Eric Johnson (00:00)
Welcome back to Boiler Wild episode eighty-three. We are in August 2026. Just winding down the end of the month. And it is gonna start getting a little bit colder. But if you're a real boiler person, you've been busy all through the summer being a real boiler person. I don't want to offend the people who do other s work such as you know the general HVAC work or chiller work. You are
Eric Johnson (00:28)
Still loved if you listen to this podcast, but the real boiler people will know that a lot of boilers get opened up during the summer. There's a lot of boiler maintenance during the summer. because I talked to a lot of people. Well, I don't want to exaggerate, it's not a lot, but when I whenever I talk to people, they'll say like, what do you do during the warm months for boiler work? Because boilers aren't needed. And yes, boilers are not needed for the heating, but the
Eric Johnson (00:58)
people who are real in depth with the boiler work are either servicing heating boilers, heating systems, and or installing heating boilers during the summer, or they're working on process boilers, which don't know what the temperature is outside and are running twenty four seven unless they are down and they will run right through the summer. Typically companies don't care.
Eric Johnson (01:23)
With the process boiler, what the temperature is outside. Sometimes the process boilers will also pick up the load for some heating, maybe some heating on the shipping docks. They I've seen some steam coils with fans on them for shipping docks for the winter, but that is just a minor load versus the actual process load of the plant. But we are gonna be entering September pretty soon and it'll start getting colder and hopefully.
Eric Johnson (01:53)
You are ready for primetime boiler season and hopefully your customers are too because we all know people love to wait till the first cold, the first frost, and then they go in their boiler room or they wonder why their boilers don't work correctly, but you know, they walk in the boiler room and they see the boilers are apart or they're off or not working and they go, right. We told them to get us through the last spring.
Eric Johnson (02:22)
And then we shut these down in May and then told ourselves that we had till September and it's now September. yeah, hopefully they can get us that part real quick. then you have a bunch of people calling at the same time, and all of a sudden you're super busy, and then they wonder why you can't get out there the same day and have the part the same day. And even if you ordered the part the same day, manufacturers for some reason, there seems to be this
Eric Johnson (02:51)
lax on sto stocking parts. It has become normal to not stock parts. Lead times have become normal. And I don't want to go into why I think that is 'cause I don't know exactly why it is. So I would just be guessing. But there are still shortages here and there of random parts and that always makes it hard for the companies that are servicing boilers 'cause now you look like the bad person.
Eric Johnson (03:21)
And that is why if you are an end user, you need to be stocking parts that are critical and you also need to be ordering parts or ordering service and doing projects ahead of time and scheduling ahead of time. This is not the nineteen eighties or nineties where you can get somebody out the same week and they have just your boiler has a couple random boiler parts on it and most boilers had those same parts and then
Eric Johnson (03:50)
You just go down to your local supply house, get the parts, come back, and change out. Boilers nowadays are a little bit more complicated and they require a couple bit more parts that are not always stocked at your local supply house. So, anyways, I'm excited for September, October, November. Not that my business changes too much because I don't service boilers, but I'm excited to see everybody start starting up boiler systems.
Eric Johnson (04:18)
To start getting into some hoodie weather and yeah, hopefully you're prepared too. Let me know what you do for preparation. If you wanna email me, eric.johnson at boilearn.com or DM me on LinkedIn. If you're on LinkedIn, we haven't connected yet, please connect with me. Love to hear from you. I love talking to people who are working in the boiler industry or any other industry that is similar, even if you're an operator.
Eric Johnson (04:47)
And you just take care of the boiler at your plant and your listen. I love to hear from you. I love to hear people's experiences. It helps me get a better worldview of what is going on and what the world needs for training, education, and maybe some different problems to solve. So today we're gonna talk about the five ways to control steam pressure in in a boiler. And this is gonna be a steam boiler. Hopefully you're not making steam in a hydronic boiler.
Eric Johnson (05:14)
If you are, you are operating your boiler very incorrectly and you should get that checked out. But the five ways to control steam pressure in a boiler, and you may be, if you're a little bit more experienced, may be checking them off in your head and you're like, five, man, what what are the five? Yes, there are five ways to do this. And I'm gonna right off the bat say that this is gonna be for a boiler parallel positioning or electronic control control system. I guess you could do
Eric Johnson (05:44)
like a plc it's gonna be kind of the same thing. but most boilers that are in the market that I like to work around are parallel positioning. I still love the linkage boilers. I will talk about those, but the linkage boilers will actually only have four ways to control steam pressure because of how they are typically wired up and ran.
