INTERNATIONAL STANDARD 3966

Transcription

1 INTERNATIONA STANDARD 3966 INTERNATIONA ORGANIZATION FOR STANDARDIZATION 4vlEX~YHAPO~HAR OPrAHW3AUMR IIO CTAHAAPTZ13A~I;IM[~ORGANISATION INTERNATIONAE DE NORMAISATION Dit docuent ag slechts op een standalone PC worden geinstalleerd. Gebruik op een netwerk is alleen. toestaan als een aanvullende licentieovereenkost voor netwerkgebruik et NEN is afgesloten. This docuent ay only be used on a standalone PC. Use in a network is only peritted when a suppleentary license agreeent for us in a network with NEN has been concluded. Measureent of fluid flow in closed conduits Velocity area ethod using Pitot static tubes Mesure du d&bit des fluides dans les conduites fere es Methode dflexploration au o yen de tubes de Pitot doubles First edition du chap des vitesses ~~~ UDC Ref. No. IS (E) Descriptors : flow easureent, fluid flow, pipe flow, Pitot tubes. Price based on 39 pages Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

2 FOREWORD ISO (the International Organization for Standardization) is a worldwide federation of national Standards institutes (ISO eber bodies). The work of developing International Standards is carried out through ISO technical coittees. Every eber body interested in a subject for which a technical coittee has been set up has the right to be represented on that coittee. International organizations, governental and nongovernental, in liaison with ISO, also take part in the work. Draft International Standards adopted by the technical coittees are circulated to the eber bodies for approval before their acceptance as International Standards by the ISO Council. International Standard ISO 3966 was developed by Technical Coittee ISO/TC 30, Measureent of fluid flow in closed conduits, and was circulated to the eber bodies in January lt has been approved by the eber bodies of the following countries : Australia Hungary Belgiu Italy Chile Korea, Rep. of. Finland Mexico France Netherlands Gerany Portugal Roania South Africa, Rep. of Turkey United Kingdo U.S.S. R. Yugoslavia No eber body expressed disapproval of the docuent. 0 International Organkation for Standardkation, 1977 l Printed in Switzerland ii Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

3 . CONTENTS Page 1 Scope and field of application... 2 Sybols and definitions... 3 Principle... 4 Design of Pitot tubes... 5 Requireents for use of Pitot tubes... 6 Positioning of Pitot tube I Velocity coputation Deterination of the discharge velocity by graphical integration of the velocity area.... D U Deterination of the discharge velocity by nuerical integration of the velocity area l.... I I. IO Deterination of the discharge velocity by arithetical ethods Corrections of local velocity easureents Errors Annexes A Pitottubes.... B Correction to the easuring Position of Pitot tubes used in a transverse velocity gradient... C Study concerning turbulente correction.... D Daping of pressure gauges.... E Measureents with a Pitot tube in a copressible fluid.... F Deterination of coefficient for extrapolation near the wall G Exaple of calculation of the uncertainty on the flowrate easureent by eans of Pitot tubes , I Ill Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

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5 conduits Veloci area 1 SCOPE AND FIED OF APPICATION This International Standard specifies a ethod for the deterination in a closed conduit of the volue rate sf flow of a regular flow (see 5.1) : of a fluid of substantially constant density or corresponding to a Mach nuber not exceeding 0,25; with substantially unifor Stagnation teperature across the easuring Crosssection; running fuii in the conduit; under steady flow conditions. In particular it deals with the technology and aintenance of Pitot static tubes, with the calculation of local velocities fro easured differential pressures and with the coputation of the fiow rate by velocity integration. The ethod sf easureent and the requireents defined in this International Standard ai at reaching, at the 95 % confidence level, an uncertainty on flow rate not greater than + 2 %. To attain this result it ay be necessary, according to easureent conditions, to take into account the corrections given in clause 11. If any of the requireents of this International Standard are not fulfilled, this ethod ay still be app lied in special cases but the uncertainty on flow rate will be larger. Sy boi H h kb kg kt I M Ma P gv R R r Re S iuantity Rectangular conduit height Reight of a particufar Point above the botto Blockage coefficient of a cylindrical ste Coefficient depending of the nose shape Coefficien correction t of tu rbu Oence Rectangular conduit width Qistance fro a particulat Point ta the sidewaii Molar Roughness Mach nuber ass of fluid coefficient Absolute static pressure of the fluid Volue flow rate Molar constant of gas Pipe radius Measuring circle radius Reynolds nuber Frontal projected area of the ste inside the conduit >iensions M MIT* T fl32@1 2 Corre sponding SI unit J.ol kg Pa % 2 Kl 2 SYMBOS AND DEFINITIONS T u Absolute teperatu re Discharge velocity 0 T K /s 2.1 Sybols u Mean velocity circuference ent line along a or a easure T /s Sybol a, a D d d di Crosssectional conduit Quantity area of the Distance of the extree easuring Point to the nearest wall Pipe diaeter Head diaeter Stern diaeter Total pressure tapping hole diaeter Diensions Corresponding SI unit =o v X Y z Cl! Y AP E ocal velocity Pipe diension of the fluid Distance of a easuring Point to the wall Gas law dcviation factor Calibration factor of the Pitot tu be Ratio of the specific heat capacities Differential pressure easured by the Pitot tube Expansibiiity Copressibihty factor factor correction T M1T2 ls Pa 1 Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

