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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Know All About Diffusion Pump Part-3

 

Q.21 What is the alternative of CVC4 diffusion pump oil.

Ans. Supervac Industries, New Delhi manufactures silicone diffusion pump oil SV-SIGMA. This oil is a perfect alternative of CVC4. Infact, SV-SIGMA has a much larger satisfied customer base and has been in use much before CVC4 started getting promoted.

 

Q.22 Can somebody supply me DC-702/DC-704/DC-705 diffusion pump oil?

 

 

Ans. Dow Corning the company that manufactured DC-702/DC-704/DC-705 has stopped production of this oil since last 2 years as such this oil is not available. Supervac Industries a leading supplier of vacuum oils worldwide supplies the following-

1) SV-SIGMA FAST which is an alternative of DC-702.

2) SV-SIGMA which is an alternative of DC-704.

3) SV-SIGMA PLUS which is an alternative of DC-705.

 

Even when DC-702/DC-704/DC-705 was available a large number of customers were using SV-SIGMA, SV-SIGMA+ and SV-SIGMA FAST because of considerable technological and cost advantage.

 

Q.23 Do I need to clean jets of my diffusion pump, every time I change diffusion pump oil in it?

Ans. If you are using good quality silicone diffusion pump oil then you need not waste precious machine time in cleaning of jets every time you change diffusion pump oil. Cleaning them once in about 5 to 7 years is enough.

Q.24 I am using silicone diffusion pump oil. Can I top up with hydrocarbon diffusion pump oil?

Ans. No, Silicone diffusion pump oil should never be mixed with hydrocarbon diffusion pump oil. So if you are using silicone diffusion pump oil then top up with silicone diffusion pump oil and if you are using hydrocarbon diffusion pump oil then top up with hydrocarbon diffusion pump oil.

Q.25 What should be the basic properties of silicone diffusion pump oil?

Ans. Silicone diffusion pump oil should have very low vapor pressure, should have high thermal stability and should be resistant to oxidation. These are very basic properties of silicone diffusion pump oil.

 

Q.26 What should the be basic properties of Hydrocarbon diffusion pump oil?

 

Supervac Hydrocarbon Diffusion Pump Oil

 

Ans. Hydrocarbon D.P. Oil should have very low vapor pressure, should have high thermal stability and should be resistant to oxidation. These are very basic properties of hydrocarbon D.P. oil. Apart from these qualities, hydrocarbon diffusion pump oil should not have aromatic compounds and artificial additives as these may turn to gel form or black tar like substance and harm the pump interiors.

Q.27 What is Holding Pump?

Ans. Holding pump is a simple rotary vacuum pump of small capacity. Its function is to create holding vacuum for diffusion pump, hence the name Holding Pump. It is invariably connected to diffusion pump and not to the vacuum chamber.

 

Q.28 Can I use diffusion pump without using a rotary pump in my vacuum system?

Ans. No, Diffusion pump cannot be used alone. Reasons are explained below.

A diffusion pump merely creates a vacuum gradient along its length where vacuum in top portion connected to vacuum chamber is very high and vacuum at the bottom of diffusion pump is low. Since even this low vacuum is better than atmospheric pressure so diffusion pump cannot discharge in atmosphere. Rotary pump therefore is added to evacuate the air from bottom portion of diffusion pump.

Q.29 What is stalling of Diffusion Pump?

Ans. A Diffusion pump works on the principle of gas diffusion. This pump requires Diffusion Pump Oil, which evaporates, gets compressed and then escapes through jets, trapping air on its path down to the bottom of Diffusion Pump and from there the air is evacuated by a Rotary Pump or a Booster Pump-Rotary Pump combination.

For this arrangement to work, vacuum of minimum 10^-1Torr in required to be maintained in the main body of Diffusion Pump. Otherwise the Diffusion Pump Oil escaping from jets in gaseous form collides with too much air available and further raises pressure in Diffusion Pump body. This stops creation of vacuum and is called Stalling of Diffusion Pump.

Q.30 What is the work of baffles in a diffusion pump?

Baffle in diffusion pump

 

Ans. Baffles are provided at the top of diffusion pump. They stop diffusion pump oil from escaping into vacuum chamber. Baffles should be properly water cooled to perform their function effectively.

 

 

 

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Know All About Diffusion Pump Part-2

Q.11 How to prevent back streaming of diffusion pump oil?

Diffusion pump diagram 

Ans. To prevent back streaming of Diffusion Pump oil it must be ensured that-

1.     System is "Roughed out" before Diffusion Pump is put in line.

2.     Rotary vacuum pump is maintaining a vacuum of at least 10^-2 Torr.

3.     If process is causing release of a large amount of gas, then sometimes it     may be required to slow down the process.

4.     Cooling water output volume and temperature must be checked. If this is not as per the manufacturer's recommended values then descaling and increasing cooling water circulation is required.

5.     If problem of back streaming is frequent then increasing the capacity of backing pump is required to solve it.

6.     Diffusion pump must have a cryotrap or at least baffles on top to trap gasses trying to flow out of pump into chamber. 

