Threaded connections on process chemicals and utilities piping can be difficult to seal. Leaks from threads in steam and compressed air service are common. This article looks at pipe thread design and the proper sealing of threads.
Thread Design Principles
Figure No. 1 is a cross-section of a parallel thread profile. To prevent threaded parts from binding on their mate, the female thread is cut slightly larger than the male thread. This provides clearance to prevent clashing. The necessary differences in size and shape create a cavity. The cavity spirals all way along the thread.
Threads allow one component to rotate and move along the other until one comes up against a stop. The application of an additional tensioning torque locks the thread flanks against each other and stretches the components like a spring. The spring force jams the two threads into place and binds them together. If the tensioning torque is excessive the induced stresses exceed the yield stress of the material and the threads strip or the part snaps. If the torque is insufficient the threads lose their spring tension and separate.
The thread shape and size influence both the tightening and loosening torque. The pitch and thread angle can be varied to make the contact freely moving or self-locking.
Pipe threads are closer pitched and of steeper thread form than a bolt and nut. The close pitch and steep flanks act to prevent the joint loosening. To aid with sealing, the threads can be tapered so the two parts are forced mechanically together when tightened. Also the thread root radii can be altered to produce a clash to make a seal. Such designs still do not guarantee a leak-free joint. They may seal the root gap but they still leave the flank gap.
The root and flank cavities along the pipe thread are a pathway for leakage unless they are sealed. A threaded joint requires sealant to close off the cavities between the male and female parts.
Thread Sealant Properties
The thread seal will need the following properties to stop leaks.
- Permit dynamic movement without failing. Inside and outside of a joint see different temperatures and pressures and move varying amounts. If the pipe vibrates so will the joint.
- Be chemically compatible with the pipe line contents and the metal of the thread.
- Permit the necessary torque to pull-up the joint while acting as a thread lubricant.
- Slip between tight clearances that remain when the pipe joint is made.
- Not crack, shred or be pushed out of the cavity to produce a leakage path.
- Stop very low viscosity (very slippery) gases, like high pressure oxygen and ammonia, from squeezing pass.
- Attach it self to the metal surfaces of the threads and bond.
- Still be able to remove the parts when necessary.
Issues with Threaded Pipe Connections
Threaded pipe joints are not suitable for high-pressure gases or flammable contents. In such cases use ferrule connections (like the fittings from Swagelock or Hoke) in which metal cones mechanically deform into each other to seal off the contents.
Teflon thread tape is not a universal thread seal as it shreds and can block fine pilot holes in instruments. Being a plastic it can be deformed by high pressures and allow fluids to be forced pass. Teflon thread tape can insulate one thread from the other and prevent static earthing through the pipe.
Don’t mix thread standards. BSP and NPT are totally different threads and are not compatible. They have different flank angles and except for the 1/2” and 3/4” sizes, they have a different pitch. On jobs with mixed threads use the right fittings or the right adapter fittings. Good practices always save a lot of problems in the future.
Clean and deburr threads. Get plenty of thread engagement. Don’t use threaded joints on corrosive fluids; or where crevice corrosion can occur; or in shock/stress failure positions; or where the mating parts are of materials that expand by markedly different amounts.
Pressure Test Piping
Always pressure test threaded pipe work to 1.25 times its working pressure before putting it into service. While the threads are under pressure ‘soapy water’ test them for leaks by spraying on a mixture of water and liquid detergent and looking for bubbles forming as the air leaks out. If bubbles appear remake the joint.
Mike Sondalini – Maintenance Engineer
We (Accendo Reliability) published this article with the kind permission of Feed Forward Publishing, a subsidiary of BIN95.com
If you found this interesting you may like the ebook Process Control Essentials.