12 replies to this topic

[rajarshi2016]

Hi All,

 

My query is as follows:

 

There is a dry screw compressor for off-gas service. The specs that we provided are as follows:

 

Flow: 7500 Nm3/hr, Suction pressure: 4.1 kg/cm2a, Suction temp: 44 degC, Discharge pressure: 9.1 kg/cm2a.

 

Composition (mol%): H2: 31 , C1: 2, C2: 2.5, C3: 9, i-C4: 11, n-C4: 28, C6+: 10, H2S: 5.3, H2O: 1.3 and other trace components.

 

Now, in the plant, the H2 content in the gas is more than design (~ 42 mol%) resulting in a decrease in MW (29 actual vs. 38 in design).

 

Under such circumstances, the compressor vendor is refusing to start the machine telling that it might damage the machine. However, we have not received any specific input from the vendor, like what parts can get damaged and why?

 

We have very limited knowledge on screw compressor and so would like to know the following:

 

1. What kind of damage (if any) is expected in a dry screw compressor for this type of variation in MW?

 

2. In general, what is the percentage variation in MW that a dry screw compressor can usually handle without any major problem?

 

Any input will be highly appreciated.

 

Regards,

Rajarshi

[breizh]

Hi ,

 

Consider this resource to support your work . Anyway it's good to wait for the vendor's detailed reply .

 

Hope this helps.

 

Breizh

[Erwin APRIANDI]

Hi rajarshi2016,

 

The change of molecular weight will affect lots of things in any compressor of any type, as it will change the actual volumetric flow, power, and also the thermodynamics of the process.

 

However, if your compressor is equipped with variable speed drive usually the vendor will try to handle it and try to commissioned the compressor by starting it at lower speed. but remember as your inlet composition changes the process guarantee from the vendor are void already.

[Bobby Strain]

I think your vendor rep sent for startup doesn't know enough to advise you. The machine is positive displacement, so the only effect is that lower power will be used in compression. And you can easily calculate the discharge temperature with the actual gas. If you don't have the tools, post the complete information and someone will surely respond. 

 

Make sure your vendor gives a prompt response and give a warranty based on the actual composition. Don't let them just walk away from their obligations.

 

Bobby

[Art Montemayor]

Rajarshi:

 

I've dealt with screw compressors many times in the past - both in design applications and in operation.  I've experience with both the dry and the oil-flooded type.

 

The screw compressor is actually an old concept and design.  However, the first design was the dry type and it was fabricated with machining expertise of the years 1930 - 1960.  This design was a dismal failure in the industrial gas industry until the english licensor of the swedish design developed the oil-flooded version in the 1970 -1980 era.  This, and the sudden increase in machining precision and much-improved fabrication techniques and instrumentation allowed the fabrication of screws with tolerable "slippage" and efficiency.  The screw has replaced a lot of reciprocating machines since the 1980's - but the design that has done this is the oil-flooded design - not the dry screw.

My experience with the dry screw was a sad one and I had to resort to oil-flooded applications - most of these in the compressed gas industry and in industrial refrigeration packaged units.  I would suspect that your compressor vendor is not communicating correctly.  The fear of excessive hydrogen gas slippage is probably what he/she is trying to express - besides the higher tip speed needed for lighter gases like hydrogen.  I personally would never use a dry screw in hydrogen-rich gas service.  I would apply an oil-flooded type.

 

One thing you must accept:  you cannot expect to apply a variety of different gas compositions to an existing screw compressor - especially a dry one.  A screw machine is a wonderful device when applied correctly; and by correctly, I mean that you must supply the manufacturer with the correct gas composition and conditions prior to selecting the type of machine.  I would never resort to using the VENDOR of the machine as my correct source of compressor information.  I would deal  ONLY WITH THE MANUFACTURER on technical issues.  The interest, obligation, and knowledge of the vendor is over and done with once the sales invoice is paid.

[manojkaila]

Dear Raj,

 

Involve top management & give assurance to vendor. We can not change the fuel gas composition very fast and same time keep it in flare is also painful for management as a loss. We need to take some calculated risk with involvement of vendor. There are very few vendor's are their who will be ready to take risk on their system. Many time this vendor representative are also out soured.

 

Let me share with one of very reputed compressor vendor we did almost 50 trial / Trip before successful start.

 

Good Luck

 

Manoj 

[rajarshi2016]

Hi All,

 

Firstly, thanks a lot for your valuable inputs.

 

We (licencor) have received a reply from the compressor manufacturer (not directly, but via client), which is more or less as follows:

 

• due to the lower MW of gas the compressor may be overheated and the gap between the screw and shell will become smaller and consequently the screw will get stuck. 

Is this explanation a justified one? 

