Positioners are used to control the air load pressure on the valve actuators. The positioner forces the actuator to move the valve until the desired position is reached and balanced according to the output signal from the controller. The control signal to the positioner could be electrical or pneumatic. They can be 3-15psi, 4-20 mA, 0-10V, digital or different protocols like HART or Profibus.

Why use positioners
The main purpose of a positioner is to enhance the control resolution of the valve. They ensure the valve position gets as close as possible to commanded position by the control system. They can mitigate the effect of dynamic variations that cause adverse controllability, such as changes in packing friction arising from corrosion, dirt, lubrication, sloppy linkage (causing dead band) and non-linearities in the valve actuators. They can also improve the response speed based on the received signal enabling faster air loading and venting.

How positioners work in the control loop
Depending on the type of valve, positioners are normally mounted either on the side-yoke for linear valves or at the top of the rotary valves body. However, there is always a mechanical linkage to the valve stem. When the process variable (e.g. flowrate, temperature, pressure) differs from the set point, a control signal from the controller inputs the positioner. The positioner sends a corresponding air pressure load to the actuator. Once the valve position is altered and sensed by the mechanical linkage, the feedback signal is sent to the controller and compared to the set point to make sure that the valve has reached the desired position. No position feedback could be achieved without a positioner in the control loop, resulting in less control precision.

Depending on the actuation configurations, positioners can fit both double acting and single acting (spring return) actuators. Positioners for single acting actuators have only one air output to the actuator while double acting types have two air outputs to be used with actuators without spring.

Ramén offers a number of analog and digital valve positioners from Flowserve, Siemens and Schneider Electric. Please contact us for more information on the positioners in our control flow packages.

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PMV D3 positioner, supplied by Flowserve, is the most intelligent digital positioners among PMV D-series; however, it is extremely easy to use. This type of positioner can be locally adjusted to fit both single and double acting pneumatic actuators. D3 type features a zero bleed pneumatic relay reducing the air consumption which results in less operational (electricity) cost.

D3 series can offer wireless communication lowering the amount of wiring while eliminating wire damaging and preventing connectors from corrosion in the plant.  D3 wireless utilizes dual backbone routers to deliver extra radio paths as a safety feature and can be configured by the user when the signal or power is lost. D3 model has different versions for explosion proof, fail freeze, intrinsically safe, remote mounted and version for SIL2 requirements.

PMV D30 is a sturdy and advanced digital positioner, supplied by Flowserve, delivering a great air capacity to fit applications with larger valves. Suitable for both single and double acting pneumatic actuators, this digital positioner offers reliably and easy operation with low maintenance cost and downtime using ValveSight DTM predictive diagnostics system.This system enables a communication dashboard between operator and the valve. It helps operators to know the valve status and showing unexpected trend to avoid sudden plant stoppages. D30 series has also remote mounting option for harsh environment.

PMV D20 is a compact, robust and intelligent digital positioner supplied by Flowserve, offering high control resolution (even on small valves). With simple design and easy operation this positioner is a proper choice for single acting actuators. It is also available in standard and explosion-proof housing for hazardous areas.

D20 series can calibrate itself automatically by pressing a bottom in just a few seconds. Its modular design also provides various options fitting different requirements.

All PMV D-series can offer ValveSight DTM predictive diagnostics system. This system enables a communication dashboard between operator and the valve. It helps operators to know the valve status and showing unexpected trend to avoid sudden plant stoppages.

Foxboro SRD998 series is a smart and highly reliable digital positioner supplied by Schneider Electric. Designed for both single and double acting actuators, SRD998 can be operated from control systems, controllers and PC-based configurations.

SRD series functions applying nozzle and flapper mechanism. This principle is all about a pressure signal produced by a mechanical movement. That is a fluid flow restriction through a nozzle covered by a flat plate called flapper generates the pressure signal.

PMV P5 and EP5 series are pneumatic and electro-pneumatic analog positioners supplied by Flowserve. These types of PMV series, with a modular and robust design, allow easy installation of I/P convertor and/or feedback units and can provide high gain, long lasting and accurate performance.

They can be supplied in various types proper for explosion proof, fail freeze and fail safe requirements. Perfectly fit both single and double acting pneumatic actuators.