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Pump Characteristics

Pump Characteristics In industry, the performance specifications for a particular pump may be known, but the tests are usually based on water as the pumping medium. For liquids of significantly higher viscosity than water, these performance curves may only be accurate at certain flow rates, or they might not be valid at all, and it might be necessary to recalibrate the specifications for higher viscosity liquids. The purpose of this experiment is to examine the performance and characteristics of a centrifugal pump, its motor, and the corresponding piping system, used to pump an ethylene glycol solution. The pump used in this experiment has a performance curve based on water, so pump performance curves, as well as motor performance curves, will be developed over a wide range of flow rates to determine the effect that the liquid viscosity and density have on the performance of this pump. Also, the pressure drop of the piping system will be examined to determine the extent of, if any, fouling in the pipes. · Develop pump performance curves over a wide range of flow rates. This involves relating total head, horsepower input, efficiency, and NPSH as a function of pumping capacity (in gpm), similar to Figure 3-36A (Lab Manual). · Develop motor performance curves over a wide range of flow rates. This involves relating the current supplied to the motor, motor shaft rotational speed, motor efficiency, and the power factor as a function of the load of the motor, similar to the figure on page 21 of the Lab Manual. · Develop the friction factor chart for the piping system. This chart shows the relationship between the fanning friction factor and the Reynolds number over a wide range of flow rates, from which the roughness parameter (e/D) for the piping system can be estimated. · Determine the optimal flow rate delivered by the pump. This value is estimated by neglecting all of the friction head losses outside the loop of the piping system. · Examine the effectiveness of the pump. The performance curves based on the ethylene glycol solution will be compared to those developed using water as the base to examine the effect, if any, the viscosity and density of the liquid has on the pump. A process flow diagram of the pump system is shown in Figure 1. The main components of the system are a centrifugal pump with a 4½-inch impeller, a 2-horsepower motor, a piping system with an effective length of about 285 feet, a rotameter for low liquid flow rates (0-2 gpm), a magnetic flow meter for high liquid flow rates (0-90 gpm), and a tank. The liquid is pumped from the tank and sent to either the rotameter or the magnetic flow meter, depending on what flow rate is desired. After the flow rate is set, it is either sent to the pipe loop for the determination of pressure drop and pipe fouling, or it is recycled back to the tank. An inverted manometer is used to measure the small pressure drops (2 psi or less) at the low flow rates. Figure 1: Process Flow Diagram for Pump and Piping System EXPERIMENTAL PROCEDURE AND DATA COLLECTION First, make sure the valves on the suction side of the pump are wide open, and remain that way throughout the course of the experiment. Next, close the valves on the pump discharge side and the valve at the bypass line from the pump to the feed tank. Measure the pressure the liquid exerts at the pipe exiting the tank while the pump is off to obtain the static head. Take a sample of the ethylene glycol solution and measure both the specific gravity and the temperature of the solution. Open valves A, B, and C on the manometer. If the liquid level in both standpipes is not the same, then the lines need to be bled as follows. Close all the valves to the standpipes, except for valve C, placing a bucket at the end of the tube to catch the fluid. Open the valve to the magnetic flow meter, start the pump, and adjust the flow to about 10 to 12 gpm. Once the fluid purges the line, close valve A, open valve B, and repeat the procedure. Stop the pump and open valves A, B, and C again to make sure that the air has been purged from the system, then close valve C. Close the valve to the magnetic flow meter and open the valve to the rotameter. For five different flow rates, measure the flow rate, suction and discharge pressure, power, voltage, and current to the motor, force exerted by the motor on the spring (by the torque-arm), and the pressure drop across the piping system using the manometer. Close valves A and B, then close the valve to the rotameter and open the valve to the magnetic flow meter. For 5 different flow rates, take the same measurements as were taken with the rotameter, except pressure drop, which is now measured using the pressure gauges on the piping lines. Close the valve to the piping system and open the bypass line valve to the tank (the rightmost valve to the tank in Figure 1). Repeat the measurements using the magnetic flow meter to regulate the flow, with the exception of the pressure drop, which does not need to be measured in this section of the experiment. Turn the pump off, lock out the power to the pump as discussed in Appendix C (Lab Manual), and disconnect the motor from the pump. Turn the power back on, turn on the motor, and measure the power, voltage, and current of the motor during this no-load condition. Turn the motor off, lock out the power, reconnect the motor to the pump, turn the power back on, and turn on the pump to make sure it is working properly. Take a sample of the ethylene glycol solution and measure both the specific gravity and the temperature. Liquid