Variable medium speed pumps combine superior performance with reduced life cycle cost (LCC)

Transcription

1 Variable medium speed pumps combine superior performance with reduced life cycle cost (LCC) Dr. Christian Rossegger Flowserve (Austria) GmbH ABSTRACT Flowserve s new medium speed pump, is a vertical inline single or two stage pump, running at speeds up to 8000 rpm. The pump is rigidly coupled to a high speed induction motor driven by a frequency converter. The frequency drive allows the pump to operate at variable speeds to meet different system characteristics. In many cases the flow control valve can be eliminated and a significant amount of energy is saved, which results in a short pay back time for the slightly higher cost of the frequency drive. Delivery of all components mounted in an electrical cabinet provides additional savings in installation costs. Control of main operating parameters result in higher reliability and longer maintenance intervals for the motor. 1. INTRODUCTION The Medium Speed Pump (MSP) is a vertical inline single stage pump, running at speeds up to 8000 rpm. The pump is especially designed for high heads at low flows and is rigidly coupled to a high speed induction motor. This very compact unit can replace multistage pumps or high speed integral gear pumps, which are being used in the past for these operation parameters.

2 The motor is driven by a frequency converter and is available in EExd design for the use in explosive environments. The frequency converter and an optional pump monitoring system give a lot of control features, e.g. speed control, to reduce operational cost as well as maintenance cost savings due to monitoring of main pump parameters. 2. PUMP DESIGN The pump is designed for vertical inline arrangement and the design follows API th ed. It is available as single stage or two stage opposed impeller version. For low NPSHA applications an inducer is available. The hydraulics consists of an impeller with straight radial vanes (Barske impeller) (1) and a modified concentric volute with a single diffuser channel. To balance the axial thrust the impeller of the single stage version is equipped with balance holes. Figure 1. Sectional drawing of 2-stage MSP

3 2.1 Performance data The following table gives an overview about the pump specification. Performance data of MSP pumps Performance Capacity 1.5 to 25 m3/h 6.6 to 110 USgpm Single stage Head 37,5 to 600 m 125 to 1970 ft Speed 2000 to 8000 rpm Efficiency max. 35% Performance Capacity 1.5 to 25 m3/h 6.6 to 110 USgpm Two stage Head 56,5 to 900 m 188 to 2955 ft Speed 2000 to 8000 rpm Efficiency max. 42% Operating Temperature (fluid) -40 to 200 C -40 to 392 F Conditions Temperature (ambient) -40 to 40 C -40 to 112 F Single stage Suction pressure max. 40 bar 580 psi Two stage Suction pressure max. 10 bar 145 psi Discharge pressure max. 100 bar 1450 psi NPSHR (8000 rpm, Q MAX ) 3,5 m 11.5 ft -with Inducer 2,3 m 7.5 ft The specific speed of the impeller is Table 1. Performance data Ns = n H Q = , = 0,75 5,5 The performance curves (Fig.2) are relatively flat in comparison to hydraulics with higher Ns and drop immediately after reaching BEP (Best Efficiency Point) flow. But this is common for impellers with straight radial vanes. To reach an overall optimum hydraulic efficiency it is essential to reduce the disk friction losses. Based on a research work by Smutny (2) the gap between the impeller shroud and the casing were optimised, resulting in a relatively large gap at the back of the impeller. This gap also has an influence on the pressure distribution and in consequence to the axial thrust developed by the impeller (3). Fig.3 shows the bearing load, which includes rotor weight and hydraulic thrust. The shape is relatively flat due to the large rear gap. For the two stage version the axial thrust is nearly balanced due to the opposed impeller arrangement.

4 Head [m] Efficiency [%] Power [kw] NPSHR [m] Inducer Flow [m³/h] Figure 2. Performance of MSP pumps (single stage)

5 Axial thrust For SG = Due to power limitation SG = 0.8 above 7000 rpm Thrust bearing load [N] rpm 5000 rpm 8000 rpm 6000 rpm 7000 rpm Flow [m³/h] Figure 3. Axial thrust of MSP pump (single stage) 3. MOTOR A Medium speed induction motor, suitable for horizontal and vertical arrangement, is used. The grease lubricated bearings must also take the radial and axial hydraulic forces created by the impeller. The bearing life meets the requirements of API th ed. The following table shows the design parameters of the 4 used motors. Voltage [V] 400 Input frequency [Hz] Power [kw] ([HP]) 22 (29.5) 37 (49.6) 55 (73.8) 80 (107.4) Current [A] Speed [rpm] Power factor 0,89 Efficiency [%] Operation Class S1 Enclosure IP55 Ambient conditions Temperature = 40 C, Altitude = 1000 m 1) Explosion proof 0102 II 2 G EEx d (e) IIC T4 Insulation class / utilized F/F Bearings (grease lubricated) Grease type: Asonic GHY 72 Thrust 6212 P6 C3 TB Line 6212 P6 C3 TB Hyb. Weight [kg] ([lb]) 250 (550) 260 (572) 440 (968) 460 (1012) Table 2. MSP Motor design parameters

