IMI STI DigitalTrak DT Compact Digital Valve Positioner

DigitalTrak DT loop-powered compact electro-pneumatic positioner

The IMI STI DigitalTrak DT is a compact, microprocessor-based electro-pneumatic valve positioner that takes its power from the 4-20 mA control loop. It reads the setpoint from the control system, compares it with the real valve position from a local sensor and adjusts the air to the actuator until the two match. It is fitted to single and double acting pneumatic actuators, on linear stems or rotary shafts, and is supplied by Seven Star LLC in Oman and the GCC as an independent supplier in Muscat.

The model selection sheet shows that the DT code is built from execution, action, feedback output, signal-fail position, gauges, connections and sensor type. The reading is as follows.

• Execution S: IP65 housing.
• Action 0 or 1: single acting (0) or double acting (1).
• Feedback output 0 or 1: not present (0) or 4-20 mA position transmitter (1).
• Signal-fail position S or F: fail safe (S) or fail in place, freeze (F).
• Gauges 0 or 1: none (0) or three gauges, on the actuator side and on the air feeding ports (1).
• Connections S or M: 1/2″ NPT electrical, 1/4″ NPT air supply and air ports (S) or M20 electrical with 1/4″ NPT air ports (M).
• Sensor 0 or 1: integrated (0) or remote (1).

DigitalTrak DT technical specifications

The brochure and the instruction manual give slightly different hysteresis figures (0.5 % and 0.2 %), so use the brochure figure as a conservative value and ask for confirmation if the loop is critical.

How the DigitalTrak DT positions the valve

A position sensor reports the stem or shaft position to the microprocessor. The software compares this with the 4-20 mA setpoint and controls the feeding and exhaust of air to the actuator chambers until the error is within the dead band. The result is a controller that can overcome friction and unbalanced forces on the control valve and improve its response speed. The dead band is adjustable from 0.1 % to 10 %, and the valve characteristic can be linear, equal percentage, quick opening or freely adjustable.

Because the positioner is loop-powered, it needs no separate supply. The minimum signal that keeps the display alive is around 2 mA, and the manual lists a signal below that as a cause of a blank display, so check loop voltage and compliance of the control output when wiring.

Autotuning, display and diagnostics on the DigitalTrak DT

A two-line LCD and three keys on the front panel provide all local setting. After mounting, the positioner must be initialised; the auto-tuning routine moves the valve automatically through five steps, determines zero, span and dead band, and optimises the control parameters. A manual tuning mode is also available, where an installer defines the start and end points. Diagnostic pages show the input current, stroke, up and down travel times, dead band and other tuned values, and a range of alarm messages helps pin down installation issues such as a feedback sensor out of range, insufficient measurement span or an actuator error.

If the signal fails, the fail action is either fail safe or fail in place, depending on the version ordered. The position transmitter option provides a separate 4-20 mA feedback, which needs an external 24 V supply.

DigitalTrak DT in the IMI STI positioner range

The DT is the compact, loop-powered option. Within the IMI STI range, the QuickTrak TM is a high-flow digital valve control system for large actuators, the FasTrak and SHP are smart positioners with extended functions, and the UP-2 and SA/CL are analogue positioners. For slow-moving valves on standard actuators the DT gives a modern autotuned loop; for large volume actuators, add a volume booster or choose a higher-flow positioner, because the DT Cv is 0.12.

Selection and ordering checklist for the DigitalTrak DT

• Actuator action: single acting or double acting.
• Need for a 4-20 mA position transmitter.
• Fail action: fail safe or fail in place.
• Local gauges or not.
• Electrical entry: 1/2″ NPT or M20.
• Integrated sensor on the actuator or remote sensor with 5 m or 10 m cable.
• Actuator type and mounting: linear lever, motion converter, or rotary shaft, with the stroke or angle.
• Air quality and supply pressure within 1.4 to 7 bar.

Installation, commissioning and maintenance

Mount the positioner on a bracket matched to the actuator. For linear actuators the manual describes a feedback lever with an adjustable threaded rod, or a motion converter (model MC) when the actuator needs a rotary feedback shaft. The feedback lever should be set so that uninitialised feedback reads around 50 % at mid stroke. Do not leave the exhaust port covered by a bracket, since exhaust restriction slows the response; the troubleshooting table suggests drilling a relief hole where the bracket blocks it.

Keep the supply dry and clean and drain the filter regulator regularly. Check the feedback linkage periodically, because long-term vibration can loosen it, and re-initialise if zero or span have shifted. Keep the pressure gauges legible so the supply and output can be verified at a glance. The manual specifies storage between -20°C and 80°C at no more than 90 % humidity.

Typical DigitalTrak DT applications in Oman and the GCC

The manufacturer lists petroleum, chemical, power generation and metal production among typical users. In Oman and the GCC that translates to control and shut-off valves on process skids, utilities, cooling water and chemical dosing lines, and fuel systems. The metallic anodised housing is described as suited to sandstorm and copper-free ambient conditions, which helps in dusty coastal and desert plants, and the -20°C to +70°C range covers the normal ambient swing, though exposed units in direct summer sun should be shaded or use the remote sensor option.

Information to send with a DigitalTrak DT request

• Part name DigitalTrak DT and quantity.
• Model selection code, or the options in the checklist above.
• Actuator make and size, linear stroke or rotary angle.
• Supply pressure and tubing size.
• Whether a feedback transmitter, gauges and remote sensor are needed.
• Area classification, if relevant, so the suitability can be confirmed.

Documents and quotation

The IMI STI model selection sheet for the DigitalTrak DT (document 7005) is attached on this page and shows the option codes summarised above.

Send the part number, option code and quantity to Seven Star LLC and we will confirm lead time and any further documents, such as the instruction manual, for the DigitalTrak DT.