Variable speed wastewater pumping

Transcription

1 WHITE PAPER Variabe speed wastewater pumping November 2013 Variabe speed wastewater pumping During the ast years the industry has seen a significant increase in the adaptation of variabe drives (VFD s) in wastewater transport systems. A genera desire is to better contro the wastewater fow through the pump stations. Variabe speed systems can provide a more fexibe and powerfu soution compared to using constant-speed pumps. Introduction Wastewater pumps have traditionay been operated in an on-off mode for sma as we as arge wastewater pump stations. Because the infow of wastewater is varying over time and is often ony a fraction of a pump s required capacity engineers have ooked for ways to operate wastewater pumps at a reduced capacity in order to optimize performance whie reducing operating costs. Before soid state variabe drives became commonpace speed contro soutions such as twospeed motors and variabe votage windings were used to achieve variabe fow pumping. During the ast years the industry has seen a significant increase in the adaptation and acceptance of variabe drives (VFD s) in wastewater transport systems. A genera desire is to optimize the wastewater fow through the pump stations. Variabe speed systems can provide a more fexibe and powerfu soution compared to using constant-speed pumps. Pumping with variabe speed pumps can resut in better process contro, energy savings, smoother operation and reduced maintenance costs for the pump station, when appied correcty. Some wastewater operators that have instaed variabe speed pump systems have noted that there were no savings in energy usage and some municipaities actuay reported an increased energy usage. An increased incidence of pump cogging has aso been experienced. This is directy attributed to reduced operating speed. The power absorbed by the impeer wi drop by the cube of the speed change and a wastewater pumps abiity to pass arger soids wi diminish as speed is reduced for conventiona noncog pumps. The increase in energy usage comes from two phenomenon, partia cogging of the impeer and operation away from the pump s best efficiency point. The first is a CONTENTS PAGE Pump and pump system aspects... 2 Pump sumps... 2 Pump suction and discharge piping... 2 Check vaves... 2 Water hammer... 2 Minimum speed... 2 Pump contros and variabe speed drive aspects... 3 Controing pumps... 3 Drive starting current and starting torque... 3 Drive cooing and ventiation requirements... 4 EMC and motor requirements... 4 Specific energy... 5 Process contro aspects... 6 Variabe speed pump systems... 6 Minimizing energy usage... 7 Pump sump eve contro... 8 On-Off contro... 8 Constant eve contro... 8 Variabe eve contro... 9 Minimum fow contro... 9 Pump mechanica aspects Reverse rotation Pump hydrauic aspects The hydrauic end Concusion resut of extended run times as VFD driven pumps wi have ong operationa cyces at ower speeds, the other is a resut of pumping in systems where the static head accounts for a high degree of the overa system head. It is important to consider a pump system and pump station aspects in order to achieve a we operating pump station. These aspects incude system curves, pump and motor seection, process contro, eectrica aspects, potentia for energy saving, contro strategies, pipe system components and more. In doing so, it is possibe to maximize the energy savings from variabe speed wastewater pumping whie ensuring cog free operation. This paper discusses many of these aspects. P 1

