Chapter Eleven: Audit and Conservation of Energy

1 Chapter Eleven: Audit and Conservation of Energy

1.1 Introduction

Energy is very scarce and short supply commodity particularly in many of the areas in Tanzania and its cost is spirally increasing day-by-day. Generally, pumping installations consume a huge amount of energy wherein proportion of the energy cost can be as high as 40 to 70% and even more of the overall cost of operation and maintenance of water works. The need for conservation of energy, therefore cannot be ignored. All possible steps should be identified and adopted to conserve energy and reduce energy consumption and loss, and cost so that water tariffs can be kept as low as possible and gaps between high cost of production of water and prices affordable by consumers can be reduced. Some adverse scenarios in energy aspects as follows are quite common in pumping installations:

(a) Energy consumption is higher than the optimum value due to reduction in efficiency of the pumps,

(b) Operating point of the pumps is not in-line with the best efficiency point (b.e.p.),

(c) Energy is wasted due to increase in head loss in pumping systems, (e.g. clogging of strainers, encrustation in column pipes, encrustation in pumping mains),

(d) Selection of uneconomical diameter of sluice valves, butterfly valves, reflux valves, column pipe, drop pipe in pumping installations,

(e) Energy wastage due to operation of electrical equipment at low voltage and/or low power factors.

Such inefficient operation and wastage of energy should be avoided to cut down energy costs. It is therefore, necessary to identify all such shortcomings and causes. The following measures should be adopted in management of energy:
(a) Conduct thorough and in-depth energy audit covering analysis and evaluation of all equipment, operations and system components which have bearings on energy consumption, and identifying scope for reduction in energy costs,

(b) Implement measures for conservation of energy,

(c) Energy audit as implied is auditing of billed energy consumption and how the energy is consumed by various units, and sub-units in the installation and whether there is any wastage due to poor efficiency, higher hydraulic or power losses, etc. and identification of actions for remedy and correction,

(d) In respect of the sources like infiltration wells, open wells, collector wells, the working head can be decided based upon the suction head, delivery head, frictional loss with reference to the pipe materials used and other losses,

(e) In respect of borehole sources, while submersible pump sets are used, the pump suction depth may be fixed with reference to the final spring achieved during drilling,

(f) Working of head of pumps should be made in a conservative way,

(g) If the head of the pump is excess of the actual requirement, then pump impeller shall be trimmed as recommended in section 6.2.2.3 of this DCOM Manual,

(h) In large pumping stations, pumps with variable frequency should be used,

(i) With low power factor loads, the current flowing through electrical system components is higher than necessary to do the required work. In order to achieve power factors greater than 0.9 power capacitors of required capacity should be installed on all the installation of pumping machinery,

(j) Electric motors usually run at a constant speed, but a Variable Frequency (speed) Drive (VFD) allows the motor’s energy output to match the required load. This achieves energy savings depending on how the motor is used. When one uses a control valve or regulator, one lose energy because the pumps are always operated at high speed.

1.2 Scope of the Energy Audit

Energy audit includes the following actions, steps and processes:
(a) Conducting in depth energy audit by systematic process of accounting and reconciliation between the following:
(i) Actual energy consumption,
(ii) Calculated energy consumption taking into account rated efficiency and power losses in all energy utilizing equipment and power transmission system as explained below.

(b) Conducting performance test of pumps and electrical equipment if the difference between actual energy consumption and calculated energy consumption is significant and taking follow up action on conclusions drawn from the tests,
(c) Taking up discharge test at rated head if test in (b) above is not being taken,
(d) Identifying the equipment, operational aspects and characteristic of power supply causing
inefficient functioning, wastage of energy, increase in hydraulic or power losses etc. and evaluating increase in energy cost or wastage of energy,
(e) Identifying solutions and actions necessary to correct the shortcomings and lacunas in (d) and evaluating cost of the solutions,
(f) Carrying out economic analysis of costs involved in (d) and (e) above and drawing conclusions whether rectification is economical or otherwise,
(g) Checking whether pump operating point is near best efficiency point and whether any improvement is possible,
(h) Verification of penalties if any, levied by power supply authorities e.g. penalty for poor power factor, penalty for exceeding contract demand. Broad review of following points for future guidance or long term measure:
(i) C-value or f-value of transmission main,
(ii) Diameter of transmission main provided,
(iii) Specified duty point for pump and operating range,
(iv) Suitability of pumps for the duty conditions and situation in general and specifically from efficiency aspects,
(v) Suitability of ratings and sizes of motor, cable, transformer and other electrical appliances for the load.

