How to Blow Down a Boiler
You know what’s going to happen when you open that last valve on the boiler for your blowdown, you’ll feel that high pressure steam go frothing and rumbling past the valve disc and seat. You know you’ll catch that familiar smell of the pipes and insulation heating up during those few steam filled seconds.
You don’t? Good thing you’re here then, today we are going to cover how to blowdown a boiler. Blowing down a boiler is easy, exciting, and there are reasons we do it as building engineers and maintenance people.
Bottom blowdowns involve periodically opening valves tied to the mud drum to allow boiler steam pressure to force accumulated sludge out of the boiler.
Why We Blow Down a Boiler
Before we list out the steps in blowing down a boiler, we should probably discuss why boiler blowdowns are important. Performing regular and routine boiler blowdowns extends equipment life. How?
A boiler blowdown is water that we as building engineers, Navy guys, homeowners purposely discharge from a boiler. We do this to avoid the concentrated build-up of impurities inside the boiler. These impurities and byproducts appear due to the continuous evaporation of steam.
Kind of confused? Isn’t boiler water pure Steven? It’s chemically treated but not pure. Kind of like the water that comes out of your faucet but different. If you were to take a pot, fill it with water, and boil it off what would you find?
Peeking into the bottom of the pot, you’d probably find some sort of brown/black, maybe smelly, crusty stuff. What is this? Impurities from the water that couldn’t boil away!
A steam boiler produces steam from water. This requires frequent replenishment of boiler feedwater, otherwise we couldn’t make any steam big chief!
Water has the ability to capture tiny, miniscule pieces of solids that may be present in equipment such as boilers. Piping and components like valves are never manufactured or sealed completely perfectly. There will always be some amount of particulate or impurity floating around in boiler water. Then we add in the high temperatures and pressure…
The repeated evaporation of water, over and over again, concentrates these dissolved impurities. This can show up in many forms – from a hard, calcium like scale to a soft sludge that accumulates in the bottom, or mud drum, of the boiler. Scale, sludge, impurities – they all reduce the ability for the boiler to transfer heat. This then reduces efficiency of your equipment.
Imagine the radiator on your car for a sec. What would happen if we added in some gel or something that hardened slightly in hot water? Some of this make believe gel would stick to your radiator tubes, preventing the water and air from exchanging heat with each other, causing your engine to overheat. And yes, you can have sludge/scale/an impurity build-up inside your radiator. Pretty much anything where there is a sustained or repeated heat exchange. The hot water heater in your home is another and is why they recommend to drain and flush your heater every so often.
Without a blowdown, impurities would reach saturation levels and begin to precipitate within the boiler.
What does precipitate mean? It means to like, fall out. Water, just like air, can only “hold” so much stuff or so many things. Eventually the substance will become saturated and be unable to hold anymore. Now is when the impurities begin to collect, gather, and assemble – through precipitation.
The concentration of impurities are the greatest in low flow areas and where the steam is being produced near the heat exchange surfaces. Precipitation would be expected to occur in the form of scale deposits on those heat exchange surfaces or as sludge in the bottom/mud drum (which is why it is called a mud drum.)
This is how NOT conducting regular boiler blowdowns can reduce the efficiency of your equipment.
Scale deposits thermally insulate heat exchange surfaces, in a bad way. Not only does this decrease the rate of steam production, it could potentially cause boiler metals to reach a failure or fatigue point.
The hard, calcium scale just sits and sits. Slowly it exchanges electrons and ions with the surrounding metal, water, other impurities. Oxidation, reduction, Leo the lion goes grrr. Yeah. Eventually the material the boiler is made out of, usually the boiler tubes, cannot exchange any more electrons and the metal fails. We may get a small boiler tube leak or we may get a huge boiler failure and explosion.
Here is the wikipedia link to boiler blowdowns – it’s not nearly as helpful or informative as mine though.
Some boiler water treatments cause the precipitation of impurities as insoluble particles. We expect those particles to settle at the bottom of the boiler, as a sludge, before they become entrained in water. In the dcase of boilers, a nice, soft sludge is way more preferable to the hard scale.
Similar blowdown connections at the bottom of water wall headers are blown down less frequently. Bottom blowdown piping drains the lowest parts of the boiler so it can be used to drain the boiler for servicing.
There are a couple advantages to doing several short blowdowns rather than one long, continuous blow. Continuous blowdowns have their place but several rapid blowdowns will:
- remove sludge more effectively.
