Various Types of Vacuum Pump Oils: A Detailed Explanation

Vacuum Pump Oils can basically be classified into two categories based on their composition –


  1) Mineral Vacuum Oils
  2) Synthetic Vacuum Oils


Synthetic Vacuum oils can further be divided into four types:

  A) Esters
  B) Silicones
  C) Ethers
  D) Fluorochemicals

Types of vacuum oils have been described in detail below so that you are able to choose the right vacuum oil for your application.

1. Mineral Oils
What are mineral oils made of?

Mineral Vacuum Pump Oil SV-68

The mineral oils used in vacuum pumps are mixtures of aromatic hydrocarbons, paraffin and naphthene. Carbon analysis shows the typical mineral oil to be composed of approximately 65% paraffin, 30% naphthene, and 5% aromatic hydrocarbons.

The paraffins with high boiling point make very good lubricants. They are stable at high temperatures, are fluid at low temperatures, have fairly high viscosity index and are adherent enough to not break down under high shear loads. They tend to oxidize at high temperatures. Paraffins have many possible isomers which have differing properties. Aromatic compounds contain phenyl groups with straight- or branched-chain structures. They form sludge at high temperatures and have an undesirably low viscosity index.


Naphthenes contain rings and chains with no double bonds. Naphthenes have properties between those of paraffins and aromatics. Preparation of a vacuum fluid begins with vacuum distilled base oil that is further purified by solvent extraction and dewaxing. The oils supplied to the vacuum fluid distiller are either single cut with one peak in the molecular weight distribution, or blends made from two of the relatively few refined single cuts.

The oil is further purified by additional distillations. The distillation conditions are chosen to produce fluids of the desired viscosity and vapor pressure. Supervac make SV-68 is mineral oil that is triple distilled with molecular distillation.

Are all mineral oils the same? 

No. The base oil is a natural product. Hence, it has characteristics as per its place of origin; as a result, all mineral oils are not the same. We divide mineral oils for vacuum pumps into four grades: mechanical pump, diffusion pump, fully saturated paraffin, and inhibited fluids. The “rotary pump” grade is, composed of blended fluids or single-cut fluids that have not been refined to remove light ends and tars. Vapor pressure requirements in a mechanical pump are not as severe as in a diffusion pump. This grade is typically used in rotary pumps used for rough pumping chambers and backing turbomolecular and diffusion pumps. “Diffusion pump” grade fluids may be characterized as having a single peak in the molecular weight with narrow mass dispersion. Nowadays, with the advent of silicone oils, mineral oils are rarely used for diffusion pumps.

2. Synthetic Fluids

SV-77 - Synthetic Vacuum Pump Oil
(Supervac make)

Mineral oils lack many properties of the ideal fluid. When used in a diffusion pump, their ultimate pressure is unacceptably high for many applications. They are not stable in oxygen, have some tendency to sludge and foam, and do not offer adequate protection in boundary layer lubrication. Synthetics were developed to overcome the shortcomings of hydrocarbon oils. Synthetic fluids (SV-77) with low vapor pressure, high viscosity index, a high degree of lubricity, and chemical inertness have been specially designed and formulated.

Comparison of Molecules

Synthetic Vacuum Oils can be classified in four categories -

A) Esters

Esters are chemicals formed by the reaction of an organic acid and an alcohol. The esters used in vacuum pump fluids all contain the same ester chemical bond, but have differing structures and rather widely varying properties. Sebacate esters (derived from sebacic acid) are organic esters that were originally developed as jet engine and aircraft instrument lubricants and today used to lubricate turbomolecular pumps. Environmental concerns have resulted in the disuse of many older ester lubricants.

Temperature/ Viscosity graph of different vacuum oils

B) Silicones

What are Silicone oils/fluids?

Molecular structure of silicone oil

Silicones, or siloxane polymers, are made up of repeated silicon oxygen groups with silicon bonds to side groups. The unique character of the silicon-oxygen bond gives similarly unique properties to silicone-based fluids. 

The type of side groups (methyl, phenyl, alkyl, chloro, etc.) and the number of silicon atoms determine the properties and applications of the fluid. The large size of the silicon atom allows the phenyl and methyl side groups great mobility. The high flexibility of the siloxane chain accounts for the high viscosity index of silicones. As a class, silicones have the highest viscosity index of any fluid. 

