pneumatics

[indycc]

Just wondering if anybody can help with this question. What protects the controller from high pressure in a pneumatic control system?

[i b cool]

Pressure relief valve protectc devices if regulator fails.T-stats and most controllers max psi is 30 but some need 60 psi or more.

[Control Man]

60 PSI is a bit TOO HIGH for Commercial Pneumatic devices 30 PSI is the MAX that " most" devices will handle.

Main air pressure is reduced to 30 PSI by a PRV , control air pressure depending on the manufacturer runs 13 - 22 PSI.

On a pilot positioner for a Valve or Damper actuator you would use both the 30PSI to drive the actuator and the control pressure to operate the positioner.

safety relief valves should be on the downstream side of any PRV to protect the system if the PRV fails or is incorrectly set.

[Edited by Control Man on 04-14-2005 at 10:57 AM]

[powerhead]

Originally posted by Control Man
60 PSI is a bit TOO HIGH for Commercial Pneumatic devices 30 PSI is the MAX that most devices will handle.

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I have a system where a 20# line goes to "normal" devices such as thermostats and small actuators and a 60# line goes to large "Commercial" devices --Cooling tower by-pass valves, Bray valves on boiler headers etc.

Very common applications. 20# circuit is protected by a 30# relief valve and 60# circuit is protected by 75# relief.

Also have systems with a 100# riser and each floor has a pressure reducing station to knock the pressure down to what is required for each branch.

[i b cool]

I have several 60 psi devices is some of the larger buildings I service, usually a large (8 inches and up) chilled water valve or bldg deny valve requires higher psi for close off force againsed 140 psi water.Same setup as powerhead stated, 100 psi 1/2 riser (15 story bldg) with regulator/relief station at each floor with end of mains looped into itself.

[Control Man]

Your VALVES ( BRAY and 8" and up ) are not the NORMAL COMMERCIAL application type devices and the part about MOST DEVICES in my posting would cover that.

In nearly 30 years of pneumatics very seldom ran across anything from the normal COMMERCIAL control companies operating above 30 PSI in a basic commercial installation.

I have ran across loads of devices in central heating / chiller plants operating above 30 PSI but those are not what you would call NORMAL applications as what my post was about.

The 30 / 20 PSI was a basic reference to a TYPICAL pneumatic system , I have systems that are 100 PSI + on the Mains and stepped down to required range throughout a 185,000 sq foot 1 storey building and others that have a 30 PSI main that supplies a 24 storey building.


A typical BARBER COLMAN 6" 3 way valve will close off against 125 PSI Water with an actuator that takes 10 PSI , and yet a 2 way 6" will only close off against 21 PSI with a 20 PSI actuator , so you see not everything is TYPICAL

[Edited by Control Man on 04-14-2005 at 11:10 AM]

[i b cool]

Exuuuuuuuuuuuse me

[Control Man]

I posted a valid response no need for the CHILDISH reply

[i b cool]

U posted an insecure defence, I was just stating that there are high psi devices, but it doesnt matter because i have ony 17 years control experience so u must know more than me.

[Control Man]

Like I said no need for CHILDISH response , never disputed the fact there are HIGH PRESSURE devices just that they are RARE on TYPICAL commercial pneumatic applications.

It was far from a INSECURE DEFENCE unlike you reference to 17 years experience and how I know more, again very childish.

[i b cool]

I was refering to ur previous reply stating "nearly 30 years pneumatic experience" So u started the whole "childish" thing. Also no one said anything about typical, cause whats typical for one may not be typical for another.............Im done arguing with u!!!

[key]

maybe just to add to this I have always be curious. If air pressure is lost will the pneumatic valve fail in the open or closed position?

[fastfred]

Originally posted by key
maybe just to add to this I have always be curious. If air pressure is lost will the pneumatic valve fail in the open or closed position?

It depends on which type is selected and how they are applied. They can be purchased,N.C.--requires air to open,N.O.--requires air to close. these two types make up most of the typical applications.

[flange]

in most control systems, you will run anywhere from 60 psig on up to 100 plus depending on how the distribution piping etc was designed. its very commonplace for the distribution to be at say 100 psig and get reduced by floor or zone to 20 or less. USUALLY the relief device is at the downstream of the prv and sometimes contained within, especially for honeywell prv's. most commercial controllers will be damaged at either 25 or 30 psig. in addition, this air should be clean down to about 3 microns, especially on the newer(plastic) controls which contain very small orifices. the valves talked about requiring 60,80 or even 100 pound air to the cylinder are used extensively around here on comercial jobs, even though most wholesalers refer to them as industrial valves. the position of an end device is determined by its normal rating i.e. normally open means thats how it is as it comes out of the box. you apply air to close it.