DP flowmeters measure the drop in pressure across a flow element in the piping, such as an orifice plate. The measured flowrate is a function of the pressure drop. So the flowmeter consists of the flow element in the piping, as well as a nearby differential-pressure meter. Small tubes, called impulse lines, on either side of the flow element lead to the DP meter for measurement.

Professional organizations, such as ISA, ANSI, API, ASME, and AGA, offer installation guidelines for DP flowmeter installations. These guidelines help to minimize disturbances to the fluid-velocity profile. Often, for example, measurements of temperature and static pressure are desirable in addition to flowrate. To minimize flow disturbances from an added pressure sensor, install a tee for connection to the appropriate DP impulse line. Similarly, install a thermowell used for temperature measurement at least 10 pipe diameters downstream of the flow element. Such installations should be characterized by smoothly ground welds and trimmed gaskets to eliminate protrusions into the fluid stream.

Straight-pipe runs upstream and downstream from the DP flow element help to guarantee a fully developed fluid-velocity profile that permits a predictable pressure drop. For an orifice, the length of straight run required depends on both the beta ratio of the installation and on the nature of the upstream components in the pipeline. (Beta ratio is the diameter of the orifice divided by the pipe diameter.) For example, when a single 90-degree elbow precedes an orifice plate, the straight-pipe requirement ranges from six to 20 pipe diameters as the beta ratio increases from 0.2 to 0.8.

The size and orientation of the impulse-line pressure taps depend on both the pipe size and the process fluid. The recommended maximum diameters of pressure-tap holes through the pipe or flange are:

¼ inch for pipes under two inches in diameter;
3⁄8 inch for two- and three-inch diameter pipes;
½ for four- to eight-inch diameter pipes; and
¾ inch for pipes larger than eight inches in diameter.
Size both pressure taps and leads with the same diameter. Where the hole breaks through the inside pipe surface, make sure it is flush with the inside pipe surface with no roughness, burrs, or wire edges. Connections to pressure taps should be made by nipples, couplings, or adaptors welded to the outside surface of the pipe.

On services where the process fluid can plug the pressure taps or might gel or freeze in the impulse lines, consider chemical seals. Connection sizes are usually larger in these cases. When using chemical seals, assure that the two connecting capillaries that route to the DP meter experience the same temperature and keep them both shielded from sunlight.

Flowmeter Piping Requirements for DP Transmitter In Steam, Liquid & Gas
Figure 1. Recommended orifice and DP transmitter configurations for steam, liquid, and gas service.
Locate the DP transmitter as close to the primary element as possible with short lead lines of the same diameter. Figure 1 shows recommended configurations for various DP applications. In steam service, the horizontal lead lines should be kept as short as possible, and they should be tilted (with a minimum gradient of one in/ft with respect to the piping) towards the tap, so that condensate can drain back into the pipe. In clean liquid or gas service, purge the lead lines through the DP cell vent or drain connections. Flush them for several minutes to remove all air, because entrapped air can offset zero calibration.

If the process temperature exceeds the maximum temperature limitation of the DP meter, either use chemical seals or make the lead lines long enough to cool the fluid. If more cooling is necessary, install a coiled section of tubing (pigtail) in the lead lines.

The frequency of inspection or replacement of a primary element depends on the process fluid’s erosive and corrosive properties, as well as the overall accuracy required. If no previous experience exists, remove the orifice plate for inspection during the first three, six, and 12 months of its operation. Based on visual inspection of the plate, develop a reasonable maintenance cycle from the findings. Keep orifices used for material balance calculations on the same maintenance cycle.