Centrifugal Pumps in a Piping Network
Objective The objective of this experiment is to illustrate the practical application of pumping and piping theory. Additionally, this experiment will exercise your ability to deal with open-ended problems. Introduction Consider a centrifugal pump used to deliver a fluid through a pipeline that contains various fittings and an elevation change. A typical performance curve for the pump is given in Figure 1. Figure 1 indicates that the head increase, ∆h, across the pump declines as the flow rate, Q, increases. However the head loss increases with increasing flow (why?) for the piping system and is typically of the form ∆h = α + βQ 2 for turbulent flow. The constant, α, is a hydrostatic term and if positive would imply that the fluid is being pumped to a higher elevation. The constant, β, represents the frictional losses through the system due to the resistance of the pipe and associated fittings. In this particular example, there is s stable point of operation, P, where the two curves cross. Clearly the same flow passes through the pump and piping system, and the head increase provided by the pump must equal the head loss in the pipeline.
Pump Head Loss or Increase ∆h P
Q Figure 1. Centrifugal Pump Performance in a Piping Network
In addition to exploring the behavior of a centrifugal pump in a piping system, you will also study frictional losses through pipes, valves and fittings in this experiment. Equipment The equipment is a pumping-piping system located in the Unit Operations Laboratory. It allows flow to be directed through any of six flow paths with different restrictions. Flow is measured using rotameters, paddle wheel flow meters and/or timed weighings. The pressure drop across a
CHEE 4500 Summer 2002
pipe, valve or fitting is obtained by subtracting direct pressure measurements made before and after a restriction using an Omega PCL4000 digital pressure calibrator. Experimental Procedure Startup: 1. Fill the feed tank about half full of water. 2. Verify the system is leak-free 3. Plug the system in, turn on the pump, and check for proper operation. 4. Clear the system of air bubbles, using appropriate valves. They rise to the clear pipe on top, and then to the atmosphere through the top valve near the tank. This valve requires periodic venting. Data Measurement: You must decide what data to collect to complete the requested tasks described below. 5. For each flow restriction (pipe, valve, or fitting), set the flow at the desired setting and measure the pressures before and after the restriction. This procedure will also work for the pump to measure the pressure rise across the pump for various conditions. Be sure to record the system flow rate as well as the pressure readings in your data. 6. At several flow rates, verify the flow meter readings by checking the paddle wheel meter and rotameters using timed weighings. Data and Analysis Please complete the following tasks as part of your report. 1. Produce a friction factor chart for smooth pipe based on flow through the ½”, 1”, and 1½” pipes using your experimental data. 2. Perform an analysis of pressure drop through globe and gate valves, piping ells, contractions and expansions. Plot the losses as a function of Reynolds Number. Calculate the loss coefficients and compare them to accepted values in the literature. 3. One way of representing pressure drop in fittings, rather than the number of velocity heads, is the use of equivalent pipe length. Would this be a better method of representation? Calculate the equivalent length of pipe for your fittings and valves and present your data in this form as well. 4. Produce a pump curve (see figure 1) for the pump from experimental data. 5. On the same graph with the pump curve, plot a predicted system curve using literature loss coefficients and friction factors (for a path of your choosing through the network and one for which you have collected the necessary data). Compare the predicted operating point, P, with the experimentally determined flow rate. Comment on your results.
CHEE 4500 Summer 2002
Figure 2. Physical Schematic of Pressure Measurement System
Figure 3. Manometer Schematic Equations for the Manometer Force balance equations:
PA − (hH − hT )( g / g c ) ρ H 2O + (hH − (hx + hA ))( g / g c ) ρ H 2O = PM A PB − (hH − hT )( g / g c ) ρ H 2O + (hH − (hx + hB ))( g / g c ) ρ H 2O = PM B
Subtracting the second equation from the first we get:
CHEE 4500 Summer 2002
PA − PB + (hB − hA )( g / g c ) ρ H 2O = PM A − PM B
∆P = ( PA − PB ) = ( PM A − PM B ) + (hA − hB )( g / g c ) ρ H 2O
(pressure drop across device) = (difference in meter readings) + (correction for difference in fluid height in clear tube)