IMI STI ARA Axial Rotary Actuator, Compact Quarter-Turn

IMI STI ARA axial rotary actuator for compact quarter-turn valve service

The IMI STI ARA is an axial rotary actuator for heavy-duty on/off and modulating valve service. A cylinder and piston sit directly above the valve, and an internal cam arrangement converts the piston movement into the rotary, roto-translatory motion the valve needs. Because the cylinder is mounted in line with the valve axis and not at the side, the ARA takes a smaller footprint than a conventional rack-and-pinion or scotch-yoke quarter-turn actuator of similar duty. Seven Star LLC supplies the IMI STI ARA in Oman and the GCC as an independent supplier in Muscat.

The ARA is a made-to-order series, not an off-the-shelf catalogue item. Each actuator is sized and configured for the valve, the pressure and the safety function it must serve, so the ordering information in this page is aimed at a clear technical enquiry.

ARA actuator series coding and construction

The instruction manual defines the series code as ARA-X-YYYY, where X is the version and YYYY is the nominal cylinder diameter in millimetres. S denotes a single acting (spring return) version and D denotes a double acting version. The remaining part of the code is a selection table that the manual leaves to be defined per project, so the final designation is confirmed against the datasheet issued for the order.

The actuator has three main assemblies: the external cylinder with its piston and stem, an internal cylinder group with cams that turn the linear thrust into rotation, and the yoke that fixes the actuator to the valve. Pneumatic accessories such as positioners, solenoid valves and limit switches are added depending on the duty.

IMI STI ARA technical specifications

The manual and the web page differ in some details. The manual defines the standard operating temperature as -20°C to +70°C with extended range possible for special applications. The page highlights -60°C to +100°C as the overall capability. Treat the standard range as the default and state any colder or hotter duty in the enquiry.

Why choose an axial rotary actuator like the ARA

• Space: in-line cylinder reduces the width of the actuator package, useful on FPSOs, offshore platforms and compact skids.
• Weight and skid size: a smaller envelope can reduce skid dimensions and support steel.
• Torque curve: the cam is tailored so that the torque curve matches the valve, which is high at the seating end for ball, plug and butterfly valves.
• Flexible power: air, oil or spring is available depending on version.
• Safety: SIL 3 is stated as achievable when the system is designed accordingly.

A torque curve matched to the valve avoids oversizing. Quarter-turn valves need high breakaway and seating torque but much less in mid-travel, and a purpose-made cam can follow that pattern instead of using the same ratio across the stroke.

Selecting an ARA actuator

Sizing starts with the valve data: type, size, pressure class, maximum differential pressure, breakaway and running torque from the valve manufacturer, and the fail-safe direction. From this the actuator supplier derives the cylinder diameter, the spring (for single acting) and the supply pressure. Give the supply pressure honestly, including the minimum value that can occur in the plant air header, because the actuator must still deliver the required torque at that value.

• Version: single acting with spring for fail-safe, double acting for non-fail positions.
• Fail position: open or closed on loss of air.
• Valve data: type, size, torque, interface flange and stem dimension.
• Supply: air or oil, minimum and maximum pressure.
• Cylinder material: carbon steel, stainless steel or fibre for corrosion.
• Accessories: positioner, solenoids, limit switches, handwheel or override.
• Safety function: SIL requirement and proof-test needs.

Installation and handling of the ARA

The manual gives practical rules that apply to any heavy actuator. Lift the actuator using its own eyebolts, never the valve and actuator assembly together, and keep the slings away from tubing and accessories. Check the nameplate against the order confirmation on receipt. For storage, keep the unit off the ground, keep plastic plugs in the connections, and for long-term or outdoor storage replace them with metal plugs, coat the valve coupling area and exercise the actuator from time to time.

When mounting, clean the mating surfaces so that no grease remains, fit the insert bush into the drive sleeve without force and tighten fasteners to the table in the manual, which gives for example 150 Nm for M16 and 300 Nm for M20 with the stated bolting materials. Flush supply tubing before connecting, fit a pressure regulator if supply can exceed the maximum, and ground the actuator if it is not grounded through the mounting.

Maintenance and spares for the ARA

The manual recommends an inspection about every two years, adjusted for the duty, covering operation and stroke time, supply pressure, leaks in pneumatic connections, paintwork and any dust build-up. The mechanism needs no lubrication during its life; special grease grades by temperature are given for overhauls. Repairs use original spare parts, and the serial number from the nameplate must be given when ordering spares. Keep a record of seal kits for critical valves.

Typical ARA applications in Oman and the GCC

The ARA is a good fit where space and weight limit the choice of actuator and the valve is large: offshore platforms and FPSOs, gas compression and processing trains, pipeline block valves, and large butterfly or ball valves in power and desalination water systems. In the Gulf climate, specify the cylinder material and paint system for humidity and salt, and consider the temperature range for solar heating of exposed actuators. Fibre or stainless cylinders can reduce corrosion concerns in coastal service.

Information to send with an ARA request

• Version (single or double acting) and fail position.
• Valve manufacturer, type, size and torque data.
• Supply medium and pressure range.
• Ambient temperature range.
• Accessories required.
• Certification and SIL requirements, and any applicable standards.

Documents and quotation

The IMI STI instruction manual for the ARA is attached on this page. It covers handling, installation, start-up, maintenance and spare-parts ordering.

Send the valve details and duty data to Seven Star LLC and we will confirm the actuator configuration, lead time and any additional documents available for the ARA.