Friday, December 8, 2017

Groundwater Monitoring Instrumentation

groundwater level temperature pressure measurement instrument
The Level TROLL groundwater monitoring instrument can be
suspended in a well and log temperature, pressure, and level.
Image courtesy In-Situ
In-Situ specializes in the manufacture of instruments used for monitoring groundwater and other water testing functions. In the fields of aquaculture, remediation, energy, mining and research, In-Situ is regarded as an innovator, providing reliable top quality water monitoring equipment.

The company's Level TROLL line of water level data loggers are specifically intended for aquifer characterization. The instruments provide continuous level, pressure and temperature readings when suspended in a well. The battery operated units can function for very long periods, holding up to 260,000 data points.

The Level TROLL Instrument is completely sealed and contains no user-serviceable parts. The instrument includes pressure and temperature sensors, a real-time clock, microprocessor, sealed lithium battery, data logger, and memory. Instruments are available in a variety of pressure (depth) ranges with vented and non-vented sensors. Placement within a well can be accomplished with In-Situ's Rugged Cable System, custom built for each application to provide direct reading operation. The cable assembly includes titanium twist-lock connectors, desiccant to prevent condensation in the vent tube, shielded connecting cable, and a placement grip for secure deployment. A second option for instrument deployment is suspension via a polyurethane coated stainless steel cable for applications that do not require direct reading, only data logging.

There are several models from which to choose, and other products suitable for groundwater monitoring throughout many industries. Share your water monitoring requirements of all types with instrumentation specialists. Combine your own knowledge and experience with their product application expertise to develop effective solutions.

Monday, December 4, 2017

Open Path Laser Spectroscopy Gas Detection

MSA has been a leader in hazardous gas detection instruments and equipment for many years. Their addition of the Senscient ELDS™ open path gas detector to their already extensive array of gas detection instruments reinforces the MSA leadership position in the field.

The Senscient ELDS™ system consists of an infrared laser transmitter unit and a receiver. The distance between the two components can be up to 200 meters. The function of the system is to provide detection of a targeted hazardous or flammable gas.

The basic operating principle centers around the selective absorption of specific wavelengths of light by the target gas. The transmitter infrared output is selected to match signature absorption patterns of the gas to be detected, with the receiver unit and its signal processing electronics tuned for the same signature. The output signal from the detector indicates the amount in target gas present in the beam path, which is proportional to the degree of absorption at the signature wavelength .

The Senscient ELDS™ incorporates advancements and technology that make its operation more resistant to the adverse effects of the operating environment and other less than ideal installation conditions than previous generations of open path gas detectors.

The included video provides more insight into the operation of the Senscient ELDS™ system. Share your hazardous and flammable gas detection challenges with process instrumentation specialists, leveraging your own knowledge and experience with their product application expertise to develop an effective solution.

Vortex Flowmeters

vortex or multivariable flowmeter flow meter for process measurement
Multivariable vortex flowmeter combines flow, temperature
and pressure measurement into a single compact instrument
Image courtesy Krohne
Vortex shedding flowmeters provide process operators with consistent fluid flow rate measurements across a wide range of applications. These flowmeters measure the volumetric flow rate of steam, gas, and low viscosity liquids, boasting both versatility and dependability when used in conjunction with process systems.

Vortex shedding refers to the phenomenon wherein flowing gas or liquid form vortices around a solid object placed in the flow path. The measurement technology returns an indication of the process fluid velocity, which can then be used to provide volumetric or mass flow data. Vortex technology is well suited for many applications involving cryogenic liquids, hydrocarbons, air, and industrial gases. Vortex flow measurement requires contact between portions of the measurement instrument and the process media, so these flowmeters are commonly fashioned from a range of corrosion resistant materials.

The process of measuring the flow involves both the flowmeter and the ability for other instrumentation to measure the vortices themselves in order to calculate velocity. Ultrasonic sensors have become popular tools for measuring vortices. Applications involving flow measurement of high viscosity fluids are not suited for vortex technology because extremely viscous fluids do not behave in the same manner as lower viscosity fluids when their flow path is obstructed. Splitting higher viscosity fluids into concordant vertices is extremely difficult due to the internal friction present in highly viscous liquids.