Eric Johnson (06:12)
and they don't have multiple brains in They only have the flame safeguard. So let's get into it. If you have any questions, once again reach out to me. If you haven't already, please rate the podcast five stars. It costs you nothing, but helps me immensely. Helps other people find this podcast. Share this podcast with your coworkers. I I'm not a big self promoter and marketing person, trying to get more into it, but
Eric Johnson (06:39)
If you just say, Hey, I learned something from this podcast. And if you tell me that and tell your coworkers that, tell them to listen, it helps me out so much. It energizes me every time somebody reaches out, says, Hey, I love the podcast. because sometimes I just feel like I'm shouting out into the void. But that is all right. Here for love of the game. There are five ways to control steam pressure in a boiler. The first one is going to be your modulation, and that is going to be
Eric Johnson (07:08)
this is a often misunderstood, but the modulation of steam pressure is the changing of the fuel input rate in order to achieve a steam pressure. And that steam pressure is going to be your desired steam pressure set point, and that is what you are going to want in your boiler.
Eric Johnson (07:30)
it depends on how your system is ran, but let's just say that we don't have a big pressure drop across our non-return valves. this is gonna be what you want in your plant. So in your steam header, if you want a hundred psi, you're gonna want your boiler to run at a hundred psi, and that's assuming that there's not a huge pressure drop across your non-return gate valves and your spool piece. So we're just gonna assume that the boiler drum pressure is also a hundred psi. That is not correct in all cases.
Eric Johnson (07:59)
But you may need like a plant master controller in order to make that run correctly. But we're just talking simple boilers, simple boilers, steam systems. If you don't already know, a steam header is a large collector pipe that multiple boilers feed into, and then on the outlets of the pipe, it goes to all the loads. So that is a collector.
Eric Johnson (08:27)
Collection of steam from all the boilers and helps even out all the boiler loads so that the load is not concentrated on one boiler or the other. If everything is evenly divided, the steam header should be where all the steam pulls from and it should equally pull from all the boilers. So the modulating steam pressure set point is the set point in your steam boiler that's going to change the fuel. And think of this as your
Eric Johnson (08:56)
gas pedal of your car. If you want to go faster, you press down the gas pedal. If you want to slow down, you pull up on the gas pedal. Same thing with your steam boiler. If you need more steam pressure, that's going to be more fuel. So it's going to add more fuel. And we're talking about fuel typically natural gas could be propane, could be fuel oil. With electric boilers this works a little bit differently. It'll bring on elements and
Eric Johnson (09:25)
Turn off elements depending on how many elements you have. So if you have 10 elements, it'll it will click on maybe five elements. And then if it needs more, it'll click on a couple more elements, and then maybe all the way up to 10 elements. If the boiler is really far from the steam pressure, and then as it gets closer to the steam pressure, it'll start turning off the elements. But
Eric Johnson (09:52)
it is not going to essentially modulate the electricity to the elements. It is just going to be turning on the elements on and off. And out of the ten in that example, it is going to be modulating how many are active and powered.
Eric Johnson (10:08)
I'm gonna mostly reference fuel boilers because a lot of people use fuel boilers as in natural gas. So modulating is your gas pedal, just like your car. Once again, more fuel is more steam pressure because we have more heat going into the boiler, which is going to boil more water, which is going to make more steam. If we reduce heat,
Eric Johnson (10:33)
Into the boiler, and if we reduce it down to the minimum firing rate, we will be at the minimum amount of heat input to the boiler. We will be still making steam, but if the load has increased above that minimum point, now we'll need to add more fuel to balance everything out. And we're trying to balance everything at 100 psi in the boiler or in the header. And when your plant goes to
Eric Johnson (11:03)
Say zero load at the end of the day, or it gets warm outside and you don't need any more heating. Your minimum load will fall below the minimum or turn down of your boiler. Turn down of your burner is essentially the minimum heat input that the burner can handle. Not every burner is infinite turn down. I don't think any of them are infinite turn down, but five to one is super common, typically up to ten to one turn down.
Eric Johnson (11:33)
on burners is sold for steam boilers. If you have a ten to one that is on the higher end. But five to one is your average burner. Sometimes they can be three to one. So if you have a heat input that is ten million BTUs and you have a ten to one turn down, your minimum fire would be a one million BTUs to keep the math simple.
