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Pneumatic Actuators for Valves and Dampers

Pneumatic Actuators for Valves and Dampers

Video: 
May 19, 2010 - 11:00am

Session III of the pneumatic series follows on our first two sessions that addressed pneumatic air systems, clean air requirements, thermostats, transmitters and receiver/controllers.  Now we move to actuators where we will look at such things as:

  • Spring range
  • Sequencing
  • Close-off pressure
  • Normally open vs. normally closed and why

We will also touch briefly on how DDC often controls pneumatic actuators.

John Graham and Carl Johnsen (instructors from Session I) return to conduct this practical session on the “workhorses” of the pneumatic system.

What You Will Learn:

The course will teach you how to understand the workings of actuators as discrete devices and how they are used to control dampers and valves.  It will also lead to an understanding of their role in the entire pneumatic system, and how they interface with other devices, such as thermostats.  Expect this to be a “practical” session geared towards applications vs. engineering.

Who Should Attend:  Maintenance people at the user level and service technicians

Speaker(s): 

John Graham is a senior engineer at Industrial Controls and has close to 30 years experience in the industry. In that time, he has worked with mechanical contractors and industrial and commercial customers selecting discrete components and designing HVAC and industrial solutions. Pneumatics and combustion are two of his many areas of expertise. John did his undergraduate work at the Milwaukee School of Engineering.

Carl Johnsen is a Commercial Distributor Rep for Honeywell and has been with them for 37 years.  Carl was previously a New Construction Technician, Service & Installation Manager, and a National Accounts Manager.  Carl graduated from the University of Illinois with a BS degree in Industrial Education.  Carl has his FAA - Airframe & PowerPlant Mechanic License, FCC - 1st Class License with Ship Radar Endorsement, and is a Certified Energy Manager.

Transcript: 

Jennifer:

Good morning everyone and welcome to the webinar. My name is Jennifer Adlestein and I am the marketing assistant for Industrial Controls. Today's webinar is pneumatic actuators for valves and dampers. This is the third webinar in the pneumatic series which followed part two on thermostats and controllers. The presentation will take about forty-five minutes and after we will take some time to answer your questions. During the presentation feel free to enter your questions into the chat interface on the right-hand side of your screen. We will also open it up to voice questions where you can raise your hand but this option is only for people with phone connections and not those using their computer microphone. Now I'll introduce our speakers. John Graham is a senior engineer at Industrial Controls and has close to thirty years experience in the industry. In that time, he has worked with mechanical contractors and industrial and commercial customers collecting discrete components and designing HVAC and industrial solutions. Pneumatics and combustion are two of his many areas of expertise. John did his undergraduate work at the Milwaukee school of engineering.

Our second speaker is Carl Johnson. He is a commercial distributor rep for Honeywell and has been with them for thirty-seven years. Carl was previously a new construction technician, service and installation manager, and a national accounts manager. Carl graduated from University of Illinois with BS degree in industrial education. Carl has his FAA airframe and power plant mechanic license, FCC first class license with ship radar and endorsement, and is a certified energy manager. Now at this time I will pass it over to John to get us started. He and Carl will be alternating comments on each of the slides as we move through the presentation.

John:

Thank you, Jennifer. Welcome back everyone to this third session, Carl and I here again. We had shared a little bit on air stations in the first session. Second session we talk more about the thermostats and relays and devices that operate between the air system and what we are talking about today which is damper actuators and valve actuators. We are going to talk about actuators or valves and dampers of course, spring ranges. How these are selected for sequencing operation, the difference between normally open and normally closed valves. We will touch a little bit on three-way valves and wrap up with damper actuators and how they are linked and selected, spring range and all that kind of stuff. Really we are kind of completing the loop if you will between our air supply and the controllers that control these devices and these being the final control devices on a typical pneumatic system.

Carl:

I award to try to advance the slide here, John.

