IMI STI CRP-3/M Analog Valve Positioner

IMI STI CRP-3/M analog valve positioner: what it is

The IMI STI CRP-3/M is a compact analog valve positioner for control valves with pneumatic actuators. It compares the signal from the controller with the real position of the valve and adjusts the air to the actuator until the two agree. It is built in a pneumatic version, which takes a 3-15 psi (0.2-1 bar) signal, and an electro-pneumatic version, which takes 4-20 mA.

The positioner uses the balanced-forces principle, and air to the actuator chambers is distributed by an internal spool. It is double acting by design but can be used on single acting actuators. In the manufacturer weight list the pneumatic version is CRP-3/M and the electro-pneumatic version is CRP-3/M-EP. Seven Star LLC, an independent supplier in Muscat, supplies the CRP-3/M in Oman and the GCC.

CRP-3/M technical specifications

How the CRP-3/M positioner works

A balance of forces inside the instrument links the input signal to the feedback from the actuator. The feedback comes from a lever or a bare shaft and passes through a cam. When the signal changes, the balance is upset and the internal spool valve moves, directing air into one actuator chamber and venting the other. The actuator moves until the feedback force brings the balance back to zero. Since the spool is the only moving air element, air distribution is simple and the positioner holds position with very little static consumption.

The cam is where the positioner adapts to different actuators. A linear cam is supplied, with angle up to 270°, for actuators of different stroke lengths, and the operating angles and range adjustments are printed on it. Special cam profiles can be ordered to characterise the valve or to change the flow characteristic. In the electro-pneumatic version an I/P converter sits ahead of the pneumatic stage to convert the 4-20 mA input to pressure.

Mounting and feedback options for the CRP-3/M

The positioner can be ordered with a lever or with a bare shaft, and in standard or NAMUR (VDI/VDE 3845, EN 15714-3) mounting. It is intended for linear actuators with a minimum stroke of 8 mm and for rotary actuators, using different feedback systems such as a lever or a motion converter. On long or short linear strokes the IMI STI MC motion converter provides the rotary feedback shaft. Optional accessories include pressure gauges, custom cams and an intrinsically safe electro-pneumatic version.

Selection and ordering checklist for CRP-3/M

• Signal type: pneumatic 3-15 psi or electro-pneumatic 4-20 mA.
• Linear or rotary actuator, and the stroke or angle.
• Single or double acting, and spring-return direction on air failure.
• Lever feedback or bare shaft; standard or NAMUR mounting.
• Cam: linear standard or a special characterised cam.
• Supply air available within 2.5 to 7 bar.
• Intrinsically safe version if the area is classified.
• Pressure gauges for supply and output, if wanted.
• Spool material: standard, or full stainless steel spool valve option.

Installation, calibration and maintenance of the CRP-3/M

Mount the positioner on the actuator with a bracket that suits the lever or NAMUR interface, connect supply and output tubing, and fit the electrical cable to the 1/2″ NPTF entry on the electro-pneumatic version. Calibration is done mechanically: set zero with the valve at one end, set the span at the other, and check mid-travel. Check the cam angle printed on the cam against the actual actuator travel before fixing.

The case is protected to IP65, but it should still be placed so that cable entries face down and vents are not obstructed. Instrument air quality is the main maintenance driver. Moisture or oil causes spool sticking and drift, so keep the supply filter regulator drained. If the valve hunts, check the feedback linkage for play before touching the positioner.

Typical applications in Oman and the GCC

The brochure lists oil and gas, refining, chemical, power and metal production as typical fields. In Oman and the GCC the CRP-3/M is relevant on older control valves that still run on pneumatic or 4-20 mA analog loops, on offshore platforms where a rugged non-electronic option is preferred, and in plants where a like-for-like positioner replacement is easier than a digital retrofit. The manufacturer says it suits standard, sandstorm and copper-free ambient conditions and low and high ambient temperatures.

Pneumatic versus electro-pneumatic CRP-3/M

Information to send with your CRP-3/M enquiry

• Input signal type and range, including split range if used.
• Actuator type, size, stroke or angle, and acting direction.
• Mounting interface (lever, bare shaft, NAMUR).
• Cam requirement and any characterisation.
• Hazardous area classification.
• Quantity and delivery location.

Documents and quotation

A manufacturer brochure for the CRP-3/M is attached on this page. It contains the specification table and dimensional drawing; the instruction manual is published separately by the manufacturer.

Send the part number CRP-3/M with your actuator and signal details and Seven Star LLC will prepare a quotation for Oman and the GCC.