1.2.1 Study and Verification of Energy Consumption

(a)All Pumps Similar (Identical)
(i) Examine few electric bills in immediate past and calculate total number of days, total kWh consumed and average daily kWh (e.g. in an installation with 3 numbers working and 2 numbers standby if bill period is 61 days, total consumption 549,000 kWh, then average daily consumption shall be 9000 kWh),

(ii) Examine log books of pumping operation for the subject period, calculate total pump - hours of individual pump sets, total pump hours over the period and average daily pump hours (Thus in the above example, pump hours of individual pump sets are: 1(839), 2(800), 3(700), 4(350) and 5(300) then as total hours are 2989 pump-hours, daily pump hours shall be 2989 ÷ 61 = 49 pump hours. Average daily operations are: 2 numbers of pumps working for 11 hours and 3 numbers of pumps working for 9 hours),

(iii) From (i) and (ii) above, calculate mean system kW drawn per pump set (In the example, mean system power drawn per pump set = 9000 / 49 i.e. 183.67 kW),

(iv) From (i), (ii) and (iii) above, calculate cumulative system kW for minimum and maximum number of pumps simultaneously operated. (In the example, cumulative system kW drawn for 2 numbers of pumps and 3 numbers of pumps operating shall be 183.67 x 2 = 367.34 kW and 183.67 x 3 = 551.01 kW, respectively),

(v) Depending on efficiency of transformer at load factors corresponding to different cumulative kW, calculate output of transformer for loads of different combinations of pumps. (In the example, if transformer efficiencies are 0.97 and 0.975 for load factor corresponding to 367.34 kW and 551.01 kW, respectively, then outputs of transformer for the loads shall be 367.34 x 0.97 i.e. 356.32 kW and 551.01 x 0.975 i.e. 537.23 kW, respectively),

(vi) The outputs of transformer, for all practical purpose can be considered as cumulative inputs to motors for the combinations of different numbers of pumps working simultaneously. Cable losses, being negligible, can be ignored,

(vii) Cumulative input to motors divided by number of pump sets operating in the combination shall give average input to motor (In the example, average input to motor shall be 356.32 ÷ 2 i.e. 178.16 kW each for 2 pumps working and 537.23÷ 3 i.e. 179.09 kW each for 3 pumps working simultaneously),

(viii) Depending on efficiency of motor at the load factor, calculate average input to the pump. (In the example, if motor efficiency is 0.86, average input to pump should be 178.16 x 0.86 i.e. 153.22 kW and 179.07 x 0.86 i.e. 154.0 kW),

(ix) Simulate hydraulic conditions for combination of two numbers of pumps and three numbers of pumps operating simultaneously and take separate observations of suction head and delivery head by means of calibrated vacuum and pressure gauges and/or water level in sump/well by operating normal number of pumps i.e. 2 number and 3 numbers of pumps in this case and calculate total head on the pumps for each operating condition. The WL in the sump or well shall be maintained at normal mean water level calculated from observations recorded in log book during the chosen bill period,

(x) Next operate each pump at the total head for each operating condition by throttling delivery valve and generating required head. Calculate average input to the pump for each operating condition by taking appropriate pump efficiency as per characteristic curves,

(xi) If difference between average inputs to pumps as per (viii) and (x) for different working combinations are within 5% - 7%, the performance can be concluded as satisfactory and energy efficient,

(xii) If the difference is beyond limit, detailed investigation for reduction in efficiency of the pump is necessary,

(xiii) Full performance test for each pump shall be conducted as per procedure,

(xiv) If for some reasons, the performance test is not undertaken, discharge test of each single pump at rated head generated by throttling delivery valve needs to be carried out,

(xv) If actual discharge is within 4% - 6% of rated discharge, the results are deemed as satisfactory,

(xvi) Test for efficiency of pumping machinery after each repairing shall be taken. If necessary inefficient machinery should be replaced by energy efficient / star rated machinery.

