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DspEdu 2.1 General Description of the System Reactive Power Management in Motors/System Transformer Loading and Loss Analysis Recommendations to Save Energy in Motors
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ELECTRICAL ENERGY AUDIT OF AN INDANE (LPG) BOTTLING PLANT – A CASE STUDY
Suresh Kumar. K.S. Department of Electrical Engineering National Institute of Technology Calicut Calicut-673601, Kerala State , India [ I conducted an Energy Audit Study at an Indane (LPG) Bottling Plant located in Kerala State in India in 1997. This Plant received Electrical Power from K.S.E.B Grid (the Kerala State Electricity Board ,referred to as KSEB in this article) at 11 kV level with a Contract Demand of 300 kVA . The Connected Load of the Plant was 380 H.P + 41 kW + 150 kVAr in 1997. The Recorded Maximum Demand varied between 225 kVA and 250 kVA.The firm maintained 2x250 kVA D.G. Sets and a 60 kVA D.G. Set as stand by units.This article covers the energy consumption details and conservation opportunities in this Plant as in 1997.] The Electrical Energy Audit at an Indane Bottling Plant is considered in this article. An abstracted version of the Energy Audit Report follows. 1.1
Summary of Recommendations
Total Annual Energy Savings = 93812 Units 1.2
Savings Anticipated by Proposed Schemes
Total Annual Energy Savings
= 93812 Units
1.3
Investment Required and Payback Period
Total Annual Energy Savings
= 93812 Units
1.4 Impact of Proposed Schemes The current value of Specific
Energy Consumption = 25.4 Units/MT
1.5 A Special Note 1. The firm is expected to pay Rs 0.025 per unit of self generation to K.S.E.B and K.S.E.B is expected to read the energy meters of D.G.Sets to calculate this amount. However, examination of K.S.E.B bills during audit revealed that the self generation figures entered in the bill are grossly in error and simply could not have been generated in the D.G. Sets. Hence in the period 1996 Apr-1997 Mar itself the firm paid an extra amount of Rs. 22,878/- as duty for self generated energy which it really did not generate and could not have generated. 2. The Audit team measured 3.8 Units/Lit in old D.G. Set and 3.0 Units/Lit in the new D.G.Set during the audit. There is no real reason why the firm should not get the same rate at other times too. However the self generated energy data furnished by the firm indicated a much lower efficiency for D.G. Sets. The Audit feels that there took place some error in reading the energy meters. The Diesel consumption figures provided by the firm and the efficiency measured by the Audit are taken as the valid data for drawing up consumption analysis in this report. It is recommended that proper and accurate readings of energy generated in the 3 D.G. Sets be maintained in future. 2. General Description of the System The Indane Bottling Plant receives LPG in Bullet Trucks from Refineries and produces filled cylinders of 14.2 kg and 19 kg variety. The installed capacity of the Plant is 3000 MT/month for two shift operation. It has 400 T storage capacity at present.The Plant was working on single shift with 3 extended hours till Feb 1997 and changed over to two shift operation since then. The Plant employs 46 blue collar employees,2 supervisors,4 white collar employees and 9 officers. There is one Chemical Engineer,2 Electrical Engineers and 4 Electricians as technical staff. The installed capacity corresponds to 28 loads of filled cylinders per day. One load will take 4.6MT. The energy inputs are electricity from K.S.E.B and Diesel for running the D.G. Sets. The firm has installed a 500 kVA transformer to convert 11kV to 415 V and there is a suitably rated Stabiliser installed. It also maintains 2x250kVA and a 60 kVA D.G. Sets as standby units. Over the last three years, half of the total electrical energy requirement was generated in the D.G. Sets due to low quality power supply received from K.S.E.B.The Connected Load of the Plant is 385 HP + 40 kW +150 kVAr and the contract demand is 300 kVA. The average Recorded Maximum Demand level is 230 kVA. 2.1 Process Description The different processes in the Plant are described briefly below. Bulk Receipt - Liquefied Petroleum Gas received in Bullet Trucks is transferred using differential pressure method, i.e pressure at the Bullet Truck is increased to create a differential of about 2.5 kg/sq.cm between the truck and the storage vessel. The product transfer is effected by connecting both liquid lines. Once the liquid transfer is completed the vapour pressure inside the truck is brought down to 2 kg/sq.cm. The operations are performed with help of vapour compressors. One set of trucks comprising 4 Bullet Trucks is handled at a time and complete transfer takes about 5 hrs. Three such sets are transferred in a day. Cylinder Filling - The LPG stored in the vessel in the vessel is pressurised to about 7 kg/sq.cm. The LPG pressure is boosted to about 12-13 kg/sq.cm and sent to filling shed through pipe lines using an LPG Pump. Measured quantity of LPG is filled into cylinders at the filling shed under pneumatic control. Filled cylinders are subjected to the following quality control checks. 1. 100% weight check.
