Improving Energy Efficiency
Motorized fluid handling machinery such as air compressors and pumps consume nearly 40 to 60% electric power used by the industry. A large number of such systems installed in our process and power plants, either operate under degraded efficiency conditions due to service induced deterioration of the equipment – or operate far from the optimal operating point on the characteristic curve due to over design or mismatch with the demand or process requirements…
- Dr G S Grewal,
Energy conservation is an optimal operational practice, which improves the efficiency of the equipment associated with both supply side and demand side i.e., at the user end. Since the industries account for a greater share of power consumption, energy conservation programmes will benefit them by reducing their Specific Power Consumption and making them more competitive in the market. ‘Negawatt’ (a negative ‘Megawatt’), a term coined by the renowned energy analyst Amory Lovins, highlights the fact that a Negawatt produced by reducing energy need saves more than a Megawatt generated. Energy audits lead to the generation of ‘NEGAWATTS’ and thus result in virtual augmentations to the installed power generation capacity of the Nation! Energy Audit is the basic tool of energy conservation, and is defined as a systematic exercise to identify the practices & processes that consume a significant amount of energy, estimate the efficiency in each of these practices & processes – and based on these estimates devise methods of improving efficiency and curbing loss of energy.
Motorized equipment such as air compressors and pumps are the workhorses of power & process plants. Nearly 60% of total power consumption in the process industry happens to be related to pumping systems. Typically, for such motorized process equipment, about 2% of total life cycle cost is the initial cost, about 8 to10% is spent on repairs and maintenance and the balance 90% is the energy cost for driving the system.
Consequently, a huge potential exists for energy savings related to air compressors and pumping systems. Systematic surveys conducted by the energy management group at ERDA indicate a savings potential of nearly 25% of the present day energy consumption. This translates into the equivalent saving of nearly 178 MW (approximately ` 6 million). In the present paper, we present five singular case studies pertaining to audit of air compressors and pumping systems, where significant energy savings were realised.
During audit of compressors, the current drawn by motor, pressure reducing valve opening position, discharge and user end pressure, controlling mechanism and load - unload time can give ideas for energy savings. Compressor motor’s loading depends on pressure developed and flow delivered. Proper mechanism for controlling the output of compressor always gives efficient operation. Pressure drop across valves, more unloading time than the loading time, motor over loading, under utilisation of design capacity etc., are indicators of energy loss.
Case Study 1 (Nuclear Power Plant)
Two Generating Units have three Air Compressors for each Unit to supply compressed air for Instrument Air, Service Air and Mask Air requirements. Two Air Compressors for each Unit are kept in operation to meet the air requirement. Load-unload time duration data for each compressor is given below:
It can be seen that compressors are remaining in unloading condition for around 50% time, which indicates that air requirement is much lower than the present capacity of compressors. Hence, the energy savings solution found is to open the interconnection valve between two units, and feed the complete requirement of both generating units with only three compressors – instead of running all four compressors – so that reliability of air supply will be maintained and energy can also be saved.
After switching off one compressor, load on other compressors will get somewhat increased. At least No load power consumption of one compressor is clearly saved on a conservative basis. Energy saving potential is computed below:
Motor Rating = 140 kW
Average Power Consumption:
At 50% load = 84.8 kW
At No load = 27.0 kW
Power Saving Potential = 27.0 kW
Energy Saved/Year = 1,06,920 kWh
(Considering 12 hrs/day & 330 Days/Year No load operation)
Amount Saved/Year@Rs.2.0/kWh = Rs. 2.13 Lakhs
Investment Required = Nil
Flow Measuring Unit...
During audit of pumps, the current drawn by motor, throttling of discharge, recirculation and user end valves, discharge pressure and controlling mechanism can give an idea for energy savings. One major reason observed for decrease in efficiency of motor driven auxiliaries is the deviations in the operating conditions from design parameters.
Normally at design stage, selection of motors is done such that they operate at 80 to 85% loading even if driven equipment is operating at full rated condition. Following case studies present energy conservation opportunites for pumps with motor operating at full load condition.
