Limit Switches

A limit switch detects the physical motion of an object by direct contact with that object.
An example of a limit switch is the switch detecting the open position of an automobile door,
automatically energizing the cabin light when the door opens.The “normal” status of a switch is the condition of minimum stimulus. A limit switch will be in its “normal” status when it is not in contact with anything (i.e. nothing touching the switch actuator mechanism).

Limit switches find many uses in industry, particular in robotic control and CNC (Computer
Numerical Control) machine tool systems. In many motion-control systems, the moving elements
have “home” positions where the computer assigns a position value of zero. For example, the axis
controls on a CNC machine tool such as a lathe or mill all return to their “home” positions upon
start-up, so the computer can know with confidence the starting locations of each piece. These home
positions are detected by means of limit switches. The computer commands each servo motor to
travel fully in one direction until a limit switch on each axis trips. The position counter for each
axis resets to zero as soon as the respective limit switch detects that the home position has been
reached.

A typical limit switch design uses a roller-tipped lever to make contact with the moving part.
Screw terminals on the switch body provide connection points with the NC and NO contacts inside
the switch. Most limit switches of this design share a “common” terminal between the NC and NO
contacts like this:

Hand Switch

A hand switch is an electrical switch actuated by a person’s hand motion. These may take the form of toggle, push button, rotary, pull-chain, etc. A common form of industrial push button switch looks something like this:

The threaded neck inserts through a hole cut into a metal or plastic panel, with a matching nut
to hold it in place. Thus, the button faces the human operator(s) while the switch contacts reside
on the other side of the panel.

When pressed, the downward motion of the actuator breaks the electrical bridge between the
two NC contacts, forming a new bridge between the NO contacts:

The schematic diagram symbol for this type of switch looks much like the real thing, with the
normally-closed contact set on top and the normally-open contact set below:

Temperature Switch

A temperature switch is one detecting the temperature of an object or Atmosphere at times. Temperature switches often use bimetallic strips as the pressure-sensing element, the motion of which actuates one or more contacts(which can be a switch).
The “normal” status of a switch is the condition of minimum stimulus. A temperature switch will be in its “normal” status when it senses minimum temperature

If the trip setting of a temperature switch is below ambient temperature, then it will be “actuated” at ambient temperature and in its “normal” status only when the temperature falls below that trip point (i.e. colder than ambient).

Some Useful Conversions

Conversion formula for temperature

• oF = (oC)(9/5) + 32
• oC = (oF - 32)(5/9)
• oR = oF + 459.67
• K = oC + 273.15

Conversion factors for distance

1 inch (in) = 2.540000 centimeter (cm)
1 foot (ft) = 12 inches (in)
1 yard (yd) = 3 feet (ft)
1 mile (mi) = 5280 feet (ft)

1.3.3 Conversion factors for volume
1 gallon (gal) = 231.0 cubic inches (in3) = 4 quarts (qt) = 8 pints (pt) = 128 fluid ounces (fl. oz.)
= 3.7854 liters (l)
1 milliliter (ml) = 1 cubic centimeter (cm3)

Conversion factors for velocity

1 mile per hour (mi/h) = 88 feet per minute (ft/m) = 1.46667 feet per second (ft/s) = 1.60934
kilometer per hour (km/h) = 0.44704 meter per second (m/s) = 0.868976 knot (knot – international)

Conversion factors for mass

1 pound (lbm) = 0.45359 kilogram (kg) = 0.031081 slugs
1.3.6 Conversion factors for force
1 pound-force (lbf) = 4.44822 newton (N)
1.3.7 Conversion factors for area
1 acre = 43560 square feet (ft2) = 4840 square yards (yd2) = 4046.86 square meters (m2)
1.3.8 Conversion factors for pressure (either all gauge or all absolute)
1 pound per square inch (PSI) = 2.03603 inches of mercury (in. Hg) = 27.6807 inches of water (in.
W.C.) = 6.894757 kilo-pascals (kPa)

Conversion factors for pressure (absolute pressure units only)

