When a stream is dirty enough that even an offset round bore would eventually clog, the opening itself can be reshaped. A segmental orifice plate does exactly that: instead of a circular hole it cuts a segment, a partial circle or crescent, that hugs one side of the pipe wall and leaves the rest of the plate solid. Positioned so the open segment sits at the bottom for slurries or the top for gas-laden liquids, it gives the heavy or light phase an unobstructed path along the pipe wall. This guide explains how a segmental plate is shaped, how it differs from eccentric and concentric plates, why it is used in the dirtiest services, and how its opening must be oriented to work.
Segmental Orifice Plate in one line: A segmental orifice plate has an opening shaped like a segment of a circle, a crescent that sits flush against the pipe wall rather than a full round bore. It is used in heavy slurry, sediment-laden, or gas-carrying service to let solids or gas pass along the wall without accumulating. Compared with eccentric and concentric plates it handles the dirtiest streams but is the least accurate and relies on empirical coefficients, so its opening orientation and its lower precision must both be respected in the flow calculation.
A segmental plate takes the idea of moving flow area toward the pipe wall further than an eccentric plate does. Rather than a round bore slid off-center, its opening is a chord-bounded segment: imagine a straight line drawn across a circle and the smaller region below that line opened up, leaving a crescent of clear area tangent to the pipe wall along its whole curved edge. Because the opening follows the wall for its full width, there is no rim at all on that side for material to collect behind, giving the widest possible unobstructed path for the phase that needs to pass.
That geometry is what makes the segmental plate the choice for the heaviest fouling. In slurry service, where the stream may be thick with suspended solids, a full-width opening along the pipe floor lets the solids sweep through continuously rather than settling. In gas-laden liquid service the same opening placed at the top of the pipe lets gas run out along the crown across the full width of the segment. The plate still restricts the flow enough to generate a measurable differential, but it does so without presenting the kind of shelf that a round bore, even an offset one, still leaves.
The price of that open geometry is that the flow through a crescent-shaped opening is far less symmetric and less well behaved than flow through a round hole. There is no clean, axisymmetric jet and vena contracta of the sort a concentric bore produces, so the plate's behavior cannot be predicted from the tidy theory that governs round orifices. Its coefficient has to be established empirically, and it is understood to be a coarser measurement from the outset.
The three plate types form a progression in how aggressively they trade accuracy for the ability to handle dirty fluid. A concentric plate, with its centered round bore, is the most accurate and the standard for clean single-phase custody metering, but it fouls readily because its full ring of face acts as a shelf on every side. An eccentric plate keeps a round bore but slides it against one wall, removing the shelf on that side so a second phase can pass while still offering a reasonably characterized round opening. A segmental plate goes furthest, abandoning the round bore entirely for a wall-hugging segment that maximizes the clear path.
Roughly speaking, the dirtier and more solid-laden the stream, the further along this progression you move. A gas line with a little condensate might be handled with an eccentric plate; a genuine slurry, or a stream carrying a heavy solids load, points to a segmental plate because only its full-width opening reliably passes the material. In exchange, accuracy degrades along the same progression: concentric is best, eccentric is a step down, and segmental is the coarsest, so a segmental measurement is understood as an approximate flow indication for a service that admits nothing better.
This is why plate selection is a deliberate engineering decision recorded with the meter, not a field substitution. The plate type sets the coefficient, the expected accuracy, and the fouling behavior all at once, and swapping a concentric plate for a segmental one, or the reverse, without updating the flow calculation and the accuracy expectation produces numbers that are either wrong or misleadingly precise. The choice is documented on the meter specification so the reading is interpreted for what it is.
Like the eccentric plate, a segmental plate is directional and must be installed so the open segment faces the correct way. For a slurry or sediment-laden liquid the segment goes at the bottom, so solids follow the pipe floor out through the full-width opening. For a gas-carrying liquid the segment goes at the top, so gas sweeps out along the crown. A segmental plate fitted with the opening on the wrong side is worse than useless: it presents the solid part of the plate exactly where the material is trying to go and clogs almost immediately, so orientation is checked carefully at installation and the plate is marked to make the correct position unambiguous.
Because the segmental plate lives in the roughest services, its readings are treated as coarse process and allocation indications rather than precise fiscal figures. Operators pair it with the knowledge that the stream is genuinely dirty and that separating the phases to meter them cleanly would be impractical or uneconomic, so a stable, approximate flow signal is the realistic goal. The value lies in having a continuous number from a line that would otherwise be nearly impossible to meter at all.
In a cloud SCADA platform such as Merobix, monitoring a segmental-plate installation focuses on trends and health rather than absolute precision. A differential that climbs steadily can indicate the opening is finally starting to load up despite its full-width path, or that the plate was oriented wrong; watching that trend centrally alongside other sites helps distinguish a real flow change from creeping fouling. Recording the plate as segmental in the meter configuration, together with its orientation and its empirical coefficient basis, keeps the accuracy expectation realistic so the reading is used for what a dirty-service measurement can honestly support.
An eccentric plate keeps a round bore but slides it off-center against the pipe wall, while a segmental plate abandons the round bore entirely for a crescent-shaped opening that hugs the wall across its full width. The segmental opening leaves no rim on that side at all, so it passes heavier solid loads and thicker slurries than an eccentric plate can, at the cost of even lower accuracy.
Segmental plates suit the dirtiest services, heavy slurries, streams thick with suspended solids, and liquids carrying significant entrained gas, where even an offset round bore would clog. The full-width wall-hugging opening lets the solids or gas sweep through continuously. They are chosen when separating the phases to meter them cleanly is impractical and an approximate flow signal is the best realistic outcome.
The crescent-shaped opening does not produce the clean, symmetric jet and vena contracta that a round bore does, so its behavior cannot be predicted from the tidy theory that governs round orifices. Its discharge coefficient has to be established empirically and is understood as coarser from the start, making the segmental plate an approximate process and allocation measurement rather than a custody-grade one.
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