Tamper proof seals are fixed on the valves of sound cylinders. These cylinders are loaded to stake trucks and dispatched to various markets. Cylinder movement inside the Plant is achieved using chain conveyors driven by electrical motors through worm gear reducers/planetary gear reducers. 2.2 Energy Consumption Profile The energy inputs are electrical energy from K.S.E.B and Diesel for the DG Sets. The annual energy consumption for the audit period (1994 Apr -1997 Mar) is summarized below. Year Electrical Energy (MWh)
Diesel(lit) Diesel (MWh) Total Eqvt
Energy(MWh)
1994-95 345.6 242.1 71200 730.7 1076.3 1995-96 326.9 293.8 86400 886.7 1213.6 1996-97 260.3 399.2 117400 1204.9 1465.2
In the average, 50% of electrical
energy requirement is met from K.S.E.B and 50% is met by Self Generation.
The average monthly Energy Cost during the three year block was Rs.1,17,000/-
and Diesel cost accounts for 58 % of it.
The Indane Bottling Plant has an installed capacity of 3000 MT per month (for two shift operation). The Plant was running single shift with 3 extended hours till Feb 1997 and started 2 shift operation from that month onwards. The average monthly production during the audit period (1994 Apr to 1997 Mar) was 2043 MT with an average monthly electrical energy consumption of 51881 Units. The average monthly energy cost was Rs. 1.17 Lakhs. The average Specific Energy Consumption was 25.4 Units/MT of LPG bottled and the average Specific Energy Cost was Rs. 57.27/MT of LPG bottled. 3.1 Specific Energy Consumption & its Monthly Variation The monthly variation of Specific Energy Consumption is shown in Fig 3.1.The Specific Energy Consumption is more or less constant at 25 Units/MT except in the month of March when it is abnormally high.
The firm maintained the Diesel purchase
data; but no separate log is maintained for the Diesel that goes into the
D.G. Sets and for the Diesel used for other purposes. There are two Diesel
Engine driven pumps that are always maintained in a ready to run condition
for fire fighting operations. The fire fighting rehearsal to verify the
readiness of the Plant to meet a fire hazard effectively takes place in
March. Hence Diesel consumption in those months will be large and this
accounts for excessively high Specific Energy Consumption in March.
3.2 Specific Energy Consumption of Processes The various processes that go into
the making of Specific Energy Consumption are (1) Bullet Transfer (2) Bottle
Filling & Finishing (3) Lighting (4) Water Pumping.
