How to Size a Steam Flow Meter

Calculation, Examples, and a Free Sizing Chart for STLU-G Vortex Flow Meters

Quick Answer to Steam Flow Meter Sizing

You need four numbers to size a steam flow meter: steam pressure, steam temperature, pipe size, and flow range (min, normal, max). The meter size is not always the same as the pipe size. For low flow applications, the meter often goes one or two sizes down so the steam velocity stays inside the sensor working range. For the STLU-G vortex flow meter, this range is 2 to 76 m/s, and the extended range goes down to 1.98 m/s. This article walks through the math, gives two real examples, and ends with a complete saturated-steam mass flow table for the STLU-G series.

Why Steam Sizing Is Not Water Sizing

Water is easy. Its density stays close to 1000 kg/m³ no matter what the pressure does. Steam is the opposite. Compress it and the density jumps. Heat it and the density drops.

Take saturated steam at 0.4 MPa(g). Its density is about 1.39 kg/m³. Raise the pressure to 1.0 MPa(g) and the density climbs to about 2.95 kg/m³. Same volumetric flow on paper, but the actual mass flow more than doubles. This is the trap most first-time buyers fall into.

So when someone tells me "I have a DN100 pipe, send me a DN100 vortex flow meter", I always ask for the pressure and the flow rate first. Nine times out of ten, the pipe size answer is wrong.

The Four Numbers You Need to Collect

Before you touch a calculator, get these:

 No data on flow rate? Use the boiler rating. Take 80% of the rated capacity as your normal flow and 110% as your maximum. It is not perfect, but it gets you close enough to start.

The Calculation, Step by Step

Step 1. Find the density.

Saturated steam: density depends on pressure only. Look it up in a saturated steam table.

Superheated steam: you need both pressure and temperature. Use a superheated table or the IAPWS-IF97 formula.

Click here for saturated steam and superheated steam density information: Steam density table download

Step 2. Convert mass flow to volumetric flow.

Qv = Qm / ρ

where Qv is in m³/h, Qm is in kg/h, and ρ is the steam density in kg/m³.

Step 3. Calculate the velocity.

v = Qv / (3600 × A)

A is the cross-sectional area of the pipe in m², so A = π × (D/2)² with D as the inner diameter in meters.

Step 4. Check against the working range.

 Below 2 m/s with an STLU-G vortex meter? Drop the meter one size or check whether the extended range (down to 1.98 m/s) is acceptable for your accuracy requirement. Above 76 m/s? Go up a size or accept the higher pressure loss.

Reference: STLU-G Series Volumetric Flow Range for Gas and Steam

STLU-G series Vortex Flow meter

The table below lists the volumetric flow range of the STLU-G series vortex flow meter from silverinstruments.com. We will use these values as the reference for the sizing examples that follow, and as the base for the saturated-steam mass flow chart at the end of this article.

 Note: "Standard" range covers velocity 2 to 76 m/s with full accuracy. "Extended" range covers velocity down to 1.98 m/s with relaxed accuracy. Insertion-type meters are available for DN200 and above.

Example 1. Saturated Steam, DN100 Pipe

Conditions:

• Saturated steam
• 0.6 MPa(g)
• DN100 pipe, inner diameter ≈ 0.1 m
• Normal flow 800 kg/h, max 1200 kg/h

Density. At 0.6 MPa(g), ρ ≈ 1.916 kg/m³ (from the saturated steam table).

Volumetric flow.

Qv (normal) = 800 / 1.916 ≈ 417.6 m³/h

Qv (max) = 1200 / 1.916 ≈ 626.3 m³/h

Velocity.

A = π × (0.05)² = 0.00785 m²

v (normal) = 417.6 / (3600 × 0.00785) ≈ 14.8 m/s

v (max) = 626.3 / (3600 × 0.00785) ≈ 22.2 m/s

Both numbers sit comfortably inside the STLU-G standard range of 2 to 76 m/s. Cross-check against the mass flow chart at the end of this article: at 0.6 MPa(g), a DN100 STLU-G covers 153 kg/h to 3065 kg/h. The 800 kg/h normal flow and 1200 kg/h max flow both fall inside this window. A DN100 vortex flow meter matches the pipe directly. No reducer needed.

