Ways in which Air Lines Can be Utilized

Transcription

1 Ways in which Air Lines Can be Utilized A good irrigator should always be aware of both the static water level (SWL) and pumping water level (PWL) of his irrigation well. Taking a measurement of the water table depth is referred to as sounding the well. An air line is an excellent tool to sound the depth of water in a well. Sounding the water table depth with an air line (or any other sounding method) provides insightful information on the well/pump and a number of very useful management and diagnostic tools. Table 1 lists some of the procedures air lines (or other sounding methods) can be used for. Table 1. Various procedures an air line can be used for. [A] Item Procedure Notes A. Determine the specific capacity (SC) of a well. B. Using SC: determine if a new well has been properly developed. C. Using SC: is the well capable of increased flow? Provides information on quality of well, occurrence of clogging, and well economics. Poorly developed wells will exhibit low SC values. A quick estimate to determine if additional flow is possible. Item A takes longer, but is more accurate. D.. Using SC: is the well clogging over time? When the pumping rate of a pump begins to decline, it is not known if it is due to a worn-out pump or clogged well screens. E. Monitor against pumping off. If pumping water level is at level of pump cavitation can occur. F. Determine aquifer characteristics. G. Calculate pump efficiency. H. Use PWL to estimate flow rate. Some characteristics, such as well recovery rate, provide valuable information to the irrigator. Data on the pumping water level (PWL) is needed to calculate efficiency and are, often times, the information that is the hardest to obtain. Applicable with electric pumps on furrow or flood fields. PWL also provides estimates of pump efficiency and cost of pumping. [A] Besides an air line, other well sounding devices could be used performing these procedures. Data shown in the schematic of an aquifer (figure 1) will be used to illustrate some of the procedures that airlines can be used in and how this information can make you a better irrigation manager Determining Specific Capacity Many of the items relating to water well management center around a hydraulic parameter of the well called specific capacity (SC). SC, in effect, is a report card on how efficient your well is performing. The more efficient the well, the less draw down it experiences as it pulls out water from the aquifer. The unit of SC is GPM/ft. The equation for SC is: Where, SC Q = Flow Rate, GPM PWL = Pumping Water Level, ft SWL = Static Water Level, ft Q Eq. 1 PWL SWL

2 Fig. 1. Diagram of well and its various features.

3 The reason high values of SC are desirable is that they result in less distance to lift water. This distance, the PWL, impacts the cost for pumping by affecting the seasonal amount of KHWs, stand-by charges (based on peak KW used), and initial equipment costs. The SC values of wells will normally change during the season if they are located in aquifers having a fluctuating SWL. The SWL creates head that "pushes the water through the aquifer media and into the well screen, where it can be pumped out. 1 Early in the season when SWL is raised, the SC of the well will be higher; SC then decreases through the season as the water level drops. Additionally, SC values decrease for wells that are being pumped hard. Figure 2 shows results from field tests on eleven diesel irrigation pumps located in the Mississippi Alluvium Aquifer in the southeast Missouri region (SEMO). Each well was evaluated at four different pumping levels by using four incrementally higher RPM rates. As can be seen, average SC values decrease as pumps are revved up; there was an 11% reduction in SC that occurred between the 1250 and 1550 RPM rate. Even though seasonal SWL differences and pumping rates can cause the SC value of a particular well to flutter over time, the stake holders (i.e., the irrigator, well driller, and irrigation equipment dealer) should strive to produce and maintain a well with as high a SC value as possible. Over the 20-year life of a well having only a SC value of 106 GPM/ft (SEMO average) versus having a value of 225 GPM/ft (best tested in SEMO) will end up costing the producer nearly $7,000 more in energy costs. Fig. 2. The average specific capacity at four different RPMs for eleven different wells powered with diesel pumping plants, SEMO. 1 The Darcy Equation describes water movement through a media of different levels of resistance as a function of head.

