haydraulic design of pipe
Answers
Answer:
Water pressure pipes can be laid at any depth below the hydraulic gradient line, the velocity in the pressure pipes directly depends on the pressure head at the point of reference. If the velocity of the water is kept very low, large diameter pipe will be required to carry the required quantity of water from one place to another.
On the other hand if too high velocity is allowed in the pipes, cost of pumping will be too high to develop the required pressure, as well as the cost of pipe and its fittings will increase to bear the extra pressure developed. Therefore, it is most necessary to design the pressure pipes in such a way, as the overall cost of the project should be lowest possible, both from constructional and maintenance point of view.
For economy, usually the pipe line should hug the hydraulic gradient line in profile and the straight line in plan. The hydraulic gradient line should neither too high nor too low. In addition to this requirement velocity should be self-cleaning i.e., no silting should be allowed in the pipe line. While designing the pressure pipes acting as gravity main for conveyance of water, the normal velocity of water is kept between 0.9 m/sec. to 1.5 m/sec. for satisfactory results.
The common practice is to design the pressure pipes acting as gravity mains such that the available pressure head between the source and the city is just lost in overcoming the friction losses.
Essay # 2. Determination of Loss of Head in Pipes:
The loss of head in the pipes can be determined by the following formulae:
(A) Manning’s formula:
The formula is usually used in determining the loss of head in the gravity conduits. This is equally applicable to the turbulent flow in pressure pipes.
This formula is:
Where, m = Manning’s rugosity coefficient
L = Length of the pipe line in metres
R = Hydraulic mean depth of pipe
V = Velocity of flow in m/sec.
If d be the diameter of pipe, the value of R will be.
(B) Hazen-Wiliiam’s formula:
Where, CH = Coefficient of hydraulic capacity as given by Table 8.1
S = Slope of the energy line.
V and R are the as in formula 8.1.
This formula is widely used now a days in designing the pipe lines.
Values of the Coefficient of Hydraulic Capacity 'CH'
The value of coefficient ‘CH‘ is more for smoother pipe and less for rough pipe. As with the age the inner surface of most of the materials becomes more and more rough. Therefore, the carrying capacity of the pipe lines decrease with the age.
Modified Hazen’s Williams’s Formula:
Sometimes, the most widely used Hazen’s William’s Formula is not preferred due to its limitations:
(i) The Hazen William’s Coefficient CH is not a dimensionless parameters but has the units of L-0.37 T-1. Hence, its value changes with change in the employed units.
(ii) The numerical constant of 0.85 (in M.K.S. units) has been calculated for an assumed hydraulic mean depth (R) of 0.3 m and friction slope of 1/1000. However, the formula is used for all ranges of pipe dia and friction slopes. This practice may lead to an error up to ± 30% in the evaluation of velocity, and ± 55% in head loss due to pipe friction.
Considering these limitations of Hazen William’s formula, a modified Hazen William’s formula has been derived for use from Davey Weisbach and Cole-brook-white equations, which obviates those limitations the modified Hazen-William’s formula states that