IMI STI SC/VLT Pneumatic Lift and Turn Actuator

SC/VLT pneumatic lift and turn actuator for rotary-motion control valves

The IMI STI SC/VLT is a double acting pneumatic valve actuator that produces a combined lift and turn motion: a short linear stroke followed by a rotation of up to 90°. This kind of movement is used on valves whose plug or disc must first lift off its seat and then rotate, so seat wear is avoided. The SC/VLT is built from a cylinder group and a yoke, and it is listed by the manufacturer as intended for control valves in on-off or regulating duty. Seven Star LLC supplies the SC/VLT in Oman and the GCC as an independent supplier in Muscat.

The name belongs to a family. The manufacturer product page describes an SC/VSC/VLT range, and the instruction manual covers the SC/VLT version used for lift and turn. Other linear versions of the SC cylinder, the SC and SC/V, are used on globe-type and damper valves, and the SC cylinder is also the core of the CML enclosed control drive.

SC/VLT technical specifications

Different manufacturer documents quote different stroke and size ceilings for the family, because each unit is built to order for a valve and a load. The data above are the general limits; the stroke, bore and design pressure for your valve are fixed by the order and recorded on the label.

How the SC/VLT lift and turn actuator works

Inside the cylinder a piston divides the volume into two chambers. The actuator stem connects the piston to the valve stem. In the yoke, a guided mechanism converts part of the linear stroke into rotation, so the valve stem first lifts a short distance to free the plug from the seat and then turns up to 90°. The yoke fixes the cylinder to the valve and defines the combined linear and rotary movement. Adjustable travel stops during opening are listed on the product page, which allows the opening stroke to be set during commissioning.

Pneumatic accessories such as positioners, solenoid valves, boosters and limit switches are mounted on the actuator depending on the required performance, and the supply circuit follows the pneumatic diagram supplied with the order.

SC/VLT construction, materials and greases

The cylinder is available in carbon steel with nickel or chrome plating or in stainless steel, which reduces friction and corrosion on the bore. The manual specifies different greases for each cylinder material and temperature class: for a standard temperature range between -20°C and +70°C, nickel plated carbon steel uses Molyguard Idrosfer, while chrome plated carbon steel and stainless steel use Polimer 400/1 with a Tecnolube seal. Low, very low and high temperature classes call for separate greases, and the yoke moving parts use a multipurpose EP grease. Specifying the correct temperature class on the order is therefore important.

Where the SC/VLT fits and related IMI STI actuators

If the valve needs only a straight stroke, the SC or SC/V linear actuators are the natural choice. For quarter-turn valves without lift, the RTC and RTQ scotch-yoke actuators are available. The SC/VLT specifically addresses valves with a lifting and turning plug. Control accessories for this actuator come from the same maker: the FasTrak and SHP positioners, the UP-2 analog positioner, and boosters such as the BR booster relay and the AV volume booster.

Selection and ordering checklist for the SC/VLT

• Valve data: manufacturer, size, lift distance, rotation angle and operating torque or thrust.
• Cylinder material: carbon steel with nickel or chrome plating, or stainless steel.
• Design pressure and supply pressure, including any special 12 bar requirement.
• Fail position and spring requirement if the actuator is configured for spring return.
• Temperature class for the grease and seals, including any extended range.
• Supply medium: instrument air, nitrogen or process gas.
• Accessories: positioner, solenoid, limit switches, volume booster.
• Certification: PED, ATEX or other marking needed for the site.

Installation and commissioning of the SC/VLT

Lift the actuator with belts through its eyebolts and never lift the valve by them. Before installation check that the model matches the order confirmation, that the pneumatic and wiring diagrams and the dimensional drawing are supplied, and that there is no transport damage. After mounting, verify that the actuator is aligned with the valve stem and that the full movement is free of binding, sticking or jumping. Supply connections should be made with suitable pipework that is deburred and flushed, a 5 micron filter and a shut-off valve on the supply, and no lubricator is needed in the line. Increase the supply pressure gradually to the maximum operating pressure and leak-test the connections, accessories and cylinder, then carry out a complete functional test.

Maintenance, storage and troubleshooting of the SC/VLT

The manual lists a periodic routine: grease the yoke moving parts every six months, check the paint every three months and touch up damage, keep the outside free of heavy dust, and replace the gaskets and grease every two to three years depending on the duty. Store in a dry, clean place between -20°C and +40°C with connection protections and mechanical locks in place.

If the actuator does not move, the checks are supply line, supply pressure, valve seizing, undersized actuator or an obstruction. Non-linear movement points to stem misalignment or the wrong lubricators. Slow opening and closing time points to the pneumatic circuit, sizing or springs. The customer may disassemble only for gasket and grease replacement, and a gasket kit is requested from the manufacturer.

Typical SC/VLT applications in Oman and the GCC

Lift and turn valves are used on high-duty service in oil and gas, gas processing, chemical and power plants, and the actuator is designed for on-off and regulation applications in process industries. In Oman and the GCC this means large control valves on process lines, with outdoor installation in heat, sand and humidity. The IP66 listing, carbon or stainless cylinders and the option of extended temperature classes help in coastal and desert sites, and the product page lists operation on sour gas as well as air and nitrogen.

Information to send with an SC/VLT request

• Part name SC/VLT, quantity and the valve manufacturer and type.
• Stroke, rotation angle and thrust or torque at the valve.
• Supply pressure, design pressure and medium.
• Temperature range, including the extremes.
• Cylinder material and plating, and painting specification.
• Required accessories and fail action.
• Certification and area classification.

Documents and quotation

The IMI STI instruction, installation and maintenance manual 5023 for the SC/VLT lift and turn actuator is attached on this page, covering the technical data, greases, maintenance and troubleshooting summarised above.

Send the part number and valve data to Seven Star LLC and we will confirm lead time and any further documents for your SC/VLT actuator.