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LAB 7: MINOR HEAD LOSSES IN PIPE FLOW

LAB 7: MINOR HEAD LOSSES IN PIPE FLOW

7.1. Objective This lab considers the minor head loss associated with changes in flow pattern. The corresponding minor loss coefficient will be determined.

7.2. Theory of Experiment Minor head losses occur in piping systems because of changes in the fluid`s flow pattern. This occurs in flow through valves or fittings such as expansions, contractions, and bends. An energy loss occurs at the point where the valve or fitting is located. For a piping system with a large amount of valves and fittings, the sum of these minor head losses can be significant. Therefore, it is important to have a method for determining these head losses

where: hL-m = head loss due to changes in flow pattern (minor loss), (ft, m, etc) k = minor head loss coefficient, (dimensionless) g = acceleration due to gravity (ft/s2 , m/s2 , etc) V = average velocity of the fluid in motion (ft/s, m/s, etc)The minor head loss coefficient, k, is varied for each type of valve or fitting. If the minor loss coefficient is previously known for a certain type of valve or fitting, the minor loss can be readily determined using Equation 7.1. However, if it is not known, it can be determined using the Energy equation:where: p = pressure at the respective location (psf, N/m2 , etc) � = specific weight of the fluid (lb/ft3 , N/m3 , etc) � = elevation above the datum at the respective location (ft, m, etc) V = average velocity of the fluid in motion at the respective location (ft/s, m/s, etc) g = acceleration due to gravity (ft/s2 , m/s2 , etc) hp = energy added by a pump or pump head (ft, m, etc).

ht = energy extracted by a turbine or turbine head (ft, m, etc) hL = energy losses or head loss (ft, m, etc) Head loss (hL) is energy lost from the fluid system and can exist as either major (hL-f) or minor (hL-m) losses:

7.3. Equipment Description

A vertical panel with several pipe sections and fittings will be used for this laboratory exercise (Figure 7.1). A hydraulic bench will be used to supply water to the panel. Depending on the section of pipe to be analyzed the supply line can be connected at different inlets. A flow meter is connected to the desired pipe section outlet. There are a number of different fittings and valves that can be tested on this apparatus. For this lab, pipe sections 1, 7, 8 and 9 will be used. They include a 90 degree knee (miter) bend, a 90 degree elbow (threaded), a 90 degree smooth bend, a sudden expansion, a sudden contraction, a slanted seat valve and a gate valve (Figure 7.2). The two valves have distinct internal features that produce different flow patterns (Figure 7.3). The flow rate through the device is controlled with the hydraulic bench. There are two types of manometers used to measure the static head drop across each fitting. A six manometer bank will be used for the bends, and a larger dual manometer will be used for the other fittings.

7.4. Experimental Procedure Data collection for this lab will occur simultaneously by using two hydraulic benches to supply water for two distinct pipe sections. Students will be responsible for collecting the data corresponding to the pipe section connected to the assigned hydraulic bench. After lab is completed, data collected for the different pipe sections will be shared. The following is a general procedure on how to collect data for each section.

7.4.1. Equipment Setup and Manometer Calibration Connect the inlet pipe to the bench feed. Turn on the hydraulic bench. Open the flow control valve on the bench so that the flow is sent to the apparatus, wait until no more air bubbles are seen and reduce the flow rate to calibrate the manometers. Make sure the vent and draining valves of the 6-tube manometer bank are closed. Connect the manometer lines to the six taps on pipe section 1, and connect the other end of the lines to the manometer bank in the same order. Open the drain valve on the bottom of the bank to remove trapped air, and then close it. Close the hydraulic bench valve. Slightly open the vent valve and draining valve of the manometer bank until manometer levels drop to about 60 mm, and close the valves. At this point all manometers should read equal values because there is no flow through the pipe (i.e. static conditions). Note: the lab GTA will have the manometer bank already adjusted, procedure is described so that you have a guide to write your report.

7.4.2. Data Collection Adjust the flow rate to collect your first set of data. Based on historic equipment performance, flow rates should be above 300 L/h to obtain accurate measurements. Verify that your initial conditions satisfy this constraint by measuring the time required to collect 10 L of water in the bench reservoir. If needed increase the flow rate. Data will be collected for 4 different flow rates in total for each fitting. When collecting data for the valves, make sure that all are fully open. To improve data quality, wait 2 minutes after each flow rate is adjusted. Enter the time and manometer levels corresponding to each fitting in the appropriate tables.

7.5. Data and Results – Discussion Questions

a) Determine the head loss for all fittings at each flow rate.

b) Make a plot of head loss versus velocity head for the three 90° bends (knee or miter, threaded elbow and smooth bend). Include all three series on the same figure with the proper legend.

c) Make a plot of head loss versus velocity head for the sudden expansion and sudden contraction. Include them on a second figure with the proper legend. Note: be sure to use the proper velocity head for the case of the expansion and contraction.

d) Make a third figure with a plot of head loss versus velocity head for the fully open slanted seat valve and gate valve (use the pipe velocity head for the slanted seat valve). Include both on the same figure and add the proper legend. However, if one of your graphs is significantly smaller and looks indistinct as compared to the other, you may make two separate plots.

e) Perform a linear regression analysis for each of the seven series setting the y-intercept to 0. Include the equation and R2 value for each. Did you obtaina good linear model from the data based on the R2 value? Evaluate each case separately. Give reasons for possible low values, if any.

f) Provide a summary table of all your minor loss coefficients

(k) compared to the corresponding theoretical values found in the appendix section (Note: there is no reference for the slanted seat valve, include only the obtained coefficient). Is there evidence of systematic or random error in your results?

g) For the three bends, which type is generating the greater head loss for a given flow rate? Which one the lowest? Does this makes sense to you? Why or why not?

h) For the expansion and contraction: which fitting is generating the greater head loss for a given flow rate? Does this makes sense to you? Why or why not?


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