Understanding the demand for better energy monitoring and management

Sept. 17, 2014
Impacts of variable speed drives, aero-derivative gas turbines, and thermal monitoring.

Many governments are now requiring plants to track and report their energy use and carbon emissions. In fact, in some countries, heavy energy users should report annually if they emit more than certain levels of CO2 equivalent (CO2e) from different sources such as combustion sources and machinery. For many organizations and companies, it’s a daunting task to reduce CO2e emissions while increasing production capacity. This leads to an energy supply and demand dilemma. Many parts of the world are seeing their peak margin (reserve) disappear with a robust economy and increasing demand for more products and power. In each developed or developing country, average electricity demand is growing up to 5% annually; electricity prices, and generally energy prices, will also rise constantly over the next five years due to different reasons, such as new carbon and environmental policies put in place. All of these lead to a demand for better energy monitoring and management in any plant and facility, offering relevant information to key individuals, engineers, and departments that enable improved energy performance. Energy management solutions should include comprehensive energy production and sustainability services leveraging a common platform.

Opportunities for savings

There are usually multiple opportunities for energy savings and emission reductions across each plant’s operations. An energy study should offer a single-source solution with the right combination of operational details, process design, technology, modeling, and energy and process optimization and control to achieve defined energy goals. Improved monitoring, operation, and control can capture hidden opportunities and identify areas that can be optimized with multivariable predictive control strategies without sacrificing throughput or yield.

Better heat recovery can help to reach higher levels of energy efficiency. A practical methodology that not only considers value and cost of improved heat recovery, but also seeks synergy in enhancing throughput and product quality, should be used.

Advanced process technologies should be considered and involved in a successful energy management exercise. These can offer significant opportunities for improved energy and plant performance. Technologies such as enhanced heat exchangers, high-performance internals, new reaction internals, power recovery turbines, and improved catalysts are part of an effective energy management plan.

Another key area is utilities optimization. In other words, one of the keys to reducing energy costs is balancing changing energy demands from the process with adequate supply from the utilities. Effective energy management should provide solutions to manage the supply/demand equation and optimize machinery and equipment operation such as steam turbine and boiler performance.

Fuel management is also important. In many plants 10-45% of input gases or hydrocarbons are used as fuel. Saving and recovering valuable components in fuel gas system, such as C2 recovery of fuel gases in some petrochemical plants, not only reduces emissions significantly, but also enhances plant performance, such as catalyst utilization, and product yields.

Energy monitoring and management exercises

To optimize energy consumption, the first step is monitoring; compact and powerful monitoring units for recording all consumption data related to energy and power generation, distribution, and consumption can be used. They can be linked to the automation, energy, and power management system.

The next step is “energy evaluation.” For energy management purposes, the energy data should be processed further. There are available energy management packages, and purpose-built or modified packages can be used. For instance, a suitable package should evaluate and display the generation, distribution, and consumption of energy and the associated energy costs of a plant. Thus, energy costs can be divided according to the consumers, permitting identification of high consumers and potential savings. Innovative energy and power management systems can provide the basis for economical and optimized energy management; such a system comprises the recording of energy data, data processing, versatile analysis, and forecasting options, as well as prediction of relevant energy data and parameters.

While energy monitoring and management can reduce energy costs by up 30% in new plants, the use of comprehensive technologies and tools such as energy-efficient electric motors or variable-speed drives can provide energy savings up to 60% in older plants.

Energy monitoring and management workshops, meetings, and one-to-one discussions: Energy workshops to identify new projects and review performance should be planned periodically. Frequent virtual meetings to share best practices and successes between locations can also be very effective if planned properly. An important consideration is, for some teams and groups, HAZOP-type meetings for energy management might not be effective; for instance, some technicians and engineers might not share their impressions or potential improvements in a formal meeting. Often one-to-one meetings can offer better opportunities for some energy management discussions.

Energy monitoring and management in maintenance and operations: This includes better operation of steam systems, insulation, and cooling towers for superior energy efficiency.

New technologies and facilities: Implementation of advanced process and machinery control technologies is an important consideration; waste heat recovery and combined heat and power concepts should be considered.

Machinery thermal monitoring and management

Over the past decade, thermal monitoring and management of machineries have started to receive more attention. In fact, with the increasing requirements for compactness, energy efficiency, cost reduction, lightweight design, and extreme temperatures, along with the need to fully exploit new topologies and materials, it is now necessary to analyze, monitor, and manage the thermal circuit of machinery to the same extent as the other aspects such as fluid dynamic.

A considerable portion of the electrical and mechanical power is eventually converted to heat. In many machines and facilities, the maximum temperatures reached during operation can impact reliability, sometimes forcing users to de-rate or modify thermal arrangement of a facility or machinery. Designers of high-performance machinery or equipment for use in thermally demanding applications require accurate thermal models of the heat-transfer processes. With a thorough understanding of thermal arrangement, designers and operators can take steps to optimize thermal structure and minimize the temperature differences between the machine casing and the environment, or the heat sink, for more efficient and more reliable operation.

