Custom Search
|
|
|
||
 inserted at right angles to the flow. The impact port must meet the flow
squarely. Placement of the tube is critical. The pitot tube measures
velocity at only one point. The single point is used to calculate average
flow velocity, but does not necessarily represent average velocity
(v
)
critical . In small lines, it may be impossible to place the pitot tube
at `critical because of the pipe wall. It may be necessary to q easure flow
at multiple locations to establish a relationship between the final pitot tube
location and the actual flow. It is preferable to locate the pitot tube in a
horizontal line. To ensure accurate flow measurement, the fluid MUST enter
the pitot tube with a fully-developed velocity profile, free from swirls or
vortices. Such a condition is best achieved by the use of adequate lengths of
straight pipe, both preceding and following the pitot tube. The minimum
recommended length of upstream piping is 7.5 times the pipe diameter. For
specific lengths for a particular installation, consult manufacturer. The
length is necessary to limit errors, due to piping configurations, to less
than %0.5 percent. If minimum distances are not observed, the pitot tube
meter may produce inaccurate data. In ducts that do not meet straight length
requirements, it is necessary to take multiple samples to obtain an adequate
flow profile.
5.1 Permanent Mounting With Separate Static Tap. In this option, the pitot
tube impact tap is permanently mounted facing directly upstream. It is
located at a point where the velocity measured represents average velocity.
The static tap is drilled through the wall of the pipe less than one-half pipe
diameter upstream of the impact tap. Both taps are for screw-fitted
components.
6. MAINTENANCE. The following procedures are the minimum required for the
most common types of units. When developing maintenance schedules, refer to
the manufacturer's instructions. To ensure reliable flow data, the following
actions must be undertaken quarterly or when data accuracy is suspect:
q Check
for clogged orifices.
q Check
tip for wear.
q Sensor
lines should be blown down on a regular schedule.
q Rescale
dp transmitter if necessary, based on flow of previous year.
q Test
dp transmitter with dead weight pressure tester and rescale if
necessary.
7. ACCURACY AND RELIABILITY. Accuracy is typically %5.0 percent of full
scale. For specific applications, accuracy can be up to %0.75. Pitot tubes
are reliable as long as the fluid being measured is sufficiently clean to
avoid clogging the orifices. With contaminated or dirty fluids, Increased
maintenance may be necessary to ensure accuracy.
5-23
|
 
|
|
|
||
