Intro4u2u

CONTROL VALVE SELECTION – (STEP – 1)

1. Select a valve type:

A. Try single seated valve 0.9 CF to determine CV required

B. Flow Direction: Always try flow-to open first, then try flow-to-close for actuator process. Flow direction has to be decided before step ‘A’ selecting CF for desired type can be made.

2. Calculate CV required

A. Sizing on normal condition unless customer stipulates “Sizing Condition”

B. If only one condition is given and if it is labeled maximum, sizing should based on that condition.

3. Select valve type

A. Use CV selection of 85% of maximum rated CV.

B. Select alternate valve type by using correct CF for style selected.

Select flow direction, failure position and air action.

A. If fail – close is required, valve is usually Flow-to-close with Air-to-open action.

B. If fail – open is required, valve is usually Flow-to-open with Air-to-close action.

C. If failure position is non critical, flow and air action can usually be in either direction however Flow-to-open with Air-to-close action is preferred because it generally results higher Delta P Capacity, higher Cf  factor and higher plug stability at low flows. It is to be noted that larger number of valves are designed for Flow-to-open operation.

Note: Some valves are designed for flow in one direction only

CONTROL VALVE SELECTION – (STEP – 2)

1. Select Valve Size
2. Select Trim Size
3. Select Trim Characteristic.

1.SELECT VALVE SIZE, TRIM SIZE AND TRIM CHARACTERISTIC.

Use CV calculated in Step One, of product bulletin for valve type selected in Step One and the following Rule of Thumb for Step Two selections.

. Minimum Flow – Valve at least 10% open.
. Normal Flow – Valve approximately 25 – 75% open.
. Maximum Flow – Valve not more than 90% open.

2.VALVE AND TRIM SIZE

A. Full size valve with full area trim: Most economical selection and most common  (especially if maximum flow conditions are accurately known)

B. Oversized valve with reduced area trim: Less economical selection but frequently used especially in small valve sizes and when future flow requirements may be higher.

Note: If reduced area trim is selected the Cf factor may be changed requiring a recalculation of Cv. Repeat part III above using reduced area Cf factor.

3.TRIM CHARACTERISTIC

Use the rule of Thumb, the following descriptions in selecting the characteristic.

A. LINEAR: Flow is linearly proportioned to valve lift (distance plug is pulled away from seat ring) Should be used when a large portion of the system pressure drop can be taken across the valve. Most low flow trims (Cv less than 1.0) are linear due to close tolerances precluding non-linear characterization.

B. EQUAL PERCENTAGE:
“Non-Linear characterization in which a unit change in valve lift produces a flow change proportional to the quantity flowing before the change was made.”
When flow is small, change in flow is small, when flow is large, change in flow is large for equal changes in lift. Equal percentage trim is further open than linear would be at a given flow condition. This is particularly effective in low flow ranges where, for example the valve in 10% open at only 3.7% of its full capacity. This can alleviate plug slamming at low flows. Should be used when over sizing is necessitated by limited flow data, when only a small portion of the system drop can be taken across the valve, or when the valve pressure drop is high at low flows or vice-versa.

C. MODIFIED PERCENTAGE: Compromise characteristic falls between linear equal percentage. Usable in most applications.

D. SEMI THROTTLE (QUICK OPENING) : First quarter of lift results in linear change in flow, balance of lift produces maximum flow. Used in on-off (non-throttle) service where initial linear characteristic reduces hammer and shock effects.

E. LO-DB : A trim construction (rather than characterization) which is designed alleviate noise problems in gas flows and often has the dual ability to alleviate cavitation problems in liquids as well. This is generally accomplished by a multi-ported cage of multi-stepped plug design. LO-DB trim characteristic is usually linear and has a low flow coefficient, Cv, than standard trim of the same nominal size.

