Guided wave radar transmitters with dynamic vapour compensation functionality are able to provide accurate and reliable level measurements in challenging high-pressure saturated steam applications. The ability to carry out such measurements plays a vital role in helping reduce a plant’s heat rate, contributing to increased profitability. By Denny Nelson, Emerson Automation Solutions

---

Most fossil fired power plants have a specified target efficiency (%) or heat rate (Btu/kWh), with the difference between the actual heat rate and the target referred to as the heat rate deviation (HRD). Although the HRD is often only small, understanding how improvements can be made to reduce it presents real opportunities for process optimisation and cost reductions, leading to increased profitability.

Because fuel expenditure constitutes the major part of a fossil fired power plant’s total operating budget, even a small improvement in heat rate will have a positive impact on plant profitability. The following formula helps in understanding the financial impact of heat rate reduction, by translating it into the effect it has on annual fuel costs:

HRD/BE x FC x CF x UGC x T = change in annual fuel costs (dollars per year).

where BE is boiler efficiency, FC is fuel cost ($ per million Btu), CF is plant capacity factor, UGC is unit gross capacity (kW), and T is operating hours per year.

A typical coal-fired power plant may have a boiler efficiency of 85% fuel costs of $2 per million Btu, and a plant capacity factor of 80%. Under these circumstances, a 500 MW coal-fired power plant operating for a full year could reduce its fuel costs by $8245 for each unit of heat rate reduction (1/0.85 x 2/1000000 x 0.80 x 500000 x 24 x 365 = $8245/year).

By lowering its heat rate, a plant will also reduce its emissions, and by doing so will have a lower operating cost associated with air quality control systems.

Feedwater heaters

Sometimes, only small investments are needed to improve plant heat rate and reduce costs, and feedwater heaters are an area where such investments can provide a quick return. The performance of feedwater heaters is critical to the thermodynamic efficiency and heat rate of a power plant. Feedwater heaters are designed with a target temperature for optimal performance. When the temperature is lower than it should be, this will result in an increased heat rate and reduced plant efficiency. The feedwater level inside the heater has a significant effect on the temperature, so it is crucial that the level is accurately monitored and controlled.

When operators are alerted to the feedwater level being too high, they would typically respond in one of two ways to lower the level. The first would be to over-fire the boiler to increase the temperature. This will increase fuel consumption and emissions. It will also increase the gas temperature exiting the furnace, which in turn increases the reheat and superheat spray (used for live steam temperature control), resulting in steam that is too hot. This steam increases the steam flow in the turbines and can cause damage to the drain cooler section and can possibly also cause thermal damage to the tubes. The second typical response to lower the level would be opening the emergency drains. However, this will cause an immediate reduction in plant efficiency and can possibly cause damage due to water induction into the turbine.

If the feedwater level is genuinely too high, these operator responses are unavoidable. However, if the level measurement is erroneous there will be occasions when such responses, and their negative impact on plant performance, could have been avoided by employing more accurate and reliable measurement technology.

In a report from the Electric Power Research Institute, it was found that improved feedwater heater monitoring led to an average heat rate improvement of 30-60 Btu/kWh and an annual fuel cost reduction of between $240 000 and $500 000. As well as helping to reduce costs, accurate and reliable level measurement is also vital from a safety perspective.

If the level gets too high, the condensing zone of the feedwater heater is decreased, and the tubes that should condense the steam will instead sub-cool the condensate. This can lead to turbine water induction, which can result in water droplets being sent to the turbine, potentially causing substantial damage. Operating with the level too low also creates risks. When this happens, the drain cooler can become exposed to high temperature steam, causing the condensate to flash to steam. This can damage the heater, resulting in increased downtime and higher maintenance costs. When the level is too low this also reduces the heat transfer, due to the mix of steam and water blowing through the heater.

Because of these financial and safety implications, it is recommended that multiple devices should be used to provide redundancy in level measurement. According to the American Society of Mechanical Engineers, at least two separate level control loops should be used on the feedwater heater.

Level measurement challenges

Accurate level measurement and control is critical not only for feedwater heaters, but also for boiler drums, deaerators and steam separators. However, these high-pressure saturated steam applications are challenging for level measurement technologies. It is common during start-up to experience varying temperature and pressure. Both the liquid and steam phases will undergo density changes as the system reaches its operating temperature and pressure, and this can create measurement errors of as much as 30% for temperatures up to 600°F (315°C).

