1. What is a vacuum pump?
Removes gas molecules from a closed system to create vacuum
Reduces pressure below atmospheric for process operations
Used in drying, distillation, filtration and solvent recovery
Essential for heat-sensitive and low-boiling chemical/pharma processes
2. What are the types of vacuum pumps?
Rotary vane pumps
Liquid ring pumps
Dry screw pumps
Roots booster pumps
Diaphragm pumps
Steam jet ejectors
Diffusion pumps
Selection depends on vacuum level, vapor load and process compatibility
3. What is ultimate vacuum?
Lowest pressure a vacuum pump can achieve with no gas load
Indicates maximum vacuum capability of the pump
Measured in mbar, torr or micron
Depends on pump design, sealing quality and maintenance condition
4. What is vacuum range (low, medium, high)?
Low vacuum: atmospheric to 1 mbar
Medium vacuum: 1 mbar to 10⁻³ mbar
High vacuum: 10⁻³ mbar to 10⁻⁶ mbar
Helps in choosing suitable pump and measuring instruments
5. Difference between rotary vane pump and liquid ring pump.
Rotary vane pump: oil-sealed, deeper vacuum, not suitable for high vapor load
Liquid ring pump: uses seal water, handles vapors and moisture easily
Rotary vane ideal for dry vacuum; liquid ring ideal for wet/corrosive service
Choice depends on process environment and required vacuum depth
6. How does a liquid ring vacuum pump work?
Impeller rotates inside a casing partially filled with seal water
Centrifugal force forms a liquid ring around the impeller
Gas gets trapped in variable-volume chambers and is compressed
Compressed gas is discharged, continuously creating vacuum
7. What is cavitation in vacuum pumps?
Formation and collapse of vapor bubbles inside the pump
Causes vibration, noise and erosion of internal surfaces
Reduces vacuum performance and damages pump components
Triggered by high temperature or low pressure conditions
8. What causes cavitation in liquid ring pumps?
High seal water temperature
Insufficient seal water flow
High vapor load entering the pump
Air leakage or sudden pressure drop in the system
Excessive pump speed (RPM)
9. What is the function of seal liquid in liquid ring pumps?
Forms liquid ring required for gas compression
Provides sealing between impeller and casing
Absorbs heat generated during compression
Helps condense vapors and stabilize vacuum
10. Why water temperature affects pump vacuum?
Higher water temperature increases vapor pressure
Limits how deep a vacuum the pump can achieve
Cooler seal water improves condensation of vapors
Maintaining recommended seal water temperature ensures strong and stable vacuum
11. Explain working of a rotary vane vacuum pump.
Uses a rotor with sliding vanes inside an eccentric cylindrical housing. Vanes create expanding and contracting chambers that trap air.
Trapped air is compressed and discharged through the outlet. Oil provides sealing, lubrication and cooling for deeper vacuum.
12. What is an oil-sealed vacuum pump?
Uses oil to seal internal gaps and prevent air backflow. Oil improves vacuum level by reducing leakage.
Provides lubrication and heat removal during operation. Commonly used in rotary vane and piston vacuum pumps.
13. Difference between dry vacuum pump and wet vacuum pump.
Dry pumps operate without oil or water inside the compression chamber. Wet pumps use oil or water as the sealing/working fluid.
Dry pumps are cleaner and ideal for contamination-sensitive processes. Wet pumps handle vapors, moisture and corrosive gases better.
14. What is a roots blower vacuum pump?
Uses two counter-rotating lobed rotors to move gas. Handles large gas volumes with minimal internal friction.
Provides medium vacuum and acts as a booster. Used with backing pumps for fast evacuation and deeper vacuum.
15. Explain two-stage vs single-stage vacuum pump.
Single-stage pumps compress gas once and provide moderate vacuum. Two-stage pumps compress gas twice and achieve deeper vacuum.
Two-stage design improves ultimate pressure and process efficiency. Preferred for low-pressure applications.
