Frictional Head Loss along a Pipe

OBJECTIVE

To study the variation of friction factor f and estimate pipe roughness element.

INTRODUCTION

Frictional head loss is termed major head loss, because it usually accounts for most of the pressure drop in dynamic water systems. For a fluid flowing between two points in a pipe, the frictional head loss is involved in the energy equation

APPARATUS

Figure (4.11) shows the arrangement in which water from a supply tank is led through a flexible hose to a tube along which the frictional head loss is measured. Piezometric tubes are installed at upstream and downstream pipe sections. The Piezometric tubes are connected to U-tube manometer, which reads the differential pressure directly in millimeter of water, or a U-tube which reads in millimeter of mercury. The rate of flow along the pipe is controlled by a needle valve at the pipe exit and is measured by timing the collection of water in a measuring cylinder (the discharge being so small as to make the use of bench weighing tank impracticable).

PROCEDURE

1. Connect the bench supply hose to the inlet of the apparatus and direct the flexible outlet pipe into the bench drain.

2.  Open the needle valve, start the bench pump, and slowly open the bench supply valve so that water flows through the apparatus.

3. Open the bleed screws at the top of the mercury U-tube, then slowly close the needle valve so that air is expelled from the piezometric Open the air valve to release air from the water manometer. When all air bubbles have been driven out, close the bleed screws and air valve.

4. With the needle valve closed, check that the mercury levels in the U-tube are in balance. If not, then repeat the process of expelling air.

5.  Close the water manometer isolating tap.

 6.  Open the needle valve and read the heights of the two columns of mercury in the U-tube. Measure the flow rate by timing the collection of water into the measuring cylinder.

7. Repeat (6) until you have different sets of readings over a range of flow rates.