Eric Johnson (12:01)
So low fire would be one million BTU's input and high fire would be ten million BTUs input, ten to one turn down. same thing with all the other ones, five to one, three to one, it's the same math problem.
Eric Johnson (12:14)
So the st if the steam load or if the load falls below the minimum fire rate or minimum turn down of the boiler, the modulation control can only take the boiler burner down to its minimum position, which is going to be X amount of heat input. And if the steam pressure keeps rising, the second way to control steam pressure in a steam boiler is going to be
Eric Johnson (12:41)
Your electronic operation control. And this is once again going to be on parallel positioning systems or PLC systems systems with a brain beyond a flame safeguard. And this is going to be your electronic operation control. And that is going to be inside the settings of your parallel position controller. And that is going to shut down the boiler. I will also note your modulation control is going to be inside the settings of your
Eric Johnson (13:11)
electronic control, your parallel positioning system. And by parallel positioning system, I'm talking about like a Fireye Nexus system, Siemens LMV5 five, Auto Flame, Mark 8, Mark 9. Those are parallel positioning systems. And though the settings are gonna be inside the systems and that is gonna be opposite of a traditional boiler. A traditional boiler with linkage and I'll say traditional steam boiler.
Eric Johnson (13:40)
You will have three pressure controls on your control tree. Typically they're gonna be Honeywell Pressuretrols Pressuretrols once again is a trademark name and a standard name just like Kleenex and Tissue. Kleenex is a trademark name. Honeywell has trademarked the name Pressuretrol although that is a standard name used in the industry. But you're gonna have three physical controls on your steam.
Eric Johnson (14:05)
Header tree that is going to control your steam pressure. And then two of those three are going to be your physical modulation control and your physical operating control. Typically, that modulation control will be the 0 to 135 ohm coil that is going to drive a mod motor or a Honeywell Module 4 motor that is going to have the 0 to 135 control in it.
Eric Johnson (14:32)
And then the second one will be the operating control, which will just be a pressure switch that is going to be single pull, single throw, and is going to open as the steam pressure rises above the set point of that control. However, with a parallel positioning system or a PLC system, wherever you want to call it, or whatever you have on your boiler, you will have the operating control, your first operating control, and your modulating control. Those were those will just be settings inside of.
Eric Johnson (15:02)
The electronic control system, your parallel positioning system, and your parallel positioning system will know what the steam pressure is in your boiler from a pressure transducer that is on the control tree of the boiler. So that is how that knows. If you look up on your control tree and see a little cylinder like object that is probably three to four inches tall, maybe two inches in diameter, that is probably your
Eric Johnson (15:27)
Pressure transducer and it'll have some wires coming off of it. And if you trace the wires back, that will go to your parallel positioning system. So that is how your parallel positioning systems knows what the steam pressure is inside the boiler. going back to the the electronic operator, that is going to be when the boiler shuts down. So if we have the minimum fire rate turned all the way down and we are at minimum fire and the steam pressure keeps rising.
Eric Johnson (15:54)
Inside the boiler, we can't infinitely just keep rising up on the steam pressure. So say our set point is gonna be 100 psi. We're gonna have 150 psi mawp of a boiler that is gonna be maximum allowable working pressure in the boiler. However, we don't wanna raise our steam pressure all the way up to 150, and we actually can't due to all the safety controls on the boiler. But say our plant can handle up to 110 psi, so we're gonna maintain
Eric Johnson (16:24)
With our modulation control, 100 PSI. If the boiler pressure drops below 100, the modulation control will add fuel. If it rises above 100, the modulation control will take away fuel until it's at its minimum firing rate. If the boiler steam pressure keeps increasing up to 110 PSI, and once again, these are just examples. If it keeps increasing up to a 110 psi, the electronic operator we have set.
Eric Johnson (16:53)
For 10 PSI over our modulating steam set point or the set point we want to maintain. Sometimes people get that confused. They think an operating control, that's where you're gonna operate your steam boiler. That is just what the boiler control is called, the operating control, but that is not where your steam pressure is going to live on how these systems work. So a modulating steam pressure control is actually where the boiler is going to operate.
Eric Johnson (17:22)
And try to maintain that steam pressure. But the steam pressure keeps rising up and it hits 110. And then our electronic operator says, Hey, we're 10 psi above our modulating steam pressure. We are at 110. All right, we're good. Cut it out. And it's going to turn off the boiler. The boiler's going to go to post purge and then to standby. And depending on what the load is, the steam pressure may quickly drop or may it may just slowly drop.