4 Has your new well been fully developed? The question of whether a well has been adequately developed is, in essence, a question concerning its SC value. The previously mentioned research in Missouri (SEMO) region has shown SC values to average 106 GPM/ft, with the maximum value being 225 GPM/ft, and the minimum value being 55 GPM/foot (90 evaluations). Figure 3 shows the histogram of SC values encountered in SEMO tests in recent years. For whatever reason, the SC values on the east side of the Mississippi River, even though in the same alluvial aquifer, are lower. A private irrigation company recently conducted about a dozen pumping plant evaluations in western Tennessee in this same aquifer and reported SC values ranging from 26 to 91 GPM/ft, with the average being 60.5 GPM/ft. After a well driller is finished drilling your well, he goes through the process of well development. During this process the well is pumped at a high rate to dislodge and purge from it any fine particles originating from both the native media and the gravel pack. He may turn the well on-and-off (a process called raw hiding ) creating a two-way movement of water into the well screens and surrounding media. Pumping the well, even at rates higher than what will be used, will not develop the well fully since this preferentially cleans out the top portions of the screen, leaving the bottom parts not fully developed. Bidirectional flow is required during some points in the development process. This purging is supposed to clear the well of any drilling muds that were used during the drilling process, as well as, dislodged native clay particles. Originally, these muds were developed by the petroleum industry, in part, to prevent water from entering the oil well and to shore up the sides of the well to prevent the sides from sloughing in. Unless these muds and the clay particles dislodged during the drilling process are adequately removed, water flow from the aquifer into the well through its screens will be reduced. Other processed, such as jetting with high pressure water or air, mechanical, and chemical means can be also be used to develop wells. As the well is cleaned out (a process that might last from a few hours to a day or two) the SC value should begin to increase. Well development should continue until the water being pumped to the surface is void of grit. Since the well driller may likely develop the well using his own equipment, an air line must be attached to his pump rig or he must use an e-line; again, a measurement of the flow rate is required. Being aware of the range of SC values in your area (the average SEMO SC value is around 100 GPM/ft.) will help you to determine if the SC value of your well is on par with other wells in your area. If your well s SC value is comparable to similar wells 2 in your area then this is an indication that the well development was successful. It is advised to find out if your well has been adequately developed before the well driller removes his rig from on site. Also, have the driller keep records of what processes were used and for what lengths of time. The development method data regarding your well should be included on his report to the state, but keep a separate copy for yourself, which will probably have more detail then the few summation lines concerning development that are on the official report. 2 Note that other factors (e.g., length of screen, diameter of well, when performed, etc.) affect SC values.

5 Figure 3. Histogram of SC values from pump testing in the Bootheel of Missouri. Can your well produce additional flow? In some cases a producer wants to know if his well can produce additional volumes of flow. The desire for increased flow rate (Q) could be for several different purposes: to irrigate an additional field, or to better meet his current crops water needs, to increase his furrow irrigation efficiency by getting hs rows out faster, or to just water fields in less time, thus qualifying for load management programs. An airline could help solve the question of whether (and how much) more water is available. The term well yield reflects the maximum amount of water a well can sustain; its units are in GPM. With diesel- or propane-pumping units, this questioned is easily answered by simply running the engine at higher RPMs, while measuring PWL and Q as the water level begins to drop. Care should be taken to keep the water level above where the pump is set. This procedure would provide a fair --but not totally accurate-- answer since it normally takes several hours (or even) days before the aquifer s water table is equilibrium. It is best to do this test late in the season when the water table is at its lowest. If the well feeds a pivot, information could also be garnered by running the unit at open discharge and recording the results. However, should the well be equipped with an electric pump running at or near open discharge, such as a furrow irrigation system, there is no practical way to increase the current flow rate to inquire wheter additional flow volume is a possibility. In this case, estimates of what additional volume of flow is possible can be made with known information. Example 1 uses the data from figure 1 to estimate the additional amount of water that could be produced from that well.