16. What is vacuum pump capacity (m³/hr or CFM)?
Indicates the volume of gas removed per unit time. Higher capacity reduces pump-down time. Selected based on vessel size, leakage rate and gas load. Expressed in m³/hr, LPM or CFM.
17. What is Pump Down Time?
Time required to evacuate a system from atmospheric pressure to the target vacuum.
Depends on pump capacity, system volume and gas load. Shorter pump-down time improves productivity. Critical for dryers, filters, reactors and distillation units.
18. What is ultimate pressure and how is it measured?
Lowest pressure a pump can achieve under zero gas flow.
Indicates maximum vacuum capability. Measured using Pirani, Penning or Bourdon gauges. Affected by pump wear, sealing and oil quality.
19. What is micron, torr and mbar?
Micron: 1 micron = 0.001 torr. Torr: 1 torr = 1 mm Hg. mbar: 1 bar = 1000 mbar. These units define vacuum levels for various industrial applications.
20. Why are vacuum boosters used?
Increase pumping speed at low pressure. Reduce pump-down time in large systems. Handle high gas loads efficiently. Allow deeper vacuum when paired with backing pumps.
21. What is backstreaming in vacuum pumps?
Backstreaming is the migration of pump oil vapors back into the process chamber. It contaminates product surfaces, affects purity and interferes with processes like drying or coating.
It occurs mainly in oil-sealed pumps when oil quality is poor or traps are missing.
22. How to avoid oil backstreaming?
Use oil mist filters, cold traps and foreline traps. Maintain correct oil level and replace degraded oil regularly.
Use dry pumps where contamination risk is high. Keep pump temperature and operating conditions stable to reduce vaporization.
23. What are common vacuum pump applications in pharma industry?
Used in vacuum drying, vacuum distillation, solvent recovery, filtration, freeze-drying and material transfer.
Essential for low-temperature evaporation, handling moisture-sensitive materials and improving batch turnaround time.
24. Why filters are installed before vacuum pumps?
To prevent dust, vapors or liquid droplets from entering the pump. Protects internal components from corrosion and wear.
Reduces contamination of pump oil and increases pump life. Ensures stable vacuum performance.
25. What is NPSH in vacuum systems?
NPSH (Net Positive Suction Head) indicates how much pressure is available at the pump suction.
Ensures the pump suction pressure stays above vapor pressure to avoid cavitation. Critical for liquid ring pumps where seal water circulation matters.
26. What causes loss of vacuum?
Leakage in pipelines or gaskets, high vapor load, improper sealing, hot seal water, worn vanes, blocked filters or damaged valves.
Poor maintenance or incorrect pump speed also contribute to vacuum drop.
27. How to troubleshoot low vacuum level?
Check for system leaks, gasket condition and loose joints. Verify seal water temperature and flow.
Inspect filters for choking and oil condition for contamination. Ensure pump speed, direction and cooling system are correct.
28. Why is gas ballast used in vacuum pumps?
Gas ballast introduces a controlled amount of air into the compression chamber. Helps remove condensable vapors without saturating the oil. Prevents moisture accumulation, oil degradation and improves pump life.
29. What is vacuum leak test?
A test to detect unwanted air entry into the system. Performed by isolating the pump and monitoring pressure rise over time.
A rapid pressure increase indicates leakage. Essential before drying or distillation operations.
30. Difference between mechanical vacuum pump and ejector.
Mechanical pumps use rotating parts for gas removal, provide stable vacuum and require electricity or motors.
Ejectors use high-pressure steam or air to create vacuum with no moving parts. Pumps offer better control; ejectors handle large vapor loads and corrosive gases easily.
31. What is a steam jet ejector?
A steam jet ejector uses high-velocity steam to entrain and compress gases, creating vacuum without moving parts.
It is simple, reliable and suitable for handling large vapor loads, corrosive gases and high-temperature applications.
32. Why are ejectors used in evaporators and distillation?
Ejectors handle large volumes of vapors efficiently, tolerate moisture and corrosive gases, and provide stable vacuum without mechanical wear.
They maintain low boiling temperatures, improve evaporation rates and reduce thermal degradation of sensitive products.
