(Partial Descriptive and
Procedural Write-Up)
Art Madsen, M.Ed.
Published research in the field of pipe-flow pressure loss is reviewed. Attention is subsequently devoted to the importance of accurate prediction; and lab procedures for conducting a five-section pipe-flow analysis are presented. Results indicate that prediction calculations, using universal equations for pressure loss, fall within a predictable and acceptable range of accuracy compared against experimental testing. Discussion indicates that the standard pressure loss equations are essentially adequate predictors of anticipated pipe-flow pressure loss. It is concluded that these findings can apply to laminar as well as turbulent conditions, both in the field and under controlled lab conditions.
The objective of this experiment is to resolve the problem of accurately predicting pipe-flow pressure loss utilizing a range of pipe types, fittings and dimensions, and subsequently verifying these theoretically calculated predictions against actual tests conducted using the same pipe characteristics. Prediction of pressure loss is a critical factor in pipe design, in pipeline system performance, and in ensuring overall efficiency and cost effectiveness. Prior research conducted in the field of pipe-flow pressure loss has been considerable. Among significant articles surveyed are the following relevant studies, published in major engineering and technical journals.
Chilton and Stainsby (1998) identify the major names in prediction of pipe flow pressure loss. The primary authors who have researched and published extensively in this area are, among others, Poiseuille, Reynolds, Prandtle, and White, they indicate. These researchers have formulated several standard equations, and their well-known methods of prediction have been firmly established for decades.
Durand (1997) focuses on pressure drop dynamics related to friction loss by defining a new rule, which he names the “Rule of Fours.” His rule is especially useful when standard tables are not available, particularly at times when engineers in the field are attempting to estimate pressure drop due to friction. He postulates that under ordinary conditions pressure loss is on the order of 4 psi/100 feet in clean 4-inch piping. He refers to the Reynolds number and discusses it in relation to the Darcy equation. He demonstrates that his Rule correlates extremely closely to the Darcy equation when verified against the Moody chart. It is therefore quite apparent that prediction of pressure loss is possible by employing Durand’s Rule.
Writing in Chemical Engineering, Cashwell (2001) addresses the concept of flow measurement within the context of either horizontal or vertical piping systems. The author states that accurate flow rate measurements can be obtained through use of differential/pressure transmitters, specifically designed to adapt to all piping systems and applications. Accuracy when using this device is frequently subject to certain conditions, notably maintaining full pipe volume at time of measurement. In his explanation of the d/p transmitter’s operational characteristics, the author clarifies the relationship between flow rate and pressure measurement. He indicates that the d/p transmitter can be calibrated to adjust to all measurement conditions. The present experiment will utilize a similar device from which data can be generated and analyzed for predictive purposes.
The limitations of the cited articles include their partial non-applicability to exact laboratory conditions where this experiment was conducted. The foregoing authors did not utilize 1.00” or 1.25” piping, tee-pipes, bell reducers or any of other fittings that were used in the experiment at hand; however, they did provide technical insight into the theoretical principles that were uniformly applied for purposes of the present experiment.
Because it will prove
useful to confirm the accuracy of prediction of pressure loss, this experiment
will build on Chilton, Durand and Cashwell’s findings and will strive to
demonstrate the high degree of predictability derived from standard pressure
equations, in relation to actual tests conducted. This experiment, therefore, will further bolster already achieved
results, and will lend more credibility to pressure loss dynamics, notably in
PVC and galvanized iron piping.
Procedures (successfully performed within parameters
stipulated):
à After prediction and estimation of pressure loss, on the
basis of standard equations, and using the same parameters and conditions that
will be employed in the actual experiment, the first step in the lab experiment
is the installation of the pressure transducer in the correct position. The use of a traveling transducer
facilitates measurement of pressure drop along five sections of pipe, namely:
1”, 1.25”, PVC, 1” tee-flow, and a 1.25” to 1” bell reducer.
à Position the pump next to the platform scale on which the
blue barrel is located. Ensure that the
discharge pipe is properly positioned so that liquid discharge returns to the
barrel from which liquid is being drawn by the pump.
à Fill the supply barrel to within 3” of the top.
à Record the weight of the empty recipient barrel, with the
liquid at sump-pump shut-off level.
à Install the correctly fitting orifice plate in the
pipe-flow loop.
à Activate the pump with the control valve fully open,
making sure that the discharge-return is flowing into the supply barrel. The
maximum flow through the loop will be obtained in this way. Close valve slowly
to vary flow through loop.
à For each valve setting, turn the flow return to the
recipient barrel. Use the scale and a
stopwatch to conduct the measurement of flow.
The sump pump will empty the recipient barrel into the supply barrel.
à Record readings for each flow setting, at each of the 5
sections, using the pipe loss transducer, the orifice plate flow meter, the
Doppler flow meter, and the turbine and rotameters. Allow the pump to run until
the recipient barrel is 6” from the top.
Then switch the discharge back to the supply barrel, and record the time
elapsed. The pump should then be
unplugged.
à If there is no time to complete all of the pressure drop
readings, they will have to be finished after the discharge line has been
switched over to the supply barrel. If
you do not change the valve setting, the flow rate should not be different.
à Proceed with weighing of the recipient barrel (including
water and sump pump) and then subtract the original weight when the procedure
was started. Calculate tare weight and divide by the time to determine rate of
flow. GPM will be the volumetric
measure, so use the density of water to accurately complete the calculation.
à Pour out all water from the recipient barrel (to sump
shut-off point) and then reweigh the barrel.
The supply barrel should be full; add spilled water to it, if necessary.
à Re-perform the
above procedures until readings have been taken for all five pressure drop
sections, with five flow settings, using four differing flow meters. Continue up to the maximum loop flow
capacity.