 

Another thing that I would like to know in this regard is as follows:

 

Dry screw machine was selected because we wanted to prevent dissolving of light hydrocarbons into the oil of the oil-flooded machine. Is there actually any chance that light hydrocarbons will get dissolved in the oil? Or was dry-screw compressor a wrong choice?

As far as I know, synthetic oil especially formulated for this type of service is used in oil-flooded machines and there is no chance of a hazardous mixture formulating with the oil.

 

@ Erwin: There is no VFD. Capacity is controlled via recycle line.

 

@ Art: Thanks a lot for the elaborate explanation. However, by vendor, I actually meant the compressor manufacturer. Sorry for any confusion. 

 

Awaiting your replies.

 

Regards,

Rajarshi

[manojkaila]

Dear Rajarshi,

 

Your compressor manufacturer is right for the temp rise. But do he has clear value of how much temp will increase and what is the safe limit. What is other alternative for you ? May be he want you to purchase some new model with different requirement.

 

My suggestion take small trial run with actual condition and watch the performance. If you get amp start increase stop machine in safe mode. What about manufacturer simulation with different feed properties if they have some reliable model.

 

Good Luck

 

Manoj 

[Art Montemayor]

Rajarshi:

 

The nature of your beast involves an inherent, built-in slippage on the rotors.  The level - or precision - of the manufacturing procedure used on that particular model and the precision of the timing gears is what normally fixes that feature on a dry screw compressor.  It is something you cannot change or modify in the field - and most times, in the factory.  Your dry screw is like a reciprocating compressor without any piston rings - with only a labyrinth seal, like that used on Sulzer reciprocating compressors of that design: you have a certain % slippage (or leakage in the rotors) that is being "recycled" within the machine (or "re-compressed").  This is typical of all dry screws and why I prefer to stay away from them.

 

The only way you are going to have a "seal" (or analogous piston ring) on the moving rotors in a screw compressor is by flooding the rotors with a lubricating fluid - such as an oil.  This contributes 3 things to the screw:

1. It helps to seal the necessary clearances between the rotors (you can never have rotors touching rotors);
2. The circulating oil absorbs some of the heat of compression, making the machine much better and quasi-isothermal;
3. The oil also lubricates the surfaces of the rotors for better operation.

Common sense tells us that the smaller the molecule (the "lighter" the gas), the more the slippage between rotors.  Hydrogen being one of the smallest molecules in existence, always tends to leak or escape from containment - and this is what happens in a dry screw compressor.  Without a positive means of containing the lighter gas between the rotors, you are going to suffer more slippage AND subsequent RECOMPRESSION as the gas tries to move down the path of the rotating rotors.  This recompression effect is what raises the temperature of the traveling gas and causes excess heat in the chamber & rotors, which leads to thermal expansion of the metal components and lots of mechanical troubles - remember, I said the rotors cannot touch each other???  Well that can certainly occur in an over-heated screw and the consequences are mechanical destruction.  I believe this is what your manufacturer is trying to tell you.

 

I have tried to explain what happens in a dry screw compressor in order to facilitate your understanding of the machine and the process it is being applied to.  That is why it is so important to fully understand the basic principles of isentropic gas compression and the limitations and trade offs that are inherent in each type of compressor.  I hope I have helped.

[Hassan seif]

Dear Rajarshi,

MW change of-course will increase discharge temperature of the compressor i have simulated that roughly using N2 and Co2 (MW is 28 & 44) so actually that has molecular weight difference more than you got and it shows temperature rise of 33 C 

ok in your case that temperature rise will be less than 33 of-course beside you can do the following

1-try to lower the suction temperature and contact the compressor vendor again

2-i have overhauled alot of screw compressor (most of the time oil flooded) anyway there is nothing the MW will do to the rotor clearances

that gap they are talking about is adjusted during compressor assembly and it never change (only when wear or some parts are changed then you have to re adjust) so actually gap between screw and shell should be the same with different MW !!!

 

 

Art Montemayor 

nice explanation but i think that manufacturer should be able to change gaps in the compressor after MW change to prevent the compressor getting overheated then people in field will have to disassemble the compressor and adjust the gap as required 

 

regards

Edited by Hassan seif, 12 May 2016 - 09:20 AM.

[Art Montemayor]

Hassan:

 

Sorry to burst your simulation bubble, but you don't find out the temperature existing inside a screw compressor by running a computer program.  The clearances existing in the machine have a lot to do with the ultimate results.  And these clearances CHANGE once the machine reaches a stable, steady operation.  You or I are not privy to knowing what those characteristics are in a screw compressor - only the manufacturer.  So the value of your computer printout on the discharge temperature means nothing.  The real temperatures are a result of actual, hands-on, operation of the machine with the actual gas composition on a steady state basis.  No computer can accurately simulate that reality.

