Most Common Mistakes When Using Vacuum Pumps and How to Avoid Them

MOST COMMON MISTAKES WHEN USING VACUUM PUMPS AND HOW TO AVOID THEM

Heinz Barfuss / PM Dry & Roots Pumps Pfeiffer Vacuum GmbH, Asslar, Germany Introduction

. Name: . Heinz Barfuss . Age 63 . Located in Asslar, Germany . Product responsibilities: . Air cooled roots pumps . Roots pumping units . Dry Screw pumps . Diaphragm pumps

. Rotary Vane Pumps

. Dry Pumps . Multi-stage Roots Pumps ACP 15 – 40 . Diaphragm Pumps MVP 003-2 – MVP 070-2

. Turbomolecular Pumps HiPace 10 – 700

. Rotary vane pumps are the most common used vacuum pump. Some of the main features are mature technology, gas independent pumping performance, over 6 decades compression to atmosphere and a superior cost/performace ratio, . Since rotary vane pumps work with internal compression, are oil sealed and oil lubricated, pumping of vapours and its chemical attack of the oil are one of the major critria for the lifetime and reliability of the pumps. . The use of magnetic coupled rotary vane pumps avoids unwanted leaking shaft seals, improves reliability and availability and keeps the maintenance cost to a minimum.

. Most manufacturers present the so called „water vapour tolerance“ in hPa and/or water vapour capacity in grams/hour in their technical specifications. They are only valid for water vapours. . With open gas ballast valve, pump at operating temperature and inlet pressure below water vapor tolerance, condensation of water vapours in the pump will be avoided. . In a drying process the amount of water to be removed defines the size of the pump by its water vapour capacity. If high amounts of water to be removed, a precondenser at the inlet of the pump is an effective solution, since it acts like a pump. . It is a must to warm up the pump with open gas ballast for 15 – 30 minutes to reach the operating temperature of 80 – 90 °C (oil). . Using a precondendenser, the condensed amount of water will reduce the size of the pump significantly.

. Only with Pump at operating temperature and open gas ballast, water vapours can be pumped through the pump. . Dependency of vapour pressure/operating temperature and the resulting effects:

pumped Temperature Condensation Results Media soluble in oil Tp > Ts no minimal suluble Tp < Ts yes soluble in oil

not soluble in Tp > Ts no oil Tp < Ts yes forms an emulsion

Tp - Pump Temperature Ts - Saturation Vapour Temperature

. After the process is finished, it is recommended to close the inlet valve above the pump, let the pump run with open gas ballast for at least 15 minutes to push the remaining vapours through the exhaust out of the pump. . This regenerates the pump oil, avoids stand-still corrosion and extends the oil change intervals. . If a vacuum level with open gas ballast of < 2 hPa is required, a two Picture 1 + 2: rotary vane pump operated without stage pump is to be used. gas ballast

. The operating agent (oil) has 3 functions: . Sealing . Lubricating bearings and vanes . Transfer compression @ friction heat to the outside . Monitoring the oil level is important for all oil sealed pumps and pumps with gear boxes. The level to be checked on a weekly basis. . If the oil level is too low: . lubrication will stop and pump may seize . Oil will overheat and the aging will occur fast

. The oil to be checked regularly for colour, viscosity and particles. A water content of > 5 % will lchange the oil milky. This can happen when gas ballast is closed, pump was not warmed up and more water than the max. water vapour capacity was pumped. . Picture shows the change of oil colour towards „black“ during aging. An oil change is to be made latest with oil colour „6“.

Picture 3: Pfeiffer Vacuum oil P3 6

© Pfeiffer Vacuum 2016 • Analytica 2016 • PM Dry & Roots Pumps • Heinz Barfuss • 2016-05-10 ■ 9 Rotary Vane Pump – Oil Misting . By function rotary vane pumps mist small oil droplets together with the gas though the exhaust port. The amount of oil depends on the inlet pressure, the higher the more. A ruel of thumb is ca. 3 cm3 oil/m3 gas at an inlet pressure > 100 hPa. . To avoid oil misting and resulting low oil level, which may cause the pump overheat and seize, oil mist eliminators are used. They contain one or more fine porous cartidges, which trap the oil mist. A return device transports the trapped oil back into the pump. . If a lot of vapours are pumped, an oil return device is not to be used, since the water enters backwards and accumulate in the oil.

Picture 4: Oil mist filter (Pfeiffer Vacuum)

. In the final vacuum range < 0,5 hPa rotary vane pumps by principle generate oil molecule backstreaming to the suction side. In most processes hydrocarbons are not acceptable. . To eliminate the backstreaming molecular seeves (Zeolite), catalytic or cold traps to be installed at the suction side to trap the molecules. Regeneration of the zeolite is done with a heating rod for bake out at 250 °C. The catalytic trap is selfregenerating and the cold trap to be brought to room temperature and cleaned.

Pictures 5 - 7: Inlet traps (Pfeiffer Vacuum)

. In processes which are particle loaded or generate particles it makes sense to protect the pump from particles entering the inlet flange. Abrasives will wear rotor and vanes, might clogg the internal oil passes (lack of lubrication) and block the high vacuum safety valve. . Depending on the application inlet filters with cartrdges made of paper, polyester or carbon, filters with so called Raschig-rings (particles stick on the oil film of the rings) or a combination cyclon/ cartridge filters can be utilized to protect the pumps.

