IMI STI AV Volume Booster, 1/4 in. and 3/4 in.

IMI STI AV volume booster in 1/4 in. and 3/4 in. sizes

The IMI STI AV is a pneumatic volume booster for high-speed control valve applications. It takes the small pilot signal from a positioner and repeats it as a much larger air flow to the actuator, so that the valve can move quickly even when the actuator is large. A stabilising bypass valve with a screwdriver adjustment is built into the body and lets the commissioning engineer trade speed against loop stability. Seven Star LLC supplies the IMI STI AV in Oman and the GCC as an independent supplier in Muscat.

The AV is offered in two sizes, 1/4 in. and 3/4 in., and in painted aluminium or stainless steel 316. The size determines the flow coefficient and weight, while the material determines suitability for corrosive environments.

IMI STI AV technical specifications

The product page lists ATEX and IECEx under certification for the AV. Confirm the applicable certificate and marking for the version you need before ordering for a hazardous area.

How the AV volume booster works

A positioner can only deliver a limited flow through its output stage. On a large actuator this makes the valve slow, because the chamber volume must be filled or emptied through a small orifice. The AV is a diaphragm-operated relay. The pilot signal acts on the diaphragm, and the diaphragm moves a plug that opens a large inlet path from the supply to the output (or a large exhaust path from the output to atmosphere). The output pressure follows the pilot signal, but the volume of air moved is limited only by the booster size.

A booster in a positioner loop can cause oscillation, because the positioner and booster together introduce additional delay and gain. The built-in bypass valve is the remedy. Opening it lets part of the signal reach the actuator directly, which damps the response and improves stability. The brochure notes that the regulation screw is accurate and lockable and cannot be ejected by internal air pressure. A conical plug seat with controlled deformation gives an elastomeric air-tight seal, which means less leakage and better repeatability.

Manifold mounting and exhaust options on the AV

Like other IMI STI accessories, the AV uses manifold mounting. Machined faces with O-ring seals connect the booster to the positioner and actuator without nipples and fittings, saving assembly time, inventory and space. The exhaust is protected by a double differentiated mesh, and options are available for collected exhaust or a silencer. On a double acting actuator one air connection can feed both boosters, which simplifies tubing.

Choosing between AV 1/4 in. and AV 3/4 in.

Do not choose the larger size simply for margin. An oversized booster on a small actuator can make the loop twitchy, while an undersized one gives only a small speed improvement. Provide actuator volume and required stroke time so the size can be checked.

Where the AV fits with related IMI STI products

The AV works with any positioner that provides a pneumatic output, such as the FasTrak FTC digital positioner or the SR/CCK analog positioner. For fast exhaust, combine it with the UD derivative unit, and for the final vent use the SL exhaust protection system. For even larger actuators the IMI STI range also offers the BV and BW boosters.

Selection and ordering checklist for the AV

• Size: 1/4 in. or 3/4 in., based on actuator volume and required speed.
• Material: painted aluminium or stainless steel 316.
• Temperature option covering minimum and maximum ambient.
• Actuator type: single or double acting, and the number of boosters required.
• Supply pressure within 3 to 7 bar.
• Exhaust: standard, collected or fitted with a silencer.
• Hazardous-area requirement and documents.

Installation, tuning and maintenance of the AV

Mount the booster on the manifold with O-rings in place, connect the pilot line from the positioner and the supply and output lines according to the circuit diagram, and make sure the supply is filtered and dry. Start commissioning with the bypass valve closed, check stroke speed and overshoot on small steps, and open the bypass gradually until the loop is stable. Lock the screw afterwards. If the response is still oscillatory, check the positioner gain settings before opening the bypass further.

Maintenance is limited to periodic checks for leaks at the manifold, blocked exhaust mesh and diaphragm condition. A diaphragm that has aged shows up as slow response, constant air leakage from the exhaust or reduced output pressure. Keep a diaphragm and seal kit available for critical valves.

Typical AV applications in Oman and the GCC

Volume boosters are common on large control valves, anti-surge valves, fast-closing shutdown valves and compressor recycle valves in gas processing, petrochemical and power plants. In desalination plants they speed up large butterfly valves on seawater and brine lines. The AV is offered for standard, offshore, sandstorm and copper-free conditions, with stainless steel 316 for coastal and offshore exposure and wide temperature options for desert summers.

Information to send with an AV request

• Part name AV, size and quantity.
• Housing material and temperature option.
• Actuator brand, size, volume and action.
• Positioner model.
• Supply pressure and required stroking time.
• Exhaust arrangement and hazardous-area requirement.

Documents and quotation

A brochure for the IMI STI AV is attached on this page, with specifications and dimensional information for both sizes.

Send the part number, size and quantity to Seven Star LLC and we will confirm lead time and any other documents available for the AV volume booster.