3.3 Target Specific Energy Consumption Level The energy conservation proposals, if implemented, will result in an energy saving of 7818 Units per month. This will bring down the Specific Energy Consumption by 3.82 Units/MT. Additional savings will result from overhauling and engine tuning of the new 250 kVA D.G. Set . Hence this Audit fixes the Specific Energy Consumption target at 21.6 Units/MT of LPG bottled. The Indane Bottling Plant receives electrical power from K.S.E.B at 11 kV level with a Contract Demand of 300 kVA . The Connected Load of the Plant is 380 H.P + 41 kW + 150 kVAr. The Recorded Maximum Demand varies between 225 kVA and 250 kVA. The firm maintains 2x250 kVA D.G. Sets and a 60 kVA D.G. Set as stand by units. The Plant is located in an area that suffers from low voltage and generally unreliable power supply and as a consequence the Plant is on its D.G. Sets for half its operating period in the average. The K.S.E.B ACB is set to trip at 330 V and the Plant is put on one of the 250 kVA D.G. Sets whenever K.S.E.B voltage goes below this value or K.S.E.B supply is not available. However the Plant is exempt from power cuts and Units restriction which other HT Consumers of the State are subjected to during periods of power shortage. The Plant runs on a two shift basis now (Since Feb 1997) and shift periods are 6 A.M to 2 P.M and 2 P.M to 10 P.M. The installed capacity for two shift operation is 3000 MT of LPG bottled. This corresponds to about 28 Loads of LPG cylinders. The average monthly electrical energy consumption is about 51881 Units and half of it is Self Generated. The average monthly energy cost is about Rs. 1.17 Lakhs and Diesel Cost accounts for 58 % of it. The average kW loading of the system during the 16 hr operating period is about 127 kW and the peak value measured during the audit was 150 kW. The corresponding kVA loading levels were about 180 kVA and 205 kVA . The average daily consumption is about 2033 Units of energy, half of which is Self Generated.The current tariff applicable to the firm is Rs. 1.345 per unit and Rs. 133/- per kVA per month for R.M.D. 4.1 Layout of Power Supply Fig 4.2 shows the kW and kVA load curves of the system as measured on 25-08-1997.The 150 kVAr Capacitor at the Main Switch Board was not available on the day of measurement and hence the kVA load curve in fig 4.2 shows the loading before capacitive compensation.
4.2 Reactive Power Management in Motors/System Motors constitute the major load in the system and almost all the motors are of flame proof design due to the enhanced degree of protection needed in this plant. There is no capacitor across any motor. However, there is 150 kVAr Capacitor banks connected at the Main Switch Board and this will be available when the system is on K.S.E.B. However, the Capacitor is not available when the system is on D.G. Sets. The 150 kVAr is distributed in three banks with independent switches;2x25 kVAr , (5x5 + 1x25)kVAr and 10x5 kVAr are the bank ratings. The reactive power load curve as measured on 25-08-1997 is shown in fig 4.3.It shows an almost constant reactive power demand of about 150 kVAr with about ? 10 % variation around that value throughout the 16 hr shift period. This data was measured at a point before the 150 kVAr Capacitor connection point and represents the actual reactive power demand of the uncompensated system at 420V level. About 6 kVAr will get added to this when we move over to the HT side of the transformer due to the transformer magnetising kVAr and leakage kVAr requirements. Thus the 150 kVAr Capacitor installed at the Main Switch Board is the right rating and if these banks were always kept on whenever the Plant is functioning , the Maximum Demand of the Plant would have contained only the kW component.
Refer to fig 4.4 that shows the monthly variation of the Recorded Maximum Demand of the Plant for the audit period of 36 months. Clearly the Recorded Maximum Demand was always between 225 kVA and 250 kVA during this 36 months' period. The kW requirement of the Plant is 127 kW average and 150 kW peak. This measurement was verified for two days - on 25-08-1997 and 26-08-1997- and discussions with Plant Engineers confirmed that there was nothing unusual about the load on those two days and that the load measurements represented typical load behaviour. Also an examination of Connected Load and processes revealed that there are no inactive loads of enough capacity to explain the mismatch between the Recorded Maximum Demand level registered and the level expected. The expected level is between 150 to 160 kVA if all the 150 kVAr Capacitor is active always. This leads to the conclusion that either the Capacitors are defective or they are not always kept on when the system is on K.S.E.B and Plant is functioning. The Capacitors are not defective as verified during the audit. Hence it is concluded that the plant functioned without the Capacitor compensation at the Main Switch Board at least for a minimum period of half an hour in a month.
