Orifice Type Flow Transmitter: How It Connects to Your Orifice Plate

Orifice Flow Transmitter

The orifice flow transmitter senses the pressure loss due to the orifice plate and gives out an electrical signal that is readable in your control room. Simple! The connections are typically where problems occur.

This guide details how to install the orifice flow transmitter in a step-by-step manner. It is designed for buyers and plant engineers or EPC contractors for EHS who wish to install the transmitter correctly.

What Is an Orifice Type Flow Transmitter?

An orifice type flow transmitter is a differential pressure (DP) transmitter paired with an orifice plate. The plate creates the pressure drop, and the transmitter measures it and sends out a flow signal.

A typical DP flow system has three parts, as Aivon’s installation guide explains:

• Primary element: the orifice plate sitting in the pipe.
• Impulse tubing: small tubes that carry pressure from the pipe taps.
• DP transmitter: the device that reads the difference and gives you an output.

Think of the plate as the one who squeezes and the transmitter as the one who reads the squeeze. If they disagree, your flow number is just a polite guess.

Flow follows the square root of the measured pressure difference, according to Pumps & Systems. The transmitter or a flow computer does that math for you.

How Does the Orifice Meter Working Principle Fit In?

The orifice meter working principle is simple: a restriction speeds up the fluid and drops the pressure. The transmitter reads that drop.

1. Fluid reaches the orifice restriction and speeds up.
2. Pressure falls just after the plate.
3. The upstream tap feeds the high side of the transmitter, and the downstream tap feeds the low side, as Emerson’s manual describes.
4. The orifice type flow transmitter reads the difference and converts it into an output signal, such as 4 to 20 mA.

When flow through an orifice plate doubles, the pressure difference becomes four times bigger, says Automation.com. That is why range setting matters so much.

How Do You Connect an Orifice Type Flow Transmitter to the Orifice Plate?

Run the upstream tap to the transmitter’s high side and the downstream tap to its low side, usually through a manifold. An orifice type flow transmitter can sit on remote impulse lines or mount directly on a compact assembly.

Manufacturer datasheets, such as ABB’s compact orifice flowmeter sheet, note that an integral manifold and transmitter remove the need for separate impulse piping. Some people call these compact units an orifice flow sensor, though the plate and transmitter still do separate jobs.

What Does the Manifold Do?

• Manifold allows for isolation and zapping of the transmitter without closing off the line.
• The most common configuration is that of a 3-valve manifold, although ABB provides a 5-valve manifold.
•  Two block valves isolate the transmitter from the process.
• One equalizing valve balances both sides for zero checks.
• Aivon’s guide adds a key rule: never let condensate or fill fluid drain out of the impulse tubing.

How Should You Lay Impulse Lines for Gas, Liquid, and Steam?

Keep each impulse line full of only one thing. Gas lines hold gas, while liquid and steam lines hold liquid. Endress+Hauser says the same in its Deltatop guide.

• Gas: Put taps on the upper half of the pipe and the transmitter above the plate.
• Liquids: Mount the taps in the bottom half of the vessel and the transmitter below the plate. If this is not possible, provide a vent at the top of the vessel.
• Steam: Mount the taps along the horizontal centerline and provide a condensate pot near the taps. The transmitter should be mounted below the condensate pots.
•  Tubing size: API RP 551 suggests a minimum tube size of ½-inch OD tubing, as stated in Processing Magazine.
• Steam flushing: do not blow down impulse lines through the transmitter, warns Emerson.

What Mistakes Cause Wrong Readings?

Most wrong readings on an orifice type flow transmitter come from the impulse lines, not the transmitter itself. Watch for these:

Incorrect readings on an orifice type flow transmitter are usually a result of impulse line problems rather than the transmitter itself. Consider the following:

• Gas bubble in the liquid line: This causes an imbalance in the pressure reading from one side; hence, the transmitter registers a signal despite the unchanging process pressure differential (ICS Schneider).

• Asymmetric liquid heads: Keep the liquid heads equal on both sides, advises Emerson.

• Frosting/freezing: In cold outdoor environments, freezing can cause a rupture in the transmitter diaphragm; therefore, plants employ steam or electric heating (Process & Instrumentation).

• Plate edge wear: Even minor wear can affect accuracy (Automation.com).

• Old parameters: When there is any change in operating conditions, Siemens recommends recalculating and programming the transmitter with new parameters.

Does the Orifice Plate Type Change the Transmitter Setup?

Yes. The orifice plate type and tapping style change the pressure signal, so the orifice type flow transmitter range must match the sizing sheet.

• ISO 5167-2 covers flange taps, corner taps, and D and D/2 taps.
• Endress+Hauser notes that annular chamber tapping gives slightly more accurate DP values than corner or flange tapping.
• A small bore gives a strong signal but more pressure loss and edge wear, says Fabrico.

Whichever types of orifice flow meter you shortlist, ask the supplier to share the sizing sheet that sets the transmitter range.

What Should EHS and EPC Teams Check Before Ordering an Orifice Type Flow Transmitter?

Check range, mounting style, ratings, and paperwork before you compare prices. A cheap transmitter on the wrong range is still the wrong transmitter.

• The orifice-type flow transmitter range matches the sizing sheet
• Remote or compact mount decided early
• Pressure and temperature ratings for the manifold, tubing, and fittings
• Material compatibility with the process fluid
• Datasheet and calibration certificate with tags that match the P&ID
• Hazardous area approvals, if your plant needs them
• Heat tracing plan for cold-climate sites
• Turndown: a single transmitter on an orifice plate usually gives about 3:1, though E2G says parallel transmitters can widen it

Optional read: if leak safety is also on your plate, see this guide on how to choose an infrared gas detector for methane, propane, and LPG leaks.

FAQs

1. What is a flow transmitter using an orifice plate type?
It is a pressure transmitter, which measures the pressure difference across the orifice plate and transmits the flow.
2. How does an orifice type flow transmitter connect to the orifice plate?
Through impulse lines or a direct mount, with the upstream tap on the high side and the downstream tap on the low side.

3. Where should the transmitter sit on a steam line?
Below the condensate chambers, with condensate pots close to the plate taps.

4. How is the orifice meter different from the venturi meter?
The orifice meter is cheaper but uses more pressure than the venturi meter, which uses little pressure and is expensive.
5. How accurate is orifice meter flow measurement?
About 1% to 5% for a basic setup, and 0.5% to 1.5% with compensation, according to Pumps & Systems.

6. Why does my orifice type flow transmitter read something at zero flow?
Usually a trapped gas bubble or unequal liquid in the impulse lines, so vent them and re-zero.

Ready to Match Your Setup?

Share your pipe size, fluid, pressure, and temperature with the team at pcd-flowmeter.com to pair the right orifice type flow transmitter with your plate.