Fricción En Tuberías

Frictional losses in hydraulic pipelines Reynolds No. where v = fluid velocity, D = pipe inside diameter, ρ = fluid density, and µ = absolute viscosity of the fluid fluid. ƒ If NR is less than 2000, the flow is laminar. ƒ If NR is greater than 4000, the flow is turbulent. ƒ Reynolds numbers between 2000 and 4000 cover a critical zone between laminar and turbulent flow. It is not possible to predict the type yp of flow that will exist within the critical zone. Thus if NR lies in the critical zone, turbulent flow should be assumed. Since turbulent Si t b l t flow fl results lt in i greater t losses, l h d li systems hydraulic t should normally be designed to operate in the laminar flow region. 1

If turbulent flow is allowed to exist,, higher g fluid temperatures p will occur due to greater frictional energy losses. Therefore, turbulent flow systems suffering from excessive fluid temperatures can be i improved ed by b increasing i e i the pipe i e diameter di ete to t establish e t bli h laminar l i fl flow. Friction is the main cause of energy losses in fluid power systems. The result is a loss of potential energy in the system, and this shows up as a loss in pressure or head. The head loss (HL) in a system actually consists of two components: 1. Losses in pipes 2. Losses in valves and fittings Head losses in pipes can be found by using Darcy Darcy'ss equation: 2

Darcy's equation can be used to calculate the head loss due to friction in pipes for both laminar and turbulent flow. The difference between the two lies in the evaluation of the friction factor f. f For laminar flow: To determine the value of the friction factor for use in Darcy,s equation, we use Moody diagram. It gives values of friction factor as a function of Reynolds number for a given value of relative roughness. 3

Head losses in valves and fittings g are pproportional p to the square q of the velocity of the fluid:

The constant of proportionality (K) is called the K factor (also called loss coefficient) of the valve or fitting. Typical K-factor values for several common types of valves and fittings are:

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Pressure drop p versus flow rate For some fluid power valves, in addition dditi to t specifying if i K factors, f t empirical curves of pressure dropp versus flow rate are ggiven by the valve manufacturer. Thus, if the flow rate through the valve is known, known the pressure drop can be determined by referring to the curve. This is normally done for directional control valves and also for flow control valves for various opening positions. positions 5

Equivalent length We ccan findd a length e g oof ppipe pe that for o thee ssamee flow ow ratee wou would d produce the same head loss as a valve or fitting. This length of pipe, which is called the equivalent length of a valve or fitting, can be f found d by b equating ti the th head h d losses l across the th valve l or fitting fitti andd the th pipe:

where Le is the equivalent length of a valve or fitting whose K factor is K. Note that K and f are both dimensionless. Therefore, Le and D 6 will have the same dimensions.

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