CONTROL VALVE SELECTION – (STEP – 3)

SELECT END CONNECTIONS

1. End Connection Type

2. End Connection Rating

SELECT END CONNECTION TYPE AND RATING END CONNECTION TYPE

A. Threaded : Available through 2″ usually ANSI 600lb. rated limited use in corrosion service

B. Welded : Most applicable to high pressure, high temperature hazardous fluid vibrating systems.

1. Socket Weld :  Available through 2″. Specify pipe schedule.

2. Butt Weld : Available above 2″. Specify pipe schedule.

C. Flanged : Most common due to easy installation/removal.

1. Raised Face: Available in all sizes, ANSI 150-2500lb rated generally used at higher pressure services.

2. Ring Joint: Available in all sizes, ANSI 150-2500 lb. Rated generally used at higher Pressure services.

D. Flangeless : Available primarily on rotary type valves in all sizes, ANSI 150 lb. rated, general service.

END CONNECTION RATING

A. Normally determined by existing system design specifications.

B. Can be determined directly from ANSI B 16.5 specifications per TDSN page 5 using pressure and temperature conditions.

CONTROL VALVE SELECTION – (STEP – 4)

SELECT SEAT LEAKAGE

1. ANSI B16 104 (FCI STD 70-2)

A. CLASS I
B. CLASS II
C. CLASS III
D. CLASS IV
E. CLASS V
F. CLASS V

SELECT SEAT LEAKAGE (FCI STD 70-2)

I. CLASS I

A. Leakage Rate – Comparable to class II, III or IV

B. Valve Type – Same as class II, III or IV.

II. CLASS II (DOUBLE SEAT LEAKAGE)

A. Leakage Rate – 0.5 of rated valve capacity.

B. Valve Type – Double seated globe, some balanced single seat globe, some metal lined butterfly.

III. CLASS III

A. Leakage Rate – 0.1% of rated valve capacity.

B. Valve Types – High quality double seated globe, balanced single seated globe with metal seals.

IV. CLASS IV. (SINGLE SEAT LEAKAGE OR TIGHT SHUT OFF)

A. Leakage Rate – 0.01% of rated valve capacity.

B. Valve Types – Single seated globe, balanced single seated globe with elastomer seals, rotary eccentric plug, rotary ball with metal seat.

V. CLASS V. (EXTRA TIGHT SHUT OFF)

A. Leakage Rate – 5 x 10 exp (-4) cc/Min. per inch of orifice diameter per psi differential pressure.

B. Valve Types – Class IV globe valves with increased actuator seating force.

VI CLASS VI (BUBBLE TIGHT OR DROP TIGHT)

LEAKAGE RATE

B. Valve types – Globe and rotary eccentric plug with soft seat, elastomer line butterfly, rotary ball with resilient seat.

Note: Lack of visible air bubbles using soap solution indicates leakage less to 1x10exp (-3) to 1x10exp(-4) cc/sec. Zero leakage less than 1×10 exp (-8) cc/sec.

CONTROL VALVE SELECTION – (STEP – 5)

SELECT ACTUATOR TYPE AND SIZE

1. ACTUATOR TYPE

A. SPRING DIAPHRAGM

B. PISTON/CYLINDER

C. ELECTRO HYDRAULIC

D. HANDWHEEL

E. ELECTRIC

2. CHECK EITHER P1 OR MAXIMUM PSI FOR SHUT OFF PRESSURE FOR ACTUATOR SIZE

ACTUATOR TYPE

A. Spring Diaphram : Pneumatic pressure acts on the diaphragm causing strain movement, an opposing spring provides returning forces.

Advantages :
Simple sturdy design, postive fail safe action and ability to operate without a positioner.

Disadvantages :
Slow response, hysteresis, and low power-to-size ration.

B. Piston/Cylinder : Pneumatic pressure is set on one side of the piston plate varied on the other to effect motion. 

Advantages:
Fast response, flexibility, and power-to-size ration.

Disadvantage : is positioner pre-requisite and lack of inherent safe action.

C. Electro – Hydraulic : Double acting servo valve loads and unloads hydraulic pressure from both sides of a piston plate. Positioning is accomplished with electronic signals.

D. Handwheel – Some times employed with Control Valve bodies for a precise manual control or when pneumatic actuator is to be added later.

