Valve Selection and Sizing

Transcription

1 Valve Selection and Sizing SECTION OF ENGINEERING MANUAL OF AUTOMATIC CONTROL Contents Introduction... 2 Definitions... 2 Valve Components... 2 Valve Flow Caracteristics... 2 Valve Flow Terms... 3 Valve Ratings... 3 Valve Types... 4 Valve Material And Media... 6 Valve Selection... 6 Globe Valve... 7 Ball Valve... 7 Butterfly Valve... 7 Two-Way Valve... 8 Quick-Opening Valve... 8 Linear Valve... 9 Equal Percentage Valve... 9 Tree-Way Valves Mixing Valve Diverting Valve Valve Sizing Water Valves Quantity Of Water Water Valve Pressure Drop Water Valve Sizing Examples Steam Valves Quantity Of Steam Steam Valve Pressure Drop Proportional Applications Two-Position Applications Steam Valve Sizing Examples U.S. Registered Trademark Copyrigt 1998 Honeywell Inc. All Rigts Reserved

2 INTRODUCTION Tis section provides information on valve selection and sizing. Valves must be selected for ability to meet temperature, pressure, flow control caracteristic, and piping connection requirements of te ydronic system. Valve sizing is critical to ensure support for eating and cooling loads wit adequate valve capacity, yet able to control system flow to provide stable building conditions efficiently. DEFINITIONS VALVE COMPONENTS Actuator: Te part of an automatic control valve tat moves te stem based on an electric, electronic, or pneumatic signal from a controller. Te actuator and valve can be two separate devices or togeter tey can be one device. Body: Te valve casting troug wic te controlled fluid flows (Fig. 1). STEM BONNET DISC HOLDER DISC SEAT IN BODY PLUG Fig. 1. Globe Valve Components. OUT M12225 Bonnet: Te part tat screws to te top of te valve body and contains te packing tat seals and guides te valve stem. Disc: Te part of te valve assembly tat contacts te valve seat to close off flow of te controlled fluid. Some valve assemblies are built so te disc is replaceable. Replaceable discs are usually made of a composition material softer tan metal. Plug: Te part tat varies te opening for te fluid to flow troug te valve body. Te following describes te tree most common types of plugs: A contoured plug as a saped end tat is usually end-guided at te top or bottom (or bot) of te valve body. Te saped end controls fluid flow troug te valve wit respect to stem travel. A V-port plug as a cylinder, called a skirt, tat rides up and down in te valve seat ring. Te skirt guides te plug and varies te flow area wit respect to stem travel via its saped openings. A quick-opening plug is flat and is eiter endguided or guided by wings riding in te valve seat ring. Te flat plug provides maximum flow soon after it lifts from te valve seat. Port: Te opening in te valve seat. Seat: Te stationary part of te valve body tat as a raised lip to contact te valve disc wen closing off flow of te controlled fluid. Stem: Te saft tat runs troug te valve bonnet and connects an actuator to te valve plug. Trim: All parts of te valve tat contact te controlled fluid. Trim includes te stem, packing, plug, disc, and seat; it does not include te valve body. VALVE FLOW CHARACTERISTICS Direction of flow: Te correct flow of te controlled fluid troug te valve is usually indicated on te valve body. If te fluid flow troug te valve is incorrect, te disc can slam into te seat as it approaces te closed position. Te result is poor control, excessive valve wear, and noisy operation. In addition, te actuator must work arder to reopen te closed valve since it must overcome te pressure exerted by te fluid on top of te disc rater tan ave te fluid assist in opening te valve by exerting pressure under te disc. Equal percentage: A valve wic canges flow by an equal percentage (regardless of flow rate) for similar movements in stem travel (at any point in te flow range). 2

3 Linear: A valve wic provides a flow-to-lift relationsip tat is directly proportional. It provides equal flow canges for equal lift canges, regardless of percentage of valve opening. Quick-opening: A valve wic provides maximum possible flow as soon as te stem lifts te disc from te valve seat. Valve flow caracteristic: Te relationsip between te stem travel of a valve, expressed in percent of travel, and te fluid flow troug te valve, expressed in percent of full flow. VALVE FLOW TERMS Rangeability: Te ratio of maximum flow to minimum controllable flow. Approximate rangeability ratios are 50 to 1 for V-port globe valves and 30 to 1 for contoured plug valves. EXAMPLE: A valve wit a total flow capacity of 100 gpm full open and a rangeability of 30 to 1, can accurately controls flow accurately as low as 3 gpm. Tigt sut-off/close-off: A valve condition in wic virtually no leakage of te controlled fluid occurs in te closed position. Generally, only single-seated valves provide tigt sut-off. Double-seated valves typically ave a one to tree percent leakage in te closed position. Turndown: Te ratio of maximum flow to minimum controllable flow of a valve installed in a system. Turndown is equal to or less tan rangeability. EXAMPLE: For te valve in te rangeability example, if te system requires a 66 gpm maximum flow troug te valve and since te minimum accurately controllable flow is 3 gpm, te turndown is 22. VALVE RATINGS Flow coefficient (capacity index): Used to state te flow capacity of a control valve for specified conditions. Currently, in te control valve industry, one of tree flow coefficients Britis Av, Nort American Kv, or United States Cv is used depending upon te location and system of units. Te flow coefficients ave te following relationsips: Av = Kv Av = Cv Kv = Cv Te flow coefficient Av is in cubic meters per second and can be determined from te formula: Q = volumetric flow in cubic meters per second. ρ = fluid density in kilograms per cubic meter. p = static pressure loss across te valve in pascals. Te flow coefficient Kv is water flow in cubic meters per our wit a static pressure loss across te valve of 10 5 pascals (1 bar) witin te temperature range of 5 to 40 C and can be determined from te formula: Av = Q Kv = Q Q = volumetric flow in cubic meters per our. ρ = fluid density in kilograms per cubic meter. ρ w = density of water in kilograms per cubic meter. = static pressure loss of 10 5 pascals. p = static pressure loss across te valve in pascals. p Kv Te flow coefficient C v is water flow in gallons per minute wit a pressure loss across te valve of one pound per square inc witin te temperature range of 40 to 100F and can be determined for oter conditions from te formula: Cv = Q Q = volumetric flow in US gallons per minute. ρ = fluid density in pounds per cubic foot. ρ w ρ p ρ K v p 1 p ρ ρ w ρ ρ w = density of water in pounds per cubic foot witin te temperature range of 40 to 100F p = static pressure loss across te valve in pounds per square inc. Close-off rating: Te maximum pressure drop tat a valve can witstand witout leakage wile in te full closed position. Te close-off rating is a function of actuator power to old te valve closed against pressure drop, but structural parts suc as te stem can be te limiting factor. 3

