Page 86 - Fluid mechanics, heat transfer, and mass transfer

FLOW MEASUREMENT 63

Pressure tap location alternatives.
FIGURE 4.6
& In order to fully develop the flow (and the pressure & Do not require direct fluid flow calibration.
drop to be predictable), straight pipe runs are required & They have no moving parts.
both upstream and downstream of the differential & Standards for construction and calibration are readily
pressure measuring element.
available.
& The length of straight run required depends on both
& Relatively cheap to fabricate and install.
the b-ratio of the installation and on the nature of the
. What are the bad points in the use of orifice meters?
upstream components in the piping.
& Not good for viscous liquids.

& For example, when a single 90 elbow precedes an
& Accuracy is 1% with calibration and less if
orifice plate, the straight pipe requirement ranges
uncalibrated.
from 6 to 20 pipe diameters as the diameter ratio is
increased from 0.2 to 0.8. & They involve high pressure losses. Large part of DP
(50–80%) is not recovered.
& In order to reduce the straight run requirement, flow
straighteners such as tube bundles, perforated plates, & Long, straight run of pipe is required to avoid effects
or internal tabs can be installed upstream of the of upstream conditions.
primary element. Flow straighteners are illustrated & DP versus flow rate relationship is not linear so that
in Figure 4.7. the range is limited to a 4:1 ratio between maximum
. What are the good points in using orifice meters for flow flow and minimum flow.
measurement? ➢ The square root relationship limits the range of
flow rates that realistically can be measured in a
& Orifice meters are simple, rugged, widely accepted,
particular application.
reliable flow measuring devices.
& Orificeplatescanbeknockedoutofpositionbyimpur-
& Can be used for clean and unclean liquids and for
ities in the flow stream, and they are subject to wear.
some slurries.

FIGURE 4.7 Flow straighteners.