33. What is a vacuum condenser?
A vacuum condenser cools and condenses vapors before they reach the pump or ejector.
It reduces load on the vacuum system, prevents pump overload and improves vacuum level.
Used extensively in drying, distillation and solvent recovery operations.
34. Why vacuum drops after some time during operation?
Due to rising seal water temperature, increased vapor load, oil contamination, filter choking or system leakage.
As operating conditions change, pump performance decreases, causing gradual vacuum reduction.
35. What is a cold trap and why used?
A cold trap condenses vapors before they enter the pump by cooling them to very low temperatures.
It protects the pump from corrosive vapors, solvents or condensables. Essential for freeze-drying, solvent-rich systems and high-vacuum applications.
36. What materials can damage a vacuum pump?
Corrosive vapors, acidic gases, particulates, solvent fumes and liquids entering the pump can damage internal components.
Incompatible chemicals degrade oil, seals and metal surfaces. Proper filtration and traps prevent such damage.
37. What is the effect of high temperature on liquid ring pump?
High seal water temperature increases vapor pressure, reducing vacuum depth. It promotes cavitation, decreases efficiency and causes thermal stress.
Maintaining proper cooling ensures stable and deep vacuum.
38. Why oil level is important in rotary vacuum pumps?
Correct oil level ensures proper sealing, lubrication and cooling.
Low oil level lowers vacuum performance and causes vane wear. Excess oil can carry over into the system and contaminate product.
39. What is the function of a check valve in a vacuum line?
A check valve prevents reverse flow of air or process vapors back into the equipment when the pump stops. It protects product, prevents contamination and avoids pump flooding or damage.
40. Why vacuum pump creates noise or vibration?
Caused by misalignment, worn bearings, cavitation, loose components, imbalance or contaminated seal liquid/oil. Excess vibration indicates mechanical issues, reducing pump life and vacuum performance.
41. What is vacuum pump seal failure?
Seal failure occurs when mechanical seals or shaft seals wear out, allowing air or liquid to leak into the pump.
It reduces vacuum level, causes oil contamination and increases vibration. Common causes include chemical attack, dry running and improper lubrication.
42. How to maintain a vacuum pump?
Regularly check and replace oil, inspect filters, clean strainers, monitor seal water temperature and verify RPM. Inspect seals, bearings and gaskets for wear. Perform leak checks and ensure proper cooling and lubrication for reliable performance.
43. What is vacuum pump overheating?
Overheating happens when cooling water flow is low, oil is degraded, load is excessive or pump vents are blocked. It increases wear on vanes, seals and bearings. Maintaining proper cooling and operating conditions prevents heat-related failures.
44. What safety precautions are required while operating vacuum pumps?
Ensure proper earthing, avoid running pump with closed suction, check oil/ seal water levels, secure all connections and follow lockout–tagout before maintenance. Keep pump area ventilated and avoid handling corrosive vapors without traps.
45. Explain vacuum pump performance curve.
A performance curve shows the relationship between vacuum level and pumping speed. It helps determine actual capacity at different pressures. Pumping speed decreases as vacuum level increases. Useful for selecting the right pump for process load.
46. What is displacement and swept volume?
Displacement or swept volume is the volume displaced by the pump per rotation. It represents theoretical capacity, not actual pumping speed. Actual speed depends on internal leakage, gas type and operating pressure.
47. Why suction filter choking reduces vacuum?
A choked filter restricts airflow to the pump, causing reduced suction capacity. It increases load on the pump, raises temperature and drops vacuum level. Regular cleaning ensures stable performance.
48. Why vacuum pump requires cooling water?
Cooling water removes heat generated during compression. It maintains seal water temperature in liquid ring pumps and prevents oil degradation in rotary pumps. Proper cooling ensures deeper vacuum and longer pump life.
49. What is seal water recirculation system?
A recirculation system reuses seal water after cooling and filtration. It reduces water consumption, stabilizes water temperature and maintains consistent vacuum. Helps prevent cavitation and improves pump efficiency.
