Pumps | Energy Conservation in Plants
Pumps can be classified according to its theory of operation to positive displacement and reciprocating pumps.
- Vane pumps
- Gear Pumps
- Screw Pumps
- Hydrodynamic Pumps
- Centrifugal pumps or redial flow pumps
- Mixed pumps
- Axial flow pumps
Positive displacement pumps are suitable for low flow rate high pressure applications, while hydrodynamic pumps are more suitable for high volume flow rate and medium to low pressure applications.Pumps can also be classified according to type of fluid being pumped:
- Newtonian fluid pumps (i.e., water, fuels,…etc.)
- Non Newtonian fluid pumps (i.e., molten plastics,….etc.)
Pumps can be single stage for low and medium pressure applications or multi-stages for high pressure applications. Pumps are normally driven by electric motors, but for high capacity pumps or for certain applications pumps could be driven by a diesel/gas engine, gas turbine or steam turbine.
Pump performance is generally described in terms of:
- Rate of flow or capacity “Q” expressed in units of volume per unit of time (ft³/sec, gal/min, m³/hr)
- Increase in energy content in fluid pumped or Head “H” expressed in units of ft or m.
- Input power “P” expressed in units of work per unit time such as kW or hp.
- Speed expressed in rpm
Centrifugal pump represents the most commonly used pumps due to their wide range of applications and design characteristics. Both of the head and input power as well as efficiency are changed due to the change in the pump flow rate. Specific speed is of interest to both designer and users.
All geometrically similar pumps regardless of their size will have identical specific speed but all pumps of the same specific speed are not necessarily of similar geometry.
Within reasonable limits, pump geometry and performance can be predicted as a function of Ns and Q. (speed and flowrate)
Pump flow control can be achieved by:
- By pass
- Speed control
Speed control is the most efficient technique where it reduces the system friction head.
The pump capacity should never be reduced to zero because the fluid within the pump would have to absorb the entire power output and therefore, would heat up rapidly with damaging effects on the pump.
In the petroleum refining industry for instance, about 59% of all electricity use in motors is for pumps. This equals 48% of the total electrical energy in refineries, making pumps the single largest electricity user in a refinery. Pumps are used throughout the entire plant to generate a pressure and move liquids. Studies have shown that over 20% of the energy consumed by these systems could be saved through equipment or control system changes.
It is important to note that initial costs are only a fraction of the life cycle costs of a pump system. Energy costs, and sometimes operations and maintenance costs, are much more important in the lifetime costs of a pump system. In general, for a pump system with a lifetime of 20 years, the initial capital costs of the pump and motor make up merely 2.5% of the total costs. Depending on the pump application, energy costs may make up about 95% of the lifetime costs of the pump. Hence, the initial choice of a pump system should be highly dependent on energy cost considerations rather than on initial costs. Optimization of the design of a new pumping system should focus on optimizing the lifecycle costs.
Pumping systems consist of a pump, a driver, pipe installation, and controls (such as adjustable speed drives or throttles) and are a part of the overall motor system, discussed above in the Motors Section. Using “system approach” on the entire motor system (pumps, compressors, motors and fans) in the coming part, the pumping systems are addressed; for optimal savings and performance, it is recommended that the systems approach incorporating pumps, compressors, motors and fans be used.
Operations and Maintenance of Pumps
Inadequate maintenance at times lowers pump system efficiency, causes pumps to wear out more quickly and increases costs. Better maintenance will reduce these problems and save energy. Proper maintenance includes the following:
- Replacement of worn impellers, especially in caustic or semi-solid applications.
- Bearing inspection and repair.
- Bearing lubrication replacement, once annually or semiannually.
- Inspection and replacement of packing seals. Allowable leakage from packing seals is usually between two and sixty drops per minute.
- Inspection and replacement of mechanical seals. Allowable leakage is typically one to four drops per minute.
Wear ring and impeller replacement. Pump efficiency degrades from 1 to 6 points for impellers less than the maximum diameter and with increased wear ring clearances.
Pump/motor alignment check.
Monitoring of Pumps
Monitoring in conjunction with operations and maintenance can be used to detect problems and determine solutions to create a more efficient system. Monitoring can determine clearances that need be adjusted, indicate blockage, impeller damage, inadequate suction, operation outside preferences, clogged or gas-filled pumps or pipes, or worn out pumps. Monitoring should include:
- Wear monitoring
- Vibration analyses
- Pressure and flow monitoring
- Current or power monitoring
- Differential head and temperature rise across the pump (also known as thermodynamic monitoring)
- Distribution system inspection for scaling or contaminant build-up
Holding tanks can be used to equalize the flow over the production cycle, enhancing energy efficiency and potentially reducing the need to add pump capacity. In addition, bypass loops and other unnecessary flows should be eliminated. Energy savings may be as high as 5-10% for each of these steps. Total head requirements can also be reduced by lowering process static pressure, minimizing elevation rise from suction tank to discharge tank, reducing static elevation change by use of siphons, and lowering spray nozzle velocities.
