Samson Type 3766 Pneumatic Valve Positioner (3-15 psi)

Type 3766 pneumatic positioner for control valves

The Samson Type 3766 is a pneumatic positioner that assigns the position of a control valve to a pneumatic control signal of 0.2 to 1 bar (3 to 15 psi). It is a mechanical, force-balance device with no electronics, which makes it a durable choice for plants that still run a pneumatic control loop or that value simplicity. Seven Star LLC supplies the Type 3766 in Oman and the GCC as an independent supplier based in Muscat.

It can be used on single-acting or double-acting actuators, handles rated travels from 7.5 to 120 mm or opening angles up to 90 degrees, and is designed to work in any mounting position. The Type 3767 on the same data sheet is the electropneumatic version with an i/p converter in front of the same pneumatic stage.

Reading the Type 3766 article code

The Type 3766 code is a string of digits after the type number. The options printed on the data sheet are summarised here.

Type 3766 technical specifications

How the Type 3766 balances forces to position a valve

The valve travel is carried to a pick-up lever that sets the force of a range spring. This is compared with the force created by the input pressure acting on a measuring diaphragm. If the set point or the valve position changes, a diaphragm lever (flapper plate) moves relative to a nozzle and the back pressure changes, which controls a booster. The booster output, the signal pressure, passes through a volume restriction to the actuator until the forces balance again. An adjustable Xp restriction and the volume restriction tune the loop.

The pick-up lever and range spring must match the rated valve travel and the nominal span of the input signal, so these two items are the main selection details. Direction of action is reversible by selecting the nozzle, and span and zero are set with separate adjusters.

Where the Type 3766 fits in an instrument-air loop

The 3766 pairs naturally with Samson pneumatic accessories. A Type 3755 volume booster speeds up actuators with a large volume, a Type 3709 lock-up valve can be attached to hold the actuator in place when supply air falls, and the Type 3767 replaces the pneumatic signal with an electric one. The optional solenoid valve lets a binary signal override the positioner and vent the actuator, so the spring moves the valve to its fail-safe position. The data sheet states that a version with a position transmitter cannot also carry limit contacts or a solenoid valve, as the transmitter fills the inner space.

Direct attachment to the Type 3277 actuator (175 to 750 cm²) gives a protected travel pick-off and a simple connection, NAMUR brackets suit IEC 60534-6-1 yokes, and a pair of universal brackets with a cam disk adapt it to the Type 3278 or other VDI/VDE 3845 rotary actuators. A VDI/VDE 3847 version permits replacement while the process runs.

Selection checklist for the Type 3766

• Control signal: confirm that the loop is pneumatic 3 to 15 psi; if not, consider the Type 3767 instead.
• Valve travel or angle, to select lever, cam disk and range spring.
• Single or double action, and whether a reversing amplifier is needed.
• Hazardous area: intrinsically safe, Zone 2 or flameproof arrangement with the Type 6116.
• Accessories: limit contacts, solenoid valve voltage, pressure gauges for supply and signal.
• Connection thread: ¼-18 NPT or G ¼.
• Low-temperature requirement, if the site is cold.

Installation, calibration and maintenance of the 3766

Mount the positioner and set the lever, cam disk and range spring according to the Samson mounting and operating instructions, connect clean supply air at 1.4 to 6 bar, and adjust zero and span against the control signal. Fine-tune the Xp band and volume restriction to remove hunting. Because the unit is purely pneumatic, regular maintenance concentrates on air quality: drain filter regulators, replace clogged filters and watch for oil in the supply, which blocks nozzles. Inspect the diaphragm and seals during scheduled shutdowns. The data sheet describes the device as low-maintenance and very insensitive to mechanical vibration.

Typical 3766 applications in Oman and the GCC

Pneumatic positioners remain in service on legacy control valves in oil and gas facilities, refineries, power stations and desalination plants across Oman and the Gulf. They are common replacements where the original unit has failed and the loop is still pneumatic, and in Zone 2 areas where intrinsically safe or Ex nA versions are specified. Heat, dust and humidity make air dryness and enclosure sealing the main concerns, so consider the filter check valve to reach IP65.

Information to send with an RFQ for the 3766

• Existing nameplate or the full 3766 article code
• Valve and actuator type, stroke or angle and fail-safe action
• Input signal range and supply air pressure
• Hazardous-area classification and certificate
• Required accessories, solenoid voltage and connection thread
• Quantity

Documents and quotation

The Samson datasheet for the Type 3766 is attached on this page. Send the 3766 nameplate data or the article code to Seven Star LLC and we will prepare a quotation for Oman and the GCC.