A flow element is the sensing component inside a flow meter that creates a measurable physical change in response to fluid movement. In most industrial applications, this means a device installed inside a pipe that restricts the flow path, creating a pressure difference that can be translated into a flow rate. The four most common types are orifice plates, venturi tubes, flow nozzles, and pitot tubes.
How Flow Elements Work
Most flow elements rely on a simple principle: when fluid passes through a narrowed section of pipe, it speeds up and its pressure drops. By measuring the pressure on both sides of the restriction (upstream and downstream), you can calculate how fast the fluid is moving and how much is passing through.
The relationship between the pressure drop and flow rate follows a square root pattern. If the pressure difference quadruples, the flow rate only doubles. This is why these devices are sometimes called “differential pressure” or “DP” flow elements. A complete measurement system has two parts: the primary element (the physical restriction in the pipe) and the secondary element (the transmitter that reads the pressure difference and converts it to a flow reading).
When properly installed, these meters can achieve accuracy within 1 percent. Under typical field conditions with normal wear and less-than-perfect piping, accuracy tends to land in the 3 to 5 percent range.
Orifice Plates
An orifice plate is the simplest and most widely used flow element. It’s a flat disc with a precisely machined hole in the center, clamped between pipe flanges. Fluid passes through the hole, creating a pressure drop that corresponds to the flow rate. Orifice plates are popular because they’re inexpensive, rugged, and straightforward to install. If they’re built and installed to the international standard (ISO 5167), they don’t require individual calibration.
The trade-offs are real, though. Orifice plates have a limited turndown ratio of about 4:1 to 5:1, meaning they can only accurately measure flow across a relatively narrow range. The sharp edge of the hole erodes over time, especially in wet or dirty fluid streams, which gradually degrades accuracy. They also need significant straight pipe runs for reliable readings: at least 10 pipe diameters of straight, unobstructed pipe upstream and 5 downstream. In tight spaces, that can be a dealbreaker. Orifice plates are commonly used for superheated steam and liquid measurement, particularly with fluids that have lubricating properties.
Venturi Tubes
A venturi tube is a gradually tapered section of pipe that narrows to a throat and then widens again. This smooth, streamlined shape gives it a major advantage over orifice plates: pressure recovery. The total unrecovered pressure loss rarely exceeds 10 percent of the measured pressure difference, which means far less energy is wasted pushing fluid through the system. For high-volume, continuous flow applications, that efficiency adds up.
Venturi tubes have discharge coefficients ranging from 0.9 to nearly 1.0 in turbulent flow, making them inherently efficient. Their upstream pipe requirements are slightly lower than orifice plates (5 pipe diameters at a 0.5 beta ratio compared to 6 for an orifice). The main limitation is that classical venturi tubes are restricted to clean, non-corrosive liquids and gases. They’re also bulkier and more expensive than orifice plates, which makes them a harder sell for applications where space or budget is tight.
Flow Nozzles
A flow nozzle sits between an orifice plate and a venturi tube in both cost and performance. It has a smooth, curved inlet that accelerates the fluid, similar to the front half of a venturi, but without the gradual expansion section on the downstream side. This means it handles higher flow capacity than an orifice plate and costs less than a venturi, but recovers less pressure than a venturi does.
Flow nozzles are dimensionally more stable than orifice plates at high temperatures and high velocities, which is why they’ve long been a go-to for measuring superheated steam at high flow rates. Their discharge coefficients range from 0.96 to 1.2 in turbulent flow, depending on the throat-to-pipe diameter ratio. Like venturi tubes, they aren’t suitable for slurries or dirty fluids.
Pitot Tubes and Averaging Pitot Tubes
A pitot tube works differently from the other three types. Instead of constricting the entire flow path, it’s a probe inserted into the pipe that measures the difference between the total pressure (fluid pushing directly into the probe’s opening) and the static pressure of the surrounding flow. That difference indicates velocity at the probe’s location.
A standard pitot tube only reads velocity at a single point, which limits accuracy if the flow profile isn’t uniform. Averaging pitot tubes solve this by using multiple sensing ports across the pipe’s diameter to sample the flow at several points. Some designs, like the Emerson Annubar, use a T-shaped sensor with a flat upstream face that creates a fixed separation point, increasing the pressure signal and improving performance over a wider flow range. The back side of the sensor includes stagnation zones that reduce signal noise and resist clogging from particles in the fluid.
Averaging pitot tubes need less straight pipe than most other flow elements: roughly 8 diameters upstream and 3 downstream. Some advanced versions combine differential pressure, temperature, and flow readings in a single device, reducing the number of holes that need to be cut into the pipe.
Installation and Straight Pipe Requirements
Every flow element needs a certain length of straight, undisturbed pipe before and after it to produce accurate readings. Bends, valves, and fittings upstream create swirling or uneven flow profiles that throw off measurements. The required distances are typically expressed as multiples of the pipe’s diameter.
- Orifice plates and nozzles: 6 to 10 diameters upstream, 3 to 4 downstream, depending on the ratio of the restriction to the pipe size
- Venturi tubes: 5 to 9 diameters upstream, 2 to 4 downstream
- Averaging pitot tubes: 8 diameters upstream, 3 downstream
These are conservative minimums. Longer upstream runs generally improve accuracy. If you can’t meet the minimum distances, most manufacturers can perform a detailed analysis of your specific piping layout to determine whether acceptable accuracy is still achievable. Some meter types, like Coriolis meters, aren’t affected by upstream piping at all, which is one reason they’re chosen for cramped installations despite their higher cost.
Choosing the Right Flow Element
The best flow element for a given application depends on the fluid, the accuracy you need, how much pressure loss you can tolerate, and the physical space available. Orifice plates cover the widest range of general-purpose applications at the lowest cost. Venturi tubes make sense when you need to minimize energy loss in high-volume systems carrying clean fluids. Flow nozzles are the strongest choice for high-temperature, high-velocity steam. Averaging pitot tubes work well when you need a compact installation with minimal pipe penetrations.
All of these devices are governed by ISO 5167, which was most recently updated in 2022. The standard applies to subsonic, single-phase flow in circular pipes running full. It doesn’t cover pulsating flow or two-phase mixtures like steam with entrained water droplets. Staying within the standard’s guidelines is what allows many of these devices to be used without individual calibration, saving significant time and cost over the life of the installation.