How to Order Control Valves
Control valves are the final control elements in a process control loop. Unlike isolation valves that are either fully open or fully closed, control valves modulate continuously to regulate flow, pressure, temperature, or level. They receive a signal from a controller (typically 4-20 mA or digital) and position the valve trim to deliver the required process variable.
The governing standards are IEC 60534 (industrial process control valve terminology, sizing, testing, and noise) and ISA S75 (flow equations, sizing procedures, and face-to-face dimensions). Control valves are ordered with a process data sheet that provides the operating conditions needed for sizing; the valve manufacturer uses this data to calculate the required Cv and select the valve, trim, and actuator.
Control Valve Types
| Type | Configuration | Typical Application |
|---|---|---|
| Globe, single seat | Single plug, cage or post guided | General flow control, small-to-medium Cv, tight shutoff (Class IV–V per ANSI/FCI 70-2) |
| Globe, double seat | Two plugs, balanced design | High Cv requirements, reduced actuator thrust, where Class II–III shutoff is acceptable |
| Globe, cage guided | Plug guided by a cage with characterized openings | Noise attenuation, anti-cavitation (multi-stage cage), precise characteristic shaping |
| Globe, angle | Inlet and outlet at 90 degrees | Flashing, high-pressure-drop, erosive, or slurry service |
| Rotary, ball | Segmented or full-bore ball | High Cv, large flow, slurry, fibre-containing media |
| Rotary, butterfly | Disc with offset shaft | Large-diameter moderate-pressure control, HVAC, water treatment |
| Rotary, eccentric disc/plug | Eccentric rotating plug or disc | Dirty service, on/off with limited throttling |
Core Attributes
Every control valve line item in an RFQ or purchase order must include all of the following attributes. A process data sheet (per ISA S20 or IEC 60534-7) should accompany the order.
| # | Attribute | Values / Options | Notes |
|---|---|---|---|
| 1 | Design standard | IEC 60534, ISA S75 | IEC 60534 covers sizing, testing, noise prediction, and terminology; ISA S75 covers flow equations and face-to-face dimensions |
| 2 | Control valve type | Globe (single seat, double seat, cage guided, angle), rotary (ball, butterfly, eccentric disc/plug) | Globe cage-guided is the most common for general process control |
| 3 | Size | NPS (line size and valve size) | Line size and valve size may differ when reduced trim is used; specify both (e.g., “NPS 4 valve in NPS 6 line” or “NPS 4 x 6 reducers by others”) |
| 4 | Pressure class | 150, 300, 600, 900, 1500, 2500 | Must match the piping class; Class 300+ is common in high-pressure hydrocarbon and steam service |
| 5 | End connection | Flanged RF, flanged RTJ, butt weld (BW), socket weld (SW), threaded (THD) | RF is standard; RTJ for Class 900+ and critical hydrocarbon service; BW/SW for small-bore high-integrity connections |
| 6 | Body material | ASTM A216 WCB, A351 CF8M, A217 WC6/WC9, Hastelloy C276, Monel | WCB for general carbon steel service; CF8M for stainless; WC6/WC9 for high-temperature chrome-moly; exotic alloys for severe corrosion |
| 7 | Trim materials | Plug/ball, seat, cage; specify each separately | SS 316, Stellite 6 (hard facing), tungsten carbide, ceramic, 17-4PH; trim material drives shutoff class, erosion resistance, and cost |
| 8 | Flow characteristic | Linear, equal percentage, quick opening | Equal percentage is the default for most process loops; linear for liquid level control; quick opening for on/off safety applications |
| 9 | Cv (or Kv) | Calculated from process data sheet | Cv is the flow coefficient at design conditions; the manufacturer sizes the valve to deliver the required Cv across the specified operating range (min, normal, max) |
| 10 | Rangeability | Typically 50:1 for globe, 100:1 for rotary | Rangeability defines the ratio of maximum to minimum controllable Cv; ensure it covers the required turndown |
| 11 | Actuator type | Pneumatic diaphragm, pneumatic piston, electric, electro-hydraulic | Pneumatic diaphragm is the most common for process control; electric for remote locations without instrument air; electro-hydraulic for very large or high-thrust applications |
| 12 | Fail position | Fail-open (FO), fail-close (FC), fail-last position (FL) | Defined by the process safety analysis (HAZOP); FC is most common for hydrocarbon isolation on loss of signal or air |
Optional and Special Attributes
| Attribute | When Required | Details |
|---|---|---|
| Positioner | Virtually always for control duty | Smart positioner (HART, Foundation Fieldbus, Profibus) for diagnostics and precise positioning; conventional I/P positioner where digital communication is not needed |
| Handwheel | Manual override requirement | Top-mounted or side-mounted; allows manual valve positioning when the actuator is de-energized or during maintenance |
| Limit switches | Remote position indication (open/closed) | Mechanical or proximity type; specify voltage, contact rating, and explosion-proof classification |