Diffusion pump oil SV-SIGMA+ (Alt.of DC-705)

Which Silicone fluids are used in Diffusion pumps?

Trisiloxanes and polysiloxanes are two fluids used in vacuum pumps. Trisiloxanes are widely used diffusion pump fluids. They do not adhere to steel and cannot be used as lubricants in rotary/mechanical pumps. They are manufactured by controlled hydrolysis of silanes and addition of phenyl groups, followed by distillation. The first silicone diffusion pump fluids, IX-702 and DC-703, were mixtures of closely related molecular species with similar boiling points. Further separation leads to the isolation of two specific chemical compounds - Tetraphenyl tetramethyl trisiloxane (DC 704/ SV-SIGMA) and Pentaphenyl trimethyl trisiloxane (DC-705/ SV-SIGMA+). One compound of Pentaphenyl silicone is DC705/ SV-SIGMA PLUS and has one of the lowest vapor pressures of any diffusion pump fluid.

C) Ethers

An ether may be regarded as a derivative of a water molecule in which the hydrogen has been replaced by alkyl or aryl groups. Polyphenyl ethers were synthesized in an attempt to develop high temperature jet engine lubricants. Hickman was the first to use them as diffusion pump fluids. He found the five-ring phenyl to be stable and have extremely low vapor pressure. Commercially available fluids are mixed meta- and para-isomers of the pentaphenyl ether, which contain trace impurities of the four-ring compound. The four-ring compound has a high vapor pressure, while the six-ring compounds are either solids or glasses. Pentaphenyl ether is very viscous at low temperatures but is stable and has excellent high-temperature lubricating properties. Its wear, friction, and load capacity are in some cases equal to mineral oil. Its chemical stability and low vapor pressure make it an outstanding fluid for critical diffusion pump applications. 

D) Fluorochemicals

Fluorochemical fluids are characterized by their inertness to a wide range of chemical compounds. Partially and fully fluorinated fluids have found use as lubricants for space applications, oxygen compressors, and liquid oxygen systems. 

Fluorinated pump fluids, perfluoro alkyl polyethers (perfluoropolyethers or PFPE for short), are currently manufactured by two techniques. Fomblin fluids are prepared by the UV-stimulated photooxidation of hexafluoropropylene and oxygen. It is a random copolymer of C3F6O and COF2.  ranges from 10-to-40. Krytox fluids are prepared by the polymerization of hexafluoropropylene epoxide. Krytox consists of 20-30 repeating C3F60 groups. 

Raw perfluoropolyethers have a distribution of molecular weights extending as high as 10,000 AMU. They are distilled to yield cuts with average molecular weights in the range 1800-3700 that are suitable for use in mechanical, turbomolecular, and diffusion pumps. 

Perfluoropolyethers are stable Lewis bases that react with few chemicals. They should not be placed in contact with ammonia, amines, liquid fluorine, liquid boron trifluoride, or sodium or potassium metal. Laboratory experiments have shown PFPE fluids to decompose when heated sufficiently (>lOO°C) in the presence of Lewis acids. 

Molecular structure of PTFE vacuum pump oil

3 Secrets that Mineral Vacuum oil manufacturers will not tell you

First of all, Understand - Not All Vacuum Pump Oils Are The Same

There are three types of vacuum pump oils-

1. Mineral Vacuum Pump oil – traditional oils made by refining crude oil – still being used by those who haven’t woken up to the countless benefits that synthetic oils offer.

2. Semi-Synthetic Vacuum Pump Oil- Also known as ‘blends’- made by mixing mineral and synthetic oils – not recommended for vacuum pumps.

3. Synthetic Vacuum Pump oil – made by advanced refining process of the base oil in which individual molecules of the oil are customized to the extreme demands of the vacuum pump.

DID YOU KNOW? Supervac Industries is the only company in India manufacturing state-of-the-art synthetic vacuum pump oil with the brand name SV-77.

Secret # 1 -

1) Mineral oil tends to turn viscous (thick) at low temperatures, taking time to circulate within the pump. This causes friction, and therefore, wear within the vacuum pump. 

