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Technical Specification and Selection Guide of Control Valve
Created on 10.21
I. Applicable Standards
Standard Type | Standard No. and Title |
International Standard | ASME B16.34 Valves — Flanged, Threaded, and Welding End |
International Standard | ASME B16.104 Control Valve Seat Leakage |
International Standard | IEC 60534-2-4, IEC 60534-3-1 — Control valve flow capacity and face-to-face dimensions |
Domestic Standard | GB/T 4213 Pneumatic Control Valves |
Test Standard | API 598, ISO 5208 — Valve inspection and pressure testing |
Reference Standard | GB/T 17213.2 Industrial-process control valves — Flow capacity — Sizing equations for fluid flow under installed conditions |
Note: JB/T 5296-1991 Test Methods for Flow Coefficient of General Valves was withdrawn in 2017 and is no longer listed.
II. Structural Principle
The eccentric rotary control valve consists of an eccentric rotary valve body throttling assembly and a pneumatic actuator.
Working Principle: The spherical plug and the valve shaft are designed with an eccentricity. When the valve shaft rotates, the plug moves eccentrically along a cam trajectory. When the valve closes, the flexible arm of the plug undergoes elastic deformation and tightly contacts the seat to form a seal. The 50° angular travel provides opening and closing with inherent shearing and anti-jamming capability, making it suitable for fibrous, particulate, and high-viscosity media. It is therefore also called a camflex valve.
Actuator Selection
Model | Structural Type | Applicable Service |
PDR | Spring-return rolling diaphragm type | Conventional service, fail-safe applications |
PCR | Double-acting piston type | Large diameter, high differential pressure, severe service |
Core Design Advantages
Design Feature | Technical Advantage |
Straight-through body flow passage | Low flow resistance, high flow capacity |
50° angular travel | Fast open/close response, good control linearity |
Integral body and bonnet | Reduced external leakage points |
Involute spline | Good centering, high torque transmission capability |
Symmetrical window design | Air-to-open / air-to-close freely interchangeable on site |
Eccentric shearing structure | Anti-jamming by particles, suitable for viscous media |
Compact and lightweight | Can be installed at any angle, easy maintenance |
III. Three Sealing Structures and Allowable Pressure Drop
1. Sealing Structure Definitions
Sealing Type | Structural Definition | Abbreviation |
TS | O-ring embedded in seat, soft-seated structure | Soft Seal |
SS | Local Stellite hardfacing overlay on plug and seat | Partial Hard Seal |
SF | Full Stellite hardfacing overlay on plug and seat | Full Hard Seal |
As described above, TS indicates an O-ring clamped in the seat; SS indicates Stellite hardfacing applied to the sealing areas of the plug and seat (partial overlay); SF indicates full Stellite hardfacing on both plug and seat (full overlay).
2. Performance Comparison
Sealing Type | Leakage Class | Applicable Temperature | Wear and Erosion Resistance | Applicable Media / Scenarios | Advantages and Disadvantages |
TS Soft Seal | ASME B16.104 Class VI (bubble-tight zero leakage) | -45 to 230°C | Moderate; not resistant to particle erosion | Clean gas, clean water, non-corrosive clean media | Best sealing performance, low cost; low temperature limit, prone to aging |
SS Partial Hard Seal | ASME B16.104 Class IV | -195 to 400°C | Moderate; resistant to slight particulate | Ordinary oil products, steam, mildly corrosive media | Good cost-effectiveness; balanced sealing and wear resistance |
SF Full Hard Seal | ASME B16.104 Class IV | -195 to 400°C | Excellent; resistant to erosion, crystallization, and particles | Media containing fibers, particles, high viscosity, high temperature, severe corrosion | Longest service life, widest service adaptability; relatively high cost |
Leakage Class Notes: Class VI is bubble-tight sealing with extremely low leakage. Class IV is the normal achievable level for metal-seated valves; the leakage is 0.01% of rated Cv and is not zero leakage. Metal hard seals are not intended to achieve bubble-tight sealing; Class IV is a reasonable expectation for metal-to-metal sealing.