The Plant switches between K.S.E.B and D.G. Set at least two to three times a day almost on all days due to the low quality of power received from K.S.E.B. Neither the electricity rules nor the bus arrangement at the substation in the Plant allow the connection of 150 kVAr Capacitor at Main Switch Board during the D.G. Set operation. This means that the station operator has to switch on the Capacitor once K.S.E.B supply is restored. There is a possibility that this switching fails to get done due to some reason or other once or twice in a month for periods greater than 30 minutes. This results in the recorded level of M.D. With the Contract Demand at 300 kVA the minimum chargeable M. D. of the Plant is 225 kVA and the firm would not have gained financially by operating the Capacitor properly. Nevertheless, if the Capacitor were operated properly the Plant would have recorded an M. D. Level of around 150 to 160 kVA. In that case the firm could have persuaded K.S.E.B to revise the Contract Demand to 225 kVA and the firm could have realised monetary benefits thereby. 4.2.1 A Proposal for Reactive Power Management at System Level Refer to fig 4.5 for the variation of system power factor during the working time without the 150 kVAr in action. The average power factor is around 0.65 only.
The D.G. Sets are rated at 0.8 p.f. With the power factor at 0.65 they will have to have a higher excitation level in the field than the rated value in order to maintain the rated voltage at terminal. Higher excitation level results in greater field losses and lowers the D.G. Set efficiency. Also, low power factor operation results in greater winding losses in armature and brings down the efficiency. Hence it will be desirable to have the Capacitor compensation available to the D.G. Set also. Fixed Capacitor banks at Main Switch Board are not allowed as per regulations when the system is on D.G. Set. However an automatic power factor correction panel can be. The 150 kVAr capacitor in service is somewhat old and has given its useful service life. It is proposed that the 150 kVAr bank be replaced by an Automatic Power Factor Correction Panel of 100 kVAr rating, complete with Capacitors, switches, power factor sensing system, automatic power factor correction logic and suitable panel with metering and annunciation. Suitable cabling should be done to make this system available during the D.G. Set operation too. The panel should go on automatically whenever the Main Switch Board is energised either from K.S.E.B or from the D.G. Sets. With this system in operation along with certain other recommendations detailed in later section s of this report implemented will result in the M.D. level of the coming down to about 150-160 kVA. The firm can then get the Contract Demand revised from 300 kVA to 225 kVA. Financial Viability of this Proposal The average Recorded Maximum Demand
over last three years = 230 kVA
Locating Capacitors at motor terminals inside the Plant can not be considered in the case of this Plant since all these motors are located in the protected zone of the Plant. Capacitors are prone to damage and explosion and cannot be deployed in a flammable environment. However, they can be connected right at the output of the DOL Starters at the substation. But the Motor Control Centres PMCC1 and PMCC2 in the substation will not permit easy retrofitting of capacitors. Hence an automatic power factor correction system recommended above is thought to be the best solution for reactive power management in this Plant. The losses in the main cables connecting the PMCC1 ,PMCC2,PMCC 3 and LSB to the Main Switch Board are calculated and are shown in Table 4.1.The losses in the motor cables from the three motor control centers are shown in Tables 4.2 & 4.3. The resistance factor in these tables have been calculated by using the per km resistance value of the corresponding cable. The Loss equation is Cable Loss = (kVA)2 x L x RF x LLF x HR where kVA is the maximum kVA which goes through the cable, L is the length of cable in meters, RF is the resistance factor, LLF is the load loss factor and HR is the number of hours over which the energy loss is calculated. LLF is calculated from load factor in the cable as LLF = 0.7 x LF + 0.3 x LF x LF where LF is the load factor. The total cable loss in the system is approximately 36 Units per day and is within limits. It is about 1.76 % of average daily consumption .There are no overloaded cables in the Plant. No cables need replacement from loading and loss point of view. 4.4 Transformer Loading and Loss Analysis The 500 kVA transformer is loaded only for 8 hrs in the average during the 16 hr operation period on most of the days due to low voltage problem. However, the loading and loss analysis of the transformer in this section is carried out assuming that the Plant works on K.S.E.B for the entire day. The peak load on the transformer is around 211 kVA and average load is around 180 kVA during the 16 hr period. Thus load factor for this period is 0.85.The transformer is loaded to 42 % when the plant is running. A 500 kVA transformer consumes about
70 Units per day as no load loss and it will have about 5 kW full load
copper loss. The Load Loss Factor value for a Load Factor value of 0.85
is = 0.7 x 0.85 + 0.3 x 0.85 x 0.85 = 0.812 . Thus energy loss due to Copper
Loss during 16 hrs of plant operation = 0.42 x 0.42 x 5 x16
x 0.812 Units = 11.5 Units . The copper loss during the remaining 8 hrs
in the day is negligible due to very low loading level.