Once the vacuum pump runs for a few hours, the oil inside tends to heat up. Mineral oils under such conditions tend to go thin and runny giving no lubrication to the pump parts.

SV-77 molecules, on the other hand, have been engineered for high-temperature stability. So it retains its viscosity and lubricity even in very hot conditions. Furthermore, it is designed to protect the vacuum pump from deposits and prolongs the vacuum pump life.

• Performance of SV-77 is exceptionally robust at both low as well as high temperatures, which is rare in vacuum pump oils.
• This protects the pump from unnecessary wear and tear thus saving the user from loss of money and precious time wasted in the maintenance of the pump.
• 79% less maintenance required by using SV-77 synthetic vacuum oil.

Secret # 2 -

2) Mineral vacuum oil is made through the process of hydrocracking crude oil which eliminates most of the impurities – but not ALL impurities. 

This means that the offered mineral vacuum oil still contains a large amount of sulfur, reactive, unstable hydrocarbon molecules and many other unwanted contaminants. 

Under extreme working conditions of the vacuum pump, these contaminants turn into sludge and harm the pump. Working life is greatly reduced.

Secret # 3 -

3) During the initial working of the vacuum pump, mineral oil is much thicker and sluggish. Hence it circulates much slowly and is unable to deliver fast vacuum. The pump has to work extra hard at such times which lowers its efficiency considerably. 

Compared to this, SV-77 vacuum oil needs no time to ‘wake up’. It delivers a fast vacuum, thereby increasing the pump efficiency.

• 43% Faster vacuum time

Vacuum Pump Oil SV-77

SV-77 has been designed by the best engineering minds of India and Europe to deliver the following great benefits to the vacuum pump users:

• Outstanding protection of the pump

• Minimizing oil degradation

• Faster lubrication at start-up in low temperatures

• Excellent protection at high temperatures

• Exceptional resistance to thermal breakdown

• Clean and fast vacuum

• Outstanding protection against harmful deposits

• Cleaner vacuum pumps

• Vacuum pump efficiency

• Greater resistance to oil oxidation (thickening), which reduces mechanical drag

• More efficient pump operation over a wider temperature range

• Reduces friction and protects moving parts within the pump

• Prevents rust

• Exhibits hardiness against process gasses and moisture

CONCLUSION:

When selecting vacuum oil for your pump, do so after making sure that it will deliver the full value of its cost and at the same time protect your valuable pump. The oil is not just meant to create sufficient vacuum for your process. Its long-term effect on the pump & the maintenance just cannot be ignored.

With science and technology developing at such a fast pace, it makes perfect sense to let go of outdated products like mineral vacuum oils. 

Also, latest technology is no longer confined to just Europe and the U.S. Technologically advanced Indian products like our SV-77 have done us proud. Along with supply to many major Indian industries and the Defense Department, it is currently being exported to 17 different countries of the world. 

Related Articles

• Top 20 Questions About Rotary Vacuum Pumps and Oils
• Why Should Vacuum Oils be changed at specified intervals?
• Caution! Vacuum Pump Users

How To Get Maximum Life From Your Diffusion Pump Oil

Diffusion Pump Oil is one of the costliest consumables required by vacuum users. In the present day scenario of cut throat competition, engineers are under a lot of pressure to reduce costs without compromising on quality. Based on my experience of almost 20 years in the vacuum line, I have compiled a list of do's and don'ts as to how one can get maximum life from the diffusion pump oil. For the benefit of our valued customers, the facts are given below -

1.   Topping up at regular intervals is most important factor in getting maximum life from Diffusion Pump Oil. Care should be taken to top up the oil in the pump in a timely manner. In the absence of topping up, volume of oil in the Diffusion Pump reduces, whereas heaters keep producing the same amount of heat. This causes overheating and accelerated loss of DP oil. This further reduces the volume of oil, which in turn causes more overheating and loss of DP oil is further accelerated and so on.

Diagram of Diffusion Pump

2.   Keep track of the water flow through the cooling lines. Check the inlet and outlet water temperature as well as the flow rate. Lines can become clogged or scaled over time. As Diffusion Pump oil vapor can only condense on coming in contact with cold walls of DP. Any reduction in cooling causes overheating and increase of pressure inside DP. This may result in loss of oil by back streaming.