4 LINE PRESSURE IN PSI VALVE SELECTION AND SIZING EXAMPLE: A valve wit a close-off rating of 10 psi could ave 40 psi upstream pressure and 30 psi downstream pressure. Note tat in applications were failure of te valve to close is azardous, te maximum upstream pressure must not exceed te valve closeoff rating, regardless of te downstream pressure. Te valve close-off rating is independent of te actual valve body rating. See definition of BODY RATING (ACTUAL) in tis section. Close-off rating of tree-way valves: Te maximum pressure difference between eiter of te two inlet ports and te outlet port for mixing valves, or te pressure difference between te inlet port and eiter of te two outlet ports for diverting valves. Critical pressure drop: See Pressure drop (critical). Pressure drop: Te difference in upstream and downstream pressures of te fluid flowing troug te valve. Pressure drop (critical): Te flow of a gaseous controlled fluid troug te valve increases as te pressure drop increases until reacing a critical point. Tis is te critical pressure drop. Any increase in pressure drop beyond te critical pressure drop is dissipated as noise and cavitation rater tan increasing flow. Te noise and cavitation can destroy te valve and adjacent piping components. Body rating (nominal): Te teoretical pressure rating, expressed in psi, of te valve body exclusive of packing, disc, etc. Te nominal rating is often cast on te valve body and provides a way to classify te valve by pressure. A valve of specified body material and nominal body rating often as caracteristics suc as pressure-temperature ratings, wall tickness, and end connections wic are determined by a society suc as ANSI (American National Standards Institute). Figure 2 sows ANSI pressure-temperature ratings for valves. Note tat te nominal body rating is not te same as te actual body rating. Body rating (actual): Te correlation between safe, permissible flowing fluid pressure and flowing fluid temperature of te valve body (exclusive of te packing, disc, etc.). Te nominal valve body rating is te permissible pressure at a specific temperature. EXAMPLE: From Figure 2, a valve wit an ANSI rating of 150 psi (ANSI Class 150) as an actual rating of 225 psi at 250F. Maximum pressure and temperature: Te maximum pressure and temperature limitations of fluid flow tat a valve can witstand. Tese ratings may be due to valve packing, body, or disc material or actuator limitations. Te actual valve body ratings are exclusively for te valve body and te maximum pressure and temperature ratings are for te complete valve (body and trim). Note tat te maximum pressure and temperature ratings may be less tan te actual valve body ratings. EXAMPLE: Te body of a valve, exclusive of packing, disc, etc., as a pressure and temperature rating of 125 psi at 335F. If te valve contains a composition disc tat can witstand a temperature of only 240F, ten te temperature limit of te disc becomes te maximum temperature rating for te ANSI CLASS o F ANSI CLASS 150 ANSI CLASS 250 ANSI CLASS150 (STEAM) 275 o F 337 o F FLUID TEMPERATURE IN o F NOTES: 1. FOR HIGH FLUID TEMPERATURES, THE VALVE AND/OR PIPING SHOULD BE INSULATED TO PREVENT AMBIENT TEMPERATURES FROM EXCEEDING ACTURATOR RATINGS. M12224 valve. Fig. 2. ANSI Pressure-Temperature Ratings for Valves. VALVE TYPES Ball valve: A ball valve as a precision ball between two seats witin a body (Fig. 3). Ball valves ave several port sizes for a give body size and go from closed to open wit a 90 degree turn of te stem. Tey are available in bot two-way and tree-way configurations. For HVAC applications, ball valve construction includes brass and cast iron bodies; stainless steel, crome plated brass, and cast iron balls; resilient seats wit 4

5 STEM SEATS BODY BALL PORT larger port area for a given pipe size. A limitation of double-seated valves is tat tey do not provide tigt sut-off. Since bot discs rigidly connect togeter and canges in fluid temperature can cause eiter te disc or te valve body to expand or contract, one disc may seat before te oter and prevent te oter disc from seating tigtly. various temperature ratings. Fig. 3. Ball Valve. M12228 Ball valves provide tigt sut-off, wile full port models ave low flow resistance, and reduced port models can be selected for modulating applications. Butterfly valve: A valve wit a cylindrical body, a saft, and a rotating disc (Fig. 4). Te disc rotates 90 degrees from open to closed. Te disc seats against a resilient body liner and may be manufactured for tigt sut-off or made smaller for reduced operating torque but witout tigt close-off. Butterfly valves are inerently for twoway operation. For tree-way applications, two butterfly valves are assembled to a pipe tee wit linkage for simultaneous operation. Fig. 4. Butterfly Valve. STEM BODY RESILIENT SEAL DISC M12247 Double-seated valve: A valve wit two seats, plugs, and discs. Double-seated valves are suitable for applications were fluid pressure is too ig to permit a singleseated valve to close. Te discs in a double-seated valve are arranged so tat in te closed position tere is minimal fluid pressure forcing te stem toward te open or closed position; te pressure on te discs is essentially balanced. For a valve of given size and port area, te double-seated valve requires less force to operate tan te single-seated valve so te doubleseated valve can use a smaller actuator tan a singleseated valve. Also, double-seated valves often ave a Flanged-end connections: A valve tat connects to a pipe by bolting a flange on te valve to a flange screwed onto te pipe. Flanged connections are typically used on large valves only. Globe valve: A valve wic controls flow by moving a circular disk against or away from a seat. Wen used in trottling control a contoured plug (trottling plug) extends from te center of circular disk troug te center of te seat for precise control (Fig. 1). Pilot-operated valve: A valve wic uses te differential between upstream and downstream pressure acting on a diapragm or piston to move te valve plug. Pilotoperated valves are suitable for two-position control only. Te valve actuator exerts only te force necessary to open or close te small pilot port valve tat admits fluid flow into te diapragm or piston camber. Reduced-Port valve: A valve wit a capacity less tan te maximum for te valve body. Ball, butterfly, and smaller globe valves are available wit reduced ports to allow correct sizing for good control. Screwed-end connection: A valve wit treaded pipe connections. Valve treads are usually female, but male connections are available for special applications. Some valves ave an integral union for easier installation. Single-seated valve: A valve wit one seat, plug, and disc. Single-seated valves are suitable for applications requiring tigt sut-off. Since a single-seated valve as noting to balance te force of te fluid pressure exerted on te plug, it requires more closing force tan a double-seated valve of te same size and terefore requires more actuator force tan a double-seated valve. Tree-way valve: A valve wit tree ports. Te internal design of a tree-way valve classifies it as a mixing or diverting valve. Tree-way valves control liquid in modulating or two-position applications and do not provide tigt sut-off. Two-way valve: A valve wit one inlet port and one outlet port. Two-way valves control water or steam in twoposition or modulating applications and provide tigt sut-off in bot straigt troug and angle patterns. 5