The motor control center PMCC 1 supplies 8 motors - CM1 to CM 5 and P3 to P5.CM1 and CM2 are air compressor motors,CM3 to CM 5 are vapour compressor motors and P3 to P5 are LPG pump motors. Both air compressors run all the time. The three vapour compressors driven by 25 H P motors also run all the time. The vapour compressors are used for three purposes (a) Bullet truck to Storage Tank transfer and reverse vapour transfer (b) Bullet to Bullet pressurization to obtain proper filling pressure in the filling section and ( c ) Evacuation of over filled cylinders or defective cylinders. Of the three LPG pumps only one runs normally and two pumps are kept as stand by units. The loading analysis of these motors is summarised in Table 4.4.The measured data for each motor were voltage, current, kVA, kW, kVAr and power factor .Standard efficiencies were assumed and the motor losses were estimated. Output was estimated by subtracting estimated losses from measured input power. The motor control center PMCC 2 supplies about 45 motors in the HP range 2-7.5. All these motors are located in the finishing section and most of them are conveyor drive motors. Not all of them run always. Load analysis was carried out for the motors which run throughout the Plant operation period. The infrequently used ones and the spare ones were left out from the load analysis. The result of load analysis on this motor control center is summarized in Table 4.5. The motor control center PMCC 3 supplies 5 Bore Well pumps and two of them run for about 8 hrs per day. Table 4.6 covers the load analysis of this control center. The vapour compressor motor
CM5 and LPG pump motor P5 are identified as the over sized motors on PMCC2.
The LPG pump motor is oversized. The audit verified that the filling process was proceeding normal and filling pressure was right at the time of metering the motor. Hence this is a clear case of Oversizing. The motors C1,C4,C9,C11,C13,C15,C17 all 7.5 H.P except C9 which is a 5 H.P motor are identified as oversized among the motors in PMCC1. PMCC 2 shows a power factor close to 0.8 throughout the day indicating satisfactory loading of motors in that bus. The daily kW and kVA loading curve for this bus is given in fig 4.4 and similar curves for PMCC 1 is given in fig 4.3. Fig 4.3 shows that the average power factor of this bus is around 0.5 and this indicates under loading of motors connected to this bus.PMCC2 is a major contributor to the system reactive power requirement and contributes about 60 kVAr. Also note the prominent increase in kW and kVA loading at both buses during 2:00 P.M to 5:00 P.M .This was due to the low frequency of the new D.G. Set which was operating during this period on the day of measurement (25-08-1997). TABLE 4.1
TABLE 4.2
TABLE 4.3
TABLE 4.4
TABLE 4.5
TABLE 4.6
4.7 Recommendations to Save Energy in Motors 1) The 30 H.P,2950 r.p.m,415 V LPG Pump Motor P5 may be replaced by a new 15 H.P ,2950 r.p.m,415 V High Efficiency Motor. After replacement the Pump P5 should be used always with the other two maintained as stand by units. Financial Analysis of this Proposal. The 30 H.P motor currently in service will have about 86% full load efficiency. It was found to take 11kVA,8.7 kW,6.7kVAr as measured during the audit. Estimated Loss
= 2.1 kW
A 15 H.P,2950 r.p.m High Efficiency motor will have 86% efficiency and 0.88 power factor as per Siemens data sheet. Estimated Loss of this motor
The plant runs on D.G. Set half the time. The D.G. Sets yield 3.4 Units/Lit of Diesel. Diesel costs about Rs.11/- per litre. The average daily consumption from K.S.E.B is 1016.5 Units and average monthly M.D is 230 kVA. The M.D Charge is Rs.133/- per kVA per month. Using these figures the average cost of one unit of energy can be worked out as Rs. 2.8 per unit. Annual Financial Gain
= Rs. 14,227/-
2) The 30 H.P ,1450 r.pm,415V Air Compressor Motor CM 1 may be replaced by a new 25 H.P ,1450 r.p.m,415 V High Efficiency Motor. Financial Analysis of this Proposal. The 30 H.P motor currently in service will have about 88% full load efficiency. It was found to take 25 kVA,18.9 kW,16.0 kVAr as measured during the audit. This motor takes excessive reactive current and is in a bad condition. Estimated Loss
= 2.6 kW
A 25 H.P,1450 r.p.m High Efficiency motor will have 90.5% efficiency and 0.83 power factor as per Siemens data sheet. Estimated Loss of this motor
The average cost of one unit of energy
has been worked out as Rs. 2.8 per unit.