1 atmosphere (Atm) = 14.7 pounds per square inch absolute (PSIA) = 760 millimeters of mercury
absolute (mmHgA) = 760 torr (torr) = 1.01325 bar (bar)
1.3.10 Conversion factors for energy or work
1 British thermal unit (Btu – “International Table”) = 251.996 calories (cal – “International Table”)
= 1055.06 joules (J) = 1055.06 watt-seconds (W-s) = 0.293071 watt-hour (W-hr) = 1.05506 x 1010
ergs (erg) = 778.169 foot-pound-force (ft-lbf)

Conversion factors for power

1 horsepower (hp – 550 ft-lbf/s) = 745.7 watts (W) = 2544.43 British thermal units per hour
(Btu/hr) = 0.0760181 boiler horsepower (hp – boiler)

Terrestrial constants

Acceleration of gravity at sea level = 9.806650 meters per second per second (m/s2) = 32.1740 feet
per second per second (ft/s2)
Atmospheric pressure = 14.7 pounds per square inch absolute (PSIA) = 760 millimeters of mercury
absolute (mmHgA) = 760 torr (torr) = 1.01325 bar (bar)
Atmospheric gas concentrations:
• Nitrogen = 78.084 %
• Oxygen = 20.946 %
• Argon = 0.934 %
• Carbon Dioxide (CO2) = 0.033 %
• Neon = 18.18 ppm
• Helium = 5.24 ppm
• Methane (CH4) = 2 ppm
• Krypton = 1.14 ppm
• Hydrogen = 0.5 ppm
• Nitrous Oxide (N2O) = 0.5 ppm
• Xenon = 0.087 ppm
Density of dry air at 20oC and 760 torr = 1.204 mg/cm3 = 1.204 kg/m3 = 0.075 lb/ft3 = 0.00235
slugs/ft3
Absolute viscosity of dry air at 20oC and 760 torr = 0.018 centipoise (cp) = 1.8 × 10−5 Pascalseconds
(Pa·s)

Process Control System information at your fingertips

Consol Energy, a publicly owned Pittsburgh-based producer of coal and natural gas, is one of the leading diversified energy companies in the U.S. Due to their various operations, they have a vastly distributed control system network. Multiple remote SCADA/HMI branches all connect back to a main control center in Claypool Hills, Virginia. They were experiencing a problem with blind spots (i.e., areas of their process they did not have vision into). This included the operator’s HMI screens at any number of their remote stations. Frequent SCADA/HMI-related problems caused a great deal of time to be put into supporting these remote operations over the phone or making frequent site visits to these extremely remote locations. The time and manpower required to support this kind of remote application was significant.

One such site was a gas processing facility in West Virginia, the next state over. The entire remote processing system was operated by a skeleton crew. The data from the remote location was sent back to the main control center in Virginia, but it did not provide enough information to properly troubleshoot and find the root cause of issues.

Visual confirmation of operator activity

Consol Energy decided to implement a video management system that included the capability to record operator consoles. The console recorder is a software module that enables automatic recording of the HMI or SCADA operator’s console display. With this tool, Longwatch archives exactly what the operator was seeing, because it records the video that is being sent to the display itself. Playing back what the operator was seeing proved to be a very valuable method for troubleshooting, training, and process improvement.

The recorder software provides access to live and recorded video of the operator’s screens. In this way, the control room in Virginia can have access to exactly what the remote operator’s screens look like at any given point in time. Managers can view their operator’s screens live or go back in time to any point to see what the screen looked like in the past.

The operator’s console should be looked upon as an asset, just like an important piece of equipment or part of your process. Consol Energy is now able to troubleshoot issues in real time or find out why an operator did not acknowledge an alarm. Was it due to operator error or did the HMI screen malfunction?

Remote emergency management

After the success of the console recording approach, Consol Energy wanted to add visual monitoring of their remote assets. At the same plant where the operator’s consoles were being recorded, Class I Division 2 cameras were put in place to monitor critical, potentially hazardous areas of their manufacturing process. When emergency situations arise, there is a need to determine the whereabouts of site personnel and also establish which part of the plant is affected.  The Class I Division 2 pan-tilt-zoom cameras provide remote “eyes” necessary to accurately assess the situation.

