What Advantages Do Displacer Transmitters Have Over Differential Pressure Level Transmitters?

Displacer
Transmitter
(Magnetrol)

Many technologies have been available over the years have helped the process control industry with level measurement. From basic mechanical float-operated level switches, the process automation industry has been developing new technologies to make industry safer and more efficient.

An example of a "tried and true" technology that was commonly used in the process automation industry is the DP (differential pressure) level transmitter. First introduced in the 1950s, DP transmitters measures the hydrostatic (head) pressure of a liquid in a tank or vessel and interprets this as level, based on the density/specific gravity of the liquid and programmed in by the user. A newer, alternative technology to DP transmitters is the displacer level transmitter, a device also based on specific gravity. While they both are dependent on specific gravity, they are significantly different in areas of installation, accuracy, and maintenance requirement.

Application/Calibration

Applying a DP transmitter or displacer level transmitter requires experience and there are many factors to be considered. Here are a few:

DP transmitters use inferential measurement to determine level measurement from the hydrostatic pressure.  Despite requiring the specific gravity variable having to be programmed into the transmitter electronics, the level displacer transmitter is in contact with the process media and the level measurement is direct.

DP transmitters requires time consuming and expensive calibration/re-calibration if any of the set-up parameters change or if the same DP transmitter is used on different materials in the same tank.
Displacer transmitters only require two variables to be programmed (temperature and specific gravity), making it easier when running multiple products in the same tank.

Many displacer transmitters do not require liquid to be present for calibration. They are programmed (wet or dry) using software. A huge time and money saving over DP transmitters.

Mounting

The physical mounting of DP transmitters is limited, which can in some situations can become downright problematic. DP transmitters require (2) side-mounted entry locations on the vessel or tank, with one having to be near the bottom. As a general rule, the fewer the entry points of a tank or vessel, the better, because of leakage. Tank bottom entries all the more so.

Displacer transmitters are mounted to meet the requirements of the application and do not require a connection at the bottom of the tank.

Installation Cost

While DP transmitters have a lower unit cost, adding ancillary components such as tubing and heat tracing can quickly "level" the installation cost playing field. Furthermore, don't discount the time cost savings when setting up, calibrating and re-calibrating displacer transmitters.

Temperature Range

DP transmitters have a normal operating temperature of up to 250°F, with an upper limit of 650°F when special options are specified.

Displacer transmitter can be used up to 850°F, very helpful particularly with level measurement in a hot oil separator application.

There are many options and variants to accommodate industrial level applications. Share your level application challenges with instrumentation specialists, leveraging your own knowledge and experience with their product application expertise to develop the most effective solution.

Cashco / Valve Concepts Model 3100 Packing Material Removal and Weight Installation Instructions

Valve Concepts Model 3100

The Model 3100 end-of-line conservation breather vent is part of the Valve Concepts, Inc. modular vent product line. The Model 3100 can easily be field converted to a vacuum only vent, a pressure/vacuum vent with pipe away and can either be direct acting or pilot operated.

The Model 3100 end-of-line conservation breather vent is designed for use on atmospheric and low-pressure storage tanks where pressure and vacuum relief is required.

How it Works

Weight loaded pallets in the vent housing allow the intake of air and the escape of vapors as the tank breathes due to thermal changes and product movement in and out of the tank. The pallets open and close to permit in breathing and out breathing necessary to maintain the tank pressure within permissible limits to avoid damage to the tank.

Paperless + Wireless Process Control Recorder

Paperless and Wireless
Yokogawa GX20W

There are probably few among us who long for the days when process recorders used paper. Video based graphic (also known as paperless) recorders, aside from being paperless, provide a broad range of functionality that empowers a user to display and deliver process information in whatever way is most useful for the application.

High sampling speed of multiple inputs that can be configured to accommodate a range of input signals, along with data storage, network communications and programmable alarm outputs power up the usefulness of these instruments for monitoring and documenting process operation. Many utilize touch sensitive screens, eliminating any buttons or keys on the operator panel.

