Calculate gas purge time, purity and fill volume for pipelines from line length, diameter, pressure, flow rate and temperature inputs.

Estimate flow-through time from pipe volume and actual flow.

Ideal-gas estimates only. Purging can create fire, explosion and oxygen-deficiency hazards. Trained personnel must follow the applicable procedure and verify the atmosphere; these results cannot certify safe conditions.

Volume exchanges

Blank uses 4 exchanges as an estimate, not a safety criterion.

Gas Purge Formula

For well-mixed pressure-cycle purging with pure purge gas, the following relation estimates the complete cycles needed to reach a target impurity. Use absolute pressures.

GP = ceil(ln(Ct / C0) / ln(Pa / Ph))

Variables:

  • GP is the number of complete pressurize-and-vent cycles; use zero if the initial impurity already meets the target. C0 and Ct are the initial and target impurity fractions.
  • Pa is the low absolute vent pressure, in the same units as Ph.
  • Ph is the achieved high absolute pressure; Ph must exceed Pa.

For nonzero initial impurity and a lower positive target impurity, divide ln(Ct/C0) by ln(Pa/Ph) and round up. The special case โˆ’4/log10(Pa/Ph) applies only to a 10,000-fold impurity reduction. Exactly 100% purity cannot be reached in finite ideal cycles from nonzero impurity. The time mode uses t = N V / Q with actual flow at line conditions. The fill mode reports total final gas inventory at the stated reference conditions, not added gas.

How to Calculate Gas Purge?

The following steps outline how to calculate the Gas Purge.

  1. First, determine the low absolute vent pressure Pa.
  2. Next, determine the achieved high absolute pressure Ph, greater than Pa.
  3. Determine initial impurity C0 and target impurity Ct = 1 โˆ’ target purity, then use GP = ceil(ln(Ct/C0)/ln(Pa/Ph)).
  4. Calculate complete pressure cycles. The model assumes pure purge gas, complete mixing, constant temperature and venting to Pa after each cycle.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Vent absolute pressure Pa = 20 psia.

High absolute pressure Ph = 40 psia; initial impurity = 100%; target purity = 99.99%. For the calculator, use line length 100 ft, outer diameter 2.5 in and wall thickness 0.25 in.

GP = ceil(ln(0.0001/1)/ln(20/40)) = 14 complete cycles. Estimated final purity is 99.99389648%. This estimate cannot certify a safe atmosphere; trained personnel must follow the applicable procedure and verify actual conditions.