Dry Claw Vacuum Pumps

Required asset: Dry Claw Selection Matrix

R01 decision: Assess contact free pumping characteristics process limits and support needs.

R01 release gate: Separate dry internal operation from universal contamination tolerance.

1 Direct answer

A dry claw vacuum pump should be assessed from its documented pumping-speed curve at the actual inlet pressure and gas condition, then checked against the full process stream, vapour and particulate exposure, temperature, discharge backpressure, cooling, purge, noise, controls and maintenance access. Contact-free, non-lubricated compression in the pumping chamber can reduce process-side lubricant contact, but it does not make every dry claw pump tolerant of liquids, condensable vapours, sticky deposits, abrasive dust, corrosive gas or polymerising material.

It does not create a universal ultimate pressure, capacity, vapour tolerance, dust loading, purge rate, discharge-temperature limit, service interval or noise value.

2 Controlled scope

It covers the inlet gas state, required pumping speed, documented performance curve, process exposure, purge and cleaning needs, cooling, discharge, utilities, controls, noise and maintainability.

Detailed rotor design, clearances, materials, coating selection, hazardous-area compliance, process safety, relief, emissions and final equipment approval remain with the exact manufacturer and competent project disciplines.

4 Contact-free pumping principle

Dry claw machines use synchronised rotors intended to operate without contact in the pumping chamber under documented conditions. Timing gears, bearings and other components can still require lubrication outside the gas path. The exact sealing arrangement determines possible migration paths.

The operating concept can offer clean compression for suitable duties, but clearances and temperature make the machine sensitive to deposits, distortion and foreign material. Contact-free operation is a design condition, not permission to ingest any contaminant.

5 System boundary

Define the process connection, isolation valve, inlet separator or filter, pump, purge, cooling system, silencer, discharge pipe, treatment, drain and emissions destination. Identify pressure and temperature measurement points.

Include check valves, bypasses, relief devices, standby pumps, controls and utility interfaces. A bare-pump selection cannot close an enquiry governed by upstream liquid carryover or downstream backpressure.

6 Operating states

7 Vacuum pressure definitions

Use absolute pressure for pumping-curve and gas-state work. Record gauge pressure only as a secondary operational value. Identify local atmospheric pressure when converting a negative gauge reading.

State suction pressure at the selected pump inlet reference point and discharge pressure at the documented outlet point. Vessel pressure and pump-inlet pressure can differ because of piping, valves and filters.

8 Pumping-speed curve

Use the exact model and configuration curve for the handled gas or an approved correction. Read pumping speed at the actual inlet absolute pressure, not at ultimate pressure or a headline nominal rating.

Check the full operating range, including start-up and upset conditions. Identify curve tolerances, test standard, frequency, motor rating, speed, cooling condition and discharge backpressure where supplied.

9 Actual inlet flow

Volumetric flow expands as absolute pressure falls and temperature rises. Convert a mass or reference flow to actual inlet volume using an appropriate gas model and stated composition. For an idealised screening conversion, actual flow equals reference flow multiplied by reference absolute pressure over inlet absolute pressure and inlet absolute temperature over reference absolute temperature.

This conversion is a screening step. Non-ideal gas behaviour, changing composition, condensation and leakage can require a more complete process model.

10 Gas load and pump-down duty

Continuous gas load and vessel pump-down are different duties. Continuous selection uses inlet flow at operating pressure. Pump-down time depends on vessel and piping volume, pumping speed across pressure, gas release, leakage and conductance.

A single pumping-speed value should not be used across a broad pressure range. Integrate or segment the documented curve when predicting pump-down.

11 Conductance and inlet losses

Filters, separators, valves, long pipe and small fittings reduce pressure at the pump inlet and can limit effective pumping speed at the vessel. Record clean and service-condition pressure loss.

Place instruments so the difference between vessel and pump inlet can be assessed. Increasing pump size will not necessarily correct a restrictive inlet path.

12 Ultimate pressure versus operating pressure

Ultimate pressure is obtained under specified test conditions and is not the same as sustainable process operating pressure. Gas load, leakage, vapour, discharge conditions and conductance govern the installed point.

Do not select from ultimate pressure alone. Use the documented pumping curve and process load at the actual inlet condition.

13 Vapour exposure

Record every condensable constituent, concentration, dew point, batch profile and temperature. Determine whether vapour can condense during compression, cooling, shutdown or in the discharge.

Obtain exact supplier limits and required purge, warm-up, cool-down or gas-ballast-type provisions where applicable. A dry chamber does not mean unlimited vapour handling.

14 Condensation and liquids

Liquid droplets, slugs or in-pump condensation can create deposits, corrosion, hydraulic stress or clearance loss. Install separators, knock-out vessels, drains or condensers when the process design requires them.

Define drain destination and safe handling. Do not assume a pump can clear liquid because it survives humid air.

