Skimming a Steam Boiler

Skimming a Steam Boiler

Why a steam boiler surges

When a steam boiler is installed there is an oil residue from the manufacturing and installation process. All steam carrying pipes should be iron pipes and the piping below the water line of the boiler can be iron or copper. The reason for this is two fold. The steam will leach copper molecules from the copper tubing and they will end up back in the boiler which sets up a galvanic reaction and may shorten the life of the boiler causing the bottom of the boiler to leak. The second reason is the expansion is fairly great and all copper joints are brazed or silver soldered which has no play or give. The near boiler piping calls for swing joints which absorb the expansion by rotating slightly when heated and cooled.

All the new piping, fluxes and boiler manufacturing causes oil to end up in the boiler when completed. The water is then put into the boiler and the switch is turned on to operate the boiler. As the water heats up this oil will gradually float to the water surface. It creates an oil slick on top of the water. The oil slows down the escape of steam into the piping as the steam bubbles will collect under the oil slick and it is difficult for the steam bubbles to break through the oil. When the steam bubble gets large enough it will break through the oil at a higher than normal rate of speed and allows other steam bubbles to also exit the hole in the oil slick. The steam at a higher velocity finds the boiler supply tapping's and starts squeezing into the openings the velocity increases even more. Now we also start producing a venturi effect and as the steam rushes to a lower pressure zone (the system piping) it sucks water up into the pipe with it.  The water level in the sight glass starts to drop. Most of the water is too heavy to continue up the pipe and drops back into the boiler. The water level in the sight glass goes up. We now have wet steam which moves slower and gives up the heat a lot quicker and causes the steam to turn into condensate which now needs to be turned back into steam at 970 BTU's plus. The water leaving and dropping back into the boiler is the surging, water level bouncing up and down more than about 1/4" to 1/2". When the water bounces an inch or more it is considered severe surging. Some of the water gets into the system and runs down the main pipe condensing steam on it's way. More money wasted. Enough water may leave the boiler if there is an automatic water feeder it will add more water. When all this condensate and added water gets back into the boiler it is now overfilled.

When this water level bounces more than 1/2" the boiler needs skimmed. Below I have listed a way to skim the boiler properly.

Material needed;

2 - Buckets at least one being a 5 gallon bucket

1 - Pipe nipple at least 6" long 

1 - Ball valve the same size as the pipe nipple.


1. Turn on the boiler and run the boiler until it gets hot but not making steam.

2. Turn the boiler off. DO NOT TURN THE BOILER BACK ON DURING SKIMMING PROCESS. This is important.

3. Find a plug you can safely remove from the boiler above the water level or remove the relief valve and use that tapping as long as it is on the side of the boiler only after lifting the relief valve to verify there is no pressure in the boiler. The tapping must be at least 3/4".

4. Install a nipple with a valve into the boiler tapping. The valve is a just in case valve you will not close this after the skimming process starts. This pipe nipple must be horizontal, and must come straight out. It cannot go up hill or elbow up. We are trying to skim the surface water so the water must come out a horizontal pipe. There should not be water at this level yet.

5. Open the valve you installed.

6. Find the manual fill valve.

7. Open the manual fill valve until the water starts to flow out of the pipe nipple and valve you just installed. The water can be filled quickly, until it starts to flow out the pipe. Closing the manual fill valve will allow the water drain down to the skim pipe tapping.

8. When the water stops flowing from the pipe nipple, it is at the bottom of the skim tapping. Reopen the manual fill valve slightly so the stream of water coming from the pipe nipple is no larger than a wooden pencil. The slower the better.

9. Continue to skim until the bucket is almost full and remove the full bucket and use the second bucket while you dump the first bucket. Do not shut off the valve while doing this. You must not stop the water flow.

10. Keep this up until you can comfortably keep your hand under the skimmed water flow. I know jobs that were skimmed 2 - 3 hours.

11. When done close manual water feed, remove pipe nipple and valve, reinstall plug or relief valve, drain water to proper fill level.

12. Turn the boiler back on. Let the boiler create steam. Did the surging Stop? If not it may not be a surging problem. Check near boiler piping.


Option Two

Skimming the boiler from the header

When skimming is complete remove nipple and valve and install a plug

Disclaimer: The information found on this web site is for informational purposes only. Any and all preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this web site as it should be reviewed by a professional.