3) The 20 H.P ,1450 r.pm,415V Air Compressor Motor CM 2 may be replaced by a new 15 H.P ,1450 r.p.m,415 V High Efficiency Motor. Financial Analysis of this Proposal. The 20 H.P motor currently in service will have about 86% full load efficiency. It was found to take 14.7 kVA,13.2 kW,11.3 kVAr as measured during the audit. Estimated Loss
= 1.9 kW
A15 H.P,1450 r.p.m High Efficiency motor will have 88.5% efficiency and 0.83 power factor as per Siemens data sheet. Estimated Loss of this motor
The average cost of one unit of energy has been worked out as Rs. 2.8 per unit. ?Annual Financial Gain
= Rs. 7196/-
4) The following motors
in PMCC2 may be reconnected in Star instead of Delta. The overload setting
of these motors should be set at 60 % of present value after reconnection.
Savings in energy that will
result
In addition , there will be a reduction of 2 kVAr from each 7.5 H.P motor put in star and a reduction of 1.4 kVAr from 5 H.P motor. Thus the reactive demand will come down by 13.4 kVAr at PMCC1. 4.8 Industrial Machinery Losses. The three vapour compressors CM4,CM5,CM6 ,the two air compressors CM1,CM2,and one LPG Pump use V Belts for transmission. Modern Flat Belts have 97% transmission efficiency compared to 88% of V Belts. Employing Flat Belts instead of V Belts result in about 10% reduction in shaft output required. It is proposed that the V Belts and pulleys in the drives CM1,CM2,CM4,CM5 and CM6 be replaced by Flat Belts with suitable pulleys. Financial Analysis of this proposal. CM1 5x0.75 inch V Belts to be replaced
by Flat belts
CM2 5x0.75 inch V Belts to be replaced
by Flat belts
CM4 7x0.75 inch V Belts to be replaced
by Flat belts
CM5 7x0.75 inch V Belts to be replaced
by Flat belts
CM6 7x0.75 inch V Belts to be replaced
by Flat belts
Net Annual Energy Saving
= 29784 Units
There is about 26 kW connected lamp load in the Light Switch Board. 32 Sodium Vapour Lamps of 400 W rating (total of 12.8 kW) constitute the major component followed by 102 Flame Proof GLS of 80 W each(8.16 kW total).The SVL are mounted in six towers and are used for yard lighting for security and safety purposes. The 80 W flame proof lamps are used for Plant lighting and are of special construction to satisfy explosive area certification norms. Not all SVL were functional at the time of audit. Actual measurements during audit revealed about 9.7 kW load from all the tower lights put together. The SVL installed and operational now are required in the plant now and no compromise can be made in the general illumination level in the yard considering the sensitive nature of Plant operations.Replacing the 80 W GSL fittings in the Plant by CFL fittings would have resulted in energy savings. However, it is felt that achieving the same degree of fireproof protection in CFL fittings may not be feasible economically. Hence this audit does not recommend any changes in the lighting system.
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