6 ISO (E) Sybol Quantity Diensions Corresponding SI unit h Universal coefficient for head loss P Dynait viscosity of the fluid M Y Pa5 v Kineatic viscosity of the fluid 2Tl % t Head loss M1T2 Pa P Density of the fluid M3 kg/3 9 Pitot tube inclination 2.2 Definitions relative velocity : The ratio of the flow velocity at the considered Point to a reference velocity easured at the sae tie and being either the velocity at a particular Point (for exaple, the centre of a circular conduit) or the discharge velocity in the easuring section straight length : A conduit section the axis of which is rectilinear and the surface and Crosssection of which are constant. NOTE The shape of this section is usually circular, but it ay be rectangul ar or annular. The definitions in the following subclauses are given only for ters used with a special e aning or for ters the eaning 0 f which ight be usefully recall ed Pitot static tube : A tubular device consisting of a cyl indrical head attached perpendicularly to a ste allowing easureent of a differential pressure fro which the flow rate of the fluid in which it is inserted tan be deterined. It is provided with static pressure tapping holes (drilled all around the circuference of the head at one or ore Crosssections) and with a total pressure hole (facing the flow direction at the tip of the axially syetrical nose of the head). NOTE Throughout this International Standard the expression Pitot tube is used without aplification to designate a Pitot static tube since no confusion is possible static pressure tapping : A gr oup of holes for the easureent of fluid static pressure total pressure tapping : A hole for the easureent of fluid Stagnation pressure (the pressure produced by bringing the fluid to rest without Change in entropy) differential pressure : The differente pressures at the total and static pressure taps. between the i rregularity : Any eleent or con figu ratio n of a cond uit which akes it different fro a stra ight Ie ngth. NOTE For the purpose of this International Standard, those irregularities which create the ost significant disturbances are bends, valves, gates and sudden widening of the section. 3 PRINCIPE 3.1 General principle The principle of the ethod consists of : a) easuring the diensions of the easuring section, which ust be noral to the conduit axis; this easureent is necessary for defining the area of the Crosssection (see 3.2); b) defining the Position of the easuring Points in the Crosssection, the nuber of easuring Points having to be sufficient to perit adequate deterination of the velocity prof ile; c) easuring the differential pressure existing between the total and static pressures of the Pitot tube placed at these easuring Points (see 3.3) and deterining the density of the fluid in the test conditions; stationary rake : A set of Pitot tubes, ounted on one or several fixed supports, which explore the whole diaeter or easuring section siultaneously peripheral flow rate : The volue flow rate in the area located between the pipe wall and the contour defined by the velocity easuring Points which are the closest to the Wall. d) deterining the Iocal velocity of the flow, fro given forulae, on the basis of previous easureents (see clause 7); e) deterining the discharge velocity fro these values; f) calculating the volue rate of flow equal to the product of the Crosssectional area and the discharge velocjty discharge velocity : The ratio of the volue rate of flow (integral of the axial coponent of Iocal velocities with respect to the Crosssectional area) to the area of the easuring Crosssection. Errors in the techniques described in a) to f) contribute to the error in the flow rate easureent; other sources of error (such as the shape of the velocity distribution and the nuber of easuring Points) are discussed in clause Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

