All About Orifice Plates and Orifice Flow Meters

Learn all about Orifice Plates and Orifice Flow Meters: what they are, how they work, the different orifice plate types, orifice flanges and pressure taps, how to build a complete orifice flow meter, advantages, disadvantages, accuracy, installation, orifice plates versus venturis, restriction orifices, pricing, and more.

What Is an Orifice Plate?

An orifice plate is a thin metal plate with a precisely machined opening that is installed in a pipeline to restrict fluid flow creating a drop in pressure. It is the most common and most cost-effective example of a primary element in a differential pressure flow meter. A differential pressure transmitter reads the pressure on each side of the plate and converts the pressure differential into flow. Together, the plate and the transmitter form a complete orifice flow meter. KOBOLD's KPL orifice plates are available in stainless steel, carbon steel, and other materials for line sizes from 2″ to 24”, at pressures up to 6090 psi and temperatures up to 930 °F, covering liquid, gas, and steam service.

How Does an Orifice Flow Meter Work?

An orifice flow meter works on Bernoulli's Principle, forcing the fluid through a smaller opening makes it speed up, and as its speed increases, its pressure drops. The faster the flow, the bigger the pressure drop across the plate.

Pressure taps on each side of the orifice plate route the upstream and downstream pressures to a differential pressure transmitter. The transmitter measures the difference and applies a well-established relationship: flow rate is proportional to the square root of the differential pressure, written as Q ∝ √(ΔP). Transmitters like the KOBOLD PAD perform the square-root calculation internally and output a direct flow signal.

For the full story of the physics and the broader technology family that orifice plates belong to, see our companion article: All About Differential Pressure Flow Meters article once published.

What Are the Types of Orifice Plates?

Not all process fluids behave the same, so orifice plates come in several opening styles. Choosing the right style for the fluid is a big part of getting accurate, trouble-free measurement. KOBOLD manufactures a variety of orifice styles to fit many applications:

• Concentric: The standard, general-purpose style. A cylindrical opening centered in the plate, with a sharp inlet and a conically tapered outlet. This is the workhorse for clean liquids, gases, and steam.
• Bi-directional: A cylindrical opening without an inlet or outlet bevel, so it measures equally well whether flow runs forward or in reverse.
• Quarter circle: A quarter-circle tapered inlet designed for small Reynolds number flows.
• Conical entrance: A conically tapered inlet and outlet for very small Reynolds number flows.
• Segmental: A half-circle opening positioned so that slurries, liquids carrying solids, or liquids with gas bubbles can pass through without collecting at the plate.
• Eccentric: A circular opening positioned off-center for the same reason, allowing solids and bubbles to pass rather than accumulate.

The Reynolds number is a standard fluid dynamics value that describes whether a flow is turbulent (fast, churning, well mixed) or laminar (slow, smooth, layered). Thick fluids and very slow flows have small Reynolds numbers, and the quarter circle and conical entrance styles exist to keep orifice measurement accurate in exactly those conditions.

What Are Orifice Flanges and Pressure Taps?

To measure the pressure drop, the piping needs small openings called pressure taps on each side of the plate. Where those taps sit relative to the plate is standardized, and the common arrangements for orifice installations have names:

• Flange taps: Located 1″ before and 1″ after the orifice plate.
• Corner taps: Located immediately before and after the plate.
• D-D/2 taps: Located one pipe diameter upstream and half a pipe diameter downstream.

The easiest way to get the taps right is not to drill them in the field at all. Pre-tapped orifice flange unions arrive with precisely placed taps already machined in, which reduces installation labor and protects measurement accuracy. KOBOLD offers pre-tapped orifice flanges for use with KPL orifice plate installations.

What Do You Need to Build a Complete Orifice Flow Meter?

An orifice flow meter is really a small system, and knowing the parts list makes the technology much less mysterious:

• The orifice plate: Creates the pressure drop. Custom manufactured for your pipe size and flow conditions.
• Orifice flanges with pressure taps: Hold the plate in the line and provide the measurement points.
• Impulse lines: Small-bore tubing that carries the two pressures from the taps to the transmitter.
• A manifold valve: A 3-way or 5-way valve block that lets you isolate, zero-check, vent, and service the transmitter without shutting down the process.
• A differential pressure transmitter: Measures the pressure difference and converts it to a flow signal.

