2.1 Square-Edged Orifice. The orifice is sized to meet one specific
anticipated flow rate. The upstream face of the orifice is flat with a square
edge where the orifice meets the plate surface (Figure 5-2). If the side with
a beveled or recessed edge is facing upstream, erroneous data will result.
2.1.1 Recommended Applications. Square-edged orifice metering is applicable
on gas, liquid, and steam flow systems when pipe sizes are greater than 2
inches in diameter.
3. SPECIAL ORIFICES. These orifices are designed for special flow
situations. One-quarter circle and conical entrance devices address low-flow
and high-viscosity situations. Their use is limited. In an effort to prevent
a buildup of debris on the upstream side of an orifice plate, eccentric
orifice plates are used where moisture-laden gases are flowing and segmental
orifice plates are used where a liquid containing a large percentage of gas is
flowing (Figure 5-2).
4. LIMITATIONS. Orifice meters cause some permanent pressure loss due to
friction. Pressure loss, increased friction, and increased pumping costs may
make orifice metering undesirable. The range of this metering system is
limited from 3:1 to 4:1. Turndown ratio can be increased by using two or more
dp transmitters of different rangeabilities with an obvious increase in cost.
They may be mounted in series and connected to a decision processor that will
select the appropriate transmitter dependent upon the differential pressure
(alp) . Since transmitters are expensive, the use of multiple transmitters is a
tradeoff between precision requirements and cost effectiveness. Other
limitations are as follows:
Temperature range is to l,OOOF.
Pressure limit is 6,000 psig.
5. INSTALLATION. The location of the orifice plate in the system is
important. Whenever possible, it is preferable to locate the primary element
in a horizontal line. For accurate flow measurement, the fluid must enter the
primary element with a fully developed velocity profile, free from swirls or
vortices. In addition, fluid must exit the bevelled side of the orifice.
Such a condition is best achieved by the use of adequate lengths of straight
pipe, both preceding and following the primary element. The minimum
recommended lengths of piping are shown in Figure 5-3. The diagram in
Figure 5-3 that corresponds closest to the actual piping arrangement for the
meter location should be used to determine the required lengths of straight
pipe on the inlet and outlet. These lengths are those necessary to limit
errors due to piping configurations to less than %0.5 percent. If these
minimum distances are not observed, or if the orifice plate is installed with
the bevel on the inlet side, flow equations and resultant flow calculations
may produce inaccurate data.
5.1 Meter Installation. Common methods of installing orifice plate meters
are described in the following paragraphs.