Temperature Controlled and Pressure Controlled

[learner08]

Hi All,

Any help is welcome.

Talking to ppl, I understand that for steam systems, there is a difference when a person says they want pressure control or when they want temperature control. But then, for sat steam, isn't pressure and temp fixed?

Also, in this context, certain valves are better for temp control and certain valves are better for pressure control. What is the idea behind this?

I do not know if what I wrote makes sense to you but hopefully it does and I can get meaningful replies from it.

 

[MikeHalloran]

Indeed, if you are heating stuff by condensing steam, the temperature is locked to the pressure, so that's all you need to control.

Which doesn't prevent a good salesman from selling temperature limiting and alarm equipment into a condensing steam system anyway.

I don't know if it's still true, but forty years ago, virtually every US paper mill had a dancer roll operated steam shutoff valve, "so that a broken web wouldn't cause the dryers to overheat and start a fire", which is essentially what one exceptional peripatetic salesman told gullible plant managers. The dryer rolls were then universally heated by condensing steam, and just flat couldn't get hot enough to ignite the paper, whether it was moving or not.

Of course, if you are heating with superheated steam, or fire, or hot gas, or other stuff that's not condensing, you need temperature controls.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

And I thought I was going to learn a new derogative word, but all is means is travelling.

 

[25362]

As for the valve characteristics visit:

http://www.spiraxsarco.com/learn/modules/6_5_01.asp

 

[davefitz]

although sat temp and press are interconnected, it is far safer to control using pressure control. In the final analysis, the boiler's pressure vessels ( and downstream system's) are designed to prevent failure due to overpressure, so using a pressure controller with a 1% error is a safe practice.

Thermocouples can have as high as an 8F error, and this worsens due to corrosion. Also, add'l temperature errors occur due to conductivity to other systems , and large transient errors can occur due to thermal inertia of thermowells, etc.

 

[25362]

Nevertheless steam temperature control is needed with superheaters and attemperators.

 

[davefitz]

Yes, for superheated steam, there is no direct relationship between temp and press. For sat steam, an 8F error in temp measurement implies a corresponding 50 psi error in pressure.