An informational site dedicated to process control education. Posts on this blog contain interesting and noteworthy information about process instrumentation, valves, and equipment used in industrial and municipal facilities. This blog is sponsored by Instrument Specialties, Inc. (ISI). For more information, please visit ISI.GROUP
Self-regulating heat tracing systems are the preferred choice for most complex pipe-tracing applications. This is due to their parallel construction, which allows them to be cut to length and spliced in the field, and their self-regulating output, which provides more heat where it is needed.
Self-regulating cable uses conductive-polymer technology in both monolithic (solid core) and fiber (polymeric fiber wrap) heating cables. The heating element is made of polymers mixed with conductive carbon black. This special blend of materials creates electrical paths for conducting current between the parallel bus wires along the entire cable length.
In each heating cable the number of electrical paths between the bus wires changes in response to temperature fluctuations. As the ambient temperature surrounding the heating cable decreases, the conductive core or fiber contracts microscopically. This contraction decreases electrical resistance and creates numerous electrical paths between the bus wires. Current flows across these paths to warm the core or fiber.
The document below provides an excellent detailed description of self-regulating cable, its application and selection.
A globe control valve is a specific type of valve used for regulating flow in a pipe. The design includes a movable plug, connected to a stem, which can be moved linearly to close or open the valve. Globe control valves are referred to as “linear” valves because of this movement, whereas ball valves and butterfly valves are referred to as “quarter-turn” valves. Generally, globe control valves provide better overall flow control than quarter-turn valves due to the design of their flow path.
The Valtek Mark One globe control valve is know for superior performance in liquid and gaseous services, while also permitting easy, fast and inexpensive maintenance to its major components.
The video below provides an excellent look inside a globe control valve (in this case the Flowserve Valtek Mark One).
Most industrial applications require the monitoring of pressure. Pressure measurement can be accomplished by transmitters, gauges or by switches. This post will provide a quick introduction of industrial electromechanical pressure switches.
An industrial pressure switch is made up of the three main components: 1) the sensor, 2) the housing and 3) the switching element.
Sensor
The sensor is located above the pressure port and process connection. For pressure and differential pressure switches, there are several varieties of pressure sensors to choose. The most common types of pressure sensors are:
Metal Bellows - an accordion-like device that provides linear expansion and contraction based upon the application of pressure or vacuum. Bellows are excellent sensors because they provide good overall pressure range and are fairly sensitive to small changes in pressure.
Piston - A rod and o-ring combination that moves linearly in direct response to applied pressure. Piston sensors are normally only applied to only very high pressure ranges. They have very small surface areas and wide deadbands (the change in pressure required to change the position of the switch output).
Diaphragm - A thin, elastomer or metallic membrane, often with a rolled lip that allows for greater movement. The diaphragm has a large surface area and provides the most sensitivity to pressure change, making it ideal for low to mid-range pressure sensing.
Housing
Housings are classified and selected based on the atmosphere in which they’ll be used. Housing ratings are classified by several national and international agencies such as NEMA and CENELEC. Very generally put, housings can be rated as general purpose, dust & water resistant, water tight, corrosion resistant and hazardous (explosive) environments. Proper selection of the housing is important to the operation and life expectancy of the device. In hazardous environments, proper selection is absolutely critical. If unsure about the housing classification, consultation with an applications expert is required.
Switching Element
The switching element refers to the signaling device inside the enclosure that responds to the movement of the sensor. It can be either electrical or pneumatic, and provides an on-off signal (as opposed to an analog, or proportional signal produced by transmitters).
The switching element is most times a “micro” type single pole, double throw (SPDT) electrical switch. These microswitches come in many configurations and electrical ratings, such as double pole, double throw (DPDT), 120/240 VAC, 12VDC, 24VDC, and hermetically sealed.
For the switching element and the sensor, it is very important to know the cycling rate (number of on vs. off times over a period of time) the instrument will see. Since both of these elements are mechanical, they will eventually wear out and need to be replaced. Switches are an economical and strong performing choice for low to medium cycle rates. For extremely high cycle rates, the use of solid state transmitters are a better choice.
