to use the top-entry version - all of the internals are accessible by removin removing g the top flange. flange. Anothe Anotherr method method is to use a three three piece body body based based on a central central piece sandwiched sandwiched between two pieces connectin connecting g the valve valve to the pipe work. The central piece can be released and pivoted away from the two outer pieces allowing access to all of the valve components. The ball valve can be engineered as a multi-port valve for flow diverting duties. An important advantage of all full bore valves is that the valve allows certain pipe cleaning operations e.g. ridding. Ball valves can also be used used on branches to enable instruments to be fed into pipe systems during operating periods.

Ball valves valves are availa available ble in all material materials s in sizes from from 5mm to over 600mm. 600mm. The valves valves can be used at pressures pressures up to 700 bars. The main components components limiting limiting the performance performance of ball valves are the ball seals and valves are available with metallic seals.

Ball Valve Trunnion Mounted Ball Valve - With Floating Ball



Butterfly Valves: The butterfly valve has head head loss characteristics of a full bore valve. The design is based on use of an engineered disc of the same dia as the bore of the pipe arranged to pivot such that when it is across the bore is closes off off the flow path. When turned through through 90o the disc provides minimum minimum resistance to the the flow. The valve is a quarter turn valve.

The main variations for this valve are the methods of sealing the perimeter of the disc in its closed closed positi position. on. The simple simplest st variatio variation n is to use an elastome elastomerr lined lined bore bore which is an interference fit on the disc. disc. The other variations are are based on offsetting the disc disc plane from the axis of rotation allow the disc to close against a circular face seal such that the fluid pressure increases the seal effect. effect. Metallic seals are available available allowing the valve to to be used for a wide range of fluids at high temperatures. The butterfly valve has been developing such that for many duties it now provides optimum solution for a leak tight tight on-off valve supplanting supplanting the gate valve. The butterfly valve can be engineered as a small valve of 25mm bore and can be made for extremely large sizes above 5000mm 5000mm bore. Depending Depending on the valve size working pressures pressures up to 100 bars can be handled.

Butterfly Valve – Lined

Diaphragm Valves:

The diaphragm valve has a significant advantage over most of the other available designs, apart from from the pinch pinch valve, valve, in that there there is no gland gland seal requirement requirement.. The fluid fluid flows straig straight ht through through the valve via a chambe chamberr over over which which is an elastom elastomer er diaphragm diaphragm.. This This diaphragm is normally arranged arranged to provide no resistance to the the flow. The perimeter of the diaphragm is simply clamped to a seal face of the valve body as a static seal.

To close off the valve the diaphragm is simply forced down into the chamber to block off  the the flow. flow. The The cham chambe berr can can incl includ ude e a weir weir across across the flow flow path again against st which which the the diaphragm can be pressed to affect a more efficient seal with r educed diaphragm distortion. The straight through variation is effectively a full bore valve design with all the associated benefits. benefits. However However this option option results results in a much more more arduous duty duty on the diaphragm diaphragm which has to be a softer material.

This type of valve is manufactured in sizes from 6mm to 400mm and is generally limited to relati relativel vely y low fluid fluid pressures pressures (less (less than 7 barg). barg). However However in the smalle smallerr siz sizes es (up to 50mm) 50mm) valves valves can be special specially ly engineer engineered ed for use at pressures pressures up to 30barg. 30barg. The diap diaphr hrag agm m must must be chos chosen en to be comp compat atib ible le with with the the flui fluid. d. What Whatev ever er the the flui fluid d the the diaphragms must be replaced at regular intervals and it is advisable to operate the valves frequently. These valves are often used for duties which require a high degree of cleanliness as they can be supplied lined, and polished and can be very conveniently cleaned.

Diaphragm Valve - straight Diaphragm Valve - With weir



Pinch Valves: The pinch valve is a theoretically ideal solution for fluid on-off duties. The valve is simple a length of pipe made from an elastomeric material with a mechanical system for squeezing the tube closed closed when a shut shut off is required. The valve is is a true full full bore valve - there are no mechanical mechanical parts in contact with the fluidfluid- The operation operation of the valve is ideally ideally simpleThe valve can be easily engineered as a tight-shut off valve.

The valve is often supplied with the pinch tube contained within a outer pipe between the end flanges. flanges. This option option provides a method of monitoring monitoring for tube leaks and provides a degree of secondary containment.

The valve has similar similar limitations limitations to the diaphragm diaphragm valve. The diaphragm diaphragm valve is really really a variation on the pinch valve principles. Pinch valves are supplied is for diameters 25 mm 1000 mm, temperatures -50 C - +160 C, and pressures 0 - 100 bar.

Pinch Valve

Check Valves:

Check valves are automatic in operation and designed to prevent reversal of flow in fluid piping piping system systems. s. The valves valves are mainta maintaine ined d open open by the flow flow of fluid fluid in the forwar forward d direction and are closed by back pressure of the fluid or by the weight of the closing mechanism or by a spring force. Various designs are are available as listed listed below. • • • • • •

Swing check Tilting disc Ball lift type Disc lift type Piston check Stop check

The range of check valve sizes range from 6mm to massive units of 3000mm dia and more. The swing check variation is a low low pressure drop unit based on a hinged disc. This type of  valve is suitable for low velocity applications applications with infrequent velocity reversals. The valve can be fitted with external weights to allow faster closure to reduce water hammer or shock pressure on flow reversal. External systems can also also be included to force the valve closed in the event of a local fire.

The lift check valve valve and piston check variations variations are used for higher duty applications. applications. The valve is forced open by the fluid flowing up through the valve and is closed on fluid reversal by gravity, back pressure or by spring force.

The tilting disc variation on the swing check valve provides improve speed of operation and pressure performance and is probably the most popular design of check valve used in the process industry.

Swing Check Valve Lift Check Valve