Pressure Measurement · Glossary

What is Blowdown?

% of set pressurePressure Measurement

Blowdown is the difference between the set pressure of a relief valve and the pressure at which it closes again.

Blowdown Explained

After lifting, a relief valve stays open until pressure falls below set pressure by the blowdown. Too little blowdown causes chattering; too much wastes fluid and lowers system pressure.

Blowdown is adjusted with rings on many spring loaded valves and is usually stated as a percentage of set pressure.

Blowdown Formula

Blowdown % = (P_set − P_reseat) ÷ P_set × 100

P_set	set pressure
P_reseat	reseating pressure

Example: Set 10 bar g, reseat 9.3 bar g: blowdown = 7 %.

Calculators for Blowdown

CalculatorRelief Valve Sizing Calculator for GasFree online calculator with formula and worked example.

Free Downloads

Free downloadRelief Valve Test Record Template in ExcelFree download in Excel and PDF.

Frequently Asked Questions

What is Blowdown?

Blowdown is the difference between the set pressure of a relief valve and the pressure at which it closes again.

What is relief valve chatter?

Rapid opening and closing, often from oversizing, high inlet pressure loss or too little blowdown. It damages seats and guides.

Is blowdown the same as a boiler blowdown?

No. Boiler blowdown removes water to control dissolved solids; relief valve blowdown is a pressure difference.

Learn more on the blog

Instrumentation and control standards

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

Read Full Article

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