Some of the great features available on today's paperless recorders:

• Multiple channels of input
• Start/stop recording by batch, and create data files
• Ample internal memory
• Creation of template-based Excel spreadsheets
• Custom display function
• Historical data with date and time calendar search function
• Ethernet interface
• PROFIBUS-DP and EtherNet/IP protocols
• Dust and splash-proof front panel
• Front panel door lock and login function

Wireless Seals the Deal

Wireless technology is a serious contributor in the effort to improve plant efficiency, lower risk, and increase productivity. Wireless recorders are now available for monitoring virtual all process variables such as pressure, temperature, level, and flow (plus many more).  The use of wireless recorders provides a compelling argument when you consider installation cost savings and convenience.  Savings estimates as high as 70% are realized when compared to the cost using cables for the same application.

There are many options and variants to accommodate every conceivable process control application. Share your data acquisition and process monitoring challenges with instrumentation specialists, leveraging your own knowledge and experience with their product application expertise to develop an effective solution.

For more information on paperless recorders, contact Miller Energy by visiting https://millerenergy.com or calling 908-755-6700 in NJ or 610-363-6200 in PA.

Pressure Switches - The Stalwart of Pressure Instrumentation

Pressure switch in an
explosion-proof housing.
(United Electric)

A pressure switch is an electromechanical device that detects the presence of fluid pressure and responds by opening or closing an electrical or pneumatic circuit.

In heavy industry, pressure switches are used in virtually every power plant, refinery, chemical plant, paper mill, steel mill, or other manufacturing plant that blends ingredients.

Pressure switches are simple devices. They can be broken down to their major parts: a pressure port or connection; a sensor that moves in relationship to changing pressures; an electrical or pneumatic switch that opens or closes upon movement; and finally a housing that protect the internals of the pressure switch from the ambient conditions.

Differential pressure switch.
(United Electric)

Pressure switches use a variety of sensing elements such as diaphragms, bellows, bourdon tubes, or pistons. In most cases, the movement of these sensors, caused by pressure fluctuation, is transferred to a set of electrical contacts to open or close a circuit. Normal status of a switch is the resting state. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.

One of the earliest and most common designs of pressure switch was the bourdon tube pressure sensor accompanied by a mercury switch. A mercury switch is a position sensitive glass bulb containing mercury that flows over, or away from, the electrical contacts. When pressure is applied, the bourdon tube attempts to straighten, and moves enough to slightly tilt the mercury switch. Many of these kind of pressure switches were sold on steam boilers, and while they became a de facto standard, they were sensitive to vibration and breakage of the mercury bulb.

NO vs. NC electrical switch contacts.

The most common electrical switch used in pressure switches are "microswitch" type. These are also called "snap switches" because they are actuated by very little physical force, through the use of a tipping-point mechanism. These type of switches offer reliability and repeatability. They also are available in many different voltages and current ratings

One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to reset the switch to its normal state after it has tripped.  The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.

When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).

It's always a good idea to discuss your application with an expert before specifying or installing a pressure switch. You'll end up saving time and money, and ensure long, safe operation.

For more information on pressure switches, contact Miller Energy by visiting https://millerenergy.com or by calling one of these numbers: In New Jersey 908-755-6700. In Pennsylvania 610-363-6200.

Miller Energy - Industrial Instrumentation & Process Control Equipment

Miller Energy is a Manufacturer's Representative and Distributor of Industrial Instrumentation and Process Control Equipment. Since 1958, we have been committed to exceeding our customers expectations by providing an unparalleled level of customer service and local technical support. We offer the most comprehensive line of measurement, control, and communication solutions in the Industry today. The products we represent solve challenging applications in the Industrial Gas, Power, Refining, Chemical / Petro-Chemical, Food & Beverage, Water/Wastewater, and Pharmaceutical markets.

https://millerenergy.com 

NJ 908-755-6700 

PA 610-363-6200

Bimetal Thermometers for Industrial Process Measurement

Bimetal thermometers have a place in modern process
measurement systems.
Image courtesy Wika

Temperature measurement is everywhere, with broad ranges of accuracy, range and other operational requirements to bring the measurement data into a process management or control system. The process could be as simple as measuring a cooking temperature, or a part of a complex refining operation. Temperature provides an indication of heat energy level that is used in many ways throughout process control.

