Pumping Plant Performance Evaluation

Transcription

1 Pumping Plant Perfrmance Evaluatin Prepared by: Rbert Evans, Extensin Agricultural Engineering Specialist R. E. Sneed, Extensin Agricultural Engineering Specialist J. H. Hunt, Extensin Agricultural Engineering Specialist Published by: Nrth Carlina Cperative Extensin Service Publicatin Number: AG Last Electrnic Revisin: June 1996 (KNS) Intrductin Irrigatin requires large quantities f energy. Mst f the energy is cnsumed by the pumping plant. High pumping csts usually result frm pr maintenance, excessive wear, r mismatched cmpnents. Annual energy csts run abut $25per acre in Nrth Carlina where applicatin rates average 8 inches per acre per year. Energy cst may be several times higher when water is applied with prly maintained and perated systems. This publicatin discusses the causes f pr irrigatin system perfrmance. Sme f these causes can be identified and crrected by the irrigatr. Others require specialized evaluatin equipment, and the crrectins shuld therefre be perfrmed by specially trained technicians. The energy required t pump a given quantity f water is affected by: 1. pumping lift-the vertical distance frm the water surce t the discharge pint (elevatin) in the field; 2. perating pressure-pressure n the discharge side f the pump necessary t prvide the prper pressure t sprinkler nzzles in the field; 3. pumping plant efficiency. Energy requirements decrease as pumping lift and perating pressure decrease and pumping plant efficiency increases. T maximize pumping plant efficiency, the cmpnents shuld be matched t the site-specific cnditins f pumping lift, perating pressure, and flw rate.

2 Pumping Plant Cmpnents Irrigatin Pumps Pump designs vary depending n the applicatin. Centrifugal pumps (self-priming r nn-self-priming) are used when pumping water frm surface surces and shallw wells (Figure 1). Turbine pumps (vertical-shaft r submersible) are used when pumping frm deep wells (Figure 2). Specific features f each pump are discussed in Extensin Service publicatin AG-389, Water Supplies fr Subirrigatin. Figure 1. Electric-mtr-driven centrifugal pump drawing water frm a surface water supply. Figure 2. Electric-mtr-driven turbine pump drawing water frm a deep well. Sme pumps are mre efficient than thers (Figure 3). Mst types f pumps are available ver a wide range f perating efficiencies. Pump efficiency is the rati (expressed as a percentage) f the water hrsepwer f the discharge water divided by the energy delivered t the pump by the pwer unit. Turbine pumps tend t have the highest efficiencies but ver a relatively narrw range f perating cnditins. Efficiencies f sme vertical-shaft turbine pumps apprach 90 percent.

3 Figure 3. Typical efficiency ranges f irrigatin pumps. Mst irrigatin dealers ffer a variety f pumps spanning the entire efficiency range shwn. Values are representative fr mst pumps, but there are sme exceptins in all categries. Pump characteristic curves shuld be cnsulted t select a pump that prvides the desired efficiency fr a particular set f perating cnditins. Submersible pumps are generally lwer in efficiency. Depending n the size and design, they are typically 3 t 5 percent less efficient than vertical-shaft turbine pumps f the same capacity. Centrifugal pump efficiencies vary the mst, ranging frm 55 t 85 percent. As a general rule, high-head-pressure centrifugal pumps-thse perating at a pressure greater than 40 punds per square inch (PSI) -tend t be less efficient than mderate-head pumps perating at a pressure less than 40 PSI. As the size f centrifugal pumps decreases, the efficiency decreases. Als, sme f the less expensive small pumps are fairly lw in efficiency. Self-priming centrifugal pumps tend t be least efficient. Nrmally, pump cst increases as efficiency increases (Figure 4). Operating csts fr fuel increase as pump efficiency decreases (Figure 5). Thus, savings in perating csts f mre efficient pumps usually cmpensate fr higher initial csts after a few years f use. Target pump efficiencies shuld be in the range frm 75 t 85 percent.

4 Figure 4. General relatinship between initial cst and efficiency f irrigatin pumps. Figure 5. Influence f pump efficiency n fuel cst. Minimum cst (zer percent increase) based n drive unit perfrming at Nebraska Standard. Once the perating pressure (head) and system capacity (flw rate) have been determined, cnsult pump characteristic curves t select the mst efficient pump fr the specific cnditins. Mst pump manufacturers prvide characteristic curves fr each f their pumps. These curves shw the relatinship between head, hrsepwer, capacity, and efficiency. Irrigatin pumps shuld be selected t match specific cnditins f pumping rate and perating pressure (expressed as ttal dynamic head, r TDH).