50. Why is an air ballast valve important?
An air ballast valve introduces controlled air into the compression chamber to help remove condensable vapors. It prevents moisture buildup, reduces oil emulsification and improves pump reliability during vapor-rich operations.
51. What are advantages of dry screw vacuum pumps?
Dry screw pumps operate without oil or water inside the compression chamber. They handle corrosive gases, solids and vapors easily. They provide clean, contamination-free vacuum, require low maintenance and achieve deeper vacuum than liquid ring pumps.
52. Why vacuum pumps trip on overload?
Overload occurs due to high torque from choked filters, viscous oil, blocked discharge, motor issues, or liquid ingress. Excessive vapor load or high seal water temperature also increases load. Tripping protects the motor from damage.
53. What causes moisture entry in vacuum pump oil?
Moisture enters due to handling wet vapors, poor condensation in the system, low oil temperature or incorrect gas ballast setting. Moisture forms emulsions, reduces vacuum level and damages pump internals.
54. What is the effect of using hot water in liquid ring pumps?
Hot water raises vapor pressure, reducing achievable vacuum. It increases cavitation risk, lowers volumetric efficiency and causes unstable vacuum. Maintaining cool water ensures proper sealing and deep vacuum.
55. Why vacuum pump capacity drops over time?
Capacity drops due to vane wear, oil degradation, seal leakage, filter choking or corrosion. Internal clearances increase with wear, reducing compression efficiency. Regular service restores performance.
56. What is the difference between absolute pressure and vacuum level?
Absolute pressure measures pressure relative to perfect vacuum (zero reference). Vacuum level measures how much the pressure is below atmospheric pressure. Higher vacuum means lower absolute pressure.
57. How does atmospheric pressure affect vacuum?
At higher altitudes, atmospheric pressure is lower, reducing the maximum achievable vacuum. Pump performance drops as suction pressure decreases. Sea-level operation provides the deepest vacuum.
58. What is compression ratio in vacuum pumps?
Compression ratio = Discharge Pressure ÷ Suction Pressure. Higher compression ratio means better vacuum capability. Pumps with high compression ratio achieve lower suction pressures and deeper vacuum.
59. What is base pressure?
Base pressure is the lowest stable pressure a system can reach with the vacuum pump operating. It depends on leaks, outgassing, pump condition and system cleanliness. It is higher than the pump’s ultimate pressure.
60. What is the effect of altitude on vacuum pump performance?
Higher altitude lowers atmospheric pressure, reducing differential pressure across the pump. This lowers vacuum depth, reduces capacity and increases pump-down time. Pumps perform best at lower altitudes.
61. Why vacuum relief valve is used?
A vacuum relief valve protects equipment from excessive vacuum by allowing controlled air entry when pressure drops below a safe limit. It prevents vessel collapse, gasket damage and structural failure during sudden vacuum surges.
62. What are common failures in vacuum pumps?
Common failures include vane wear, seal leakage, bearing failure, cavitation damage, overheating, corrosion, oil contamination and motor overload. These failures reduce vacuum level and require timely maintenance to avoid breakdown.
63. Why pump speed (RPM) is important?
RPM controls pumping speed, vacuum depth and heat generation. Too high RPM causes overheating, wear and cavitation; too low RPM reduces capacity and delays pump-down. Correct RPM ensures efficient and stable vacuum performance.
64. Why vacuum pipelines need proper sizing?
Properly sized pipelines minimize pressure drop and leakage. Undersized lines restrict flow and reduce vacuum level, while oversized lines increase dead volume and slow pump-down. Correct diameter ensures efficient evacuation and stable vacuum.
65. What are vacuum pump interlocks?
Interlocks prevent unsafe pump operation by ensuring conditions like correct rotation, proper seal water flow, adequate oil level, open suction valves and temperature limits. They protect pump, equipment and operators from failures.
66. What is an oil mist eliminator?
An oil mist eliminator removes oil vapors and aerosols from pump exhaust. It prevents environmental pollution, protects utilities and reduces oil loss. Essential for clean-room and pharmaceutical applications.
