Pneumatic Bonnet Steam Valve

Owen

I run seven boilers for a school district. We have four systems that have pneumatic controls. There are about 50 unit ventlators in classrooms. Lots of fin/pipe and fin/tube radiators also used in the buildings in halls and such. The uni-vents all have a pneumatic bonnet which modulates between full open and fully closed as dictated by the thermostat. Some also have an odd venting scheme with 1/4" copper no one around here understands anymore.

Question 1. Does a modulating steam valve (the bonnet) require additional pressure at the boiler (above Dan's 2# maximum; try selling THAT to my knuckleheads) because it is often less than full open? In other words, if only 1/2 open, pressure is 1/2 that at the boiler? (I was told that by a vendor's rep.)

Question 2. What is the 1/4" piping and what does it do and how does it work?

I'll post photos tomorrow.

Steamhead

They don't need more pressure

as long as the heating unit is full of steam when the valve is open. That's the idea behind the modulating valve- open wide when full steam is needed, then close partially as needed.



The 1/4" vent lines may be a variation of the Paul system. Do they lead to a vacuum exhauster?

Oak Park Electric

Pneumatic valves

Definitely post pictures.  We need to see the valves, the control tubing, pic of a room thermostat won't hurt either.  Also, is this a 2 pipe system with traps?  Are there powered vacuum pumps on the condensate side? ( In the basement or boiler room, not at the units )  What is current operating pressure?  Most importantly, are there any noticeable problems with the system?  

Owen

Problems?!!

Photos coming this afternoon. All seven systems are two pipe w/traps, w/o vacuum, about half of the traps are over 20-30 years, & assumed bad. I've replaced about 120 of maybe 250-300 traps.

Because of the ongoing debate over pressure, it varies. Yesterday at the building in question I turned it up to 6-7# because there are many rooms not heating, and my super thinks that is the answer to that problem.

I just received a dozen thermostatic traps yesterday and will be starting on replacing those in rooms not heating. I did one yesterday and ta-da, heat! Money is very tight and this stuff is "out of sight, out of mind" to the admin., but I get a bone thrown my way ever so often.

Most uni-vents throughout all the buildings are Herman-Nelson, some AAF, with Johnson Controls pneumatics.

Interestingly, on the original blueprints for this building (Sixth Grade School) in the Mechanical section, the stated operating steam pressure is, guess what, 2#.

Owen

Ma, I'm Having a Hard Time With My Unit-Vent Homework!

Pic 1. Plumbing schematic;

Pic 2. This is a Unit Ventilator, the picture is blurry because they make my head spin;

Pic 3. Back end of steam coil core;

Pic 4. 1/4" vent piping, old trap, old bonnet thermostat, pneumatic outside-air piston/actuator, another thermostat, pressure switch.

Pic 5. New trap, aquastat.

Pic 6. Close-up of 2nd thermostat

Dave in QCA

Control Prints

Johnson Controls would have created temperature control prints for their system. Do you have them?  They will be invaluable to help you understand the controls in the univent.  I am quite familiar with various setups and can help you through the sequence of operation.  Additional informtion that I need. 

Is there a wall mounted thermostate that provides an air line to these units, or is the control coming from the integral thermostat on the valve top?

Is there more than one copper air line coming into the  unit?

Is there a central switching point that allows your to turn the univents on and off?  If so, is that a pneumatic switch, or an electrical switch?



Regarding the Steam Pressure, the building prints are the best indicaiton of what pressure the system was designed to be operated at.  Cv factors of the control valves and other piping arrangements that could necessitate higher pressures would have been based on the design pressure.

I managed the operation of a school building many years ago.  It was designed to operate at 5 PSI.  There were 2 air handling units hanging from the ceiling in the gym.  The condensate lights went up about 3 feet from the bottom of the air handlers to the steel trusses that supported the roof, and then crossed the gym before they dropped in a piping chase and eventurally to a condensate receiver station.  If the pressure was operated too low, there was not enough to lift the condensate and the air handlers would hammer like hell.  At 5 PSI, everything was fine.   I suspect that you do not have any situations like this, but keep it in the back of your mind.  You should be OK at the design pressure of 2 PSI.

Owen

Reply to Dave in QCA

Answers:

1. No Johnson Control blueprints that I know of; I'll look in our blueprint library, which we actually do have.

2. Thermostats were originally (1973) inside the units but about half have been replaced by wall mounts.

3. I don't think so. One copper line in. I'll check.

4. No central switching point other than the breaker panel.

Questions:

1. What is "Cv"?

2. The most condensate has to lift at at any of the seven buildings with boilers is about 14" (inches). Would that not require less than one half pound to do the lifting and half a pound for pipe loss plus a pound to give up the latent heat to the fan coil unit or radiator, so three or at most four would be oodles and gobs, right?

