Charge (hydraulic)

See also: Load

In Hydraulic, the load is the constant which constitutes the member of right-hand side of the equation of Bernoulli.

\ frac {v^2} {2} + gz + \ frac {p} {\ rho} = constant

It typically is expressed:

  • in the form of a Pressure: constant × ρ
  • in the form a height of the water column: constant / G
where ρ is the Density fluid, G the acceleration of the Gravité, Z the height to which the fluid is, p static pressure and v the speed of the fluid.

Definition

When one is in the presence of frictions, the theorem of Bernoulli does not apply any more and charges it is not more constant. One speaks then about pressure loss .

One uses in this case the theorem of generalized Bernoulli, who is written:

\ frac {v^2_1} {2 G} + z_1 + \ frac {p_1} {\ rho G} = \ frac {v^2_2} {2 G} + z_2 + \ frac {p_2} {\ rho G} + \ Delta H

where the term \ Delta H represents the pressure loss in meters between item 1 (downstream) and 2 (upstream of the flow). This pressure loss can be expressed as being the difference in height between the geometrical height between items 1 and 2 and the useful height which will determine real energy to provide to pass from 1 to 2.

This term will be positive in the case of a flow in a led , but could be negative if the two points considered are generating turboshaft engine on both sides (pump, ventilator, generating turbine…)

Case of an incompressible fluid in a fixed drain

In the case of an incompressible fluid, ρ is a constant, and the flow is a constant. If the section of the Tuyau is constant, then, speed is also constant. Altitude Z being imposed by the installation of the drain, one sees that the pressure loss results in a pressure decrease.

Two types of pressure losses

Linear pressure losses

These losses are proportional to the length of pipe to traverse. They in the following way are calculated: \ Delta H = \ lambda \ frac {L.c^ {2}} {2.g.D_ {H}} with

\ lambda the loss ratio of regular load

C the speed of the fluid in pipe (m/s)

L the length of the pipe (m)

D_ {H} is the hydraulic Diamètre defined by D_ {H} = \ frac {4S} {P_ {m}}

where S is the section of the pipe and P_ {m} the wet perimeter

The loss ratio of regular load depends on the type of flow and thus on the Reynolds. For a laminar flow one uses the correlation of One tenth of a poise: \ lambda = \ frac {64} {Re} For a turbulent flow one uses the correlation of Colebrook: \ frac {1} {\ sqrt {\ lambda}} = -2log (\ frac {2,51} {Re \ sqrt {\ lambda}} +0,27 \ frac {\ epsilon} {D})

Singular pressure losses

The singular pressure losses are primarily due to the accidents of drain, i.e. any modification of a rectilinear way. One can count there the elbows, the valves or taps, the measuring devices, etc… The singular pressure loss of an accident can be determined by calculation or using tables (Abaque S) where a graphic construction starting from simple sizes will give a result.

The pressure losses are added according to the number of these accidents

See too

  • Fire hose > Calculation of the pressure losses

Simple: Head (hydraulic)

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