Showing posts with label Level Control. Show all posts
Showing posts with label Level Control. Show all posts

Tuesday, May 29, 2018

Basic Set up of the Schneider Electric / Foxboro LG01 Guided Wave Radar Level Transmitter

Foxboro LG01 Guided Wave Radar Level TransmitterThe Foxboro Model LG01 Radar level measurement transmitter provides very accurate and reliable level measurement for the widest choice of installation and application.

Guided Wave Radar Technology 

Electromagnetic pulses are emitted and guided along a probe. These pulses are reflected back at the product surface. The distance is calculated by measuring this transit time. This device is perfect for high-end applications. It is suitable for applications with foam, dust, vapor, agitated, turbulent or boiling surfaces with rapid level changes.

This video demonstrates the quick set up of the instrument. 

Tuesday, January 30, 2018

Steam Boiler Water Level Control

Steam boiler level control diagram.
Steam boiler level control diagram.
Click on image for larger view.
Steam boilers are very common in industry, principally because steam power is so useful. Common uses for steam in industry include doing mechanical work (e.g. a steam engine moving some sort of machine), heating, producing vacuums (through the use of “steam ejectors”), and augmenting chemical processes (e.g. reforming of natural gas into hydrogen and carbon dioxide).

The process of converting water into steam is quite simple: heat up the water until it boils. Anyone who has ever boiled a pot of water for cooking knows how this process works. Making steam continuously, however, is a little more complicated. An important variable to measure and control in a continuous boiler is the level of water in the “steam drum” (the upper vessel in a water-tube boiler). In order to safely and efficiently produce a continuous flow of steam, we must ensure the steam drum never runs too low on water, or too high. If there is not enough water in the drum, the water tubes may run dry and burn through from the heat of the fire. If there is too much water in the drum, liquid water may be carried along with the flow of steam, causing problems downstream.

The first instrument in this control system is the level transmitter, or “LT”. The purpose of this device is to sense the water level in the steam drum and report (“transmit”) that measurement to the controller in the form of a signal. In this case, the type of signal is pneumatic: a variable air pressure sent through metal or plastic tubes. The greater the water level in the drum, the more air pressure output by the level transmitter. Since the transmitter is pneumatic, it must be supplied with a source of clean, compressed air on which to operate. This is the meaning of the “A.S.” tube (Air Supply) entering the top of the transmitter.

This pneumatic signal is sent to the next instrument in the control system, the level indicating controller, or “LIC”. The purpose of this instrument is to compare the level transmitter’s signal against a setpoint value entered by a human operator representing the desired water level in the steam drum. The controller then generates an output signal telling the control valve to either introduce more or less water into the boiler to maintain the steam drum water level at setpoint. As with the transmitter, the controller in this system is pneumatic, operating entirely on compressed air. This means the output of the controller is also a variable air pressure signal, just like the signal output by the level transmitter. Naturally, the controller requires a constant supply of clean, compressed air on which to run, which explains the “A.S.” (Air Supply) tube connecting to it.

The last instrument in this control system is the control valve, operated directly by the air pressure signal output by the controller. Its purpose is to influence the flow rate of water into the boiler, “throttling” the water flow more or less as determined by controller. This particular type of control valve uses a large diaphragm and a large spring to move the valve further open with more signal pressure and further closed with less signal pressure.

When the controller is placed in the “automatic” mode, it will move the control valve to whatever position necessary to maintain a constant steam drum water level. The phrase “whatever position necessary” suggests the relationship between the controller output signal, the process variable signal (PV), and the setpoint (SP) is complex. If the controller senses a water level above setpoint, it will close off the valve as far as necessary to decrease the water level down to setpoint. Conversely, if the controller senses a water level below setpoint, it will open up the valve as far as necessary to raise the water level up to setpoint.

What this means in a practical sense is that the controller’s output signal (equating to valve position) in automatic mode is just as much a function of process load (i.e. how much steam is being used from the boiler) as it is a function of setpoint (i.e. where we wish the water level to be). Consider a situation where the steam demand from the boiler is very low. If there isn’t much steam being drawn off the boiler, this means there will be little water boiled into steam and therefore little need for additional feedwater to be pumped into the boiler. Therefore, in this situation, one would expect the control valve to hover near the fully-closed position, allowing just enough water into the boiler to keep the steam drum water level at setpoint. If, however, there is a high demand for steam from this boiler, the rate of evaporation will be much greater. This means the control system must add feedwater to the boiler at a much greater flow rate in order to maintain the steam drum water level at setpoint. In this situation we would expect to see the control valve much closer to being fully-open as the control system “works harder” to maintain a constant water level in the steam drum. Thus, we see how the controller automatically positions the control valve to react to different boiler operating conditions even when the setpoint is fixed.

A human operator supervising this boiler has the option of placing the controller into “manual” mode. In this mode the control valve position is under direct control of the human operator, with the controller essentially ignoring the signal sent from the water level transmitter. Being an indicating controller, the controller faceplate will still show how much water is in the steam drum, but it is now the human operator’s sole responsibility to move the control valve to the appropriate position to hold water level at setpoint – in manual mode the controller takes no corrective action of its own. Manual mode is useful to human operators during start-up and shut-down conditions. It is also useful to instrument technicians for troubleshooting misbehaving control systems. Placing a controller into manual mode is akin to disengaging the cruise control in an automobile, transferring control of engine power from the car’s computer back to the human driver. One can easily imagine an automobile mechanic needing to throttle a car’s engine “manually” (i.e. with the cruise control turned off) in order to properly diagnose an engine or drivetrain problem. This is true for industrial processes as well, where instrument technicians may need to place a controller into manual mode in order to properly diagnose transmitter or control valve problems.

Reprinted from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Friday, April 29, 2016

Choose Guided Wave Radar for Your Challenging Process Level Application

Guided Wave Radar transmitters (GWR)
Guided Wave
Radar Transmitter
Courtesy of
Foxboro/Schneider
Electric
Designed to perform continuous level measurement in a wide range of industries and applications, Guided Wave Radar transmitters (GWR) are unaffected by changes in temperature, specific gravity, pressure and with no need to recalibrate, offering a highly available measurement at low maintenance cost. GWR transmitters provide level measurement solutions in a variety of process applications, providing a universal radar measurement solution for all liquids including corrosive, viscous, sticky and other difficult media such as foam and turbulent surfaces, and solids.

Electromagnetic pulses are emitted and guided along a probe.  These pulses are reflected back at the product surface.  The distance is calculated by measuring this transit time. This device is perfect for high-end applications.  It is suitable for applications with foam, dust, vapor, agitated, turbulent or boiling surfaces with rapid level changes.

Common features include:
Easy configuration via digital communication; Wide selection of materials facilitates service under harsh/corrosive conditions; Solutions for density/pressure variations and rapid level changes; Empty Tank Spectrum filtering; Quick Noise scanning reduces false radar reflections.

Applications: Steam Generation /Boiler Drum; Oil/Water Separator; BioDiesel Production; Overflow Protection; Interface and Density; Process tanks; Storage tanks; Polyester/Nylon fiber production; Claus Process


For more information on Guided Wave Radar level instruments, contact:

Mead O'Brien
(800) 892-2769
www.meadobrien.com