67. What is a knock-out drum in vacuum systems?
A knock-out drum removes entrained liquids, condensate and droplets before they reach the pump. It protects the pump from damage due to liquid slugging and improves vacuum stability in drying or distillation operations.
68. Why vacuum should not exceed equipment design limit?
Excess vacuum can collapse vessels, deform jackets, damage sight glasses or cause leakage. Equipment is designed for specific pressure ratings, and exceeding them compromises safety and structural integrity.
69. What is a priming chamber in liquid ring pumps?
A priming chamber helps maintain adequate seal liquid during startup. It ensures the pump builds vacuum quickly and prevents dry running. Essential for applications with intermittent operation or low suction head.
70. What are multi-stage ejectors?
- Multi-stage ejectors use series-connected nozzles to achieve deeper vacuum levels.
- Each stage boosts vacuum by entraining gases from the previous stage.
- Common in evaporators, crystallizers and solvent recovery systems for achieving high vacuum without moving parts.
71. What is a barometric condenser?
A barometric condenser uses direct-contact cooling water to condense vapors from an ejector system.
Installed at a height above 10 meters to prevent backflow of water.
Reduces ejector load and improves vacuum efficiency in evaporation and distillation systems.
72. Why sealing water should be free from solids?
Solids cause abrasion and erosion of impeller and casing.
Lead to clogging of internal passages and poor sealing.
Clean sealing water ensures stable vacuum, smooth operation and longer pump life.
81. What is the purpose of a vacuum gauge (Bourdon, Pirani, Penning)?
Vacuum gauges measure pressure inside the vacuum system at different ranges.
Bourdon gauge measures low vacuum, Pirani gauge measures medium vacuum and Penning gauge measures high vacuum.
Ensures accurate monitoring and control of vacuum levels during operation.
82. Why is the pump installed below the process vessel sometimes?
Installing the pump lower than the vessel improves liquid draining by gravity.
Prevents liquid from flowing back into the equipment during shutdown.
Reduces risk of pump flooding and ensures stable vacuum startup.
83. Why inlet temperature affects vacuum?
Higher inlet temperature increases vapor load entering the pump.
Causes cavitation, reduces vacuum depth and raises pump temperature.
Cooling inlet vapors improves pump efficiency and vacuum stability.
84. What is the effect of foam in vacuum systems?
Foam carries liquid droplets into the vacuum line, leading to pump flooding.
Reduces vacuum level, causes contamination and damages pump internals.
Controlled using antifoam agents, reduced agitation and knockout drums.
85. What is hydrotest vs vacuum test?
Hydrotest checks vessel integrity using water pressure, ensuring no leaks under positive pressure.
Vacuum test checks tightness under negative pressure by monitoring pressure rise.
Both ensure equipment safety and leak-free operation.
86. Why flexible hoses are avoided in vacuum lines?
Flexible hoses collapse under vacuum and restrict flow.
Increase leakage risk and reduce achievable vacuum.
Rigid metal pipelines provide better sealing and stable performance.
87. What is startup and shutdown procedure for vacuum pumps?
Startup includes checking oil/seal water level, opening suction valves and verifying rotation.
Shutdown requires isolating suction, allowing pump to unload and stopping motor safely.
Proper procedures protect pump components and maintain reliability.
88. What causes pump oil to turn milky?
Oil becomes milky due to moisture contamination from wet vapors.
Indicates emulsification, reducing lubrication and vacuum performance.
Requires oil change and better vapor handling or gas ballast use.
89. Why liquid slugging damages vacuum pumps?
Liquid hitting pump internals causes sudden mechanical impact.
Damages vanes, rotors and seals, leading to failure.
Knock-out drums and proper condensate removal prevent slugging.
90. What is vacuum pump skid?
A skid is a complete modular unit containing pump, motor, piping, filters and controls.
Provides plug-and-play installation with improved reliability.
Commonly used in pharma and chemical plants for easy maintenance.