6 EXAMPLE 1 How much additional water is the well in figure 1 capable of producing? Given: Q = 1,065 GPM SWL = 46 ft PWL = 82 ft Well depth = 158 ft Pump 130 ft What is the current specific capacity? Q 1,065GPM SC 29.6 GPM / PWL SWL 82 ft 46 ft ft What additional length (AL) of well is available to produce water? Assume that 3.0 ft is needed above pump intake as the submergence requirement for preventing vortex entry into the pump. AL = (Pump depth Submerge requirement) PWL AL = (130 ft 3 ft) 82 ft = 45 ft What additional Q is available? Q = AL SC = 45 ft 29.6 GPM/ft = 1,332 GPM ANSWER Recall that the value of SC decreases as a well is pumped harder, therefore, to be safe reduce the calculated amount by 1/3, giving: Safe additional Q = 1,332 GPM 67% = 888 GPM. NOTE 1: This well might be able of producing even more water if the pump was to be dropped further. As shown, there is 28 feet of clearance between the bottom of the well and where the pump is set at. NOTE 2: A recovery test can also be used to determine if the well is capable of producing additional flow. Is your well screen clogging up? Over time your well screen, and even your media, may -- and probably will-- start to clog up. This is especially so if the well water is high in iron (Fe) or bicarbonates. If your pump flow rate begins to decline it can be due to screen clogging, but it could also be due to pump problems. One method to determine if the decline stems from screen clogging or pump wear is to examine the SC value over time. If the SC value has stayed consistent --even when the flow rate has decreased markedly-- then chances are

7 the problem has more to do with the pump than with the screen. Example 2 illustrates the procedure to determine if decreased flow is screen- or pump-related. EXAMPLE 2 Is the drop off in flow shown in the data set below due to screen clogging or wearing out of the pump? Previously Q = 1,065 GPM SWL = 46 ft PWL = 82 ft Currently Q = 806 GPM SWL = 49 ft PWL = 75 ft What was the previous specific capacity? Q 1,065GPM SC 29.6 GPM / PWL SWL 82 ft 46 ft ft What is the current specific capacity? Q 806 GPM SC 31.0 GPM / PWL SWL 75 ft 49 ft ft ANSWER The current SC is comparable to the earlier SC, so even though Q has dropped nearly 25%, this decrease in Q is probably not due to screen clogging issues. Look for other potential problems. Many parts of Canada have high Fe levels. Iron-related bacteria (IRB) flourish in water like this. Because of the very high cost of drilling new wells, many Canadian municipalities have made it a regular practice to take frequent SC value readings (fig. 4). SEMO also has high native amounts of Fe in our water; this is the reason center pivot spans can turn a rusty color after just a few months of use. Likewise, irrigators in the SEMO region, because of the high level of Fe in our water, should conduct annual SC tests on their wells. The commonly accepted time to do this is during August. Over time IRB will build up coral reefs on well screen mesh that consist of dead bacteria under which the healthy bacteria actually live. However, instead of being skeletal reefs, they are slime reefs. Not only can they shield, but their glutinous nature allows particles to stick to them like fly paper. Thus, it is important to treat wells before the slime reefs become so thick and impenetrable that they shield the

8 bacteria beneath from chemicals being injected into the well to kill them. If the SC value of your well decreases by 10-15%, it is time to investigate and treat the well if needed. If wells experience a SC loss of 25% or more they are very difficult to rehabilitate successfully (Scott and Keevil, 1997); a SC decline of more than 15-25% will require extraordinary measures to restore them to their original level of performance (Umble and Smith, 1999). Fig. 4. Plotted SC values and treatments taken over time for a Canadian municipality. Monitoring against pumping off. If the water level in the well drops close to the pump intake, it may start to draw in water from the vortex area created as water enters the pump. This might cause the pump to behave strangely. Some manufacturers recommend that the water level on a vertical turbine pump always remain at least two feet above the bottom of the pump; the recommended clearance level for different pumps can often times be found on the manufacturer s published pump curve for that pump. If the water level drops further and goes below the pump intake, it will suck air. This is called pumping off. Pumping off causes cavitation to occur inside the pump which can result in damages to the impeller surfaces. Carefully monitoring an airline can keep this from happening. One caveat, however, is that when the water level is close to the top of the pump where the airline was probably strapped in at, the pressure gauge reading will be only one or two PSI; inaccuracies in the gauge at this point may result in not having a correct reading, and allow cavitation to occur. Also, in unconfined aquifers make sure that

