Level monitoring in power plants

Ensuring that power plants have adequate supplies of coal and lime for No_x scrubbers is vital.
Ravi Jethra of Endress+Hauser explores the challenges facing power plant operators in terms of level measurement together with possible solutions.

Power plant operators are constantly challenged with not just running plants safely and reliably but doing that on a 24h, seven-days-a-week and 365-days-a-year basis. There are numerous issues impacting power plants such as spiralling maintenance costs, emissions regulations requiring compliance that seem to get stricter with each passing year and ensuring minimal downtime. Level measurement is crucial to ensure smooth operation and requires level devices to not only work accurately but also provide a reliable and fail-safe (dual redundant) operation. An example would be to ensure an ammonia tank is not leaking. The level measurement devices would need to be able to work in a hot, turbulent and corrosive environment.

The outsourcing/cost-cutting campaigns have had the effect of engineers in power plants not just wanting to get devices, but someone that can provide the complete package. This would include accessories and application assistance related to start-up/configuration as well as training for the people responsible for maintenance. Accessories could include panel or field displays, power supplies or recorders. It is generally perceived that power plants are a huge source of emissions and a vendor that uses environmentally-friendly packaging materials and energy-efficient solutions is preferred, all else being equal.

From a supply chain management standpoint, it is key to implement solutions that make the life easier such as direct ordering of materials or alerting operations and purchasing.

In a typical power plant there is storage of primarily two types of materials:
1. solids, such as coal, fly ash, limestone, caustic soda etc for different operations within a plant
2. liquids, ie chemicals for feedwater treatment, cleaning boiler tubes, corrosion control, fuel treatment, polymer/sludge dispersant.

Can you imagine a power plant running out of coal or chemicals used to treat boiler feedwater? A situation like that with, say, US$10,000 worth of chemicals could cause a shutdown of a unit where the financial loss itself could be in millions of dollars and would be a public relations disaster. Depending on the size of the plant, a typical plant in US utilises about 20-50 railcars filled with coal per 24h run time. It is critical to monitor levels of these and order “refills” to ensure smooth operation.

Knowing current inventory levels at any time and anywhere in the supply chain helps optimise the storage and supply process. Current technology makes this possible. A fieldgate-type of device can even allow for remote monitoring access of storage/inventory levels. Fieldgate data can be transmitted in web format via Ethernet and a whole host of other methods. Fieldgate devices also enable remote configuration of instruments. Software tools with advanced diagnostics capability allow faults to be flagged immediately.

Level measurement can be divided into the following technologies:
• continuous level – hydrostatic or differential pressure, time of flight such as ultrasonic or radar, capacitance, nuclear (gamma)
• point level – float (electro-mechanical), ultrasonic (gap), thermal dispersion, capacitance, tuning fork (vibration).

The perfect level technology that works for every liquid level and point level application does not exist. Hydrostatic pressure has been around, it seems forever, and is proven in many applications. Non-contact radar and guided wave radar level instrumentation are popular and growing. Capacitance and conductivity have long histories of success in certain applications. However, it is still true that certain level technologies work better than others for some applications.

A good example is the use of lime in scrubbers. In this case, powdered lime is added, which is stored for use. The levelflex continuous TDR (guided wave radar) transmitter is an excellent choice for a reliable measurement. The measurement reliability and accuracy is independent of dust created from filling, large angles of repose, and inconsistencies of the product.

Another example is the storage and use of liquid additives for boiler feed water. It is costly and time-consuming to have this water treated improperly or not at all because of an unexpected outage of these raw materials. The Micropilot utilises a simple menu driven interface for fast and easy commissioning. It is non-contact and the measurement is independent of vapours and changing temperatures or pressures. Most importantly the antenna is completely encapsulated in Teflon, with absolutely no metallic wetted parts.

Levelflex radar

Radar technology in general has been introduced to the process industry as being a measurement technology using high-frequency electromagnetic waves that are not influenced by the gas phase it travels through and the temperature and pressure conditions in process vessels. As processes get more extreme in temperature and pressure it is time to have a closer look at radar behaviour in those critical applications and the solutions available on the market that overcome the obstacles.