Example 2. Same Pipe Idea, but the Flow Is Too Low

Conditions:

• Saturated steam
• 0.4 MPa(g)
• DN150 pipe, inner diameter ≈ 0.15 m
• Normal flow 200 kg/h

Density. At 0.4 MPa(g), ρ ≈ 1.394 kg/m³.

Volumetric flow.

Qv = 200 / 1.394 ≈ 143.5 m³/h

Velocity in a DN150 meter.

A = π × (0.075)² = 0.01767 m²

v = 143.5 / (3600 × 0.01767) ≈ 2.26 m/s

2.26 m/s is just barely above the STLU-G standard minimum of 2 m/s. Checking the mass flow chart at the end of this article, a DN150 STLU-G at 0.4 MPa(g) starts working at 265 kg/h. Our 200 kg/h is below the standard minimum for DN150. The meter would still read, but only inside the extended range with reduced accuracy.

The fix. Size down to DN80.

A (DN80) = π × (0.04)² = 0.005027 m²

v = 143.5 / (3600 × 0.005027) ≈ 7.9 m/s ✓

A DN80 STLU-G at 0.4 MPa(g) covers 77 kg/h to 1533 kg/h, so 200 kg/h sits well inside the standard range. Install with DN150 to DN80 reducers, keep 20D of straight pipe upstream and 5D downstream, and the reading will be stable and accurate.

We had a brewery customer in Indonesia last year with almost this exact case. They had bought a DN150 vortex meter from another supplier, watched it read zero for two weeks, and only then sent us the numbers. We shipped a DN80 unit with reducers and the reading stabilized within an hour of restart.

Free Sizing Chart: Saturated Steam Mass Flow Range (kg/h) for STLU-G

The table below shows the saturated-steam mass flow range (kg/h) that the STLU-G vortex flow meter can measure at common gauge pressures. Values are calculated from the STLU-G standard volumetric range and the saturated steam density at each pressure. Use it as a quick selection reference for boiler outlet, steam header, and distribution-line applications.

Boilers needs steam flow meter

Pressure range: 0.1 to 0.8 MPa(g)                                                                         Flow unit: kg/h

Pressure range: 0.9 to 1.6 MPa(g)                                                                         Flow unit: kg/h

 Notes:

1. All pressures are gauge pressure (MPa(g)). Add 0.1013 MPa to get absolute pressure.
2. Mass flow values are calculated as: volumetric flow range × saturated steam density at the given pressure.
3. DN15 to DN40 values at low pressure correspond to low steam velocity. For best accuracy, choose a DN size where your normal flow sits in the middle of the listed range.
4. For superheated steam or pressures outside this table, send us your operating conditions and we will calculate a custom range for you.

Five Mistakes We See Every Week

1. Sizing by pipe size alone. Calculate the velocity first. A DN100 pipe does not automatically need a DN100 meter.
2. Sizing to the maximum flow only. Most plants run at 50 to 70% load most of the time. If the minimum flow puts you below 2 m/s, the meter is dead weight half the day. Size to the minimum.
3. Mixing up gauge and absolute pressure. Saturated steam tables work in absolute pressure. Add 0.1013 MPa to your gauge reading before you look anything up. Forgetting this is the single most common mistake we see in customer specs.
4. Ignoring turndown. The STLU-G typically gives you 8:1 to 20:1, with 30:1 available on request. If your flow swings from 100 kg/h to 5000 kg/h on the same line, no single vortex meter covers that. Two meters in parallel, or a different technology, is the answer.
5. Skipping the straight pipe rule. A perfectly sized meter installed right after an elbow will still read wrong. 20D upstream and 5D downstream is the minimum for the STLU-G. Less than that, the error grows fast.

Need Help Sizing Yours?

Vortex flow meter for steam

Send us these four lines and we will reply with a sizing report and a quotation within 12 hours:

1. Steam pressure (MPa or bar, gauge or absolute, please mark which)
2. Steam temperature (°C), and saturated or superheated
3. Pipe size (DN or inch)
4. Flow range: min, normal, max (kg/h or t/h)

If you also tell us the signal output you need (4-20 mA, RS485 Modbus, HART, or pulse) and the power supply (24 VDC or 220 VAC), we will give you a full STLU-G model number on the quote, ready to order.