John:

Okay, thank you. Well, the first image that you see up on your screen is a picture of a pneumatically actuated normally open valve. You will see at the bottom we have got globe valve body and above it are the actuator and spring and linkage. This is typically shipped as one piece but quite often can be serviced as individual pieces being the linkage, body, and actuator. What you will see at the top is the branch pressure line which emits air from your branch device via your thermostat or your receiver controller. That air fills the chamber the rolling diaphragm and pushes down against the spring which ultimately actuates the valve stem which operates the plug inside the valve body. The valve that we are looking at here is normally open which means in the absence of air pressure, the spring is applying an upward force to the spring cup which ultimately pulls the stem and plug up. Valves can be normally open or normally closed. Valves can also be provided in a three-way configuration which essentially has a plug between two seats. So as the stem goes down it closes one plug. As it rises, it closes the other but again we are looking at here is a normally open pneumatically actuated valve.

Carl:

On the maintenance side, John, we've talked a little bit about before just to share what your experience has been at these bladders that we have talked about here. They can be easily replaced on a lot of these devices where the top can open up and unscrew out, right?

John:

Exactly, yes, the wear parts because of course the diaphragm is in continuous movement. The edges will eventually crack, they will dry up so it is not a real big deal to remove the top and replace the diaphragm. The other thing that can happen through time is the actual bonnet and packing can begin to leak and wear. Many manufacturers do offer renewal kits that provide essentially a new stem, new packing so you don't necessarily need to replace the entire valve. You can just replace the parts that have failed- very simple to service, very dependable design.

Carl:

Okay, let's see if we can move on to the next slide. This one is for you also John.

John:

We talked briefly about valve bodies and we are going to talk about valves first and the damper actuators second. Depicted on the left, is a normally open or direct acting valve. By that we mean, as the stem is driven downward by the actuator the valve closes. The one on the right although it looks like parts of it have been missed, that is a reverse acting valve. If it were all there, you would see a plug that is actually below the port so as the stem is drawn upward, the valve closes. The normally open valve is typically called direct acting. A normally closed or stem closed valve is called reverse acting. The three-way valves on top, it is maybe perhaps a little hard to see but, actually no. Now that I see it better it looks like it's a two-way valve normally closed. That would be very similar to what you would see on a three-way with the port on the bottom.

Carl:

Okay, now to the next slide. The control of the two actions with either direct acting or reverse acting actuator would give us our final control configuration. You really don't know what that is until you add both the and look at the combination of both the actuator and the valve together. So right now, John was just talking about valves and now let's talk about the actuators. The actuators here for air pressure, for a direct acting actuator is that the air pressure comes in through the chamber above here and this is kind of exaggerated because it looks like a great amount of space here as far how it pushes down. Normally they may only maybe ¾ to a 1 ½ inches of travel as far as this air space that is above here. Nonetheless, the air pressure pushes up here and build of air pressure which is against the spring. It pushes down on the stem. The stem starts to extract out. When that air pressure goes away this spring pressure then starts to go back and then the stem goes back up into the actuator retracting the stem. Then from the opposite action, is where the air pressure comes in underneath the bladder. In this particular case, we can see that this air pressure coming in underneath there and therefore the fitting that would be on the side of this actuator is physically further down from the top of this actuator and you can tell often as far as looking at those valve actuators this might be a reverse acting actuator. Air pressure comes in underneath this bladder and actually stretches this spring and win the air pressure is released then that spring returns back to its normal position. Once it starts to build up pressure, that stem is retracted and goes up into itself and once the air pressure goes away then the spring returns back and then that stem goes back out itself outside. So that's how you would actually see the operation of this direct acting or reverse acting actuator. Also this picture here is maybe a little bit easier to see. In the upper right-hand corner, the fitting that is attached to this particular actuator is above this round top and therefore the air is being introduced into the top above the bladder. If there are other ones that might be designed that might come underneath here and bring in the air pressure around or underneath the bladder so just things to look for.

John:

The other neat thing about that picture is that you can clearly see the spring. Some actuators are fully enclosed. You don't actually get to see the spring. This one here is kind of neat in that you can see the yoke where couples to be valve body. You can see the spring and if you look up into the top part of it you can almost see the actuator itself.

