Manifold Connector Technology – Why Settle for Second-best?

Securing leak-free connection of impulse lines to manifolds for applications that use differential pressure flowmeters is a subject that has taxed instrumentation engineers for more than a century. Back in 1910, when the very first orifice plate installations made an appearance, they involved 33 connections and 16 lengths of tubing! Thanks to the development of highly integrated manifolds, todayÂ’s installations often only require just two tube connections. However, ensuring the long-term integrity of these connections remains a contentious issue. The first manifolds on the market used NPT taper threads for their tube connections. Despite being the bane of installersÂ’ lives, this type of technology continues to enjoy widespread use today, with most manifold manufacturers still offering it as an option. But much better tube connector technologies are now available.

WhatÂ’s wrong with taper threads?

Unlike compression type tube fittings with one or more ferrules, taper thread fittings rely on the threads themselves to provide the seal. During make-up, progressively larger diameter threads on the fitting are compressed into progressively small diameter threads on the manifold, until eventually there is no clearance left between the crests and roots of the threads and they effectively form a metal-to-metal seal. NPT taper thread fittings are popular because they are relatively inexpensive, but they also have distinct disadvantages. The fittings cannot easily be installed with a specific torque, which makes it all too easy to crack or distort the female part by applying too much torque, or to apply too little, resulting in potential leak paths due to incorrect thread cling. There is also always some thread clearance due to manufacturing tolerances, which means that if the fitting is not tightened to the point where thread deformation creates a metal-to-metal seal, there is a spiral leak path. Furthermore, the upper and lower machining limits of NPT taper threads mean that there might only be two turns of thread engagement in an assembled connection; the most reliable means of preventing this is to use, if possible, a matched pair of male and female parts produced by the same manufacturer.

Taper thread can suffer from limited thread engagement

Another major disadvantage of NPT fittings is that their radial orientation cannot easily be adjusted without compromising connection integrity. Most installers of NPT fittings elect to use some form of thread sealant to help prevent leaks. This usually comprises a fluid carrier which transfers a filler compound into the threads and then cures. Unfortunately, not all sealants act as lubricants and they are also very easy to misapply. Too much sealant can cause system contamination, which can result in unseated valves or blocked lines, while too little can allow the threads to gall (cold weld) during installation, requiring replacement of