6 VALVE MATERIAL AND MEDIA Valves wit bronze or cast iron bodies aving brass or stainless steel trim perform satisfactorily in HVAC ydronic systems wen te water is treated properly. Failure of valves in tese systems may be an indication of inadequate water treatment. Te untreated water may contain dissolved minerals (e.g., calcium, magnesium, or iron compounds) or gases (e.g., carbon dioxide, oxygen, or ammonia). Inadequate treatment results in corrosion of te system. Depending on te material of te valve, te color of te corrosion may indicate te substance causing te failure (Table 1). Table 1. Corrosive Elements in Hydronic Systems. Brass or Bronze Component Corrosive Substance Corrosion Color Cloride Ammonia Carbonates Magnesium or Calcium Oxides Sulpide (Hydrogen) Iron Ligt Blue-Green Blue or Dark Blue Dark Blue-Green Wite Black (water) Black (Gas) Rust Iron or Steel Component Corrosive Substance Corrosion Color Magnesium or Calcium Iron Wite Rust Glycol solutions may be used to prevent ydronic systems freezing. Glycol solutions sould be formulated for HVAC systems. Some available glycol solutions formulated for oter uses contain additives tat are injurious to some system seals. In addition, ydronic seals react differently to water and glycol suc tat wen a new system is started up wit water or glycol te seals are effective. Te ydronic seals are likely to leak if te system is later restarted wit media canged from to water to glycol or glycol to water. To prevent leakage part of te process of media cangeover sould include replacing seals suc as, pump and valve packing. VALVE SELECTION Proper valve selection matces a valve to te control and ydronic system pysical requirements. First consider te application requirements and ten consider te valve caracteristics necessary to meet tose requirements. Te following questions provide a guide to correct valve selection. Wat is te piping arrangement and size? Te piping arrangement indicates weter a two-way or tree-way mixing or diverting valve is needed. Te piping size gives some indication of weter te valve requires a screwed end or a flanged end connection. Does te application require two-position control or proportional control? Does te application require a normally open or normally closed valve? Sould te actuator be direct acting or reverse acting? In its state of rest, te valve is normally open or closed depending on te load being controlled, te fluid being controlled, and te system configuration. For cilled water coils, it is usually preferable to close te valve on fan sutdown to prevent excessive condensation around te duct and coil, and to save pumping energy. Tis may be accomplised wit eiter normally closed valves or a variety of oter control scemes. Lower cost and more powerful normally open valve assemblies may be used wit te close-onsutdown feature and allow, in te case of pneumatic systems, te capability to provide eating or cooling in te event of air compressor failure. Converter control valves sould be normally closed and outdoor air preeat valves sould be normally open. Is tigt sut-off necessary? Wat differential pressure does te valve ave to close against? How muc actuator close-off force is required? Single-seated valves provide tigt sut-off, wile doubleseated valves do not. Double seated valves are acceptable for use in pressure bypass or in-line trottling applications. Te design and flow capacity of a valve determine wo muc actuator force is required for a given close-off. Terefore, te valve must first be sized, ten, te valve and actuator selected to provide te required close-off. Wat type of medium is being controlled? Wat are te temperature and pressure ranges of te medium? 6

7 Valves must be compatible wit system media composition, maximum and minimum temperature, and maximum pressure. Te temperature and pressure of te medium being controlled sould not exceed te maximum temperature and pressure ratings of te valve. For applications suc as clorinated water or brine, select valve materials to avoid corrosion. Wat is te pressure drop across te valve? Is te pressure drop ig enoug? Te full open pressure drop across te valve must be ig enoug to allow te valve to exercise control over its portion of te ydronic system. However, te full open pressure drop must not exceed te valves rating for quiet service and normal life. Closed pressure drop must not exceed valve and actuator close-off rating. GLOBE VALVE Globe valves are popular for HVAC applications. Tey are available in pipe sizes from 1/2 in. to 12 in. and in a large variety of capacities, flow caracteristics, and temperature and pressure capabilities. Tey provide wide rangeability and tigt sutoff for excellent control over a broad range of conditions. Globe valves are made in two-way, straigt or angle configurations and tree-way mixing and diverting designs. Globe valves close against te flow and ave arrows on te body indicating correct flow direction. Incorrect piping can result in stem oscillations, noise, and ig wear. A two-way globe valve as one inlet port and one outlet port (Fig. 5) in eiter a straigt troug or angle pattern. Te valve can be eiter pus-down-to-close or pus-down-to-open. Pneumatic and electric actuators wit linear motion to operate globe valves are available for operation wit many control signals. BALL VALVE Ball valves are available for two-position applications eiter manual (and) or power operated or for modulating applications wit direct coupled electric actuators. Ball valves are relatively low cost and provide tigt close off and available in two-way and tree-way configurations. As wit all oter valves, ball valves must be properly sized to provide good flow control. Wen used in modulating service, ball valves must be specifically designed for modulating service as compared to two-position service. Packing must provide leak-free sealing troug tousands of cycles to ensure trouble-free HVAC service. Te ball and stem sould be made of stainless steel or similar material tat minimizes sticking to te seat. Two-way ball valves ave equal percentage flow control caracteristics and flow can be in eiter direction Tree-way ball valves can be used in eiter mixing or diverting service. Tey ave linear flow control caracteristics for constant total flow. BUTTERFLY VALVE Butterfly valves (Fig. 6) control te flow of ot, cilled, or condenser water in two-position or proportional applications. Butterfly valves are available in two-way or tree-way configurations. Tigt sutoff may be acieved by proper selection of actuator force and body lining. Te tree-way valve can be used in mixing or diverting applications wit te flow in any direction. Te tree-way valve consists of two butterfly valves tat mount on a flanged cast iron tee and are linked to an actuator wic opens one valve as it closes te oter. Minimum combined capacity of bot valves occurs at te alfopen position. IN IN PUSH-DOWN-TO-CLOSE PUSH-DOWN-TO-OPEN C2328 Fig. 5. Two-Way Globe Valves. M10403 Fig. 6. Butterfly Valve. 7