Carl:

Moving on to the next slide here and we will talk about putting together the normally open versus the normally closed configurations. This can be a little bit confusing so we are going to just up there in a little bit at a time to see how this operates. The application engineer that is actually piecing this together is designing this to be able to say what happens and what is the best way for this valve to fail if the air pressure in a system is lost in the building. In this particular case, we are talking about a normally open valve and this application here is the assembly. The assembly of this actuator with the bladder and air pressure coming on top of the bladder. For the arrow I am drawing back and forth then and then pushing down when the air pressure comes in and starts to build up pressure. And then the, my arrow is a little bit slow right now, I'm not really sure why. Bare with me here for a second. There it goes. This then extends down as the air pressure starts to build up above the bladder and pushes this plug and the plug then meets on this particular side of the valve and closes and does not allow the water to flow through. So the water that would be going from left to right here would be stopped. In the same situation, we also have a normally open configuration by putting a reverse acting actuator with the bladder and air pressure coming underneath the bladder pulling and pushing down with that spring with a reverse acting valve. When you piece these two together a reverse acting actuator along with a reverse acting valve, you do get a normally open valve configuration allowing water to come from left to right through here. John, you had pointed out before in our practice session that this plug is really and the direction of the flow is not really accurate is it?

John:

Right, generally what you will see is the air on any side of valve that points along with the flow. If there is ever a doubt, the water should be going against the plugs. So in other words, in this picture the arrow should be flopped. So flow would enter from the right, go towards the left down against the plug and out the left. So it is just something that you will see if it is backwards, if it is installed backwards you will find that as the plug approaches the seat it will slam and the valve will start to hammer and make all sorts of racket. Then you will lose your ability for smooth modulating control.

Carl:

So let’s take a look then at the normally closed configuration, again the configuration being the combination of the valve and the actuator together. In this particular case we have a direct acting valve and so the port is down, the plug is sitting on the seat and now is the air pressure comes underneath this bladder, it is going to stretch the spring, pull up on the stem and allow water to go through. But if this was just sitting in the pipe and there was no air pressure, the whole system didn't have any air pressure available, this valve would sit and be closed and be so therefore normally closed, no air pressure. In the same situation here, again, we have a reverse acting valve with a direct acting so they are opposite. This water flow that is coming through here would be stopped without air pressure. You need air pressure on top of the bladder to be able to push down on this stem and therefore open this plug up to the seat and allow water to flow through on this valve and actuator combination.

John:

So one of the benefits that we talked about before is not only are these valves inherently modulating but they are very simple to predict their failed position. As opposed to electrics where you are familiar you have to have a spring return feature added.

Carl:

Okay, so let’s advance the slide.

John:

Okay, so we are getting into a little more detail here on spring range. The springs in many actuators are color-coded based on the spring range themselves. Here we see an example of a 4 to 11 pounds spring essentially what that means that 4 pounds the spring will be completely extended. The valve, in this case being a normally open valve will be fully open. As we apply pressure to the top side of the diaphragm, there will be no downward movement until we hit roughly that 4 pounds value. These are not exact. You might find a spring that might start to move at 3 ½ or maybe 4, 4 ¼, 4 1/2. Essentially the downward movement will begin at about 4 pounds. The other end of the spin, the 11 pound value tells us that the spring will be as compressed as it can be. The valve will be fully close in this normally open valve case at about 11 pounds. So the beauty of the valve spring range is that we can predict when the stroke will begin and predict essentially where the stroke will end. That becomes important in our next slide where we look at an application, a very simple one, with air handling unit. So we have the fan on the left-hand side blowing air through a heating coil and then through a cooling coil into the space. The space is equipped with a direct acting thermostat. From the last session you'll remember that as the temperature rises above set point the branch pressure raises meaning direct action. The normally open valve on the left for our heating is going to be open again in the absence of any air. The right-hand valve, are cooling valve, being normally closed will be closed in the absence of any air from the branch of the thermostat. Looking at the diagram on the top right, we are looking at set point being about 8 pounds. As the space temperature falls, the branch pressure falls and that valve begins to vent back through the thermostat, the stem rises, the valve opens and supplies heat to the zone. As the temperature returns towards set point, the branch pressure from the thermostat rises, drives that normally open valve stem downward reducing the amount of heat added to the space. If the temperature should begin to rise above set point of course the branch pressure will continue to rise but now it will drive the normally closed cooling valve open in small increments. So you can see here that we will have a dead band between the heating and cooling valves which is established based on the selection of the spring ranges. That's why that's so important. When replacing an actuator particular attention has to be paid to that so that you don't end up with overlap between heating or cooling. That is typical in this application in that a loss in air will give you full heat. That's just how you'd want your zone to fail.

