Globe Control Valve — WCB Body NPT Thread with Pneumatic Diaphragm Actuator and 4–20 mA Positioner

The WCB body globe control valve with NPT thread is the lowest installation cost proportional control solution for flow, temperature, and pressure control loops in sizes ½" to 2" where a threaded connection is the installation standard. The WCB carbon steel body provides the necessary mechanical strength for steam, hot water, gas, and general process fluid services, while the SS304 stainless steel trim ensures corrosion resistance on surfaces in contact with the fluid.

The assembly includes a WCB globe body with NPT thread, a pneumatic diaphragm actuator with spring return, an electropneumatic positioner with a 4–20 mA signal input, and graphite packing in the stem — assembled, calibrated, and tested at the Cematic workshop before shipping.

When to specify NPT thread and not ANSI flange

The NPT threaded control valve is the correct choice in the following cases:

• Sizes from ½" to 2" where the existing or new design installation uses NPT thread as the connection standard — the threaded valve installs directly without the need for additional flanges, bolts, or gaskets
• Installations with limited axial space where the shorter face-to-face length of the threaded valve is an advantage over the flanged version
• Projects where installation cost is a relevant factor — the threaded connection eliminates the cost of flanges, bolts, and gaskets required in the ANSI version
• Instrumentation and auxiliary services installations where NPT thread is the established standard

For sizes larger than 2", for higher pressure services, or when frequent valve disassembly is a requirement, the ANSI 150 flanged control valve → is the correct choice.

Construction Materials

• Body: WCB Carbon Steel — high mechanical strength for steam, hot water, gas, and general process services. Nominal pressure up to ANSI 150 (19.6 bar at ambient temperature)
• Trim (plug and seat): SS304 Stainless Steel — corrosion resistance on control surfaces in contact with the fluid. SS304 trim covers the most frequent services for water, steam, gas, and neutral or slightly corrosive process fluids
• Stem Packing: Expanded Graphite — resistance to high temperatures and aggressive process fluids. Graphite is the first choice for control valves in steam service and high-temperature processes where PTFE is not sufficient
• Connection: NPT (ANSI B1.20.1) Tapered Thread — industrial threaded standard in Mexico
• Actuator: Pneumatic Diaphragm and Spring — linear movement proportional to the positioner's signal. Normally Closed (NC) configuration — the spring closes the valve upon loss of air supply or signal
• Positioner: Electropneumatic with 4–20 mA input — converts the analog signal from the controller into signal pressure to the actuator's diaphragm to precisely position the plug at any point in its stroke

Technical Specifications

• Type: Straight-through globe, linear movement
• Connection: NPT (ANSI B1.20.1) tapered thread
• Body Material: WCB carbon steel (ASTM A216)
• Trim: SS304 stainless steel
• Stem Packing: Expanded graphite
• Nominal Sizes: ½" (DN15), ¾" (DN20), 1" (DN25), 1½" (DN40), 2" (DN50)
• Nominal Pressure: ANSI 150 — up to 19.6 bar at ambient temperature; reduced at higher temperatures
• Operating Temperature: Up to +425 °C with graphite packing (depending on pressure and size)
• Flow Characteristic: Standard equal percentage — most common in temperature and pressure loops
• Positioner Input Signal: 4–20 mA
• Actuator Action: Normally Closed (NC) — fail-close. Air opens, spring closes.
• Air Supply Pressure: 1.4–4 bar to the positioner
• Design Standard: IEC 60534

Flow Coefficient Cv per size

The Cv is the flow rate in gallons per minute (GPM) of water at 60 °F that passes through the valve with a pressure drop of 1 psi in the fully open position. The selected size must provide a normal operating Cv between 20% and 80% of the maximum Cv to ensure control stability throughout the range.

Need to calculate the required Cv for your process? Share the fluid, operating flow rate, upstream and downstream pressure — our technical team will size the correct diameter at no cost. Consult via WhatsApp →

WCB with SS304 trim — chemical compatibility

• ✅ Steam — the most frequent service for this valve in Mexican industry
• ✅ Hot water up to 200 °C
• ✅ Natural gas, LPG, and dry industrial gases
• ✅ Neutral to slightly alkaline process water
• ✅ Mineral oils and lubricants at moderate temperatures
• ✅ Steam condensate
• ⚠️ Water with high chlorides (>500 ppm): SS304 trim may show pitting — specify SS316 trim
• ⚠️ Diluted acids: verify concentration and temperature with our technical team
• ❌ Concentrated acids, HF, seawater: use control valve with SS316 body or special materials

Fail-safe NC — why it is the standard configuration

This valve has a Normally Closed (NC) configuration as standard — the actuator spring closes the valve upon any loss of compressed air supply or the 4–20 mA signal from the controller. This is the correct safety configuration for most control loops where fluid cutoff is the safe state in the event of a system failure:

• Heating steam to heat exchangers — failure closes to prevent process over-temperature
• Reagent dosing — failure closes to prevent over-dosing
• Feed to reactors — failure closes to stop feeding until control is restored

For services where continuous flow is the safe state (cooling water, lubrication), specify Normally Open (NO) configuration — available upon request.

Main applications

• Steam control in process equipment: Regulation of heating steam flow in heat exchangers, kettles, process reactors, and autoclaves. The graphite packing and WCB body cover the pressure and temperature range of low and medium-pressure steam. This is the highest demand application for this valve in Mexico.

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Technical Data Sheet