73. What is stall condition in vacuum pumps?
Stall occurs when the pump cannot move gas due to overload or internal blockage.
Causes sudden drop in vacuum and increase in temperature.
Requires immediate shutdown and inspection to prevent damage.
74. What is vapour handling capacity?
Vapour handling capacity indicates the amount of vapor the pump can manage without losing vacuum.
Liquid ring and dry screw pumps have high vapor tolerance.
Exceeding capacity leads to cavitation, oil contamination or vacuum drop.
75. Why vacuum pumps fail to start?
Caused by electrical issues, motor overload or wrong rotation.
Suction blockage, seized bearings or liquid lock can prevent startup.
Checking power supply, system pressure and pump internals resolves most issues.
76. What is vacuum system leakage rate?
Leakage rate is the amount of air entering the system per unit time.
High leakage prevents achieving deep vacuum and increases pump load.
Identified during leak tests and must stay within acceptable limits.
77. What is running vacuum vs blank-off vacuum?
Blank-off vacuum is the maximum vacuum when suction is fully closed.
Running vacuum is the vacuum level during actual process load.
Running vacuum is always lower due to gas load and system leakage.
78. What is purge cycle in dry vacuum pumps?
A purge cycle introduces inert gas to remove condensables and corrosive vapors.
Prevents rotor seizure, corrosion and contamination.
Essential for chemical, solvent and reactive gas applications.
79. Why chemical compatibility is important for vacuum pump selection?
Incompatible chemicals corrode pump internals and damage seals.
Lead to oil degradation, leakage and performance drop.
Correct material selection ensures safety and long service life.
80. What is a pressure–vacuum breaker?
A pressure–vacuum breaker protects tanks from excess vacuum or pressure.
Allows controlled airflow to prevent tank collapse or deformation.
Used during filling, emptying or temperature fluctuations to maintain equipment safety.
91. What spare parts are critical for vacuum pumps?
Critical spares include vanes, seals, bearings, gaskets, filters, O-rings and oil.
Liquid ring pumps require impeller, shaft sleeves and mechanical seals.
Keeping spares reduces downtime and ensures quick maintenance
92. What are the signs of pump bearing failure?
Increased noise, vibration and temperature rise.
Irregular rotation or grinding sound during operation.
Drop in vacuum level due to internal friction and poor alignment.
93. What is vacuum hold time?
Vacuum hold time indicates how long a system maintains vacuum after isolation.
Shows tightness of vessel, piping and seals.
Short hold time suggests leakage or outgassing issues.
94. Why is variable frequency drive (VFD) used with vacuum pumps?
VFD adjusts pump RPM to match process load.
Reduces energy consumption and prevents overheating.
Allows smooth start-up, protects motor and improves vacuum control.
95. What is deep vacuum?
Deep vacuum refers to very low pressure levels, typically below 1 mbar.
Required in freeze-drying, solvent recovery and high-efficiency drying.
Achieved using two-stage pumps, boosters or multi-stage ejectors.
96. How vacuum affects boiling point of liquids?
Lower pressure reduces boiling point of liquids.
Allows evaporation at lower temperatures, protecting heat-sensitive products.
Increases drying and distillation efficiency.
97. Why vacuum pump should not run with closed suction?
Running with closed suction causes internal overheating and oil breakdown.
Creates high load on vanes or rotors, damaging the pump.
Must always ensure suction is open before starting.
98. What is priming and why is it required in some pumps?
Priming fills the pump casing with liquid before operation.
Required for liquid ring pumps to form the sealing ring.
Prevents dry running and ensures immediate vacuum generation.
99. Why does vacuum fluctuate during operation?
Caused by variable vapor load, leakage, choked filters or unstable seal water temperature.
Sudden boiling or foaming inside the equipment also affects vacuum.
Stabilizing operating conditions prevents fluctuations.
100. What parameters are used to select a vacuum pump?
Required vacuum level, pumping speed, vapor load and chemical compatibility.
Type of gas handled, temperature, maintenance needs and operating cost.
Material of construction, sealing type and safety requirements also influence selection.