By William C. Livoti, Baldor Electric Co.; Sean McCandless, Colfax Corp.; and Ray Poltorak, EagleBurgmann Industries
The most cost-effective approach to addressing our power generation energy crisis is energy conservation. Improving the efficiency of energy systems will play a major role in solving the energy and environmental problems of the future.
Energy efficiency is the least expensive approach to our energy shortage at the power generation and distribution level. The cost of efficiency programs averages about 2 to 3 cents per avoided kilowatt hour, roughly one-fifth the cost of electricity generated from new nuclear, coal and natural gas-fired plants, and energy efficiency does not require new power lines or generate greenhouse gas emissions or radioactive waste.
Serious energy efficiency is not a one-shot deal where you pick the low-hanging fruit and you’re done. In fact, the “lessons learned” can be passed on to other plants while the energy savings continue to reap benefits. Furthermore, the efficiency resource never gets exhausted because technological advances continue to spread throughout the industry.
Since the first modern coal-fired power plant was placed on-line in the 1890s, there has been little improvement in overall plant efficiency. Yes, coal consumption per kilowatt has improved slightly over the last 100 years, but not on the magnitude one would expect when compared to other industries and technologies.
Today’s coal-fired power plants convert electricity at approximately 33 percent efficiency, wasting 66 percent of every unit of fuel. That’s the same efficiency we had back in the 1950s.
While efficiency hasn’t increased, the cost of coal has. Coal, which accounts for almost half of all the power produced in the United States today, has risen 20 percent in delivered price in the last two years alone. In some areas, prices have increased 200 percent since 1996.
Coal may be the nation’s major fuel for electric power generation, but natural gas is the fastest growing. More than 90 percent of the power plants built in the next 20 years will most likely be fueled by natural gas. Natural gas is also likely to be the primary fuel for distributed power generators. Fuel price increases affect the cost of power purchased by utilities and the price of purchased spot power has increased between 200 percent and 300 percent in many power markets across the U.S. Add the ever-increasing cost of new plant construction and the inevitable battle with special interest groups, and the most cost-effective approach to addressing our power generation energy crisis is energy conservation.
Here is our “current reality” checklist:
- Moving ahead, there is going to be a renewed emphasis on efficiency by utilities.
- The forecasted cost associated with greenhouse gas emissions and carbon sequestration will be astronomical.
- Plant efficiency is the key. There are no other options available to meet the future needs of this country in the required timeframe.
- The most economical option is to defer the expense as long as possible. This, however, is only available with “energy efficient” choices.
- We will see a renewed emphasis at utilities on marketing and demand side energy programs.
- On the generation side, the role of “asset optimization” will play a key role. A major push toward maximizing overall plant efficiency is now a priority in order to reduce new plant construction by maximizing the efficiency of existing units.
If the average efficiency of a coal-fired power plant is now 33 percent and target efficiency should exceed the 53 percent mark by 2020, according to Department of Energy research, there is indeed room for improvementbut only if we embrace new technology.
To improve efficiency, there must be a rapid acceptance and transfer of technology. This is difficult for an industry that lives and dies by uptime, availability and reliability. In order to survive into the next decade and beyond, the industry must use ultramodern technologies when building or upgrading power plant pumping systems, thereby using fuels more efficiently so that less damage is done to the climate and our precious resources are not used up.
Gaining Pump System Efficiency
The most important indicator of the energy efficiency at a power plant is its electrical efficiency. For coal-fired power plants, improvements in efficiency depend primarily on two variables:
- Increasing the two steam parameters, pressure and temperature
- Reducing losses in the steam water cycle
Although there isn’t a great deal that can be done with pumping systems to improve steam parameters, we can have a significant impact on the steam water cycle, which comprises the boiler feed, condensate, heater drain, circulating water, closed cooling water and open cooling water. In this cyclethe Rankine cyclethe steam is produced in the boiler and then taken to the steam turbine. From the turbine, the steam is cooled back to water in the condenser. The resulting water is fed back into the boiler and the cycle is repeated all over again.
Where can we find energy improvements? Following are just a few areas where the utility can gain efficiency and, typically, reliabilitybecause if the system is efficient, it should be more reliable.
Taking into consideration that most fossil power plants in North America are more than 40 years old, there is plenty of opportunity to incorporate energy efficient solutions as motors and other critical equipment are replaced.
