Pneumatic HVAC Systems/Fail-safe Valves/HELP

Factual Questions
1

Here’s the short version:
I direct a bio-ethics committee that deals with issues in animals testing in medical research. Most animals at where I work are housed in a building separate from the hospital and labs the animal research facility that has its own HVAC system. We had gotten into trouble because our HVAC system was not “fail safe” A contract was written up to replace the valves so that they failed in the off position (cooling)

Last weekend AFTER the room in question had had its valves replaced the pneumatic tube that connects the controller to the valve just blew off. With a lack of signal or air pressure, the valve should have failed to off or cooling but instead they failed on. Heat goes full blast, room gets over 100 degrees over night…very valuable research transgenic mice dead. Many many people upset.

Now I’m the one that has to deal with this. I have to first explain this to our internal committee and then write an incident report later on. I need someone to just really help me understand the basics of pneumatic HVAC systems.

It was determined that the contractors did NOT install the right valves. From what I gather, there are some valves that fail safe in certain situations but not all.

If someone who knows about pneumatics, HVACs or both would be willing to maybe chat with me online to explain it to a degree that I can pass the info on to others and write about it I would be forever grateful. All the information on the interwebs is pretty advanced.

What is the mechanism that makes a fail off valve stay off? How does this physically differ than a fail on valve? How does a pneumatic system use airpressure exactly to regulate heating and cooling. I need to know pretty specifically. Our hospitals engineering department has NOT called me back all week and I need to get this in my head immediately.

2

I don’t pretend to be an expert in HVAc or pneumatically controlled valves; however, we routinely specify that pneumatic (or motor operated valves for that matter) valves fail either or open or close on loss of signal. If you specified that the valves should fail closed on loss of signal and they didn’t then I suspect that the valve response was never tested during startup and shame on whomever was performing the role of construction oversight.

3

This is pretty much right up my alley as I have worked in the HVAC controls industry for the last 11 years.

First, the definition of fail-safe varies depending on the application. In some instances, open is the safe position and in others (yours), closed is the preferred fail mode. The normal convention, especially in pneumatics, is for cooling valves to fail closed and heating valves to fail open. This is meant to protect against a worst case scenario where an air pressure failure in the winter leads to frozen and burst heating coils with the concommitant flooding damage. I suspect this convention was formulated in one of the sub-Arctic states, like Michigan or New York, where frozen water is known to fall right out of the sky - probably not a big concern in San Francisco. However, since your application is contraryto this convention, it would be wise to closely examine the project specifications. It is possible that a generic specification was issued which called for the normal “fail to heat” application. Or there was no spec and the contractor chose the wrong definition of “fail-safe”.

To your specific question, pneumatic actuators use air pressure to drive a valve open OR closed, not both. When the air pressure is removed, a spring drives the actuator back to it’s normal position, where normal is defined as the unpowered state. Actuators generally fail in the up or retracted position. Thus, for a valve to fail closed, it would need to close with the stem up, generally referred to as a “normally closed” valve. A valve that opens stem-up is “normally open”.

It would seem that your contractor got the normally open/normally closed part wrong. As to how they failed to catch it, it can be difficult to tell if a valve is opening or closing as the stem rises, especially if it is being tested when the boiler and pumps are turned off and there is no change in pipe temperature to indicate the valve’s position. Please don’t ask me how I know this.

If you need to know more, my e-mail addy is in my profile.

[shameless plug] I, meaning the company I work for, would be happy to sell you a small webserver that interfaced to the control system which would give you remote monitoring and control functions and would send e-mails to phones or pagers on any alarm conditions. And if you don’t have a control system, we’d be happy to sell you that part too.[/shameless plug]

4

[QUOTE=Rhubarb]
This is pretty much right up my alley as I have worked in the HVAC controls industry for the last 11 years.