In KOBOLD terms, that is a KPL orifice plate paired with a PAD differential pressure transmitter. The PAD delivers ±0.075% of calibrated span accuracy, outputs 4-20 mA with HART, and includes a standard 5-digit local display, continuous self-diagnostics, and remote configuration.

It is also worth knowing that the transmitter half of this system is one of the most versatile instruments in process measurement. Plumbed differently, the same PAD measures tank level or filter condition instead of flow. Our article on differential pressure transmitters for flow, level, and pressure covers that versatility in depth.

Are There Orifice Flow Meters with Everything Built In?

Yes. If you would rather skip the component list entirely, an integrated orifice flow meter combines the orifice element and the readout in one instrument that arrives as a pre-calibrated, matched set. KOBOLD's RCM is an orifice-type direct reading flow meter for liquids, gases, and steam, with a compact design that installs in horizontal or vertical pipes, handles flow rates up to 3,000 GPM for liquids or 20,000 SCFM for gases, and offers digital or mechanical displays. Its sibling, the RCD, takes the integrated approach with a venturi element for liquids in ½″ to 3″ lines.

The trade-off between integrated and component-built systems comes down to simplicity versus flexibility, and we walk through it in detail in the All About Differential Pressure Flow Meters article. The short version: integrated meters win on speed and simplicity, while component systems win on serviceability and extreme conditions.

What Are the Advantages and Disadvantages of Orifice Plate Flow Measurement?

Orifice plates have earned their position as the most widely used primary element for good reasons:

• Cost-effective and rugged: A precisely machined plate is inexpensive compared to nearly any other flow technology, and there is very little that can break.
• No moving parts: Nothing rotates or wears in the flow stream, which means minimal maintenance and long service life.
• Highly repeatable: The pressure drop an orifice plate produces is consistent and predictable, which is the foundation of reliable measurement.
• Works with almost any fluid: Liquids, gases, and steam, with material options for corrosive media and opening styles for dirty media.
• Scales affordably to large pipe: A plate for a 24″ line costs less than a full-bore flow meter for the same line.

The trade-offs to keep in mind:

• Permanent pressure loss: The abrupt restriction consumes more pressure than a venturi or pitot tube. If your system cannot afford the pressure loss, another primary element may fit better.
• Straight pipe requirements: Like all DP measurement, orifice plates need stable, fully developed flow, which means straight runs of pipe before and after the installation.
• Fluid property sensitivity: Density changes impact the reading, which matters most for gases and steam.
• Edge wear: The accuracy of an orifice plate depends on its sharp inlet edge. Abrasive or dirty media can gradually round that edge and drift the measurement, so plates in harsh service should be periodically inspected.

Can Orifice Plates Measure Gas and Steam?

Yes. The KPL is rated for liquid, gas, or steam. Gas and steam are core orifice plate territory, and the technology is often chosen for them specifically, particularly in large pipe sizes.

The caveat is density. Unlike liquids, gases and steam change density significantly as temperature and pressure change, and density directly affects the flow calculation. The meter must be configured for your actual operating conditions, and if those conditions vary in service, the reading needs real-time compensation from external temperature and pressure sensors. If your process has genuinely variable density, KOBOLD's DVE and DVH multivariable vortex flow meters build that compensation in.

How Accurate Is an Orifice Plate Flow Meter?

Accuracy depends on three things: the precision of the plate, the quality of the installation, and the transmitter reading it. Each KOBOLD KPL orifice plate will have unique accuracy specifications as they are custom manufactured for your specific pipe diameter and flow conditions, with sizing calculations and flow coefficient determination included. Paired with the PAD transmitter at ±0.075% of calibrated span, a properly installed orifice system according to the DIN EN ISO 5167-2 standard delivers precision suitable for demanding process control.

For integrated monitoring meters, the RCM delivers ±3% of full scale, which is well matched to duties like cooling water, chillers, reverse osmosis, and lube oil systems. Whatever the system, accuracy is only delivered when straight-run requirements are respected and the calibration matches the actual fluid and operating conditions.