It is not good business practice to indiscriminately replace standard efficient motors with premium, efficient ones, unless they have failed. However, it does make good financial sense to replace 40-year-old motors that have been rewound several times and have lost a few points of efficiency over the years. To justify the change-out, simply “dyno” test the motor (a motor repair shop typically has this capability) and compare its performance to original design data. Consider the additional auxiliary power drain due to environmental controls such as flue gas desulphurization and selective catalyst reduction and it is fairly easy to justify the cost.
Positive Displacement Pumps
Pump users can contribute to their bottom line by using the most efficient technology to move liquid products. In a power plant, positive displacement pumps can typically be found in these applications:
- General lubrication of rotating machinery (turbines, engines, pumps, and so on)
- Seal oil supply to hydrogen-cooled generators
- Low pressure fuel oil unloading, transfer and forwarding in tank farms
- Fluid coupling circuit oil for boiler feed water pump speed control
- High pressure burner services in coal or oil-fired steam boilers and to combustion gas turbines
- Hydraulic governor service to control water flow to hydroelectric generating units
- Underground forced-oil cooling of high power cables using high dielectric strength oil
- Transformer oil cleaning.
Today’s three-screw high performance pumps can deliver liquids to pressures above 4,500 psi (310 bar) and flows to 3,300 gpm (750 m3/h) with long-term reliability and excellent efficiency. Power levels to 1,000 hp (745 kW) and higher are available. Twin-screw pumps are available for flow rates to 18,000 gpm (4,000 m3/h) and pressures to 1,450 psi (100 bar) and can handle corrosive or easily stained materials, again at good efficiencies. Power ranges to 1,500 hp (1,100 kW) are available on critical applications.
Compared with centrifugal pumps, multiple-screw pumps provide remarkably good operating efficiencies when handling viscous liquids such as heavy crude oil, bunker or residual fuel oils, low sulfur fuels and distillate fuels. The high efficiency performance is a clear advantage over centrifugal pumps where liquid viscosity exceeds 100 SSU (20 centistokes). (See Figure 1, page 46.)
For pressures requiring two or more stages in a centrifugal pump, multiple-screw pumps will frequently be very competitive on a first-cost basis as well. Not only are annual power savings substantial, but the initial driver, starter and cabling costs will also be lower for multiple-screw pumps.
The typical performance curve of multiple-screw pumps shows excellent efficiency over a broad range of discharge pressures. Properly sized, multiple-screw pumps also handle dual liquid fuels such as distillate and residual oil using the same pumps and drivers, which can provide a utility with some diversification of available oil supplies. Combustion gas turbines can be started and stopped using light distillate fuel while running on less costly (and higher heat value) heavy fuels.
Most plants ignore the potential energy savingsand reliabilitypossible in a positive displacement and control system because the positive displacement pump is assumed to have a higher efficiency rating than a positive displacement pump typically found in a low horsepower application. Positive displacement pumps above 25 horsepower warrant attention if the bypass valve is always lifting, a clear indication the pump is oversized for the system requirements. Consider the losses across the bypass valve as well as the additional maintenance for it.
A positive displacement pump, unlike a centrifugal pump, will produce the same flow at a given RPM, regardless of the discharge pressure. A positive displacement pump cannot be operated against a closed valve on the discharge side of the pump because it does not have a shut-off head like a centrifugal pump does. If a positive displacement pump is allowed to operate against a closed discharge valve without pressure relief, it will continue to produce flow that will increase the pressure in the discharge line until the line bursts, the pump is severely damaged or both.
The horsepower required to operate a positive displacement pump depends on several factors: the liquid or hydraulic horsepower necessary to overcome the mechanical friction drag of all the moving parts within the pump and the viscous losses from the fluid viscous-drag effects against the internal pump parts plus the shearing action of the fluid itself.
The liquid horsepower (Pw) is the power imparted to the liquid by the pump:
Pw = Q (gpm) x Δpressure (psid) / 1714.
Therefore the pump input power is defined as:
Pp = Pw + mechanical loss + viscous loss
If the pump is sized correctly for the system, the operating point will intersect with the system curve and the pump design point. Unfortunately, this is not always the case (Figure 2, page 48). There are several ways to control the excess pressure: discharge throttling, suction throttling, recycle control and pump speed control.
Discharge throttling rotates the system curve counterclockwise so that the modified system curve intersects the pump curve higher up (Figure 3, above). The additional pressure is dropped through the valve so the pressure and flow to the process is almost exactly the same as before.