6 4. FREQUENCY CONVERTER The frequency converter allows the connected motor to operate with variable speed. Generally the converter is supplied as a completely wired unit including all contactors and fuses. A power choke is included to limit peak voltages. Therefore the cable length between motor and frequency converter can be extended to 350 m without further measurements. Also a RFI filter is included to meet the EMC standards. All necessary safety features are included, so that the unit need only be connected to the electrical power supply. 4.1 Control features The control panel is located in the front door of the casing. The control structure can be switched between: - speed control with or without sensor - torque control with or without sensor (optional) - frequency control The VFD is equipped with an interface for external control. The frequency converter, including all auxiliary systems, like short circuits, filters,... is mounted in an electrical cabinet, which only needs to be connected to the mains supply. This provides significant savings in installation costs. With the sensorless speed control of the frequency drive it is easy to integrate the pump into the main process control. Also the frequency converter monitors the operation of the motor, such as low / high load protection, winding temperature, etc..., which can be implemented into the safety control chain. A smooth motor start up is provided by using a free programmable time - speed ramp. The temperature of the motor is controlled by an electronic evaluation of the signal from the PTC - resistor. A rotating motor will be caught by the frequency converter and the torque will be established again smoothly - this is important if a machine is started automatically after an interruption. All this results in higher reliability and longer maintenance intervals for the motor. 5. PUMP CONTROL AND MONITORING SYSTEM 5.1 General The pump itself can be equipped with an advanced monitoring system. Based on the pump characteristics, this system checks, if the current operating point of the pump is in the allowable operation range. This includes minimum and maximum flow protection and cavitation protection. Additional other parameters such as bearing temperature, mechanical seal monitoring, vibration, etc... can be included in this system. As a result, the customer operates a single package for both pumping and system monitoring, which offers a high reliability and long maintenance intervals.

7 5.2 Function The pump control and protection system uses the standard control of the VFD in combination with an independent SPS to create a real Multi-Purpose Control Application. A lot of features can be activated by the customer himself. As standard the measurement of the suction and discharge pressure is used to calculate the differential head, which is the base for further calculations. For the evaluation of the operation parameters the performance and NPSH characteristics of the pump are stored as polynoms in the CPU of the SPS. The actual operation point is given by the differential head calculated from the measured pressures in conjunction with the pump characteristics. Based on the following operation data - Suction pressure - Discharge pressure - Differential head - Flow - Fluid temperature - Motor speed the SPS decides, if there is a malfunction or not. Additionally the customer can choose to monitor characteristic data of the - Pump - Mechanical seal - Drive The limits used for the "failure decision" are given in the menu "Settings". 5.3 Mathematical Model This is based on the measurement of suction and discharge pressure (velocity head is neglected). p = pd ps p... differential head in bar pd... discharge pressure in bar ps... suction pressure in bar H p 10 = ρ g 5 H... differential head in m ρ... density in kg/m³ g... acceleration due gravity in m/s² Subsequent to this result, the SPS calculates the reference head with the following formula:

8 2 nref. Href. = H n Href.... reference head in m nref.... reference speed in rpm n... real speed in rpm Now the reference flow Qref. will be calculated (based on the theoretical pump H/Q curve). The formula for the calculation is: 3 2 Qref. = a Href. + b Href. + c Href. + d The constants a, b, c and d for the third power polynom and nref. are set in the menu "Factory Settings". The real flow is also calculated by: Q = Qref. n nref. As option the measurement of the real flow by a flowmeter is also possible. In this case the above calculation (no. 5) is not needed. Based on Href. and Qref. the SPS monitors the pump operation. 5.4 Control Panel The Touch Panel has several menu levels. The first level shows the introduction screen with three options, "Back", "System" and "Continue". The system links automatically to the system parameters. For entering the level 2 the continue button must be selected. There the pump can be started, stopped and reset. Additional the client is able to change the location for control (VFD, Local and Remote). VFD... Frequency drive panel Local... Control box directly to the pump on site Remote... Touch Panel Simatic TP170BC Remote shall include: Start/Stop, Speed up/down, Manual/Automatic The third level shows the "Monitoring", "Settings" and "Fault and Warnings" menu Monitoring Several subroutines lead through the menu. Subroutine Operation Data I. Suction pressure analogue input (0-20mA, Hart) II. Discharge pressure analogue input (0-20mA, Hart) III. Differential head calculated