9 the water table does not drop below the top of the top screen or air will be introduced that can lead to oxidation of Fe and rapid growth of IRB. Determining aquifer characteristics well recovery test. Transmissivity. Information on water level depth in your well (plus in any surrounding wells) provides useful information about your aquifer and your well. One aquifer attribute, transmissivity (T), is the rate which groundwater flows horizontally through an aquifer (given in gal/day/ft). It can be estimated for confined and unconfined aquifers using SC. Based on certain assumptions, Driscoll (1986) estimated transmissivity of an aquifer by using well SC values as: T = 2000 SC for a confined aquifer, and T = 1500 SC for an unconfined aquifer. Therefore, using the average SC value found in the SEMO studies (106 GPM/ft) then we can estimate the average T for our confined Mississippi Alluvium aquifer to be: T = 2000 SC = GPM/ft = 212,000 gal/day/ft Recovery Test. An air line (or other sounding device) can be used to perform another extremely useful test, the recovery test, which records the time it takes for the water level in your well to recover, moving from its PWL position back to its original SWL position after the pump has been turned off. The faster it does it the more water your well has! The recovery test is comprised of two parts. In the first part, after the pump has been off for several days, it is turned back on and the depth to water is measured at fixed intervals for several hours. This part of the test provides information on the inflow of water moving into your well while pumping is taking place (I p ). Ideally, the pump will be run long enough/be pumped hard enough (but, at a constant Q) so that water level approaches the bottom of the well. If this is done then the recovery test should represent the maximum rate that the well can produce; this rate represents well yield. The pump is then turned off, immediately after which begin frequent soundings of the water table depth. The rate of recovery (in feet per minute) will be rapid and linear for a while then the rate of recovery slows down. This part of the test provides information on the inflow of water that moves into the well during recovery (I r ). It is the rapid, linear period that we are interested in. The equations for I r and I p for are seen in equations 2 and 3. Where, I r = inflow during recovery, GPM r = well radius, ft I 7.48 r w Eq. 2 2 r l r

10 wl r = water-level change rate during recovery, ft/min I 2 Q (7.48 r w ) Eq. 3 p l p Where, I p = inflow during pumping, GPM The first portion of Eq. 3, Q, represents the water that is being removed from the well by action of the pump being run at a constant flow rate. If the well soundings show that the water table is not in equilibrium, but continues to drop throughout the test, indications are that the level of Q being used in the test would not be able to be indefinitely sustained by your well. The second portion of Eq. 3 represents just how much the well was being overdrawn (its units are also GPM). Subtracting this value from the first portion of the equation provides you with the maximum sustainable flow rate for the well. Although it was shown earlier how your SC value can provide an estimate of well yield, the recovery test is actually a more accurate way to answer the well yield question. Example 3 illustrates a recovery test. Although the data are taken on a recovery test for a small well in Pennsylvania, the same strategy applies to our larger SEMO wells. EXAMPLE 3 From the data below calculate the maximum flow rate this well can produce. r = 0.25 ft wl r = 1.7 ft/min (Note: Occurred in first 4 minutes of recovery) Q = 2.5 GPM wl p = 0.16 ft/min (Note: Occurred from minute 140 to 180) ~ After Risser, I I 7.48 r w = fpm = 2.50 GPM 2 r l r 2 Q (7.48 r w ) = 2.50 ( fpm) p l p = (0.24) = 2.27 GPM Answer: Maximum Q for the well is 2.27 GPM.