3. Allowable Pressure Drop Rules
Sealing Type | Differential Pressure Capability | Air Supply / Actuator Selection Constraints |
TS Soft Seal | Lowest; more than 30% lower than hard seal at the same size | Suitable for conventional PDR spring actuators; high differential pressure and particulate service strictly prohibited |
SS Hard Seal | Moderate; sufficient differential pressure margin | PDR or PCR both acceptable; general medium-high differential pressure service |
SF Full Hard Seal | Highest; best differential pressure resistance | Large diameter preferably with PCR piston actuator; first choice for high differential pressure and erosion service |
Key Constraints:
- Actual shutoff differential pressure must not exceed the manufacturer's catalog table value.
- Inlet pressure must not exceed the body ANSI class temperature-pressure rating.
- Allowable differential pressure for gas media is greater than that for liquid media.
IV. Main Technical Performance Parameters
Item | Technical Specification |
Size Range | 1″ to 12″ (DN25 to DN300) |
Pressure Class | ANSI 150 / 300 / 600 LB |
Applicable Temperature | Depends on sealing type: TS soft seal -45 to 230°C; SS/SF hard seal -195 to 400°C |
Basic Error (with positioner) | ±2% (reference GB/T 4213; refer to manufacturer's catalog for details) |
Basic Error (without positioner) | ±5% |
Hysteresis (with positioner) | 1% |
Hysteresis (without positioner) | 3% |
Dead Band (with positioner) | 1% |
Dead Band (without positioner) | 5% |
Rangeability | 100:1 (refer to specific manufacturer's catalog; some models 50:1) |
Inherent Flow Characteristic | Linear |
Connection Type | Wafer type (recommended), flanged type |
Coating | Carbon steel: PLA9006 silver-white; stainless steel: unpainted |
V. Design Calculation
1. Rated Cv Flow Coefficient Table
Size (in) | Full Cv | 60% Stroke Cv | 40% Stroke Cv | 20% Stroke Cv (Metal Seat Only) |
1 | 14 | 8.4 | 5.6 | 2.8 |
1-1/2 | 30 | 18 | 12 | 6.0 |
2 | 50 | 30 | 20 | 10 |
3 | 135 | 81 | 54 | — |
4 | 230 | 138 | 92 | — |
6 | 500 | 300 | 200 | — |
8 | 850 | 510 | 340 | — |
10 | 1300 | 780 | 520 | — |
12 | 1750 | 1050 | 700 | — |
The Cv values in the table are typical product data. Products of the same size from different manufacturers may differ; the manufacturer's catalog shall govern during selection.
2. Service Cv Calculation
Liquid Media (non-choked flow, no flashing/cavitation):
Where:
Q = volumetric flow rate (m³/h)
ρ = relative density (relative to water, dimensionless)
ΔP = pressure drop across the valve (bar)
Applicable Premise: This simplified formula is applicable to non-choked flow with Newtonian fluids. If flashing or cavitation exists, the pressure recovery factor FLFL and critical pressure drop must be introduced for correction in accordance with IEC 60534-2-1 (GB/T 17213.2); this formula cannot be applied directly.
Gas Media: Calculate according to the dedicated formulas for low-pressure or high-pressure compressed gas, with correction for operating temperature and pressure. For the discrimination between low-pressure gas (ΔP<0.5P1ΔP<0.5P1) and choked flow, as well as the values of expansion factor YY and pressure drop ratio factor XTXT, refer to IEC 60534-2-1 or GB/T 17213.2.