3.   Strange as it may sound a very low fore- pressure can also cause loss of DP oil. This can be prevented by switching on heaters before fore pressure becomes too low.

4.   Faulty gauges for pressure/heater too, can cause accidental overheating of the oil or the oil not getting the required heating. Check that all the gauges are functioning properly.

5.   Silicone diffusion Pump Oils have a life of about 4 times that of Hydrocarbon Diffusion Pump Oils. Also Silicone Diffusion Pump oil does not blacken and clog the jets of DP so Silicone DP oil works out more economical in the long run.

6.   Poorly working Rotary Pump will put extra burden on diffusion pump. It must be ensured that Rotary pump is working properly to get full life from Diffusion Pump Oil.

7.   Do not expose the oil to air when it is heated. Doing so will affect the viscosity of the oil – it thickens and vacuum reduces. To avoid air exposure valves must be checked from time to time.

8.   To avoid having the oil migrate into the vacuum chamber, a cold cap should be fitted at the top of the nozzle assembly inside the diffusion pump. This cold cap condenses the vapor in that area.

9.   A concentric circular chevron baffle should be present at the mouth of the pump. It allows air molecules to wander in, but traps the heavier oil vapors as they try to escape.

10. There should be no gross vacuum leaks as this will result in deterioration of oil quickly. Vacuum leaks also cause the pressure to drop and in extreme cases may cause stalling of Diffusion Pump. Stalling will cause loss and deterioration of diffusion pump oil. Such leaks can be plugged by applying silicone vacuum grease to O rings and joints.

11. Sometimes back streaming of contaminants from the vacuum chamber takes place causing the oil to form sludge. This is preventable by installing filters above the chevron baffle of the diffusion pump.

12.    Silicone diffusion pump oil starts to decompose if it comes in contact with even trace elements of alkali metals (e.g. hydroxides  of sodium, potassium, cesium etc.). If process requires use of these then care should be exercised to prevent them reaching silicone DP oil. 

13.  In processes like PVD, coatings requiring plasma or transparent metalizing, gas load can cause vacuum of diffusion pump to fall below 10^-1Torr. This can result in back streaming of DP Oil or even stalling. This is preventable by reducing the process speed or increasing the capacity of backing pumps.

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Boron Nitride Powder: SV-BNP+ (Supervac Industries): Properties and FAQs

Q1) What is the particle size of SV-BNP+?

Answer: The average particle size of SV-BNP+ boron nitride powder is 5 micron.

Q2) Is it water soluble?

Answer: Yes, SV-BNP+ can readily dissolve in water to form a smooth lubricious paint-like liquid. Quantity of water can be as per the user’s requirement. Thereafter it can be applied with a paint brush, roller or sprayed.

Q3) Can it be mixed with a solvent?

Answer: Yes, SV-BNP+ can be dissolved in any solvent if so required.

Q4) What is the purity of SV-BNP+?

Answer: SV-BNP+ comes with the purity >99%. This makes it suitable not only for high-precision, high-technology fields, but also for advanced research purposes as well.

Q5) What is the crystal structure of SV-BNP+?

Answer: SV-BNP+ boron nitride powder’s crystal structure is hexagonal.

Q6) We need good quality boron nitride powder for sintering. Can SV-BNP+ withstand high temperatures?

Answer: SV-BNP+ powder can withstand temperatures till 1800 degree centigrade in an inert atmosphere or under vacuum without losing any of its properties.

Q7) We are a thermoplastic compound manufacturer. Is SV-BNP+ suitable for us?

Answer: Absolutely. SV-BNP+ is added to thermoplastic compounds for adding thermal conductivity. SV-BNP+ dissipates heat while retaining electrical insulation properties.

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Boiling Temperature of Diffusion Pump Oil Explained

A lot of confusion exists in the vacuum industry about certain things, for example-

·         What should the optimum temperature be set in diffusion pump heaters so as to get maximum level of vacuum in shortest possible time?

·         How can the temperature in diffusion pumps be set in such a way that the engineers are able to get maximum life from costly diffusion pump oil?

·         Is boiling temperature of silicone diffusion pump oil different from hydrocarbon diffusion pump oil?