Eric Johnson (17:49)
But the steam pressure will slowly drop and then there'll be a cut in. This will be different than the cut out. The cut out will be 110. The cut-in we may say is five PSI differential. So it's gonna be 105. So the steam pressure will hit 110, the boiler's gonna shut off, and then it'll go 109, 108, 107, 106, 105, and then the boiler starts. It'll go from standby to pre-purge.
Eric Johnson (18:17)
And then when the boiler lights, hopefully it's around a hundred psi. So the boiler doesn't have to do this big catch up. And then it'll just maintain and modulate to a hundred psi. And then once again, if it repeats that cycle, as most plants do, if they don't have a steady state load, or if they're in a low period of load, the boiler will eventually rise up to 110 and shut down. There is no limit to how high you can make that steam pressure. Just know that
Eric Johnson (18:47)
Every part in your system needs to be rated for it. So if you have parts in a steam system that are only good to 125 psi, you obviously can't go above 125 PSI. But in a 150 PSI boiler, typically I would recommend all your parts be good for 150 PSI. There is some issues with that if you have 125 PSI parts and you have 150 PSI boiler.
Eric Johnson (19:16)
I would always recommend having 125 PSI safety valves. That's a side tangent. So the operator, shut it down, and I will say that is the electronic operator. If you have a linkage control, that is going to be the physical operator. So then the third way to control the steam pressure is going to be your physical operator on your boiler. And this is going to be the third one. And this is
Eric Johnson (19:44)
Different than a linkage boiler because a linkage boiler doesn't have the electronic operator. They have a physical modulator and they have a physical operator. But even with all our safety controls, ASME requires a physical operator to be present. So even though your parallel positioning system or your PLC system is turning the boiler on and off with your internal operating set point measured with a pressure transducer.
Eric Johnson (20:13)
ASME still says, hey, you still need a physical operating control. So you're gonna have a physical operating control that is set just higher than your electronic operator. so in this example, if we're modulating to 100 PSI and the electronic operator is set at 110 Psi, we may set our
Eric Johnson (20:35)
physical operator to 115 PSI. And that will allow us or keep the boiler safe if somebody changes the electronic operator. But that will allow us a little bit of s a room for the electronic control system work properly just in case it overshoots a little bit that it's not turning off on the elect on the physical operator. But if we have a little bit of space in there, that is fine. If we don't, you know we can back it down to
Eric Johnson (21:05)
Hundred and twelve if you really ran up against the safety valves. there's no requirement be of the space in between the electronic operator and the physical operator. And that physical operator is going to be on the pressure control tree. And it's typically gonna be a Honeywell pressure troll or a Dan Foss. The the white controls, I don't I forget the model number. I don't know if they have a trade name.
Eric Johnson (21:31)
But I know Cleaver Brooks likes to use the Dan Foss ones. Fulton likes to use the Dan Foss. But or the Mercoid Mercoid was bought by Dwyer. You can use the Mercoid Mercoids, they're a little bit weird. They seem super complicated and they're kind of big, but you can also use the Mercoid Or I've also seen I think it's Ashcroft. Ashcroft. Yeah, the black.
Eric Johnson (22:00)
I I think it's Ashcroft. They're black pressure switches. I've also seen those. Although those they are not super great with being able to set a precise set point with a scale. Typically you have to bench test them and bench set them. But really I've only seen those used on larger water tube boilers. They are a little bit more durable controlled. They also come in like explosion proof or fully waterproof housings.
Eric Johnson (22:29)
You can get that option, but the set point control on them is not going to be as fine as you can see on a Honeywell pressure troll. But they they still work, but you would just have to set them on the bench, pump them up with a calibrator, and make sure that they are set correctly, and then write down where the control is going to trip. But you can use any of those, but that is going to be your physical operating control.
Eric Johnson (22:56)
And then the fourth way to control steam pressure on a boiler is going to be your high steam pressure control set point. And ASME, once again, mandates two steam pressure controls on a boiler, even if you have an electronic operating system. So we're going to have two physical steam pressure controls. We're going to have one physical operator. In this case, that is going to be set at 115. Then we're going to have a physical high steam pressure control. And that is going to be
Eric Johnson (23:25)
set higher than the operator. So in this case, let's set it at 125 just to make sure it doesn't trip, just in case we overshoot our operators for some reason. Or if we're playing around with our set points and our steam set point goes up by a little bit, we don't want to have to move our high steam pressure control set point. And that high steam pressure control will differ than the operator in that it will be a manual reset.