Density-based level measurement technologies, such as displacers or differential pressure transmitters, must compensate for these changes to discern the actual level from the density-associated errors. Algorithms have been developed that enable modern control systems to make this compensation. However, the compensation procedure is complex and can be slow, which can result in erroneous readings.

Because they are completely independent of density, devices based on guided wave radar (GWR) technology do not require any compensation relating to density changes. GWR has the added advantage over other technologies of having no moving parts that can freeze, stick or cause noisy signals due to vibration. This increases their stability and minimises the need for maintenance.

Dynamic vapour compensation

When applying radar technology, it is important to understand that the dielectric properties of the feedwater will change in both the liquid and steam phases. For example, when steam is under high pressure and temperature, its dielectric constant increases. This variability affects radar level measurement technology because the propagation speed of the radar signal used to perform level measurements will decrease. This can then create a measurement error of up to 20% if there is no compensation. 

Devices based on GWR technology can compensate for dielectric changes by using static vapour compensation, in which the expected operating pressure and temperature are manually entered during configuration of the transmitter. However, GWR transmitters offering dynamic vapour compensation (DVC) simplify the process of compensating for dielectric changes, which gives devices offering this technology a significant advantage in high-pressure saturated steam applications.

DVC works by using a target at a fixed distance to measure the vapour dielectric continuously. The transmitter knows where the target is and will expect a corresponding pulse at this location when there is no vapour present. When vapour is present, the pulse appears to move further away. The transmitter determines the difference between where the pulse should occur and where it actually occurs, to calculate the dielectric constant of the vapour space. This calculation is performed within the transmitter, eliminating the need to perform any compensation in the control system. The continuous onboard compensation is always performed in the same way, making it more accurate and repeatable.

Error rates can be high when measuring level in high pressure and high temperature applications. However, DVC provides constant, reliable measurement up to pressures of 5000 psi and at temperatures between -320°F and 752°F, reducing error rates to 2% or less. This increased accuracy helps to optimise the process, which has a direct impact on plant profitability.

By recognising the importance of level measurement and using the right technology to achieve accurate and reliable measurements, significant savings can be made. 

Case study: DVC helps plant achieve reliable level control

Let’s look at an example of how DVC helped plant owners to improve the accuracy and reliability of level measurement monitoring and control at a US natural gas combined cycle power plant (installed capacity of over 900 MW).

During the winter months, the area where the plant is located regularly experiences sub-freezing temperatures, and this creates challenges for level measurement instrumentation. The primary level measurement technology for the plant’s boiler drums was differential pressure transmitters with impulse tubing (wet legs). During the harsh winter months, frozen wet legs were causing errors in the indicated levels, which resulted in unit trips. The impulse tubes were insulated and heat-traced to try to alleviate the problem, but they continued to freeze.

As a result, the gas site systems engineer decided to investigate alternative technologies to improve the reliability of the boiler drum level measurement. With support from Emerson experts, RosemountTM 5300 GWR level transmitters (Emerson.com/Rosemount5300) with DVC were installed, along with Rosemount chambers. This solution provided fully compensated level measurement, independent of pressure and temperature, resulting in accurate and reliable level readings during all start-up and shutdown conditions, regardless of the weather.

The plant used a two-out-of-three voting system with three redundant radar installations. Emerson technicians performed on-site start-up of the transmitters, as well as providing formal training for the site operators, thereby ensuring confidence in the new product. Since the transmitters were installed, level measurement accuracy has increased and the devices have proved to be extremely reliable in all weather conditions, helping to reduce maintenance requirements, improve the plant heat rate, and increase profitability.

New mobile app for configuring radar level transmitters

A new mobile is app is now available for configuring RosemountTM radar level tansmitters. The Radar Master app, for the AMS TrexTM Device Communicator, makes it easier for technicians to carry out the configuration in the field.

Dynamic graphics and an intuitive touchscreen interface in the Radar Master app help ensure that users can more quickly configure tank measurement devices with the correct settings. Tank geometry is drawn to match the physical parameters of the actual tank, allowing technicians to more easily tune measurement settings to prevent false echoes from static objects in the tank such as ladder rungs, agitator blades, and baffles, which can delay the process of obtaining accurate measurements.

Users can track, view, and trend tank level and alert information to identify issues and aid troubleshooting by using the new built-in historian. Technicians can also view and compare snapshots of past configuration data in a timeline, giving better insight to operational impact of changes.