Plastic piping and threaded fittings are extremely reliable at providing a leak free service. Issues with stress cracking of PVC threaded fittings can be easily avoided by adhering to proper installation guidelines.
In this post, we will take a look at three do's and don't when making threaded connections with PVC fittings and PVC pipe to ensure that you have a smooth and stress free assembly process. Let's get started!
1. Do Not: Over tighten joints by turning once more just to make sure.
Do: Finger tighten, then add one or two turns, no more
Threaded Male PVC fittings (MPT) have threads which increase in diameter whereas female threaded fittings (FPT) decrease in diameter. This is referred to as taper. Since the threads are tapered, any additional turns can cause the female part to undergo strain causing the female fitting to split. More strain is applied on smaller diameter fittings, so smaller fittings are more prone to stress cracking. Thread sealants act as a lubricant making it extremely easy to over torque.
The proper way to assemble threaded PVC Sch 40 or Sch 80 fittings is to finger tighten, then add one to two turns, but no more. Two turns past finger tight, plus the stress of system pressure will ensure it is within the tensile strength of PVC. The below table explains the strain and tensile stress levels according to the pipe diameter.
2. Do Not: Use sealant tape, paste or pipe dope to lubricate the joint.
Do: You can use a sealant designed specifically for threaded joints.
PTFE tape is intended for metal pipe and fittings, not plastic. Pipe dope and PTFE paste are suitable for metal and plastic. Metal to metal fitting joints are harder to tighten compared to plastic because the surfaces are physically harder and can gall without any PTFE tape or pipe dope. When PTFE tape is wrapped around PVC male threads, it will add strain. Thicker premium grade tapes will also increase the strain.
When you work with threaded plastic fittings, you should use a sealant specifically designed for plastic fittings. A non-hardening compound is forced into potential areas where leaks could happen by water pressure, sealing it completely. Be sure to check that the sealing compound you are using is compatible with plastics. Some brands of pipe sealant contain oils that can damage plastic. The sealing compound must also not lubricate the joint to the point that over tightening is encouraged.
3. Do Not: Assume the use of "stronger" Sch 80 fittings will solve the problem of splitting of threaded connections when over tightened.
Do: Use Sch 80 PVC nipples with both Sch 40 and 80 PVC fittings.
It is true that schedule 80 fittings are stronger and have higher pressure ratings than schedule 40 fitting systems. However, this is only true when comparing systems with components that have been solvent welded together. Although, the wall thickness on the female sch 80 threaded fittings is thicker, they do not solve the problems caused by over tightening. The stress and strain levels remain the same and threaded sch 80 fittings perform similarly to sch 40 from a stress cracking standpoint.
Did you know that sch 40 PVC pipe nipples do not exist? Threaded pipe nipples are only available in sch 80. Cutting threads into sch 40 PVC pipe results in a thin wall section, compromising strength. Sch 80 pipe nipples must be used with both sch 40 and 80 threaded PVC pipe fittings. While the general rule of thumb is to use the same sch of fittings and pipe, this refers to systems using solvent weld fittings. The below chart shows the maximum static pressure for a type 1120 PVC fitting at 73°F.
PVC fittings are very common and preform extremely well when assembled correctly. By keeping these do's and don'ts in mind, you can avoid unnecessary headaches and extra costs due to improperly installed systems.