Eric Johnson (23:55)
And I know CSD one off the top of my head, I don't know NFPA, but it has to be a manual reset. And CSD one says the manual reset can be inside or part of the control. So on a Honeywell pressure troll, on the new ones, it'll have that little red button on the front. You'll press that little red button. The Dan Foss, the white housing controls will have a green slide on the top right of the control, and you'll push that green slide back.
Eric Johnson (24:25)
Mercroid pressure switches will have the reset at about the five o'clock position on the side of the housing. You'll so you'll press a little metal rod in and it'll reset the control. So those are some different controls for the high steam pressure limit.
Eric Johnson (24:45)
Also, the manual reset can be part of the control system. So it doesn't have to be part of the control. That is a common misconception. So if you want to have an automatic high limit, as long as your
Eric Johnson (25:02)
Control system locks out and requires a manual reset. That is also good for CSD one. I am not sure on NFPA. I am not w as well studied off the top of my head for NFPA on that. And that would be NFPA eighty five. If you want to look it up and let me know, but I'll probably look it up after this recording. Just so that I can get educated because I know this will seem crazy, but I don't know everything and I'm not afraid to admit that I don't know everything. I am just here trying to spread.
Eric Johnson (25:31)
What I know, what I've learned, so that you can be better and help others learn as well. So going back, so the modulation control controls the amount of fuel or the amount of heat input into the boiler. In this example, we're modulating to control 100 psi. That is going to be our steam set point. That is what our plant requires: 100 psi. That's what we're gonna set our modulation control to, not our operator control. If we set our operating control to 100 psi.
Eric Johnson (26:00)
The boiler will get up to 100 and cycle and turn turn off and we'll never maintain a hundred Psi. So every time we hit set point, we'll be we'll be right there and then cycle and turn off. And then by the time the boiler turns back on, the steam pressure will have dropped and it'll be this this cycle. And if you were to look at the steam pressure, it'd never be consistent. But I have seen pressure controls set up like that by people who didn't know better. They've they thought, operating control, that's where we want to operate.
Eric Johnson (26:30)
No, actually the operating control is gonna be where the boiler is gonna actually shut off and it's gonna be your highest safe operating steam pressure for the boiler. But typically that is gonna be set about 10 Psi above the modulating pressure control. So in this case, the electronic operating steam pressure control is going to be set at 110.
Eric Johnson (26:53)
And then ASME requires we have a physical operating steam pressure control, which is going to be on our control tree. That is in this example going to be set at 115 PSI. And then ASME also says that we have to have a manual reset high limit pressure control that is physical as well. And that is also going to be on our pressure control tree. And we are going to set that at 125 psi in this example.
Eric Johnson (27:22)
And that is the only control that is manual reset. So if that trips, it requires an operator to go in and either press the manual reset on the front of the panel or press the manual reset on the control. I always recommend having a manual reset on the control so that no matter how the boiler is wired, you will always know that your high limit tripped and you have to push it on the high limit and actually look at the high limit.
Eric Johnson (27:45)
Because some operators will just push reset on the front of the panel. And sometimes you don't have the best air history. Or sometimes the boiler is not wired in a way that you will know that the high limit tripped. So if you have all or not all of them, but if you have a couple limits that are wired in series and you don't have any way to distinguish between the limits, anywhere in that limit string, if it opens, the boiler's gonna turn off and fail.
Eric Johnson (28:14)
But you as an operator won't exactly know what is in that limit. You'll have to check each and every control. So you'll have to pull out a wiring diagram. Hopefully, you have a wiring diagram and you'll have to see what is in that control string. And then eventually you'll find that the high limit control is open. But if it is open and it has a manual reset on it, you can hear the click when you push the manual reset. And then the boiler will start and you'll know that.
Eric Johnson (28:41)
It was the high limit pressure control, and then you can take the appropriate action when diagnosing the high steam pressure control and why it tripped. So the fifth way to control steam pressure on a boiler is going to be your safety valves. Safety valve is the proper term for vapor service, not relief valves.