Additionally, in order to split these process liquids, the piping through which the process material flows must be straight, and any disturbance or vibration in the pipe may impact the measurement. A vortex flowmeter will be in a fixed installation. This stationary element, operating without electrodes, can be advantageous for flow measurement in chemical applications utilizing low viscosity fluids.

The vortex shedding flowmeter is widely used for the measurement of steam flow. The high pressure and elevated temperature of steam, along with the variation that exists in most steam systems, have little negative impact on the operation of a vortex flowmeter. Vortex shedding flowmeters are often volumetrically based in terms of measurement, but their output can be combined with other fluid information to calculate mass flow. A product variant commonly available will combine the vortex flow measurement with temperature and pressure compensation, delivering three process measurements from a single installed device.

Share your process and flow measurement challenges with instrumentation specialists, leveraging your own knowledge and experience with their product application expertise.

Pressure Switch Design Details

Industrial process control applications present dynamic and varied requirements for measuring, monitoring and control. Each point calls for specific evaluation of the information needed from the process point for use in monitoring process performance, or control to be applied at the process point to regulate an outcome. Sometimes, a continuous analog signal is needed to provide indication across a range of values. Other times, it is only necessary to have notification of, or take action when, a certain temperature or pressure related event occurs. In those cases, a simple and reliable device can adequately meet the project requirements.

Pressure and differential pressure switches connect to a process and change their switch position when a setpoint condition is reached. The are simple to understand, easy to install, low in cost, and require little maintenance of attention. The switches are available in an extensive array of configurations, with options to fill out almost any application requirement.

SOR, Inc., globally recognized manufacturer of temperature and pressure switches, has produced this video outlining some of the distinctive features of their pressure switches for industrial process control applications. Share your process measurement and control requirements and challenges with product application specialists, and leverage your own process knowledge and experience with their product expertise to develop effective solutions.

Friday, November 10, 2017

Best Practices for Routing Control Signal to Multiple Devices

process signal conditioner modules isolator
Isolating transmitters are part of best practice for
retransmitting process signals
Image courtesy of Acromag
Acromag is a globally recognized leader in the design and manufacture of signal conditioning equipment for process measurement and control. On a daily basis they get calls on understanding 4-20 mA current loops and how to wire them with or without power supplies. The application note provided below can serve as a template for wiring Acromag modules, or those of similar design from other manufacturers, in applications. Download and save it for use as a reference when connecting field devices to a PLC, DCS, HMI, etc.

Share your process measurement and control challenges with application specialists, leveraging your own knowledge and experience with their product application expertise to develop effective solutions.

Friday, November 3, 2017

Eight Selection Criteria for Control Valves

globe valve with actuator and positioner
Control valve shown with actuator and positioner
Image courtesy Flowserve - Valtek
Proper selection with respect to a number of factors plays an important role in obtaining the desired performance from a control valve. Failure to make a properly considered selection can impact operation, productivity and safety. Here is a quick checklist of basics that need to be considered:
  • Control valves are not intended to be a an isolation valve and should not be used for isolating a process. 
  • Always carefully select the correct materials of construction. Take into consideration the parts of the valve that comes in to contact with the process media such as the valve body, the seat and any other wetted parts. Consider the valve's exposure to operating pressures and temperatures. Finally, also consider the ambient atmosphere and any corrosives that can occur and effect the exterior of the valve. 
  • Put your flow sensor upstream of the control valve. Locating the flow sensor downstream of the control valve exposes it to an unstable flow stream which is caused by turbulent flow in the valve cavity.
  • Factor in the degree of control you need and make sure your valve is mechanically capable. Too much dead-band leads to hunting and poor control. Dead-band is roughly defined as the amount of control signal required to affect a change in valve position. It is caused by worn, or loosely fitted mechanical linkages, or as a function of the controller setting. It can also be effected by the tolerances from mechanical sensors, friction inherent in the the valve stems and seats, or from an undersized actuator. 
  • Consider stiction. The tendency for valves that have had very limited travel, or that haven't moved at all, to "stick" is referred to as stiction. It typically is caused by the valves packing glands, seats or the pressure exerted against the disk. To overcome stiction, additional force needs to be applied by the actuator, which can lead to overshoot and poor control.
  • Tune your loop controller properly. A poorly tuned controller causes overshoot, undershoot and hunting. Make sure your proportional, integral, and derivative values are set). This is quite easy today using controllers with advanced, precise auto-tuning features that replaced the old fashioned trial and error loop tuning method.
  • Don't over-size your control valve. Control valves are frequently sized larger than needed for the flow loop they control. If the control valve is too large, only a small percentage of travel is used (because a small change in valve position has a large effect on flow), which in turn makes the valve hunt. This causes excessive wear. Try to always size a control valve at about 70%-90% of travel.
  • Think about the type of control valve you are using and its inherent flow characteristic. Different types of valve, and their disks, have very different flow characteristics (or profiles). The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. Globe control valves have linear characteristics which are preferred, while butterfly and gate valves have very non-linear flow characteristics, which can cause control problems. In order to create a linear flow characteristic through a non-linear control valve, manufacturers add specially designed disks or flow orifices which create a desired flow profile.
These are just a few of the more significant criteria to consider when electing a control valve. You should always discuss your application with an experienced application expert before making your final selection.

