Why You Can't Use High-Pressure Steam Boilers on Low-Pressure Steam Systems

steam gauge

This was going to be an easy job – one of those in-and-out deals where a guy can make a fast buck, but the truth was he didn't really care whether he got this one or not. He was already plenty busy. But this one looked so easy. All he had to do was convert a high-pressure steam boiler to low-pressure operation so the building owner could use it for heating instead of process. The owner also wanted him to reduce the boiler's load by 75 percent, and all that involved was changing the burner, right? Nothing to it!

The contractor figured he could install the new burner, hustle the old one away for its parts, replace a few steam traps throughout the system and get the heck off the job. Easy. He wasn't quiet sure what to charge, because he knew he might run into a few unexpected problems, so he just threw out a price that seemed right, and he got the job.

Now, whenever this contractor got a job he always liked to go back and look at his numbers. He wanted to compare them to what his competitors had bid – just to see if he had made a big mistake. When he asked the owner about the competitors the owner told him there hadn't been any. He had been the only bidder on the job. "No one else was interested," the owner told him. "We accepted your price because we had no choice. How soon can you get the job done?" This, of course, gave him a queasy feeling.

But all he had to do was take a 400-HP boiler that had been operating at 125-psi and make it work as a 100-HP boiler that would release a mere 10-psi steam pressure into the system. The owner had just taken over this building, and he had no need for such a big boiler. The seller had told him he could get plenty more years out of the big boiler by just turning it down a bit. That had made sense to the new owner. "It's just like turning down a stove," the seller had told him. "You just turn down the knob, just like you would on a stove." The seller made a twisting motion with his hand and smiled. “Everything will be fine!"

This had made sense to the contractor, as well as the new owner. It seemed so easy.

But someone really should have noticed how small the 400-HP boiler’s nozzle was. You see the thing about high-pressure steam is that it takes up a lot less space than low-pressure steam. It also moves more slowly. The people who make boilers know this. Unfortunately, the owner and the contractor didn’t know this.

When the steam's velocity increases (which it surely will when it's at low pressure and aimed at such a little nozzle), the steam will pull the water from the boiler and send it vomiting into the system piping. That causes water hammer, which can break just about anything standing in its way. It will also lead to wet steam, which is not of much use when you're trying to heat something. This is why you can't run a high-pressure boiler on low-pressure settings. The boiler manufacturer told the contractor about this when the contractor called to ask.

The contractor thought he could get around the high-velocity problem by cutting into the boiler and welding a bigger nozzle onto the shell. The boiler manufacturer said this would probably work, but the contractor hadn't mentioned the pipes. They were still as small as ever. The boiler manufacturer hadn't asked about the pipes because the contractor hadn’t brought them up. The boiler manufacturer's curiosity ended at the flanges of his equipment.

The contractor would cut the bigger hole, and that would allow the steam to leave the boiler more slowly, but then the steam would just speed up again when it hit those little pipes. The contractor hadn't planned on any of this. He figured that when he installed the smaller burner Mother Nature would take care of everything else. "It’s only 25 percent of the load!" he said. "The pipes should be all right. Shouldn't they? I mean, I haven't checked because I don't know how to, but they should be all right. Shouldn’t they?"

Another problem was that the boiler's internal surface area hadn't changed one stinking bit. Sure, the contractor had lowered the firing rate by installing the smaller burner, and he had increased the size of the nozzle to accommodate the relatively enormous volume of steam, but he still had all that internal surface area inside the boiler to deal with.

The boiler looked down on that tiny flame in its belly, laughed at it, and then settled down simmers away. It ran, and ran, and ran, and ran without producing much steam. This, of course, solved the excessive velocity problem for the time being. Where there is hardly any steam, there will be hardly any steam velocity.

But the owner wanted steam. It was, after all, steam for which he had contracted, was it not?

The contractor began to get very concerned. This had seemed so easy.

And then there were the flue gases to consider. They began to condense inside the boiler and inside the flue, because there were so many boilers and so little fire. The contractor knew instinctively that this was not good because this was not a condensing boiler. Well, at that point it actually was, but it wasn't supposed to be.

He solved the problem by increasing the load. He did this by installing a bigger burner, which he now had to explain to the owner, who thought the smaller burner would save him money. The bigger burner, of course, caused the steam’s velocity to increase as it crammed itself into those little pipes.

And then there were the steam traps. The contractor had changed some of the steam traps from high pressure to low pressure by replacing the seats. What he hadn't counted on, though, was that some of the traps discharged to overhead returns. He was going to need steam pressure to push the condensate up and over that hump. He'd need 1-psi pressure to lift condensate every two feet, and he would have to figure that as backpressure when he was resizing the traps. The contractor didn't actually know much about sizing steam traps; he was depending on a local steam trap manufacturers rep for help. It was the rep that had given him the bad news about the backpressure. The rep had stared at the piping for a moment and then said, "Uh oh." The contractor hated when reps said that.

The contractor also learned from the trap rep that when you lower the pressure differential across a steam trap the trap generally has to be larger to get the job done. "There's not as much push behind the condensate," the rep had said. "Water's going to back up. That's not good."

The contractor thought he was just going to have to change the trap seats. He hadn't figured the price of new traps when he had quoted the job. But it was too late to go back for a recount at this point. He would just have to eat them.

It was around then that the contractor called and asked if there was anything I could suggest to get him out of all of the trouble he was in. I sat back and listened as he told about his great adventure, and then I told him what he already knew. He was screwed.



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