9 IV. Flow analogue input (0-20mA, Hart) or calculation V. Fluid temperature analogue input (0-20mA, Hart) VI. Motor speed analogue input (4-20mA) from frequency drive Subroutine Pump Data I. Bearing temperature digital input II. Vibration x and y indicator digital input Subroutine Mechanical Seal Data (API 52, 53) I. Pressure switch digital input II. Level switch low / high analogue input (4-20mA) III. Temperature analogue input (4-20mA) Subroutine Drive Parameter / Data Linked to the Profibus-DP connection of the frequency drive Settings The settings menu offers subroutines, which can be configured by the customer and others, which only can be configured by the factory. A) Customer Subroutine Language I. English (Standard) II. German, French, Spanish, Italian (additional Software Package) Subroutine System Parameter Fluid density ( kg/m³) Vapor pressure ( bar) Subroutine Options I. Fluid monitoring (Selection of Temperature, Flow with Yes or No) II. Pump monitoring (Selection of Temperature, Vibration with Yes or No) III. Mech. Seal monitoring ( Selection with Yes or No) Subroutine Calibrate Offset for High and Low Limit B) Factory settings Subroutine Parameter 1 nref., Qmin., Qmax., constants for polynomia, g and line diameter

10 Subroutine Parameter 2 a) Inducer Selection with Yes or No including the correct constants The constants will be set directly Fault and Warnings: In this menu the limits for the hydraulic operation range are set. Subroutine - Active Faults I. Dry Run (pd 0.9 * ps and n 1000 rpm) II. Cavitation according to formula ps 10 ρ g 5 pv 10 ρ g ,94 Qref. + 4 ds g 2 NPSHRref. NPSHRref. is calculate: 3 2 e Qref. + f Qref. + g Qref. + h with inducer If the pump is equipped with an inducer NPSHRref. is: 3 2 i Qref. + j Qref. + k Qref. + l without inducer III. Bearing Temperature (Pt100 wired to the motor terminal box) IV. Fluid Temperature (Pt100) V. Mech. Seal Fault (directly linked to the instruments) VI. Flow (calculate by SPS or measured directly by a flowmeter) VII. Vibration (directly linked to the instruments) VIII. Frequency Drive Fault (digital input via Profibus-DP) Overview over all parameters Monitoring features - Suction pressure ps in bar - Discharge pressure pd in bar - Differential head H in m - Flow Q in m³/h - Fluid temperature Tm in C - Motor speed n in rpm - Bearing temperature Tb - Vibration switch S(x,y) - Pressure switch PS - Level switch LSL, LSH - Mech. Seal temperature Ts in C - Drive data like motor current, torque, power, etc.

11 Setting features Customer: - Density ρ in kg/m³ - Vapor pressure pv in bar - Options activ/inactiv - Calibrate high/low limit Factory Settings: - Parameter 1 consist of nr, Qmin, Qmax, constants for polynomia, g and ds - Parameter 2 consist of inducer selection and constants settings for reference curves Protection features - Dry Run observation - Mech. Seal temp., Seal fluid level and pressure - Cavitation observation - Flow observation - Bearing temperature - Vibration observation - Fluid temperature - Frequency drive fault controlling Other features - Three control methods available (speed, capacity and pressure) - Easy parameter setting and monitoring via Touch panel - Individual configuration via Profibus-DP - Instruments could be in EExia (favourable price) - Three control stands (VFD panel, Touch panel and Control panel) - Prepared for Ethernet communication to servicing the system via Brunn 6. ADVANTAGES OF THE MSP PUMPING SYSTEM REGARDING LIFECYCLE COST Within the power range up to 80 kw the price for a complete package, consisting of pump, VFD and control system is competitive to traditional pumping systems. The main total lifecycle cost (4) depend on - Price for the equipment - Installation cost - Operation cost - Maintenance cost 6.1 Price for the equipment The price for a pump plus VFD is comparable to similar multistage or high speed geared pumps. The VFD offers as standard a lot of control and monitoring features ( see chapter 5.4).