Picture 8 + 9: inlet flange of a pump not using an inlet filter Picture 10: inlet filter with 10 µ Polyester and damaged rotor. Pump system needs to be cartidge (SAS by Pfeiffer Vacuum) replaced

. The frictionless pump module is operating without oil lubrication, has a non contact gap sealing between rotor and stator to generate a hydro-carbon and particle free vacuum. . Pumps provide long term vacuum stability and long service intervals. . Gas ballast function and silencer drain allow to pump quite a high amount of water and consensibles.

. After the process is finished, it is recommended to close the inlet valve above the pump, let the pump run with open gas ballast for at least 15 minutes to push the remaining vapours through the exhaust out of the pump. Picture 11: ACP15 operated without gas ballast . This avoids stand-still corrosion and extends the oil change intervals. . To avoid damage from particles getting into the pump an inlet filter Picture 12: ACP15 operated without inlet to be installed filter Picture 13: inlet filter 25 µ (Pfeiffer Vacuum)

. Membrane is moved back and outlet valve forth by means of a excentric reciprocating drive inlet valve . Single (1) up to (4) head pump versions are available membrane . 2 – head version excentric . Heads in parallel for high piston pumping speed, in series for low ultimate vacuum . 4 – head version . 3 stages – 2 heads in parallel for high pumping speed, 4 in series for low ultimate vacuum

. When pumping vapours it is highly recommended to operate the pump with gas ballast open. When the process is finished, it makes sense to close the inlet valve above the pump, let the pump run with open gas ballast for at least 15 minutes to push the remaining vapours through the exhaust out of the pump.

Picture 14 + 15: diaphagm/valves damaged due to water condensing in the pump

. The working range of turbomolecular pumps is the high and ultra - high vacuum down to < 10-11 hPa. A pair of a rotating and a static disc are forming a pump stage, generating a defined compression for gases. . Lining up several pump stages in series on a shaft, whose 13 compression to be multiplied, compression ratios of up to 10 for N2 can be reached. . Turbomolecular pumps cannot compress to 1013 hPa and need a forvacuum pump. Pumps with e.g. Holweck-stages can handle foreline pressures > 30 mbar and can reduce the size of the forevacuum pumps by a factor of up to 50. This makes the system compact and cost effective. . Processes which tend to condensation, sublimation and particle generation require turbomolecular pumps w/o drag stages.

Upper passive magnetic bearing

Turbo-blades

Holweck-stage

Motor Purge connection Lower ceramic ball bearing

Picture 16: Hybrid Turbomolekular Pump HiPace 80™ (Pfeiffer Vacuum)

. Pumps with Holweck-stages can tolerate a forevacuum > 30 hPa. For pumps with only turbostages the max. forevacuum is 2 - 3 hPa ( for

N2 < 2 and H2 < 0,5 hPa). . When the forevacuum is above the maximum, due to gas friction and high compression the pump might overheat and fail. . Dry pumps have no outlet valves. In case of a dry pump failure and the turbo has no speed dependent vent control system, diring slowing down the forevacuum will rise quickly and the chamber is vented backwards. With large chambers a sudden pressure rise, the ratating rotor touches the non moving stator discs and pump will crash. This is a so called Helicopter effect. . The installation of a fast acting foreline valve will avoid backwards venting and resulting pump damage.

. High vacuum piping & valves should have at least the diameter of the pump inlet, be as short as possible, to avoid conductance and performance losses. . In the following an example to demonstrate the influence of the conductance loss with a piping of D = 25 and lenght L = 100 mm: . Eff. pumping speed for a turbomolecular pump with a nom. pumping speed of: . 60 l/s: 10 l/s (pump inlet flange – 63 mm) . 5000 l/s: 14 l/s (pump inlet flange – 500 mm) . As can be seen, with a 80 x larger turbopump only 40 % higher pumping speed can be achieved. . Important are short inlet piping, no reductions, least number of elbows, valves and flanges (leaks).

© Pfeiffer Vacuum 2016 • Analytica 2016 • PM Dry & Roots Pumps • Heinz Barfuss • 2016-05-10 ■ 20 Turbomolecular Pumps – Splinter Shield/Venting . To avoid pump damage due to foreign objects falling into the pump, a splinter shield or protective screen to be inserted in the inlet flange. Depending on the gas the pumping speed reduction can be up to 30 %. If possible an upside down installation might be an option. . Without venting the turbo pump can get contaminated due to a pressure equalization between fore and high vacuum when using rotary vane pumps. A missing high vacuum valve will continue the contamination into the chamber. . Large chambers under vacuum pull in atmospheric air very fast and can cause the pump to crash.

. Venting the pump with dry gas e.g. N2 avoids contamination and vents the forevacuum pump to atmospheric pressure. . Automatic venting is the best way to avoid contamination. The

optimum rotational speed for venting is at 50 % of nnom.

. Dry Pumps . Rotary Vane Pumps . Dry compression . Oil lubricated/sealed . < pumping speed @ 1000 hPa . > pumping speed @ 1000 hPa . Not gas independent . Gas independent . High gas temepratures . Low gas temperatures . Mostly water cooled . Air cooled . Higher noise level . Lower noise level . Expensive PM/overhaul . Inexpensive PM/overhaul . High initial cost . Low initial cost . Complex monitoring . Simple monitoring

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