PROCESS & CONTROL | FEBRUARY 2018 19

PUMPS, VALVES & ACTUATORS FEATURE

/ PROCESS&CONTROL

Valve actuator instrument tubing(Figure 1) can have a tough life. It can

be damaged by rough handling duringshipping or transport. Workers tend tolean ladders against it, stand on it, pick upvalves by grabbing onto the tubing,overtighten connections and hang onto itfor support. Excessive vibration can damage a control

valve’s actuator tubing by causing fatiguedamage at or near the nut/ferrule location,resulting in leakage. Leakage causespressure to be lost, and without pressurein the tubing, the actuator cannot becontrolled. Leakage also requires increasedsupply pressure, adding to operationalcosts.Valve actuator tubing that is exposed to

the elements can freeze in coldtemperatures at low spots that collectcondensation, thus blocking pressuresignals. Freezing can also cause cracks andleaks. As you may know, the valve actuator

controls the position of a valve plug. Valveactuators can be powered by hydraulics,pneumatics or electric motors. In a springand diaphragm pneumatic actuator,compressed air acting upon a rubberdiaphragm moves the valve plug/stemassembly. A pneumatic actuator can bespring-closed or spring-opened, with airpressure overcoming the spring to providemovement. A "double acting" actuatoruses air applied to different actuator portsto move the valve in the opening orclosing direction. A central compressed airsystem is usually the source of the clean,

Mike Sommerlot, Emerson Automation Solutions,says valve actuator instrument tubing can be subject toabuse, and damage from vibration and environmentalextremes, which is why the company’s 667 size iactuators now eliminate this problematic element

done so through various solutions.Emerson, for example, created an internalair passage in the actuator yoke toeliminate external tubing. Emerson cast an internal air passage into

the yoke of a Model 667 size i pneumaticactuator (Figure 2). This eliminates theneed for external tubing from Emerson’sFisher FIELDVUE digital positioner to theactuator diaphragm casing. Thisconfiguration eliminates potential leakpaths that can develop at tubing andfitting connections, as well as reduces thepossibility of damage to tubing throughimproper use. Now that the positioner is mounted

integrally to the cast iron yoke boss –instead of the former approach of boltingto a mounting bracket, then to the yokeboss – this configuration is much morerobust and the total assembly becomesmore resistant to vibration.The internal air passage also simplifies

installation. The Model 667 size i allowsthe FIELDVUE positioner to mount using50% fewer mounting parts, reducinginstallation time and expense. Also, thereis no tubing to bend or cut, no ferrules ornuts to attach, and no chance of losing ormismatching those parts.Eliminating external tubing is a

significant improvement in actuatortechnology. By eliminating tubing, valveactuators become more reliable, less proneto vibration problems, and immune todamage from misuse and inclementweather.

Emerson Automation Solutions www.emerson.com

Figure 2: A standard 667pneumatic actuator(left) has externalactuator tubing runningfrom the positioner tothe diaphragm.Emerson’s Fisher Model667 size i actuator(right) replaces thistubing with an internalair passage

Mike Sommerlot holds aBA in BusinessManagement. He isproduct marketingmanager for FisherSliding Stem Valves atEmerson AutomationSolutions

Figure 1: Externalinstrument tubing on acontrol valve can beeasily damaged byimproper use — such aspicking up the valve bythe tubing line

dry, compressed air needed for pneumaticactuators. The actuator is usually paired with a

positioner. The positioner is connected toa control system, which sends the actuatorcommands to open, close or adjust theposition of the valve plug via a 4-20mAsignal, fieldbus signal or wireless signal.For example, a digital positioner takes a

control command from a 4-20mA HARTconnection and adjusts the pneumaticsignal to the actuator diaphragm from 0to 60 psig. The pneumatic signal travelsfrom the positioner to the actuatordiaphragm via tubing. Emerson conducted market research,

including a customer analysis along withsurveys to our sales offices and servicesector, in order to identify what issuescustomers bring up most often withrespect to spring and diaphragmactuation. Along with these activities, wereviewed our Quality Assurance archive fora three-year period to understand theroot-causes and frequency of actuation-related warranty claims.We determined that the two biggest

problems for customers weretubing/fitting issues and actuators beingassembled incorrectly, often directlyrelated to tubing and fittings. We alsoidentified that end users had two prioritieswith respect to valve actuation: increasedreliability and robustness, and reducedassembly complexity.A solution to both of these top-two

identified issues was eliminating externaltubing. Several valve manufacturers have

ELIMINATING TROUBLESOME TUBING

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