Carl:

So this is that band right down on the bottom?

John:

Uh-huh, down at the bottom, sure.

Carl:

Great, we will advance the slide. So close off rating or what is valve close off as far as what the definition of valve close off is. Discussing pneumatic actuator we introduced the concept of the tight valve close off. A valve must be able stop the flow of water or steam when it is controlled and told to do so. A valve and actuator combination can be selected to close off tightly against the normally operating conditions but cannot leak if the water or steam pressure increases. Have to make sure that the valve is not leaking therefore has tight close off. You need to understand that the fact that pumps may increase pressure at times and also understand that water is not compressible so had pressure can easily exceed a valves close off rating and push the disc or the plug off the seat and open the valve and let steam and water flow through. This water and steam leakage can get excessive and even small amounts of leakage will cause a loss of control or wasted energy and certainly premature valve wear. We guess that is allowed to continue can cause the entire system to fail so that valve close off is really important.

John:

As you can kind of picture from that last example at the air handling unit where the valves were sequencing, picture in your mind if you will, an actuator that doesn't have sufficient close off to allow the steam valve to close. Just small amounts of heat will have to be removed by the cooling coil because of course the thermostat doesn't know if that is real space load or if that's artificial load introduced by the steam from the air handling unit itself so it is very important.

Carl:

The next slide we continue to talk about valve close off. The actuator force that is required to prevent the flow of leakage is what's going to be required and looked at so here, meeting all the valve selection and sizing criteria, will be lost if the valve cannot stop the flow of steam or water in the fully closed position. So really the way to state this is closed off is the actuator force required to prevent flow leakage and this force can be and must be met with the proper selection of the pneumatic actuator. The valve must be able to close off against the water completely so as this, I am trying to get the cursor to come across here, so this red area here as far as the flow and we need to have this plug be seated down in here to stop the flow of water in this particular case. This actuator with its specific spring range is determined and engineered to be able to make sure that this can close off against this amount of water pressure or differential across this valve that the valve will be seen.

John:

In this picture we see the valve fully open. It is a 5 to 10 pound spring and with a 5 pound branch pressure applied the plug is fully up so here we are heating.

Carl:

So we will go to the next slide here talking again as the valve source close with the increase in air pressure it will push the stem down causing the valve to move towards a closed position. We will have the valve fully closed at 10 psi. This area in here once this air pressure that's coming into this port coming up on top of here pushing down, pushes down on the stem at 10 pounds this plug is fully seated on this valve seat and therefore there should be no flow through that section. John, you want to take the next one here.

John:

When you look at the net force applied to the valve stem downward it’s really the force developed by the air acted upon the diameter of the diaphragm but being opposed by the spring and the dynamic forces on the plug. What that means essentially is only a certain portion of the energy delivered by the air is actually used as thrust to drive the stem downward. If we apply a high differential pressure to the valve or one that in excess of what the valve was selected to close off against, we will end up with the small amount of leakage past the seat.

