4-20mA 3-15psi ? 3

[Herman2]

We are replacing chilled water and steam valves that have pneumatic actuators. The steam valve operates between 4 and 8 psi. The chilled water valve operates between 9 and 13 psi. We still want to use pneumatic actuators, but we have an electronic control system that can drive both valves between 3 and 15 psi.

Any reason not to spec new actuators that operate over the full pressure range of 3 to 15 psi? I don't see why not, since we are trashing the pneumatic thermostats and using I/P transducers.

What do the experts say?

Thanks.

 

[valvendamprguy]

Why do you want to go pneumatic? You get superior accuracy via resolution and repeatability plus less hysteresis, and no spring range shift (since there is no spring)
For accurate control of pneumatic actuators, you need to have positive positioners and high quality IP's not just any transducers.
Electronic control valves have so many advantages over the other way.

 

[imok2]

If you have been getting acceptable results with pneumatics and extreme accuracy is not a criteria then ,yes I would stick with pneumatics and the original spring ranges otherwise when you get to 8 lbs on heating to close the valve and you have a 12 lb spring range the valve will still be in a heating mode as you try to open the cooling valve...No?

 

[Herman2]

Pneumatic is a given on this project, I don't have a choice. I planned to put each valve (chilled water and steam) on separate air lines and pressure sources (I/P transducers). So each could be operated over the entire 3 - 15 psi pressure range and there would be no interaction.

I assume the only advantage of the 5 to 8 and 9 to 13 psi scheme is that you can use one pressure source.

Or, maybe I should use one pressure source to be sure I never have both valves open?

I was thinking I could get better resolution over 3 - 15 psi.

Thanks for the fast responses, I actually need to make a decision tomorrow morning.

Any more thoughts?

Cheers!

 

[pablo02]

I not familiar with your application but we have used split range controllers to control separate valves in power plants that will have the same ranges you refer to... as you allude to and others point out, you want the operation of these valves to be dependent upon each other, not independent -- this assures that these valves do not operate at the same time... I think the split range was used initially on purpose and it would be wise to maintain that funtionality...

 

[Whoops]

I did much the same some years ago on a 1970's Satchwell pneumatic control system. I had to retain the pneumatic actuators due to a lack of finance. We controlled them with electro/pneumatic transducers controlled from a Trend BMS system. The control of the MTHW heating and the chilled water valves had to be separated so that the cooling & heating could be on simultaneously for de-humidification and re-heat. The amended system worked fine for many years with no major problems, thanks to a competent controls contractor.

All the 1970's 3-port heating valves eventually started to fail and the system was eventually converted to all electric actuators.

In your position, I think I'd specify 3-15psi actuators, but the 3-6/8-13 psi actuators worked fine. They had a very fast response and the room temperature graphs were flat lines for many years.

 

[ChasBean1]

I agree with Whoops & others (now that it's too late). You really can go either way. The separate spring ranges fed from the same E/P will guarantee no overlap, but the 3-15 fed by separate E/Ps I think would be likely to provide better throttling. You can still ensure overlap doesn't occur through programming and using deadbands in your control setpoints.

 

[okidataman]

http://www.sea.siemens.com/instrbu/docs/pdf/77 & 771.pdf

I understand the pros and cons of using electronics to control pneumatics from the perspective of a control technican and that of an operating engineer.

If all else fails, the watch engineer can pull off the control devices and apply supply air pressure (20 - 25 p.s.i) or use a Ben Casey (sphygmomanometer) to override complete system failure.

I particularly prefer devices that utilize a current signal over voltage or resistance as it pertains to ease of troubleshooting. My digital multimeter can measure current, voltage and resistance as well as generate a 0-30 mA signal.