AN fitting torque
Hi,
some information for you from Mechanical Toolbox CD (great source !).
Hope this help
-------
There is no standardized method of tightening a tapered pipe fitting. One advantage of tapered pipe fittings is that you can position an elbow at any angle you desire by tighening a little more or a little less. Your best bet is to follow the standards for your industry. What follows is a rather lengthy discussion of problems you might encounter using all of the commonly used methods. This will help keep you out of trouble when your "industry standard method" fails.
LEAKS:
Tapered pipe threads leak because there is a gap between the top of the male thread and the bottom of the female thread. Tightening doesn't remove this gap, so tightening isn't done to stop the fitting from leaking. Proper application of thread sealant prevents the fitting from leaking.
TORQUE:
Torque works fine until you use a fitting that is out of spec and bottoms out in the boss before reaching torque; Or you reach the specified torque and the fitting is pointing in the wrong direction, or the threads are of poor quality or damaged and torque is reached before the fitting is properly engaged. Torque is not always practical with tapered pipe fittings because of the wide differences in friction (material, pipe dope, Teflon tape). Torque will sometimes get you in trouble; understand its weakness and use other quality assurance methods along with torque.
FEEL:
Most screwed pipeing is tightened until it feels "right" and the fitting is pointing in the desired direction. What the experienced mechanic is often "feeling" is how the fitting is getting tight. Screw it into until it starts to seat. Then up the force a little by yanking. If each yank gives less movement, you probably have a sound joint. If the movement stops suddenly, you have probably bottomed out. The experienced plumber knows when to stop before he damages the fitting or boss.
Caution is advised, tapered pipe fittings can crack an aluminum boss if over tightened. This is especially true when using Teflon tape because the low friction of teflon makes it easy to over-tighten.
Number of Rotations:
Included in the standards for NPT and ANPT are engagement length, both straight and wrenched. For example, a 1/4 inch tapered pipe fitting should screw in 4.1 threads until finger tight (handtight engagement), and 3 threads for wrench makeup. One problem is the wide variance in quality of the fittings and threads. Few 1/4 inch fittings screw in 4.1 threads before they reach finger tight. As a general guideline, after hand-tight engagement, tighten 3 full turns for sizes up to 2 inches for NPT fittings.
Cross-treading:
Perhaps the most common fault a beginner makes is cross-threading the fittings and not realizing it. The male thread fitting needs to be aligned with the axis of the tapped hole. Rotation force should gradually increase with tightening.
Comment:
Torque alone is an unsuitable method of specifying how a NPT fitting should be installed. Other conditions, such as type of thread sealant, direction of elbow, number of turns, and recognition and pre-inspection of thread quality also need to be included with any torque value.
-------