(b)Dissimilar Pumps
Procedures for energy audit for dissimilar pumps can be similar to that specified for identical pumps except for adjustment for different discharges as follows:
(i) Maximum discharge pump may be considered as 1(one) pump-unit,
(ii) Pump with lesser discharge can be considered as fraction pump-unit as ratio of its discharge to maximum discharge pump. (In the above example, if discharges of 3 pumps are 150, 150 and 100 litres per second, respectively, then number of pump-units shall be respectively 1, 1 and 0.667). Accordingly the number of pumps and pump-hours in various steps shall be considered as discussed for the case of all similar pumps.

1.3 Measures for Conservation of Energy

Measures for conservation of energy in water pumping installation can be broadly classified as follows:

(a) Routine Measures
The measures can be routinely adopted in day to day operation and maintenance.

(b) Periodical Measures
Due to wear and encrustation during prolonged operation, volumetric efficiency and hydraulic efficiency of pumps reduce. By adopting these measures, efficiency can be nearly restored. These measures can be taken up during overhaul of pumps or planned special repairs.

(c) Selection Aspects
If during selection phase, the equipment i.e. pumps, piping, valves etc. are selected for optimum efficiency and diameter, considerable reduction in energy cost can be achieved.

(d) Measures for System Improvement
By improving system so as to reduce hydraulic losses or utilized available head hydraulic potentials, energy conservation can be achieved. Example is the use of rainwater harvesting through storages as supplementary to the main water supply system, saves lot of energy.

1.3.1 Routine Measures

(a) Improving Power Factor
Generally as per guidelines of power supply authority, average power factor (PF) of more than 0.9 is to be maintained in electrical installations. The power factor can be improved to level of 0.97 or 0.98 without adverse effect on motors. Further discussion shows that considerable saving in power cost can be achieved if PF is improved. The low power factor may attract penalty by respective power supply authorities.

(b) Operation of Working and Standby Transformers
As regards operation of working and standby transformers, either of two practices as below is followed: (i) One transformer on full load and second transformer on no-load but, charged, (ii) Both transformers on part load.

(c) Voltage Improvement by Voltage Stabilizer
If motor is operated at low voltage, the current drawn increases, resulting in increased copper losses and consequent energy losses.

(d) Reducing Static Head (Suction Side)
A study shows that energy can be saved if operating head on any pump is reduced. This can be achieved by reducing static head on pumps at suction end or discharging end or both. One methodology to reduce static head on pumps installed on sump (not on well on river/ canal/lake source) is by maintaining WL at or marginally below FSL, say, between FSL to (FSL - 0.5 m) by operational control as discussed below.
(i) Installation where inflow is directly by conduit from dam,
(ii) In such installations, the WL in sump can be easily maintained at FSL or slightly below, say, FSL to (FSL - 0.5 m) by regulating valve on inlet to sump,
(iii) Other installations.

(e) Keeping Strainer or Foot Valve Clean and Silt Free
Floating matters, debris, vegetation, plastics, gunny bags etc. in raw water clog the strainer or foot valve creating high head loss due to which the pump operates at much higher head and consequently discharge of the pump reduces. Such operation results in:
(i) Operation at lower efficiency as operating point is changed. Thus, operation is energy wise inefficient,
(ii) Discharge of the pump reduces. If the strainer/foot valve is considerably clogged, discharge can reduce to the extent of 50% or so,
(iii) Due to very high head loss in strainer/foot valve which is on suction side of the pump.