Tuesday, October 10, 2017

Two-Wire vs. Four-Wire Transmitter For Analog Process Signals - What to Consider?

industrial I/O modules for process signal conditioning
I/O modules are an integral part of process signal connectivity.
Image courtesy of Acromag
Transmitters are everywhere in process control. They take a sensor output signal,amplify and condition it, then send it to monitoring and decision making devices. The most common analog electrical signal used for transmitting process control signals is a 4-20 mA (milliampere) current flow. It has succeeded in its adoption for a number of reasons, not the least of which are its resistance to interference and ability to transmit a signal across a substantial length of cable.

Aside from the sensor connection, there are two basic wiring schemes for these devices. The simplest employs just two conductors to transmit the signal and coincidentally provide operating power for the transmitter electronics. This type of transmitter is commonly referred to as a "loop powered" or "two-wire" device. A DC power supply, typically 24 volts, is wired in series with the 4-20 mA output signal and the transmitter derives its operating power from this source. Loop powered devices generally consume very little power, but process designers must consider the total resistance imposed on the loop by all connected devices. The cable, unless the length is monstrous, poses a measurable but comparatively small resistance. Careful consideration should be given to the resistance imposed by receiving devices, especially if there are several in series, receiving the loop signal. The output voltage of the power supply and the maximum tolerable voltage of the connected devices will serve as limiting factors on loop instrument quantity. Where they can be applied, two-wire transmitters offer a straight forward solution for delivery of analog process measurement signals.

A "four-wire" transmitter gets its name from, you guessed it, the two pairs of wires used to provide operating power and a signal transmission path. Provided with a separate power source, possibly even 120 volts AC, this transmitter type will often be found in applications where the sensor may have power requirements that cannot be met with the limitations inherent in the loop powered device. While it may seem that the separate power supply negates the need to consider total resistance load on the signal loop, this is not the case. The signal loop still will be limited by the DC power supply that serves as the driving force of the loop.

In many cases, the question of "two-wire or four-wire" will be answered by the transmitter manufacturer. Since the two-wire scheme is a less burdensome installation, it may be the only product offering when a suitable device can be designed for an application. That said, a diligent search will probably find two and four-wire versions of transmitters for almost every application.

What are some decision making guidelines?
  • Some types of transmitters have sufficiently high power requirements that they cannot be loop powered. In this case, four-wire may be the only option.
  • For low resistance loads, use 2 wire transmitters for a simpler installation.
  • Allow some headroom in the loop resistance to accommodate at least one added receiving device in the future. For example, a temperature signal may serve as an input to a controller now, but need to service a recording device potentially added in the future.
  • Distance should not be mindlessly overlooked, but is generally not a limiting factor, as most installations would be compatible with the distance limitations for two- or four-wire device output signals.
  • When signal transmission distances become unwieldy, due to cabling costs or other factors, consider a wireless transmitter instead of a wired device.
An important aspect of applying 4-20 mA signal loops is to maintain the capability to add another receiving device to the circuit. The use of information in the form of process signals has been growing for a long time and is likely to continue. It is certainly easier to wire an additional device into an existing loop, than to install an additional sensor, transmitter, power supply, and cabling to accommodate the additional device.

Share your process measurement requirements and challenges with process instrumentation experts, leveraging your own process knowledge and experience with their product application expertise to develop complete and effective solutions.