Radar devices used today for level measurement operate with electromagnetic radiation at much shorter wavelengths – 1.5-26Ghz – commonly known as microwaves. Non-contact radar and guide wave radar operate using the same principle of microwaves. Non-contact is considered free space (not contacting the media) while guided wave radar does contact the process media because it uses a long wave guide to better direct the microwave energy. Since the speed of light is well-known, the level of the liquid can be measured by measuring the amount of time it takes for the microwaves to travel to the liquid, and reflect back to the radar.

Radar signals

All radar technologies available on the market to measure level use the “time-of -flight” principle. This means that the radar measurement device measures the elapsed time between emitting and receiving a pulse consisting of a bundle of high-frequency electromagnetic waves. Frequency of the waves varies between 1GHz for guided wave devices and 6-26GHz for free space radars.

Speed of radar signals

Radar signals travel at the speed of light when travelling through a vacuum. However, the pressure and temperature of a specific gas phase or liquid has some influence on the speed of radar signals. The amount of influence depends on how polarised those gases are – in other words, how much the dielectric constant changes. Hydrocarbon vapours show little effect even under high-temperature and or high-pressure process conditions. But highly polar steam does impact the speed of radar signals. The dielectric constant of steam at 212°F is 1.005806 and at 691°F it is 3.086.

In a typical steam application the level of the water in a condenser or boiler is of utmost importance. Radar measurement devices offer a great alternative with advanced diagnostics and insensitivity to build up and temperature fluctuations.

Steam is highly-polarised, which means that the speed of radar signals in high-pressure and -temperature steam applications are subject to a reduction in speed. In a boiler, for instance, this leads to a lower water level reading then there actually is. This can be dangerous and influences the performance of boilers and causes a reduction in the quality of steam. The error can easily be as large as 30-40 per cent depending on the pressure and temperature of the steam and distance from the launch of the signal to the actual water level.

Overcoming the effects of process changes on radar speed

The simplest method to overcome this problem is to put a fixed offset in the measurement device, by simply entering the temperature or pressure and having the radar unit calculate the “offset”. The problem with doing that is there will be rather big “errors” during the start-up of an installation. The normal operating conditions have not yet been met and thus the unit will be over compensating. One could also programme a compensation table in a DCS or PLC and connect this to a pressure or temperature transmitter.

The optimal method – “built-in” dynamic compensation

The most accurate method is through the dynamic compensation circuit on a guided wave radar. A reference signal at a known distance is used to compensate for the delay in speed of the radar signal measuring the water level. This is done dynamically – for example, when the reference pulse signal shows a small shift in time, the level signal will be compensated for this small shift. In converse, if the reference signal shows a large shift, then the level signal will be compensated for this large shift.
Free space radar (non-contact) non-contact radar devices use microwaves in the 6-26Ghz range to measure liquid level in tanks. The level can be calculated by knowing the dimensions of the tank and measuring the amount of time it takes for the microwaves to reflect off the process media. Microwaves are used in preference to other wavelengths as they have little effect from type of gases, temperature, pressure, build up and condensate. However, the ability for the process medium to reflect or not reflect microwaves needs to be taken into account.

One can determine this ability to reflect light or microwaves by looking at the dielectric number of the media. The dielectric number is a measure of the polarisation power of an insulating material or how much charge can be stored in a type of material versus air. Water has a dielectric number of 80 and is considered a great reflector of microwaves. Air has a dielectric number of 1 and is considered a poor reflector of microwaves. Aqueous mixtures tend to work well with radar due to the high dielectric number. However, while hydrocarbon based liquids can be measured, the measuring ranges may be lower due to lower dielectrics numbers. Petroleum oil has a dielectric number of 2 while gasoline has a dielectric number between 2 and 3. Ambient conditions have little effect on microwaves, and so radar devices are accepted as the most accurate level devices – some can measure level to +/-0.5mm or +/-0.02 inches. This is one of the main reasons why suppliers, processors and sellers of crude oil and other high cost materials will use a radar device as part of their tank gauging equipment to accurately measure level.