8 Wen butterfly valves are used for proportional control, tey must be applied using conservative pressure drop criteria. If te pressure drop approaces te critical pressure drop, unbalanced forces on te disc can cause oscillations, poor control, and/or damage to te linkage and actuator, even toug te critical flow point is not reaced. Butterfly valves are usually found in larger pipe sizes. For example, two butterfly valves could be piped in a mixing application to control te temperature of te water going back to te condenser. Te valves proportion te amount of tower water and condenser water return tat is flowing in te condenser water supply line. PERCENTAGE OF FULL COOLING NONLINEAR SYSTEM RESPONSE 0% TEMPERATURE RESULTANT LINEAR SYSTEM CONTROL EQUAL PERCENTAGE CONTROL VALVE C2330 Fig. 8. Linear vs Nonlinear System Control. TWO-WAY VALVE Two-way valves are available as globe, ball, or butterfly valves. Te combination of valve body and actuator (called valve assembly) determines te valve stem position. Two-way valves control steam or water in two-position or proportional applications (Fig. 7). Tey provide tigt sutoff and are available wit quick-opening, linear, or equal percentage flow caracteristics. SUPPLY TWO WAY VALVE LOAD RETURN C2329 Fig. 7. Two-Way Valve Application. Ideally, a control system as a linear response over its entire operating range. Te sensitivity of te control to a cange in temperature is ten constant trougout te entire control range. For example, a small increase in temperature provides a small increase in cooling. A nonlinear system as varying sensitivity. For example, a small increase in temperature can provide a large increase in cooling in one part of te operating range and a small increase in anoter part of te operating range. To acieve linear control, te combined system performance of te actuator, control valve, and load must be linear. If te system is linear, a linear control valve is appropriate (Fig. 8). If te system is not linear, a nonlinear control valve, suc as an equal percentage valve, is appropriate to balance te system so tat resultant performance is linear. QUICK-OPENING VALVE A quick-opening two-way valve includes only a disc guide and a flat or quick-opening plug. Tis type of valve is used for two-position control of steam. Te pressure drop for a quickopening two-way valve sould be 10 to 20 percent of te piping system pressure differential, leaving te oter 80 to 90 percent for te load and piping connections. Figure 9 sows te relationsip of flow versus stem travel for a quick-opening valve. To acieve 90 percent flow, te stem must open only 20 percent. Linear or equal percentage valves can be used in lieu of quick-opening valves in two-position control applications as te only significant positions are full open and full closed. 90% FLOW QUICK-OPENING CONTROL VALVE 0% 20% STEM TRAVEL C2331 Fig. 9. Flow vs Stem Travel Caracteristic of a Quick-Opening Valve. 8

9 LINEAR VALVE A linear valve may include a V-port plug or a contoured plug. Tis type of valve is used for proportional control of steam or cilled water, or in applications tat do not ave wide load variations. Typically in steam or cilled water applications, canges in flow troug te load (e.g., eat excanger, coil) cause proportional canges in eat output. For example, Figure 10 sows te relationsips between eat output, flow, and stem travel given a steam eat excanger and a linear valve as follows: 90% 90% 90% HEAT OUTPUT FLOW HEAT OUTPUT 20% 20% 20% 0% 20% 90% 0% 20% 90% 0% 20% 90% FLOW STEM TRAVEL STEM TRAVEL GRAPH A GRAPH B GRAPH C C2332 Fig. 10. Heat Output, Flow, and Stem Travel Caracteristics of a Linear Valve. Grap A sows te linear relationsip between eat output and flow for te steam eat excanger. Canges in eat output vary directly wit canges in te fluid flow. Grap B sows te linear relationsip between flow and stem travel for te linear control valve. Canges in stem travel vary directly wit canges in te fluid flow. NOTE: As a linear valve just starts to open, a minimum flow occurs due to clearances required to prevent sticking of te valve. Some valves ave a modified linear caracteristic to reduce tis minimum controllable flow. Tis modified caracteristic is similar to an equal percentage valve caracteristic for te first 5 to 10 percent of stem lift and ten follows a linear valve caracteristic for te remainder of te stem travel. Grap C sows te linear relationsip between eat output and stem travel for te combined eat excanger and linear valve. Canges in eat output are directly proportional to canges in te stem travel. Tus a linear valve is used in linear applications to provide linear control. EQUAL PERCENTAGE VALVE An equal percentage valve includes a contoured plug or contoured V-port saped so tat similar movements in stem travel at any point in te flow range cange te existing flow an equal percentage, regardless of flow rate. EXAMPLE: Wen a valve wit te stem at 30 percent of its total lift and existing flow of 3.9 gpm (Table 2) opens an additional 10 percent of its full travel, te flow measures 6.2 gpm or increases 60 percent. If te valve opens an additional 10 percent so te stem is at 50 percent of its full travel, te flow increases anoter 60 percent and is 9.9 gpm. Table 2. Stem Position Vs Flow for Equal Percentage Valve. Stem Flow Cange Position Rate Cange 30% open 3.9 gpm 10% increase 40% open 6.2 gpm 60% increase 10% increase 50% open 9.9 gpm 60% increase An equal percentage valve is used for proportional control in ot water applications and is useful in control applications were wide load variations can occur. Typically in ot water applications, large reductions in flow troug te load (e.g., coil) cause small reductions in eat output. An equal percentage valve is used in tese applications to acieve linear control. For example, Figure 11 sows te eat output, flow, and stem travel relationsips for a ot water coil, wit 200F. entering water and 50F entering air and an equal percentage valve, as follows: Grap A sows te nonlinear relationsip between eat output and flow for te ot water coil. A 50 percent reduction in flow causes a 10 percent reduction in eat output. To reduce te eat output by 50 percent, te flow must decrease 90 percent. Grap B sows te nonlinear relationsip between flow and stem travel for te equal percentage control valve. To reduce te flow 50 percent, te stem must close 10 percent. If te stem closes 50 percent, te flow reduces 90 percent. 9

