A standard orifice plate has its bore centered in the pipe, which is fine for clean single-phase fluid but becomes a problem when the stream carries sediment, condensate, or entrained gas. Anything heavier than the main fluid settles against the upstream face of a centered plate, and anything lighter collects at the top, so the plate slowly dams up whatever it is supposed to let pass. An eccentric orifice plate solves this by moving the bore off-center, usually flush against the bottom or top of the pipe, so the troublesome phase drains straight through the opening instead of piling up. This guide explains how the offset bore works, when to bias it low versus high, the accuracy cost compared with a concentric plate, and where it fits in dirty-service metering.
Eccentric Orifice Plate in one line: An eccentric orifice plate is an orifice plate whose bore is deliberately offset from the pipe center, typically positioned tangent to the bottom or top of the bore, so that solids, condensate, or gas can pass through without accumulating against the plate. The bore is biased low to drain sediment and liquids in a gas line, or high to vent gas and light phases in a liquid line. The offset trades some measurement accuracy for the ability to keep metering a stream that would foul a conventional concentric plate.
In a concentric plate the circular bore is centered on the pipe axis, leaving a ring of plate face all the way around that acts as a shelf. Denser material carried in the flow, sand, scale, or heavy condensate, cannot climb over the lower rim of that ring and so settles into a growing pile against the upstream face; lighter material such as gas bubbles in a liquid gathers against the upper rim in the same way. As the deposit builds, it changes the effective flow area and the approach conditions the plate sees, and the measurement drifts away from what the coefficient assumes.
An eccentric plate removes the shelf on one side by sliding the bore off-center until its edge is tangent to the pipe wall. If the bore is pushed down so its lower edge meets the bottom of the pipe, there is no lip for sediment or liquid to collect behind; the heavy phase simply follows the pipe floor straight out through the opening. If the bore is pushed up so its upper edge meets the top of the pipe, gas and light phases sweep out along the pipe crown rather than pooling under a shelf. The plate keeps producing a usable differential while staying self-cleaning for the phase that would otherwise foul it.
The offset also shifts the meter's characterization. Because the bore is no longer symmetric within the pipe, the flow pattern through it and the pressure recovery downstream differ from the concentric case, so an eccentric plate carries its own discharge coefficient behavior rather than borrowing the concentric one. That is why the plate type, and the direction of the offset, are recorded as fixed properties of the meter run and reflected in the flow calculation.
The direction of the offset is chosen to match which unwanted phase the stream carries. When the main fluid is a gas that carries liquid droplets, condensate, or fine solids, the bore is biased low so its lower edge is flush with the bottom of the pipe. The heavier liquid and particulate then run along the floor of the line and pass straight through, keeping the plate clear. This is the arrangement for wet gas gathering and other gas services where a little liquid is unavoidable but a centered plate would slowly load up with it.
When the main fluid is a liquid that carries entrained gas or vapor, the logic flips: the bore is biased high so its upper edge meets the top of the pipe, letting gas bubbles sweep out along the crown of the line rather than collecting under a centered plate's upper shelf. This suits liquid streams that occasionally flash or carry dissolved gas that breaks out, where trapped gas against the plate would distort the reading and could give an unstable, noisy differential.
Getting the direction right is not optional, because an eccentric plate installed upside down defeats its own purpose, biasing the bore high when the stream carries sediment simply moves the shelf to the bottom and traps the very material the plate was meant to pass. The plate therefore has a defined orientation, usually marked, and the installation must respect it. This is one of the failure modes worth checking during commissioning, since a plate that measures acceptably at first can foul steadily if it was fitted the wrong way up.
An eccentric plate is a compromise instrument. Because its bore sits against the pipe wall rather than symmetrically in the flow, the velocity profile it presents is less clean than a concentric plate's, and the coefficient behavior is not characterized as tightly. The consequence is that an eccentric plate is generally accepted as less accurate than a concentric one and is not the first choice for custody transfer of a clean fluid, where a centered plate in a proper meter run would always be preferred. It is chosen because the alternative, a fouling concentric plate whose reading drifts as it clogs, is worse.
That framing matters for how the reading is interpreted in a control system. An eccentric plate is typically a working measurement for allocation, process monitoring, or field balance rather than a high-precision fiscal number, and it is often paired with an expectation that the stream is genuinely dirty or multiphase and cannot be metered any other way without separation. The value it delivers is a continuous, reasonably stable flow signal from a service that would otherwise give an unreliable one.
For an operator monitoring many such sites through a cloud SCADA platform like Merobix, the eccentric plate's dirty-service role shapes what to watch for. A differential that grows steadily over days can signal that the plate is fouling despite the offset, or that it was installed with the bore biased the wrong way, and centralizing the trend where it can be compared against other sites makes that drift easier to spot early. Recording the plate as eccentric, and its offset direction, in the meter configuration also keeps the accuracy expectation honest, so nobody treats a dirty-service allocation reading as if it carried custody-grade precision.
You use an eccentric plate when the stream carries a second phase, sediment, condensate, or entrained gas, that would accumulate against a centered concentric plate and foul the measurement. The offset bore lets the troublesome phase pass through instead of piling up. A concentric plate remains the better choice for clean single-phase fluid because it is more accurate.
Bias the bore low, tangent to the bottom of the pipe, when the main fluid is a gas carrying liquid or solids, so the heavy phase drains straight through. Bias it high, tangent to the top, when the main fluid is a liquid carrying entrained gas, so the gas vents out along the crown. Installing the plate with the offset in the wrong direction traps the very phase it was meant to pass, so orientation must be respected.
Yes. Because the bore sits against the pipe wall rather than symmetrically in the flow, the velocity profile is less clean and the discharge coefficient is not characterized as tightly, so an eccentric plate is generally less accurate. It is chosen for dirty or multiphase service where a fouling concentric plate would drift worse, making it a working allocation and monitoring measurement rather than a custody-grade one.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.