ISO 7194:2008 pdf download – Measurement of fluid flow in closed conduits – Velocity-area methods of flowmeasurement in swirling or asymmetricflow conditions in circular ducts by means of current-meters or Pitot static tubes.
7.1 Number and position of measuring points
The number and position of measuring points in the measuring section shall conform to the specifications of ISO 3354 or ISO 3966, taking into account the integration technique chosen. However, the minimum number of measurements per radius shall be five (excluding any measurement on the centre line) and, when there is reason to believe that the flow is asymmetric, the minimum number of radii shall be six. Also, at least one measurement of local velocity shall be made in each of the folowing zones within the pipe on each radius in
addition to any measurement which might be made on the centre line:
This condition is fulfilled automatically when the log-linear or log-Ufleoysflev memoas or integration are usea, but care has to be taken to choose the measuring positions in accordance with this requirement when either the numerical or graphical inteWation method is used
Often, especially when there is reason to believe that the flow may be asymmetric, the uncertainty of flow measurement is reduced more by increasing the number oh radii along which measurements are made than by increasing the number of points per radius For example, if 48 current-meters are available for installation wi a conduit, it is often slightly better to use six on each of etght radii rather than eight on each of six radiL
72 Effect of pressure fluctuations
In any conduit subject to flow covered by ISO 3966 or this International Standard, there are pressure fluctuations dNectly linked to the turbulent components of the local velocities superimposed on the mean flow, The traversing Pitot static tube transmits these to the manometer or pressure transducer as components of the instantaneous differential pcessure Sufficient damping in the manometer oircuit helps the operator to estimate the average differential pressure, but such damping shall be symmetrical and linear, in order to avoid an additional error which cannot be assessed The error in the mean velocity estimated from the mean differential pressure reading over time in the presence of turbulence is considered separately in Clause 8.
There shall be sufficient symmetrical and linear damping in the manometer circuit to ensure that fluctuations of the manometer reading at each point of measurement do not exceed ±3 % 01 the average reading at that point.
Recommendations on ensuring that damping is symmetrical and linear are given in Annex B.
Pressure fluctuations of acoustic origin, quite unrelated to the local flow velocities. may be present in some conduits, particulady those subject to gas flows. Such pressure fluctuations are usually much greater than those arising from turbulence and the smallest departure from linearity in damping of the manometer circuit inevitably leads to a considerable error in the local velocities estimated from the average manometer reading. Therefore, before measurements can be carried out in accordance wth this International Standard, the user shall check that no significant regular pressure fkictuations are present in the conduit and, if there are, shall eliminate them. Advice on detection and removal is given in Annex A.
7.3 Axial velocity measurement using a Pitot static tube
Guidance on the use of Pitot static tubes is 9lven in Annex D. The Pitot static tube used shall be one of those specified m ISO 3966, and measurement may be made by one of the two following methods,
In method A (see 7.3.1), the probe shall be aligned with the axis of the pipe at each measunng position, and use made 01 a knowledge of the response of the particular Pitot static tube at vanous angles of inclination to the local flow dWectlon Thes method may be used only for swirl angles up to 20.