Carl:

That is what that red arrow is trying to represent. Once you start having any kind of leakage across there, you have all kinds of issues. Again, valve close off or the close off rating is high end pressure from the water comes through here you start to have problems with leakage therefore you are not actually controlling the water temperature going into the coil. You are getting valve wear and what we mean by that is that as the water or steam that is coming across this plug has got such a small area to come across as it is leaking it actually starts to cut into the valve seat and starts to cut into the actual plug assembly and the erodes away a certain section of the metal there. If it happens over a long period of time you will get a valve that is continuing to leak all the time and will not be able to control the system.

John:

When troubleshooting something like that oftentimes it is helpful to remove the bonnet and actually inspect the plug and the seat and use that as maybe some evidence to select a different valve and actuator combination when you replace it. What we will see next is what is typically done to counteract that in a normal system. As the upward force that is working in opposition to the actuator becomes greater with the greater differential across the valve. If the valve seat is still good, the temperature and the space will continue to rise. The branch pressure from your thermostat will increase and apply more downward force. Again, if the valve seat and plug are good it will apply enough pressure to ultimately seal that valve off. But as Carl had said before, wear drawn or damaged or otherwise compromised, additional branch pressure on the actuator will not change or reduce the flow of steam or water through the valve.

Carl:

Let’s take a look then at the next slide which talks about the close off tables. This one is a little bit difficult to read so bare with us here. As somebody, if an application engineer is looking at designing and applying what is the right size value and what is the right size actuator. They go to a table that might look similar to this. First part that we are looking for is that I'm a size here they are looking for a particular two-way valve and this is a 2 inch valve with a part number of V, as in Victor, 5011 and has a name 1099 it looks like. If you take that same valve and take look at what this column coming over here represents we then apply a particular valve actuator of whether it is normally open or normally closed, stem up for stem down without air pressure. We find that model number and in this particular selection you're looking at MP9563C1067. We come down in this column, matched the left and right and we see at this red circle 122 psi of pressure which means that this will close off this actuator at this valve combination against 122 pounds of pressure differential across the valve. It would take a lot of pumping pressure in order to start push this up and start to move the plug off of its seat, so 122 pounds of close off pressure. John also talked a little bit about as far as that this is a different rating than that static rating or static pressure rating. We want to differentiate right?

John:

Right, exactly, as we look at a valve they close off is a matter of the differential at the valve seeds. In other words, if you were to look at a set of pressure gauges across that valve both sides of the valve with the pump off might be at 30 pounds. The valve doesn't really see any differential to act upon: the pump starts the differential created by the pump is ultimately in part what this valve needs to close off again. When you are selecting a valve you need to advise those that are part of the selection process what's your actual differential pressure will be so that they can select the appropriate size actuator and spring.

Another interesting thing on this table if you look, just one number above the 122 pound value that Carl had pointed at you'll see that they close off goes up to 210 pounds for a 1 ½ inch body. Well, it is the same actuator and the only difference essentially is that plug inside 1 ½ inch valve is smaller. There is less surface area for the water or steam to act upon and that's why our net force available for closure is higher with a smaller body valve. The big issue that you have to be careful of when retrofitting or selecting valves especially as they get larger is that we need to keep in mind the spring range and the actuator size so that we have enough net force available to close the valve.

What is interesting here and we might not been real clear on it before but the normally open valve examples the branch pressure aids and closing the valve. Meaning as the pressure rises, the stem is driven down against the seat and it closes. Normally closed valves, however, don't rely on the air pressure to close them. It is the spring range and the actuator size that ultimately gives you your close off there. In the absence of air, again, being that it is a normally closed valve, the valve closes but its closure pressure differential is a function of the spring range and actuator size so we just wanted to clarify that.

Carl:

Great point John. Well, John for the next table-

John:

This is for electric actuators. We won't touch on this much but as you might imagine and electric actuator and valve combination has to be selected with the same concern and care to allow that to be closed or open when it should be. The next slide shows a visual wrap up of what we just talked about in the table. So across the bottom, although it is very difficult to see, there is the close off pressure ratings. Diagonal lines are actually the different valve flow coefficients or valve sizes, the equivalent of it essentially. Every valve has got a flow coefficients and a valve size. What you can see is the lighter slopes, the lower CVs have the higher close off pressures and obviously the larger CVs, larger valves and this graph kind of depicts the culmination of the spring range, they close off pressure and the valve size. Anyone helping you to troubleshoot a valve here at ICD will probably be using a graph very much like this if you should happen to call and need to specify a replacement or if you are troubleshooting a valve that isn't acting properly.

