Vacuum Pump Down and Vessel Sizing

Required asset: Pump down calculation case

R01 decision: Distinguish volume evacuation gas load and effective pumping speed.

R01 depth: Vessel volume; effective pumping speed; pressure-dependent speed; gas load; initial state.

R01 worked case: Contrast ideal exponential pump-down with a loaded vessel.

R01 release gate: Label ideal assumptions and require curve-based analysis for selection.

1 Direct answer

Vacuum pump-down time is not determined by vessel volume divided by nominal pump capacity. Define the complete connected volume, initial and target absolute pressures, pressure-dependent pump speed, inlet conductance, valve sequence, leakage, outgassing, vapour release, temperature and measurement response. Use effective pumping speed at the vessel and integrate across pressure.

It creates no universal evacuation time, gas-load allowance, speed margin, conductance, ultimate pressure or acceptance criterion.

2 Controlled scope

It separates removable stored gas from continuing gas throughput and distinguishes pump-port speed from vessel-side effective speed.

Pressure-vessel integrity, process chemistry, hazardous-area compliance, pump compatibility, control-system safety and final supplier selection remain with competent disciplines.

4 Stored gas versus continuing load

Volume evacuation removes the gas initially contained in the isolated system. Leakage, outgassing, permeation, vapour generation, purge and process flow continue to add gas during evacuation.

The ideal exponential equation treats only stored gas with constant effective speed. A loaded-vessel model adds continuing throughput and can establish an equilibrium pressure that cannot be crossed.

5 System boundary

Define main vessel, jackets or cavities connected to vacuum, piping, manifolds, filters, traps, valves, instruments, hoses, tooling, receivers, pump inlet, pump and exhaust.

Exclude volumes that remain isolated and include volumes opened during the sequence. Draw each valve state and measurement location.

6 Operating states

7 Pressure definitions

Use absolute pressure in pump-down equations and curves. State local ambient separately. Gauge pressure near atmosphere and vacuum magnitude are not substitutes for absolute pressure.

Record vessel, manifold and pump-inlet pressure where conductance matters. State gauge technology, range, gas correction, accuracy, response and calibration.

8 Volume inventory

Measure or calculate vessel, nozzle, pipe, filter, trap, manifold, instrument, hose and tooling volumes. Record drawings, dimensions, fill states and uncertainty.

A nominal vessel nameplate volume may exclude connected space or internal displacement. Maintain a controlled volume register.

9 Initial state

State initial absolute pressure, gas composition, temperature, humidity, vapour saturation and whether surfaces or products were recently exposed to atmosphere.

Starting at 1000 mbar is not universal. Initial conditions affect stored gas mass and vapour release.

10 Target state

Define target pressure, measurement location, allowed time, stability period and process purpose. Separate a pump-inlet target from a vessel target.

Confirm whether the target is below the equilibrium pressure created by continuing gas load. A mathematical time is meaningless if the target is unreachable.

11 Pump-port speed

Use the exact pump model curve versus inlet pressure, speed, frequency, gas and test condition. Record tolerance and ultimate-pressure context.

A nominal or maximum pumping speed is not constant through the pressure range. It cannot be applied unchanged to the full evacuation.

12 Effective pumping speed

The speed available at the vessel is reduced by inlet conductance and restrictions. In a simplified applicable regime, reciprocal effective speed equals reciprocal pump speed plus reciprocal conductance.

Conductance changes with geometry, gas, temperature and pressure regime. Use the correct model or measurement.

13 Valves filters and traps

Valves, filters, traps, separators and long small-bore lines can dominate effective speed. Compare clean and service states.

Opening sequence changes connected volume and gas load. Include valve travel and bypass paths in the pressure trace.

14 Ideal exponential equation

For an isothermal ideal gas, fixed volume, constant effective speed, no continuing gas load and a negligible ultimate-pressure term, pressure decays exponentially. Time equals volume divided by speed multiplied by the natural logarithm of initial pressure divided by final pressure.

Every assumption must be labelled. The equation is a screening tool rather than an equipment-selection proof.

