Steam Velocity

Dec. 14, 2020
The critical time for steam velocity is the point that the steam wants to start moving out of the boiler.

Steam can move fast, sometimes faster than you want. There isn’t a traffic cop inside the pipes to monitor its speed. The steam just moves as fast as it can, based on its volume and the size of the pipe it is moving through.

It moves from the boiler to the radiation because high pressure wants to move to low pressure. The high pressure is the pressure at the boiler, which can be as low as a few ounces in a “vapor” system. The low pressure is the atmospheric pressure at the open air vent, at the end of the supply and/or return mains or one-pipe radiators. Whatever the pressure is on the inside of the system, it is more than the pressure on the outside.

If the boiler is under a vacuum, then all bets are off. Vacuum was used back in the good old days of coal-fired steam boilers, and no thermostats. Who has a system like that anymore? Well maybe the Amish. With automatic fired steam systems, there should always be some amount of pressure at the boiler while steaming.

The critical time for steam velocity is the point that the steam wants to start moving out of the boiler. Enough steam has released from the water and is now building up a slight amount of pressure in the steaming chest, which is the space inside the boiler above the water line.

In the early days of steam heating, the manufacturers were designing their heat exchangers with large steam chests to produce dry steam. Modern manufacturers have shrunk the steam chest to minimum proportions, which cause the steam to move away from the boiler sooner. For the bean counters there is less cast iron and therefore less cost. Fortunately for the guy hauling it down the steps, it’s lighter than its ancestors.

If the steam is nice and dry, a smallish steam chest is not a problem. If the steam is wet because there is cutting oil floating on the waterline, then that is a dirty boiler problem, which is aggravated by the smallish steam chest. A couple of articles ago, I discussed the merits of a clean boiler.

So now the steam is moving up the vertical portion of the near boiler piping to the header. This pipe size determines how dry the steam will be. Steam is moving up from the boiler, while the condensate that forms as the pipes warm up flows down the same pipe. For steam and water to move in different directions in the same pipe without problems, the pipe needs to be big enough.

If the pipe is too small, the velocity quickly builds and the water that is flowing down is carried up to the header. If the velocity builds too much, water is pulled up from the boiler, bringing the waterline down to the point of shutting off and/or bringing on the direct style feeder. Prematurely shutting off the boiler wastes fuel and leads to uneven heat. Bringing on the feeder floods the boiler when all the water finds its way back to the boiler.

To find out what pipe size you need, check the boiler’s installation manual. It will tell you what the minimum pipe size should be. I will tell you that the manufacturer’s minimum pipe size is too close to being too small. If given the opportunity, always use as large a pipe size as you can for the vertical riser(s) and header. The larger the pipe, the slower the velocity, the drier the steam.

Exit velocities from the early boilers were as low as 10 feet per second (fps) with their large steam chests and multiple large boiler outlets. With modern boilers, exit velocities are up to 40 to 50 fps. The higher velocities don’t leave much room for error before the steam starts getting wet. Today, the near boiler piping determines how dry the steam will be.

I go into more detail on pages 58 through 63 of my field guide, available as a free download at Don’t worry, I’m not selling anything, I’m paying forward. The equations are there for you to calculate the sizing before your next install or for that one job that is giving you trouble.

On steam boilers with multiple outlets, try to use the full size of each, since that keeps the velocity down. If you are planning to use the existing header from the original installation back in 1923, it was probably sized for low velocity, and is also probably well over the minimum height above the waterline. Just make sure your new vertical riser(s) are sized for low velocity. The higher the header is off the waterline, the drier the steam.

These are some of the details that make an installation trouble free and reduce call backs. Can you afford call backs? You avoid the problems of water hammer, wet steam, poor steam distribution and high fuel bills when you pay attention to the exit velocity.

I got a call the other day from Al in Cleveland. He claimed he had actually read my last article and wanted to offer some advice. Al worked in the trades and likes to keep informed about the current doings in the hydronic side. His suggestion was about getting a reliable outside temperature for the high efficiency boilers, since so often, the location of the outdoor air temperature sensor can cause problems.

Why not, Al suggested, use the local weather data that is broadcast on the internet? Most homes have wireless service now, so tap into that info with the boiler controls. I’m not an internet genius, but the idea sounds like it could work. Some manufacturer might already be doing this or have it in the planning stages. Thanks for the call Al in Cleveland, where there are a lot of boilers.

Down here on the south shore of Ohio in Cincinnati, I am in the thick of my third last full-time heating season, for those that are keeping track. I mention it because I get asked when I’m going to retire on a near daily basis. Got a few good years to go. Comments from Al or anyone else always welcome. Happy Holidays!

Patrick Linhardt is a thirty-five-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.

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