Suction throttling has the same effect on the characteristic curve as discharge throttling, but this is not a good idea since positive displacement pumps have a net positive inlet pressure requirement (NPIPR) similar to net positive suction head (NPSH) for centrifugal pumps. In fact, their requirements are more stringent. Therefore, restrictions and pressure drops in the suction lines must be similarly avoided.
With recycle control, the valve is installed in a line from the discharge and led back to the source of the pumpage. The system curve is rotated clockwise around its intersection with the pressure axis, an efficient method of control for positive displacement pumps. Since the flow rate is essentially constant, the power requirement is essentially proportional to the discharge pressure. The effect of recycle is to drop the discharge pressure, resulting in significant reduction in power requirements (Figure 4, page 52). Nevertheless, there is still wasted power proportional to discharge pressure times recycle flow.
By controlling the speed of the pump, the pressure can be controlled by sliding up and down the system curve. Any point on the system curve can, in theory, be reached. The system will still require a recycle valve to compensate for a severe pressure rise should a valve close suddenly.
In the final analysis, the most practical solution is to control the speed of the pump. When all factors are taken into consideration (valve damage, wasted energy, pump reliability, oversized motor and oversized pump) speed control makes the most sense.
Centrifugal pumps have the greatest potential for energy savings simply because there are more of them. Centrifugal pumps should be selected for and normally operated at or near the manufacturer’s design-rated conditions of head and flow. This is usually at the best efficiency point (BEP).
Pump impeller vane angles and the size and shape of the internal liquid flow passages are fixed and can only be designed for one point of optimum operation. For any other flow conditions, these angles and liquid channels are either too large or too small. Any pump operated at excess capacity (that is, at a flow significantly greater than BEP and at a lower head), will surge and vibrate, creating potential bearing and shaft seal problems as well as requiring excessive power. When operation is at reduced capacity (that is, at a flow significantly less than BEP and at a higher head), the fixed vane angles will cause eddy flows within the impeller, casing and between the wear rings. The radial thrust on the rotor will increase, causing higher shaft stresses, increased shaft deflection and potential bearing and mechanical seal problems. Radial vibration and shaft axial movement will also increase. Continued operation in this mode will result in the accelerated deterioration of the mechanical and hydraulic performance and may ultimately result in the failure of the pump.
As is the case in most pumping applications, the pump and/or motor (or both) are typically oversized to the system. This is not unusual when you take into account the “fudge factor” added to the sizing process, including:
- Poor or nonexistent specifications from the end user
- Lack of a systems approach during the design process (EPC, end user, integrator, OEM)
- Pumping systems supplied by system integrators (cooling towers, filtration systems, boilers, and so on)
- Overly conservative or improper pump selection, resulting in poor performance
- Improper installation or operation
- Poor maintenance
- Change of system requirements over time.
Sealing systems that feature maximum operational reliability, convenient maintenance, low leakage rates and necessary environmental protective measures are standard equipment in today’s power plants. The power consumption of the seal will affect the efficiency of the pump and the overall efficiency of the plant. Within the seal chamber, the power consumed or produced by the seal consists of three elements: seal-generated heat, heat soak and turbulence. This includes considerations for torque consumed by the seal at startup as well as running conditions, coefficients of friction based on tribological seal face material selection for mating pairs, and the fluid being sealed.
Unfortunately, the power utility has the task of maintaining and absorbing the excess energy costs of this equipment for the next 40-plus years.
Identifying Opportunities for Improvement
Here is a short list of pumping system “symptoms” to look for that could indicate opportunities for improvement:
- Systems controlled by throttle valves/dampers
- Recirculation line normally open
- Cavitation noise at valve, piping or pump
- Systems with multiple parallel pumps with the same number of pumps always operating (condensate)
- Constant pump operation in a batch environment or frequent-cycle batch operation in a continuous process
- Systems that have undergone a change in function, like increased plant load or load cycling
- High system maintenance“bad actors”
- Motor tripping-out.
Proper specifications, equipment selection, process control, installation and operation will dramatically impact the profitability of electric power generation. Energy efficiency and life-cycle costing will be critical factors in the success of the power generation industry.
For additional information on energy savings, life-cycle cost and system reliability, refer to “Optimizing Pumping Systems, A Guide for Improved Energy Efficiency, Reliability and Profitability,” by the Hydraulic Institute and Pump Systems Matter, at www.Pumps.org or www.PumpSystemsMatter.org.