More Efficient Pump
Pump efficiency may degrade 10 to 25% in its lifetime. Newer pumps are 2 to 5% more efficient. However, industry experts claim the problem is not necessarily the age of the pump but that the process has changed and the pump does not match the operation. Replacing a pump with a new efficient one saves between 2 to 10% of its energy consumption. Higher efficiency motors have also been shown to increase the efficiency of the pump system 2 to 5%.
A number of pumps are available for specific pressure head and flow rate capacity requirements. Choosing the right pump often saves both in operating costs and in capital costs (of purchasing another pump). For a given duty, selecting a pump that runs at the highest speed suitable for the application will generally result in a more efficient selection as well as the lowest initial cost. Exceptions to this include slurry handling pumps, high specific speed pumps, or where the pump would need a very low minimum net positive suction head at the pump inlet.
Correct Sizing of Pump(s)
Pumps that are sized inappropriately result in unnecessary losses. Where peak loads can be reduced, pump size can also be reduced. Where pumps are dramatically oversized, speed can be reduced with gear or belt drives or a slower speed motor. This practice, however, is not common. Paybacks for implementing these solutions are less than one year.
Use of Multiple Pumps
Often using multiple pumps is the most cost-effective and most energy efficient solution for varying loads, particularly in a static head-dominated system. Variable speed controls should also be considered for dynamic systems. Parallel pumps also offer redundancy and increased reliability. One case study of a Finnish pulp and paper plant indicated that installing an additional small pump, running in parallel to the existing pump used to circulate water from the paper machine into two tanks, reduced the load in the larger pump in all cases except for startup.
If a large differential pressure exists at the operating rate of flow (indicating excessive flow), the impeller (diameter) can be trimmed so that the pump does not develop as much head. In the food processing, paper and petrochemical industries, trimming impellers or lowering gear ratios is estimated to save as much as 75% of the electricity consumption for specific pump applications.
The objective of any control strategy is to shut off unneeded pumps or reduce the load of individual pumps until needed. Remote controls enable pumping systems to be started and stopped more quickly and accurately when needed, and reduce the required labor.
Adjustable Speed Drives (ASDs)
ASDs better match speed to load requirements for pumps where, just as for motors, energy use is approximately proportional to the cube of the flow rate. Hence, small reductions in flow that are proportional to pump speed may yield large energy savings. New installations may result in short payback periods. In addition, the installation of ASDs improves overall productivity, control, and product quality, and reduces wear on equipment, thereby reducing future maintenance costs.
Similar to being able to adjust load in motor systems, including modulation features with pumps is estimated to save between 20 and 50% of pump energy consumption, at relatively short payback periods, depending on application, pump size, load and load variation. As a general rule of thumb, unless the pump curves are exceptionally flat, a 10% regulation in flow should produce pump savings of 20% and 20% regulation should produce savings of 40%.
Throttling valves should always be avoided. Extensive use of throttling valves or bypass loops may be an indication of an oversized pump. Variable speed drives or on off regulated systems always save energy compared to throttling valves.
Correct Sizing of Pipes
Similar to pumps, undersized pipes also result in unnecessary losses. The pipe work diameter is selected based on the economy of the whole installation, the required lowest flow velocity, and the minimum internal diameter for the application, the maximum flow velocity to minimize erosion in piping and fittings, and plant standard pipe diameters. Increasing the pipe diameter may save energy but must be balanced with costs for pump system components. Correct sizing of pipes should be done at the design or system retrofit stages where costs may not be restrictive.
Precision Castings, Surface Coatings, or Polishing
The use of castings, coatings, or polishing reduces surface roughness that in turn, increases energy efficiency. It may also help maintain efficiency over time. This measure is more effective on smaller pumps. In one case study in the steel industry, investment in surface coating on the mill supply pumps (350 kW pumps) cost nothing and would be paid back in months by energy savings.
Seal failure accounts for up to 70% of pump failures in many applications. The sealing arrangements on pumps will contribute to the power absorbed. Often the use of gas barrier seals, balanced seals, and no-contacting labyrinth seals optimize pump efficiency.
Curtailing Leakage through Clearance Reduction
Internal leakage losses are a result of differential pressure across the clearance between the impeller and the pump casing. The larger the clearance, the greater is the internal leakage causing inefficiencies. The normal clearance in new pumps ranges from 0.35 to 1.0 mm. With wider clearances, the leakage increases almost linearly with the clearance. For example, a clearance of 5 mm decreases the efficiency by 7 to 15% in closed impellers and by 10 to 22% in semi-open impellers. Abrasive liquids and slurries, even rainwater, can affect the pump efficiency. Using very hard construction materials can reduce the wear rate.