12 The control system together with the necessary instruments is on the same price level as a conventional pumping system equipped with different kind of sensors, like temperature, pressure, vibration, etc. 6.2 Installation All components are installed in an electrical cabinet, these include short circuits, RFI Filter to comply with EMC standards, control switches, etc. So in the field the system must be wired up between pump and VFD, the same as by conventional pumping systems, but there are no other costs, like motor starter, installation of transmitters for sensors, etc. The VFD just requires connection to the mains supply. In many processes a flow control valve can be eliminated by using pump speed control to meet the required flow and head. Furthermore, traditional pumps need a minimum flow valve to protect the pump from overheating and dry running due to vaporisation of the pumped fluid. The minimum flow is usually pumped back to the suction tank, which results in additional cost for the required piping and valves. Equipped with the control system the MSP can use the minimum flow protection feature. The system checks the actual operation data against the allowable operation range and sets necessary actions to avoid operation below a certain flow rate. This is done by reducing speed or shutting down if operation within the allowable range cannot be reached. So there is no need for a minimum flow valve and the corresponding piping, which gives a further cost advantage compared to conventional pumps. 6.3 Operation cost By using pump speed control instead of a flow control valve to maintain the required flow and head for the process a lot of energy can be saved. Considering that the majority of installed pumps are running at part load the following example gives an overview about the possible energy savings. Base is a 80 kw (107.4 HP) pump/motor with the requirement to keep the process pressure constant over the flow range. Constant pressure operation is required for many processes.

13 Let us take an assumption for a load profile, which may be typical for some kind of industries: Load profile P=80 kw (107.4 HP) Energy E for VFD (speed control) Energy E for standard drive (throttle control) Load Operation Hours OH E = P L/100 OH E = P (0.5+L/200) OH 1) [% of flow] [h] [kwh] ( [HPh] ) [kwh] ( [HPh] ) 20 % (8582) (25747) 40 % (34330) (60077) 60 % (180230) (240307) 80 % (240307) (270346) 100 % (128736) (128736) (592185) (725213) 1) A rising linear power curve from shut off (50% P) to rated point (100% P) is assumed, which is quite common for centrifugal pumps Table 3. Typical load profile Using assumed energy cost of 0,25 /kwh for Europe and 0,3 $/kwh for USA, the energy savings are = kwh ( HPh) per year. This is equivalent to or $ The example shows a quite high cost savings potential especially for processes with a lot of flow variation. This is quite common for industrial boilers, where often a back up pump is installed to meet the high flow requirement. For example: A boiler is rated for a flow of 5 m³/h and a head or 300 m. Sometimes a flow of 20 m³/h at 250 m is required. These operation data can hardly reached by one multistage pump, so a second pump is needed for parallel operation to reach the flow of 30 m³/h. A speed controlled MSP can handle these flows without problem. The technology allows an MSP pumping system to be easily integrated into the main process control. There is no need anymore for local control rooms with the necessary equipment and manpower. 6.4 Maintenance cost. The mechanical design of the pump is relatively simple and consists of only a few parts. The motor bearings, which take the hydraulic loads, are grease lubricated with regreasing intervals of 5000 hours. So no lubrication system is needed. The control system protects the pump from dry run and cavitation. Additionally vibration and bearing temperature sensors observe the mechanical integrity. For hazardous fluids the seal

14 system is also monitored, by leakage detection based on pressure or level measurement in the seal vessel. If one of the parameters is above or below the set limit the system creates an alarm or shuts down to avoid catastrophic damage. Besides, monitoring the parameters over a longer period of time allows an operator to create successful maintenance planning and to avoid unplanned plant shut down due to pump failure. All this together contributes to low maintenance cost and high MTBF (Mean Time Between Failure) in comparison to conventional pumps. 7. CONCLUSION The MSP is a modern pumping system with outstanding hydraulic and control features. It is an alternative to conventional pumps in many applications. It should be considered for new plants as well as for refurbishing existing ones, where the MSP creates additional value for the user. It can be configured to customer needs and optimised for the process. 8. REFERENCES 1) Dahl, Tryg, Centrifugal pump hydraulics for low specific speed applications, Proceedings of the sixth international pump users symposium, Texas, ) Smutny, Andreas, Der Einfluss verschiedener Parameter auf den Axialschub einer mehrstufigen horizontalen Kreiselpumpe, Thesis of Diplome, Technical University Vienna, ) Daily, J.W., Nece R.E., Chamber Dimension Effects on Induced Flow and Frictional Resistance of Enclosed Rotating Disks, Journal of Basic Engineering, March ) Pump Life Cycle Costs: A guide to LCC analysis for pumping systems, Europump and Hydraulic Institute, 2001