(f) Replacement of existing Mercury Vapour Lamps & Sodium Vapour Lamps by LED or solar lamps

1.3.2 Periodic Measures

(a) Restoring Wearing Ring Clearance
Due to wear of wearing rings, the clearance between wearing ring increases causing considerable reduction in discharge and efficiency. Reduction in discharge up to 15-20% are observed in some cases. If wearing rings are replaced, the discharge improves to almost original value. Initial leakage through wearing rings is of the order of 1 to 2% of discharge of the pump. Due to operation, wearing rings wear out causing increase in clearance which increases leakage loss and results in consequent reduction in effective discharge of the pump. A study shows that even though discharge is reduced, power reduction is very marginal and as such the pump operates at lower efficiency. Reduction in discharge up to 15% to 20% is not uncommon. Thus the pumps have to be operated for more number of hours causing increase in energy cost.

(b) Reducing Disk Friction Losses
Disk friction losses in pump accounts for about 5% of power consumed by the pump. A study shows that if surfaces of the impeller and casing are rough, the disk friction losses increase. If casing is painted and impeller is polished, disk friction losses can be reduced by 20% to 40% of normal loss. Thus as disk friction loss is about 5% of power required by the pump, overall saving in power consumption will be 1% to 2%. For large pump the saving can be very high.

(c) Scrapping down Encrustation inside Column Pipes
Due to operation over prolonged period, encrustation or scaling inside the column pipe develops causing reduction in inside diameter and making surface rough. Both phenomenon cause increase in friction losses. If scrapping of encrustation is carried out whenever column pipes are dismantled energy losses can be minimized.

1.3.3 Election Aspects

(a) Selection of star rating motor pump
Nowadays, three star/five star rating pump sets are available in the market, which can save 10-15% of power, can be used in place of normal pumping machinery.

(b) Optimum Pump Efficiency
Optimum efficiency of pump can be ensured by appropriate selection such that specific speed is optimum.

(c) Optimisation of Pipe appurtenance
Sluice Valve/Butterfly Valve and Non-Return Valve on Pump Delivery ‘K’ values of sluice valve and non-return valve are 0.35 and 2.50 respectively which amount to combined ‘K’ valve of 2.85. Due to very high ‘K’ value, head loss through these valves is significant and therefore, it is necessary to have optimum size of valves.

(d) Delivery Pipe for Submersible Pump
As delivery pipe for submersible pump is comparatively long and therefore, head loss in delivery pipe is considerable, it is of importance to select proper diameter. Optimum design velocity is around 1.1 - 1.5 m/s. However, pipe diameter should not be less than 50 mm.

1.3.4 Concept for Energy Audit

Energy Audit is a vital link in the entire management chain. The energy man¬ager, while proposing various courses of action and evaluating their conse¬quences, requires a detailed information base to work from energy audit at¬tempts to balance the total energy inputs with its use and serves to identify all the energy streams in the system and quantifies energy usages according to its discrete function.

Energy audit is an effective tool in defining and pursuing comprehensive energy management programmes. It has positive approach aiming at continuous improvement in energy utilization in contrast to financial audit which stresses to maintain regularity. Energy audit provides answer to the question – what to do, where to start, at what cost and for what benefits?

Energy audit helps in energy cost optimization, pollution control, safety aspects and suggests the methods to improve the operating and maintenance practices of the system. It is instrumental in coping with the situation of variation in energy cost availability, reliability of energy supply, decision on appropriate energy mix, decision on using improved energy conservation equipment, in¬strumentations and technology. It has been established that energy saving of the order of 15 to 30% is possible by optimizing use of energy by better housekeeping, low cost retrofitting measures and use of energy efficient equipment at the time of replacements. The developed countries' industry consumes more energy as com¬pared to the developing countries. The energy audit provides the vital information base for overall energy conser¬vation programme covering essentially energy utilization analysis and evalua¬tion of energy conservation measures.

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