Unbalanced and Pressure-balanced Trim

1 8-1 Rev 3/93 Unbalanced andPressure- balanced TrimUnbalanced trim TypesValtek offers three Unbalanced trim designs: 1) stan-dard full area trim which provides maximum Cv with aremovable seat ring; 2) reduced trim which provides alower Cv in a wide variety of sizes or when larger bodiesare required; and 3) integral seat trim which utilizes theseat machined into the body and an oversized plug toprovide additional Cv beyond the capabilities of stan-dard full area One, Mark Two and Mark Eight valves can beconverted from one Unbalanced trim type to anothersince all seat rings and plugs within a given size andpressure class are completely interchangeable.

2 Inte-gral seat trim is available by changing the plug andremoving the seat ring. Table 8-III lists the values forunbalanced, full : All Mark One, Mark Two, Mark Eight, and MarkEleven bodies are machined with integral seats, exceptANSI Classes 900, 1500 and 8-1: Unbalanced trim DesignPressure- balanced TrimSome high pressure drop applications or valves withlarge seat diameters may require pressure -balancedtrim. Pressure-balanced trim reduces the actuatorthrust requirement by reducing the trim s effective off-balance area. However, with high-thrust cylinder actua-tors, Pressure-balanced trim may not be required.

3 Of-ten, an over-sized cylinder actuator may be the mosteconomical the Pressure-balanced plug fits closely to thesleeve (or retainer), the trim should be used in relativelyclean a standard, Pressure-balanced trim is designed toANSI Class II shutoff for valve sizes 1 2 to 3-inch andClass III shutoff for valve sizes 4-inch and larger specifications call for a maximum seat leakage and percent (respectively) of rated valve capac-ity. Flow direction is normally under the plug for fail-closed applications and over the plug for fail-openapplications. The sleeve area minus the stem area isdesigned slightly larger than the seat area, creating anoff- balanced area that assists closing with flow underfor fail-closed applications.

4 The off-balance area forvarious valve sizes are given in Table 8-2:Typical Pressure-balanced DesignSizing & Selection8 Full AreaReducedE0050E0052 SleevePlugSeat RingTransferHoleSeal8-2 Table 8-I: Shutoff Capabilities ofPressure- balanced TrimMetal SeatsNiResist rings(.5 to 3-inch)Class II(4-inch and above)Class IIIM uskegon rings(.5 to 4-inch)Class IIIM uskegon rings(6-inch and above)Class IVTFE SealsUp to 10% of Class IVO-Ring sealClass VSoft SeatsTFE SealsUp to 1% of Class IVO-Ring SealClass VITable 8-II: Seat Loading Rates to ObtainSpecified Leakage ClassClass IVMetal Seats*.

5 5 to 4-inch50 lbs/linear inch6-inch and above 75 lbs/linear inchSoft SeatsAll sizes50 lbs/linear inchClass VMetal Seats* .5 to 4-inch250 lbs/linear inch6-inch and above 400 lbs/linear inchSoft SeatsAll sizes50 lbs/linear inchClass VISoft to 4-inch50 lbs/linear inch6-inch and above 100 lbs/linear inch* When hardened trim is used, add 100 lbs / linear inch to the listed loading Seals with Backup RingsBuna-N O-rings are standard for those applicationswith temperatures between -60 to 250 degrees Fahren-heit and pressures to 6000 psi. Viton O-rings can beused for higher temperatures between -40 to 435degrees Fahrenheit and for pressures up to 6000 rings are used in conjunction with materials are available in O-rings and back-uprings for temperatures up to 500 degrees (TFE) SealsThe maximum pressure /temperature usage for Teflonseals in Pressure-balanced applications is shown inFigure 8-3.

6 Leakage RequirementsTable 8-I provides the class shutoff available with pres-sure- balanced trim and Table 8-II provides the seatloading ratings which are necessary to obtain the speci-fied leakage rate. ANSI standard discussesshut-off classes in section 8-3: pressure /TemperatureLimitation for Teflon SealsAllowing the fluid pressure to act on both sides of theplug results in a net force equal to the pressure times theoff-balance area. This balancing force is made possibleby transfer holes in the plug. Leakage past the plug isprevented by a seal around the top of the plug 8-2 shows a typical Pressure-balanced following list provides information about the typesof plug seals available for Pressure-balanced service:Metal Rings for all sizes and pressures, andtemperatures up to 800 degrees Fahrenheit.

7 Fortemperatures over 600 degrees Fahrenheit, the sur-face of the Pressure-balanced sleeve bore must behardened. Pressure-balanced sleeves for tempera-tures up to 800 degrees Fahrenheit are constructedfrom the following materials;-Carbon steel with electroless nickel plating(up to 600 degrees Fahrenheit)-410-416 high temperature stainless steel(up to 800 degrees Fahrenheit)-316 stainless steel with Stellite overlay or(up to 1600 degrees Fahrenheit) Multi-sealTM Rings for temperaturesfrom 300 to 1600 degrees Fahrenheit. WhenMuskegon rings are used, the bore surface of pres-sure- balanced sleeves must be hardened with oneof the following materials:-410-416 high temperature stainless steel(up to 800 degrees Fahrenheit)-316 stainless steel with Stellite overlay(up to 1600 degrees Fahrenheit)8-3 Table 8-III: Unblanced trim Full AreaValve Size RatingFull AreaSeat AreaStem AreaStd.

8 Act. Stroke(inches) ClassTrim Size*(sq. in.)(sq. in.) Size**(inches) 1/2150-600 .50 .196 .248 25 .753/4150-2500 .72 .405 .248 25 .751150-600 .81 .518 .248 25 .751900-1500 .81 .518 .248 25 .7512500 .72 .405 .248 25 .7511/2150-600 .601 .601 .785 .601 50 .752150-600 .601 .601 .601 .99 ..99 .99 *This data does not always apply to ChannelStream or MegaStream trim . **Minimum standard actuator size. Oversized actuators may be required for large pressure DirectionUnbalanced trim design generally requires that thedirection of flow should assist the motion of failure, forexample, flow-over for fail-closed and flow-under for fail-open.

9 The force required to fail-open or closed is afunction of the off- balanced area. This area is equal tothe seat area in fail-open applications and the seat areaminus the stem area in fail-closed applications. Valuesfor Unbalanced , or standard, full trim are listed in DirectionPressure- balanced trim design generally requires thatthe direction of flow be opposite to standard (unbal-anced) trim to oppose the motion of failure--for example,flow-over for fail-open and flow-under for force required to fail-open or closed is a function ofthe off- balanced area.

10 This area is equal to the sleevearea minus the stem and seat area in fail-closed appli-cations. When flowing under the seat, the net off- balanced area pushes downward to assist closure. Forfail-open applications, the off- balanced area is equal tothe sleeve area minus the seat area, thus there is anupward force with flow over the seat to assist failingopen. Values for full area, first and second reduction arelisted in Tables 8-IV, 8-V and 8-IV: Pressure-balanced trim Full AreaValveRatingFull Area Seat StemSleeve Off-balance AreaStandard StrokeSize Class trim Area Area Area ( )( )Actuator(inches)(inches) Size*( )( )( )to closeto open SIze**2 600.