5 Figure 6 shws a typical characteristic curve fr an electric-mtr-driven pump. Figure 6. Typical pump characteristic (perfrmance) curve fr a mtr-driven pump. There is a narrw range (shaded area) f pressure and vlume within which the pump will perfrm at maximum efficiency. This pump is best suited fr peratin in the range frm 400 t 500 gallns per minute (GPM) at a TDH f 100 t 150 feet with a suctin lift (NPSHR) f 14 t 19 feet. In this perating range, the pump efficiency is greater than 80 percent. Maximum efficiency fr this pump is 82 percent when pumping 460 t 480 gallns per minute at a TDH f 135 t 145 feet and a suctin lift f 17 feet. Pump wear and reduced efficiency ccur quickly when the water cntains sand r ther abrasives. Many wells in Nrth Carlina are drilled int water-bearing sand r sandstne. T prevent mvement f sand int the pump, it is equipped with an intake screen. Imprper sizing f well-screen penings will accelerate pump wear and reduce its efficiency. Imprper screening f the suctin line f centrifugal pumps can allw abrasives in surface water t enter. When water is pumped frm a surface surce, pnd sediments may be picked up, thus accelerating pump wear, especially near the end f the irrigatin seasn when the water level is lw. Pwer Units Pwer units are f tw basic types: electric mtrs and internal cmbustin engines (either direct-drive engines adapted slely fr irrigatin r tractrs with a pwer takeff (PTO) used t drive a pump). Where electricity is available, it is the mst efficient pwer surce. As shwn in Table 1 n page 4, the efficiency f electric mtrs ranges frm 80 percent fr mtrs under 7.5 hrsepwer t ver 90 percent fr mtrs f 75 hrsepwer r larger. One disadvantage f electrical systems is that electric lines in many rural areas d nt run near the water surce, and three-phase pwer is ften less available in these areas. Mst electric cmpanies require three-phase pwer fr mtrs that deliver mre than 5 r 10 hrsepwer, depending n the applicatin and the cmpany prviding the service.

6 Typical Attainable Electric Mtr Full Lad Mtr Matched Size Pumping Plant Size Efficiency Pump Efficiency* Efficiency** (hrsepwer) (percent) (percent) (percent) and larger Table 1. Typical Electric Mtr and Pumping Plant Efficiencies as Influenced by Mtr Size *Values shwn are typical fr centrifugal pumps. Submersible pumps range 3 t 5 percent higher and turbine pumps range 5 t 10 percent higher fr sizes under 75 hrsepwer. Abve 75 hrsepwer, a few centrifugal pumps may apprach efficiencies f 88 percent, whereas large submersible and turbine pump efficiencies peak at abut 90 percent. Since pump efficiencies are quite variable depending n type, manufacturer, and size, cnsult pump perfrmance curves fr a specific applicatin. **Values btained by multiplying pwer surce efficiency range (in this case, fr an electric mtr) by the matched pump efficiency range. Internal cmbustin engine drives are available t meet mst pwer requirements f 3 hrsepwer r mre. Generally, small engines are gasline perated, whereas larger units are usually diesel. Mst pwer units with less than 40 hrsepwer plus sme f the larger diesel units are air cled. Internal cmbustin engines are inefficient. Gasline engines are nly 20 t 26 percent efficient, while diesel efficiencies range frm 25 t 37 percent. The advantage f the diesel's higher efficiency and lnger perating life must be balanced against the higher initial cst. If the engine is t be perated ver a lng perid f time, the diesel engine is usually the mst ecnmical, but if the pump is perated fr nly a few hurs each year, a gasline engine is mre ecnmical. Perfrmance standards fr engines and pumping plants are published annually in the Nebraska Tractr Test Reprt. The reprt cntains perfrmance data fr mst agricultural pwer equipment marketed in the United States. These unbiased perfrmance data are determined by University f Nebraska engineers fr equipment adjusted t manufacturer's recmmendatins. Standards fr irrigatin pumping plants are given in Table 2. Hrsepwer- Hurs(1) Per Unit Water Hp-hr(2) Per Energy Surce Energy Unit f Energy Unit f Energy(3) Diesel Galln Gasline(4) Galln Prpane(4) Galln Natural Gas(6) 1,000 cu ft 88.9(5) 66.7 Electricity kwh 1.18(7) 0.885(8) Table 2. Nebraska Perfrmance Standards fr Irrigatin Energy Surces