7 ISO (E) This International Standard presents three ethods for deteri ning the di seharge velocity : types of 3.3 Measureent of local velocities Graphicai in tegra tion of the veiocity (see clause 8) This ethod consists in plotting the velocity Profile on a graph and evaluating the area under the curve which is bounded by the easuring Points closest to the Wall. To the value thus obtained is added a calculated ter which allows for the flow in the peripheral zone (the area between the wall and the curve through the easuring positions closest to the Wall) on the assuption that the velocity Profile in this zone satisfies a power law Method of expioring traverse section It is soeties proposed that several Pitot tubes be ounted on a stationary rake in Order to explore siultaneously the whole easuring Crosssection. However, the experiental data at present available are insufficient to allow the design of certain details (such as shape of head and of Stern) which would ensure that easureents by a rake would achieve the accuracy required by this International Standard. For this ethod the easuring Points ay be located at whichever positions are required in Order to obtain a satisfactory knowledge of the velocity prof ile. Nuericai in tegra tion of the veiocity (see clause 9) The differente between this ethod and the previous one lies in the fact that the graphical velocity Profile is replaced by an algebraic curve and the integration is carried out analytically. Aritheticai ethods (see clause IO) The arithetical ethods assue that the velocity distribution follows a particular law and the ean velocity in the conduit is then given by a linear cobination of the individual velocities easured at the locations specified by the ethod. For the arithetical ethods described in clause 10, the assuption is ade that in the peripheral zone the velocity distribution follows a logarithic law as a function of the distance fro the Wall. 3.2 Measureent of the easuring Crosssection Circuiar Crosssections The ean diaeter of the conduit is taken as equal to the arithetical ean of easureents carried out on at least four diaeters (including the traverse diaeters) at approxiately equal angles to each other in the easuring section. Should the differente between the lengths of two consecutive diaeters be greater than 0,5 %, the nuber of easured diaeters shall be doubled Rectanguiar Crosssections The conduit width and height shall both be easured at least on each straight line (at least four) passing through the easuring Points. Should the differente between the widths (or heights) corresponding to two successive easuring lines be greater than 1 %, the nuber of easured widths (or heights) shall be doubled. Therefore, this International Standard deals only with velocity area ethods using a Single Pitot tube placed successively at each easuring Point Reference easureen t Reference easureents shall be ade in Order to check the steadiness of flow and to correct individual velocity easureents for slight changes in flow rate during traversing; any reference easuring device inserted in the conduit shall be placed in such a way that there is no interaction with the traversing Pitot tube. The reference easureent shall be ade as far as possible siultaneously with each velocity easureent. However, if only one easuring device is available, the steadiness of the flow shall be checked by repeating easureents at the reference Point after each local velocity easureent. lt is essential that the shape of the velocity Profile in the easuring Crosssection reains stable and is not affected by possible variations of the flow rate whilst easureents are being taken. When the curve of reference velocity Variation v, has been plotted against tie, this curve is used to relate all traverse easureents to the sae reference flow rate q. (preferably that which corresponds to the ean of velocity easureents at the fixed Point). For coparatively sall changes of the reference velocity, the velocity v,~ easured at any Point i at tie t tan be transposed by ultiplication by the ratio of velocity v, 0 at the reference Point corresponding to flow rate &, at velocity v,,~ at this reference Point at tie t : Vi,0 V r,o = Vi,, X c,t NOTE Where the reference easureent is a quantity directly proportional to the flow rate (for instance, the rotational Speed of a shaft driving a fan or a pup), this easureent tan be substituted directly for v,~ and V, t in the above equation. Where the reference reading is in the for of a pressure differente (for instance across a fixed feature of the flow circuit, or the differential pressure of a reference Pitot tube), the Square root of each reference reading tan be substituted for ~r,~ and r+t in the above equation. 3 Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