10 Grap C sows te relationsip between eat output and stem travel for te combined coil and equal percentage valve. Te combined relationsip is close to linear. A 10 percent reduction in eat output requires te stem to close 10 percent, a 50 percent reduction in eat output requires te stem to close 50 percent, and a 90 percent reduction in eat output requires te stem to close 90 percent. Te equal percentage valve compensates for te caracteristics of a ot water application to provide a control tat is close to linear. 90% 90% HEAT OUTPUT 50% FLOW 50% HEAT OUTPUT 50% 10% 10% 0% 10% 50% 0% 50% 90% 0% 10% 50% 90% FLOW STEM TRAVEL STEM TRAVEL GRAPH A GRAPH B GRAPH C C2333 Fig. 11. Heat Output, Flow, and Stem Travel Caracteristics of an Equal Percentage Valve. THREE-WAY VALVES Tree-way valves (Fig. 12) control te flow of liquids in mixing or diverting valve applications (Fig. 13). Te internal design of a tree-way globe valve enables it to seat against te flow of liquid in te different applications. An arrow cast on te valve body indicates te proper direction of liquid flow. It is important to connect tree-way valve piping correctly or oscillations, noise, and excessive valve wear can result. Treeway valves are typically ave linear flow caracteristics, altoug, some are equal percentage for flow troug te coil wit linear flow caracteristics for flow troug te coil bypass. Ball valves are also available in a tree-way configuration, wile two butterfly valves can be made to act as a tree-way valve. HOT WATER SUPPLY SUPPLY LOAD BYPASS THREE-WAY DIVERTING VALVE LOAD BYPASS THREE-WAY MIXING VALVE HOT WATER RETURN A. LOAD BYPASS IN MIXING VALVE APPLICATION RETURN B. LOAD BYPASS IN DIVERTING VALVE APPLICATION C2335A Fig. 13. Tree-Way Valve Applications. MIXING VALVE DIVERTING VALVE MIXING VALVE A mixing valve provides two inlet ports and one common outlet port. Te valve receives liquids to be mixed from te inlet ports and discarges te liquid troug te outlet port (Fig. 12). Te position of te valve disc determines te mixing proportions of te liquids from te inlet ports. IN I N OUT IN Fig. 12. Tree-Way Valves. O U T OUT C2334 Te close-off pressure in a mixing valve equals te maximum value of te greater inlet pressure minus te minimum value of te downstream pressure. EXAMPLE: A mixing valve application as a maximum pressure of 25 psi on one inlet port, maximum pressure of 20 psi on te oter inlet port, and minimum downstream pressure of 10 psi on te outlet port. Te close-off pressure is 25 psi 10 psi = 15 psi. Te application requires a mixing valve wit at least a 15 psi close-off rating. Te actuator selected must ave a ig enoug force to operate satisfactorily. 10

11 In globe mixing valve applications, te force exerted on te valve disc due to unbalanced pressure at te inlets usually remains in te same direction. In cases were tere is a reversal of force, te force canges direction and olds te valve disc off te seat, cusioning it as it closes. If te pressure difference for te system is greater tan te pressure ratings of available globe mixing valves, use a ball mixing valve or two butterfly valves in a tee configuration. Globe mixing valves are not suitable for modulating diverting valve applications. If a mixing valve is piped for modulating diverting service, te inlet pressure slams te disc against te seat wen it nears te closed position. Tis results in loss of control, oscillations, and excessive valve wear and noise. Mixing valves are acceptable using about 80 percent of te close-off rating, but not recommended, in two-position diverting valve applications. Te close-off pressure in a diverting valve equals te maximum value of te inlet pressure minus te minimum value of te downstream pressure. Globe diverting valves must not be used for mixing service. As wit mixing valves used for diverting service, media pressure drop across te valve can cause it to slam sut wit resulting loss of control. EXAMPLE: A diverting valve application as 20 psi maximum on te inlet port, one outlet port discarging to te atmospere, and te oter outlet port connecting to a tank under 10 psi constant pressure. Te pressure difference between te inlet and te first outlet port is 20 psi and between te inlet and second outlet port is 10 psi. Te application requires a diverting valve wit at least 20 psi close-off rating. DIVERTING VALVE A globe diverting valve provides one common inlet port and two outlet ports. Te diverting valve uses two V-port plugs wic seat in opposite directions and against te common inlet flow. Te valve receives a liquid from one inlet port and discarges te liquids troug te outlet ports (Fig. 12) depending on te position of te valve disc. If te valve disc is against te bottom seat (stem up), all te liquid discarges troug te side outlet port. If te valve disc is against te top seat (stem down), all te liquid discarges troug te bottom outlet port. VALVE SIZING Every valve as a capacity index or flow coefficient (Cv). Typically determined for te globe and ball valves at full open and about 60 degrees open for butterfly valves. Cv is te quantity of water in gpm at 60F tat flows troug a valve wit a pressure differential of 1 psi. Sizing a valve requires knowing te medium (liquid or gas) and te required pressure differential to calculate te required Cv. Wen te required Cv is not available in a standard valve, select te next closest and calculate te resulting valve pressure differential at te required flow to verify to verify acceptable performance. After determination of te valve Cv, calculation of te flow of any medium troug tat valve can be found if te caracteristics of te medium and te pressure drop across te valve are known. WATER VALVES Determine te capacity index (Cv) for a valve used in a water application, using te formula: Q Cv = Q G = Flow of fluid in gallons per minute required to pass troug te valve. G = Specific gravity of te fluid (water = 1). = Pressure drop in psi. See Figures 14 and 15 for glycol solution correction values. Determining te Cv of a water valve requires knowing te quantity of water (gpm) troug te valve and te pressure drop () across te valve. If te fluid is a glycol solution, use te pressure drop multipliers from eiter Figure 14 or 15. See te sections on QUANTITY OF WATER and WATER VALVE PRESSURE DROP. Ten select te appropriate valve based on Cv, temperature range, action, body ratings, etc., per VALVE SELECTION guidelines. 11

12 PRESSURE DROP CORRECTION FACTOR PRESSURE DROP CORRECTION FACTOR % 30% 20% 50% BY MASS 10% WATER ETHYLENE GLYCOL SOLUTION TEMPERATURE, F M12226 REPRINTED BY PERMISSION FROM ASHRAE HANDBOOK 1996 HVAC SYSTEMS AND EQUIPMENT Fig. 14. Pressure Drop Correction for Etylene Glycol Solutions % 30% 10% 40% 50% BY MASS WATER PROPYLENE GLYCOL SOLUTION TEMPERATURE, F M12227 REPRINTED BY PERMISSION FROM ASHRAE HANDBOOK 1996 HVAC SYSTEMS AND EQUIPMENT Fig. 15. Pressure Drop Correction for Propylene Glycol Solutions. QUANTITY OF WATER To find te quantity of water (Q) in gallons per minute use one of te following formulas: 1. Wen Btu/r is known: Btu/r K TDw Q = Btu/r K x TDw = Heat output. = Value from Table 3; based on temperature of water entering te coil. Te value is in pounds per gallon x 60 minutes per our. = Temperature difference of water entering and leaving te coil. Water Temp F Table 3. Water Flow Formula Table. 2. For ot water coil valves: Q = cfm x 1.08 x TD a K x TDw cfm = Airflow troug te coil = A scaling constant. See Note. TDa = Temperature difference of air entering and leaving te coil. K = Value from Table 3; based on temperature of water entering te coil (pounds per gallon x 60 minutes per our). TDw = Temperature difference of water entering and leaving te coil. NOTE: Te scaling constant 1.08 is derived as follows: 1.08 = 1 lb air ft 3 = Te specific volume of air at standard conditions of temperature and atmosperic pressure. Simplifying te equation: 1.08 = 0.24 BTU lb air F x 60 min 1 r To find te scaling constant for air conditions oter tan standard, divide Btu by specific volume of air at tose conditions. 3. For fan system cilled water coil valves: Q = K Btu min F r f t 3 cfm x Btu/lb 113 x TDw Water Temp F cfm = Airflow troug te coil. Btu/lb = Heat per pound of dry air removed. Includes bot sensible and latent eat. 113 = A scaling constant. TDw = Temperature difference of water entering and leaving te coil. x 1 lb air ft 3 K