Carl:

Okay, we will move on to this description of valve actuators for positive positioners. So this positive positioner is used and applied for these valves in order to be able to have the come I'm going to take my cursor over here, it is very difficult to see it in this particular picture. Inside and through this valve actuator there is actually a pen or rods that comes through this area and tells this positive positioner which is this gray box with the black knob on top. It tells this positive positioner what position physically the disc and plug are in. What we want to do is be able to give feedback about the actual physical position of this valve and plug and sitting on the seat and what position is this actually in. You may be sending a pneumatic signal from the thermostat or controller to say this valve ought to be at 50% but whether or not it is at 50% unless you have feedback coming from this valve going up and through this actuator up into this positioner we really don't know that this valve is actually sitting at 50%. They've invested these positive positioners to be able to take this valve signal coming in and it will be on the pilot side of this positive positioner. The branch line would come in here and you also have available full main air pressure coming into this positive positioner. Then to be able to say if I'm not at this 50% position, the positive positioner says I need to be at 50% because the controller is telling me to be a 50% and it will then take the full 20 pounds of air and either push up or pull down on this valve actuator to be able to make sure that the valve down here towards the bottom is at 50%. This positive positioner has a couple of functions that actually make sure that the valve is where it is supposed to be and the example that I was giving at 50%. It also has the ability to be able to what we call start the sequence of operation from whatever this controller branch line air pressure is coming in. The positive positioner, it can actually be dialed to say I want to start at 3 pounds or want to start at 4 pounds or want to start a 5 pounds and then the spring range of the actual valve actuator will take over from there. So that is the purpose of this black knob sitting on top of here to be able to move that back and forth.

John:

So if you might imagine, I like to use the word “sticktion”, it's a combination of the word sticking and friction. As that stem becomes sluggish or becomes sticky in the packing, the positioner will oppose that and maintain smooth modulating control. The other neat thing about the positioner is that it allows you to very specifically select the range of modulation whereas before we were looking at the natural spring range. That dictated the stem movement and pressure relationship. This device allows you to make that acts over any range that you wish with great precision.

Carl:

If we and maybe I think we are going to try to go back to that original slide that you were going through with the blue and lines to show people how important this is to be able to make sure that valve is where it is supposed to be. Hopefully people will be with us see this and give them a couple of seconds.

John:

Essentially, your screen might be updating a little slower than ours but on the graphics that we looked at before, the 2 to 7 pounds spring range for the heating valve in the 8 to 12 pounds spring range for the cooling valve were just that. It's just the relationship or compression rate of the spring in those actuators. What you're going to end up with, in this example at least, is a 1 pound dead band because it is in that range that the heating valve is fully closed, the cooling valve is fully closed and you're just left with that dead band. In the case of these valves being fitted with positive positioners, we can make that dead band anything that we want to be. When you flip open your Honeywell or Johnson or anyone's catalog, you're going to find that there are some very standard spring ranges but for example if you needed to sink when several valves you would have to have positioners only because you'd never find the spring ranges you wish. This allows you to not only maintain very precise motion of the valve stems but it allows you to sequence outside of spring ranges that are commercially available. What you often can see on valves, heating coils that are very, very large, you might see two steam valves. We call it a 1/3, 2/3 scheme where you'll have one very small valve that handles very light heating loads, a larger valve that opens with an increased need for heat. In that scenario, you would need to have a positioner on each because we want to control the spring range, the start points of those two valves so that they never overlap into the cooling range. That is a typical example for that. Or if you have an economizer cycle you might need to shift spring ranges to accommodate the economizer operating sequence with heating and cooling valves so just an additional detail there, thank you.

