Vessel Holdup and Surge Time Calculator
Find the liquid volume and the time available between two levels in a vertical or horizontal vessel, such as normal level to low low trip or high alarm to high high trip, at a given flow.
- V between levels = V(h₂) − V(h₁) = 11.81 − 2.684 = 9.125 m³
- Time = V ÷ Q = 18.25 min
Holdup Time Formulas
Horizontal: V(h) = L × [R² cos⁻¹((R − h)/R) − (R − h)√(2Rh − h²)]
Vertical: V(h) = πR²h
h₁ and h₂ are the two levels measured from the vessel bottom, R the inside radius, L the shell length and Q the flow that empties (or the net flow that fills) the vessel. Dished heads add volume; the result here is conservative for horizontal vessels with heads.
Worked example
In a 2 m × 6 m horizontal separator, the volume between 400 mm and 1200 mm is 9.13 m³, which gives 18.3 min of holdup at 30 m³/h outflow.
Frequently Asked Questions
What is holdup time?
The time a vessel can supply or absorb flow between two levels, usually normal and low level, at the design flow.
What is surge time?
The time to fill between normal and high level when the outlet stops or inflow rises.
How much holdup is needed?
It is set by process design practice and company standards for each service, often several minutes for separators and drums.
Learn more on the blog
Basics of level measurement
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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