For many machines and critical equipment, an accurate temperature distribution is critical for operational and safety reasons — to control the peak temperature for hazardous locations, to predict the fatigue under the heating-up and cooling-down processes, and to calculate the number of consecutive starts. Dimensional changes can occur due to temperature deformations at the machinery elements. Considering tight working fitness of various components in modern rotating machines and complex equipment, it is necessary to take into account the actual variation of the installation and the operation gaps considering thermal arrangement/monitoring.

The thermal lumped-parameter method is used for equipment and machinery thermal monitoring and analysis. The machinery or equipment can be described as a thermal model including many lumped elements, and with the help of linear networks of the heat resistances of each element, neighboring elements generate and store heat. Lumped parameters have been calculated from the size-related information, thermal characteristics of the materials, and various heat transfer coefficients. Detailed features are lumped into models with averaged properties. The temperature at every node in the thermal model can be calculated by solving energy balances. It’s necessary to account properly for the radial heat transfer, heat generation distribution, heat radiation, contact conductance, and surface convection modeling. The convectional heat transfer usually plays an important rule in equipment or machinery thermal analysis. For some rotating machines, the forced convection heat transfer due to the rotor rotation and the convection around the machine ends should be properly considered. Thermal analysis software exists for rotating machines and various mechanical equipment. The users enter geometric data for the design under consideration using the radial and cross-sectional graphical editors.

Entropy balances, based on the second law of thermodynamics, allow for a calculation of the entropy generation or, equivalently, the energy destruction in a piece of equipment. Evaluation of the entropy generation, the energy destruction, and their distribution throughout a rotating-machine or mechanical-equipment thermal system allows for identification of the mechanisms that contribute most to the overall irreversibility and destruction of the useful work. This analysis can provide a diagnostic tool for identification of areas where potential thermal performance improvements can be made. The consequent thermal monitoring can show how effective the improvements are.

Generally high temperature differences between components and fluids — main stream fluid, cooling water system, lubrication oil stream, surrounding environment — can result in the energy destruction. For example, machine arrangements that result in high temperatures at bearing systems can cause a high temperature difference between the lubrication oil and the surfaces in contact with the lubrication oil; a considerable amount of heat can be transferred to the lubrication oil, which can cause lubricant problems, heat waste, requirements for a very expensive lubrication oil system, large lubrication oil pumps, expensive oil coolers, or special lubricant requirements. The same is true for closed-loop water-cooling systems. A considerable amount of energy is also destroyed near the outer housing or casing.

Variable speed drives

Variable speed drives (VSDs) can be used to operate machinery such as pumps and compressors at their optimal speeds based on operating conditions, which can result in energy benefits. Using VSD electric motors seeks to eliminate steam turbines, gas engines, and other traditional drivers in plants. The main benefit would be a better overall efficiency and added productivity because of the elimination of unscheduled and scheduled outages of traditional drivers. For instance, electric motors and compressors in a clean-gas service theoretically do not need maintenance that would require a shutdown for five to eight years. Other benefits associated with electric-motor-driven compressors are derived from better control of variable speed drives and the unlimited number of soft starts. Electric motors are up to one-third less expensive compared to other traditional drivers with same ratings.

Gas turbine energy management and monitoring

Since about 40 years ago, the turbine temperature capability (allowable temperature at turbine first stage) has advanced approximately 10 °C per year, which is corresponding to about a 1.5-2% increase in power output (in the gas turbine size) and around 0.4-0.6% improvements in the simple-cycle efficiency every year on average. The gas turbine business is a dynamic market. The aero-derivative gas turbine is a superior selection for both the mechanical drive and the power generation application.

Typically, the gas-turbine hot-gas exhaust has ample oxygen. The supplementary firing at the waste heat recovery unit could be a feasible option to achieve the maximum possible efficiency. This design is also becoming popular in plants that require better operational flexibility. However, this complex design requires special care. For example, the transfer duct between the gas turbine exhaust and the waste heat recovery unit should be of sufficient length to ensure complete combustion and avoid direct flame contact on the heat transfer surfaces. On the other hand, the duct system length should be optimized to limit the heat loss, the cost, and the footprint.

A gas turbine energy monitoring system usually includes a technique, which is more specific for gas turbines, namely the gas path analysis (GPA). The GPA algorithms should be based on a well-developed gas turbine model and gas path measurements, specific to the gas turbine type and model. In addition to energy and performance monitoring, the GPA provides a deep insight into gas turbine component performances, revealing gradual degradation mechanisms and abrupt faults. This can allow estimating core performances like the shaft power, overall efficiency, specific fuel consumption, and air-compressor surge margin, which are important health indicators. Since modern gas turbines are complex and the amount of data to be interpreted is huge, a precise and fast method should be formulated, case-by-case, considering each gas turbine’s specific details. This is necessary in order to arrive at the best recommendations to monitor the efficiency and energy structure, identify problems, prevent catastrophic failure, and prolong the life of the gas turbine.

