Requirement of Valve Postioner
Requirement of Valve Postioner
(OP)
Why valve positioners are required in Pneumatic Control Valves ??
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Requirement of Valve Postioner
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Requirement of Valve PostionerRequirement of Valve Postioner(OP)
Why valve positioners are required in Pneumatic Control Valves ??
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RE: Requirement of Valve Postioner
David Baird
mrbaird@hotmail.com
Sr Controls Engineer
EET degree.
Journeyman Electrician.
RE: Requirement of Valve Postioner
I'll go a little farther -- I think the term you meant is "positive positioner". Since pneumatically operated valves have fixed-diameter diaphragms, the force available to operate a valve or damper varies with the control signal pressure. Thus a 10-square-inch diaphragm operating a valve over a 3PSI to 15PSI signal range will develop 150 pounds of force at the high end of the control band, but only 30 pounds at the low end. A positive positioner takes the control signal as an input, and outputs whatever it needs (up to main air pressure) to put the end device in the postion demanded by the control signal. It's a position controller, in other words. It can deliver the same operating force at either end of the stroke.
Hope this helps!
Old Dave
RE: Requirement of Valve Postioner
RE: Requirement of Valve Postioner
#If the control valve is operated without positioners,there will be hysterisis in the operation of the valve.Control will not be accurate.In this case positioners helps in exact positioning of the control valve.
RE: Requirement of Valve Postioner
I have found and used electric actuators that use a simple spring to obtain fail safe action. They worked for us.
The website is: http://www.neptronic.com/
Richard Neff
Richard Neff
Irrigation Craft
RE: Requirement of Valve Postioner
valve positioners are used in those processes where the production conditions can vary. If you assume the process is constant you only need a i/p transducer. Between selection of the shaft spring and the actuactor spring you have a large ability to "fudge" the set up. To decrease the stroke tighten the shaft spring then reset the connection between the valve plug shaft and actuator shaft to set up zero. both of these adjustments are usually found "in" the yoke area of the valve. Fail safe is provided by setting (selecting?) the air inlet to the bonnect and the springs under the in bonnet of the actuator. bearing in mind which way the plug or valve operates and lot of units can't be changed over after purchase.
Now if the valve is in a closed loop control situation the PID (assumed = all functions) will vary the signal to suit the changes in process. eg pipe pressure for flow head changes on level etc etc etc. This is a common cost saving measure and I'm not saying it's nice or elegant but there's a lot around. The draw back is the i/p deals with a lot more air (or pnuematic controller of direct connect like some of the really old systems were) and generally need to be a bit more robust and the air a bit cleaner.
I have seen (and it is common) split valve pairs and i/p units set to a huge variety of pressures.
Now having given all that waffle the must have reason for a postioner is a process that varies. You can bench set your valve for a given pressure under the valve plug then find that when the process changes you use 80% of the controlrange to lift the valve 5% which really gets operations people wound up.
I think DRWieg hit the nail on the head with the phrase "positive positioner" you have available all the motive power of your air supply to put the valve in the postion you want and hold it there. This is why It's good to put reasonable guauges on all your positioners. They will tell you a lot about whats ahppening in the processes.
Phew that's a lot for me today so I'll jump off my soap box but if I need to clarify anything please point it out
All the best
Don
RE: Requirement of Valve Postioner
Valve actuators are not all selected for 3-15 psig. More force is available with 6-30 psig. For very large valves the actual output to the valve may require the 80-100 psig instrument air supply. If you are using a 4-20 mAdc signal, you already need a signal converter to convert the electronic signal to a pneumatic signal. We work with many valves on a project and some are quite large. In general, we have moved to the digital valve controllers. Most of my experience is with Fisher-Rosemount. Similar products are available from ABB, Valtek and others. The added cost is not significant for a major facility project. However, if you are using 1-inch valves in a very small project the digital valve positioner becomes significant. Also, the 1-inch valves tend to not require a positioner.
John
RE: Requirement of Valve Postioner
http://www.emersonprocess.com/fisher/products/valvepositioners/
etc. for more info