Pond Boss Forums AERATION Systems questions Ecoplus 7???

I scratched my head when JKB said the "T" is for Taper and not to use sealants on PVC pipe...it ran counter to all I ever remembered, but the reason given sounded good (But LASCO Fittings Inc says otherwise). I did know to make sure any liquid sealant is PVC or ABS safe...So I'm bored and like to educate myself and satisfy my personal curiosity. Plus, I'd hate to get a really weird look asking someone at a Menards where they stock the National Pipe Taper at. I did find one engineering blog post using taper for the T, I also found a few places saying National Pipe Taper Thread Here are the best two I found...



From the ANSI (American National Standards Institute) Definition of Pipe Thread Acronyms...



Definition of pipe thread acronyms
NPT National Pipe Thread (tapered)
FPT Female Pipe Thread (interchangeable with NPT)
FIP Female Iron Pipe (interchangeable with NPT)
MPT Male Pipe Thread (interchangeable with NPT)
MIP Male Iron Pipe (interchangeable with NPT)
IPS Iron pipe size (interchangeable with NPT)
PTF SAE short taper pipe thread
NPTF National Pipe Thread Fine (American National taper pipe thread for dryseal pressure-tight joints)
NPSM National Pipe Straight Mechanical (American National straight pipe thread for mechanical joints)
NPSI American National straight intermediate pipe thread
GHT Garden Hose thread
NST National Standard Thread (Fire Hose thread)
BSPT British Standard taper pipe thread (metric dimensions)



All pipe thread types used in plumbing are specified by the American National Standard for pipe as accredited by the American National Standards Institute (ANSI).
NPT tapered pipe thread

National Pipe Threads (NPT) have tapered threads. These are the most common threads used for general purposes. NPT threads are designed with a 60 degree thread angle, and are used for joining and sealing pipe to fittings in low pressure air or liquids and also mechanical applications. The tapered thread is 3/4" over one foot of length. Tapered threads are deeper at the end of the pipe and are increasingly shallower the further they are from the end of the pipe. The taper on the pipe only allows the pipe to screw inside the fitting until it is forced to stop because of the taper. The distance the pipe can be screwed into the fitting is specified by the ANSI standard. After tightening with a wrench the threads may have slight spaces between the pipe and fitting which could cause a leak so a pipe sealant must be used to ensure any gaps are filled.

The Dry-seal thread (NPTF) also have tapered threads. NPTF threads are used when the application is such that pipe sealing compounds may fail due to higher heat or pressure than normal NPT threads can withstand. The threads are designed to seal mechanically by slightly, but sufficiently, crushing the threads when tightened with a wrench. This allows for joining the pipe and fitting without sealants.

The NPT and NPTF threads can be interchanged if sealants such as PTFE tape or suitable pipe joint compounds are used. None of the other thread standards are fully interchangeable. Female NPT threads can be designated as "FPT" or "FIP" and male NPT threads can be designated as "MPT" or "MIP".

National Standard Free-Fitting Straight Mechanical Pipe Threads (NPSM) have straight threads which are only used for joining. A washer or gasket is needed to seal this type of threaded connection.

There are also three less common thread types used in the plumbing industry. The Garden Hose Thread (GHT) and the Fire Hose Thread (NST) have coarse threads. The seal is made with a gasket or washer and are used mainly for attaching (joining) hoses to valves quickly, without the use of a wrench. The British Standard Taper Pipe Thread (BSPT) has a 55 degree thread angle (NPT are 60 degree) and is used internationally as a standard thread for joining steel pipes.


Also from LASCO Fittings Inc (PVC Pipe manufacturer's Fitting DO's and Dont's page...note the manufactures says DO use a sealant, but not to use it just take up gaps or as a lubricant [on bad threads}......

The "Do's and Don'ts" of Assembling Threaded Plastic Fittings

Today, there are millions of miles of plastic piping with threaded fittings, providing reliable, leak-free service. However, a tiny percentage of those threaded plastic fittings may leak or break. The reason for this is improper assembly of threaded joints.

Here are some of the Do's and Don'ts of PVC joint assembly:
•Don't over-tighten joints by giving them "one more turn to be sure. "Do finger tighten plus one or two turns - No More.
•Don't wrap Teflon tape or Teflon paste or pipe dope to add bulk to or to lubricate the joint. Do use a sealant for threaded joints.
•Don't use "stronger" Schedule 80 threaded fittings on the assumption that they may solve the problem of splitting through over-tightening.
•Do use only Schedule 40 threaded fittings with Schedule 40 pipe and fittings.
•Don't over-tighten.
•Do finger tighten plus one or two turns.

On threaded male PVC fittings each successive thread is slightly larger in diameter than the one before it. Female threads get successively smaller. This is called taper and the amount of taper is specified (1¾ degrees) in the American National Standard B2.1. All pipe manufacturers voluntarily follow these standards to assure their customers they are receiving quality materials.

Because the threads are tapered, additional turns cause the female part to stretch or undergo "strain." This will split the female fitting just as a wedge, driven by a sledgehammer, will split a tree stump.

The amount of strain increases as the size of the pipe decreases. Therefore it is easier to split smaller diameter threaded joints than larger ones. It is also easier to over-torque smaller diameter fittings because their resistance to torque is less. Table 1 gives Strain and Tensile Stress levels according to pipe diameter.

"Stress" (tensile stress) is the force exerted by the strain of the male thread multiplied by the resistance of the PVC. The resistance of PVC is 400,000 pounds per square inch (psi). The strain per turn past finger tight for one-inch PVC pipe is .00447, so the stress per turn is 1,788 psi. Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7,152 psi. The joint is bound to fail since the stress exceeds the 7,000 psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the irrigation system (up to a maximum of 2,000 psi).