13 WATER VALVE PRESSURE DROP To determine valve pressure drop: 1. For two-way valves consider te following guidelines for valve pressure drop: a. Include te pressure drop in te design of te water circulating system. In systems wit two-way valves only, it is often necessary to provide a pump relief bypass or some oter means of differential pressure control to limit valve pressure drops to te valve capabilities. For control stability at ligt loads, pressure drop across te fully closed valve sould not exceed triple te pressure drop used for sizing te valve. To avoid ig pressure drops near te pump, reverse returns are recommended in large systems. b. Te pressure drop across an open valve sould be about alf of te pressure difference between system supply and return, enoug so tat te valve, not te friction troug te coil or radiator, controls te volume of water flow or te valve pressure drop sould be equal to or greater tan te pressure drop troug te coil or radiator, plus te pipe and fittings connecting tem to te supply and return mains. c. Verify allowable full open and full closed pressure drops for all proportional and twoposition water valves wit appropriate manufacturer literature. d. Make an analysis of te system at maximum and minimum rates of flow to determine weter or not te pressure difference between te supply and return mains stays witin te limits tat are acceptable from te stand point of control stability and close-off rating. 2. For two- and tree-way valves consider te following guidelines for valve pressure drop: a. In load bypass applications (Fig. 13) suc as radiators, coils, and air conditioning units, te pressure drop sould be 50 to 70 percent of te minimum difference between te supply and return main pressure at design operating conditions. b. A manual balancing valve may be installed in te bypass to equalize te load drop and te bypass drop. 3. Wen selecting pressure drops for tree-way mixing valves in boiler bypass applications (Fig. 13), consider te following: a. Determine te design pressure drop troug te boiler including all of te piping, valves, and fittings from te bypass connection troug te boiler and up to te tree-way valve input. b. Te valve pressure drop sould be equal to or greater tan te drop troug te boiler and te fittings. If te valve drop is muc smaller tan te boiler pressure drop at design, effective control is obtained only wen te disc is near one of te two seats. Te mid-portion of te valve lift will be relatively ineffective. c. A manual balancing valve may be installed in te boiler bypass to equalize te boiler drop and te bypass drop. WATER VALVE SIZING EXAMPLES EXAMPLE 1: A two-way linear valve is needed to control flow of 45F cilled water to a cooling coil. Te coil manufacturer as specified an eigt-row coil aving a water flow pressure drop of 3.16 psi. Furter, specifications say tat te coil will produce 55F leaving air wit a water flow of 14.6 gpm. Supply main is maintained at 40 psig, return is at 30 psig. Select required capacity index (Cv) of te valve. Use te water valve Cv formula to determine capacity index for Valve V1 as follows: Q = Flow of fluid in gallons per minute required is 14.6 gpm. G = Specific gravity of water is 1. Substituting: Cv = Q G = Pressure drop across te valve. Te difference between te supply and return is 10 psi. 50% to 70% x 10 psi = 5 to 7 psi. Use 6 psi for te correct valve pressure drop. Note tat 6 psi is also greater tan te coil pressure drop of 3.16 psi. Cv = = 6 Select a linear valve providing close control wit a capacity index of 6 and meeting te required pressure and temperature ratings. EXAMPLE 2: A bypass valve is required to prevent flow troug te ciller from dropping below 90 percent of design flow. Wen sizing valves for pump or ciller bypass applications (Fig. 16), system conditions tat cause te valve to open or close completely must be considered before a pressure drop can be selected. 13

14 PD SUPPLY CHILLER 12 PD 9.6 DP SETPOINT = 34' DP HEAT/ COOL COIL 1 HEAT/ COOL COIL 2 HEAT/ COOL COIL GPM PER AHU COIL HEAT/ COOL COIL 4 PUMP V5 V1 V2 V3 V4 B1 B2 B3 B ZERO REFERENCE PD RETURN SYSTEM STRAINER NUMBERS IN CIRCLES = GAGE PRESSURES PUMP INLET = ZERO FOR SIMPLICITY TOP NUMBERS = FULL FLOW BOTTOM NUMBER = 90% FLOW PD = PRESSURE DROP M10605 Fig. 16. Ciller Bypass Application. Assume te following: System flow at design, 1000 gpm Pump ead at design, 48 ft Pump ead at 90 percent flow, 50 ft Pressure across mains at AHU 1 at design flow, 28 ft Ciller pressure drop, 12 ft Ciller piping loop design pressure drop, 8 ft Wit full system flow, Valve V5 is closed. Pressure drop across V5 equals te pump ead minus te friction drops to V5. Pressure drop across Valve V5 is ten 48 ft 12 ft (ciller drop) 4 ft (supply drop) 4 ft (return drop) or 28 ft. Wit system flow at 90 percent, te pump ead rises to 50 ft, wile te friction drops fall to te lower values sown in Figure 16. For additional information on ciller bypass operation see Ciller, Boiler, and Distribution System Applications section. Pressure drop across V5 equals te pump ead minus te friction drops to V5. Pressure drop across Valve V5 is ten 50 ft 9.6 ft (ciller drop) 3.2 ft (supply drop) 3.2 ft (return drop) or 34 ft. Converting ft to psi, 34 ft x psi/ft = 14.7 psi (see General Engineering Data section). Substituting te flow of water, specific gravity of water, and pressure drop in te Cv formula sows tat te Valve V5 sould ave a Cv of 235. Cv = = 235 EXAMPLE 3: Sizing water valves for eating coils is especially critical. In Figure 17, a valve wit a Cv of 12 will ave 30 percent of te available pressure drop wen full open, wile a valve wit a Cv of 5 will ave 70 percent of te available pressure drop. As sown in Figure 18, te valve wit 70 percent of te available pressure drop essentially provides te equal percentage water flow control, resulting in linear coil eat transfer and stable temperature control. Te valve wit only 30 percent of te available pressure drop as a more linear flow control wic results in nonlinear coil eat transfer. See EQUAL PERCENTAGE VALVE section for furter information. 14

