Accurate and reliable level measurement in high pressure and high temperature steam applications, such as feedwater heaters, can contribute to an improved plant heat rate, leading to optimized processes and increased profitability, writes Patricia Mattsson

A higher feedwater temperature results in reduced fuel use

Credit: Emerson Process Management

Patricia Mattsson

Heat rate is a measure of plant efficiency and is calculated by measuring the combined performance of all auxiliaries, specifically the measure of energy (fuel) put into a system, divided by the electricity generated.

The Rosemount 5300

Credit: Emerson Process Management

Heat rate is the inverse of efficiency, so a lower heat rate is desirable and most plants have a target heat rate that they want to achieve. The difference between the actual figure and the target is known as the heat rate deviation. These deviations are often small, but represent real opportunities for improvement, as can be seen when the financial impact of changes in heat rate on annual fuel costs is revealed. For example, a typical power plant may have a boiler efficiency of 85 per cent, fuel costs at $2 per million Btu and a plant capacity factor at 80 per cent.

Given such circumstances, a 500 MW coal-fired power plant operating for a full year could reduce its fuel cost by more than $8000 for each unit of heat rate reduction.

Feedwater heaters

The performance of the feedwater heaters is critical to the thermodynamic efficiency of a plant and its heat rate. A higher feedwater temperature results in reduced fuel use to create steam in the boiler. In a report from the Electric Power Research Institute, it was found that by improving feedwater heater monitoring, the average gain in heat rate was 30-60 Btu/kWh, leading to annual fuel savings from $240,000 to $500,000.

There are many factors that can cause the feedwater temperature to be maintained at a sub-optimal rate. These include tube fouling, plugged tubes, inadequate venting and tube leakage. The feedwater level inside the feedwater heater has a significant impact on the feedwater temperature and must be accurately monitored and controlled.

If the level is too high, the feedwater heater condensing zone decreases and the tubes that should condense the steam instead sub-cool the condensate. This can lead to turbine water induction, potentially causing substantial damage.

Operating with the level too low also creates a risk. Here, the drain cooler will be exposed to high temperature steam, which causes condensate to flash to steam. This can damage the heater, and increase downtime and maintenance costs. When the level is too low this reduces the heat transfer due to the mix of steam and water blowing through the heater.

Should feedwater temperature decrease, an operator will respond in one of two ways. If the feedwater level is too high or the heater is out of service, the operator will probably over-fire the boiler to increase the temperature. This increases fuel consumption, emissions and gas temperature exiting the furnace. It also increases the reheat and superheat spray (used for live steam temperature control), resulting in steam which is too hot.

As steam flow in the turbines increases, this can cause damage to the drain cooler section and the tubes. Alternatively, the operator will likely open emergency drains to reduce the level. This will reduce plant efficiency and potentially cause substantial damage due to water induction into the turbine.

Guided wave radar

Accurate level is critical for safe operations in high pressure saturated steam applications and reliable measurement helps to optimize process performance. However, these applications are challenging to measure. Varying temperatures and pressures, especially during startups and media density changes, can create measurement errors of as much as 30 per cent.

Density-based measurement devices must compensate for these changes to provide accurate level readings. Special algorithms within the control system can account for density changes, but the operating pressure must also be known to be able to provide accurate results.

Because it directly measures the surface of the liquid, guided wave radar (GWR) technology provides a level measurement that is completely independent of density, thus eliminating the need for compensation relating to density changes. GWR has the added advantage over other techniques of having a robust design and no moving parts, increasing reliability and minimizing maintenance requirements.

However, it is important to understand that the dielectric properties of the feedwater will alter during the phase change from liquid to steam. For example, when steam is under high pressure its dielectric increases. This affects the use of radar level technology as the propagation of the microwaves used to perform level measurements will slow down. This can create a measurement error of up to 20 per cent if the level reading is not compensated.

GWR are able to compensate for the vapour dielectric using static vapour compensation, in which the expected operating pressure and temperatures are manually entered when the transmitter is configured and the dielectric changes are compensated for.

The enhanced functionality of Emerson’s Rosemount 5300 GWR means that it is also able to compensate for the vapour dielectric in steam applications by using dynamic vapour compensation (DVC).

This provides a better solution as it continuously compensates for changes in the dielectric constant in the vapour space. The compensation occurs in the transmitter electronics and a corrected level measurement is provided to the control system. No additional compensation is required.

Reducing measurement errors

Error rates can be high when measuring level in high pressure and high temperature applications. With DVC, error rates can be cut to 2 per cent or less. Greater accuracy helps to optimize the process, which has a direct impact on plant profitability. By recognizing the importance of level measurement and using the right technology to achieve accurate and reliable measurements, significant savings can be made.

A good example of how DVC can help to improve the reliability of level measurement occurred at a natural gas combined-cycle power plant in the US. The location of the plant regularly suffers sub-freezing temperatures during the winter months, which creates challenges in terms of its level measurement instrumentation. The primary level measurement was differential pressure transmitters with impulse tubing (wet legs). During winter months, frozen wet legs caused errors in the indicated levels, resulting in unit trips. To alleviate the problem, the impulse tubes were insulated and heat-traced, but they continued to freeze.

Accurate level is critical for safe operations

Credit: Emerson Process Management

Rosemount 5300 chambers

Credit: Emerson Process Management

As a result, the gas site systems engineer decided to investigate alternative technologies to improve the reliability of the level instrumentation. With support from Emerson experts, Rosemount 5300 GWR with DVC were installed, along with Rosemount 9901 bypass chambers. This solution provided fully compensated level measurement, independent of pressure and temperature, resulting in accurate and reliable level readings during all startup and shutdown conditions, regardless of the weather.

Emerson technicians performed on-site startup of the GWR and provided formal training for the site operators to ensure confidence in the new product. Since installation, the solution has proved to be extremely reliable.

By optimizing feedwater heater processes it is possible to reduce the heat rate of a power plant, creating significant fuel and emissions savings and helping to achieve greater profitability. The use of advanced GWR with dynamic vapour compensation can play a vital role by providing accurate and reliable level measurement in these challenging applications.

Patricia Mattsson is Marketing Engineer at Emerson Process Management. For more information, please see EmersonProcess.com/Rosemount/Guided-Wave-Radar/5300