Showing posts with label boiler. Show all posts
Showing posts with label boiler. Show all posts

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.

Tuesday, June 13, 2017

Common Industrial and Commercial Process Heating Methodologies

Gas Steam Boiler
Fuel boiler producing steam.
Process heating methodologies can be grouped into four general categories based on the type of fuel consumed:
  1. Steam
  2. Fuel
  3. Electric
  4. Hybrid systems
These technologies are based upon conduction, convection, or radiative heat transfer mechanisms - or some combination of these. In practice, lower-temperature processes tend to use conduction or convection, whereas high-temperature processes rely primarily on radiative heat transfer. Systems using each of the four energy types can be characterized as follows:

STEAM


Heat Exchanger
Tube heat exchanger.
Steam-based process heating systems introduce steam to the process either directly (e.g., steam sparging) or indirectly through a heat transfer mechanism. Large quantities of latent heat from steam can be transferred efficiently at a constant temperature, useful for many process heating applications. Steam-based systems are predominantly used by industries that have a heat supply at or below about 400°F and access to low-cost fuel or byproducts for use in generating the steam. Cogeneration (simultaneous production of steam and electrical power) systems also commonly use steam-based heating systems. Examples of steam-based process heating technologies include boilers, steam spargers, steam-heated dryers, water or slurry heaters, and fluid heating systems.

FUEL


Fuel-based process heating systems generate heat by combusting solid, liquid, or gaseous fuels, then transferring the heat directly or indirectly to the material. Hot combustion gases are either placed in direct contact with the material (i.e., direct heating via convection) or routed through radiant burner tubes or panels that rely on radiant heat transfer to keep the gases separate from the material (i.e., indirect heating).  Examples of fuel-based process heating equipment include furnaces, ovens, red heaters, kilns, melters, and high-temperature generators.

ELECTRICITY


Electricity-based process heating systems also transform materials through direct and indirect processes. For example, electric current is applied directly to suitable materials to achieve direct resistance heating; alternatively, high-frequency energy can be inductively coupled to suitable materials to achieve indirect heating. Electricity-based process heating systems are used for heating, drying, curing, melting, and forming. Examples of electricity-based process heating technologies include electric arc furnace technology, infrared radiation, induction heating, radio frequency drying, laser heating, and microwave processing.

HYBRID


Hybrid process heating systems utilize a combination of process heating technologies based on different energy sources and/or heating principles to optimize energy performance and increase overall thermal efficiency. For example, a hybrid boiler system may combine a fuel-based boiler with an electric boiler to take advantage of access to lower off-peak electricity prices. In an example of a hybrid drying system, electromagnetic energy (e.g., microwave or radio frequency) may be combined with convective hot air to accelerate drying processes; selectively targeting moisture with the penetrating electromagnetic energy can improve the speed, efficiency, and product quality as compared to a drying process based solely on convection, which can be rate-limited by the thermal conductivity of the material. Optimizing the heat transfer mechanisms in hybrid systems offers a significant opportunity to reduce energy consumption, increase speed/throughput, and improve product quality.

The experts at Mead O'Brien are always available to assist you with any process heating application. Visit http://meadobrien.com or call (800) 892-2769 for more information.

Sunday, May 8, 2016

A Better Alternative to Magmeter for Unpolished Condensate Flow Measurement

Veris Accelabar
Veris Accelabar 
A large University was using magnetic flowmeters (Magmeters) to measure the flow of boiler feed water downstream of a condensate polisher. There were occasional system upsets that required the condensate polisher to be bypassed. When this happened, large amounts of debris would be released downstream and pass directly through the Magmeter, causing a build-up of foreign material on the internal sensing surfaces.

A magnetic flowmeter uses an electric current applied to a coil which produces a magnetic field. When conductive liquid flows through the magnetic field, a small voltage, proportional to the liquid velocity, is induced. As long as the interior surfaces of the Magmeter are clean and unobstructed, the meter accurately measures flow. If they get dirty or coated, all bets are off.

It was in the above mentioned upset situations where the University maintenance people were having problems. When upsets occurred, and the condensate polisher had to be by-passed, it meant the Magmeter would also have to be serviced because accuracy could no longer be guaranteed. Servicing the Magmeter was slow and costly. It meant shutting down the line, draining the pipe, removing the flowmeter, cleaning, and then putting it all back together.

Armstrong International’s Veris Group was called in for a consult. After review,  the Veris Group recommended installing an Accelabar® flow meter to offer an alternative solution that could provide reliable flow measurement regardless of an upset condition like unpolished condensate. The Accelabar provided a flow range of 22.5:1 turndown in flow, in a limited straight run scenario. In the past, two transmitters were required to provide the best accuracy across the entire range of the Accelabar. Veris was able to use the Foxboro IDP 10S with its FoxCal™ technology in order to have a combined percent of rate accuracy solution.

The new transmitter installation has 11 separate calibrations loaded into the device. As the differential pressure from the primary element is measured, the transmitter chooses the correct calibration curve. Veris’ solution delivered performance that was previously unattainable with a single differential pressure transmitter.

The Accelabar and Foxboro combined to be best solution, and the Accelabar flow meter is now the University's standard for the boiler feed water measurements.

For more information, contact:

Mead O'Brien, Inc.
10800 Midwest Industrial Blvd
St Louis,  MO 63132
314-423-5161
314-423-5707
www.meadobrien.com