Instruments may be connected to their corresponding processes and to each other by using pipe, tube and/or wires. In this article we focus on pipe and pipe fittings; the common techniques for joining pipes together and joining pipe ends to equipment such as flow measurement instruments.

Pipe and Pipe Fittings

Pipe can be defined as a hollow structure designed to provide an enclosed pathway for fluid to flow. Pipes are normally made from cast metal however you may find pipes made from plastic in many industrial applications.

Flanged Pipe Fittings

A pipe flange is a ring of metal, normally welded to the end of a pipe, with holes drilled in it parallel to the pipe centerline to admit several bolts.

Flange joints are usually made pressure-tight by inserting a gasket between the flange pairs before tightening the bolts. Gaskets being made from materials softer than the hard steel of the flanges will be crushed when sandwiched between a pair of flanges to seal all potential leak paths.

A common technique employed when installing a flange gasket is to first install only half of the bolts (in the holes located below the centerline of the pipe), drop the gasket between the flanges, insert the remaining bolts, then proceed to tighten all the bolts to the correct torques.

Flanges differ with regard to their sealing design and the required gasket type. Typical flange face designs include raised-face (RF) and ring-type joint (RTJ). The raised face (RF) flange is designed to seal against a gasket by means of a set of concentric grooves machined on the face of the flange. These grooves form a sealing surface with far enhanced leakage path length than if the faces were smooth hence discouraging leakage of process fluid under pressure.

The ring-type joint (RTJ) flange design has a special metal ring that sits inside a groove machined into the faces of both mating flanges, crushing and filling that groove when the flanges are properly tightened together. These flanges are usually found on high pressure applications where leakage control is more difficult. The grooves in RTJ flanges must be completely free from foreign material and not distorted in order to achieve sealing.

Flanges are typically rated as per as a system of pressure classes defined in the ANSI standard 16.5. These pressure classes are designated by numerical values followed by ‘pound’, ‘lb’ or ‘#’. The common ANSI ratings include the 150#, 300#, 400#, 600#, 900#, 1500#, and 2500# pressure classes. It is important to take note that these class numbers don’t refer directly to pressure ratings in units of PSI, however they do scale with pressure. For instance a 900# flange will have greater pressure rating than a 600# flange, all other factors being kept constant. In addition to pressure ratings varying with the class of the flange, it also varies with temperature, since metals as we know tend to weaken at higher temperatures.

Piping flanges and components must have matching flange ratings and sizes in order to work properly. Take a case of a control valve with a flanged body rated as a 4-inch ANSI class 300#, it can only be properly joined together to another 4-inch ANSI class 300# pipe flange. Mismatching the pressure-class flanges puts at risk the piping system impacting safety. Correct gasket types must be chosen to coordinate with the pressure class of the mating flanges. All these considerations ensure the integrity of the flanged joint.

A special provision of flanged pipe connections is the ability to install a blank metal plate referred to as a blind between the flange faces, thereby preventing flow. This is useful in special occasions where a pipe must be blocked in a semi-permanent manner, for instance, if the section of pipe must be sealed for safety reasons during maintenance operations.

Threaded Pipe Fittings

Threaded fittings are typically used for smaller pipe sizes to create connections between pipes and between pipes and equipment and or instruments.

Tapered Thread Pipe Fittings

The tapered pipe thread design is a common threaded pipe fitting. The aim of a tapered thread is to allow the pipe and fitting to ‘wedge’ together when engaged, creating a joint that is both mechanically rugged and leak free. The male and female tapered pie threads form a loose junction when first engaged. But after tightening, the tapered profile of the threads acts to wedge both male and female pieces together.

There are several standards available for tapered thread pipe fittings. For each standard, the angle of the thread is fixed, as is the angle of taper. Thread pitch or the number of threads per unit length varies with the diameter of the fitting. For example ¼ inch and 3/8 inch fittings are 18 threads per inch, ½ inch and ¾ inch NPT fittings are 14 threads per inch, and 1 inch NPT fittings are 11.5 threads per inch.

National Pipe Taper (NPT)

National Pipe Taper (NPT) design is a common tapered standard used in US for general purpose. NPT threads have an angle of 60° and a taper of 1.7833° as illustrated below:

A form of sealant must be applied to NPT pipe threads prior to assembly to ensure pressure-tight sealing between the threads. Teflon tape and various liquid pipe dope compounds may be used for this purpose. These sealants help lubricate the male and female pieces and to fill the spiral gap formed between the root of the thread and the crest of the male thread.

The NPTF (National Pipe Thread) pipe threads are designed with the same thread angle and pitch as NPT threads, but carefully machined to avoid the spiral leak path inherent to NPT threads.

ANPT (Aeronautical National Pipe Tapered) is identical to NPT, but with a greater level of precision and quality for its planned utilization in aerospace and military applications

British Standard Pipe Tapered (BSPT)

This tapered thread standard has a narrower thread angle than NPT threads (55° instead of 60°) but with the same taper of (1.7833°) as demonstrated below:

Parallel Thread Pipe Fittings

An option to tapered threads in pipe joints is the use of parallel threads, akin to the threads of machine screws and bolts. Since parallel threads are incapable of forming a pressure tight seal on their own, the sealing action of a parallel thread is achieved with O-ring or gasket.

SAE Straight Thread

A typical parallel-thread pipe fitting is the SAE (Society of Automotive Engineers) straight thread.

Sealing is achieved as the O-ring is compressed against the shoulder of the female fitting. The threads serve only to provide force (not fluid sealing) just like the threads of a fastener.

British Standard Pipe Parallel (BSPP)

Like the BSPT (tapered) standard, BSPP thread angle is 55°.Like the SAE parallel thread standard, sealing is accomplished by means of an O-ring which compresses against the shoulder of the matching female fitting.

Sanitary Pipe Fittings

To ensure health and quality control in pharmaceuticals manufacturing, food processing and biological research processes, process piping and vessels in these industries are designed so as to ensure meticulous cleaning can take place without the need for disassembly. This is done to ensure there are no formation of microorganisms such as bacteria, fungi and algae. Routine cleaning and sterilization cycles are planned and performed between productions schedules (batches) to ensure no colonies of harmful micro-organisms can grow.

A typical Clean-In-Place (CIP) protocol consists of flushing all process piping and vessels with alternating acid and caustic solutions, then washing with purified water. For enhanced sanitization, a Steam-In-Place (SIP) cycle may be incorporated too, flushing all process pipes and vessels with hot steam to ensure the destruction of microorganisms.

A key design feature of any sanitary process is the elimination of any dead end called dead legs in the process industry, crevices or voids where fluid may collect and stagnate. This includes any instrument contacting the process fluids.

Standard pipe fittings are problematic in sanitary systems, as tiny voids between the mating threads of male and female pipe fittings may provide a haven for microorganisms. To avoid this problem, special sanitary fittings are employed. These fittings consist of a matched pair of flanges, held together by an external clamp.

Related articles:

• Standard Pipe Sizes
• Instrument Piping Systems Installation Guidelines
• Instrument Installation – Mounting & Accessibility Guidelines
• Instrumentation Systems Installation: Precautions to Take
• Instrument Installation – Cabling Guidelines