15 CASE A: 50 PSI CASE B: 62 PSI 180 F HOT WATER SUPPLY VALVE VI 30% PRESSURE DROP, Cv = 12 70% PRESSURE DROP, Cv = 5 HEATING COIL LOCAL PIPING 2.2 PSI DROP 20 GPM AT DESIGN, 4.3 PSI DROP 40 PSI HOT WATER RETURN C2339A Fig. 17. Equal Percentage Valve Hot Water Application. FLOW AT CONSTANT PRESSURE DROP 30% PRESSURE DROP 0% STEM TRAVEL 70% PRESSURE DROP IDEAL EQUAL PERCENTAGE VALVE CHARACTERISTIC C2340 Fig 18. Effect of Pressure Drop in Hot Water Valve Sizing. EXAMPLE 4: A tree-way mixing valve is needed for a eat excanger application wit a bypass line. Water flow is specified at te rate of 70 gpm. Manufacturer data for te excanger indicates a pressure drop of 1.41 ft of water troug te excanger coils. Use te water valve Cv formula to determine capacity index for Valve V1 as follows: Q Cv = Q = Flow of fluid in gallons per minute required to pass troug te valve is 70 gpm. G = Specific gravity of water is 1. G = Pressure drop across te valve. Plans of te eating system indicate tree-inc supply and return mains. From an elbow equivalent table and pipe friction cart found in te ASHRAE Handbook or oter reference manuals, te calculated pressure drop troug a tree-inc tee and te piping from te valve and te tee to te excanger is 0.09 psi. Heat excanger pressure drop is 1.41 ft of water or 1.41 ft x psi/ft = 0.61 psi. Total pressure drop from bypass connection troug te eat excanger and to te ot-water input of te tree-way valve is or 0.70 psi. Since te valve pressure drop () sould be equal to or greater tan te drop troug te eat excanger and fittings, 0.70 psi is used as te valve pressure drop. For optimum control, a manual balancing valve is installed in te bypass line to equalize te pressure drops in te excanger and bypass circuits. Substituting te flow of water, specific gravity of water, and pressure drop in te Cv formula sows tat te valve sould ave a Cv of 83.6 or 84. Cv = 70 1 = 83.6 or Select a linear valve providing close control wit a capacity index of 84 and meeting te required pressure and temperature ratings. STEAM VALVES Calculate te required capacity index (Cv) for a valve used in a steam application, using te formula: ( s)Q Cv = V 63.5 Q = Quantity of steam in pounds per our required to pass troug te valve. V = Specific volume of steam, in cubic feet per pound, at te average pressure in te valve. For convenience Table 5 at te end of te STEAM VALVES section lists te square root of te specific volume of steam for various steam pressures. Terefore, use te value in tis column of te table as is; do not take its square root = A scaling constant. = Pressure drop in psi. s = Supereat in degrees F. Determining te Cv for a steam valve requires knowing, te quantity of steam (Q) troug te valve, te pressure drop () across te valve, and te degrees of supereat. See QUANTITY OF STEAM and STEAM VALVE PRESSURE DROP. Ten select te appropriate valve based on Cv, temperature range, action, body ratings, etc., per VALVE SELECTION guidelines. NOTE: Wen te supereat is 0F, ten ( s) equals 1 and may be ignored. 15

16 QUANTITY OF STEAM To find te quantity of steam (Q) in pounds per our use one of te following formulas: 1. Wen Btu/r (eat output) is known: Btu/r = Heat output Btu/lb = A scaling constant representing te approximate eat of vaporization of steam. 2. For sizing steam coil valves: cfm TDa Q = Btu/r 1000 Btu/lb steam Q = CFM x TD a x Btu/lb steam = Cubic feet per minute (ft 3 /min) of air from te fan. = Temperature difference of air entering and leaving te coil = A scaling constant. See NOTE Btu/lb = A scaling constant representing te approximate eat of vaporization of steam. NOTE: Te scaling constant 1.08 is derived as follows: gpm = Gallons per minute of water flow troug converter. TDw = Temperature difference of water entering and leaving te converter = A scaling constant. Tis value is derived as follows: 0.49 = W1 Simplifying te equation: 4. Wen sizing steam jet umidifier valves: W ft 3 lb air ft 3 min 8.33 lb water 1 gal 0.49 = x 60 min 1 r x 0.49 min lb steam gal r F 1 lb steam 1000 Btu x 1 Btu lb water F Q = ( W1 W2 ) lb moisture 1 x lb air ft 3 x ft3 60 min x min r lb air = Humidity ratio entering umidifier, pounds of moisture per pound of dry air. = Humidity ratio leaving umidifier, pounds of moisture per pound of dry air. = Te specific volume of air at standard conditions of temperature and atmosperic pressure. = Cubic feet per minute (cfm) of air from te fan = 0.24 BTU lb air F x 60 min 1 r x 1 lb air ft 3 60 min r = A conversion factor. 1 lb air ft 3 = Te specific volume of air at standard conditions of temperature and atmosperic pressure. Simplifying te equation: 1.08 = Btu min F r f t 3 To find te scaling constant for air conditions oter tan standard, divide Btu by specific volume of air at tose conditions. 3. For sizing steam to ot water converter valves: Q = gpm x TDw x 0.49 Simplifying: Q = 4.49 ( W1 W2 ) lb moisture r 5. Wen Equivalent Direct Radiation (EDR) is known: Q = EDR (Total) x 0.24 EDR (Total) = Radiators are sized according to Equivalent Direct Radiation (EDR). If controlling several pieces of radiation equipment wit one valve, add te EDR values for all pieces to obtain te total EDR for te formula = A scaling constant, lb steam/unit EDR. See Table 4. 16