Consol Energy now has complete vision into their remote applications. The advanced video management system deployed provides video from operator screens along with video from hazardous area cameras that can be viewed anywhere on their network.

*The Article is from InTech

Securing and monitoring a crude oil pipeline

In January of 2011, the Pipeline and Hazardous Materials Safety Administration (PHMSA) invoked a requirement for Chevron to secure and monitor their remote block valve sites for pipeline leaks. PHMSA is a U.S. Department of Transportation agency that develops and enforces regulations for the safe, reliable, and environmentally sound operation of the nation’s 2.6-million mile pipeline transportation system and the nearly one million daily shipments of hazardous materials by land, sea, and air. Pipeline leaks can have a devastating cost in terms of lost production and cleanup effort, not to mention the environmental impact.

Chevron began investigating the options for security, monitoring, and leak detection. Their initial investigation included technologies ranging from 24-hour manned surveillance to radioactive isotope tracers for leak detection. After the evaluation, they decided on a remote video monitoring solution that included advanced video management software and a new hazardous area thermal sensor.

Chevron utilized an advanced video management solution that included the following system components:

• Control Room: Video control center software for integration of video into operator SCADA/HMI screens.
• Remote Site:  Remote DVR appliance for on-site recording, alarming, and direct communication to the PLC.
• Cameras:
  • Thermal imaging camera for pipeline leak detection
  • Pan-tilt-zoom camera for overall site surveillance

A new type of sensor

The solution provided integrates a thermal imaging used to detect hydrocarbon leaks and spills. The camera is optimized for persistent, 24-hour leak detection in locations where pipes rise above ground. The camera is used to detect thermal anomalies present when fluid or high-pressure gaseous leaks occur. The system provides spill detection over the full range of temperature conditions.

Smarter and faster alarm verification

When the thermal camera senses a leak detection event, it immediately alerts the PLC at the remote site via Modbus TCP. The PLC will then send a prioritized alarm message back to the control center. The Longwatch video engine simultaneously records a ten-second video clip surrounding the leak event. This snippet of video is sent over the control network back to the operator’s console. The operator now has the ability to have more information about the event in the form of video. This allows smarter and faster decision-making and gives the operator the information needed to respond to the event appropriately.

*The Article is from InTech

Signal Ranges - Graphical Interpretation

A helpful illustration in understanding analog signal ranges is to consider the signal
range to be expressed as a length on a number line. 

For example, the common 4-20 mA analog current signal range would appear as such:

If one were to ask the percentage corresponding to a 14.4 mA signal on a 4-20 mA range, it
would be as simple as determining the length of a line segment stretching from the 4 mA mark to
the 14.4 mA mark:

As a percentage, this thick line is 10.4 mA long (the distance between 14.4 mA and 4 mA) over
a total (possible) length of 16 mA (the total span between 20 mA and 4 mA). Thus:

This same “number line” approach may be used to visualize any conversion from one analog scale
to another. Consider the case of an electronic pressure transmitter calibrated to a pressure range of
-5 to +25 PSI, having an (obsolete) current signal output range of 10 to 50 mA. The appropriate
current signal value for an applied pressure of +12 PSI would be represented on the number line as
such:

Finding the “length” of this line segment in units of milliamps is as simple as setting up a proportion between the length of the line in units of PSI over the total (span) in PSI, to the length of the line in units of mA over the total (span) in mA:

Solving for the unknown (?) current by cross-multiplication and division yields a value of 22.67
mA. Of course, this value of 22.67 mA only tells us the length of the line segment on the number
line; it does not directly tell us the current signal value. To find that, we must add the “live zero”
offset of 10 mA, for a final result of 32.67 mA.

Thus, an applied pressure of +12 PSI to this transmitter should result in a 32.67 mA output
signal.