Limitorque SMB Series Multi-Turn Electric Actuator

Overview

The Limitorque SMB series is a heavy-duty multi-turn electric actuator that has been used for decades on the valves where failure is least acceptable. The manufacturer brochure describes a service life that is often more than 30 years, and a cast or ductile iron housing with weatherproof construction as standard. The Limitorque SMB is also offered for explosion-proof, submersible, Navy and nuclear requirements, which is why it appears on gate, globe, plug, ball and butterfly valves in refineries, pipelines, power plants and large water systems.

For buyers in Oman and the GCC, the Limitorque SMB is relevant to a wide installed base, from legacy refinery and terminal valves that need like-for-like replacement to new high-torque valve packages on pipelines and water transmission mains. Seven Star LLC supplies the Limitorque SMB from Muscat with engineering support, and can compare your valve data with the brochure ratings, order the correct motor and base, and obtain spares for existing units in service.

Operating Principle of the Limitorque SMB

At the heart of the Limitorque SMB is a double-reduction gear set, a helical stage followed by a worm stage, driving the stem nut or drive sleeve. The gearing is available in several ratio combinations, which lets the designer choose the speed and torque independently and avoids the usual trade-off between running time and output. A hammer-blow feature lets the motor reach full speed before the load is engaged, so the actuator can deliver strong starting force to unseat a stiff valve.

A geared limit switch controls travel in both directions, and the same mechanism drives the open and closed indicating lights. Torque is limited automatically: if an obstruction raises the load, the torque switch de-energises the motor and protects the valve stem and seat from damage. The motor disengages during handwheel operation, and the actuator can be declutched for manual operation at any time, so the valve can still be moved during a power failure.

Engineering Design

The Limitorque SMB housing is made of cast iron or ductile iron, with worm gear in alloy bronze, worm in alloy steel and helical gears in heat-treated steel, lubricated with long-life extreme-pressure grease. The standard motor is a squirrel-cage induction type with Class F insulation and a 15-minute nominal duty, suitable for common three-phase supplies at 50 and 60 Hz. The limit switch compartment carries a gear-driven snap-acting switch with 16 contacts in four trains, and the position and torque switches are rated at 300 volts and 6 amps resistive.

Optional SB and SBD spring-compensated assemblies add a spring-loaded stem nut to the gear housing. The SB version accommodates thermal expansion and absorbs seating shock when valves close at high speed, allowing operation at up to three times normal rates, while the SBD version floats in both directions for stem contraction and torque back-seating. An insertable stem nut lets the actuator be moved to another valve without major disassembly, and integral controls such as reversing starters, transformers, position transmitters, heaters, breathers and drains can be fitted into the control compartment.

Key Features

Specifying the Limitorque SMB brings these practical advantages to heavy valve duty:

• Output torque is listed up to 60,000 ft-lb (81,349 Nm) and stem thrust up to 500,000 lb (2,224 kN) across the SMB-000 to SMB-5XT size range.
• A rugged cast or ductile iron housing and precision-machined gearing support a service life that the manufacturer states is often more than 30 years.
• Nuclear, weatherproof, submersible and explosion-proof constructions are offered, including construction qualified to IEEE 323 and IEEE 344 for nuclear service.
• Automatic torque limiting de-energises the motor on an obstruction and prevents valve damage.
• SB and SBD spring-compensated versions are interchangeable with comparably sized SMB units on the same mounting bases.
• Close-coupled control stations with indicating lights and selector switches provide local open, stop and close operation.
• Mounting on Limitorque HBC and WG gearboxes extends the actuator to quarter-turn butterfly, ball and plug valves.
• An insertable stem nut allows the actuator to be moved from valve to valve with little disassembly.

Industrial Applications

The brochure lists oil and gas wells and platforms, pipelines, petroleum terminals and tank farms, refineries and hydrocarbon plants, power plants, hydroelectric stations, water and wastewater works, steam systems and metals processing among the typical applications. In Oman that translates to terminal isolation valves at Sohar and Duqm, refinery and gas-processing valves, large cooling-water valves at power and desalination plants and sluice gates on water-transmission schemes. Where valves are exposed to extreme heat and long idle periods, a rugged iron-cased actuator with a proven manual override is a reassuring specification.

Seven Star LLC is often asked to support brownfield plants where an SMB was installed years ago and a replacement or a rebuild is needed. We can help identify the frame from the nameplate, check torque and thrust against the ratings table and propose a matching Limitorque SMB or a successor, while newer projects can compare the actuator with the Flowserve Limitorque L120 electric actuator and the AUMA SA 10.2 multi-turn electric actuator for multi-turn duty. For gearing on quarter-turn valves, a AUMA GST spur gearbox may be reviewed as an alternative arrangement.

Technical Specifications

The following figures are drawn from the manufacturer brochure for commercial service; nuclear qualification data is documented separately by the manufacturer.

Request a Quote for Limitorque SMB

If you are specifying or replacing a Limitorque SMB, send Seven Star LLC the valve type, stem size, required torque and thrust, supply voltage and any hazardous-area or nuclear requirement. We will confirm the frame and motor, provide the official brochure and quote delivery to Oman and the GCC. For units already in service, our Muscat team can help with nameplate identification and spares so your Limitorque SMB fleet keeps running through the next outage.