Orifice Plate vs. Venturi: Which One Should I Use?

An orifice plate is the simple, compact, economical option: easy to install, easy to inspect, and available in styles for nearly any fluid. Its abrupt restriction, however, causes a permanent pressure loss.

A venturi narrows and widens the flow gradually, so the fluid recovers most of its pressure downstream. That makes venturis the choice for high-velocity fluids, energy-conscious facilities, and systems like water treatment where pressure recovery is crucial, at a higher cost for the element itself. If pressure loss is not a limiting factor, the orifice plate is usually the more economical answer.

How Do You Install an Orifice Plate?

Installation quality directly determines measurement quality. The guidelines:

• Always start with the documentation for your exact plate and installation as requirements vary by style and line size.
• Respect straight-run requirements, which for KPL orifice plates follow DIN EN ISO 5167-2. If your piping layout cannot provide the runs, flow conditioning plates (ZUB-FCZ) or flow straighteners (ZUB-FSB) can substitute for pipe length. Our article on straight run requirements for flow meters explains why this matters.
• Orient the plate correctly. Orifice plates can be installed backwards, and flow direction matters. Verify orientation before bolting up.
• Seal properly, ensuring correct gasket sealing at the flanges.
• Design impulse lines with care: ¼″ to ½″ tubing, kept as short as practical, sloped up for gas service and down for liquid service, in a material compatible with the process fluid.
• Install a manifold valve (3-way for block and equalize, 5-way to add venting) so the transmitter can be serviced without a process shutdown.
• Mount the transmitter close to the taps, accessible for maintenance, and below the tapping points for liquid service.
• Commission properly: leak test, perform a zero calibration, and a span verification before trusting the readings.
• Inspect periodically. Orifice plates can become damaged or blocked in service, and one advantage of the component design is that the plate can be inspected and cleaned without removing any electronics.

How Much Does an Orifice Plate Flow Meter Cost?

Orifice plates are one of the most economical primary elements in flow measurement. A complete component-built system requires the plate, the flanges and taps, a manifold, and a differential pressure transmitter, while an integrated meter like the RCM is a single purchase.

Because KPL plates are custom manufactured to your pipe diameter and flow conditions, pricing depends on line size, materials, and configuration. For current pricing on any configuration, visit the KPL product page or contact our team for a quote matched to your application.

Do Orifice Flow Meters Have Switches?

Yes. If you need an alarm or control action at a specific flow rate, low-flow protection on a pump for example, the integrated RCM is available with optional alarms and output signals including analog, switching, frequency, and HART outputs. In a component-built system, the PAD transmitter provides the signal that your control system can act on.

What Is the Difference Between an Orifice Plate and a Restriction Orifice?

They look similar and the names get mixed up constantly, but they do opposite jobs. A measurement orifice plate is engineered to create a precise, repeatable pressure drop so that flow can be calculated from it. The pressure loss is the necessary side effect, and the measurement is the point.

A restriction orifice is installed specifically to create the pressure drop or limit the flow, and nothing is being measured. The restriction is the point. If what you actually need is to control or limit flow rather than measure it, you are shopping for a different device entirely, and our article on flow restrictors and regulators covers that family in depth.

Orifice Plate Manufacturers

KOBOLD is proud to not only manufacture orifice flow measurement solutions, from custom KPL orifice plates and PAD transmitters to the integrated RCM, but to sell and support them directly, unlike many manufacturers. Every KPL is custom manufactured for the specific pipe diameter and flow conditions it will serve, with proper sizing calculations and flow coefficient determination included. Plenty of companies machine orifice plates; we believe the engineering behind ours, and the support that comes with them, make them one of the best values in the market.

How Do I Choose an Orifice Plate?

The short answer: we would love to do it for you! Orifice plate selection is genuinely an engineering task. The opening style must match your fluid, the bore must be sized for your flow range and pipe, and the materials must stand up to your process. KOBOLD includes the sizing calculations and flow coefficient determination with every KPL, so you are never guessing.

Rather than working through it alone, let us save you the time, for free. Schedule a free engineering consultation, call us at 1-800-998-1020, or email info@koboldusa.com, and our application engineers will size an orifice solution (or tell you honestly if another technology fits better) for your exact process.