How Often Should You Bottom Blowdown Boilers?
Easy question yeah? Must be a standard for bottom blowdowns everyone here should be saying. There is, sort of. The number of bottom blowdowns actually required daily is a moving target. There is not one set answer for every single boiler.
It is a simple answer though.
The quantity of bottom blowdowns required each day, for each boiler, ultimately depends on the conductivity of the boiler water.
Don’t forget that conductivity is merely a reading of the amount of solids, both dissolved and undissolved, floating around inside causing corrosion.
If that was the answer you came looking for, there you have it. Below here, we are going to dive deeper into it. Talk about why we are actually concerned with the frequency of bottom blowdowns, how to adjust and fine tune your own.
This is perhaps the most common question asked and debated about by building and stationary engineers.
How often should bottom blowdowns be performed?
And almost every answer you will ever hear is going to be different. Being a former bubblehead, I firmly believe that boilers need to be blown down more often than not. Per most boiler manuals, a boiler should get at least one bottom blowdown per day. This helps prevent sludge from settling too much on the bottom.
This is where balancing the conductivity comes in though. We can’t just keep blowing down over and over, we’d waste nice warm boiler feed water, we’d lose the chemicals that are being added to treat that feed water, and we’d lose the protective aspect of having some conductivity.
We want some conductivity in our boilers, just not too much. More often, a water treatment representative will come in and analyze your water and tell you what conductivity you should try and maintain. They will give you a range. Knowing your boiler water conductivity on a daily basis should be a big part of any water treatment program as well.
You then perform your bottom blowdowns as often as necessary to stay within that range they gave you. Back in my day…
Building and stationary engineers did all that. They analyzed the water and then made adjustments as necessary. If conductivity was too high, we would maybe blowdown twice that day. The next day we’d take a conductivity reading. If it was too low, we knew two bottom blowdowns were too much. If it was still too high, we’d maybe add a 3rd blowdown to the day.
Then to fine tune it, or dial it in, you would then analyze the amount of time you were blowing down the boiler. A bottom blow can be brief, a second or two to blow out sludge or 5-10 seconds, which is standard, or even 30 seconds.
If we fell somewhere in between 2 to 3 bottom blowdowns per day based on conductivity, but not quite either, we then mess with the bottom blowdown duration. Maybe with 2 LONGER blowdowns we could hit the reading we wanted without having to do a 3rd. Or, what if we did 3 shorter blows instead of 2 long ones? Feel me? Maybe try three daily 10 second blows instead of two daily 20 second blows.
Anyways take a week and mess around with your boilers if you happen to fill that role. If you can take a conductivity reading off of just one boiler water sample. Perform a bottom blowdown, wait like an hour, and take another reading. What happened? Next shift take a reading, bottom blow, wait the hour, reading, bottom blow, wait, reading. What now?
You just did 2 blowdowns, what is conductivity at now? How about compared to the previous day where you only did one?
What the “Book” Says About Boiler Bottom Blowdowns
Do you think the book will break it down like I did up above? It might actually. Ha ha. Of course I needed some reference material, can’t keep everything in my head. Here is what the latest edition of the Low Pressure Boilers has to say about bottom blowdowns.
They go over a brief why, a brief when, and then how. Paraphrased of course.
The reason for bottom blowdowns is to control chemical concentrations in the water and to remove sludge and sediment. How often you blowdown your boilers depends on plant operations and conditions as well as the characteristics of the feed water.
Important (from the book):
In general, boilers should be blown down at least once every 24 hours.
The authors, who are or were stationary engineer instructors, also recommend performing a bottom blowdown right before placing a boiler online to help stimulate water circulation in the boiler.
A bottom blowdown is best performed during a light load condition, as low a fire rate as possible. This used to be a major brain fart on my part because I was taught you want to do it under heavy load to ensure as much turbulent water as possible. But this way makes more sense.
We want to do it under light load conditions so that the sludge and sediment are sitting on the bottom of the boiler, ready to get blasted out of the bottom. You want to always have some amount of pressure inside the boiler as well.
Make sure you have a normal operating water level in the boiler.
You’ve heard about the exam question with the valves right? Ha ha, everyone has. And everyone messes it up. Here you go.
Boiler bottom blowdown valves can be a slow opening valve and a quick opening valve, or two slow opening valves. The quick opening valve should be located closest to the boiler. Wink wink hint hint.
The quick opening valve is opened first when both kinds are used. The slow opening valve is opened slowly to fully open. This valve is the one that takes the brunt of the blowdown, the force of the pressure and such. Also, by using the slow opening valve, you can control the rate of blowdown and thus the water level inside the boiler.
For example, if you wanted to do a 60 second blowdown, you would throttle back on the slow opening valve. Keep in mind if you do this, the blowing out of the sludge and sediment will be LESS effective. You are throttling back on the rate of flow out of the boiler.
Bottom Blowdown Good Practices
- Check the water level before, during, and after.
- The water level in the sight glass should always be visible.
- The bottom blowdown valves should be opened and closed slowly (another thing I was apparently taught incorrectly, I was taught to whip them open.)
- If there is a quick acting valve, open it first and close it last. Hint hint wink wink.
- Never leave a blowing down boiler – always watching the water level, ready to act.
Basically we answered the question of how often should a bottom blowdown be performed on a boiler. Then we got all up into the why like nobody’s business.
How often you bottom blowdown a boiler depends on the concentration of solids in the boiler – conductivity.
We then talked about ways you can control it and try to customize it to the needs of your plant and systems. Controlling this, the conductivity of the boiler water, should of course be a huge part of your overall water treatment plan. We just did a post on water treatment and its importance, click here to read that one.
Finally, we paraphrased information from some resources, mainly the one below, Low Pressure Boilers. Hey, this may be stuff you need to know for your boiler exam test yeah?
Hey, a lot of the info on this page came from one of the books I’m always talking about, Low Pressure Boilers. If you think after reading this that it would help you, you can check out the review I did here.