7.     All the valves should be checked from time to time.

8.     Strange as it may sound, very low fore pressure can also cause back streaming.

Q.12 Why is diffusion pump water cooled?

Cooling coils around diffusion pump

Ans. A diffusion pump has no moving parts and therefore depends on diffusion pump oil for creation of vacuum by action of gas diffusion.

For diffusion to occur the diffusion pump oil is heated and vaporized using heaters. It is then released through jets at high speed and in this way it traps air by diffusion process. Then it is cooled at walls of diffusion pump. This liquefies the oil vapors and these vapors again turn to liquid state. This completes the cycle and the process repeats continuously.

Q.13 I have a Chinese diffusion pump (Not Varian) do I need to use some different kind of diffusion pump oil?

Ans. Diffusion pumps as well as Diffusion pump oils are made as per a standard engineering practice of interchangeability. So a good Diffusion pump oil will be suitable for any Diffusion pump made anywhere in the world.

Just like same petrol works for cars made by Honda, Suzuki, Audi, Toyota or any other brand.

Q.14 What is the difference between Vapor Jet Booster Pump (Ejector Pump) and Diffusion Pump?

Ans. Although vapor jet booster or ejector pump works on the same principle as diffusion pump but there are many differences, these are given below:--

1. The jet design of diffusion pump and vapor jet booster pump are different. In diffusion pump oil vapors are compressed to a higher compression ratio compared to ejector pump.

2. Because of higher compression ratio diffusion pump is capable of producing vacuum up to 1x10^-10 Torr. Where as an ejector pump cannot produce vacuum above 1x10^-5 Torr.

3. Vapor jet booster pump creates vacuum in a shorter time than diffusion pump.

4. In an ejector pump the working fluid i.e. vapors jet booster pump oil has lower boiling temperature compared to diffusion pump oil used in diffusion pump.

                                                                                                             

Q.15 When Should I check the diffusion pump oil level? In hot condition or cold condition?

Ans. Level of diffusion pump oil should always be checked when pump is cold. In hot condition some oil will be in vapor state so correct level will not be indicated.

Q.16 While changing diffusion pump oil should we wait till pump cools down or is this not required?

Ans. You should always wait for diffusion pump oil to cool down to room temperature before taking it out of diffusion pump. Diffusion pump oil when heated is at a temperature above 200^oC. Trying to take out oil at this temperature can be very dangerous.

Q.17 What is shelf life of silicone diffusion pump oil?

SV-SIGMA 

Ans. If stored properly, shelf life of Silicone diffusion pump oil is 2 years.

Q.18 Which oil is more economical to use hydrocarbon DP oil or silicone diffusion pump oil?

Ans. Under normal circumstances silicone diffusion pump oil works out cheaper because of its long life and no requirement of maintenance. However if plant is very old or process involves emission of corrosive gasses, then hydrocarbon oil may work out cheaper. 

To know more about this please click here.

Q.19 Why is it important to check level of diffusion pump oil regularly?

Ans. In case level of diffusion pump oil reduces then thermal equilibrium in diffusion pump is disturbed. This causes Diffusion Pump Oil to overheat, which results in oil loss, loss of vacuum and damage to diffusion pump in extreme cases. It is therefore important to check the level of diffusion pump oil regularly and top it up if required.

For more tips click here.

Q.20 Can I reuse diffusion pump oil?

Used diffusion pump oil

Ans. No, diffusion pump oil cannot be reused. This is because while creating vacuum diffusion pump oil is subjected to very high temperature and extreme vacuum. This destroys the molecular structure of oil which cannot be corrected by filtration or distillation.

About this read more here.

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Know All About Diffusion Pump Part-1

Q.1 How does a diffusion pump work?

Ans. A diffusion pump works on the principle of gas diffusion to create vacuum. In a diffusion pump, diffusion pump oil is heated to its boiling temperature. Vapor of this oil is compressed in a vertically tapering hollow cone as it moves up. Simultaneously this oil vapor escapes from jets along the height of hollow cone. Oil vapor escaping at great speed traps air and then collides with water cooled wall of Diffusion Pump and moves down. Trapped air is released at bottom of Diffusion Pump. This creates differential pressure in diffusion pump. Pressure at top is minimum (high Vacuum) and at bottom is maximum (low Vacuum) from bottom of diffusion pump the air is sucked by rotary pump.

Q.2 Is a diffusion pump made only in cylindrical shape?

Ans. Diffusion Pump has been successfully made in rectangular shape also. In fact rectangular shaped Diffusion Pump has an advantage that its inlet connected to Vacuum Chamber can be of larger area and Diffusion Pump itself can be adjusted to occupy less space. This also works with Diffusion Pump Oil as a working medium just like cylindrical Diffusion Pump.

Q.3 I understand that Turbo molecular pump (TMP) is an alternative to diffusion pump. Is it better or worse than diffusion pump?

