PREFABRICATED METER ASSEMBLIES

previous DIFFERENTIAL PRESSURE METERS

1. Insertion Fittings: Some manufacturers produce a dual-chambered, hand-operated, gear-driven apparatus which allows one person to change an orifice plate. An insertable plate can be changed without system shutdown or removal of flanges. The mechanism is constructed so that there is no fluid spillage or loss and no danger to the operator. For specific product information consult manufacturers of fluid flow metering equipment.

2. Recommended Applications: Prefabricated meter assemblies are Particularly suited for low flow in pipes of less than 2 inches in diameter” A common application is for measuring natural gas flow to specific pieces of equipment.

FIGURE 5-5. Orifice Meter With Corner Taps

3. Limitations: Prefabricated orifice meter assemblies cannot measure over an infinite range. The turndown of an orifice meter is 3:1 and, therefore, predetermined orifice sizing is required to obtain accurate measurement data.

4. Installation: Prefabricated meter assemblies are permanently installed meters fitted in a specially designed length of pipe with permanently located pressure taps. The complete assembly is installed in the pipe where flow is to be measured. The prefabricated assembly is sized for a particular flow range and is calibrated accordingly.

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. Annually disassemble meter and inspect and perform maintenance as follows:

(a) Check orifice for wear, i.e., roundness, size, and squareness of edge.

(b) Plate should be examined for warping with a straightedge.

(d) Check pressure taps for burrs and/or debris.

e) Test dp transmitter with dead weight pressure tester and rescale if necessary.
(f) Dress off roughness on plate.
(g) Resize orifice, if necessary, based on flow of previous year. When orifice is resized, stamp the new diameter and coefficient on the holder.
(h) Flush all trapped sediment from unit.
(i) Reinstall orifice plate so that flow exits the bevelled side.
(j) Sensor lines should be blown down at regular intervals.
Since inspection of an orifice plate requires shutdown, scheduling may be predicated by system supply requirements.

ACCURACY AND RELIABILITY: Orifice meter accuracy is up to ±%0.75 percent of full scale. The major influence on accuracy is installation, where care must be taken to ensure proper installation of the run, pressure taps, and the tap tubing. Meter runs are highly reliable when used over the range of calibration. If removed for cleaning or inspection, they should be recalibrated before returning to service.

PRESSURE TAPS AND INDICATION DEVICES: A variety of pressure tap configurations are available for orifice plates. Various devices are used to quantitatively express the differential pressure. Each of the common tap options and dp devices are described in the following paragraphs.

1. Pressure Taps: The common tap options are: flange taps, corner taps, and radius taps.

1.1 Flange Taps: The most commonly recommended configuration is the flange tap (Figure 5-4). Pretapped flanges are standardized, convenient, and easily replaced.

1.2 Corner Taps: Carrier rings are drilled for pressure taps and insertion between existing flanges (Figure 5-5). This type of flange should only be used if the flange taps are unavailable. Standardization of equipment should be the goal of a well-planned system.

1.3 Radius and Vena Contracta Pipe Taps: These taps are mounted directly in the pipe (Figure 5-6). Although widely used in the past, they have been largely replaced with standardized flange taps.

2. Differential Pressure Devices: Dp devices are used to provide a quantitative display of the differential pressure; they are also called delta P and D P devices. The four most common dp devices are: manometer, diaphragm, bellows, and electronic. All new or replacement differential pressure installations should be electronic.

2.1 Manometer: The manometer is a rather simple device. One end of the manometer is attached to the high-pressure tap and the other end to the low-pressure tap of the orifice plate installation. As the dp created by the orifice plate is sensed by the manometer, a column of fluid in the manometer allows the dp to be read directly on a scale (Figure 5-7). Refer to the manufacturer’s instructions before installing a manometer.

2.2 Diaphragm: The diaphragm device includes a hermetically sealed diaphragm that is in an enclosure with one side open to the high-pressure tap and the other side to the low-pressure tap (Figure 5-8). The diaphragm moves as the dp created by the orifice meter is transmitted to the diaphragm chamber. A pointer attached to the diaphragm pivots about a fulcrum in the wall of the chamber and mechanically indicates the dp directly on a scale. Refer to the manufacturer’s instructions before installing a diaphragm.

2.3 Bellows: A bellows device is similar to the diaphragm device in that the indicator pointer is attached to a component that is subject to movement caused by the dp. In a bellows device, a partition is hermetically sealed between two bellows in a confined compartment with an opening on one side to the high-pressure tap and another to the low-pressure tap (Figure 5-9). The input ends of the bellows are fixed to the compartment walls. As the dp forces the partition to move, compressing and expanding the respective bellows, a lever system causes a pointer to directly indicate the dp on a scale. Refer to the manufacturer’s instructions before installing a bellows.

2.4 Electronic: Electronic devices are also known as capacitance devices. In an electronic device, the dp is transmitted through an isolating diaphragm to a hermetically sealed sensing diaphragm in the center of the device (Figure 5-10). The sensing diaphragm is surrounded by silicone oil contained between capacitors. As the sensing diaphragm deflects in proportion to the dp, the position of the diaphragm is detected by capacitor plates on each side of the diaphragm. The differential capacitance between the plates and the diaphragm is converted electronically to a 2-wire, 4-20 mA, or 0-10 volt data transmission signal. Electronic devices are available in both square root and linear function models. Solid state, plug-in components simplify maintenance/repairs.

2.5 Calibration: Static calibration should be performed on all dp devices at least every 6 months.

3 Data Transmission: Readings from various dp devices must often be transmitted to remote data collection and recording sites. his is because the dp device may be too remote to warrant onsite reading. Data transmission may also be necessary because there may be many widely dispersed devices to be read and it would be uneconomical to have each one read onsite; or a central data management point has been set up to collect, record, plot, reduce, and analyze all flow data. All the taps for dp will accommodate remote transmission fittings along with the various dp devices.

FIGURE 5-6. Orifice Meter With Radius and Vena Contracta Pipe Taps

FIGURE 5-7. Manometer

FIGURE 5-8. Diaphragm

3.1 Pneumatic: All dp devices, except electronic, can have readings transmitted to a remote site by pneumatic lines.

3.2 Electrical: Rather than pneumatically, the recommended means of data transmission is to have the dp electronically converted to an analog signal and transmitted electrically to a data collection center.