Question:
One of our customers has a steam system with two Smith low pressure steam boilers, each rated at 5250 MBH input.  They are having a problem with water being carried over into the steam piping when the outdoor temperature drops and steam demand increases. Their chemical treatment regime is excellent and there are no steam pipes coming off the bottom of the headers.

The steam system itself is not physically very large. Almost all of the boiler output goes to a shell/tube heat exchanger right beside the boilers, so there is not much volume in the piping system. I believe that may be part of the problem. The boilers are relatively new, and replaced existing boilers which had more volume in the boilers themselves.

When you replace an old, large volume boiler with a smaller one, you must be sure to design the steam header and "near boiler piping" in a manner that will allow it to act as an external steam separator. Each boiler manufacturer has his own ideas about how to do this, so always follow their instructions. In general, they will tell you to:
1. use all the steam risers from the boiler, and use them at full size.
2. tie all the risers together in a header, and make the header at least one size (sometimes more!) greater than the riser size. Separation occurs in the header, so we want low velocities there.
3. connect the system to the header at a point where
steam/water separation can be achieved most easily (not between the risers, not at either end of the header)
4. Install a suitably sized header drain to allow carry over to flow back into the boiler.

These are just the ballpark ideas, see your boiler manufacturer for the specifics on your boiler.
You could still have a carry over problem, even though the piping is OK. Excess water treatment chemicals or over firing of the boiler can defeat the near boiler piping. If your heat exchanger is demanding more steam than your boiler should be able to supply, that can be a cause too. Good Luck!