Gas Flow Pressure and Temperature Compensation
Correct a DP flow meter reading for gas when the line pressure, temperature or compressibility differ from the values used to size the meter.
- F = √[(Pact ÷ Pdes) × (Tdes ÷ Tact) × (Zdes ÷ Zact)] = 0.8904
- Absolute values: T in K, P absolute
- Qcomp = 1,000 × 0.8904 = 890.38
Compensation Formula
Actual volume flow: factor = √[(Pdes ÷ Pact) × (Tact ÷ Tdes) × (Zact ÷ Zdes)]
For a DP meter qm ∝ √(ΔP × ρ) and gas density ρ ∝ P ÷ (Z × T). The reading assumes design density, so the square root of the density ratio corrects it. Use absolute pressure and temperature in kelvin. This corrects density only; it does not change the discharge coefficient or expansibility.
Worked example
A gas meter designed for 6 bar a and 20 °C reads 1000 Nm³/h while the line runs at 5 bar a and 35 °C: compensated flow = 1000 × √(5/6 × 293.15/308.15) = 890 Nm³/h. Without compensation it reads 12.3 % high.
Frequently Asked Questions
Why do gas DP flow meters need compensation?
Gas density changes with pressure and temperature. The meter equation uses a fixed design density, so the reading drifts when conditions change.
Do liquids need pressure compensation?
Rarely. Liquid density hardly changes with pressure; temperature compensation is used only where density changes noticeably, such as hydrocarbons.
Why use absolute pressure and temperature?
Gas density is proportional to absolute pressure and inversely to absolute temperature. Gauge pressure or °C would give wrong ratios.
Learn more on the blog
What factors affect the stability of flow meter readings?
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article