7 Surce: Drn, T.W., P.E. Fishbach, D.F. Eisenhauer, J.R. Gilley, and L.E. Statesn, It Pays t Test Yur Irrigatin Pumping Plant. Publicatin EC-713. Lincln: University f Nebraska, Cperative Extensin Service. (1)Hrsepwer-hurs is the wrk being accmplished by the pwer unit with lsses cnsidered. (2)Water hrsepwer-hurs is the wrk being accmplished by the pwer plant at the Nebraska Perfrmance Criteria. (3)Based n 75 percent pump efficiency. (4)Taken frm Test D f Nebraska Tractr Test Reprts. Drive lsses are accunted fr in the data. Assumes n cling fan. (5)Manufacturers' data crrected fr 5 percent gear-head drive lss and n cling fan. (6)Assumes natural gas energy cntent f 1,000 Btu per cubic ft. (7)Assumes 88 percent electric mtr efficiently. Pump Drive The pump drive transfers energy frm the pwer unit t the pump. Electric mtrs are frequently cupled directly t the pump thrugh the mtr line shaft. When prperly aligned and lubricated, direct drives intrduce very little energy lss. In sme cases, pwer is transmitted thrugh a belt drive s the pump speed can be changed. Belt drive efficiencies are typically between 85 and 95 percent. When the pump is driven by an internal cmbustin engine, a gear drive may be necessary t cnvert engine speed t prper pump speed. Engine-driven centrifugal pumps drawing water frm surface surces are usually clse cupled (Figure 7). Clse-cupled gear drive efficiencies shuld exceed 98 percem. Engine-driven turbine pumps (Figure 8) require a right-angle drive t change the hrizntal directin f the engine drive shaft t the vertical directin f the pump line shaft. Right-angle drive efficiency is abut 95 percent. Figure 7. Clse-cupled engine-driven centrifugal pump.

8 Figure 8. Right-angle drive fr engine-driven turbine pump. Inefficient pumping plants cst mre t perate. Lw pumping plant perfrmance usually results frm (1) an inefficient pump r pwer unit, (2) mismatching f the pump and pwer unit, r (3) a cmbinatin f these. Pumping plant efficiency can be tested t prvide infrmatin n adjustments r retrfits needed t imprve efficiency. T determine the pumping plant efficiency, measure pumping lift, discharge pressure, flw rate, and fuel usage. Fuel usage can be read directly frm the electric meter r by measuring the liquid level in the fuel tank ver a specific time perid. A pressure gauge is usually installed n the discharge side f the pump t indicate perating pressure. Sme systems have in-line flw meters t indicate pumping rate. Thus, the nly equipment needed is a device t measure water level (relative t the pump) at the water surce and a flw meter if ne is nt present. Once the abve measurements have been made, the pumping plant perfrmance can be cmputed frm the relatinships in the bx n the fllwing page. An example f a pumping plant perfrmance analysis is shwn in Table 3 n page 6. In this example, the pumping plant is perfrming at 86 percent f the Nebraska Standard. Fr additinal explanatin f the terminlgy used in Table 3, cnsult Extensin Publicatin AG-452-5, Irrigatin Management Strategies t Imprve Water and Energy Efficiencies. In establishing perfrmance standards, it is recgnized that individual pumping plant cmpnents are nt 100 percent efficient. The perfrmance standards were established t cmpare and evaluate systems based n "reasnable efficiency expectatins." Fr an electrically driven pumping plant, the standard is based n a mtr efficiency f 88 percent and a pump efficiency f 75 percent (see the ftntes t Table 2), resulting in an actual efficiency f 66 percent (0.88 x 0.75 = 0.66). Thus, an electrically driven pumping plant perfrming at the standard is nly 66 percent efficient in cnverting electric energy t mechanical energy. In the

9 example, perfrming at 86 percent f the standard, the abslute pumping plant efficiency is nly 57 percent (0.86 x 0.66 = 0.57). Interpreting Pumping Plant Perfrmance There is n exact efficiency level that indicates when adjustments r repairs are necessary. The cst f efficiency imprvements must be balanced against the pssible energy savings. Factrs that influence the decisin are: the extent f pr perfrmance fuel cst hurs f peratin each year specific cnditins f perating pressure and pumping rate. Nrmally, adjustment csts are small, and any resulting imprvement in pumping plant perfrmance will pay dividends within ne year. Repair r replacement f minr cmpnents is mre cstly, and payback may take ne t three years. Minr repairs are usually justified when the system perfrmance is between 60 and 80 percent f the Nebraska Standard. Fr the example system perating at 86 percent f the standard, adjustments are warranted but repair r cmpnent replacement is nt. Replacement f majr cmpnents, such as the engine, mtr, r pump r changing t a mre efficient system are mst cstly. Nrmally, replacement f majr cmpnents is nt warranted unless the perfrmance rating is less than 60 percent f the standard. The payback perid fr replacement f majr cmpnents is typically five years r mre. Analyzing Pumping Plants Driven By Electric Mtrs The pumping plant analysis indicates whether r nt verall perfrmance is acceptable but des nt indicate which cmpnent is respnsible if verall efficiency is lw. Overall pumping plant efficiency can be cmputed by multiplying the efficiencies f the individual cmpnents. Electric mtrs are relatively efficient and reliable. Pr pumping plant perfrmance can usually be attributed t lw pump efficiency. When the pumping plant test indicates pr perfrmance, assume a reasnable mtr efficiency (usually 90 percent) and