Eric Johnson (28:59)
It is gonna be safety valves. So use the term safety valve, that is intentional. Relief valves are for liquid service, so that's gonna be on a hot water boiler. So a steam boiler has safety valves. There is a difference. Look it up. A SME. Safety valves are gonna be the set anywhere at the MAWP of a boiler or less. I know they can change a little bit, but typically depending on the jurisdiction,
Eric Johnson (29:28)
They will be set differently or they'll they'll be set the same. I'm not gonna go into that. We're just gonna assume that safety valves are gonna be set the same. So if we have two safety valves on our boiler and the boiler has an MAWP of 150 psi, we can order our safety valves to have the same pressure, set pressure as the
Eric Johnson (29:46)
MA WP of our boiler. So both safety valves will be set at 150 PSI. So if our modulating steam pressure control doesn't control the steam pressure inside the boiler, if our electronic steam pressure control doesn't shut off the boiler,
Eric Johnson (30:02)
If our physical operating control doesn't shut off the steam pressure or the heat input to the boiler, if our high limit doesn't shut off the steam pressure or heat input to the boiler, if all those fail, and they can, that that seems wild to some people, but they can due to how they're piped, it's a very easy failure point. But if all four of those don't work,
Eric Johnson (30:29)
Now you have our safety valves, and our safety valves are sized on our boiler in order to exceed the capacity of the boiler. So if we have a certain amount of heat input, that heat input will then translate to a certain amount of steaming capacity for the pressure vessel, and that steaming capacity will be less than the amount of steam that can be relieved out of the safety valves. Over a certain size of boiler, you need two safety valves or multiple safety valves.
Eric Johnson (30:55)
So you'll see some boilers have one safety valve on them, and then you'll see some boilers have two, or other boilers will have three. There is a certain size of boiler where the manufacturer is required to install two safety valves versus one larger safety valve. But the total combined capacity of both of those safety valves has to be greater than the capacity of the boiler. How do we know what the capacity of the boiler is?
Eric Johnson (31:21)
So every boiler has a ASME marking or ASME stamping data on it. And your boiler will be stamped with an ASME designator. That designator for a high pressure steam boiler, which is 15 psi or more, is going to be typically on a normal boiler, it's going to be S. So that is the most common one. If you have a heating boiler, which is going to be 15 PSI or down.
Eric Johnson (31:49)
That is going to be stamped with the designator H.
Eric Johnson (31:54)
But next to that, ASME information will be the rated capacity of the boiler, and that will just be a number as in pounds of hour of steam. So say it is 7,000 pounds an hour of steam. Your total capacity of your safety valves would have to add up and exceed the 7,000 pounds an hour of steam.
Eric Johnson (32:16)
So if you had a safety valve that was set at 4,000 pounds an hour of steam relieving capacity, on the safety valve tag, it will say capacity and it'll say 4,000 pounds an hour. And if you have two safety valves, the total capacity of both the safety valves would be 8,000 pounds an hour, which is greater than the total capacity of the boiler, which is in this example 7,000 pounds an hour, and therefore it would be compliant.
Eric Johnson (32:44)
So that is something that you gotta look at on every single boiler. Typically it's gonna be correct. Typically, if you have more than one safety valve, they are with a little bit of margin over capacity of what the boiler actually produces. Very rarely have I seen two safety valves add up and be like at the exact just over the the exact capacity of what the boiler does, typically since you can't exactly match
Eric Johnson (33:13)
Safety valves perfectly to the boiler, and there's really no need to do it. The capacity is gonna be much greater than what you actually need. I wouldn't say much greater, but ten to twenty percent easy, greater than what you actually need. So let's go back through these top to bottom. We went bottom up. Let's go top to bottom.
Eric Johnson (33:38)
The MAWP of a boiler is going to be the maximum allowable working pressure of the boiler. Typically, your safety valves will be set at the MAWP, but they can also be set less than the MAWP. If you have an MAWP of 150 PSI, you can have your safety valve set at 125 PSI. That is fine. You just cannot have your safety valve set higher than the MAWP of the boiler in general. I know there's some exceptions.
Eric Johnson (34:06)
With the 10% or 5% over with the modulation or changing of the set points of the safety valves. But in general, the safety valve set point will be no higher than the MAWP of the boiler. The next one down will be your high steam pressure limit. That is going to be a manual reset limit, and that is going to be a physical limit that is going to be on your control tree of your steam boiler. That physical limit will be the last line of defense before your safety valve.
Eric Johnson (34:37)
Opens and relieves steam pressure from the boiler, but that high limit is going to shut down the boiler and hopefully make it safe if all the other limits before it do not turn off the boiler. Next below the high limit is going to be your physical operating steam pressure control switch. That is going to be the physical switch right next to the high limit on the control tree. That physical switch will be set just under.
