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Break pressure manholes and calculation of the pump unit power


• Break pressure manholes

In some installations with a steep and long slope in which water goes down from a deposit, spring, etc. it is necessary to break the growing pressure which is produced in the parte inferior of the pipe with the aim of not having excessive pressure in the case that the pipe transfers the load or suffers some obstruction.
To break the pressure, it can be installed load break manholes which are small deposit fitted with the corresponding lid and an overflow which avoids the load transfer.

The inlet pipe pour its flow into the manhole braking the pressure and of the outlet pipe which it is provided if it is needed a filter to avoid the entrance of strange elements to evacuate the water. It is convenient to install an overflow in the manhole in the case of a possible obstruction in the outlet section. The overflow will be properly conducted for its exploitation or elimination.

When it is not possible or proper the installation of these short pressure manholes, it can be chosen the use of pressure reducing valves, strategically placed to keep the pressure in the conduction within the maximum stipulated limits. It must be taken into account that most of the distribution networks don’t normally work at more than 10 bars.


• Calculus of the pump unit power

In an impulsion, the manometric height is the sum of the aspiration height Ha, impulsion Hg and of the whole load losses which are produced in the pipes due to the aspiration, impulsion and other things (fittings, changes of directions, etc.):

Hm = Ha + Hg + V2 / 2g + Ja + Ji

Hm = Manometric height, in m.c.a.
Ha = Aspiration height, in m.
Hg = Geometric height, in m.
V = Velocity of the water in m/s
g = Acceleration of the gravity = 9.81 m/s2
Ja = Total load loss in the aspiration pipe, in m.c.a.
Ji = Total load loss in the impulsion pipe, in m.c.a.

The power used by the pump will be determined by the formula:

N = a . qv . Hm / 75 . b

N = Power used by the pump, in CV (1 CV = 0.736 kW)
a = Density liquid (for water = 1).
qv = Flow, in l/s
Hm = Manometric height, in m.c.a.
b = Performance of the pump (a normal value is usually 0.7)

The necessary power of the engine will be equal to the power used by the pump, divided by the performance of the engine.

It is convenient to choose the power of the engine is with a margin higher than 20% for possible overloads of the installation and voltage drops of the network.

>>> Due to their low elasticity and celerity modulus of the plastic pipes, these support better the water hammer consequently overpressures from 25 to 30% are perfectly admissible according to the TECHNICAL GUIDE ABOUT PIPES FOR TRANSPORTATION OF WATER UNDER PRESSURE of the CEDEX, written by Luis Balairón.

By |2018-11-02T12:32:59+00:00October 4th, 2017|01. Plastic pipes, 07. Installation|0 Comments

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