8 (E) However, it ust be noted that velocity Profile fluctuations ay occur without creating flow rate fluctuations. hr such a case the use of reference Point velocity ay lead to errors and it is preferable to check steadiness of flow by eans o8 any pressure differente device (standardized pressure differente flow eter, piezoetric control on a convergent, bend, spiral casing, peculiar pressure loss, etc.), even if it is not calibrated, provided that its reliability and adequate sensitivity have been ascertained. In this case the aboveentioned proportional correction will relate to the differential pressure and not to the velocity Checking of velocity distribu tion lt is recoended that the regularity of the velocity distribution be checked either by plotting or by other eans, regardless of whether or not the plotting is necessary for calculating the discharge velocity. In the sae way, when several easureents are ade on the sae Crosssection at different flow rates it is recoended that the velocity profiles be plotted in a nondiensional anner (i.e. by using the relative velocities; see 2.8) to check their consistency with each other and hence to ensure that there are no abnoral features at particular flow rates (thus, the profiles shall not Change erratically as the flow rate varies over a wide range sf Reynolds nubers). ft ay also be useful to plot the velocity distribution curves as indicated above in Order to detect any error in the easureent of a Iocal velocity. The doubtful easureent shall be repeated whenever possible; when this cannot be done, it shall be ignored and the velocity Profile drawn on the basis of the previously obtained profiles provided there are independent reasons for believing the doubtful easureent is false. 3.4 ocation Crosssection and nuber of easuring Points in the Subclauses 3,4.2 and 3.43 prescribe a iniurn nuber of easuring Points applying in particular to sall diension conduits. As it Ps necessary to define the velocity Profile as accurately as possible, the nuber of easuring points tan be advantageously increased provided that this is allowed by the operating conditions and steadiness of the flow. When a Single Pitot tube is traversed across the duct, the distance between a reference Point (fro which each Position is easured) and the wall of the duct ust Birst be obtained. This ay introduce a relatively large systeatic error in all Position easureents. In such instances it is recoended that coplete diaeters be traversed (rather than opposite radii on each diaeter) since the systeetic error will then tend to cancel out on the two halves of the traverse. 3,4.2 Circuiar Crosssec tions The easuring Points shall be located at every point of intersection between a prescribed nuber of circles concentric with the pipe axis and at least two utually perpendicular diaeters. The easureents shall be carried out in at least three Points per radius, so that there is a iniu of twelve Points in the crosssection. An additional easuring point at the centre of the conduit is desirable to check the shape sf the velocity Profile and is necessary for the calculation of the ste blockage eorrection, where applicable (see ) Ret tangular Grosssec tions The iniu nuber of easuring Points shall be 25. Unless a special layout of easuring Points is required for the use sf an arithetical ethod, their Position shall be defined by the intersections of at least five straight lines running parallel to each Wall of the conduit General requireen ts The rules to be followed for locating the easuring Points differ according to the ethods of deterination of the discharge velocity as specified in this International Standard. These rules are given in clauses 8, 9 and 18 respectively. Whatever ethod is used, the distance between the axis sf the head of the Pitot tube and the wall shall not be less than the head diaeter d. The location sf the Pitot tube shall be calculated fro the actual diension sf the conduit along each traverse line (rather than fro the ean diension) and shall be easured to : + 0,005 X, where X is the d iens ion of the duct to the easureent of Pitot tube Position, or parallel 2 0,05 y, where y is the distance of the Pitot tube fro the nearest Wall, whichever is the saller. 4 QESIGN 0F PITCIT TUBES 4.1 General description The use of one sf the types of Pitot tube described in annex A, all of which fulfil the requireents of 4.2, is recoended; this avoids the necessity of aking several corrections to the easureents. The use of any other Pitot tube which fuifils the requireents sf 4.2 is peritted provided that its catibration is known. The Pitot static tubes dealt with in this International Standard consist of a cyiindrical head attached perpendicularly to a ste which usually passes through the wall sf a conduit. The length of the head is generally between 35 and 25 ties the head diaeter. 4 Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

