IMI STI CF Fast Commutator for Positioner Speed Control

IMI STI CF fast commutator for positioner speed and fail-freeze control

The IMI STI CF fast commutator is a compact pneumatic switching block that combines two 3-way valves around one shared pilot conduit. It is designed as an accessory for double-acting actuators fitted with IMI STI positioners, and its purpose is to let the actuator move or hold quickly without relying on the small internal ports of the positioner. Seven Star LLC supplies the CF in Oman and the GCC as an independent supplier in Muscat.

Because the pilot fluid acts on a single membrane to move both valves, switching is fast, and the manufacturer notes that it stays fast even when the pilot comes from a low-CV solenoid valve. The CF is also described as holding a tight seal on the seats at up to 8 bar while operating from a pilot pressure above 2 bar.

CF configuration options and weights

CF is a model name, and the brochure defines the configurable points rather than a coded part number. Options are the housing material, the temperature band and whether the optional integrated piezo solenoid valve for fail-freeze is included.

CF technical specifications

How the CF fast commutator operates

Two 3-way valves are built into one block, each connecting one actuator chamber to either supply or exhaust. A common pilot passage commands both valves together. When the pilot acts on the membrane, the valves change over, so one chamber is pressurised while the other is vented, and the actuator moves. When the pilot is removed, the valves return and the actuator moves the other way or is held, depending on how the CF is piped.

Up to three CF units can be mounted on the same positioner to give several functions: quick open, quick close and lock in place. The lock-in-place function uses the piezo valve variant for fail-freeze, so the actuator stays at its last position when the signal or supply is lost. This is the main reason to choose the piezo option.

Where the CF fits with FT positioners and BV/BW boosters

The brochure states that the CF is designed to work with the IMI STI FT positioner alone, or together with BV or BW boosters. A booster increases the air flow to the actuator, and the CF adds the fast switching and holding behaviour. Related items in this range include the CS quick exhaust valve, which vents a single chamber, and the CO 3-way operating valve for higher-flow change-over duties. Use the CF where the double-acting arrangement and the shared pilot are what you need.

Its manifold mounting system connects accessories using machined faces with an o-ring seal, so fittings and nipples are not required. This reduces assembly time, inventory and installed dimensions, according to the manufacturer.

Selection and ordering checklist for the CF

• Actuator: double acting, with the volume of each chamber.
• Positioner model and whether a BV or BW booster is in the circuit.
• Functions needed: quick open, quick close or lock in place, and how many CF units that implies.
• Fail-freeze requirement, which decides on the piezo valve option.
• Housing: aluminium for weight, stainless steel 316 for harsh or offshore service.
• Ambient range at site, noting the narrower -25°C to +60°C range with the piezo valve.
• Port style: threaded NPT or manifold mounting.
• Pilot source and its pressure, which must exceed 2 bar.

Installation, commissioning and maintenance of the CF

Fit the CF on its manifold face with the o-rings supplied, and confirm that the pilot line is clean and dry. Orient the block so that exhaust ports are not exposed to standing water or heavy dust. The pilot connection is 1/2 inch NPT and the main ports are 3/4 inch NPT if you pipe rather than manifold-mount.

On commissioning, stroke the actuator in both directions from the real command and time each movement. If a piezo valve is fitted, test the fail-freeze behaviour by removing the signal and confirm the actuator holds position. Keep a record of the results. For maintenance, use clean instrument air, check for leaks at the manifold faces and replace o-rings if sealing degrades. The manufacturer publishes an instruction manual for the CF, which should be used for service procedures.

Typical CF applications in Oman and the GCC

Positioner accessories like the CF are used on control and safety valves in oil and gas, power, desalination and petrochemical plants. Fail-freeze is attractive on valves where an uncontrolled move on loss of signal would upset the process, for example on large control valves in gas processing. In Gulf conditions, the choice between aluminium and stainless steel 316 should consider salt-laden air, and the ambient temperature band should be checked against summer extremes in the equipment shade and sun load. The brochure lists standard, offshore, sandstorm and copper-free ambient conditions as suitable.

Information to send with a CF RFQ

• Part number CF and quantity.
• Housing material and whether the piezo valve is required.
• Positioner and booster models in use.
• Actuator size and chamber volumes.
• Ambient temperature range.
• Pilot pressure and signal type.
• Delivery location in Oman or the GCC.

Documents and quotation

A manufacturer brochure for the IMI STI CF is attached on this page. It includes the specification table and dimensional drawing referred to above.

To request a quotation, send the part number, quantity and the configuration details above, and we will confirm availability and lead time.