2 Pump and pump system aspects Pump Sumps It is important to keep the pump sump cean in order to prevent an accumuation of organic and inorganic materia resuting in odor and maintenance probems. Variabe speed contros can be programmed to perform sump ceaning cyces on a reguar basis in order to reduce the risk of sediment buidup and accumuation of foating debris. During a sump ceaning cyce, the sump eve is pumped down to a eve at which the pump starts to draw air ( snore ). As the iquid eve is drawn down surface vortices wi form and grow in strength and. The moment before the pump starts to snore strong forces add high oca veocities from surface vortices wi draw foating debris into the pump. It is essentia to imit pump operation at snore to a minimum in order to minimize the amount of air in the discharge ine and to prevent potentia exposure to high pump vibration eves. increasing the fuid veocity by increasing the pump speed. Depending on the type and concentration of heavy sediments and grease in the media, the degree of sedimentation wi differ; the higher the concentration of soids, the greater the risk of sedimentation. A variabe speed controed pump station increases the fexibiity to fush the force main by increasing the pump speed. The of fushing depends upon the system design, the degree of contaminants and the minimum veocity required to maintain optima operating conditions. Risk Optima speed Sedimentation probems Air cogging Energy consumption Pipe fushing Fuid veocity Figure 2: When fushing the discharge ine on reguar basis, it is possibe to reduce the minimum fuid veocity beow 0.7m/s without having sedimentation probems. Check vaves In appications with variabe speed pumps the iquid veocity wi often be ower than norma in the pipe system. Swing check vaves have ower friction osses at ow veocities than ba check vaves. Therefore the energy savings at reduced speed wi increase when a swing check vave is chosen instead of a ba check vave. Figure 1: A wastewater sump covered with foating debris. Pump Suction and Discharge Piping The fuid veocity in the force main affects the degree of force main sedimentation as we as the energy consumption due to friction oss. Operating with high veocities in the force main reduces the risk for sediment buidup athough it increases the energy consumption. In contrast, operating with ow fuid veocities in the force main reduces energy consumption and increases the risk of sedimentation. It is therefore important to consider optimize fuid veocity with energy consumption when seecting suction, discharge and force main pipe sizes. With variabe speed pumping it is possibe to reduce fuid veocity beow the normay recommended 0.7m/s (2.5 fps) for extended periods because of the possibiity to periodicay fush the discharge ine by temporariy Water hammer Variabe speed pumps have the possibiity to soft-start and soft-stop by graduay increasing and decreasing the pump speed thereby reducing the impact of water hammer and wear and tear on the check vaves.. When the pump stop sequence is ong (sow pump speed deceeration) the iquid wi deceerate sowy in the force main. Note that a VFD wi not function during a power faiure without immediate battery backup power avaiabe, thereby imiting its effectiveness as a water hammer protection device. Minimum speed Certain pump designs may have imitations on the owest aowabe pump speed. The minimum speed may be dependent on proper operation of a cooing system, shaft resonance frequencies and other issues. Consut the pump manufacturer to make sure any imitations are identified. P 2

3 Pump contros and variabe speed drive aspects Controing pumps Functions to start and stop pumps can be achieved with simpe reay systems or soid state controers. Dedicated pump controers and pump supervision units are commony used today. Variabe speed drives are frequenty used as a means to contro the speed of induction motors. For arger and more compex systems genera purpose Programmabe Logic Controers (PLC s) can be used. Comprehensive custom software wi then have to be written for the specific appication. A variabe speed drive must be controed by software to be abe to contro a pump, the pump station must have a master controer to propery sequence and contro a pumps. The most modern pump controer soutions for wastewater pumps are hosted inside variabe speed drives. Appication specific software can be pre-instaed and pre-configured in a drive dedicated for a specific appication for a certain pump and motor. A system start-up can be as simpe as connecting the eve sensors and power cabes to the drive and then pushing the start button for the system to be up and running. Drive starting current and starting torque The rated current of a drive must be greater than the rated motor current for the seected pump. To hande under votage and overoad situations, a current margin of 20% is generay recommended. In wastewater pumping appications, it is important that the motor can suppy the impeer with fu torque to hande potentia cogging situations. It is therefore necessary for a wastewater pump to be equipped with a variabe speed drive capabe of deivering nomina torque at startup and sustaining twice the nomina torque at fu operating speed for at east one second. There are two main types of drives: scaar contro and vector contro drives. The scaar contro drive generates a predefined votage as a function of the. A vector contro drive is based on a mode of the motor and anayzes the votage and current required to contro the pump. Using the feedback data from the motor, the starting current can be decreased whie the starting torque can be increased. (%) Starting current Direct onine Scaar Vector Figure 4: When starting a pump using a VFD with vector contro, the starting current is consideraby ower than when starting the pump direct onine. (%) 100 Torque Direct onine Scaar Vector Figure 3: Preprogrammed and pre-configured inteigent contro for a dedicated appication is quick and easy to insta and start up. Figure 5: When starting a pump at zero speed, the torque of pump motor controed by a VFD with vector contro is the same as a pump motor with a direct onine starter. P 3