Excellent accuracy and non-contact measurement are not the only benefits of free space radar.
• Non-contact radar devices can be installed in the top of the tank (unlike hydrostatic) and are not affected by liquids with changing dielectric number, conductivity or density.
• When using non-contact radar, you must also take into account the tank shape. Nozzles and other devices protruding into the tank can have some effect on measurement.
• The liquid level should be smooth and foamless. Agitated surfaces and foam can have some effect on level measurements.
• Similar to ultrasonic, some radar devices will experience a blocking distance. However, some other radar designs (pulsed radar) do not experience blocking distances. Blocking distance is when the level gets too close to the radar device. The reflections cannot be received while the radar is emitting microwave pulses.
• Last but not least, the liquid must have a minimum dielectric number. Minimum dielectric for free space installation is generally 2. A radar device with a stilling well or by pass tube however, can be used down to 1.6.

Guided wave radar devices use the same principle as free space radar devices – it has the ability to transmit and receive reflected microwave energy. Guided wave (sometimes called TDR – time domain reflectometry) operates at 1.5GHz. While the electronics are mostly the same as non-contact radar, the big difference is the wave guide. The wave guide is a metal rod or rope which guides the energy to the process media.

This increases the amount of energy directed on the process media, thereby improving the signal to noise ratio. More energy also means being able to work better with lower dielectric liquids as well as applications that might have foam. Just like free space radar, guide wave radar devices are very accurate to +/-2mm and the accuracy is independent of the liquid’s conductivity, density and dielectric number. No re-configuration is needed if changing measuring liquids in the tank. Just like non-contact radar, it’s important to take into account tank design. Agitators are an issue with guided wave because the probe/cable protrudes into the tank. While light foam is not an issue, heavy foam can affect the level measurement. Guided wave radar devices work particularly well in a tank bypass.

Case Study: TDR Level Measurement on Coal Bunker

A coal burning power plant located in the US Midwest with two units that provide approximately 730MW each. The plant used low-sulphur Wyoming coal, between 15,000-30,000tpd. Coal that is conveyed from the yard is buffered in six bunkers prior to the burner. To ensure a continuous production process, these bunkers have to be filled to a certain level at all times. Since the level measurement is critical to maintain 24/7, 365-days-a-year smooth operation, each bunker was equipped with two measurement instruments.

Coal bunker information:
Dimensions: W20ft x D50ft x H45ft;
Chambers: six per bunker

Figure 1: Levelflex TDR in coal bunker

A Levelflex TDR was used to measure the level in coal bunkers to ensure continuous trouble-free operation of the coal-fired power plant. Prior to using the TDR, the plant had tried using ultrasonic transmitters with poor results. These instruments frequently failed in extreme weather situations (for example in hot and humid summers and cold winters) causing the operator to check the contents of the bunkers manually. The ultrasonic instruments did not provide a continuous reading, especially during filling, causing overfill errors. In contrast, the TDR Levelflex provides the operators with a stable and accurate reading even during filling of bunkers. A PC-based software tool (time of flight software) that allows the plant engineers to setup their instruments via HART from the control room was used for field configuration.

Installed instruments configuration:
Levelflex device – ¼” cable diameter, 45ft long, Centring disk, Aluminium electronics housing (FM XP), 1-1/2” NPT process connection, 4-20mA (HART®) electronics with display.
The device was threaded into a flange with a nozzle (length 12”) on top of the concrete ceiling of the bunkers, see Figure 1.
As a result of the superior performance the Engineers replaced the ultrasonic units in the second 730MW unit as well.

Panel display for level reading

Case study: Monitoring level on bulk fly ash bins

The case study related to a coal-fired power plant with two 400MW generating units. Fly ash is a by-product from electrical power generation. Fly ash bins previously used a mechanical plum-bob device which continuously failed due to the cable breaking. The top of the bin is 180ft (55m) from ground level, with only ladders for access. The maintenance group had given up on the mechanical device due to frequent breakage, which forced the operators to climb the bin several times daily to check the level. With the plant being at 7000ft (2133m) elevation, cold, windy and often icy conditions made it dangerous to check levels. Levelflex is used to measure level in fly ash bins. A previously-used electro-mechanical (plum-bob type) device failed in this application.

Fly ash bins are another component of power plants where level monitoring is essential.