9 ISO (EI At one or two Crosssections along the head, staticpressure holes are drilled around the circuference, so that, in the absence of leakage, the registered pressure is transferred through the head and ste to a Point outside the conduit. A saller tube, concentric with the head and Stern, transfers the total pressure, registered by a hole facing the flow direction at the tip of an axially syetrical nose integral with the head, to a Point outside the conduit. An alignent ar, fitted to the end of the Stern, facilitates alignent of the head when this is obscured by the conduit Wall. d) The staticpressure holes shall be : 1) not larger than 1,6 in diaeter; 2) at least six, and sufficient in nuber for the daping in the static pressure circuit to be as nearly as possible equal to that in the totalpressure circuit; if necessary, on Pitot tubes the diaeter of which is sall, the orifices ay be placed in two planes; 3) tip placed not of the nose; less than six headdiaeters fro the 4) placed not less than eight headdiaeters fro the axis of the Stern. 4.2 Criteria to be fulfilled by the Pitot tube The nose (including the total pressure hole) shall be designed in such a way as to coply with the following requireents : a) The response of the differential pressure to inclination of the head relative to the flow shall eet one of the following two conditions according to the circustances (in both cases it is necessary to know the response curve of the Pitot tube) : 1) if precise alignent of the Pitot tube with the conduit axis is not possible but there is no swirl, the differential pressure should be as independent as possible of the yaw of the head in unifor flowl); 2) if precise alignent of the Pitot tube with the conduit axis is possible but swirl is present, the Variation of the differential pressure recorded by the tube in unifor flow with yaw angle q shall be approxiately proportional to coszrp. If the head is perfectly aligned axially and if swirl is less than + 3, the differential pressure shall not deviate fro this law by ore than 1 %. lt should be noted that isalignent and swirl tan occur siultaneously and efforts shall be ade to iniize each of the. b) The calibration factors for different speciens of tubes to a particular specification shall be identical, to within + 0,25 %, and shall reain so for the working life of any such tube. If the user has any doubt upon this Point, an individual calibration of each Pitot tube should be ade. c) When used in a liquid, any cavitation fro the nose shall not Cause a significant error in the static pressure reading of the tube. e) If the ste is enlarged to a diaeter d, there shall be a length of ste not less than 7 d, between the axis of the head and the coenceent of the enlargeent, for which the sterndiaeter is equal to the headdiaeter. f) The junction between the head and ste shall be either itred or curved to a ean radius equal to 3 i 0,5 ties the headdiaeter. g) An alignent ar shall be fitted to the end of the ste away fro the head, to ensure precise alignent and positioning within a conduit. Three types of Pitot tubes which are currently used and which coply with these criteria are described as exaples in annex A. 5 REQUIREMENTS FOR USE OF PITOT TUBES 5.1 Selection of the easuring Crosssection The Crosssection selected for easureents shall be located in a straight pipe length and shall be perpendicular to the direction of flow. lt shall be of siple shape, for exaple either circular or rectangular. lt shall be located in an area where the easured velocities fall within the noral working range of the apparatus used (see 5.3.2) Close to the easuring Crosssection, flow shall be substantially parallel to and syetrical about the conduit axis and contain neither excessive turbulente nor swirl; the easuring Crosssection shall thus be Chosen far enough away fro any disturbances that could create asyetry, swirl or turbulente (see 5.1.4). 1) The Pitot tubes descri bed in annex A allow independence of the differential p ressu re to within + 1,5 % up to 14 yaw in unifor flow. 5 Dit docuent is een voorbeeld van NEN / This docuent is a preview by NEN

10 Bestelforulier Stuur naar: NEN Standards Products & Services t.a.v. afdeling Klantenservice Antwoordnuer WB Delft Ja, ik bestel NEN Standards Products & Services Postbus GB Delft Vlinderweg AX Delft T (015) F (015) ex. ISO 3966:1977 en Measureent of fluid flow in closed conduits Velocity area ethod using Pitot static tubes Wilt u deze nor in PDFforaat? Deze bestelt u eenvoudig via Gratis eailnieuwsbrieven Wilt u op de hoogte blijven van de laatste ontwikkelingen op het gebied van noren, noralisatie en regelgeving? Nee dan een gratis abonneent op een van onze eailnieuwsbrieven. Gegevens Bedrijf / Instelling T.a.v. O M O V Eail Klantnuer NEN Uw ordernuer BTW nuer Postbus / Adres Postcode Plaats Telefoon Fax Factuuradres (indien dit afwijkt van bovenstaand adres) Postbus / Adres Postcode Plaats Datu Handtekening Retourneren Fax: (015) Eail: klantenservice@nen.nl Post: NEN Standards Products & Services, t.a.v. afdeling Klantenservice Antwoordnuer 10214, 2600 WB Delft (geen postzegel nodig). Voorwaarden De prijzen zijn geldig tot 31 deceber 2016, tenzij anders aangegeven. Alle prijzen zijn excl. btw, verzend en handelingskosten en onder voorbehoud bij o.. ISO en IECnoren. Bestelt u via de norshop een pdf, dan betaalt u geen handeling en verzendkosten. Meer inforatie: telefoon (015) , dagelijks van 8.30 tot uur. Wijzigingen en typefouten in teksten en prijsinforatie voorbehouden. U kunt onze algeene voorwaarden terugvinden op: Noralisatie: de wereld op één lijn. preview 2016