Eric Johnson (35:04)
the high limit steam pressure control. So if your high limit steam pressure control is set at hundred and thirty-five, your operating steam pressure control would be set at a hundred and thirty. That is going to shut down the boiler. Your operating steam pressure control is going to shut down the boiler. If your electronic operating steam pressure control doesn't shut it down, we're going to be talking about
Eric Johnson (35:32)
Parallel positioning control boilers or PLC control boilers, they'll all have electronic operating pressure control settings that will shut off the burner and that will be inside the controller. And the the settings of the cut-on, cut off, that'll be all inside the controller. But you'll have that electronic operating control. That is important to remember because if you are
Eric Johnson (35:58)
Thinking that you're gonna be cycling the boiler on the physical operating steam pressure control, you will have the boiler short cycle because that control is opening and the electronic brain, the operation, the parallel position control of the PLC is not going to be expecting that control to open if its own internal set point for its operating control is set higher than the other operating control. So you will have short cycling of the boiler and
Eric Johnson (36:27)
It won't seem like there's an issue, but there will be an issue to somebody who is keen on how boilers should be running. And then just below the electronic operator is going to be the electronic modulation control. The modulation control is your gas pedal of your car. More fuel is more steam. Less less fuel is less steam. So that is going to modulate the amount of fuel input.
Eric Johnson (36:54)
or the amount of heat input into the boiler, which is going to try to maintain the amount of steam pressure that you need in your boiler or in your header. So in our example, if we're trying to maintain 100 psi, we're going to be setting our modulation control to 100 psi. And if the steam pressure rises above 100 psi, we are going to be in the low fire position. If the steam pressure drops below 100 psi, we're going to be increasing fuel proportionally in order to try to
Eric Johnson (37:24)
Make up the steam pressure and make more steam pressure and then get back to the set point of 100 psi. And if we keep rising above that because the load is low, then in our example, the electronic operator will shut down the boiler at 110. And if that operator doesn't shut down the boiler, say the pressure transducer pigtails plugged or something's wrong with it,
Eric Johnson (37:51)
then the physical operating steam pressure control is going to shut down the boiler, and that will be set at say 115 in our example. And then the after that the next safety is the manual reset high limit. Once again, the high limit doesn't have to have the manual reset as part of the switch, it just has to cause a manual reset function. But I would highly recommend it be part of the switch.
Eric Johnson (38:17)
It just makes troubleshooting easier. Also makes identifying the control easier. without it, it will look just like the operating steam pressure control. And then you'll have to look at the wiring diagram and trace it back to see which one you should be setting higher as far as the set point. although you could also trip it and see which one causes an alarm versus which one will just shut down the boiler and cause it to go to standby. The operating steam pressure control.
Eric Johnson (38:46)
I should have said this earlier, we'll just shut down the boiler and cause it to go to standby where the high steam pressure control will cause it to go into alarm because the thinking is, is if the high steam pressure control, if that fails or if the high steam pressure control is operating and shuts off the boiler, something is wrong because the operating steam pressure control below it, and if you have in this case the
Eric Johnson (39:13)
electronic operating steam pressure control below that as well. If those two controls have not shut off the boiler, something is wrong and we want a operator to come look at it. So that is why a manual reset is required. So those are the five steam pressures. Hopefully you do not walk in and find the boiler cycling on the safety valves like I have. That was a good experience. The customer called and said
Eric Johnson (39:40)
Hey, our safety valves are periodically blowing on our boiler. And I went in and it was I I won't even say the boiler manufacturer, but this one, not a good design of boiler, but basically the whole control tree stemmed around a little nipple that never got inspected, never got cleaned, just due to how the boiler design was and the nipple was a hundred percent plugged, which then
Eric Johnson (40:10)
removed the sensing from all the controls and this was a little quarter inch nipple, not it was definitely a choke point in the boiler design. should not have a little quarter inch nipple that is then feeding all the pressure control tree piping. but due to this boiler design that was how it is. I haven't seen that on any other boiler design. But that plugged up
Eric Johnson (40:38)
And all it jumped out all the controls. So none of the safety controls were seeing boiler pressure. So the boiler was sitting in high fire and the safety valves would it would run up to the safety valves and then it would blow the safety valves and they would eventually blow down the boiler to where the b the valves would shut and then it would just keep opening the safety valves and it was essentially just cycling on the safety valves.