- cause less change to boiler water level.
- be safer during periods of high steam demand.
How to Blow Down a Boiler
Water is discharged out of the boiler due to the steam pressure inside. Two bottom blowdown valves are often used to minimize erosion. The sealing valve is generally opened first and closed last. Both are opened rapidly and fully to minimize erosion on the seat and disk faces. Care is taken to avoid trapping scale or rust particles within the valve by reopening a valve to flush the particles out.
The blowdown piping associated with a boiler gets hotter than shit during operation. As you can see from one of the pics, the water temperature is 330F something. Our boilers here are civilian boilers and run at 100 psi and 400F. In the Navy we had boilers 6x that pressure, imagine those temperatures. I can’t tell you the exact numbers due to the secrecy. Seriously. My arms have several little scars from bumping into steam piping. You get instant burns and scars just from looking at the steam piping lol.
- Check boiler water level.
- Check position of bottom blowdown valves – should all be closed.
- Wear any necessary ppe (personal protective equipment) baby.
- Check boiler fire rate – re-think blowing down.
- Open the valves closest to the boiler first (usually butterfly or ball valves.)
- Open the last valve in the piping (usually some type of throttle valve.)
- Open or close this last valve as needed to maintain water level in the boiler.
- Keep all valves open for the specified time (10 seconds to 30 seconds usually.)
- The duration of the bottom blowdown depends on what conductivity range is best for your boilers.
- For example, if your conductivity is at 3200 micromhos, and the recommendation is 3000, you wouldn’t have to blow down as long. But what if your reading came out at 6000 micromhos? Longer blowdown right?
- Close the throttling valve.
- Close the quick acting valves you opened first.
- Crack open the throttle valve to flush any pressure or sediment then close.
- Check boiler water level and fire rate.
- Ensure proper operation of the feedwater pump for that boiler or make sure that water level is being made up in the boiler.
How Often Should You Blow Down a Boiler?
This is kind of a tricky question to answer. It depends. And I don’t mean the adult undergarments for soiling yourself.
It depends on so much stuff – your local mechanical code, how the boiler is used, condition of the make up water supply, how hard the boiler works (how much steam it is asked to produce,) what your water treatment company recommends.
- At the very least – 1x per day.
- Preferably – 1x per shift.
- Even better – dependent upon conductivity.
This can be measured daily. Use this as a baseline, get a recommended conductivity range from your boiler maker or water treatment rep, and blow down your boiler accordingly.
The more often you blow your boiler down, the less impurities you will have. This means the boiler will show being LESS conductive.
And yes, you can blow down your boiler too much. Chemicals are added not only to inhibit say corrosion but they are also added to scavenge for oxygen. We don’t want any free oxygen why? Because oxygen is a major ingredient for rust. So if we blow down too much, we will also be flushing our oxygen scavenger, usually really expensive morpholine, down the drain.
Plus every time you blow down, you will most likely have to make up feedwater. This means adding cold water to the system, running your feedwater pumps, we would now be DILUTING or boiler water.
A note – the higher the steam demand, or fire rate, the more excited the water and steam will be inside the boiler. At a high fire rate, like 100%, the water will be violently boiling off (like you are eating Hershey’s Kisses (inside joke,) a bunch of foam will be present, and impurities will be riding the foam and maybe precipitating out.
So it is cool to do blowdowns at higher fire rates. For bottom blowdowns, you mainly will be getting rid of the sludge at the bottom. But keep in mind that blowing down at a high fire rate can also be slightly risky. Risky is the way wrong word but at 100% the boiler is firing at all cylinders and a blowdown does have the potential to shut the boiler down.
This would be like you going full speed in your car, pedal to the metal, and then just turning it off. Not good.
You should now have a pretty awesome handle on bottom blowdowns for boilers. Nice alliteration Steve-o. I know. Not only should you now know how to blow down a steam boiler but also why and how often we do it.
Blowdowns for boilers (bottom, surface, and continuous) are key to maintaining the efficiency of your boiler but are also vital in extending the life of one too. Blowdowns get rid of collected impurities so that they don’t have a chance to inhibit heat transfer or promote corrosion in low flow areas.
It is important that you base your blowdowns off of your recommended conductivity range (which is usually somewhere around 3000 micromhos,) or off of the recommendation of your water treament rep or Chief Engineer.