For updated ersion, please click on http://kalam.ump.edu.my Chapter 5 Flow in ipelines by Dr. Nor Azlina binti Alias Faculty of Ciil and Earth Resources Enineerin azlina@ump.edu.my
5.4 Flowrate and Velocity Measurement in ipes Numerous deices hae been deeloped oer the years for the purpose of measurin flow. Some flowmeters measure the flow rate directly by discharin and recharin a measurin chamber of known olume, howeer most flowmeters measure the flow rate indirectly (in which the aerae elocity, is measured directly). The flow rate measurement techniques rane, the simplest method to measure the flowrate are : The flow rate of water throuh a hose can be measured by collectin the water in a basin of known olume and record the time taken to fill the basin. diidin the amount collected by the collection. olume (m Q time (s) 3 )
A basic or simple way of estimatin the flow elocity of a rier is by droppin a float on the rier and measure the drift time between two specified locations. distance between two locations (m) elocity, time taken for floatin materialmoes from point A to B (s) Here, deices that are commonly used to measure elocity and flow rate such as itot and Orifice are presented.
5.4. itot A itot or impact tube makes use of the difference between the static and kinetic pressures at a sinle point. The itot-static probe measures local elocity by measurin the pressure difference in conjunction with the Bernoulli equation. It consists of a slender double-tube alined with the flow and connected to a differential pressure meter. (a) A itot probe measures stanation pressure at the nose of the probe, while (b) a itot-static probe measures both stanation pressure and static pressure, from which the flow speed can be calculated. Circular type itot tube Spherical itot tube
Measurin flow elocity with a itot static probe. (A manometer may also be used in place of the differential pressure transducer.) The inner tube measures the stanation pressure at that location (point ). The outer tube measures the static pressure. For incompressible flow with sufficiently hih elocities (so that the frictional effects between points and are neliible), the Bernoulli equation is applicable and can be expressed as: z z z
Since the static pressure holes of the itot-static probe are arraned circumferentially around the tube, z = z and = 0 because of the stanation conditions. The flow elocity = becomes: This is known as the itot formula. The actual elocity is then ien by : C h or C is the coefficient of the instrument. C
Example 5.8 A pitot-static probe is mounted in a.5 cm diamter pipe at a location where the local elocity is approximately equal to the aerae elocity. The oil in the pipe has density, = 788.4 k/m 3 and iscosity, = 5.857 x0-4 k/ms. The pressure difference is measured to be 95.8 ka. Calculate the olume flow rate throuh the pipe in cubic meter per second (m 3 /s).
5.4. Orifice An orifice meter is a conduit and a restriction to create a pressure drop. It has a small openin on the side or bottom of a tank, throuh which a fluid is flowin. The openin can be of any shape or cross-section, like rectanular, trianular and circular. A nozzle, enturi or thin sharp eded orifice can be used as the flow restriction. A known olume is passin throuh the meter while the orifice in a pipeline with a manometer measures the drop in pressure. The minimum cross sectional area of the jet is known as the ena contracta.
Consider incompressible steady flow of a fluid in a horizontal pipe of diameter D that is constricted to a flow area of diameter d. The mass balance and the Bernoulli equations between a location before the constriction (point ) and the location where constriction occurs (point ) where z = z can be written as From continuity equation: () z z () D d A A A A Q
Substitute () into () ies: 4 where = d/d is the diameter ratio. The olume flowrate/dischare can be determined by : Q A Thus : Q Q actual actual C C d d A A 4 ena contracta area < flow area of the obstruction. Losses can be accounted for by incorporatin a correction factor called the dischare coefficient
Example 5.9 The flow rate of methanol at 0 C ( = 788.4 k/m 3 and = 5.857 x0-4 k/ms) throuh a 4 cm diameter pipe is to be measured with a 3 cm diameter orifice meter equipped with a mercury manometer across the orifice place as shown in fiure. If the differential heiht of the manometer is read to be cm, ien the density of mercury, H = 3600 k/m 3. Determine the flow rate of methanol throuh the pipe and the aerae flow elocity. The dischare coefficient, C d = 0.6.
Summary There are types of flow in pipeline system, i.e: laminar & turbulent. Reynold s number (Re) is ien by: D Re D Types of flow: if Re 000, the flow is laminar if Re 4000, the flow is turbulent For both laminar & turbulent flow, head loss (h L ) can be find usin Darcy equation: fl h L D Friction factor (f) for laminar flow: 64 f Re Friction factor (f) for turbulent flow: Moody Diaram/Colebrook-White equation