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Computation of lc.s.
Outflow through outlet openings (scurgerea prin orificii)
A small outlet opening is considered two tanks having different
manometric pressure (presiuni
manometrice diferite) as it can be seen in the figure:
The section 1 of the Bernoullis equation is at free level of the
first tank while section 2 is at the
exit from the outlet, the Bernoullis equation has the form:
from which the speed v2 can be computed:
If (V=speed coefficient), the speed is given by general
form:
In the case of free exit from outlets (iesire libera din
rezervor) we have 2=0;H2=0 and:
If further on the free surface of the first tank only the a.p.
(atmospheric pressure) acts we have:
If we want to compute the flow (debitul) of the exit, we have to
introduce first the notion of
shrinkage coefficient (coefficient de contractie)=wc/w,
where:
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w=cross section of the outlet
wc=cross section of the liquid jet (see figure)
The flow (debitul) is given by:
The parameter =v is called coefficient of discharge (coefficient
de debit) and:
Where, usual values of the above coefficients are:
v= 0,960,99
= 0,620,64
= 0,600,62Computation of sh.s.
Water supply networks (w.s.n.) (de distributie a apei)
The w.s.n. are system that bring the water (drinking and
industrial) to each individual consumer
(residential buildings, schools, factories, agricultural units,
etc.). There are two types w.n.s.
which differ from each other by their operational system,
economical and hydraulic computation:
- ring network (retea inelara)
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- open network (retea ramificata,retea deschisa sau retea
arborescenta)
Comparison
1. For the same number of consumers, the total length of pipes
is much smaller in the caseof open network than in the case of ring
networks;
2. The hydraulic computation is simpler in the case of open
networks than in the case ringnetwork;
3. From the point of view of their functionality the ring
network are superior to opennetworks. For instance in the case of
ring network each consumer is supplied from at
least two parts while in the case of open networks one consumer
gets its flow from one
part only.
As it can be seen from above drawing, if in section A damage
(avarie, defectiune) is produced, inthe case of open networks all
the consumers located downstream ( in aval) are not supplied.
In
the same situation, in the case of ring networks, only the
consumers located in the immediate
neighboring of the damage pint A will not be supplied, while the
rest of consumers are normally
supplied. This situation leads to specific legislation
concerning the water supply networks: the
open networks are used only for non-important places,
agricultural consumers, etc. In the case of
towns and etc. the ring networks are compulsory.
Computation of open Networks
Take a simple open network with two pitchers (doua ramificatii)
and three consumers. In the
case of more elaborated open network only the computation volume
increases, the computation
principles are the same.
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The hydraulic computation from the above open networks is
presented.
Given:
- The network geometry (lengths, coordinates).- The necessary
flows (debite necesare) for each consumer. It is a specific value
depending
on the number of people and the required level of comfort
(comfort tehnic-edilitar)
- The height of equipment inside the buildings. It imposes the
necessary pressure (p/)Required:
- the diameters of the pipes for each pitches (tronson,
ramificatie);-
the pressure in the supplying tank (rezervor de alimentare)The
practical solving of this typical dimensioning problem depends on
the local relief:
- a mountain type relief;- a flat plane relief.
In the case of a mountain type relief (as in Cluj for instance)
we can build the water tank at the
required level using the local relief (on the top of a hill for
instance). It is the case of an efficient
network (small diameters). The procedure is as follows:
- the terminal points of the network are considered and their
pressure line ( liniapiezometrica sau presiunea de serviciu) is
computed (see laboratory classes);
- using the criteria of efficient speed ve (viteza economica),
the pipe diameter near theterminal point is computed using the
relation:
- a commercial (industrially manufactured) diameter immediately
smaller than thecomputed on is chosen;
- the pressure line (linia piezometrica) can now be computed and
therefore the necessarypressure in section i. In the following
drawing the variation of pressure along the
branches up to the tank is presented.
In this way, using the most efficient diameter each consumer us
supplied with the necessary flow
and under at least the required pressure.
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In the case of flat plane relief, the water tank has to be
artificially located at the required level.
But due to specific construction problems this level can not be
at big heights. Starting with the
necessity of assuring the required pressure at the terminal
points (critical points), and following
the way up to the water tank, the distributed head losses are
found to be smaller than the
precedent case. Using Sevelevs formula for head losses, the
parameters0 (coefficient de
rezistenta hidraulica = exchange coefficient) is computed
With =1 and knowing lij, Qijand hd, from given tables (see
laboratory classes) the commercial
neighboring diameters can be found. To ensure the required
pressure the bigger will be chosen
from the table.
Concluding remarks: in the second case (flat plane) the
diameters are bigger than in the case of
mountain type relief.