3. There is thinking that if there are rooms that won't heat, and if the boiler pressure is turned up and the problem goes away, who wouldn't? How can you argue with that? When I turn some boilers down all kinds of problems become apparent, bad traps, bad pitch, lack of venting and nobody wants to hear it. I admit I am making some decent headway but these are lean times for school districts all over the country. We are actually pretty well off. Next year is looking scary though. Could you or anybody else on this blog comment on how to deal with that mind set?

Dave in QCA

Answers / guesses

In Picture #1, the plumbing schematic, what they are showing here is the 1/4" copper tube that runs from the the coil through a device that they have labeled, "Pressure Equalizing Unit",  This is probably some kind of a check valve that would operate as a vacuum relief for the coil.  These kinds of copper fin tube coils do not tolerate vacuum well at all.  If the coil was hot, and the control valve closed, it would produce a very strong vacuum in the coil.  This pressure equalizing unit is probably simply a check valve the allows are to flow from the condensate line (note that they point out that the condensate piping must allow free air passage above the condensate)

Picture #2. no comments other than don't ever leave the cover off if you leave the room and children are in the buidling.  These things can start up on their own.

Picture #3.  Motor, speed selector switches, etc.  I am a little confused as to whether I am looking at 3 air lines, or 2 air line and 2 conductor electric line.  I am pretty sure it is the latter.  The 2 air lines are coming from the other side.  The pass through the solenoid.  Its purpose is to prevent air from reaching the damper actuator when the unit is not running.  Therefore, when the fan is running, the solenoid is powered and lets the air pass through.  When the fan is not running, the air line is shut off.  An extra port in the solenoid lets any air pressure in the damper actuator release so that the damper will close when the fan is not powered.

A note on pnuematic terminology.  The compressor which is probably located in the boiler room produces air pressure that usually runs through a pressure reducing station and usually a dyer and oil filter.  The air that goes out to the building is usually at 20 PSI.  This is called "Main Air".  Once the Main Air has gone through a thermostatic device, it is called the signal, T'stat output, control air, etc. depending on the company that installed the system, such as Honeywell, Powers, Johnson Control, and others.

Picture #4,  Ah... now we are getting down to business!  The gray device in the upper left corner is a PE switch.  (pressure electric)  Signal Air pressure on the line above the set point will close the contacts in the switch and cause the fan motor and the solenoid to be activated.    

The big silver colored cylindrical device to the right of the PE switch is the Damper Actuator.  Signal air pressure applied to the actuator will cause it to open the damper.  This is a modulating device that operates off a pressure range, typically 6-11 PSI.  at 6 PSI the fresh air dampers are fully closed and return air dampers are fully open, at 9 PSI, the fresh air dampers are fully open and the return air dampers are fully closed.  

The green device located to the right of the Damper Actuator, attached to the side of the blower assembly is the low limit thermosat.  It has a capilary tube inserted into the discharge air stream.  It usually has a set point of 60 F.  Its job is to overide the demands of the thermostat.  If the thermostat is asking for full cooling, and the outdoor air is 30F, this low limit divice will act to cause the dampers to modulate so that the discharge air is no lower than 60F 

The round green colored device fastened onto the top of the valve is actually 2 devices combined into one.  Because it is fastend to the top of the valve, it is obviously a valve actuator.  This is a modulating device and typically a heating valve operates on the range of 4-8 PSI.  At 8 PSI the valve is closed.  As the pressure falls, the valve begins to open.  At 4 PSI, the valve is fully opened.

( A point to remember with pneumatic systems is that they fail to full heat.  That means if something goes wrong with the temp controls, you get full heat.  The pneumatic system is not needed to operate the heat, merely to control the heat.)

The round device is also a thermostat.  It has a capilary tube that extends down into the return air stream, thus sensing the room temperature.  The operation of this thermostat internally operates the valve actuator.  In addition, it also puts out an air signal.  This "signal air" exits the actuator through a small bright green plastic device that looks like it is simply a hose barb, but it is more.  It is a restrictor.  It lets the signal air exit into the plastic tubing, but only at a very small flow rate.

The gray divice strapped onto the steam pipe is a thermostat.  Its purpose is to sense the temperature if the steam supply pipe, and when steam is present, it activates the fan motor and the solenoid.