Though there are many instruments and technologies available to measure temperature, one that everyone is familiar with is the dial thermometer. A familiar numeric scale and a pointer indicate the temperature at the sensing location. Even within the product range of dial thermometers, there are several differing methods utilized to produce a temperature reading. One of these is the bimetal thermometer.

A bimetallic thermometer is named for the mechanism that responds to process temperature and provides the force to position the indicator needle over the scale on the dial face. A bimetal is formed from two dissimilar metals bonded together. The metals expand and contract at different rates in response to a change in their temperature. A bimetal thermometer relies on the predictable deformation of a bimetal spring or strip in response to a temperature change. The mechanical deformation is transformed into rotational movement of the indicating needle on the instrument face where the corresponding temperature can be read by a technician or operator. This design principle has been in use throughout laboratories, kitchens, and industry for many years and has proven to be predictably accurate, stable, and rugged.

The major advantages of the bimetallic thermometer are its relative cost, ease of use, and ability to function without any external power source. This class of instruments provides operability up to +1000°F.

When applying dial faced thermometers, there are several main considerations.

• Scale - The display behind the indicating pointer. The scale divisions impact the instrument's accuracy at indicating process temperature.  
• Range - The physical suitability of the instrument to be exposed to the temperatures which may be present in the process. May be the same as scale.
• Dial Size - Larger diameter dial faces make reading the instrument indications easier.
• Connection - There are numerous options for the way in which the probe or stem, which is inserted into the process, attaches to the dial portion or head of the instrument. Common arrangements are back, side, or bottom connected. If the head cannot be rotated or angled, the connection attributes may be the sole determinant of how the dial face is oriented.
• Stem Length - The stem extends from the head into the process. Coordinating the stem length with the insertion depth into the process and the placement of the instrument is important to achieving a useful and ergonomic installation.
• Materials of Construction - Make sure the selected instrument is rugged enough to withstand expected environmental conditions at the installation site.

These are only the primary considerations. Share your operational requirements with a product specialist. Leverage your own knowledge and experience with their product application expertise to develop the optimal solution.

Guided Wave Radar - An Option for Level Measurement in Hygienic Applications

A special version of the Magnetrol Eclipse 705
is configured for hygienic applications.
Image courtesy Magnetrol

Measurements of a variety of process conditions are utilized to monitor and control operations and output. One general goal of measurement, other than answering the question "how much", is to avoid or minimize any interference with the process itself. A second goal is to not be fooled by the process into returning a false measurement result.

Guided wave radar is based upon the principle of TDR (time domain reflectometry). Pulses of electromagnetic energy travel from the emitting antenna via a fixed waveguide or probe immersed in the target medium. When it contacts the media surface, the pulse energy is reflected back along the probe to a receiving antenna. The instrument actually measures the time elapsed between the pulse transmission and the detection of the reflected return. The time measurement is used to calculate the distance from the antenna to the media surface. The distance calculation, with knowledge of the vessel, can be converted into a value indicating media level or volume. Of course, this is a simplified account of the operating principal.

Guided wave radar (GWR), as opposed to an open style radar level measurement method, uses a probe immersed in the process media to guide high-frequency electromagnetic waves into the media being measured. While it does involve contact by the sensing instrument with the media, GWR eliminates interference from fixtures or structures that may exist within the tank or vessel. The immersion probe waveguide also attenuates the impact of media turbulence and other potential disturbances. The waveguide reduces the potential impact of elements that may adversely impact the measurement accuracy, resulting in greater accuracy and reliability of the measurements.

For hygienic applications, the transmitters are available with 304 stainless steel housings designed specifically for use in facilities with the special requirements for the wetted and non-wetted materials, process connections and surface finishes of hygienic industries. In addition to high accuracy, the GWR instrument output is not impacted by media buildup on the sensing probe.

Share your level measurement challenges with process instrumentation specialists. Leverage your own process knowledge and experience with their product application expertise to develop an effective solution.

Magnetrol Guide Wave Radar Level Transmitter For Hygienic Applications from Miller Energy, Inc.