15 Loaded-vessel equation

With constant throughput and constant effective speed, equilibrium pressure equals throughput divided by speed in consistent units. Pressure approaches this equilibrium exponentially.

Time to a target above equilibrium equals volume divided by speed multiplied by the natural logarithm of initial pressure minus equilibrium divided by target pressure minus equilibrium.

16 Pressure-dependent speed

Real pumping speed varies with pressure. Divide the curve into bands or numerically integrate volume divided by effective speed multiplied by differential logarithmic pressure.

Use enough points to capture speed changes, inlet restrictions and control transitions. Preserve the exact curve revision.

17 Ultimate pressure

Ultimate pressure is a limiting result under documented test conditions. It is not the installed base pressure or the target automatically achievable under load.

The target must remain above the combined equilibrium from leakage, outgassing, vapour and process throughput with suitable verified performance.

18 Leakage

Real leaks admit external gas through openings. Quantify them by an approved pressure-rise, tracer-gas or other method with defined boundary and instrument behaviour.

Leakage can be pressure dependent. Distinguish actual leak flow from virtual leaks and desorption.

19 Outgassing and desorption

Surfaces, seals, products and lubricants release gas after pressure falls. Load depends on material, area, temperature, cleanliness and exposure history.

Outgassing can dominate the later pressure trace. A larger pump may give limited improvement until the source is addressed.

20 Permeation

Gas can permeate through elastomers, polymers and membranes without a discrete hole. Record materials, area, thickness, gas and temperature.

Permeation becomes material in low-pressure holding and long cycles. Do not treat it as an ordinary repairable leak without evidence.

21 Vapour and condensation

Liquids and wet products can flash or evaporate as pressure falls. Cooling from evaporation can change vapour pressure and release rate.

Use thermodynamic and process data. A dry-gas pump curve alone does not predict wet-vessel pump-down.

22 Deliberate process gas

Purge, seal gas, analyser flow, instrument bleed or process dosing creates continuous throughput. Record normal, start-up and failure states.

Convert mass, molar or reference flow to throughput using stated conditions. Preserve units.

23 Temperature effects

An isothermal screen assumes uniform constant temperature. Actual gas and surfaces can cool or warm during evacuation, changing pressure independently of gas removal.

Place temperature measurement where it supports interpretation. Use specialist modelling for strong thermal transients.

24 Gas composition

Molecular weight, specific heat, viscosity, vapour behaviour and gauge response vary by gas. Pump speed and conductance can also change.

Use the process mixture and exact supplier restrictions. Air data do not automatically transfer.

25 Pump and process compatibility

Check particles, vapours, corrosives, condensables, oxygen concentration and hazardous constituents against the pump, oil, seals, filters and exhaust system.

A pump that meets time and pressure calculations can still be unsuitable for the medium.

27 Control sequence

Document valve commands, pump starts, speed changes, purge, isolation and process admission with timestamps and permissives.

Control delays belong in the total cycle time even when they are outside the thermodynamic evacuation calculation.

28 Measurement methodology

Log vessel and pump-inlet pressure, temperature, valve state, pump speed, current or power and relevant flow at a sampling rate suited to the fastest event.

Use calibrated instruments that cover the pressure bands. One gauge may not provide adequate accuracy over the full range.

29 Pressure-time trace

Plot absolute pressure on linear and logarithmic scales. An ideal constant-speed region appears as a straight line on a log-pressure plot.

Changes in slope can indicate curve transitions, conductance, gas release, leaks, valve events or gauge limits. Diagnose before resizing.

30 Pressure-rise test

After reaching a defined condition, isolate the pump and record pressure rise with temperature and time. The apparent gas load is volume multiplied by pressure-rise rate when assumptions are valid.

Separate instrument drift, thermal rebound and vapour release. State the test window and volume.

31 Conductance test

Compare simultaneous vessel and pump-inlet pressures with a known or estimated throughput. A large difference indicates restriction.

Test clean and service conditions where filters or condensate can load. Avoid assigning all slow pump-down to the pump.

