The largest single cost of operation of a power plant is fuel, and the amount of fuel burned is affected directly by the efficiency of the plant. A natural gas fired plant with a net output of 500 MW can burn over $100 million in fuel each year, which means a 1% improvement in efficiency can be worth $1 million per year.
As described in our Efficiency blog (March 2018), it is explained that an Efficiency Program can address the issue of efficiency. One common element of effective Efficiency Programs is a computer system for control room operators that is commonly called a Controllable Loss Monitor (CLM).
To understand Controllable Loss Monitor (CLM) systems, consider that, as part of an Efficiency Program, there should be an efficiency assessment, which identifies the following:
The goal of an Efficiency Program is to reduce this gap between best achievable and actual performance to as close to zero as possible, and then to maintain the performance achieved. An analysis of the plant is done to identify the causes for the gap so that the program can be customized to correct each source of inefficiency
It is common that a large fraction of the causes of the difference is hardware problems, such as worn turbine seals and plugged condenser tubes. After all the hardware issues are identified, however, there is a fraction that is due to the plant operation, called controllable losses (as shown in Figure 1).
The issue of controllable losses can only be addressed by training the operators to understand how their actions in operation of the plant affect the performance of the plant. Once the operators have that knowledge, it is important for them to have a tool that can provide them with timely feedback on their success in managing controllable losses. That tool is the CLM. To understand the CLM, it is necessary to understand the concept of a controllable parameter and the information about controllable parameters needed by operators.
A controllable parameter can be defined as a parameter that the operator can control that also has a significant impact on plant efficiency. The loss in plant efficiency due to a controllable parameter is a controllable loss. It is not always obvious what should be considered a controllable parameter.
An excellent example of a controllable parameter for Rankine cycle plants with fired boilers is boiler flue gas oxygen content. The amount of oxygen in the flue gas is an indirect indication of the amount of excess air supplied to the boiler. The excess air has a great influence on the efficiency of the boiler, and also has an impact on other parameters, such as steam temperatures, ash fusion temperature (for coal-fired plants), and NOx production. The operator can control this parameter by adjusting the oxygen trim on the boiler controls.
Another example of a controllable parameter is condenser pressure. Condenser pressure may be considered the most important single parameter for plant performance and, therefore, one of the two criteria for qualification as a controllable parameter is satisfied. Unlike boiler flue gas oxygen, however, there is no control adjustment available in the control room that allows the operator to “trim” the condenser pressure. If this is the case, then how can it be considered a controllable parameter?
The answer is that there are many things under the control of the operator that has a direct influence on the condenser pressure. For instance, the operator determines how many circulating water pumps operate and when to bypass the cooling tower. As another example, the operator has the option of placing an additional vacuum pump in service. These are a few examples of how the operator can influence the performance of the condenser.
An example of a parameter that is not a controllable parameter is turbine section efficiency. Turbine section efficiency certainly has an impact upon the efficiency of the plant, and thus, the first criterion for a controllable parameter is satisfied. The operator has no control over turbine section efficiency, however. The one exception to that statement is mis-operation or problems that result in rubbing which can reduce turbine section efficiency. Once the damage is done, however, the operator cannot restore turbine section efficiency.
Another example of a parameter, or parameters, which is not a controllable parameter, is generator internal temperatures. The internal temperatures of the generator are critical and should be monitored closely. To a certain extent, the operator can control generator temperature; for instance, control generator temperature can be controlled through control of reactive power generated. These temperatures do not, however, have any significant impact on the efficiency of the plant.
For the CLM to be useful to the operator, the operators need three values for each controllable loss:
If the actual value of the controllable parameter is “worse” than the expected value, the result is an increase in heat rate and a corresponding increase in the cost of operating the plant. If, however, the actual value of the controllable parameter is “better” than the expected value, the result is a decrease in heat rate that results in a “negative cost,” or savings, to the plant.
Controllable parameter costs are commonly calculated in dollars per hour ($/hr). It is common to find that even when there is a significant penalty for off-target operation, it may have an hourly cost of $10/hr. This may seem like a small cost for a unit that burns fuel worth over $100 million/yr, but the annual cost of this parameter is over $70,000 (assuming a capacity factor of 80%).
Controllable parameters to be considered in any given plant must be determined based on an evaluation of the type of plant, its configuration, and equipment. The following table is for a 500 MW conventional Rankine cycle unit with the controllable parameters showing the actual and expected values, heat rate difference and cost; this table is similar to those commonly used in a CLM.
|Controllable Loss||Actual||Expected||Heat Rate Diff.||Cost/hr|
|Main Steam Temperature||995.3°F||1000°F||13.1||$15.07|
|Main Steam Pressure||3382.9 psig||3500 psig||13.1||$19.40|
|Reheat Steam Temperature||994.6°F||1000°F||7.1||$10.46|
|Auxiliary Power||52.19 MW||58.43 MW||-67.2||-$93.84|
|Superheater Attemperator Sprays||30 klb/hr||19 klb/hr||43||$63.33|
|Reheater Attemperator Sprays||9 klb/hr||4 klb/hr||-16||-$23.56|
|Condenser Pressure||2.18 in HgA||1.91 in HgA||21.4||$31.59|
|Boiler Flue Gas Excess Oxygen||4.37%||3.54||22.5||$31.41|
|Boiler Exit Gas Temperature||312.2°F||291.2||43.9||$61.33|
|Final Feedwater Temperature||545.3°F||534.1°F||5.4||$8.05|
Simple cycle gas turbine plants have many fewer controllable parameters because they have fewer components, and the nature of the gas turbine is such that there are few operator controllable parameters. For combined cycle plants, the parameters that are specific to fired boilers are eliminated and replaced with parameters associated with gas turbines.
It is important that a complete list of controllable parameters be provided and that the sum of the costs for those parameters is monitored. This is because nearly every parameter in the controllable parameters listing has some effect on every other parameter. Accordingly, the operator may find that optimizing one parameter by bringing its actual value to the target value affects other parameters in such a way that the overall plant efficiency goes down. Without the complete listing of parameters, the operator cannot know the overall effect of his actions on the plant.
Another benefit of having the costs of all of the controllable parameters available to the operator is that those costs can help prioritize action. If for instance, one parameter has a current cost of $100/hr while the costs of the other parameters don’t exceed $10/hr, the operator can quickly see that the parameter with the greatest cost should be addressed first.
The total cost of deviations of controllable parameters from the expected values is given at the bottom of the CLM table. This total can be used by operators to optimize plant efficiency. First, the operator can prioritize actions to improve overall plant efficiency by working on improving the controllable parameters that are costing the most. Secondly, the total can be used to show the effect of “tradeoffs.”
Tradeoffs result from the fact that controllable parameters interact. For instance, suppose that the actual value of Boiler Excess Oxygen is higher than the target value for a coal-fired unit. The operator can reduce the Boiler Excess Oxygen using the oxygen trim control. When this is done, however, other parameters are affected as well. If Main Steam Temperature is low, reducing excess air will generally drop the Main Steam Temperature, but it will also reduce the Auxiliary Power.
To summarize, the Controllable Loss Monitor (CLM) is a powerful tool for the optimization of unit efficiency. It does, however, have limitations.
FCS has long experience in computer systems like CLM from various manufacturers designed for use by operators to address controllable losses. Regardless of the vendor of the CLM, there is a common element required for successful implementation of a CLM: Operator Training. Operators must understand how the actions they take in the day-to-day operation of generating units affects efficiency, and how the CLM can be used to minimize controllable losses. FCS has developed many training programs designed to address this need, which can help assure that there is a return on the investment in a CLM.