 

 

Carl:

We are going to jump forward a couple back to our original of the pneumatic damper actuators so bare with us as we step forward and we should be able to give you a couple of seconds here for the system to catch up and we should now all be seeing pneumatic damper actuators now.

John:

On the bottom left, I should say there is one, two, three, four electric actuators toward the bottom left and then the pneumatic options are on the right and top right. The bottom right is what you would typically see in a smaller unit ventilator. You will see that the actuator diaphragm is obviously very small. It is very compact. It is not designed to overcome a lot of force with very large dampers. The one above it you will see is a little different in that the spring and piston is completely contained. You can see any of those parts. Above that is a larger model that has a positive positioner on it much like the ones that we just explored for the valves, these are available on pneumatic actuators for dampers as well. And the last one is just one that has a pivoting base. These come in any number of sizes and configurations to fit the most common applications.

Within there is just a real simple depiction here of a damper actuator. You will see there is a linkage between the actuator itself and a crank arm on the damper. We like to think of the damper really is just another type of valve but in this case for air. This damper might be a two position damper for example interlocked to an exhaust fan on a roof. It might be part of an economizer scheme or relief damper where we want continuous modulation. These damper actuators are inherently proportional. Again, it is just like the valve actuators where the two position nature or the modulating nature is a function of the control not of the actuator. Unlike valves, damper actuators always retract on a failing pressure, a loss in pressure, the stem retracts. So as you air it up, the stem extends. The fact that the actuator is going to be part of a normally open or normally closed damper is strictly in how it is linked.

Carl:

So let’s take a look at that on the next slide. John's point here as far as the rolling diaphragm again, very similar to the valve actuator that we have in this particular case, zero pounds, coming in between and on top of this bladder. Once the branch line from a controller starts to increase its pressure then this bladder starts to push against the spring and extends the shaft out. The rolling diaphragm here again is taking the changes as far as the air pressure coming in on top of the bladder and like John was saying in this particular case we have this particular damper with the pushrod being actually attached directly to the damper blade. The damper blade in this particular case is attached above the axle of the damper so this round circle that is right in the center here is the axle or center point of the damper blade. This rod then starts to push against it as branch line air pressure builds up. In this case we are actually going to be pushing this damper closed without any kind of branch line air pressure or at zero pounds like this is showing, this damper is actually going to be pulled open. So that is the area that John was talking about. There is no such thing as a normally open or normally closed damper actuator. It actually depends on where and how this pushrod is attached to the damper itself and therefore that in the field is how it's determined whether failing open or failing closed. Let’s take a look at some more damper actuators here.

John:

Right, and to Carl's point you can see the right-hand side as noted as normally closed and there the actuator stem is connected to the bottom side of the axle. As the branch line pressure increases, the stem is going to extend driving the damper open. This is what some great analogies to our valve actuators. You have to have a properly selected spring range. You have to have the proper amount of thrust to operate the damper smoothly. These are all parts of the selection process. For dampers that are very large they can be split and actuated in sections or actuators can be tied to the same branch line aid in opening very large or very tight closure dampers. That is all part of the initial selection process.

Carl:

Okay, let’s move on then to damper actuators and how they may be interfaced to a direct digital control system or any kind of electronic control system. This picture on the left hand side has main air pressure coming into the controller as you learned in the last session and then this pneumatic controller is outputting out of the branch line a variable air signal. In this particular case, it's 3 to 13 pounds psi and that may be dependent on the different manufacturers. This then changes modulating the full 3 to 13 pounds. At 3 pounds then there is no air pressure on here. This rod will be fully inside the actuator and as it is modulated from 3 pounds up to 13 pounds, the air pressure would build up and open up this or extend out stem out of the actuator. So now if we take the same controller as far as the air pressure goes, we now have air pressure of the main air coming into an EP, which is really this is actually a transducer, an electric to pneumatic transducer, that is going to convert the signal that's coming from the main air through here and change this line going to 3 to 13 pounds. It does so by changing. Instead of having an air pressure coming in here we are now showing a DDC, or direct digital control controller that is going to change the signal from 2 V DC modulating it all the way up to 10 V DC which would then be changing the air pressure going in here. This is something that we see quite often where a direct digital control system comes then on top of an existing pneumatic control system. In particular in the main air handling unit is to be able to take direct digital control, electronic control, and still keep in the existing pneumatic actuators either dampers in this case or valve actuators because the existing cost, the capital investment, is already there in place for these actuators. Quite often the size of these pneumatic actuators can be pretty large size therefore this is a better application and a better solution so you continue to use those pneumatic actuators.

