1.1 Learning Outcome
After measuring and tabulating the friction losses in the flow caused by different valves, the student should be able to draw the relationship between the different friction losses encountered in and its effect on the operations of the various valves, whilst noting the different sources of error encountered.
The valves in the pipeline control the flow rate of the air flowing within the pipeline. Based on simple deduction, we might expect that when the valve is three quarters closed, the flow rate of the air flowing through the pipe is reduced by three quarters of the maximum flow, demonstrating a linear relationship. However, not all valves would demonstrate such relationship and through this experiment determine the relationship of the gate and the globe valve.
To determine the characteristics of the individual valve, we would have to plot a graph of percentage of maximum flow against percentage opening. Assuming steady state conditions, it is given that the volumetric flow rate of air through the pipe, Q, is proportional to the square root of the differential head across the manometer for the simple case in which the valve discharges into the atmosphere:
Q = CdAv√((2gh)) where K is a constant, h1 and h2 are the different readings of the manometer.
Taking Table 3 as example, for number of turns =2,
Q = √6.05 = 2.46 cm3s-1 (2 decimal places). (as shown in Table 3)
We ignore K in the previous calculation as
Qmax = K√(h_1-h_2 )
∴ % max = Q/Q_max x 100% = (K√(h_1-h_2 ))/(K√(h_1-h_2 ) ( when valve is fully opened)) x 100% = √(h_1-h_2 )/(√(h_1-h_2 ) ( when valve is fully opened)) x 100%
Taking Table 3 as an example, when valve is fully opened,