Ultrasonic Probles Installed on Piping for Velocity Measurement...
Case Study 2 (Thermal Power Plant)
Motor of Cooling Water Pump of rating 1550 kW in Thermal power plant was operating at almost full load condition – while the pump was operating at lower loading condition. Hence the pump was taken under maintenance and overhauling was done.
Specifically, bearings were checked and repaired. Complete overhauling work of replacement of stainless steel liner, muff coupling of runner shaft and shaft sleeve were undertaken. Maintenance of the above pump improved its performance. Energy saving calculations are presented below:
Preparation in Progress for Water Flow Rate Measurement...
Case Study 3 (Hydro Power Plant)
Cooling water pumps are provided for generator cooling in Hydro power plant. Design capacity of the pumps is given below:
Measurements were taken as mentioned below:
Valves were throttled at user end and flow was found 40% lower than the design capacity. Hence, it was clear that pumps were of oversized design. Therefore, it was recommended to replace the existing pumps with lower sized pumps as per the actual requirement having specification given below:
Flow : 360 m3/hr
Head : 60 m
Pump Efficiency : 70%
Motor Rating : 110 kW
Energy Efficiency calculations are given below:
Power consumption for new pump = 84.0 kW
Investment Required = 4.0 Lakhs
Payback period = 13.1 months
Case Study 4 (Thermal Power Plant)
Two Boiler Feed Pumps rated 3500 kW each are kept in operation to meet feed water requirement of 210 MW thermal power plant. The boiler drum level is controlled by controlling feed water flow by maintaining Differential Pressure across Feed Control Valve (FCV) and scoop control is being done by maintaining fixed differential pressure of 5 to 7 kg/cm2 across Feed Control Valve. After analysing the system, initiative was taken to change the operating system to make zero pressure drop across FCV.
Keeping the FCV fully open, flow was controlled by adjusting the scoop position (scoop control). The scoop position was reduced from 79 to 72% keeping the FCV fully open (DP across FCV zero). Efficiency was observed to increase from 61.8% to 66.5%. Details are presented below:
Case Study 5 (Fertilizer Plant)
Boiler house of a fertilizer plant has 12 bowl mills. Depending on requirements, 9 or 10 mills are kept in operation. The plant has two crushers and normally one is kept in operation. Parameters measured for crushers are tabulated below:
Power consumption and coal crushed by each crusher were measured to evaluate the specific power consumption. Observation made are:
- Specific energy consumption of Crusher-2 was found very low
- Huge amount of coal rejection observed at mill end.
Hence, the whole system was analysed to identify the problem. Larger size coal at mill inlet was observed. This suggested non-optimal functioning of the coal crusher and/or damage to its outlet screen. Hence, checking of Crusher-2 and its outlet screen was suggested. Damage was found on the outlet screen, as a consequence of which large sized lumps of coal were being passed through to the coal mill instead of being recirculated back into the crusher. Repairs to the outlet screen were suggested. Further, the practice of checking coal fineness at outlet of both the crushers before supplying into mills was suggested. This exercise will reduce the power consumption by mills, reduce mill rejection and also mills will require less maintenance.
Considering at least 5% reduction in power consumption of mills, the energy saving potential is computed below:
Average power consumption of mill = 169 kW
Power saving potential @ 5% = 8.45 kW
Power saving potential for 9 mills = 76.0 kW
Units saved = 2,09,760 kWh
(24 hrs/day, 115 days/year crusher-2 operation)
Saving potential @Rs.3.00/kWh = Rs. 6.29 Lakh
Approx. investment required = Rs. 2.00 Lakh
Pay back period = 3.8 months
The audit of five process flow equipment, presented in this paper, have resulted in identification of saving potential of nearly 3450 MWh per year at a cumulative investment of only about 30 lakhs.
In other words, the specific ‘per unit’ cost for saving electrical energy is only about Rs. 0.85 per unit against per unit electricity price of nearly Rs. 5. This study thus clearly highlights the significant electrical energy savings potential existing in the country for motorized fluid handling equipment such as pumps and air compressors.
Authors are from Electrical Research & Development Association, Vadodara.
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