Sequence of operation is as follows:   When the room temperature is at the setpoint of the thermostat, the signal air will be 8 PSI.  The steam valve will be closed.  If steam is present, the Pipe mounted thermostate contacts will be closed and will cause the fan and the air solenoid to operate.  The fresh air damper and return air damper will be positioned to allow a small amount of fresh air to be drawn in for ventilation purposes.   If the temperature of the outdoor air is cold enough that it causes the discharge air temp to drop below 60F, the low limit thermostat will modulate open and will bleed off sufficient signal air to cause the dampers to modulate closed to the point that the unit delivers air no cooler than 60F.   If the temperature of the room falls, the thermostat output (signal) will drop.  As this occurs, the dampers will modulate so as to partially close the fresh air and open the return air, at the same time the steam valve will modulate partially open.  If the room temperature continues to fall, the thermostat signal will decrease, below 6 PSI, the fresh air damper will completely close and the steam valve will be over half open. If the room temperature rises above the set point, the steam valve will modulate closed, the fresh air dampers will modulate open.  If steam is not present and the room is at or below the set point of the thermostat, the unit will not operate because the pipe mounted thermostat contacts are open.  If steam is not present, but the room temperate is greater than the set point, the signal air will cause the PE switch contacts to close, starting the fan and activating the solenoid, allowing signal air to enter the damper actuator so that the unit will deliver fresh air to the room.

In normal winter operation, it is typical for the steam valve to be modulated partially open and for the dampers to be partially open at the same time.  Thus, the unit is providing space heat and ventilation at the same time.  Only slight modulations are normally necessary to produce accurate temperature control in the room.

It has been about 30 years since I have had my hands inside of one of these beasts!  They were early 1960s Trane units and 1970s Nesbitt units.  They all tend to operate on a similar setup.  They are a very small version of a large commercial air handling unit.

Dave in QCA

Answers to questions for Dave in QCA

Cv factor relates to the flow capacity of a valve.  i.e., at a given pressure, the valve will pass a certain amount of water or steam.   In control valves, in addition to the pipe size, there are internal design factors that affect the flow rate.

It would seem that if the greatest lift that you have is about 14", that 2PSI design pressure for the boilers and steam distribution system would be sufficient.

At trap that is partially clogged or has failed in the closed position may seem to operate OK if the pressure is turned up, because it seems to compensate for the VERY low flow rate going through the faulty trap.  But this approach fails to repair the faulty equipment, costs lots of extra energy, and is an irresponsible approach to maintaining the public's investment in capital buildings and equipment.  Keep in mind, that temporary quick fixes may be justified in the short term if the money is simply not in the bank.  However, sooner or later the problems must be dealt with.   Best thing you can do is put together a total list of deferred repairs and their associated costs and work closely with the persons that have to manage the money.  Communication and planning is the key.  Don't expect everything you need when they are laying off teachers at the same time.  Yes, the coming year is very serious business for many state and local governments.  The seriousness of the recession has been buffered by stimulus money, and now that is going to run out. Times are going to get worse before they get better. 

Regarding problems you have identified, pitch should have been dealt with in the original construction.  Minor issues in condensate line pitch may not be a problem.  Remember, you have 2 PSI steam and atmospheric pressure condensate lines.  It depends on the circumstances and whether is is causing hammering.

On systems where the boiler is maintained at a constant operating pressure, and the temp control is via control valves, pneumatic or otherwise, at the heating devices, venting is seldom used or needed.  The mains will vent during start-up through and drip legs, usually through a F&T trap at the low end of the steam main.   Heating devices will vent through the trap at the outlet.  All venting passes through the condensate lines and out the atmospheric vent of the condensate receiver.

This type of a system operates completely differently than an atmospheric type of system where the boiler cycle on and off, and during each cycle, the system fills with air when the boiler is off, and then has to be vented evenly, with steam flowing evenly to every radiator, under almost no pressure.  Your system does not operate under those parameters. 

In your system, control valves have to modulate open and closed as they should.  Thermostats need to be correctly calibrated and set at consistent temperatures throughout the building.  Traps have to operate correctly.  Failed closed and you have no heat.  Failed open and you will be blowing steam out the vent of your condensate receiver tank.



BTW, where are you located?   Beautiful Quad Cities here, (Davenport, IA, Bettendorf, IA, Moline, IL, & Rock Island, IL), located on the banks of the Mississippi River.

will smith_4

On the plus side

At least your unit ventilators don't have MicroTech II controls...

Dave in QCA

correction

As I recall, the damper actuators on these univents is actually a 2 stage device.  The first stage  opens a pressure around 4-5 psi.  Thus, in a full heat condition, 4 psi, steam valve fully open, there would still be ventilation air.  However, if the thermostat signal dropped below 4 psi, the ventialtion would close to speed warmup.  The second stage of the actuator is for cooling, and it runs from 9-13 psi.  Thus, the steam valve would be fully open at 4psi, fully closed at 8 psi, outdoor air dampers modulate open and indoor dampers modulate closed beginning at 9 psi.  Dampers would index to 100% outside air at 13 psi.

Hope this is of some help to you.

Owen

Thank You Dave

Many thanks Dave! It is going to take me a while to digest all the info you shared. What little that I know anything about confirms you're right in what you say.