MEDIDAS EN FLUIDOS INEL 5205 Instrumentación
SENSORES DE PRESIÓN Unidades de Presión 1 psi! 1 lbf/in 2 = 6,894.76! Pa = 6,894.76!newton/m 2 = 68.948 10 3!bar (1 bar = 10 5 Pa) = 68.046 10 3!atm = 51.715 torr! = 51.715 mm-hg
TUBO DE BOURDON
p 1 p 2 D hg, es A and B of the g = 980.665 cm/s 2 ; ρ = liquid density B A B h A h (a) (b) A B h Fig. 15.6 Manometers: (a) U-tube; (b) well type; (c) inclined type. (c)
Capacitivo - basado en membrana de Si
MEDICIÓN DE FLUJO
MEDICIÓN DE FLUJO Q = flujo volumetrico Qm = flujo de masa = ρq = dm/dt Qv = velocidad del flujo = Q/A Flujo turbulento: N > 4000 Flujo laminar: N < 2000 Re = numero de Reynolds Re = Q vd µ µ = viscosidad del fluido Ecuación de Bernoulli Q = k P 2 P 1
D 1 D t D 2 1 2 Ideally, Figure 2.1: A generalised restriction / differential pressure flow meter. u 2 1 2 + p 1 Due to non-idealities: ṁ = u 1 A 1 = u 2 A 2 ) u 1 = A 2 A 1 u 2 = u2 2 2 + p 2 (Bernoulli s equation) q 2 (p 2 p 1 ) u 2 = A 1 2 A 1 ṁ = A p 2 2 (p2 p 1 ) 1 A 2 A 1 ṁ actual = CA t p 1 4 p 2 (p2 p 1 ) where: = D t D 1, D t and A t are the diameter and area of the throat of the restriction.
[2.5] hest Re, and b, n are constants, empirically determined. As ReIn summary, the principal advantages of the orifice plate are it is simple and robust to Re - the flow is increasingly turbulent and thus the velocity standards are well established and comprehensive C = discharge coefficient K are the orifice plate, Venturi meter and flow nozzle. ments C 1 plates are cheap may be used on gases, liquids and wet mixtures (eg steam) = flow coefficient Its principal drawbacks are & max : m& min only 4:1 at best (see tutorial 1) low dynamic range: m performance changes with plate damage or build up of dirt. affected by upstream swirl large head loss 4 Figure 2.3. Orifice profile (Furness, 1989) Figure 2.4: Flow coefficients for orifice with corner taps. of the orifice and the location of the pressure taps. Figure 2.2 overed by BS EN ISO 5167: flange taps, pipe taps (both D and ws a typical orifice, with a sharp, square edge on the upstreamventuri Meter &! o-front can introduce an error 20% in m The Venturi meter (after Giovanni Venturi, 1746 1822) is designed to cause minimal head loss as the flow passes the restriction. Figure 2.5 shows a typical arrangement. Like the orifice plate, as a (relatively) simple function of pipe Reynolds number Rethe Venturi is dealt with by a British / ISO standard (BS EN ISO 5167-4) taps, " 8 November 19, 12!Monday, 0.184" + 91. 71 2.5 For a Venturi, C = 0.99 for 105 < Re < 107 is a useful approximation. BS EN ISO 5167-4 gives more accurate Figures for particular details of an installation (ranges of Re, β etc).
Figure 2.5: The Venturi meter (Furness, 1989) The disadvantages compared to the orifice are occupies longer length of pipe more expensive (manufacture and installation) C = 0.99 for 105 < Re < 107 Flow Nozzle In many respects, the flow nozzle is a compromise between the compact orifice plate and the efficient Venturi. There are two standardised designs - Figures 2.6 and 2.7. Flow nozzles have proved particularly suited to high velocity applications, eg steam metering.
Obstruction-type Presión de impacto Presión Estática tubo de Pitot Venturi meter Orifice plate http://www.youtube.com/watch? feature=player_detailpage&v=oud4wxjohky
CORIOLIS FLOWMETER http://www.youtube.com/watch? feature=player_detailpage&v=xiivianitiw
THERMAL FLOW http://www.youtube.com/watch? feature=player_detailpage&v=xiivianitiw
VORTEX FLOWMETER http://www.youtube.com/watch? feature=player_detailpage&v=gmtmdm7jhza
Example: Design an instrument to measure a flow from 20 to 150 gal/min using an orifice plate system. The orifice plate is described by Bernoulli s equation, with K=119.5(gal/min)/ psi ½. The instrument should produce a 4-20mA signal proportional to the flow. The relationship between current and flow should be linear, with 4mA and 20mA corresponding to 20 gal/min and 150 gal/min, respectively. Piezoresistive strain gages with GF=50 are attached to a diaphragm and are used to measure the pressure difference.
Metro de flujo ultrasónico Q v = L sin t UP t DOWN t UP t DOWN http://www.youtube.com/watch? feature=player_detailpage&v=bx2rnrflkqg
Figure 2.17: Construction of an Axial Turbine flow meter. Q = Kω
Electromagnetic flowmeter http://www.youtube.com/watch? feature=player_detailpage&v=yyqrlvmdzy0