First, the definition of fail-safe varies depending on the application. In some instances, open is the safe position and in others (yours), closed is the preferred fail mode. The normal convention, especially in pneumatics, is for cooling valves to fail closed and heating valves to fail open. This is meant to protect against a worst case scenario where an air pressure failure in the winter leads to frozen and burst heating coils with the concommitant flooding damage. I suspect this convention was formulated in one of the sub-Arctic states, like Michigan or New York, where frozen water is known to fall right out of the sky - probably not a big concern in San Francisco. However, since your application is contraryto this convention, it would be wise to closely examine the project specifications. It is possible that a generic specification was issued which called for the normal “fail to heat” application. Or there was no spec and the contractor chose the wrong definition of “fail-safe”.

To your specific question, pneumatic actuators use air pressure to drive a valve open OR closed, not both. When the air pressure is removed, a spring drives the actuator back to it’s normal position, where normal is defined as the unpowered state. Actuators generally fail in the up or retracted position. Thus, for a valve to fail closed, it would need to close with the stem up, generally referred to as a “normally closed” valve. A valve that opens stem-up is “normally open”.

It would seem that your contractor got the normally open/normally closed part wrong. As to how they failed to catch it, it can be difficult to tell if a valve is opening or closing as the stem rises, especially if it is being tested when the boiler and pumps are turned off and there is no change in pipe temperature to indicate the valve’s position. Please don’t ask me how I know this.

If you need to know more, my e-mail addy is in my profile.

[shameless plug] I, meaning the company I work for, would be happy to sell you a small webserver that interfaced to the control system which would give you remote monitoring and control functions and would send e-mails to phones or pagers on any alarm conditions. And if you don’t have a control system, we’d be happy to sell you that part too.[/shameless plug]
[/QUOTE]

shameless plug aside I was reading about those alarm mechanisms. I think I am really going to lobby for the money to get this installed in our building. Even if something fucks up again at least someone can be paged to get the animals out of the room before they are literally cooked.

Okay so I understand a loss of signal and the valve goes to its resting state which for us MUST BE CLOSED. San Francisco needs heat literally two days a year. My first suggestion was to turn down the fucking 200 degree water in the heating pipes. When we do need heat it seems impossible that we would need that much heating power. I don’t know if thats right though.

So because the pneumatic tube that connects the controller to the valve blew off there was a lack of signal but the heat kicked on anyways…this is a loss of signal failure situation yes? Because Research Oversight will want to know how we are preventing overheating in the future can you give me some other examples of things that can happen where the valves will need to fail off besides total loss of air pressure? Is there an opposite failure like way too much air pressure and it’ll still fail closed?

I’m looking at stuff online is a diagram of a solenoid valve relevant? Because I understand this one.

5

Okay, where to start? First, I have no idea what type of system you have. It could be a 2-pipe system where the same pair of pipes carry hot water in the winter and cold in the summer and whether the valve opens or closes on a rise in space temperature is dependent on a changeover controller. Or it could be a 4-pipe system with separate heating and cooling sections. Or a dual-duct mixing box system. Or VAV (variable air volume) with terminal reheat. In addition to all of that, your system could have 2-way valves which are either open or shut, or it could have 3-way valves which are either open to the coil or open to the bypass and the definition of closed can get confused.

Regarding this,

the fail mode generally means what happens on a loss of power or signal. For a pneumatic this means air pressure. I’ve never seen a failure caused by too much air pressure, but since that would cause the diaphragm to rupture, it would have the same effect as no air pressure at all.

Your system failed because of a bad pneumatic connection which could be seen as a catastrophic leak. Other causes could be an electrical outage which would shut down the air compressor for the control system or some other failure of the air compressor itself.

No, not really. Although, I have seen solenoids used to shut down the pneumatics so that the system went to it’s fail-safe mode. This scheme is sometimes used to prevent heating or cooling during unoccupied hours in a building. Probably not relevant to your situation.

Here’s a link to one of the webservers my company manufactures (Warning - 370 kB .pdf) Xenta 527. The cost for a system like this is in the thousands of dollars and depends on a lot of factors that I can’t glean from your posts.