15 Particulate exposure

Record particle chemistry, size distribution, concentration, abrasiveness, stickiness and credible slug load. Dry claw clearances and timing can be affected by deposits and foreign material.

Use inlet separation or filtration supported by the process and pump maker. Pressure drop, filter loading and maintenance access belong in the selection, not as later accessories.

16 Sticky polymerising and reactive material

Materials that polymerise, cure, crystallise, sublime or become sticky can accumulate on internal surfaces. Temperature cycles and shutdown residence time may govern the risk.

Require process chemistry and cleaning review. A purge can reduce exposure in some duties but is not a universal cure.

17 Corrosive media and materials

List corrosive constituents, water content, concentration and temperature for normal and upset states. Review wetted metals, coatings, seals, gaskets, silencers and downstream treatment.

A corrosion-resistant option for one chemical does not establish compatibility with mixtures or cleaning agents. Obtain written project-specific confirmation.

18 Purge purpose

Purge may protect seals, dilute process gas, reduce condensation, sweep deposits, prevent air ingress or support shutdown. Define media, quality, pressure, flow, entry point, timing and discharge consequence.

Use the exact maker or project requirement. Too little purge may fail its purpose; excess purge increases gas load and can move the operating point.

19 Purge accounting

Add purge entering upstream of the pumping chamber to the inlet gas load when it reaches the pump. A purge introduced elsewhere may affect discharge flow or containment without changing inlet load in the same way.

Include purge in treatment, emissions and utility calculations. State whether the pumping curve comparison includes it.

20 Cooling and heat rejection

Identify air cooling, water cooling or another documented arrangement. Record ambient or coolant inlet temperature, flow, pressure, quality, fouling allowance and heat-rejection path.

High inlet temperature, compression ratio, gas properties, backpressure and deposits can raise machine temperature. The exact manufacturer defines allowable conditions and protection.

21 Discharge backpressure

Silencers, catalytic or adsorption treatment, condensers, long discharge pipe and common headers create backpressure. Check maximum and normal outlet pressure against exact pump documentation.

A restricted silencer or flooded discharge system can reduce capacity and increase temperature. Include clean and service-condition loss.

22 Discharge treatment and emissions

Define where the exhaust goes and what it contains. Vapour, condensate, dust, purge gas and trace lubricant migration may require separation, recovery, scrubbing or controlled venting.

Never assume atmospheric discharge is acceptable. Environmental and process-safety owners define the basis.

23 Temperature monitoring

Record inlet, discharge, casing, coolant and ambient temperatures where relevant. Select sensor locations and trip logic from the exact pump and project requirements.

A normal casing temperature alone does not prove that internal deposits, blocked cooling or discharge restriction are absent.

24 Clearances and thermal growth

Claw rotors depend on controlled clearances. Thermal expansion, deposits, corrosion, particles or mechanical distortion can reduce those clearances.

Foundation, piping strain, cooling and operating procedure can affect alignment and temperature. Follow manufacturer installation and restart requirements after an abnormal event.

25 Bearings gears seals and lubricant boundary

Timing gears and bearings are generally outside the pumping chamber but remain critical support components. Record lubricant type, change method, seal arrangement and possible process or atmosphere interfaces from exact documentation.

Do not label the complete package oil free without a supported boundary and definition.

26 Controls and turndown

Define pressure control, speed control, inlet throttling, bypass, sequencing and standby philosophy. Check permitted speed and operating envelope. Some duties require minimum temperature or purge during low load.

Control selection must prevent operation in an unsupported thermal, pressure or contamination state.

27 Starts shutdown and restart

Record permitted starts, warm-up, purge sequence, isolation, venting, cool-down and restart checks. A sudden process trip can trap condensable or reactive material.

Restart after a contaminated or liquid event may require inspection. Do not rely on automatic restart unless explicitly approved.

28 Noise and acoustics

Use documented sound data with test condition, measurement method, enclosure and uncertainty. Installed noise depends on inlet and discharge pulsation, silencer condition, building reflection, ventilation and connected pipework.

State the site acoustic criterion and measurement position. Do not invent a universal dry-claw noise value.

29 Vibration piping and foundation

Provide a level, rigid support and piping arrangement that avoids unacceptable strain. Flexible connectors, supports and isolation must follow the project and manufacturer design.

Trend vibration where the maintenance strategy requires it. A vibration limit needs an exact source and measurement convention.

30 Motor electrical and ambient conditions

Check motor power, starting method, supply, frequency, hazardous-area requirements, ambient temperature, altitude, enclosure, ventilation and enclosure heat rejection.

A motor nameplate value is not proof of process power across the full duty. Use documented absorbed-power information and project electrical rules.

31 Maintenance access and lifecycle

Provide access for filters, separators, silencers, cooling equipment, lubricant points, inspection covers and rotor-chamber cleaning where permitted. Record lifting, isolation, decontamination and waste needs.

Service intervals depend on model, media and duty.