4 Drive cooing and ventiation requirements Variabe drives typicay have a 96-97% efficiency, which means that 3-4% of the transmitted power is ost as heat. To prevent drive overheating, these osses must be transferred to the ambient air. To enabe cooing air to fow around the drive, free space above and beow the drive is required. For ambient temperatures above 40 C (104 F), derating of the drive is typicay required. Air conditioners or ventiation fans may be required at higher ambient temperatures. For drive instaations at high atitudes (exceeding 1000 m (3300 ft.) above sea eve) derating of the drive is generay required in order to compensate for poor cooing at high atitudes. the drive chops the votage in order to transform the votage to the desired. Some induction motors require a 5-10% power margin when operated on a variabe speed drive, other motors have been designed to operate on drives up to the rated shaft power. When operating a drive, consut the motor manufacturer for power margin requirements. EMC and motor requirements In order to ensure a power suppy free from excessive harmonics and prevent eectrica equipment probems, it is important to foow good EMC (Eectromagnetic compatibiity) engineering practices when designing a drive system. Best practices when designing a VFD system incudes: Use screened power cabes and position the cabe screen as cose as possibe to the connection terminas Use screened signa cabes Use twisted signa eads aong the entire cabe route Ground the cabe screens at both ends Use the shortest possibe power cabes Separate signa and power cabes by more than 500 mm (1.5ft) in parae runs The use of Cass H motor insuation and tricke impregnation provides soid protection against corona effects for nomina motor votages up to 600 V and pump cabe engths up to 50 m (150 ft). Longer suppy cabes can cause fu wave refection, which can doube votage stresses on the windings. When cabe engths exceed 50 m (150 ft), the use of a sine wave fiter is recommended. Eectric harmonics create parasitic torques in the motor which increase the oad and stress on the motor. Harmonics can be reduced by the use of a common mode fiter. A switching of approximatey 7-8 MHz is recommended to obtain a good compromise between the eectrica noise eves and the osses in the motor and the drive. The switching determines the rate at which Figure 6: A cass H motor insuation system and VPI motor impregnation provides good protection against corona effects. P 4

5 Specific energy Introduction The most usefu measure to compare energy efficiency in pump systems is specific energy. Specific energy is defined as the amount of energy it takes to move a certain iquid voume in pump system. A ower number means ower energy consumption. The specific energy vaue takes into account a parts of a pump system, i.e. eectrica, mechanica and hydrauic efficiencies incuding osses in the pipe system. Note that the specific energy vaue is vaid for the particuar pump system that it was cacuated for; it cannot be compared to another pump system without making adjustments for system differences. Specific energy can aso be cacuated by using this aternate formua: h 1 kwh E X FORMEL s = g * X m X FORMEL X X FORMEL h = X 1 kwh E s g * X FORMEL X 639 Mga where E s = Specific energy [kwh/m³]; [kwh/mga] h = Tota head deivered from the pump [m]; [ft] η = Tota efficiency of the pump [%] ρ = Density of the fuid [kg/m³]; [b/cu-ft] g = Gravitationa constant [m/s²]; [ft/s²] Tota efficiency of the pump and motor. Energy Voume Voume Figure 7: Specific energy is a measure of how much energy it takes to move one cubic meter of iquid from one point to another. Specific energy is cacuated with the foowing formua: X EFORMEL X s = energy X FORMEL voume X kwh m 3 kwh Mga where E s energy voume = Specific energy [kwh/m³]; [kwh/mga] = Energy required to transport a given iquid voume (kwh) = Voume of pumped iquid (m³, Mga) P 5