32 Uncertainty and sensitivity

Record uncertainty in volume, speed, conductance, gas load, gauge accuracy and initial state. Test sensitivity to material inputs rather than adding an unexplained margin.

Use conditional release when the result depends on an unverified assumption.

33 Diagnostic method

34 Selection sequence

Create the volume and initial-state register. Define targets and time. Obtain the exact pump curve, calculate conductance and effective speed, then model stored gas and continuing loads.

Use ideal screening for understanding, followed by curve-based loaded analysis and an installed test. Hold selection when load or curve data are missing.

35 Commissioning and acceptance

Verify model, rotation, speed, inlet path, filters, valves, instruments, controls, exhaust and compatibility. Confirm the controlled initial state.

Run repeated pressure-time tests with representative products and operating states. Compare measured band times, target hold and pressure-rise results with the approved model. Record deviations and disposition.

36 Fictional volume register

37 Fictional pressure and pump basis

38 Worked case A ideal exponential pump-down

Assumptions. Connected volume is 5.00 m3. Initial pressure is 1000 mbar absolute, target is 10 mbar and constant effective speed is 180 m3/h or 0.050 m3/s. Gas load, ultimate pressure and temperature change are neglected.

Method. Time equals 5 divided by 0.050 multiplied by the natural logarithm of 1000 divided by 10. The result is approximately 460.5 s or 7.68 min.

Result. This is an ideal screen. Its assumptions are explicitly unsuitable for final pump selection.

39 Worked case B loaded vessel

Assumptions. The same vessel and speed have a constant continuing load of 400 mbar L/s. Equilibrium pressure is 400 divided by 50, equal to 8 mbar.

Method. Time to 10 mbar equals 5 divided by 0.050 multiplied by the natural logarithm of 1000 minus 8 divided by 10 minus 8. The result is approximately 620.7 s or 10.35 min.

Result. The continuing load adds about 160 s compared with the ideal result. A target of 5 mbar is unreachable in this constant-speed model because it is below the 8 mbar equilibrium.

40 Worked case C conductance and effective speed

Assumptions. Fictional pump-port speed is 250 m3/h, equal to 69.44 L/s. Inlet conductance is 150 L/s in the assessed regime.

Method. Effective speed is the reciprocal of one divided by 69.44 plus one divided by 150, giving approximately 47.47 L/s or 170.9 m3/h.

Result. Applying the 250 m3/h port value directly would understate pump-down time. The valid conductance model must match the pressure regime.

41 Worked case D curve-band ideal integration

The fictional exact-model effective speeds are used without continuing gas load.

Result. The curve-based ideal result differs from the constant-speed screen before any continuing gas load is added. Selection must use the exact curve and load model.

42 Worked case E low-pressure load check

At the final fictional band, effective speed is 120 m3/h or 33.33 L/s. A 400 mbar L/s continuing load creates an equilibrium of 12 mbar.

Result. The 10 mbar target is unreachable in that banded loaded model. Increasing roughing speed at high pressure would not solve the low-pressure gas-load limit.

43 Required asset Pump Down Calculation Case

Complete for every product, initial state and valve sequence. Mark Confirmed, Measured, Calculated, Estimated, Assumed, Unknown or Not applicable.

Case A boundary and volume

Case B pressure and initial state

Case C pump curve and conductance

Case D continuing gas loads

Case E calculation and test

Case F release disposition

44 Filled fictional case extract

45 FAQs

Is pump-down time volume divided by pump capacity

No. Pressure changes exponentially and pump speed, conductance and gas load vary with pressure.

Can nominal pump speed be used for every pressure

No. Use the exact pressure-dependent curve and vessel-side effective speed.

Why can a target be unreachable

Continuing throughput creates an equilibrium pressure equal to gas load divided by effective speed.

Does a larger roughing pump solve outgassing

Not necessarily. Low-pressure gas load, conductance or process source can still govern.

Is an ideal calculation useful

Yes, as a labelled screen and diagnostic reference. It is not final selection evidence.

What proves acceptance

A curve-based loaded analysis and repeatable installed pressure-time test under controlled initial conditions.