32 Monitoring and evidence

Useful evidence can include inlet and discharge pressure, inlet and discharge temperature, motor current or power, cooling status, purge flow, filter differential pressure, vibration and process-state markers.

Define alarm and trip values from approved sources. Retain trends around start-up, vapour peaks, shutdown and abnormal events.

33 Selection sequence

Define the duty and evidence status. Convert flow to the actual inlet condition. Compare required flow with the exact pumping curve. Check the full pressure and power envelope. Then apply media, vapour, particle, materials, purge, cooling, discharge, noise, utility and maintenance gates.

If any gate lacks critical evidence, use a qualification hold. Curve capacity alone cannot release the selection.

34 Worked case A dry nitrogen duty at actual inlet condition

Assumptions. A fictional documented duty requires 100 m3 ref/h of dry nitrogen. Reference state is 1.013 bar absolute and 0 degrees C. Pump inlet is 0.300 bar absolute and 35 degrees C. For screening, ideal-gas behaviour is used. A candidate dry-claw curve documents 430 actual m3/h at 0.300 bar absolute for the applicable configuration and accepted gas basis.

Method. Actual inlet volume is 100 times 1.013 divided by 0.300 times 308.15 divided by 273.15, or approximately 380.9 actual m3/h. Curve utilisation is 380.9 divided by 430, or approximately 88.6 percent.

Result and interpretation. The documented curve exceeds the converted inlet requirement in this fictional screen. Selection is not released until power, temperature, discharge backpressure, materials, purge, cooling, curve tolerance and required project allowance are confirmed. The 88.6 percent figure is not a universal allowable utilisation.

35 Worked case B condensable vapour duty

Assumptions. A second fictional duty requires 70 m3 ref/h of a process-gas mixture at the same 1.013 bar absolute and 0 degrees C reference. Pump inlet is 0.180 bar absolute and 55 degrees C. A candidate curve documents 520 actual m3/h at 0.180 bar absolute for a compatible dry gas. The stream contains a condensable vapour, but its dew point and cooling path remain open.

Method. Actual inlet volume is 70 times 1.013 divided by 0.180 times 328.15 divided by 273.15, or approximately 473.3 actual m3/h. Curve utilisation is approximately 91.0 percent.

Result and interpretation. Curve capacity alone appears above the converted flow, but the selection remains on qualification hold. Condensation, vapour compatibility, discharge temperature, purge, materials, liquid separation and backpressure have not been closed. Dry internal operation does not establish vapour tolerance.

36 Worked case C purge changes the operating point

Assumptions. The case B process requires a fictional 18 m3 ref/h protective purge that enters upstream of the pumping chamber and therefore adds to the inlet gas load. It shares the case B reference and inlet conditions.

Method. Purge expands to approximately 121.7 actual m3/h at 0.180 bar absolute and 55 degrees C. Combined inlet requirement is approximately 473.3 plus 121.7, or 595.0 actual m3/h. This exceeds the fictional 520 actual m3/h curve value before curve tolerance or project allowance.

Result and interpretation. A purge intended to protect the pump can materially change capacity selection. The duty must be re-evaluated, not passed because the unpurged process flow fitted the curve. Purge location and actual state must be confirmed.

37 Duty comparison

38 Diagnostic method

39 Commissioning and acceptance

Confirm the exact pump, curve, media approval, installation, cooling, purge, filters, separators, discharge, controls and instruments before operation. Inspect alignment, piping strain, rotation, utility flow and access.

Capture evacuation, continuous duty, vapour peak, variable load, controlled shutdown and restart as applicable. Compare measured inlet pressure, temperature, power, cooling, purge and backpressure with documented limits. Record deviations and authorised dispositions.

40 Required asset Dry Claw Selection Matrix

Use this editable matrix for screening, qualification and release. Unknown fields remain open. Do not copy hypothetical case values into a project selection.

Matrix A duty identity and boundary

Matrix B gas state and pumping curve

Matrix C contamination and compatibility

Matrix D purge cooling and discharge

Matrix E site controls noise and maintenance

Matrix F qualification and release

41 Filled fictional matrix extract

42 Frequently asked questions

Does dry claw mean oil free

It can mean no operating lubricant in the pumping chamber for the documented design. Bearings, gears and other parts may use lubricant outside that chamber. State the boundary and exact evidence.

Can a dry claw pump handle water vapour

Only within the exact model's documented limits and with the required temperature, purge, separation and operating procedure. Water vapour can condense.

Is ultimate pressure enough to select a pump

No. Use pumping speed at the actual inlet pressure and gas condition, then close process and package limits.

How should dust be handled

Define the dust and exposure, then obtain exact tolerance and protection requirements. Separation or filtration can add pressure loss and maintenance needs.

Why include purge in capacity

Purge entering the pumping path becomes part of the gas load. It can change pressure, capacity, treatment and emissions.

Can a curve comparison release selection

No. It is one gate. Power, temperature, media, contamination, discharge, cooling, controls, utilities and maintenance must also be accepted.