6 Process contro aspects Variabe speed pump systems Infow to a wastewater pump station varies consideraby over a 24-hour period. The infow is typicay ow during the night and peaks once in the morning and once again in the evening. Fow duration diagrams are often used to visuaize the fow variations (Figure 8). To minimize energy consumption, we shoud focus on two areas: a) Reducing the tota pumping head Tota head is defined as the sum of static head and osses. Since friction and point osses are directy proportiona to the fow squared, it is desirabe to reduce osses by decreasing the fow. As a wastewater pump normay is sized to hande maximum infow to the pump station (dupex stations), it is possibe to reduce the pumped fow during norma operation and thereby reduce the tota head. b) Maximizing pump efficiency To achieve maximum pump efficiency, it is important to seect pumps that deiver sustained efficiency (sef-ceaning pumps) and that operate as cose to the best efficiency point as possibe. A pump that is operated at reduced speeds shoud be seected we to the right of the best efficiency point (BEP) at fu speed in order to achieve maximum efficiency when the speed is reduced. Determining the optima performance of a typica variabe speed pump system requires the anaysis of pump curve (Figure 9). The bue pots show three different pump system curves S1, S2 and S3. Pump system S1: This system curve represents a ift system because the static head is arger than the friction osses. The energy saving potentia of variabe speed operation in ift systems is sma because the pump efficiency decreases faster than the tota head decreases when the pump speed is reduced. Pump system S2: This system has a better energy saving potentia than S1 because the tota head decreases faster than the efficiency does when the pump speed is reduced. Pump system S3: This is primary a circuation system (itte or no static head). Here the potentia for energy saving is the greatest because the pump system efficiency is constant whie the tota head decreases as the pump speed is reduced. 8 Duration diagram Infow (/s) Time (hours) Figure 8: A wastewater fow duration diagram shows infow as a function of time. P 6

7 25 VFD performance curve 20 60% 70% S3 S2 S % Head (m) 10 80% 70% 60% Hz Fow (/s) 30Hz Hz 40Hz Hz 50Hz 250 Figure 9: Possibe energy savings depends upon the system curve and the pump curve. Minimizing energy usage Specific energy for a given system varies with the pump speed. The optima speed from an energy savings perspective is when the pump runs at the that corresponds to the minimum specific energy (see figure 10). The optima is dependent on severa factors. These incude the pump performance curve and the system curve, which together generate different specific energy curves. The optima speed for system curve S3, as an exampe, is approximatey 23 Hz when using the second formua and figure 10. Finding the optima when operating a pump at variabe speed presents chaenges. One method to identify optima speed is through the use of agorithms. Inteigent pump contros ike the Fygt SmartRun have agorithms that provide automatic optimization of speed for minimum energy usage. Fygt SmartRun uses an iterative process to determine the optima speed and adapts for system changes such as reduced pump performance or increased sedimentation in the force main. Conducting a theoretica study of the pump system is another method to identify optima. However, there are drawbacks to this approach, incuding the possibiities that changes to the system occur over any given period of time. These pump system cacuations are imprecise at best or impossibe due to ack of documentation of the pipe system. Specific energy (kwh/m³) Specific energy S3 S2 S Frequency (Hz) Figure 10: Different system curves wi yied different specific energy curves. The minimum energy pump speed is ocated where the curves are at their minimum. 60 P 7