Turbo Molecular Pump

Ans. Turbo molecular pump works on the principal of positive displacement. In this pump a series of inclined blades are mounted on a shaft which rotates at very high speed. These are called Rotors. Fixed blades or stators are fixed in such a way that Rotors and Stators alternate. Because of rotation of shaft and incline rotors push air down to next row of Stators. Stators too are inclined in same direction and push this air to next row of Rotors and so on to exit, where a Rotary Pump draws away the air.

Though this pump creates good vacuum and can be used in place of Diffusion Pump but its applications are limited to critical applications like semiconductor coatings. This is because of its high cost, more requirement of maintenance and the fact that since this pump cannot be made of large size because of mechanical considerations, a diffusion pump can be replaced only by a couple of Turbo molecular pumps. Diffusion Pump on the other hand has no moving parts and requires almost no maintenance other than Diffusion Pump Oil change, is cheaper in cost and can be built in almost any size.

Q.4 Why is topping up required for Diffusion Pump Oil?

Ans. In a Diffusion Pump, DP oil evaporates to produce diffusion action. Since oil is in vapor state so part of it is lost by leakage, some of this oil vapor is also sucked away by Rotary Pump. It is therefore important that lost diffusion pump oil is leveled up by topping.

Not topping up will result in overheating of the remaining oil and loss of oil as explained above will further speed up, causing a drop in vacuum. In extreme cases Hydrocarbon Diffusion Pump Oil may burn to tar like substance and Silicone diffusion Pump oil may turn into grey colored crystals.

Read about diffusion pump maintenance here.

Q.5 Why can a diffusion pump not work alone and requires a rotary pump as backing pump?

 

Belt Driven Rotary Vacuum Pump

Ans. A diffusion pump merely creates a vacuum gradient where top of diffusion pump has a higher vacuum and bottom of diffusion pump has much lower vacuum. A diffusion pump has no moving parts to force air out, so though lower portion of Diffusion pump has a low vacuum but since it is still better than atmospheric pressure so air will not flow out.

This lower portion of diffusion pump is connected to rotary vacuum pump. Since vacuum in lower portion of Diffusion Pump is lower than what Rotary Vacuum Pump Oil is able to create so Rotary Pump evacuates the diffusion pump and diffusion pump is able to work.

Q.6 How can I get more life from my expensive Diffusion Pump Oil?

Ans. Diffusion pump oil is one of the costly consumables of the metallizer. With a few precautions, its life can be prolonged and money saved.

1. Never open the pump when the oil is still hot. Exposing the hot oil to air changes its viscosity, making it thick gel-like.

2. Take care to top up the oil in the pump in a timely way so as to avoid accidental over heating of the oil.

3. Keep a timely watch on the pressure and heater gauges.

4. Back streaming of vapors from the vacuum chamber and/or Rotary pump letting its vapors in to the Diffusion pump are two reasons for the DP oil to get contaminated. This can be prevented by installing filters above the chevron baffle of the DP and on the line in between the rotary and the diffusion pump. These filters must be cleaned from time to time.

5. Silicone DP oil decomposes on coming in contact with even trace elements of alkali metals e.g., sodium, potassium, cesium etc. Do not allow them and/or their vapors to come in contact with the DP oil.

6. Keep a check on the water flow through the cooling lines for proper cooling of the hot oil inside the diffusion pump.

Read more on this topic here.

Q.7 Do I need to open & clean jets of my diffusion pump every time I change oil in it?

Ans. If you are using a good quality silicone diffusion pump oil then there is no need of cleaning jets every time you change oil cleaning be done once in 4 to 5 years. However if you are using hydrocarbon diffusion pump oil then cleaning is a must every time you change oil.

Q.8 Is DC-702/DC-704/DC-705 Diffusion pump oil still available? If not then what is the alternative?

Ans. DC-702/DC-704/DC-705 - these oils are no longer available in the entire world as the manufacturer Dow corning stopped the production of these oils more than a year back.

Supervac Industries supplies SV-SIGMA, SV-SIGMA+ and SV-SIGMA FAST as alternatives of DC-704, DC-705 and DC-702 respectively. These oils are being used by a large number of customers in more than 20 countries.

Q.9 I observed whitish deposits in my diffusion pump. What could this be? It is serious?

Silicone crystals

Ans. On overheating silicone diffusion pump oil turns into whitish grey silicone crystals.

Yes, this is serious. You need to check cooling for diffusion pump and also make sure that level of diffusion pump oil never falls below minimum level. This requires topping up from time to time. Also a faulty temperature gauge not allowing heater cut-off to take place, can be responsible for the oil over heating.

Q.10 What is back streaming in diffusion pump?

Ans. If the pressure is too high in the diffusion pump then the emerging diffusion pump oil vapor from the jets trying to move down collides with air molecules in the pump. This results in no vapor going down the pump. Contrary to this, the vapor itself starts rising up the pump along with the air and raises pressure instead of reducing it. This undesirable process is called “back streaming”.

Because of back streaming Diffusion Pump Oil can travel to vacuum chamber or can even be removed by Rotary Vacuum pump. If it enters Rotary Pump then it contaminates Rotary Vacuum Pump Oil also.

(In the next part-2 of this series I shall explain how to prevent back streaming.)

Written by: Anshuman Punj.   

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