11 Calculating pump efficiency Energy prices have risen drastically over the last decade. The current high cost of energy compels today s irrigators to seek ways to decrease their cost for pumping. Pump-related savings can come through re-nozzling pivots, reducing friction loss by using larger pipe sizes, and (for electricity users) opting in on load management programs. Another important way to keep irrigation energy costs in tow is to check that your pumping plant runs at top efficiency. Determining if your unit is indeed efficient requires that a pump efficiency test be conducted. A pump efficiency test compares the system s generated water horsepower (power-out) to the rate that energy (electricity, propane, diesel, etc.) is being consumed, be it electricity, diesel, natural gas, etc. (power-in). Equation 4 defines water horsepower (WHP). Where: PWL H 2.31 OP / 3960 WHP Q f Eq. 4 WHP = Water Horsepower, HP Q = Flow rate, GPM PWL = Pumping Water Level, ft H f = Sum of all Friction Losses, ft OP = Operating Pressure, PSI Of all parameters required to calculate equation 4, PWL is sometimes the hardest one to ascertain. This is why it makes sense to install a $35 air line when every new deep well pump is installed. With an air line in place, pump evaluations become much easier to do (and thus more likely to be done). When using an air line, PWL is determined by: Where: PWL = L - (P * 2.31) Eq. 5 L = The air line s length, ft P = The pressure at the air line 3-way valve, PSI To calculate pumping plant efficiency, the water horsepower value (eq. 4) is divided by the power introduced into the system, which is the hourly fuel consumption rate (E R ) times a fuel-specific content constant (E C ). Table 2 lists the gross energy content in BTUs per energy unit and the deliverable BTUs per hour for various fuel sources. The table also includes information about the Nebraska Pumping Plant Performance Criteria (NPPPC) standards for each motor/engine type. The NPPPC desirable efficiency levels represent the near top best achievable efficiency for each fuel type in terms of power in/power out. For example, if the results of an efficiency test on an electric pump was 55.6%, its NPPPC rating would be 55.6% / 66.0% = 84.2%. To read more about water horsepower, see the factsheet in this series entitled, Understanding Water Horsepower.

12 The full equation to calculate diesel pumping plant efficiency is shown in equation 6. Example 4 illustrates how to calculate efficiency for a propane pumping plant. Q PWL H f 2.31 OP Eff WHP BHP E E Eq. 6 C R Where: Eff = Efficiency of pumping plant, % WHP = Water Horsepower, HP BHP = Brake Horsepower, HP Q = Flow rate, GPM PWL = Pumping Water Level, ft H f = Total Friction Losses, ft OP = Operating Pressure, PSI E C = Constant for the energy content of selected fuel, HP/hr E R = Hourly rate of fuel being consumed, fuel use/hr Table 2. Various sources of fuel for pumping, unit of measurement, intrinsic gross energy content per unit, net useable [A] energy per hour of consumption, and Nebraska Pumping Plant Performance Criteria s desirable efficiency level. Fuel Source Unit BTUs per Unit HP per Unit per hour NPPPC Desirable Efficiency Level Electricity 1 KWH % Diesel #2 1 US gal 129, % Propane 1 US gal 84, % Gasoline 1 US gal 114, % Natural Gas 1000 ft 3 1,000, % [A] All fuel sources, other than electricity, are assumed to involve a right angle gearhead which reduces deliverable energy by 5%. Direct coupled electricity experiences no reduction in deliverable energy.

13 EXAMPLE 4 Flow rate = 1,533 GPM PWL = 39 ft Friction Loss = 1.8 ft Operating Pressure = 8.0 PSI Fuel = Diesel o E C = hp-hr/gal o E R = 1.78 gph o DEL = 24.6 % Eff Q PWL H 3960 E E C f 2.31OP R , % NPPPC Test Re sult % 100 NPPPC Desirable Eff Level % 23.7 % % 96.3% Determining the flow rate for electric pumps How an irrigation pump performs is very predictable, especially if its rotational speed that is, its RPMs do not change. This is the case for all electric irrigation pumps, other than those that are equipped with variable frequency drive units. A common motor speed for irrigation motors is 1760 RPM; however, the motor s exact speed will be clearly stamped on a plate attached to the motor. The performance characteristics for these electric-driven pumps has been likened to a locomotive on a stretch of railroad track. Even should the particular location of the locomotive not be known exactly, one thing for sure is, it will be somewhere along that stretch of RR track (fig. 5)! Continuing with analogy of a lost locomotive somewhere on a stretch of RR track, an enterprising switchman might offer: it pulled out of the south station 6 ½ hours ago and its normal speed is 34 miles per hour. Now they are getting close to pin-pointing its location. Another switchman adds: it had passengers that were going to be let off at four different stations and it usually takes about 15 minutes at a stop. The RR company now has a very good idea of where exactly the train is! They started out reasoning that it has to be somewhere on a set RR track, and they developed clues to estimated where exactly on that stretch of RR, the locomotive currently is. However, if the search was for a taxi cab that wasn t tied to a set of RR tracks, its location would be total speculation. Likewise, engineering detective work could be used to find our lost locomotive --the operating point on our pump curve, but of more specific concern, the flow rate of that pump! Very few of the