Before attempting to understand optimum boiling temperature of diffusion pump oil, let us take a brief look about working of diffusion pump.

Since efficiency of diffusion pump depends solely on the working cycle of diffusion pump oil (diffusion pump has no moving parts) so this is evident that a higher diffusion pump temperature will mean a higher ultimate vacuum. Obviously a question comes to mind at this stage that why don’t we increase the boiling temperature to highest possible level. Answer to this is that increasing the temperature of diffusion pump oil involves financial implications (cost increase) because of the following reasons -

1.   Diffusion pump oil works in very difficult environment where there is vacuum and high temperature. So to increase the boiling temperature, superior (obviously costlier) oil will be required which is capable of withstanding these adverse conditions.

2.   At higher temperature O-rings and oil seals may give way so better quality sealing material is required (Costly again).

3.   For handling higher temperature, construction of diffusion pump also has to be stronger.

4.   For attaining higher temperature higher energy charges will be incurred.

5.   For obtaining higher temperature longer time is required so vacuum time increases.

Decision about boiling temperature is basically a balance between level of vacuum required and cost incurred. Our guideline in line with the established practice in the industry is as below -

S. No.	Vacuum Required	Temperature in Diffusion Pump	Supervac Products	Equivalent Products
1.	10-9 to 10-10 Torr	245oC	SV-SIGMA+	DC-705
				Hivac F5
2.	10-7 to 10-8 Torr	220oC	SV-SIGMA	DC-704
				CVC4
				Hivac F4
				Ulvoil D31
3.	10-5 to 10-6 Torr	180oC	SV-SIGMA FAST	DC-702

Some myths are prevalent in the industry about diffusion pump oils. These are clarified as under-

Myth no.1: Boiling temperature of hydrocarbon diffusion pump oil is different from boiling temperature of silicone diffusion pump oil- This myth is baseless as boiling temperature of both hydrocarbon and silicone diffusion pump oil is same. For example- Well known hydrocarbon diffusion pump oils like Supervac SV-DELTA, Lion S, Ulvoil D11 and CVP300, all have boiling temperature of 220^oC.

Myth no.2: My diffusion pump has been designed for hydrocarbon/silicone diffusion pump oil only- This is also false. You can use any diffusion pump oil (hydrocarbon or silicone) in any diffusion pump based on your requirement. For details about merits of hydrocarbon and silicone diffusion pump oil please read my blog post-http://supervacindustries.blogspot.in/2013/04/silicone-diffusion-pump-oil-vs.html

Myth no.3: My equipment manufacturer asks me to use only a particular brand of diffusion pump oil which is exorbitantly priced- Your equipment manufacturer is trying to fleece you. For details about equivalent vacuum oils manufactured by reputed and established manufacturers anywhere in the world please read our blog post at- http://supervacindustries.blogspot.in/2014/03/vacuum-oil-equivalents.html. In case you don’t find it there then please contact us. We will help you find an equivalent product.

Myth no.4: There is no need of topping up- Absolutely false and dangerous myth prevailing in the industry. Topping up at regular intervals is of utmost importance to get proper vacuum and avoiding damage to the equipment.

Myth no.5: Charging less diffusion pump oil in diffusion pump reduces vacuum time- Absolutely false. Charging less oil than required will cause back streaming, burning of diffusion pump oil and damage to costly equipment.

Myth no.6: My diffusion pump oil works at 170^oC- Diffusion pumps do not work at temperature below 180^oC. Your oil supplier is supplying you cheaper vapor jet booster pump oil.

Conclusion:-

Based on the information given above I am sure that you will be able to choose the right diffusion pump oil as per your requirement. For example- if level of vacuum required is only up to 10^-5 Torr and equipment is old then hydrocarbon diffusion pump oil will work more economical. Similarly if vacuum level required is 10^-7 Torr then there is no point in using SV-SIGMA+. Furthermore by using simple precautions like frequent topping up you can get a much longer life and value for money from your diffusion pump oil.

For more helpful tips about how to get your money’s worth from your diffusion pump oil, you can read my blog post- http://supervacindustries.blogspot.in/2013/09/how-to-get-maximum-life-from-your.html

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