Requirement of Valve Postioner 2

[abs710524]

Why valve positioners are required in Pneumatic Control Valves ??

 

[dbaird]

Positioner: To throttle the valve in response to a varying control signal. Pneumatic actuators provide a means for fail-safe action which for electric actuators may mean dc motors with battery backup which are both cumbersome and expensive. Upon power or signal failure a spring-return actuator drives the valve to a pre-determined safe position. This is a practical and inexpensive option with pneumatic actuators and is an important reason for the wide use of pneumatic actuators throughout the industry.


David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[DRWeig]

Hi ABS,

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

 

[abs710524]

What I understood that Valve positioner is used to ensure correct positioning of control valve. Any positioner will have input from I/P Convertor and send the required amount of air at required pressure to pneumatic valve. Is this correct ?? What is the mechanism / electronics inside a pneumatic valve positioner. How it works? Is this require for each and every pneumatic valve ?

 

[arunachalam]

#All pneumatic controller orI/P convertor's o/p will be only 3-15 psi.But the design of the controlvalve may have different bench set.In this case positioners are used to amplify the signal.
#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.

 

[PUMPDESIGNER]

In response to dbaird's statement about it being expensive and cumbersome to obtain fail-safe action on an electric actuator.

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

 

[don01]

hi all
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

 

[jsummerfield]

There are a few accessories of note regarding control valves. The discussion here mentioned valve positioners. Another accessory is a volume booster. Many valves with properly sized actuators require no positioner to provide satisfactory performance. A volume booster can improve the speed of response.

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

 

[jbartos]

Suggestion: Visit

http://www.emersonprocess.com/fisher/products/valvepositioners/

etc. for more info