ACTUATOR SIZE

A. Determine the maximum Delta P that could exist across the valve when closed, Delta P shut off. This is often assumed to be PI maximum to atmosphere.

B. Examine the valve pressure drop limitation tables for the given valve and trim size, air and flow directions and available supply pressure and select the actuator size (and spring range, if applicable)

Delta P shut off from part A.

Note : Other variables may be reflected in the tables such as ANSI class, whether fluid is gas or liquid, whether service is throttling or on-off, whether there is soft seating or extra tight shut off, and what type of trim is employed. Pressure drop limitation tables are generally included in the technical product bulletin for a given valve.

CONTROL VALVE SELECTION – (STEP – 6)

SELECT VALVE MATERIAL

1. VALVE BODY MATERIALS. (Ref. Table – 1)

2. TRIM MATERIALS (PLUG, SEAT STEM OR SHAFT) (Ref. Table – 2)

3. SEAT CONSTRUCTION AS DICTATED BY “STEP FOUR” SEAT LEAKAGE.

4. GUIDED BUSHING (BEARING) METERIALS. (Ref. Table – 3)

5. BUTTERFLY VALVES LINER MATERIALS. (Ref. Table – 4)

6. SOFT SEAT SEAL MATERIALS FOR CLASS VI LEAKAGE

VALVE BODY MATERIALS (CAST TYPE)

TABLE – 1

Note : Temperature ranges indicated are further restricted by:

1. Pressure temperature limitations of ANSI B16.5
2. Temperature Limitation of other valve components.
3. Manufacturer’s specified limitations.

11. VALVE TRIM MATERIALS (PLUG, SEAT, STEM OR SHAFT).

Note : In general Carbon Steel and 316 SS Bodies supplied with 316 SS trim; alloy bodies supplied with same alloy trim.

III. GUIDE BUSHING (BEARING)

Note : Guides are normally furnished in a standard material for a given valve model and should differ by a minimum 5 to 10 Rockwell hardness units from the stem or shaft material.

IV BUTTERFLY VALVE LINER MATERIALS

(CORROSIVE SERVICE AND TIGHT SHUT OFF)

Alternate designations: 
Buna -N (Nitrile) Nordel (EPT Rubber) Viton (Flurocarbon) TFE (Teflon).

Hot Liquids use 180 Degree F Maximum do not use with steam or ammonia.

V. SOFT SEAT SEAL MATERIALS (FOR CLASS IV LEAKAGE)

CONTROL VALVE SELECTION – (STEP – 7)

1.SELECT PACKING MATERIAL AND BONNET TYPE

A. STANDARD, TEFLON ASBESTOS.

B. TEFLON V – RING

C. GRAFOIL

2. BONNET TYPE

A. STANDARD BONNET

B. EXTENDED BONNET

C. BELLOWS SEAL EXTENSION

1. PACKING MATERIAL

A. TEFLON ASBESTOS : (Crane 285, JM 2012 MN)

LongAsbestos fibres coated with Teflon (285) or impregnated with Teflon suspensoid (2012) and braided. Standard packing on most valves. Used for all services except molten Alkali, Hot Hydrofluoric acid and oxygen. Typical temperature ratings:

Standard Bonnet : 20 / Deg. F to 450 Deg. F

Extended Bonnet : 320 Deg. F to 850 Deg. F

Standard Vapour Limit : 450 Deg. F

B. TEFLON V RING : (Crane 772SV, Chevron Rings)

Solid Teflon V-Ring construction. Popular alternate to Teflon Asbestos. Used in most alkalies and acids except fluorine and chlorine gas at elevated temperatures and molten materials. Typical temperature ratings.:

Standard Bonnet : 20 Deg.F to 450 Deg.F

Extended Bonnet : 320 Deg.F to 850 Deg.F

Extended Vapour Limit : 450 Deg. F

C. GRAFOIL (All – Graphite) 

High temperature packing for all services except strong Oxidizers. Typical temperature ratings.:

Standard Bonnet : -20 Deg. F to 750 Deg. F

Extended Bonnet : -20 Deg. F to 1050 Deg. F

D. OTHER CONSIDERATIONS

1. Lubricator and Isolator valve – essentially obsoleted by modern, self – lubricating packing.

2. In high temperature service, standard bonnet with grafoil is less expensive than extended bonnet with Teflon Asbestos.