3. Cv Selection Principles
Selection Range | Service Suitability |
40%–60% stroke | Optimal control range; good linearity, no oscillation |
< 10% | Small opening prone to oscillation and inaccurate control; selection must not fall within this range |
> 80% | Large opening with flow saturation; insufficient load control margin |
4. Allowable Pressure Drop Verification
Verification Dimension | Verification Content |
Structural Type | Metal seat / soft seat; carbon steel / stainless steel body |
Actuator | PDR spring-return / PCR double-acting piston |
Service Conditions | Air supply pressure, flow-to-open / flow-to-close, gas / liquid medium |
Boundary Limits | Actual differential pressure ≤ manufacturer's catalog table value; inlet pressure not exceeding body rated pressure |
5. Valve Body Material Temperature-Pressure Applicability
Body Material | Applicable Temperature | Applicable Pressure Class | Notes |
WCB Carbon Steel | -29 to 425°C | 150 / 300 / 600 LB | Above 425°C prone to graphitization |
CF8 (304) | -195 to 400°C | All classes | Mild corrosion, low-temperature service |
CF8M (316) | -195 to 400°C | All classes | Acid/alkali, severe corrosion service |
6. Packing Temperature and Pressure Limits
Packing Type | Applicable Temperature Range | Applicable Service |
Reinforced PTFE | -45 to 230°C | Ambient temperature, no high-temperature service |
Flexible Graphite | -196 to 538°C | High/low temperature, steam, corrosive service |
VI. Standard Material Combinations
Body Material | Plug / Seat Configuration | Stem Material | Applicable Service |
WCB | 304SS / SS / SF / TS | 304 nitrided | Water, steam, ordinary oil products |
CF8 | CF8M / SS / SF | 316SS / 17-4PH | Mild corrosion, low-temperature media |
CF8M | CF8M / SS / SF / TS | 316SS | Acid/alkali, severe corrosion, high temperature |
VII. Common Faults, Causes, and Remedies
Fault Phenomenon | Fault Cause | Remedial Measure |
Poor control accuracy, large flow fluctuation | Positioner improperly calibrated; air supply pressure fluctuation; long-term operation at small opening; medium impurities causing jamming | Recalibrate zero and span; install air supply pressure regulator; recalculate Cv and reselect; disassemble and clean foreign matter from plug |
Excessive internal leakage | TS seal ring aging; SS/SF sealing surface erosion and wear; plug flexible arm fatigue; debris stuck on sealing surface | Replace O-ring soft seal; lap or upgrade to full hard seal SF; replace plug assembly; install upstream filter |
Slow action, sticking | Packing over-tightened; stem corrosion; connecting rod coaxiality deviation; actuator diaphragm/piston wear | Moderately loosen packing gland; polish stem and apply anti-corrosion treatment; align transmission linkage; replace actuator seals and diaphragm |
Actuator air leakage, loss of position | Diaphragm aging or damaged; joint seal failure; spring fatigue; water/oil contamination in air supply | Replace diaphragm and gaskets; reseal piping connections; replace spring assembly; install air supply dryer/filter |
Air-to-open / air-to-close switching failure | Incorrect assembly of body symmetrical windows; directional deviation of rocker arm linkage | Reassemble linkage according to OEM structure; re-switch air-to-open / air-to-close mode on site |
High operating noise | Excessive differential pressure; flow velocity over limit; throttling vortex | Use multi-stage pressure reduction; appropriately increase valve size; optimize piping flow pattern |
VIII. Routine Maintenance and Servicing Schedule
Maintenance Interval | Maintenance Content |
Daily Inspection | Check air supply pressure, valve stroke, external leakage, and abnormal operating noise |
Every Six Months | Positioner calibration, air supply filter cleaning, packing tightening, bolt re-tightening |
Annual Maintenance | Disassembly inspection of plug and seat wear, actuator seal replacement, full-stroke calibration |
Spare Parts Stocking | O-rings, packing, diaphragm, positioner filter element, seat seal assembly |
Warranty Period | 18 months after shipment or 12 months after installation, whichever comes first; free repair or replacement for manufacturing defects |
IX. Summary of Selection and Application Points
The core selection logic for eccentric rotary control valves can be summarized in three points:
First, select the sealing type according to the medium.
- For clean media requiring bubble-tight sealing, select
TS soft seal.
- For ordinary oil products, steam, and media with slight particulate, select
SS partial hard seal.
- For media containing fibers, particles, high viscosity, high temperature, and severe corrosion, select
SF full hard seal.
Second, select the actuator according to differential pressure.
- Soft seals have the lowest differential pressure capability and are strictly prohibited for high differential pressure service.
- SF full hard seals have the strongest differential pressure resistance; for large diameter and high differential pressure, prioritize
PCR double-acting piston actuators.
Third, determine the material according to service conditions.
- WCB
is suitable for water, steam, and ordinary oil products.
- CF8
is suitable for mild corrosion and low-temperature service.
- CF8M
is suitable for acid/alkali, severe corrosion, and high-temperature service.
- For packing,
Ambient temperature applications use reinforced PTFE, while high/low temperature, steam, and corrosive services use flexible graphite.
In field operation, the most critical concern is long-term operation at small opening—this not only results in poor control accuracy but also accelerates erosion of the plug and seat. If the valve is found to operate below 10% opening for extended periods, it indicates that the Cv is oversized and the selection should be recalculated. In addition, if there is a risk of flashing or cavitation in liquid media, the simplified formula cannot be used directly for Cv calculation; choked flow verification must be performed in accordance with IEC 60534-2-1.
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