The Armstrong OB-3 series (models OB-30 and OB-31) is a line of direct-acting, self-operated temperature regulators designed for automatic temperature control of steam, water, and non-corrosive gases in industrial heating and cooling systems. Unlike electronic controllers that require power supplies, PLCs, and sensor wiring, the OB-3 series operates entirely on thermodynamic principles: an interchangeable fluid-filled capillary sensor detects the temperature of the process fluid, vaporizes internally, and directly actuates a valve stem to modulate flow—no external electricity, compressed air, or control signal required. Industry terminology calls this a "direct-acting temperature regulator" or "self-actuated temperature control valve," and it has served as the maintenance-free backbone of steam and water temperature control for over a century.

Self-Operated — No Electricity, No Compressed Air

• Thermodynamic Actuation: A fluid-filled sensing bulb and capillary tube form a sealed thermodynamic system. As the process temperature rises, the fill fluid vaporizes and expands, generating internal pressure that drives the valve toward the closed position (OB-30 heating) or open position (OB-31 cooling). When the temperature falls, the fluid condenses, pressure drops, and the valve returns to its original state. This cycle repeats automatically, maintaining a relatively constant process temperature without any external energy source or operator intervention.
• Why This Matters: In steam tracing, tank heating, and jacket water cooling applications, electrical wiring is often impractical or prohibitively expensive. Solenoid valves with external controllers require weatherproof enclosures, conduit runs, and certified electricians for initial setup. The OB-3 series installs like a standard pipe fitting—thread it in, mount the capillary bulb, and the system is ready. No spark risk, no EMI susceptibility, no control panel to program or maintain.
• Heating/Cooling Selection: OB-30 is configured for heating duty (valve closes on temperature rise, opens on temperature fall), while OB-31 is configured for cooling duty (valve opens on temperature rise, closes on temperature fall). Both share the same bronze body, connection sizes, capillary lengths, and temperature ranges—only the internal valve seat orientation differs.

Construction & Maintenance — Designed for Decades of Service

• Packless Stem Design: The valve stem requires no packing gland. The single-composition seat performs the dual role of throttling element and seal, eliminating the most common leak path in conventional control valves—eroded or dried-out stem packing. No gland follower adjustments, no periodic repacking, no fugitive steam emission from the bonnet area. In steam services, this directly reduces energy loss, maintenance labor hours, and safety incidents from hot condensate spray.
• Bronze Body (ASTM B584): Suitable for water, steam, and non-corrosive gases. The bronze construction provides excellent corrosion resistance in humid plant environments and is fully compatible with standard copper, steel, or stainless steel piping systems via NPT or BSPT threaded connections.
• Interchangeable Capillaries: The sensing bulb connects to the valve body via a union nut—loosen the nut, and the entire capillary and bulb assembly can be removed without taking the valve off the pipe. This means capillary damage or a change in immersion depth can be serviced in minutes. Three standard lengths (approximately 2 m, 3 m, 5 m) accommodate a range of tank heights and pipe diameters.
• Single Composition Seat: Provides tight shutoff with no auxiliary power. The PTFE valve disc is carried directly by the valve stem so that when the bellows pressure equalizes, the spring-loaded stem presses the disc firmly into the metal seat, achieving positive isolation.

Five Temperature Ranges — One Valve Family

• 0–35 °C: Cooling water service, chiller bypass, jacket water temperature maintenance
• 25–70 °C: Domestic hot water, low-temperature process heating, freeze protection
• 40–110 °C: General-purpose steam heating, tank heating, heat exchanger temperature control
• 60–130 °C: High-temperature water heating, clean steam applications, industrial process heating
• 70–150 °C: Saturated steam service up to 10 bar, oil heating, reactor vessel temperature control
• The range is determined by the fill fluid inside the capillary sensor and cannot be changed in the field. Select the range that matches your target operating temperature when ordering. Maximum allowable temperature for all models is 185 °C (366 °F).

Pressure/Temperature Boundaries & Typical Applications

• OB-30 (Heating): Maximum allowable pressure 150 psig (10 bar). Used on steam tracing lines, shell-and-tube heat exchangers, tank coil heating, and process hot water loops where the regulated fluid is kept at a constant elevated temperature.
• OB-31 (Cooling): Maximum allowable pressure 250 psig (17 bar). Used on cooling water returns, compressor jacket cooling, hydraulic oil cooler bypass circuits, and non-corrosive gas coolers where the valve opens to increase cooling flow when temperature rises.
• Steam Trap Integration: On OB-30 steam applications, a steam trap must be installed on the condensate side to prevent water hammer and ensure consistent heat transfer. The regulator is installed upstream of the heat exchanger or heating coil; the steam trap handles condensate removal downstream.
• Installation Notes: The sensing bulb must be fully immersed in the process fluid for accurate temperature measurement. Allow 100–200 mm of straight pipe on either side of the valve body for best flow characteristics. An isolation valve and bypass valve should be installed around the regulator to permit servicing without draining the entire system.