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CE 3620: Water Resources Engineering
Spring 2012
Lab 3: Pumps in Series & Pumps in Parallel BACKGROUND
Pumps are used to transfer fluid in a system, either at the same elevation or to a new height. The needed flow rate depends on the height to which the fluid is pumped. Each pump has a headdischarge relationship that is inversely proportional (i.e., if a higher flow rate is needed, then less head or pressure will be produced by the pump, and vice versa). This head-discharge relationship, also known as the pump characte ristic curve, is provided by the pump manufacturer. In civil engineering applications, a single pump often cannot deliver the flow rate or head necessary for a particular system. system. However, two pumps (or typically typically more in practice) can be combined in series to increase the height to which the fluid can be pumped at a given flow rate, or combined in parallel to increase increase the flow rate associated associated with a given value of head. The H32 pumping apparatus employed in this lab demonstrates how the combined pump characteristic curve (whether in series or parallel) compares with that of the single single pump. In theory, if two pumps are combined in series, the pumping system will produce twice the head for a given flow rate. Similarly, if two pumps are combined in parallel, the pumping system is expected to have twice the flow rate of single pump for a given head. LAB OBJECTIVES •
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To develop pump characteristic cha racteristic curves for a single pump, two pumps in series, and two pumps in parallel by measuring head (h) and flow rate (Q) using the experimental apparatus. To develop theoretical pump characteristic curves for pumps in series and pumps in parallel experimentally derived single pump characteristic curve. To compare the experimental and theoretical pump characteristic curves for pumps in series and pumps in parallel.
EXPERIMENTAL PROCEDURE
1. Adjust the valves on the apparatus so that a single pump is active. 2. Use the valve downstream of the pump(s) to control discharge and the corresponding co rresponding head. For a given head (pressure) reading, reading, use the volume-time method to measure measure the flow rate. Measure the flow rate three times. times. Record the values in the appropriate table on the attached data sheet. 3. Measure the head (pressure) downstream of each pump. Record the values in the appropriate table on the attached data sheet. 4. Repeat steps 2 and 3 for five different discharges/head readings, plus with no flow. 5. Adjust the valves on the apparatus such that two pumps in series or two pu mps in parallel are active. Repeat steps 2 – 4 for each pumping system.
RESULTS
Compare the experimental and theoretical pump characteristic curves for pumps in series and pumps in parallel. Comparisons should be made both graphically and in terms of the percentage error. Record measurements taken during lab in the tables on the attached data sheet. Type these results in a spreadsheet and include th em in the report. CALCULATIONS
Show sample calculations for one trial (i.e., for on e flow rate/head reading) as outlined below. Note: sample calculations for each pumping system should be provided when alternative forms of a given equation are needed. Label variables and use units in your calculations. •
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Calculate flow rate using the volume-time method. Calculate the change in head (ΔE p) based on your measured/experimental values. Note: ΔP corresponds to the head produced by the pump. Calculate the change in head (ΔE p(th)) expected based on theory. Calculate the percentage error (theoretical versus exp erimental) in the change in head for the pumps in series and in parallel. 2
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For ease of unit conversion: 1 bar = 100 kN/m , 1000 liters/sec = 1 m /sec. GRAPHS
Create two graphs showing pump characteristic curves as follows: •
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Graph 1 - Pumps in Parallel Single pump line (reference pump A) o Theoretical line for pumps in parallel o Experimental line for pumps in parallel o Graph 2 - Pumps in Series Single pump line (reference pump A) o Theoretical line for pumps in series o Experimental line for pumps in series o
DISCUSSION
1. List possible causes for differences between your predicted valu es and experimental values of the pump characteristic curves for pump s in series or parallel. 2. Given the data for reference pump A (N = 1850 rpm) collected/derived in this lab, use similarity laws to predict the values of discharge and head for pumps rotating at 3000 rev/min. Provide a table with the predicted values of discharge and head for a single pump, pumps in parallel, and pumps in series rotating at 3000 rev/min. (Similarity laws are discussed in Section 4.2.1 of Water Resources Engineering by Wurbs & James.)
DATA SHEET Table 1. Single Pump Data Trial Time (s) Volume (L) 0 1
