4-20 mA Signal

An “analog” electronic signal is a voltage or current whose magnitude represents some physical
measurement or control quantity. An instrument is often classified as being “analog” simply by virtue
of using an analog signal standard to communicate information, even if the internal construction and
design of the instrument may be mostly digital in nature. This is to distinguish such instruments
from those making use of no analog electronic signals at all (e.g. wireless or Fieldbus instruments).

The most popular form of signal transmission used in modern industrial instrumentation systems
(as of this writing) is the 4 to 20 milliamp DC standard. This is an analog signal standard, meaning
that the electric current is used to proportionately represent measurements or command signals.
Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value represents
100% of scale, and any current value in between 4 and 20 milliamps represents a commensurate
percentage in between 0% and 100%.

ON/OFF Valves

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or
measurement refers to a true-or-false condition. Thus, a discrete control element is one that has but
a limited number of states (usually two: on and off).
An on/off valve is the fluid equivalent of an electrical switch: a device that either allows unimpeded
flow or acts to prevent flow altogether. These valves are often used for routing process fluid to
different locations, starting and stopping batch processes, and engaging automated safety (shutdown)
functions.
Valve styles commonly used for on/off service include ball, plug, butterfly (or disk), gate, and
globe. Large on/off valves are generally of such a design that the full-open position provides a
nearly unimpeded path for fluid to travel through. Ball, plug1, and gate valves provide just this
characteristic:

*A plug valve is very much like a ball valve, the difference being the shape of the rotating element. Rather than a spherical ball, the plug valve uses a truncated cone as the rotary element, a slot cut through the cone serving as the passageway for fluid. The conical shape of a plug valve’s rotating element allows it to wedge tightly into the “closed” (shut) position for exceptional sealing.

Industrial Instrumentation - Measurement

Instrumentation is the science of automated measurement and control. Applications of this science
abound in modern research, industry, and everyday living. From automobile engine control systems
to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation
surrounds us.

The first step, naturally, is measurement. If we can’t measure something, it is really pointless to
try to control it. This “something” usually takes one of the following forms in industry:

• Fluid pressure
• Fluid flow rate
• The temperature of an object
• Fluid volume stored in a vessel
• Chemical concentration
• Machine position, motion, or acceleration
• Physical dimension(s) of an object
• Count (inventory) of objects
• Electrical voltage, current, or resistance

Once we measure the quantity we are interested in, we usually transmit a signal representing
this quantity to an indicating or computing device where either human or automated action then
takes place. If the controlling action is automated, the computer sends a signal to a final controlling
device which then influences the quantity being measured.

Flow Switch

A flow switch is built to detect fluid flow through a pipe. In a schematic diagram, the switch
symbol appears to be a toggle switch with a “flag” hanging below. The schematic diagram, of course,
only shows the circuitry and not the pipe where the switch is physically mounted:

This particular flow switch is used to trigger an alarm light if coolant flow through the pipe ever falls to a dangerously low level, and the contacts are normally-closed as evidenced by the closed status in the diagram. Here is where things get confusing: even though this switch is designated as “normally-closed,” it will spend most of its lifetime being held in the open status by the presence of adequate coolant flow through the pipe. Only when the flow through the pipe slows down enough will this switch return to its “normal” status (remember, the condition of minimum stimulus?) and conduct electrical power to the lamp. In other words, the “normal” status of this switch (closed) is actually an abnormal status for the process it is sensing (low flow)!

Normal Status

The “normal” status for a switch is the status its electrical contacts are in under a condition of
minimum physical stimulus. For a momentary-contact pushbutton switch, this would be the status
of the switch contact when it is not being pressed. The “normal” status of any switch is the way
it is drawn in an electrical schematic. For instance, the following diagram shows a normally-open
pushbutton switch controlling a lamp on a 120 volt AC circuit (the “hot” and “neutral” poles of the
AC power source labeled L1 and L2, respectively):

We can tell this switch is a normally-open (NO) switch because it is drawn in an open position.
The lamp will energize only if someone presses the switch, holding its normally-open contacts in the
“closed” position. Normally-open switch contacts are sometimes referred to in the electrical industry
as form-A contacts.

