Flow Measurement · Glossary

What is Permanent Pressure Loss?

Symbol: PPLmbar, kPaFlow Measurement

Permanent pressure loss is the part of the DP across a flow element that is not recovered downstream and costs pumping energy.

Permanent Pressure Loss Explained

Pressure falls through a restriction and partly recovers further downstream. The unrecovered part is lost as heat and turbulence, and must be supplied by the pump or compressor.

Orifice plates lose most of their DP. Venturis recover most of it through their diverging section, which is why they are chosen for large lines and low pressure systems.

Permanent Pressure Loss Formula

Δϖ ≈ (1 − β^1.9) × Δp

Δϖ	permanent pressure loss for an orifice plate
β	beta ratio
Δp	measured differential pressure

Example: At β = 0.6, Δϖ ≈ (1 − 0.379) × Δp ≈ 62 % of the DP.

Permanent Pressure Loss: Typical Values

Orifice plate	about 40 to 95 % of DP depending on β
Classical venturi	about 5 to 20 % of DP (ISO 5167-4)

Calculators for Permanent Pressure Loss

CalculatorOrifice Pressure Loss CalculatorFree online calculator with formula and worked example.CalculatorPump Power CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Permanent Pressure Loss?

Permanent pressure loss is the part of the DP across a flow element that is not recovered downstream and costs pumping energy.

Why does pressure loss matter?

It costs energy continuously. On large water or gas lines the yearly energy cost of an orifice can exceed the price of a low loss meter.

Which meters have almost no loss?

Full bore meters such as magnetic and ultrasonic meters.

Learn more on the blog

Mistakes in DP flow measurement

A practical guide on instrumentationblog.in with more detail on permanent pressure loss and how it is applied on real plants.

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More Flow Measurement Terms

Flow termSquare Root ExtractionSquare root extraction converts a DP signal into a linear flow signal, because flow through a DP element…Flow termTurndown RatioTurndown ratio is the maximum flow divided by the minimum flow a meter can measure within its stated accuracy.Flow termLow Flow CutoffLow flow cutoff forces the flow reading to zero below a set value, so noise and zero drift do not show as…Flow termK FactorK factor is the number of pulses a flow meter sends for each unit volume of fluid passing through it.Flow termMeter FactorMeter factor is the ratio of the true volume from a prover to the volume indicated by the meter during…Flow termFlow TotalizerA flow totalizer adds up flow rate over time to give the total volume or mass that has passed.

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