10 then cmpute the pump efficiency. In the example, the verall pumping plant perfrmance was 86 percent f the standard. The abslute pumping plant efficiency was 57 percent. Assuming a mtr efficiency f 90 percent, the abslute pump efficiency is 63 percent (the verall efficiency divided by the mtr efficiency, 0.57/0.90 = 0.63). This value shuld be cmpared with the apprpriate pump characteristic curve (like the ne shwn in Figure 6) t determine if the pump is being perated utside its highest efficiency range. The fllwing are likely causes f lw pump efficiency: Lw pressure and lw flw rate: These cnditins indicate excessive pump wear. Internal pump cmpnents shuld be evaluated by a qualified representative f the pump manufacturer. Lw pressure and high flw rate: These cnditins indicate an excessive number f sprinklers perating at ne time r excessive wear f sprinkler nzzles. Reduce the number f sprinklers until the prper flw rate is reached. If the pressure is still belw standard, the pump is wrn. If pump efficiency returns t the recmmended range, check nzzles fr wear and replace them if necessary. If there is n evidence f excessive nzzle wear, t many sprinklers are perating at ne time. Take crrective actin by reducing the number f sprinklers if pssible, adjusting the pump t deliver mre pressure fr revised flw with all sprinklers perating at ne time, r replacing the pump with ne rated fr the desired perating cnditins. High pressure and lw flw: These cnditins indicate that the discharge may be partially blcked r there are nt enugh sprinklers perating. Increase the number f sprinklers. If the prblem is nt crrected, lk fr blckage n the discharge side f the pump. If efficiency imprves t the rated range, cntinue the peratin with added sprinklers. If perating pressure and flw are in the high-efficiency range but efficiency is lw, this indicates a change in suctin lift, usually resulting frm a drp in water level at the water supply r a blcked intake (suctin hse). Analyzing Pumping Plants Driven By Internal Cmbustin Engines Mre cmprehensive testing prcedures are necessary t identify specific causes f lw pumping plant efficiency when internal cmbustin engines are used because the engine efficiency must als be determined. In additin t the measurements discussed abve, specialized equipment is needed t measure the actual pwer utput f the engine. One such device, a dynammeter, allws the engine t be perated with a knwn lad. Strain gauges can be used t measure stress/strain deflectin f the drive-shaft. This measurement can be cnverted t engine pwer utput with a calibratin curve. Once engine efficiency has been determined, the pump can be diagnsed in the same way as fr electrically driven systems. The imprtance f perfrming an engine analysis is demnstrated in Figure 9. Fr the gasline engine shwn, fuel cnsumptin increases nearly 5 percent fr each percentage decrease in engine efficiency. Pr engine efficiency cmbined with lw pump efficiency quickly increases fuel csts (Figure 10). Yu can measure the efficiency f yur engine-driven pumping plant. If verall efficiency is lw, cntact a pump r engine dealer, cnsultant, r Extensin Service agent wh has the necessary engine analysis equipment.

11 Figure 9. Increase in fuel cnsumptin (gasline) resulting frm pr engine efficiency. Figure 10. Effect f verall pumping plant efficiency n fuel csts. values are based n a price f $1.10 per galln fr gasline. A zer percent increase is the minimum pssible fuel cst with pump efficiency f 100 percent and engine perfrming at the Nebraska Standard. In general, the lwer the existing efficiency, the shrter the payback perid fr repair r replacement f system cmpnents. Higher fuel prices, increased perating pressure and pumping rate, and mre annual perating hurs shrten the payback perid required t recver majr repair r replacement csts.

12 Cntact yur cunty Extensin Service ffice fr mre infrmatin and assistance. Making a prfitable decisin abut repairs and imprvements requires a cmplete ecnmic evaluatin f existing perfrmance and the cst f alternatives. Cntact yur persnal accuntant r an Extensin ecnmist befre making cstly repairs r replacing majr cmpnents. Distributed in furtherance f the Acts f Cngress f May 8 and June 30, Emplyment and prgram pprtunities are ffered t all peple regardless f race, clr, natinal rigin, sex, age, r disability. Nrth Carlina State University, Nrth Carlina A&T State University, U.S. Department f Agriculture, and lcal gvernments cperating. AG Return t: BAE Extensin Publicatins