ISO 7194:2008 pdf download.Measurement of fluid flow in closed conduits — Velocity-area methods of flow measurement in swirling or asymmetric flow conditions in circular ducts by means of current-meters or Pitot static tubes.
Introduction
In order to carry out measurements of the flow-rate of single phase fluids in closed pipes by velocity-area methods, using either current-meters or Pitot static tubes, with satisfactory accuracy (e.g. of the order of +2 % ), it is usually necessary to choose a measuring plane where the velocity distribution approaches that of fully developed flow(see ISO 3354 and ISO 3966).
There are, however, some cases where it is practically impossible to obtain such a flow distribution, but where as good as possible a measurement of the flow-rate is desirable.
ISO 7194:2008 specifies velocity-area methods for measuring flow in swirling or asymmetric flow conditions in circular ducts by means of current-meters of Pitot static tubes.
ISO 7194:2008 specifies the measurements required, the precautions to be taken, the corrections to apply, and describes the additional uncertainties which are introduced when a measurement in asymmetric or swirling flow has to be made.
2 Normative references
ISO / TR 3313 , Measurement of fluid flow in closed conduits – Guidelines on the effects of flow pulsations on flow-measurement instruments.
ISO 3354 : 2008 , Measurement of clean water flow in closed conduits-Velocity-area method using current-meters in full conduits and under regular flow conditions.
ISO 3455 : 2007 , Hydrometry-Calibration of current-meters in straight open tanks.
ISO 3966 : 2008 , Measurement of fluid flow in closed conduits-Velocity area method using Pitot static tubes.
ISO 4006 , Measurement of fluid flow in closed conduits-Vocabulary and symbols.
ISO 5168 . Measurement of fluid flow-Procedures for the evaluation of uncertainties.
4 Principle
This International Standard describes
— methods which minimize the errors in carrying out a traverse in swirling or asymmetric flow.
— corrections which should be applied for certain sources of error.
— methods of determining the increase in uncertainty in the flow-rate measurement when it is not possible
to compensate for a particular source of error .
The origins of the errors giving rise to the uncertainties considered in this International Standard are
a ) errors in the determination of local velocities , due to the behaviour of the instruments in a disturbed flow.
b ) errors in the calculated mean pipe velocity , due to the number and position of the measuring points and the methods of integration used.
Corrections are possible for some of these errors , but , in general , the limiting uncertainty in the flow-rate measurement has to be increased according to the characteristics of the flow.
Although velocity-area integration techniques to measure flow-rate under conditions where there is swirl and / or asymmetry in the flow are described , a measuring section in the pipe where the swirl or asymmetry is as small as possible is preferred.
5 Choice of measuring plane
When the configuration of the pipe and any fittings installed in it is such that any changes of directions of the
flow are all in the same plane ( e.g. a single bend , a single valve , or two bends in an S-shape ) , no significant
bulk swirl is introduced and the disturbance to the flow results in an essentially asymmetric velocity distribution.
If , however , the pipe configuration is such that the flow changes direction in two or more different planes in
rapid succession ( e.g. two bends at 90° to each other ) , a bulk swirl is introduced in addition to the asymmetric which the individual fittings introduce.
Unlike asymmetry , swirl has a big effect on the response of Pitot static tubes and current-meters , and also persists for very much longer distances ; whenever possible , therefore , the traverse plane should not be downstream of a swirl-inducing configuration . Care should also be taken to avoid locating the traverse plane downstream of any adjustable fitting for which the geometry may change ( e . g . a flow control valve ) , especially if several different flow-rates have to be measured.ISO 7194:2008 pdf download.