John:

Right and you have probably seen this before with the electric actuator offering. To get the same amount of thrust or torque, quite often they get either very difficult to find or very expensive whereas pneumatics can very easily and inexpensively give you a good deal of thrust or torque. So this really makes the DDC retrofit a lot more realizable.

Carl:

Okay, now to the next slide.

 

 

John:

Here we are talking a little bit about spring ranges again and in this example we have got a 5 to 10 pound span spring. As we touched on before at zero pounds the actuator stem is fully retracted. The spring of course is what's doing that. At 5 pounds or thereabouts, we will begin to extend the actuator shaft to the point where at 10 pounds we will be fully extended. No course there is going to be some opposing force on the actuator stem. If that opposing force is greater than 10 pounds, that 10 pound pressure might have to rise to 10 or 12 before the actuator is fully extended. If our positive position were installed on this actuator, the relationship of stem to branch pressure would be maintained because we could admit up to a 20 pound branch to drive against the backside of that diaphragm so that we would achieve the desired position. There are different spring ranges shown on the left hand side but most of your economizers, your cooling dampers will have the higher ranges, the 8 to 12, and 8 to 13. The 3 to 13’s are shown here as well but that's uncommon for full range of control. Again it depends on the application and what other components that this actuator needs to sequence with as to what that spring range would be.

So here we see something that looks very similar to what we did before with our air handling unit but in this scenario we've got an additional device to sequence. So as before we just a heating valve and a cooling valve, now we have these very same valves with different spring ranges but we've added a damper, a cooling damper. So picture that air handling unit with maybe an economizer on it meaning we can take advantage of free cooling from the outdoors if it's suitable. As the space temperature in that application begins to rise, are heating valve would close. And on a continued rise in temperature now instead of operating the chilled water valve as we did before, we would now start to open our economizer dampers to the outside. Our cool fresh outside air would come in and start to cool the space down. If the air wasn't sufficiently cool or the space load was too high, the temperature in the space would continue to rise and our thermostat branch pressure would continue to rise. At which point our cooling valve would now begin to open at 10 or so pounds. So you can see here how the spring ranges, again, are so important in sequencing the different products so that they all operate together to achieve your final control sequence. So now we get into positioners.

Carl:

We talked before about valve actuator positioners, positive positioners and we also have the equivalent of those for the damper actuators. We take a look at the slide and follow the cursor here. On the side of this particular model we have a rod and spring that is attached directly to the stem as it extends out of this damper actuator. This is physically giving some feedback of where this is extending out or not extending out. It is actually giving feedback directly into the positive positioner. Again this is a box that may or may not look exactly like this but again has ability to be able to have main air pressure. I will say this for sake of conversation today let's pretend 20 pounds of main air and then we have from the controller we have the branch line air pressure from the controller going into the P port which is the pilot port of this positive positioner. Then with this feedback spring, the controller or thermostat is saying, again, I should be at 50% open. If this stem is not extended out 50%, the positive positioner will say I'm not at 50%. I am going to use 20 pounds of air pressure coming through this positive positioner. I am going to send that out on the branch line so these two actuators or three actuators rather him be able to actually take the 20 pounds of air and move this stem against the airflow that's going against it that is not allowing this damper to be at 50%. So once this is at 50% and the controller says I should be at 50%, there is no more movement and this positive positioner then is helping do its job with the damper actuator.

John:

Right, so not only can we sequence but we gain additional thrust by use of a positioner. Really when you see a lot of “sticktion”, you definitely see it in dampers.