Steam Boilers and Stay Bolts

Steam Pressure

Here is shown how steam pressure tends to bulge boiler head.

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Pressure within an approximately cylindrical vessel is to force the shell into a truly cylindrical shape. Thus any steam boiler surface under pressure and which is not cylindrical or hemispherical in shape has forces on it which tend to cause the surface to bend out in a curve. On the other hand a flat boiler head is subjected to a high internal pressure and has a tendency to bulge as shown by the drawing. To prevent the plate from assuming the bulged shape and possibly exploding or cracking, stays or braces must be used to hold it. Stay bolts in a steam boiler should support the plate completely. They should allow free water circulation and they should allow room for boiler inspection. The principal types are the solid screw stay bolt and the hollow bolt. The solid screw stay bolt is completely threaded and is screwed into the boiler plates so both ends extend inch. All stays in the water legs must be hollow or have holes drilled in the ends 3/16' in dia. and ' deep. If the bolt breaks, steam or water will rush out and give warning of the break. The hollow stay bolt is threaded only on the ends as the middle is smooth and resists corrosion. The hole is rolled during manufacture. All stay bolts have 12 threads to the inch and are usually the U. S. standard thread. If either end of the bolt projects over ' it should be neatly cut off. Both ends are then hammered over to form a head. This is called upsetting the stay bolt and prevents any pressure leak. The tubes of boilers on steam traction engines have another purpose other than helping raise steam pressure. This is that they have a staying effect and help hold both ends of the boiler in the desired flat shape and help to prevent bulging. This is true only when they have been properly expanded, rolled and beaded over.

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