17 Average Radiator of Convector Temperature, Deg F a b Table 4. Output of Radiators and Convectors. Cast Iron Radiator Btu/Hr/EDR a Convector, Btu/Hr/EDR b At Room Temperature. At 65F Inlet Air Temperature STEAM VALVE PRESSURE DROP Proportional Applications Wen specified, use tat pressure drop () across te valve. Wen not specified: 1. Calculate te pressure drop () across te valve for good modulating control: = 80% x (Pm Pr) NOTE: For a zone valve in a system using radiator orifices use: Were Pm Pr = (50 to 75)% x (Pm Pr) = Pressure in supply main in psig or psia (gage or absolute pressure). = Pressure in return in psig or psia. A negative value if a vacuum return. 2. Determine te critical pressure drop: Pma critical = 50% x Pma = Pressure in supply main in psia (absolute pressure) psia = psig Use te smaller value or critical wen calculating Cv. Two-Position Applications Use line sized valves wenever possible. If te valve size must be reduced, use: Were Pm Pr = 20% x (Pm-Pr) = Pressure in supply main in psig or psia (gage or absolute pressure). = Pressure in return in psig or psia. A negative value if a vacuum return. STEAM VALVE SIZING EXAMPLES EXAMPLE 1: A two-way linear valve (V1) is needed to control igpressure steam flow to a steam-to-water eat excanger. An industrial-type valve is specified. Steam pressure in te supply main is 80 psig wit no supereat, pressure in return is equal to atmosperic pressure, water flow is 82.5 gpm, and te water temperature difference is 20F. Use te steam valve Cv formula to determine capacity index for Valve V1 as follows: Q = Te quantity of steam required to pass troug te valve is found using te converter valve formula: Q = gpm x TD w x 0.49 gpm = 82.5 gpm water flow troug excanger TDw = 20F temperature difference 0.49 = A scaling constant Substituting tis data in te formula: Q Pm Pr Cv = ( s)Q V 63.5 = pounds per our = Te pressure drop across a valve in a modulating application is: = 80% x (Pm Pr) = Upstream pressure in supply main is 80 psig. = Pressure in return is atmosperic pressure or 0 psig. Substituting tis data in te pressure drop formula: = 0.80 x (80 0) = 0.80 x 80 = 64 psi 17

18 Te critical pressure drop is found using te following formula: critical critical = 50% x (psig psi) = 0.50 x (80 psig upstream psi) = 0.50 x 94.7 psi = 47.4 psi Te critical pressure drop ( critical ) of 47.4 psi is used in calculating Cv, since it is less tan te pressure drop () of 64 psi. Always, use te smaller of te two calculated values. V = Specific volume (V) of steam, in cubic feet Pavg = Pm 2 = per pound at average pressure in valve (P avg ): Te specific volume of steam at 56.4 psig is 6.14 and te square root is = A scaling constant. = = 56.4 psig Substituting te quantity of steam, specific volume of steam, and pressure drop in te Cv formula sows tat te valve sould ave a Cv of 4.6. Cv = = ( x 0) x x x 6.88 = 4.6 NOTE: If Pavg is rounded off to te nearest value in Table 5 (60 psi), te calculated Cv is 4.5 a negligible difference. Select a linear valve providing close control wit a capacity index of 4 and meeting te required pressure and temperature ratings. NOTE: For steam valves downstream from pressure reducing stations, te steam will be supereated in most cases and must be considered. EXAMPLE 2: In Figure 19, a linear valve (V1) is needed for accurate flow control of a steam coil tat requires 750 pounds per our of steam. Upstream pressure in te supply main is 5 psig and pressure in te return is 4 in. Hg vacuum minimum. SUPPLY 5 PSI VALVE VI 30% PRESSURE DROP, Cv = 41 80% PRESSURE DROP, Cv = 25 STEAM 1.96 PSI COIL (VACUUM) RETURN C2336 Fig. 19. Linear Valve Steam Application. Use te steam valve Cv formula to determine capacity index for Valve V1 as follows: Cv = Q and: Pm Pr ( s)Q V 63.5 = Quantity of steam required to pass troug te valve is 750 pounds per our. = Te pressure drop across a valve in a modulating application is found using: = 80% x (Pm Pr) = Upstream pressure in supply main is 5 psig. = Pressure in return is 4 in. Hg vacuum. NOTE: 1 in. Hg = 0.49 psi and 1 psi = 2.04 in. Hg. Terefore, 4 in. Hg vacuum = 1.96 psig. = 0.80 x [5 ( 1.96)] = 0.80 x 6.96 = 5.6 psi Te critical pressure drop is found using te following formula: critical critical = 50% x (psig psi) = 0.50 x (5 psig upstream psi) = 0.50 x 19.7 psia = 9.9 psi Te pressure drop () of 5.6 psi is used in calculating te Cv, since it is less tan te critical pressure drop ( critical ) of 9.9 psi. V = Specific volume (V) of steam, in cubic feet per pound at average pressure in valve (P avg): Pavg = Pm 2 = = = 2.2 psig Te specific volume of steam at 2.2 psig is and te square root is

19 63.5 = A scaling constant. s = 0 Substituting te quantity of steam, specific volume of steam, and pressure drop in te Cv formula sows tat Valve V1 sould ave a Cv of or te next iger available value (e.g., 25). Cv = = ( x 0) x 750 x x 2.37 = NOTE: If Pavg is rounded off to te nearest value in Table 5 (2 psi), te calculated Cv is Select a linear valve providing close control wit a capacity index of 25 and meeting te required pressure and temperature ratings. EXAMPLE 3: Figure 20 sows te importance of selecting an 80 percent pressure drop for sizing te steam valve in Example 2. Tis pressure drop (5.6 psi) approximates te linear valve caracteristic. If only 30 percent of te available pressure drop is used (0.30 x 6.96 psi = 2.10 psi or 2 psi), te valve Cv becomes: Cv = Cv = ( s)Q V x = 40.5 Tis larger valve (2 psi drop) as a steeper curve tat is furter away from te desired linear valve caracteristic. See LINEAR VALVE under VALVE SELECTION for more information. VALVE OPENING/ STEAM FLOW Cv = 41 Cv = 25 LINEAR VALVE CHARACTERISTIC 0% STEM TRAVEL C2337 Fig. 20. Effect of Pressure Drop in Steam Valve Sizing. Vacuum, Inces of Mercury Gage Pressure, psig Table 5. Properties of Saturated Steam. Boiling Point or Steam Temperature Deg F Specific Volume (V), cu. ft/lb V (For valve sizing) Maximum Allowable Pressure Drop, psi (continued) 19

20 Table 5. Properties of Saturated Steam (continued). Gage Pressure, psig Boiling Point or Steam Temperature Deg F Specific Volume (V), cu. ft/lb V (For valve sizing) Maximum Allowable Pressure Drop, psi Home and Building Control Honeywell Inc. Honeywell Plaza P.O. Box 524 Minneapolis MN Honeywell Latin American Region 480 Sawgrass Corporate Parkway Suite 200 Sunrise FL Home and Building Control Honeywell Limited-Honeywell Limitée 155 Gordon Baker Road Nort York, Ontario M2H 3N7 Honeywell Europe S.A. 3 Avenue du Bourget 1140 Brussels Belgium Honeywell Asia Pacific Inc. Room Sun Hung Kai Centre No. 30 Harbour Road Wancai Hong Kong G. C Printed in U.S.A. on recycled paper containing at least 10% post-consumer paper 20fibers.