temperature (°C) _______ _______ NO-LOAD CONDITIONS Wattage (watts) _______ PIPE SYSTEM Flow rate (gpm) _______ _______ _______ _______ _______ (Rotameter) Suction pressure (psi) _______ _______ _______ _______ _______ Discharge pressure (psi) _______ _______ _______ _______ _______ Wattage (watts) _______ _______ _______ _______ _______ Voltage (volts) _______ _______ _______ _______ _______ Current (amps) _______ _______ _______ _______ _______ Spring Force (lbf) _______ _______ _______ _______ _______ Loop inlet pressure (psi) _______ _______ _______ _______ _______ Loop outlet pressure (psi) _______ _______ _______ _______ _______ PIPE SYSTEM Flow rate (gpm) _______ _______ _______ _______ _______ (Magnetic flow meter) Suction pressure (psi) _______ _______ _______ _______ _______ Discharge pressure (psi) _______ _______ _______ _______ _______ Wattage (watts) _______ _______ _______ _______ _______ Voltage (volts) _______ _______ _______ _______ _______ Current (amps) _______ _______ _______ _______ _______ Spring Force (lbf) _______ _______ _______ _______ _______ Loop inlet pressure (psi) _______ _______ _______ _______ _______ Loop outlet pressure (psi) _______ _______ _______ _______ _______ BYPASS LINE Flow rate (gpm) _______ _______ _______ _______ _______ (Magnetic flow meter) Suction pressure (psi) _______ _______ _______ _______ _______ Discharge pressure (psi) _______ _______ _______ _______ _______ Wattage (watts) _______ _______ _______ _______ _______ Voltage (volts) _______ _______ _______ _______ _______ Current (amps) _______ _______ _______ _______ _______ Spring Force (lbf) _______ _______ _______ _______ _______ The pump characteristics and pressure drop in a pipe are evaluated in this experiment. In order to study the characteristics, the performance of the pump and the motor are studied. The performance of the pump can be determined by capacity, total head loss, brake horsepower, efficiency, liquid horsepower, pump efficiency, and net positive suction head. The capacity is the rate at which the liquid (ethylene glycol) flows through the pump and is expressed in gallons per minute. The capacity affects other parameters of the pump’s performance. The capacity is chosen such that the efficiency of the pump is high. The total head is the pressure available out of the pump that result from the change in the mechanical input energy into kinetic and potential energy, i.e. the energy that the pump transfers to the liquid. On the pump curve, the total head is the difference between the discharge head and the suction head. The total head for a liquid is dependent on rotation speed and capacity. Thus, the total head for is independent of the fluid that is being pumped and is constant for different fluid under the same capacity and rotational speed. The total head, as seen in Figure 2, decreases as the capacity increases. The efficiency of the pump can be obtained from the brake horsepower, the power required by the pump or the horsepower input to the pump, and the liquid horsepower, energy delivered by the pump to the fluid. As seen in the theoretical performance curve in figure one, the pump efficiency increases as the capacity of the pump increases. The net positive suction head (NPSH) is the absolute pressure available at the pump suction flange and is above the vapor pressure of the liquid. The NPSH ensures that sufficient head of the liquid at the entrance of the pump impeller is present to overcome internal flow losses of the pump. This allows the pump to operate free of vapor bubbles created by the boiling action of the liquid. Figure 2: Theoretical Pump Performance Curve To study the performance of the motor, the following parameters are studied: current to the motor, motor shaft rotational speed, motor efficiency, and the power factor against the load. The current supplied to the motor is the current that would be supplied if the voltage were at the rated voltage of the motor. The motor efficiency is the ratio of the HP output from the motor to the HP input to the motor. Figure 3 shows the theoretical trends of the motor performance curves. A friction factor chart represents friction versus Reynolds number for a wide range of roughness parameters. The roughness parameter has a dimension of length since it may be considered as a characteristic of the height of the projections from the pipe wall. Figure 3: Theoretical Motor Performance Curves In developing a friction chart, the friction head loss must be accounted for. The friction head loss is the energy dissipated in the system - the higher the head loss, the higher the friction factor. The velocity of the liquid also effects the friction factor as they are inversely proportional. Bibliography: Al-Dahhan, Muthanna. ChE 374 Laboratory Manual: Experiments in Heat-Mass- Momentum Transport. Washington University, 1997. Baker, Donald, and Shryock, Howark. Journal of Heat Transfer. “A comprehensive approach to the analysis of cooling tower performance.” August, 1961 Hensley, J.C., ed. Cooling Tower Fundamentals. The Marley Cooling Tower Co. 1982. McCabe, W.L., Smith, J.C., and Harriott, P. Unit Operations of Chemical Engineering. 5th edition. McGraw-Hill, 1993. Perry, R., Green, D., and Maloney, J. Perry’s Chemical Engineers’ Handbook. 6th edition. McGraw-Hill, 1984. Smith, J.M., and Van Ness, H.C. Introduction to Chemical Engineering Thermodynamics. 4th edition. McGraw-Hill, 1987. Welty, James R., Wicks, Charles E., and Wilson, Robert E. Fundamentals of Momentum, Heat, and Mass Transfer. 3rd edition. John Wiley & Sons, 1984.

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