8 Pump sump eve contro On-Off contro Pump stations with constant speed pumps operate in an on-off mode where distinctive start and stop eves enabe the wet we to draw and fi as the pumps are started and stopped. Here the pumps are operated at their intended speed and the pumps non-cog performance wi be as good as the design aows for. Variabe speed contro Two-speed pumps and variabe speed operated pumps aow the station to operate at fows coser to actua infows as opposed to design fows. Correcty impemented variabe speed operation can ead to ower energy consumption. The next sections wi discuss the benefits and drawbacks with different variabe speed contro strategies. Traditiona constant eve contro The traditiona contro method for variabe speed operation in a wastewater appication is a constant eve scheme (Figure 11). The controer uses the iquid eve as a reference vaue. At ow infows, the pump wi operate at too ow a speed and consequenty at a very ow efficiency, thereby wasting energy. In addition to this the risk of sedimentation in the pipes and pump sump increases. The pump is often operating outside of its preferred operating range with a reduced ife expectancy as a resut. This wi happen when the pump is run at a speed ower than the energy optima speed, i.e. the speed at which the pump s specific energy consumption is at a minimum. Partia pump cogging is aso more ikey to occur for non-sef-ceaning pumps, such as traditiona cosed impeer and vortex pumps. Optima constant eve contro Using a combination of reduced speed and intermittent draw-fi operation is the most energy efficient method of controing variabe speed pumps and it is recommended for wastewater pump stations. It wi eiminate the time the pump runs at a speed beow the optima speed. To achieve intermittent operation and ensure the pumps do not start and stop too frequenty, it is important to estabish a suitabe distance between the start and stop eves, which enabes the pumps to operate at the minimum energy speed for a sufficient period of time. When modern constant eve contro is used as a contro strategy, it is necessary to set the minimum energy as the optima in order to maximize energy savings and operationa reiabiity. The contro scheme can be seen in figure 12. Liquid eve Liquid eve Start pump Norma infow High infow Extreme infow Start pump High infow Norma infow Extreme infow Stop pump Pump Pump Optima Maxima Optima Maxima Figure 11: Traditiona constant eve contro. Norma infow: Operating at too ow. High infow: The wi be controed to keep the pumped fow equa to the infow. Extreme infow: Operation at maximum speed cannot hande the infow. (Green coor symboizes operation at ow specific energy and red coor symboizes operation with high specific energy.) Figure 12: Optima constant eve contro. Norma infow: Intermittent draw-fi operation at optima speed. High infow: The wi be controed to keep the pumped fow equa to the infow. Extreme infow: Operation at maximum speed cannot hande the infow. P 8

9 Start pump Liquid eve High infow Extreme infow Liquid surface downward veocity Norma infow High infow Stop pump Norma infow Min veocity Extreme infow Pump Pump Optima Maxima Optima Maxima Figure 13: Variabe eve contro. Norma infow: Operating on/off at optima. High infow: The wi increase inear to the water eve unti the pumped fow is equa to the infow. Extreme infow: Operating at maximum speed cannot hande the infow. Figure 14: Minimum fow contro. Norma infow: Operating on/off at optima. High infow: The wi increase to secure that the iquid surface veocity is equa to the minimum veocity. Extreme infow: Operation at maximum speed cannot maintain a iquid surface veocity equa to the minimum veocity. Variabe eve contro Another commony used method to contro wastewater pumps is a variabe eve contro (Figure 13). The advantage here is that the infow can be buffered in the sump, resuting in a smoother pumped fow. With this method of contro, pump speed is a function of the iquid eve in the sump. Compared to constant eve contro, variabe eve contro is a softer contro strategy, which heps smooth out shorter infow peaks. Minimum fow contro A fourth contro method is based on the use of the timederivative of the sump iquid eve [iquid surface veocity]. This entais setting a minimum fow, which ensures that the veocity of the iquid eve decrease is set at a given minimum rate. If the iquid eve does not decrease with this given rate, the pump speed wi automaticay increase. In order to be energy efficient, the speed, must not fa beow the optima speed even if the iquid eve decreases at a sower rate. The minimum fow contro can be seen in figure 14. P 9