 

The "gap" you refer to is the end gap in the rotors.  There is another clearance that is even more significant: the clearance existing between the rotors - which you do not address.  THAT is the clearance that is affected by flooding with oil.  This clearance is fixed by the degree of the precision that is obtained in the machining of the different components: gears, casing, and rotors.  The critical machine alignment also plays a critical part.  All these factors lead to an acceptable limit on the minimum clearance obtained.

 

There is a mechanical fact existing here that hasn't been acknowledged: there is no positive, rotor-to-rotor seal or a rotor-to-casing seal existing in a "dry" screw compressor.  There can't be - by simple understanding of how the compressor is expected to compress the gas involved.  There will always be slippage in a dry screw compressor.  The "heavier" the gas (higher molecular weight), the LESS the slippage.  But there will always be slippage.  There is yet today no physical or mechanical way to eliminate the required clearance between metal parts in a positive displacement compressor.  Only two efforts are known by me that attack the problem:

• the Sulzer Brothers in Switzerland invented the labyrinth piston to reduce the slippage in a reciprocating compressor, but they have to rely on a certain amount still slipping and being recompressed and the fact that they require extra-precision in their machining of the critical cylinder and piston components.
• The famous Nash Hytor Liquid Ring compressor attacked the problem with a liquid piston.

The requirement of segregating the metal compressing components exist in the manufacture of a screw compressor - whether dry or flooded.  A dry model naturally requires extra precision and very precise assembly, alignment, installation, and vibration during operation.  Naturally, in a dry screw, these requirements are much more important than those for a flooded model.  Lighter gases normally require higher tip speeds in a screw compressor.  That, I believe is published data.

 

There is inherent re-compression occurring in a dry screw - much more than that in a flooded one.  That is why the original specifications for the application should be done very carefully and with full knowledge of the trade offs.  That re-compression is what causes rapidly increasing temperatures - much faster in some cases than can be process-controlled.  The resulting thermal expansion of the various parts also occurs too fast to react in time.  The recommendation to operate the machine and then stop it safely when temperatures get too high or "rubbing" occurs would be action taken too late to save the machine, in my opinion.  I certainly would not recommend such an experiment.

[Art Montemayor]

All:

 

The following is a follow-up to what I've already stated.  Although it cites what I already learned over 50 years ago while working in the compressed gas industry, it reinforces my comments:

 

Source: COMPRESSOR HANDBOOK; Paul C. Hanlon, Editor.  McGraw-Hill Publishers
Derek Woolatt; Manager, Valve and Regulator Engineering; Dreser-Rand Company & (Screw Compressor Section)
Fred Heidrich; Dresser-Rand Company

 

"Leakage Effects
The leakage paths for a screw compressor differ from those for a reciprocating compressor in that instead of valves, rings and packing, a screw compressor relies solely on tight running clearances to establish sealing.  In a screw compressor, the most significant leakage occurs at

1. The interaction between the meshing rotors
2. The clearance between the rotors and the compressor housing

The impact of these leak paths must be determined to accurately predict screw compressor performance.  The leakage is reduced in an oil- or water-flooded compressor by the sealing effect of the liquid.

 

In a screw machine, compression is the result of two rotating rotors meshing, thereby reducing a volume in the groove of one rotor as part of the other rotor moves in the groove.  Tight clearances must be used to maintain the increased pressure in the groove. If the clearance between the two rotors is large, pressurized gas will flow back to a low pressure zone.  In a typical screw design, this usually means back to suction. This loss will cause a decrease in volumetric efficiency or capacity.

 

The potential leakage between the rotors and the housing is harder to quantify in terms of performance loss.  The main rotor lobe has two edges, the leading and trailing edges.  The leading edge faces the discharge pressure zone, while the trailing edge faces the suction pressure zone.  Leakage occurs from the leading edge to the trailing edge.  This means that during the compression cycle, the gas in the groove can leak back to the next groove in the screw since it is at lower pressure, and receive higher pressure gas from the groove that precedes it.  These two effects do not cancel each other out.  They are the direct effect of the running clearance between the rotor and the housing.  The performance effects are two-fold.  Since gas is constantly being transferred from one groove to another, this becomes a fixed flow loss.  Similarly, since the machine needs to re-compress already worked gas, this becomes a fixed power loss (& temperature increase).  It is important to determine the effect of these leakages to determine the overall efficiency of the unit."

[rajarshi2016]

Hi All,

 

Thanks a lot for all the inputs.

 

@ Art: Thank you very much for the very detailed explanation. It is much appreciated.

 

I will let you know in due course what we will finally do; Though that will definitely take some time. But I will surely let you know.

 

Regards,

Rajarshi