| Solenoid valve | Emergency shutdown (ESD), safety instrumented system (SIS) | Mounted on actuator; de-energizes to vent actuator pressure and drive valve to fail position; specify ATEX/IECEx rating |
| I/P converter | Converting electronic signal to pneumatic | Converts 4-20 mA signal to 3-15 psi (or 0.2-1.0 bar) pneumatic output; often built into smart positioners |
| Noise attenuation trim | High pressure-drop gas service (noise > 85 dBA) | Multi-stage, multi-path cage or trim that reduces velocity and noise at the vena contracta; specify maximum allowable noise level |
| Anti-cavitation trim | Liquid service with high pressure drop (P2 approaches Pv) | Multi-stage pressure reduction cage or characterized trim that maintains pressure above vapor pressure; prevents cavitation damage |
| Bellows seal | Toxic, lethal, or fugitive-emission-critical service | Metal bellows replaces conventional packing to provide zero stem leakage; specify bellows material (Inconel 625, SS 316L) and cycle life |
| Extended bonnet | Cryogenic service (below -46 deg C) or high-temperature service (above 260 deg C) | Provides thermal separation between the process and the packing; specify cryogenic or high-temperature extension length |
| SIL rating (IEC 61508 / 61511) | Safety instrumented function (SIF) | Specify SIL 1, SIL 2, or SIL 3; manufacturer must provide SIL capability certificate with PFD (probability of failure on demand) and SFF (safe failure fraction) data |
| NACE MR0175 / ISO 15156 | Sour service (H2S-containing environments) | Restricts body, trim, fastener, and actuator spring materials to NACE-compliant grades |
| Fugitive emission (ISO 15848) | Environmental regulation compliance | Specifies packing leakage class; usually Class A (tightest) for refineries and gas plants |
| Certificates | Almost always required | EN 10204 Type 3.1 (MTR), Cv test report, noise calculation per IEC 60534-8-3, actuator sizing calculation; Type 3.2 for critical services |
Example Line Items
Correct
2 pcs - Globe control valve, IEC 60534, NPS 4 x Class 300, FL-RF, A351 CF8M body, Stellite 6 plug & seat, cage guided, equal percentage characteristic, Cv 120, pneumatic diaphragm actuator, FC, HART smart positioner, MTR EN 10204 3.1
This line item defines: quantity, valve type, design standard, size, pressure class, end connection and face, body material, trim materials, guiding method, flow characteristic, required Cv, actuator type, fail position, positioner type and protocol, and certificate requirement.
Incorrect
2 - Control valve 4 inch with actuator
This is missing: the design standard, valve sub-type (globe vs rotary), pressure class, end connection, body material, trim materials, flow characteristic, Cv, actuator type and sizing, fail position, positioner, and certification. Without process data and these specifications, the manufacturer cannot size, select, or quote the valve.
Common Pitfalls
| Pitfall | Why It Matters |
|---|---|
| Not providing a process data sheet | Control valves are sized from process conditions (flow rate, inlet/outlet pressure, temperature, fluid properties). Without a data sheet, the manufacturer cannot calculate Cv, predict noise, or check for cavitation/flashing. A control valve ordered without process data is almost certainly wrong |
| Specifying line size instead of valve size | Control valves are frequently one or two sizes smaller than the line size (reduced trim) to operate in an efficient Cv range. Ordering “NPS 6” when the required Cv calls for an NPS 4 valve results in oversizing, poor controllability, and hunting |
| Omitting the fail position | The fail position (FO, FC, or FL) is determined by the process safety analysis. If not specified, the manufacturer chooses a default that may be the opposite of what the HAZOP requires, creating a safety hazard |
| Confusing Cv with Kv | Cv (US gallons/min) and Kv (cubic meters/hour) are related by a factor of 1.156 (Cv = 1.156 x Kv). Using the wrong coefficient results in a valve sized approximately 16% too large or too small |
| Ordering a positioner without specifying the communication protocol | HART, Foundation Fieldbus, and Profibus PA are not interchangeable. The positioner protocol must match the DCS/PLC input type. Shipping a HART positioner to a Fieldbus plant means the valve cannot be commissioned |
| Ignoring noise and cavitation at the sizing stage | High-pressure-drop gas applications can exceed 110 dBA without attenuating trim. Liquid applications with ΔP approaching the fluid vapor pressure will cavitate, destroying the trim and body within weeks. These conditions must be identified during sizing and addressed with the appropriate trim selection |
| Not specifying SIL rating for safety-critical loops | If the control valve serves a safety instrumented function (SIF), it must be SIL-rated per IEC 61508/61511. A non-SIL-rated valve installed in a SIF loop violates the safety integrity level requirement and will be flagged during SIS validation |
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