Eric Johnson (41:07)
That made for a very long and interesting day. the customer was very, very confused as what was happening, and I was also also confused as to what was happening because I had never seen a boiler cycle on the safety valves. But that is just a great example of the safety valves working and none of the electronic steam pressure controls working. Due to that design, and that is why when you are opening up a steam boiler and
Eric Johnson (41:38)
maybe doing an inspection on a steam boiler that you should be taking apart the control tree piping, that you should be visually looking through it. That is why we put crosses on the piping. That is why we put T's on the piping so that we can see down through the piping so that there are no plugs. Even if the piping is ninety percent blocked, it would pass a pressure test as in like pressure can get through. But we obviously don't want to close up piping that is ninety percent blocked because it is going to
Eric Johnson (42:07)
eventually become a hundred percent blocked. So we're gonna want to visually see any blockages and you will also want to, and I know it takes time and your customers don't have the time, they need production back, but you will want to pull apart all the pigtails and flush those and inspect those. It that is well worth it. And I would also highly recommend depending on the amount of steam pressure you have, you gotta be mindful of your steam pressure. But
Eric Johnson (42:36)
having pigtails that are not carbon steel, having ones that are brass is highly recommended because they won't corrode on the inside like the steel ones. I've seen steel pigtails, they will corrode and it'll almost become a pinhole right below the steam pressure control switch. And then eventually when that pinhole blocks up, sometimes it'll block up when you shut down the boiler, because that's the first time that the boiler's been shut down. But when that pinhole blocks up,
Eric Johnson (43:05)
Now that control is essentially isolated from the boiler and depending on how it blocks up, it'll either trap pressure in between the control and the pigtail, or it'll trap no pressure in between the control and the pigtail. And I've had I've seen it both ways to where a control won't reset because of that, or I've seen it where the control will not run. I've had another issue where the control tree piping was a hundred percent plugged.
Eric Johnson (43:35)
And this was a little bit special application on this boiler, but the control tree piping was plugged and I was starting up the boiler and I found it interesting that when I was firing it that the pressure gauge or anything didn't seem to be going up yet. I was making heat. but the it was open to the header, so I was like, Man, we're must be consuming a lot of steam pressure. And then all of a sudden the boiler shut down.
Eric Johnson (44:04)
And it shut down because the high limit steam pressure control was piped off a different pipe and that shut it down. But on my gauge, I was still at zero on the steam pressure. I was like, why'd the boiler shut off? And I find that the high limit steam pressure control had tripped. And then I got to looking into it and I found out the entire control tree was a hundred percent plugged. But luckily the high steam pressure control was piped on a different pipe off of that particular boiler. And that is what shut down the boiler. Otherwise I would have found out.
Eric Johnson (44:33)
that all the pressure controls didn't work and were blocked from the safety valves opening, which would have been no big deal. That is why ASME has safety valve sizing and the rating of the safety valves and that's why safety valves are have ASME designators on them, the V and you have to be a certified rebuild shop Those are calibrated safety valves and everything. So there is a lot of safety controls that go into a boiler.
Eric Johnson (45:02)
Boilers are very safe when they are operated correctly. But as a service technician, as an operator, if you are pulling apart a boiler, you need to make sure that you are making sure that you are setting those controls up for success and that when you close that boiler together, that that boiler can run for another 365 days without having any issues. If you have partial blockage and you close it up, yeah, it may be fine for a week. Yeah, it may be fine for a couple months. But if it's partially being blocked and
Eric Johnson (45:32)
We're just gonna assume that that blockage is gonna keep building and that's gonna become a service call and a problem with that plant operation. And we will always want to prevent that. And that is part of the preventative maintenance from the inspection standpoint when you are taking part a boiler on its annual, that you are cleaning all the piping, that you're looking at it, that you're either cleaning or replacing the pigtails, that you're doing everything you can to make sure that you're giving that boiler the greatest chance of running correctly with.
Eric Johnson (46:01)
out any problems for a year. So that's what I have for you today. The five ways to control steam pressure on a boiler. Hopefully you learned something. If you didn't, maybe I failed you. Maybe it was just review. If you have a positive review, please write that. I think you you know I've had some people comment on Spotify, but write the positive review, five stars on Apple or Spotify or Amazon Music.
Eric Johnson (46:29)
iHeart Radio if you're listening on those but or even YouTube. But I appreciate everybody who listens to this podcast. And if you have any questions, guest recommendations, or anything else, you can reach out Eric dot johnson at Boilearn.com or DM me on LinkedIn thank you for listening and stay wild.