An aero-derivative gas turbine consists of two parts, an aircraft-derivative gas generator section and a free-power turbine section. The gas generator is derived from an aircraft engine modified to burn industrial fuels. The aero-derivative gas turbine’s ability to start quickly, shut down fast, and properly cope with load changes, as well as its high efficiency and variable-speed capability, make it a preferable option to the a heavy frame gas turbine. High efficiency is one of the important issues that encourage the use of aero-derivative gas turbines. The gas turbine initial cost runs about 10% of the total lifecycle cost. The operational and maintenance cost is approximately 18% of the total lifecycle cost. The fuel cost is about 72% of the total lifecycle cost, which shows the critical role of the efficiency and consequently energy monitoring and energy management.

Common economic and operational advantages and benefits of the aero-derivative drivers compared to conventional heavy frame gas turbine drivers are:

  • better efficiency (10-15% improvement in efficiency)
  • smooth, controlled start of the complete compressor string (particularly startup of pressurized compressor trains)
  • higher availability and operational reliability
  • wide operating speed range permits economically optimized process control
  • operational flexibility (through aero-derivative core)
  • lower operating expenses due to reduced maintenance and spare part requirements, using modern condition monitoring methods.

One of the disadvantages for aero-derivative gas turbines is their relatively high initial cost. Since aero-derivative machines use advanced technologies and materials, they are expensive compared to heavy industrial gas turbines. Considering the higher efficiency of aero-derivative machines, their total life cost (initial cost + fuel and operation cost) can be much less compared to heavy industrial frame gas turbines. Nearly in all cases, saving in fuel consumption can compensate the cost difference (higher initial cost of an aero-derivative gas turbine compared to a heavy industrial frame machine) in less than one year.

A gas turbine expands when it becomes hot. This leads to axial and radial growth. It is necessary to model various thermal expansions accurately in the gas-turbine-driven train. Special attention should be given to coupling selection and thermal growth. The importance of coupling stiffness to manage axial, lateral and torsional movement should be emphasized.

Over the years, fuel efficiency gains have steadily taken place and continue to do so with special focus on jet engines. A significant savings in fuel has been realized for jet engines based on extensive searches in aircraft and spacecraft industries. Advancements in jet engine applications are being mainly passed on to aero-derivative gas turbines. Extremely high firing temperatures are now possible to give high efficiencies. Market pressures for new thermally efficient and environmentally friendly plants have resulted in more applications of high-performance aero-derivative gas turbines. Efficiency of modern aero-derivative gas turbines are around 41-47%. Traditional heavy frame gas turbines can only offer efficiencies around 29-36%. The temperature of the exhaust hot gases of aero-derivative gas turbines is usually lower, due to better efficiencies. In other words, a larger portion of fuel heat value is converted to useful works. When high fuel costs are expected, the selection of a high-efficiency driver, specifically aero-derivative gas turbines, becomes a strong criterion in the lifecycle cost evaluation.

Amin Almasi is a rotating equipment consultant in Australia. He’s a chartered professional engineer of Engineers Australia (MIEAust CPEng — Mechanical) and IMechE (CEng MIMechE), in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in rotating machines including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, offshore rotating machines, LNG units, condition monitoring, and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, and reliability. Contact him at [email protected].

Aero-derivative gas turbines have several features that facilitate condition-based maintenance, rather than time-based maintenance. Numerous bore-scope ports allow on-station, internal inspections to determine the condition of internal components, thereby increasing the interval between scheduled, periodic removal of gas turbines. When the condition of the internal components of the affected module has deteriorated to such an extent that continued operation is not practical, the maintenance program calls for exchange of that module. Aero-derivative gas turbines are designed to allow for rapid on-site exchange of major modules in a short time. The cool-down time for an aero-derivative machine is much less than that of a heavy-duty frame machine due to the lower overall mass. Any maintenance activity, including washing, can therefore be done with less downtime.

Efficiency comparisons are typically based on the process operating at design conditions. In an actual plant environment, this design point is elusive, and an operator is always trying to attain a sweet spot where it will operate at its peak performance under prevailing conditions. For instance, as the ambient temperature changes during the day, affecting the performance of the gas turbines, process fluid, equipment, and air coolers, the operator needs to continually adjust plant parameters to achieve an optimal performance. A plant that uses aero-derivative gas turbines, because of the variable speed capability and operation flexibility, allows an operator to continually achieve this optimal performance, which improves the plant’s overall thermal efficiency and lifecycle costs.

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