If we had used a normally-closed pushbutton switch instead, the behavior would be exactly
opposite. The lamp would energize if the switch was left alone, but it would turn off if anyone
pressed the switch. Normally-closed switch contacts are sometimes referred to in the electrical
industry as form-B contacts. :

Discrete Sensor

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or
measurement refers to a true-or-false condition. Thus, a discrete sensor is one that is only able
to indicate whether the measured variable is above or below a specified setpoint.

Discrete sensors typically take the form of switches, built to “trip” when the measured quantity
either exceeds or falls below a specified value. These devices are less sophisticated than so-called
continuous sensors capable of reporting an analog value, but they are quite useful in industry.
Many different types of discrete sensors exist, detecting variables such as position, fluid pressure,
material level, temperature, and fluid flow rate. The output of a discrete sensor is typically electrical
in nature, whether it be an active voltage signal or just resistive continuity between two terminals
on the device.

Electrical signal and control wiring

There is much to be said for neatness of assembly in electrical signal wiring. Even though the
electrons don’t “care” how neatly the wires are laid in place, human beings who must maintain the
system certainly do. Not only are neat installations easier to navigate and troubleshoot, but they
tend to inspire a similar standard of neatness when alterations are made.
Here we see 120 volt AC power distribution wiring. Note how the hoop-shaped “jumper” wires
are all cut to (nearly) the same length, and how each of the wire labels is oriented such that the
printing is easy to read:

This next photograph shows a great way to terminate multi-conductor signal cable to terminal
blocks. Each of the pairs was twisted together using a hand drill set to very slow speed. Note how
the end of the cable is wrapped in a short section of heat-shrink tubing for a neat appearance:

Bending Instrument Tubing

Tube bending is something of an art, especially when done with stainless steel tubing. It is truly
magnificent to see a professionally-crafted array of stainless steel instrument tubes, all bends perfectly
made, all terminations square, all tubes parallel when laid side by side and perfectly perpendicular
when crossing.

If possible, a goal in tube bending is to eliminate as many connections as possible. Connections
invite leaks, and leaks are problematic. Long runs of instrument tubing made from standard 20 foot
tube sections, however, require junctions be made somewhere, usually in the form of tube unions.
When multiple tube unions must be placed in parallel tube runs, it is advisable to offset the unions
so it is easier to get a wrench around the tube nuts to turn them. The philosophy here, as always, is
to build the tubing system with future work in mind. A photograph of several tube junctions shows
one way to do this:

Tube and Tube Fittings

Tubing elbows are tube connectors with a bend. These are useful for making turns in tube runs
without having to bend the tubing itself. Like standard connectors, they may terminate in male
pipe thread, female pipe threads, or in another tube end:

These elbows shown in the above illustration are all 90o, but this is not the only angle available.
45o elbows are also common.


Tee fittings join three fluid lines together. Tees may have one pipe end and two tube ends (branch
tees and run tees), or three tube ends (union tees). The only difference between a branch tee and
a run tee is the orientation of the pipe end with regard to the two tube ends:

Of course, branch and run tee fittings also come in female pipe thread versions as well. A
variation of the theme of union tees is the cross, joining four tubes together:

Special tube fittings are made to terminate tube connections, so they are sealed up instead of
open. A piece designed to seal off the open end of a tube fitting is called a plug, while a piece
designed to seal off the end of an open tube is called a cap:

Common tube fitting types and names

Tube fittings designed to connect a tube to pipe threads are called connectors. Tube fittings designed
to connect one tube to another are called unions:

If a tube union joins together different tube sizes rather than tubes of the same size, it is called
a reducing union.

A variation on the theme of tube connectors and unions is the bulkhead fitting. Bulkhead fittings
are designed to fit through holes drilled in panels or enclosures to provide a way for a fluid line to
pass through the wall of the panel or enclosure. In essence, the only difference between a bulkhead
fitting and a normal fitting is the additional length of the fitting “barrel” and a special nut used
to lock the fitting into place in the hole. The following illustration shows three types of bulkhead
fittings:

Tube and Tube Fittings- Leak

When assembling compression-style tube fittings, you should always precisely follow the
manufacturer’s instructions to ensure correct compression. For Swagelok-brand instrument tube
fittings 1 inch in size and smaller, the general procedure is to tighten the nut 1-1/4 turns past fingertight.