14 estimated 16,000 irrigation wells in Missouri have WATER METERs. It is important for an irrigator to have a meter for a host of reasons: to determine if he is meeting his crop s water needs, if see if he is keeping his pumping costs in check, to see if he s losing flow, etc. However, at two or three thousand dollars a pop, few water meters get installed. Fig. 5. A pump characteristic curve (for a unit commonly found in the SEMO region) can be likened to a locomotive on a stretch of railroad track. Even if the locomotive s exact location is not known for sure, one thing is certain it must be somewhere on that stretch of track. A $35-air line could provide some of the information a water meter supplies. Here is how: the set of RR tracks are now the pump characteristic curve. The clues used to find where the pump is operating at center around the fact that for every Q on the curve there is an associated TDH. A mathematical procedure can be used to locate that unique position on the curve by simultaneously solving an equation regarding the hydraulics of the irrigation system (e.g., length and diameter of column pipe and line shaft, length and diameter of outlet pipe, operating pressure, etc.) and an equation related to the pump curve. This procedure is done in a newly developed computer program called Q Detective, developed by the Commercial Agricultural Program of the University of Missouri. Q Detective is targeted for electric pumps used on furrow/flood systems for several reasons:

15 They have a constant RPM (a single RR track). These high flow-low head (HF-LH) pumps are greatly affected by drops in the water table. They are numerous in the SEMO area. In order for Q Detective to be successful, a component list of all hydraulic features of the well, starting at the suction end up to the location of the pressure gauge, must be made and then the friction losses must be accurately calculated. Typical hydraulic items would include: Intake screen (if any) - diameter Intake pipe (if any) length and diameter. Column pipe length, diameter, and line shaft diameter. Discharge head valve size. Discharge pipe length to location of pressure gauge and diameter. Other minor friction loss items, such as bends, check valves, water meters, etc. HF-LH can be extremely sensitive to head differences. One commonly used pump in SEMO decreases flow by 13% over just 15 ½ inches of head difference! Therefore, very accurate calculations of friction loss are needed. The estimate of column pipe loss where different diameters of line shaft might exist has been greatly improved using the Manning formula for head loss (Henggeler and Thompson, 2013). Since a pump characteristic curve includes data on Brake Horsepower, costs of operation (per hour or per acre-inch) can be determined for any PWL.

16 Fig. 6. An existing modeled pump installation showing PWL values (horizontal axis) in relation to both expected Q (vertical axis, left) and the cost of water (vertical axis, right). In this example two PWL values are illustrated: 33 and 41 feet. When PWL is 33 feet then Q = 1,230 GPM and pumping costs are $1.40/acre-inch. If PWL drops eight feet and now PWL is 41 feet then Q = 920 GPM and it costs $1.95/acre-inch to pump. References Driscoll, F.G Groundwater and Wells, Second Edition, Published by Johnson Filtration Systems Inc. Henggeler, J.C Q Detective: A Digital Tool to Estimate Flow Rate based on Pumping Water Level. Unpublished spreadsheet software. Henggeler, J.C. and A. L. Thompson Calculating Friction Loss for Column Pipe When a Lineshaft is Present. ASABE Paper No St. Joseph, Mich: ASABE. Risser, D.W Factors affecting specific-capacity tests and their application A study of six lowyielding wells in fractured-bedrock aquifers in Pennsylvania. USGS Scientific Investigations Report Reston, Va.: USGS.

17 Scott, P. and B. Keevill City of North Battleford Well 15: 1997 Field Test of UAB Water Well Treatment Technology. Prepared in collaboration between Droycon Bioconcepts Inc and AAFC PFRA. Umble,A. and S.A. Smith A Cautionary Tale: Well Rehabilitation in Elkhart, Indiana's South Wellfield. Presented at Indiana Section, AWWA, Indianapolis, Indiana, February 24, Available at: Accessed 13 October, 2013.