The video below, courtesy of SOR Inc., provides a look inside how a typical pressure switch works.
Fike rupture discs are designed to provide instantaneous pressure relief at a predefined pressure and temperature. Installation is an important consideration that can affect the performance a ruptured disk. Installation instructions are included with all Fike ruptured disc shipments. These instructions should be followed carefully and completely.
Remember to place the rupture disc assembly where it will have sufficient clearance to operate unhindered. The rupture disc assembly should be vented to a safe area where people and equipment are not at risk, as a system discharge can be hazardous or cause injury. Baffle plates used on the end of outlet piping will redirect but not eliminate potentially dangerous system discharge. The piping near the rupture disc assembly should be braced to absorb shock caused by the opening up a rupture disk. Fike provides a danger sign with all rupture disc shipments and it should be placed in a conspicuous location near the zone a potential danger. Keep the danger sign clean and unobstructed for ease of viewing.
To install a new rupture disc remove the burst rupture disc assembly from its piping. Please use caution as a ruptured disk may have sharp edges. Remove pre-assembly screws or side clips and separate the holder components. If this is an existing installation, it is important at this point to do a visual inspection of the rupture disc holder. Inspect the holder seat area for nicks, scratches, corrosion, or deposits left by the process media. If necessary, hand polish the seat area with Scotch Bright, super fine steel wool, or a fine emery cloth. Before installing a new rupture disc into the rupture disc holder, clean the seat area with the solvent compatible with your media. To verify that the holder has not become deformed, place a straight edge on the flat flange mating surfaces of the base and hold-down. If the flat surfaces are not parallel with the straight edge, the holder should be replaced immediately.
Visually inspect the rupture disc for shipping damage. Read the complete information contained on the rupture disc tag. Verify that the disk size, type, pressure, and temperature are correct for your system. After verifying that you have the correct rupture disc, carefully place reverse acting rupture discs into the hold-down of the holder. Or if the rupture disk is forward acting, carefully place the disc into the base the holder. Checked the rupture disc tag and make sure that the side labeled "vent side" faces downstream. Rupture disc tags with flow arrows must point in the same direction as the flow arrows on the ruptured disc holder. Carefully place the hold-down on top of the disc.
Install the pre-assembly screws or side clips. Do not apply torque to the pre-assembly screws. Finger tight is sufficient. Check to be sure the gap between the base and the hold-down is equal all the way around the holder. If it is not, take the rupture disc holder apart and reassemble. After assembling the rupture disc and holder, you are ready to install the rupture disc assembly into your piping system. Place gaskets on the top and bottom of the rupture disc holder as needed. Gaskets that are subject to cold-flow are not recommended. Carefully place the rupture disc assembly between the piping flanges. Check to insure the holder flow arrows point downstream. Care should be taken not to damage the dome of the rupture disc, being particularly careful if it protrudes above the hold-down of the rupture disc holder.
Check to see that all studs and nuts are the correct size. Lubricate the studs with the light, free running oil, such as 20 grade. Studs and nuts that show evidence of galling should not be used. Insert the studs into the flange and finger tighten. Check to see that the gap is still equal. If it is not, loosen the studs and the nuts and adjust flanges until the gap is once again equal.
To determine the required torque, refer to the appropriate stud torque chart included in the written installation instructions. Find a nominal rupture disc size and ANSI rating of your flange. Follow the row across the column that contains your rupture disc type to determine what is required for your rupture disc in foot-pounds. Using a criss-cross pattern, apply torque in 25 percent increments. For example if the required torque is 100 foot-pounds, torque would be applied in 25 foot-pound increments using the criss-cross pattern. After each torque step, it is recommended that you checked the gap between the base and the hold-down to be sure that it is still equal. After the final torque step has been completed, make one revolution around the flange to be sure that each stud has received the correct amount of torque.
Fike rupture discs and holders come in many sizes and types. A common requirement of all designs is proper handling and installation. Please follow the instructions included in your Fike shipment to optimize performance and service life.