Insufficient turning of the nut will fail to properly compress the ferrule around the tube, and
excessive turning will over-compress the ferrule, resulting in leakage.  Unlike pipe fittings, tube fittings may be disconnected and reconnected with ease. No special procedures are required to “re-make” a disassembled instrument fitting connection: merely tighten the nut “snug” to maintain adequate force holding the ferrule to the fitting body, but not so tight that the ferrule compresses further around the tube than it did during initial assembly.

A very graphic illustration of the strength of a typical instrument tube fitting is shown in the
following photograph, where a short section of 3/8 inch stainless steel instrument tube was exposed
to high liquid pressure until it ruptured. Neither compression fitting on either side of the tube leaked
during the test, despite the liquid pressure reaching a peak of 23,000 PSI before rupturing the tube.

Compression tube fittings

By far the most common type of tube fitting for instrument impulse lines is the compression-style
fitting, which uses a compressible ferrule to perform the task of sealing fluid pressure. The essential
components of a compression tube fitting are the body, the ferrule, and the nut. The ferrule and
body parts have matching conical profiles designed to tightly fit together, forming a pressure-tight
metal-to-metal seal. Some compression fitting designs use a two-piece ferrule assembly, such as this
tube fitting shown here  (prior to full assembly):

Just prior to assembly, we see how the nut will cover the ferrule components and push them into
the conical entrance of the fitting body:

After properly tightening the nut, the ferrule(s) will compress onto the outside circumference of
the tube, slightly crimping the tube in the process and thereby locking the ferrules in place:

Tube and Tube Fittings

Tube, like pipe, is a hollow structure designed to provide an enclosed pathway for fluids to flow. In
the case of tubing, it is usually manufactured from rolled or extruded metal (although plastic is a
common tube material for many industrial applications). This section discusses some of the more
common methods for joining tubes together (and joining tube ends to equipment such as pressure
instruments).
One of the fundamental differences between tube and pipe is that tube is never threaded at the
end to form a connection. Instead, a device called a tube fitting must be used to couple a section of
tube to another tube, or to a section of pipe, or to a piece of equipment (such as an instrument).
Unlike pipes which are thick-walled by nature, tubes are thin-walled structures. The wall thickness
of a typical tube is simply too thin to support threads.
Tubes are generally favored over pipe for small-diameter applications. The ability for skilled
workers to readily cut and bend tube with simple hand tools makes it the preferred choice for
connecting instruments to process piping. When used as the connecting units between an instrument
and a process pipe or vessel, the tube is commonly referred to as an impulse tube or impulse line

Sanitary Pipe Fittings - Process Equipments

Sanitary pipe fittings are not limited to instrument connections, either. Here are two photographs
of process equipment (a ball valve on the left, and a pump on the right) connected to process pipes
using sanitary fittings:

Sanitary Pipe Fittings - Problems

Standard pipe fittings are problematic in sanitary systems, as tiny voids between the mating
threads of male and female pipe fittings may provide refuge for micro-organisms. To avoid this
problem, special sanitary fittings are used instead. These fittings consist of a matched pair of
flanges, held together by an external clamp. An array of sanitary fittings on an instrument test
bench appear in the following photograph:

The installation of a pressure transmitter on an ultra-pure water line using one of these sanitary fittings. The external clamp holding the two flanges together is clearly visible in this photograph:

Sanitary pipe fittings

Food processing, pharmaceuticals manufacturing, and biological research processes are naturally
sensitive to the presence of micro-organisms such as bacteria, fungi, and algae. It is important in
these processes to ensure the absence of harmful micro-organisms, for reasons of both human health
and quality control. For this reason, the process piping and vessels in these industries is designed
first and foremost to be thoroughly cleaned without the need for disassembly. Regular cleaning and
sterilization cycles are planned and executed between production schedules (batches) to ensure no
colonies of harmful micro-organisms can grow.
A common 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 increased sanitization,
a Steam-In-Place (SIP) cycle may be incorporated as well, flushing all process pipes and vessels with
hot steam to ensure the destruction of any micro-organisms.
An important design feature of any sanitary process is the elimination of any “dead ends” (often
called dead legs in the industry), crevices, or voids where fluid may collect and stagnate. This includes
any instruments contacting the process fluids. It would be unsafe, for example, to connect something
as simple as a bourdon-tube pressure gauge to a pipe carrying biologically sensitive fluid(s), since
the interior volume of the bourdon tube will act as a stagnant refuge for colonies of micro-organisms
to grow:

Instead, any pressure gauge must use an isolating diaphragm, where the process fluid pressure
is transferred to the gauge mechanism through a sterile “fill fluid” that never contacts the process
fluid:

With the isolating diaphragm in place, there are no stagnant places for process fluid to collect
and avoid flushing by CIP or SIP cycles.

