Pressure · 2.6

Impulse lines: slopes, sealing, and gas versus liquid installation

26 October 2026 · 9 min read · ISO 2186

In short

An impulse line must carry only one phase: liquid-full for liquids and steam, gas-full for gas.
Liquids and steam: transmitter below the tappings, so gas rises back to the pipe. Gas: transmitter above, so liquid drains back.
Slope lines continuously, typically at least 1 : 12, with no high or low pockets.
Even 1 m of trapped gas in one leg of a liquid DP line gives an error of about 98 mbar. On a 250 mbar orifice range that is 39 %.

Many "bad transmitters" are good transmitters connected through a bad impulse line. The tubing between the tapping and the instrument is part of the measurement. If it holds a bubble, a slug of condensate or a leg of different density, the transmitter faithfully measures the error.

Where this matters

DP flow (article 4.2): the DP range is small, so small head errors are large percentages.
DP level (article 5.1): wet legs must stay full and at a known density.
Steam: condensate must fill both legs equally.
Wet gas: condensate collecting in a leg gives drift that comes and goes with the weather.
Cold climates: water-filled lines freeze without heat tracing.

How it works

Each leg of an impulse line adds its own hydrostatic head:

ΔP_error = (ρ_{H leg} × h_H − ρ_{L leg} × h_L) × g

ρ is the density of what actually fills each leg (kg/m³), h is the vertical height of the leg (m) and g = 9.80665 m/s². If both legs hold the same fluid at the same height and temperature, the heads cancel. Any difference in fill, height or temperature becomes a measurement error.

Installation by fluid

Fluid	Tapping orientation	Transmitter position	Why
Liquid	Side of the pipe, horizontal or up to 45° below	Below the tappings	Gas bubbles rise back to the pipe. Sediment stays out of the tapping.
Gas	Top of the pipe, or up to 45° above	Above the tappings	Condensate drains back to the pipe.
Steam	Side of the pipe	Below, with condensate (seal) pots or tees at equal height	Both legs fill with condensate at equal height. The cold water protects the transmitter.

What the standards and practice say

ISO 2186 covers pressure signal lines between primary elements and transmitters for flow measurement: tapping orientation, slopes, sizes and pots.
Slope lines continuously at not less than about 1 : 12 (some companies use 1 : 10). No horizontal runs, no sag between supports.
Keep the two legs of a DP line together, the same length and route, so they share temperature.
Keep lines short. Close-coupled or direct-mounted transmitters remove most impulse-line problems.
Typical tube is 12 mm (½″) OD, larger for viscous or dirty fluids. Fit vents at high points of liquid lines and drains at low points of gas lines.
Heat-trace and insulate where the fluid can freeze, condense or solidify, but avoid boiling a liquid leg.

Worked example: two impulse-line faults on an orifice meter

Input	Value
Service	Water, orifice DP range 0–250 mbar
Transmitter	1 m below the tappings

Fault 1, gas trapped in the low-pressure leg. A 1 m vertical section of the low leg holds gas instead of water. The low side loses that head: ΔP_error = 1,000 × 9.80665 × 1 = 9,807 Pa = 98 mbar. The transmitter reads 98 mbar too high.

As a share of span, 98 / 250 = 39 %. Because flow ∝ √DP, at a true DP of 100 mbar the meter would read √(198/100) = 1.41 times the true flow, 41 % high.

Fault 2, one leg warmer than the other. Hot water in one leg (958 kg/m³, near 100 °C) and cooler water in the other (1,000 kg/m³), over 0.5 m: (1,000 − 958) × 9.80665 × 0.5 = 206 Pa = 2.1 mbar, which is 0.8 % of span. It is smaller, but it changes with the weather and the process temperature.

Fault	Error	% of 250 mbar span
1 m gas pocket in a liquid leg	98 mbar	39 %
0.5 m of legs at different temperatures	2.1 mbar	0.8 %
Correct installation, legs together, vented	≈ 0	0 %

What this example shows

A single bubble can be larger than the whole span. Vent the liquid legs at commissioning and after maintenance.
Route the legs together. Temperature differences are small but never go away.
Orientation is the cure. With the transmitter below liquid tappings, bubbles leave by themselves.

When it fails: the fixes in order

Vent or drain both legs and re-zero at line pressure (article 2.3).
Correct the orientation: transmitter below for liquid and steam, above for gas.
Remove pockets: re-slope the lines and add vents or drains at unavoidable high or low points.
Shorten: close-couple or direct-mount the transmitter.
Heat-trace or insulate both legs together where needed.

Common mistakes

Gas transmitters mounted below the tappings, so condensate collects in the legs.
Liquid lines with a high point and no vent.
Long horizontal runs that sag between supports.
Legs routed separately, one in the sun and one in the shade.
Steam DP without seal pots or tees at equal height.
Tappings at the bottom of the pipe on dirty liquids, which collect sediment.

Checklist

The orientation suits the fluid: below for liquid and steam, above for gas.
Tappings are at the side for liquids and steam, and on top for gas.
Lines are continuously sloped at ≥ 1 : 12, with no pockets.
Both legs have the same length and route, and are kept together.
Vents are at high points (liquid) and drains at low points (gas).
Steam service has pots or tees at equal height, filled before start-up.
Lines are as short as possible, with close coupling where it can be used.
Heat tracing and insulation cover both legs equally where needed.
The hook-up drawing shows all of this (article 1.6).

Quick reference

Head: ρ g h. Water ≈ 98 mbar per metre.
Liquid and steam: transmitter below. Gas: transmitter above.
Slope ≥ 1 : 12, no pockets, legs together.
DP error to flow error: flow ∝ √DP.