Samson Type 3724 Electropneumatic Positioner (4-20 mA)

Type 3724 compact electropneumatic positioner for the Type 3379 actuator

The Samson Type 3724 is a single-acting electropneumatic positioner that is integrated into the Type 3379 pneumatic actuator instead of being mounted beside it. It takes a 4 to 20 mA set point, measures the valve stem travel without contact and sets the actuator signal pressure to match. Seven Star LLC supplies the Type 3724 in Oman and the GCC as an independent supplier based in Muscat.

Because the positioner sits inside the actuator package, the assembly is compact and there are no external levers or linkages to align. The 3724 covers a travel of 4 to 16 mm, and it is fully self-calibrating, so it adapts to the valve and actuator automatically after start-up.

What the Type 3724 package includes

The data sheet lists a single version: the Type 3724 electropneumatic positioner with local operation and an LCD. It is used with the Type 3379 actuator in two piston sizes. Fig. 2 of the data sheet shows the unit on a Type 3347 angle valve, an example of the valve type it is combined with.

Type 3724 technical specifications

How the Type 3724 senses and corrects position

An internal pick-up rod carries a magnet. A magnetoresistive sensor reads the magnet position without touching it, so there is no gear or potentiometer to wear. A microcontroller compares this position with the set point converted from the 4 to 20 mA signal. An analogue i/p converter with a downstream air capacity booster then fills or vents the actuator. A pressure regulator holds the i/p module on a constant upstream pressure, which compensates for supply fluctuations. If an actuator is very fast, a volume restriction can slow the signal pressure change.

The data sheet notes that the transit time function applies only to actuators with an initialisation time above 0.4 s.

Operation and on-board functions of the 3724

Three pushbuttons and an LCD form the local interface. The top and bottom buttons move through the parameter list, which is laid out on a single level, and the middle button confirms a choice. The display reading direction can be rotated by 180 degrees, although the data sheet points out that an upright or horizontal installation is only permissible depending on the valve. All parameters are stored permanently, protected against power failure, and presets mean only values that differ from the standard need adjusting.

Additional functions include two adjustable software limit contacts, a tight-closing function that can be activated, and optional continuous zero monitoring. The limit contacts are reverse-polarity protected and galvanically isolated, they conduct with a resistance of 330 Ω when responding and are highly resistive (below 100 µA) otherwise, and the maximum power for connection to a PLC input is 400 mW.

Selection checklist for the Type 3724

• Actuator: confirm the Type 3379 piston size, 63 mm or 90 mm, and that the required thrust is met at the available supply air.
• Valve: confirm the valve travel is within 4 to 16 mm and that the valve is meant to be combined with the Type 3379.
• Signal: 4 to 20 mA from the control system, or split-range signals if two valves share a loop.
• Supply air: 1.4 to 7 bar of air according to ISO 8573-1 classes; the signal pressure is lower than supply by 0.4 bar.
• Surface: micro-bead blasted or polished finish according to the process area.
• Ambient: the temperature range is –20 to +80 °C, so check the site winter and summer extremes.

Commissioning and maintenance of the 3724

After the air and the 4 to 20 mA loop are connected, start the automatic initialisation. The positioner determines the stroke and the end positions, adapts to the actuator dynamics and then starts closed-loop control. Make sure the process allows the valve to stroke during this step. Check the supply air before every start-up because oil, water and dust are the usual causes of poor control with small i/p modules. Keep the dome clean so the visual indicator can be read, and record the settings so a replacement unit can be configured quickly.

Typical 3724 applications in Oman and the GCC

Compact process valves with short strokes in utility and skid-mounted systems are the natural home for this positioner: chemical dosing and neutralisation skids, cooling water temperature control, steam trace and heat exchanger control, filtration plants and seawater treatment packages. In Oman and the Gulf, mounting the positioner inside the actuator keeps dust and humidity away from the linkage, but the –20 to +80 °C limit should be checked against sun-exposed pipe racks.

Information to send with an RFQ for the 3724

• Type 3379 actuator size and piston diameter, or the valve and actuator tag
• Valve type, nominal size, stroke and fail-safe direction
• Control signal range and any split-range requirement
• Supply air pressure
• Required surface finish
• Quantity and whether a complete valve assembly or only the positioner is requested

Documents and quotation

The Samson datasheet for the Type 3724 is attached on this page and includes the article code and dimensions. Please send the 3724 code, valve data and quantity to Seven Star LLC to receive a quotation for Oman and the GCC.