Look Beyond Orifice Plates

Consider other types of elements for differential pressure measurement.

By By Greg Livelli and Steve Pagano, ABB Instrumentation

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choosing flow meters

As an example of the wedge meter’s ability to maintain calibration in tough applications, consider two stainless steel meters — one 3-in. and one 4-in. with wafer-type seal connections — that ABB examined after 12 years of service. The meters measured steam-cracked tar, a byproduct of ethylene production. The cracked tar is kept at elevated temperature to prevent solidification of abrasive coke fines and other particles in the process stream. The meters endured temperatures in excess of 355°F and pressures up to 310 psi. Fluid viscosities of 22 cP produced Re of 1,870 and 2,850, respectively, at maximum flow rates. With more than 5 million pounds a month of the abrasive tar being produced, repeatable and reliable measurement was a prime concern.

Calibration before and after the 12 years of service demonstrated that neither unit exhibited a major change in meter coefficient. The 3-in. meter showed a deviation of 0.24% from its original testing while the 4-in. meter shifted 1.3% (based on averaged meter factor). Given typical calibration uncertainties, it’s safe to say that the meter factors remained virtually constant over the 12 years of operation.

Wedge meters can be manufactured in virtually any alloy for service temperatures up to 720°F and pressures exceeding 6,000 psi.

Flow tubes
The ASME defines flow tubes as any DP element whose design differs from the classic venturi (a definition that includes short-form venturis, nozzles and wedges). In practice, flow tubes come in several proprietary shapes; all tend to be more compact than the classic and short-form venturis. Laying lengths typically run from two to four pipe diameters. Being proprietary, flow tubes vary in configuration, tap locations, differential pressure and pressure loss for a given flow. The user must depend on the manufacturer of the particular flow tube for sizing and calibration. The ASME recommends calibration with a piping section that replicates actual use over the full range of expected flows, which may be difficult and expensive for the larger sizes.

Flow tubes can have either static or corner pressure taps. Static taps, like those of a venturi tube, sense pressure where the fluid velocity doesn’t change direction and parallels the pipe wall. Otherwise the taps are called corner taps. Three types of flow tubes are available:
flow tube
Figure 4. Flow Tube -- This Type 1 version comes close
to duplicating the behavior of the venturi tube.

• Type l (Figure 4) has static pressure taps at both the inlet and outlet;
• Type 2 has a corner tap in the inlet and a static tap in the throat; and
• Type 3 has a corner tap at both the inlet and outlet.
Laying lengths tend to decrease with type number. Flow coefficients range from 0.9797 for Type 1 to 0.75 for Type 3.

Type 1 flow tubes more closely approach the characteristics of the classic venturi. The inlet cone converges in two angles that condition the fluid as it enters the throat. Flow coefficents are relatively stable for a variety of flow conditions. For large pipe sizes, the shorter Type 3 flow tubes (such as the one shown in Figure 5) may be useful but also may require more upstream flow conditioning for good performance. Type 3 meter coefficients may change with variations in Re, line size and beta ratio.

Flow tube sizes range from 4 in. to 48 in. Justification becomes easier for the larger pipe sizes, where installed cost may be less than that of the venturi. However, accuracy depends on the manufacturer’s calibration data. Extrapolation of meter flow coefficients for large sizes from tests on smaller sizes may be problematic.

Flow tubes can be fabricated from a variety of materials. In some cases they’re available as inserts of fiberglass-reinforced plastic or metal.

Greg Livelli and Steve Pagano are senior product managers for ABB Instrumentation, Warminster, Pa. E-mail them at greg.livelli@us.abb.com and steven.g.pagano@us.abb.com.

Further Reading
1. Lipták, B., “Process Measurement and Analysis,” 4th ed., CRC Press, Boca Raton, Fla. (2003).
2. Livelli, G., “Matching the Flowmeter to the Application,” Flow Control, p. 14 (August 2007).
3. “Differential Pressure Flow Elements,” ABB Inc., Warminster, Pa. (2005). Downloadable via http://library.abb.com/global/scot/scot203.nsf/veritydisplay/ee6036f29cf9e8c68025708c0047f235/$File/SS_DP_3.pdf.
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