BLRA Pneumatic Actuator (Type 766)

DESCRIPTION

The Type 766 actuator with integral, top-mounted positioner is a pneumatically operated, spring-opposed diaphragm actuator designed specifically to fit the Research Control Valve body-bonnet assembly. The unit is available in two sizes: one for the 1/4 in. (6 mm) valve and another larger version for the 1/2…1 in. (12…25 mm) valves. The unit, when equipped with the model BLRA positioner, functions as an air-to-open actuator retracting the stem and opening the valve on an increasing instrument signal. The unit is designed to extend the stem, closing the valve, on a decrease or loss of instrument signal. A force-balance system is incorporated using the full force of the supply air to position the stem precisely and with a high degree of repeatability. This type of actuator should be used when the application calls for high positioning accuracy or when greater force is required over the standard actuator such as in the case of high shutoff pressures or excess packing friction.

FUNCTION

The Type 766 actuator normally operates in response to a 3…15 psi (0.2…1 bar) change in instrument signal or a 12 psi
(0.8 bar) range. The span, or range, of the instrument signal, is determined by the feedback range spring mounted directly under the positioner. The feedback range spring is responsible for sensing the position of the main diaphragm as the instrument signal
changes. The position is then transmitted through the spring, directly to the positioner diaphragm assembly. The valve spring, visible in the yoke area, provides the downward thrust necessary to counteract forces created within the valve. Consideration should be given to the amount of preload required for proper operation since the main valve spring only provides downward force.

Description

SPECIFICATIONS

OPERATION

The actual operation of the unit is simple. Two airlines are required: one to provide the instrument signal and one to provide supply air. The amount of supply air required is determined by the spring force necessary to overcome forces generated within the valve. The standard minimum supply pressure is 22 psig (1.5 bar) of clean, filtered, dry air. The two airlines should be connected to the ports marked Supply and Inst on the positioner. The “load” and “aux load” are not used in normal installations and are provided with “vented” stainless plugs. These plugs should not be removed. The port marked “valve” is provided with a blind pipe plug and should be left in place. Since this port is an integral part of the piping of air from the positioner to the main diaphragm, it can also be plugged with a gauge, which will indicate the actual output of the positioner to the air diaphragm.

Upon an increase in instrument signal, the position of the pilot within the positioner is shifted down, causing the supply air to be re-directed through internal passages to the main valve diaphragm cavity. As the main diaphragm travels upward, the feedback range spring is compressed. The increased force, created by the range spring, is transmitted to the diaphragm assembly in the positioner. The upward shift in the positioner diaphragm assembly causes
the pilot to re-position and assume a balanced state. The entire function creates a complete feedback loop within the unit, causing the valve to position accurately and with a high degree of repeatability.