IMI STI SR/CCK Analog Quarter-Turn Valve Positioner

IMI STI SR/CCK pneumatic quarter-turn valve positioner

The IMI STI SR/CCK is a heavy-duty analog positioner for the proportional control of quarter-turn valves such as butterfly, plug and ball valves. It receives a 3–15 psi pneumatic signal and positions the actuator so that the valve angle follows the signal. With a dedicated bracket it can also be used on linear actuators. Seven Star LLC supplies the IMI STI SR/CCK in Oman and the GCC as an independent supplier in Muscat.

Unlike the electronic FasTrak FTC, the SR/CCK is entirely mechanical and pneumatic. It has no electronics, so it needs no power, and its operation can be understood and adjusted by a technician with ordinary tools, which is the reason it is still specified for harsh, hazardous or remote locations.

SR/CCK technical specifications

How the SR/CCK works: balanced forces and spool valve

The SR/CCK works on a balanced-forces principle. The pneumatic input signal is converted into a force inside the positioner. A cam coupled to the actuator shaft produces an opposing force that depends on the valve angle. When the two forces are equal, the positioner is in balance. If the signal increases, the balance is upset, a spool valve moves and air is directed to one side of the actuator and exhausted from the other until the new angle brings the feedback force back to equilibrium.

Distribution of air to the actuator chambers is done by the spool. For corrosive or demanding service a full stainless steel spool valve is available as an option. The cam is the key to flexibility. The SR/CCK is equipped with a linear cam that covers operating angles up to 270°, with the operating angles and range adjustments printed on the positioner, and special cam profiles are available on request for non-linear valve characteristics.

Signal ranges, split range and characterisation

Split-range operation lets two or more valves share one control signal, for example one valve moving over 3–9 psi and a second over 9–15 psi. The brochure lists split-range compatibility and special characterisation or ranges as available. If your loop needs split range, a non-standard stroke or an equal-percentage characteristic, state it in the enquiry because it affects the cam and spring configuration.

Features that matter in the field

• Metallic case and heavy-duty design for hazardous plants, which the brochure associates with petroleum, chemical, power and metal production.
• Visual position indicator for quick checks from a distance.
• Intuitive mechanical and pneumatic construction that is straightforward to maintain.
• Suitable for standard, sandstorm and copper-free ambient conditions.
• Suitable for single and double acting actuators and for low and high ambient temperatures.
• Optional pressure gauges for supply and output.

Where the SR/CCK fits in the pneumatic circuit

The positioner is normally placed in the control circuit between the air filter regulator and the actuator of the final control element. Other IMI STI accessories connect to it. For large actuators an AV volume booster raises the flow, the UD derivative unit increases exhaust speed, and an SL exhaust protection system can protect the vent. If a plant wants HART diagnostics, the FasTrak FTC digital positioner is the electronic alternative.

Selection and ordering checklist for the SR/CCK

• Actuator type and rotation angle, up to 270°.
• Single or double acting action and fail position.
• Signal range and any split-range requirement.
• Cam: linear or special characterisation.
• Spool valve material: standard or stainless steel.
• Accessories: pressure gauges and mounting bracket for the actuator or for linear use.
• Supply pressure within 2.5 to 7 bar and ambient range of -20°C to +70°C.

Installation and calibration of the SR/CCK

Mount the positioner on the actuator shaft or bracket so that the cam turns with the valve, keeping the coupling free from backlash. Connect a filtered, dry supply through the filter regulator, then connect the output lines to the actuator chambers and the signal line to the controller output or I/P converter. Set zero and span against the 3–15 psi signal, check the cam angle printed on the unit and verify the valve travel at several points between 3 and 15 psi. Use the visual indicator for cross-checking.

Since static air consumption is small but not zero, check the supply for leakage during routine rounds and keep filters clean. Moisture and oil carryover are the most common causes of sticking spools and poor sensitivity. Clean the spool during overhaul.

Typical SR/CCK applications in Oman and the GCC

Quarter-turn control valves are widely used for cooling-water, seawater and utility services in power and desalination plants, and for gas and liquid hydrocarbon service in refineries and petrochemical complexes. In these plants the SR/CCK is a robust choice for butterfly valves with pneumatic actuators in dusty, humid and hot conditions, and for locations where instrument technicians prefer a purely mechanical device. The metallic case and sandstorm suitability address the typical Gulf environment.

Information to send with an SR/CCK request

• Part name SR/CCK and quantity.
• Actuator brand, model and rotation angle.
• Action: single or double acting.
• Input signal and split-range requirement.
• Accessories needed, such as gauges or a bracket.
• Supply pressure and ambient temperature.

Documents and quotation

A brochure for the IMI STI SR/CCK is attached on this page, with specifications and the dimensional drawing.

Send the part number, actuator details and quantity to Seven Star LLC and we will confirm lead time and any additional documents for the SR/CCK positioner.