Instructions for Layout and Tapping Stanley Boilers
By Don A. Bourdon, Woodstock, Vermont
Stanley boilers are shipped from Bourdon Boiler Works with two pipe tap openings on the top sheet only. One is in the center and it is tapped to 3/8 NPT. It serves as the steam outlet for the throttle. The other tapping is on the outside edge on the top sheet. This is tapped to 1/8 NPT.
When testing a boiler, water enters the boiler in the center as this serves as the inlet for the test pump. The tapping on the outside edge lets the air out of the boiler while it is being filled and serves as a place to attach the test gauge.
Boilers typically are tapped on the bottom sheet to ¼ NPT. I tap 20 HP and 30 HP boilers with 4 openings. 10 HP or smaller boilers are ok with 3 openings. They all serve as bottom blow offs. One exception, however, may be made in the case of a surface blow off used on condensing cars. One bottom blow off is connected to the water level indicator which should be plumbed so it can be shut off from the top and bottom of the boiler, as needed.
Taps on the top sheet serve to connect the throttle, water feed, safety valve, siphon, steam automatic, steam gauge, and whistle (optional). Tapping size ranges from 1/8 NPT to 3/8 NPT (boiler feed and throttle). A word of caution - Do not place the boiler feed standpipe feed line tapping on the top of the boiler near the water level indicator top and bottom tappings as water feed coming into the boiler will give you a higher water level indication than actually exists.
In laying out for the tappings, I recommend that no edge of a tap hole be closer than 5/16” from a boiler tube. No hole should be in contact with the edge of the welds on the bottom or top sheets. Space your hole locations accordingly.
The tap hole size for 1/8” NPT is 5/16”, 7/16” for ¼” NPT, 9/16” for 3/8” NPT. I follow with a pipe taper reamer. This makes the tap go easier into the hole. Next, I follow the taper reamer with an 82 degree countersink to break the sharp edge of the hole. I drill the hole free hand. To aid in directing the drill so that it penetrates the steel at 90 degrees to the tube sheet, it is best to have someone with a good eye to spot you, looking at both the x and y axes to make sure the drill is vertical and remains so during the operation. I suppose that if one had a super large drill press or mill, they could use it to drill the holes, but this would be somewhat time-consuming and cumbersome, particularly for the larger boilers.
Following the drilling, reaming, and countersinking operations, I next set up for tapping the hole. To do this, I use a HSS tap that is new or nearly so. I use a long tap wrench (21” or longer) to hold the tap. The handles of the wrench need to remain parallel to the tube sheet. To aid in doing this, I place a board on edge (4 to 5 inches wide by 25”long) which has a series of parallel lines drawn on it spaced about ½” apart on top of the tube sheet.. By placing the board next to the wrench as it is started into the hole, one can readily see if the tap handles holding the tap are parallel to the lines on the board. A careful watch on the wrench handles relative to the lines on the board will ensure that the outcome is what you want it to be.
Use a high grade of cutting oil. The boiler plate used in the construction of boilers is harder than standard hot finished materials.
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