Fly ash collected at a coal- fired power plant is different then that collected from a cement plant. Both processes even might burn the same coal, but the temperatures the coal was burned at and also the process of burning itself have an impact on the properties of the fly ash collected after wards. Fly ash collected in installations burning heavy oil or waste have different properties as well.
The properties of fly ash that tend to vary are weight, size of the particles, dielectric constant, temperature and last but not least: how sticky it is. Keep in mind, fly ash can be as fine as talcum powder, yet highly abrasive.

Levelflex radar on fly ash silo

Measurement principles used to measure fly ash

The plant was using mechanical systems, where the specific density of the fly ash determines the accuracy and reliability of the measurement. A weight was lowered on a cable or metal tape into the silo. When the weight touches the level in the silo, the electronics detect the slag in the cable or band, reverses the motor and starts counting the length of the tape which indicated the amount of free space in the silo.

This is a simple and robust way of measuring in fly ash silos, but is costly. It requires relatively high maintenance as the moving, mechanical parts wear out and need frequent replacement, especially in a dusty environment. With the current focus of modern power plants on reducing maintenance costs and cost of ownership, this technology is being rapidly abandoned.

The plant engineers also tried using ultrasonic measurement devices. Although initial results were encouraging they started to have intermittent problems and would never work during the filling operation. This technology also uses the density property of the fly ash. It makes an emitted sound wave reflect to a sensor and the distance to the level is calculated. A benefit of ultrasonic or sonic measurement devices is that they have no moving parts and are in principle maintenance-free. The problem occurs in getting the sound waves though a thick dust of blown in fly ash to the level. Sound is being deflected by the dust particles. So success is entirely dependent on the amount of fly ash pneumatically conveyed, the density of the ash and the shape of the particles. The temperature of the ash could also create a problem as the speed of sound varies. The materials used to build ultrasonic sensors limit the use of this technology in high temperature applications too.

One solution is to install Levelflex radar units which are not affected by fly ash build up on the measuring cable. The Levelflex units have been installed for over two and a half years with great success.

Case study: Lime and Soda Ash storage silos

Levelflex is used to measure level in lime and soda ash silos. The Levelflex replaced competitor’s ultrasonic instruments which failed in this application.

Company profile
The coal fired-power plant has two 400 MW generating units. A water treatment area features four large silos, (2) store bulk lime and (2) store soda ash.

Application problems
The ultrasonic instrument being used failed to perform to meet the requirements. The only access to check the levels in these silos was by climbing silo ladders to the top (110ft).

Solution: Install Levelflex radar units which are not affected by material inside the silos. The bulk lime bins have been fitted with a Levelflex for over three years, and the soda ash bins for over nine months. All installed units are successfully providing level measurement. The water treatment control room was fitted with remote displays, no more climbing to check levels.

Summary

The use of radar signals in high-temperature and high-pressure applications is not as simple as it sounds. Under these conditions the speed of radar signals can change, causing large measuring errors. A Levelflex guided wave radar or free space micropilot radar offers a unique solution to compensate for changing radar signal speeds, offering peace of mind and confidence in the accuracy of the level process measurement. Innovations in technology make it easier to install/commission.
It is worth keeping in mind that the Level monitoring solution provides a flexible inventory control system that is web-based and integrates field instruments, data visualisation and ERP systems for eg, SAP.

Level monitoring software allows for:
a. User management – access rights that can be assigned based on user authorisation
b. Activity logging and reporting – to ensure that all changes are registered and traceable
c. Customisation – simplifies handling and improves accessibility of the system
d. Plant view – enables quick retrieval of information by linking devices to specific site locations, areas etc
e. Document management – allowing manuals, SOPs, etc to be linked to a device
f. XML data from fieldgates can be scanned according to a periodic schedule or at predefined scan times. The XML data can also be received via e-mal for a specific event. Collected data can be stored in a history database, exported to files (such as .csv), OPC server and SQL.

Ravi Jethra is industry manager – Power at Endress+Hauser in Greenwood, IN. He received his Bachelor’s degree from Bombay University in Instrumentation Engineering and an MBA from Arizona State University. Ravi is a Senior member of ISA, and a member of ASME and IEEE. He can be contacted at +1- (317) 535 2147 or via e-mail : ravi.jethra@us.endress.com

For more information on this subject, visit: www.endress.com