2. BONNET TYPE

A. STANDARD BONNET: Normally used with Teflon – Asbestos packing for temperature from 20 Deg. F to between 450 Deg. F and 750 Deg. F depending on valve type used with Grafoil packing or higher temperature upto between 750 Deg. And Deg 450 Deg. F depending on valve type.

B. EXTENDED BONNET : Normally used for extreme temperatures such as dov 320 Deg. F with Teflon – Asbestos packing or upto 1050 Deg. F with Grafoil pack.

C. BELLOWS SEAL EXTENSION : Used in services of toxic, flammable, explosive radio active nature or with costly fluids or when fluid destroys regural packing consists of a flexible metal bellows which encapsulates the plug stem to seal off process fluid. The bellows is usually 300 series stainless steel or Inconel and has limited life.

CONTROL VALVE SELECTION – (STEP –

SELECT VALVE ACCESSORIES

1. POSITIONER : Valve mounted; normally required by piston and double acting Cylinder actuators, by diaphragm actuators with spring ranges that do not match controller or signal ranges, or by applications require precise valve positioning or fact valve responding. Some positioners contain a characterizing element giving selection of input signal plug position relationships.

A. Pneumatic – Most common type; usually operates on a 3-15 psi input signal (9, 3-27, 6-30, 9-15 also used)

B. Electro Pneumatic – Operators on a 4-20 Ma or a 10-50 Ma Input signal.

2. TRANSDUCER (ELECTRO PNEUMATIC) : Mounted on or off the valve; use convert a 4-20 MA or 10-50 MA electrical input signal to either a 3-15 psi or a 6-30 pneumatic output signal. The pneumatic output signal is used either to directly act the valve or is used as a positioner input signal.

3. CONTROLLER : Device whose input is indicative of the valve of process various and whose output initiates valve position changes to maintain the set point value of process variable.

A. Pressure Controller     – Mounted on or off valve.

B Temperature Controller – Mounted on or off valve.

C. Liquid Level Controller – Mounted on liquid containing vessel.

4. SOLENOID VALVE : Valve mounted; commonly three way type valve used in service when solenoid is energized, air supply is applied directly to the actuator complete opening or closing the valve; when de-energized, actuator pressure is vented to atmosphere allowing the actuator spring to fully stroke the valve to the opposite position.

5. VOLUME BOOSTER : Valve mounted; pneumatic device whose output volume is greater than its input signal thus increasing valve stroking speed.

6. AIR SET : Valve mounted; supply air pressure reducing regulator (with internal filter and relief valve) used in conjunction with any of the above air consuming devices.

7. LOCK UP VALVE : Valve mounted; pneumatic relay which locks in the actuator pressure during loss of plant air supply causing the valve to fail-in-last-position.

8. TRANSFER VALVE : Valve mounted; pneumatic relay used in conjunction with a volume tank to transfer volume tank pressure to a springle actuator to ensure proper valve failure during loss of plant air.

9. VOLUME TANK : Mounted on or off the valve; capacity tanks or accumulators used to store air pressure for transfer to a springless actuator to insure proper valve failure during loss of plant air.

10. LIMIT SWITCH : valve mounted; mechanical device which trips electrical switches at set positions of value stroke.

11. TRAVEL STOP : Mounted in the actuator; mechanical stops which limit valve travel, generally to prevent to prevent full closure.

12. BOLT SETS : Mounted across valve body; provided for thorough bolting on fangeless valves (limited to 750 Deg. F operations) 

13. LO-DB PLATES AND CARTRIDGES : Mounted downstream of valve multistage, multiport device which effectively reduce valve noise up to 20 db A by increasing the valve outlet pressure.

14. HAND WHEELS : Valves mounted; mechanical devices to allow manual over ride of valve positioning, sometimes used as a built-in travel stop