10 Pump mechanica aspects Reverse rotation Lower speeds and sower pump acceeration/deceeration are beneficia with respect to reducing the mechanica and therma oads on the pump and on the connected systems as forces on bearings, joints and seas wi be ower. Pump reversing is sometimes used as a means to uncog pumps. Drive reversing can be done without mechanicay overoading the impeer joint and risking mechanica damage. Different pump manufacturers have different imitations on reversing. Pump hydrauic aspects The hydrauic end Pump cogging is a common wastewater pumping probem. The key design criteria for a non-cog pump is its abiity to pass soids without cogging the pump. Cogging can be a fu or a partia cog of the impeer or the voute. A fu cog exists when the pump has ceased pumping; this condition is easy to detect and highy undesirabe. A partiay cogged pump however, is harder to detect and most often goes unnoticed because the pump sti deivers fow, athough it is reduced. This can go on for ong periods of time, wasting substantia amounts of energy. If the pump is operated continuousy, the pump efficiency wi tend to graduay decine to eves ess than haf of the cean water efficiency or ower. The cog may be caused by foreign objects but more often it s caused by norma wastewater contents, sometimes in arger sizes or heavy accumuations. There are severa different types of cogging phenomena, which can affect the pump performance (head, fow and input power) in different ways depending upon the pump design. Conventiona channe impeer wastewater pumps, both singe- and muti-vane as we as vortex impeers are prone to cogging due to soft and fibrous objects accumuating on the eading edges of the impeer or at the center of the impeer. This resuts in reduced pump efficiency and substantiay increased energy consumption. When the pump operating cyce ends, back fushing often ceans the impeer and pump, thereby pump efficiency is partiay restored. If a pump operates at reduced speed it is ikey to see heavier accumuation of debris because the pump operating cyce is onger and no back fushing of the impeer occurs. This is the main reason to avoid ong pump cyces Conventiona wastewater pump in continuous operation Hydrauic efficiency Specific energy Conventiona wastewater pump running intermittenty Hydrauic efficiency Specific energy (Time) (Time) Figure 15: A conventiona wastewater pump is partiay cogged and the efficiency of pump decreases over time, resuting in increasing specific energy Figure 16: When a wastewater pump is turned off, a reverse fow is generated which fushes the eading edges of the impeer and the voute. P 10

11 The most demanding wastewater pump scenario is a traditiona non-cog pump operated on a variabe speed drive. Very often the pump contro software wi operate the pump(s) at reduced speeds for ong periods of time (hours and days). This ack of pump cycing means the pump does not benefit from the back fush that occurs each time the pump is stopped. Compounding the issue for the pump, the contro software is often times programmed to perform a soft stop in addition to a soft start. This means that the pump speed is genty brought down unti the pump is stopped. The consequence is that a cogged pump wi not benefit from the important back fush generated at a hard pump stop, and the pump is ess ikey to regain its origina efficiency For pumps with sef-ceaning pump hydrauics, which now have been on the market for over 10 years, the risk of debris accumuation is very ow. This is due to the hydrauic design with extremey back swept eading edges, a reief groove, and sometimes other hydrauic and mechanica augmentations. The sef-ceaning mechanism remains constant and is independent of fow and speed. Therefore, as ong as the duty point remains within the pumps aowabe operating range, a sef-ceaning pump can hande reduced speeds, even down to as ow as 50% of fu speed without an increased risk of cogging. This resuts in sustained high efficiency. In those rare cases where a foreign object causes a compete bockage of the pump, the bockage may be removed automaticay by initiating a pump ceaning cyce via the pump controer. Inteigent pump contros can sometimes perform this automatic pump ceaning cyce. During the pump ceaning the impeer is manipuated unti it is free of the cogged materia. Sef ceaning pump hydrauics Hydrauic efficiency Specific energy (Time) Figure 17: Fygt N-pumps feature mechanica sef-ceaning, which impies that the eading edges stay cean and the efficiency is sustained at a high eve. P 11

12 Concusion Using a standard drive for variabe speed pumping in wastewater appications requires many engineering hours, investigations and impementation of the right contro agorithm for wastewater, to achieve energy savings and increased pumping reiabiity. Some drives are preprogrammed for controing pumps with agorithms for various pumping appications. These drives must sti be configured and engineered for the specific appication. Appication specific inteigent wastewater contros that are pre-programmed with advanced agorithms and preconfigured to ensure reiabe wastewater pumping and ease of commissioning are just now entering the market. These devices wi increase the pump stations reiabiity and wi assure that cacuated energy saving wi be reaized. In addition no unnecessary engineering hours wi be spent finding a custom contro agorithm to a common appication chaenge. When pumping at variabe speed, it is necessary to consider the system curve, the pumped media, the pump type, the contro method and the process requirements to achieve reiabe pumping with a high overa efficiency. Pumping with variabe speed pumps can resut in better process contro, energy savings, smoother operation and reduced maintenance costs for the pump station, when appied correcty. FWP Variabe speed wastewater pumping White Paper 12/2014 US P 12 Fygt is a brand of Xyem. For the atest version of this document and more information about Fygt products visit Xyem, Inc.