British Standard Parallel Pipe (BSPP)

Another parallel-thread pipe standard is the BSPP, or British Standard Pipe Parallel. Like the
BSPT (tapered) standard, the thread angle of BSPP is 55o. 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:

Parallel Thread Pipe Fittings

An alternative to tapered threads in pipe joints is the use of parallel threads, similar 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 pipe fitting must be achieved some other way.
This function is usually met with an O-ring or gasket.
A common design of parallel-thread pipe fitting is the SAE straight thread, named after the Society of Automotive Engineers:

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

British Standard Pipe Tapered (BSPT)

Another tapered-thread standard is the BSPT, or British Standard Pipe Tapered. BSPT threads
have a narrower thread angle than NPT threads (55o instead of 60o) but the same taper of 1o 47’
(1.7833o):

National Pipe Tapered (NPT)

Several different standards exist for tapered-thread pipe fittings. For each standard, the angle of

the thread is fixed, as is the angle of taper. Thread pitch (the number of threads per unit length) varies with the diameter of the pipe fitting.

For example, 1/8 inch NPT pipe fittings have a thread pitch of 27 threads per inch. 1/4 inch and 3/8 inch NPT fittings are 18 threads per inch, 1/2 inch and 3/4 inch NPT fittings are 14 threads per inch, and 1 inch through 2 inch NPT fittings are 11.5 threads per inch.

NPT threads have an angle of 60o and a taper of 1o 47’ (1.7833o):

NPT pipe threads must have some form of sealant applied prior to assembly to ensure pressuretight

sealing between the threads. Teflon tape and various liquid pipe “dope” compounds work well

for this purpose. Sealants are necessary with NPT threads for two reasons: to lubricate the male

and female pieces (to guard against galling the metal surfaces), and also to fill the spiral gap formed

between the root of the female thread and the crest of the male thread (and visa-versa).

NPTF (National Pipe Thread) pipe threads are engineered with the same thread angle and pitch

as NPT threads, but carefully machined to avoid the spiral leak path inherent to NPT threads.

This design – at least in theory – avoids the need to use sealant with NPTF threads to achieve

a pressure-tight seal between male and female pieces, which is why NPTF threads are commonly

referred to as dryseal. However, in practice it is still recommended that some form of sealant be

used (or at the very least some form of thread lubricant) in order to achieve reliable sealing.

ANPT (Aeronautical National Pipe Tapered) is identical to NPT, except with a greater level of

precision and quality for its intended use in aerospace and military applications.

1

Tapered thread pipe fittings

For smaller pipe sizes, threaded fittings are more commonly used to create connections between pipes
and between pipes and equipment (including some instruments). A very common design of threaded
pipe fitting is the tapered pipe thread design. The intent 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.
When male and female tapered pie threads are first engaged, they form a loose junction:

After tightening, however, the tapered profile of the threads acts to wedge both male and female
pieces tightly together as such:

Instrument Connections

All instruments connect to their respective processes and to each other by means of pipe, tube, and/or
wires. Improper installation of these connective lines can make the difference between success or
failure in an installation. Safety is also impacted by improper connections between instruments and
the process, and from instrument to instrument.

Pipe is a hollow structure designed to provide an enclosed pathway for fluids to flow, usually
manufactured from cast metal (although plastic is a common pipe material for many industrial
applications). The more common methods for joining pipes together and joining pipe ends to equipment such as pressure instruments.

Flanged pipe fittings

 A pipe “flange” is a ring of metal, usually welded to the end of a pipe, with holes drilled in it parallel to
the pipe centerline to accept several bolts:

Flange joints are made pressure-tight by inserting a donut-shaped gasket between the flange pairs
prior to tightening the bolts. A common method of installing such a flange gasket is to first install
only half of the bolts (in the holes lower than the centerline of the pipe), drop the gasket between
the flanges, then insert the rest of the bolts: