Preventive Maintenance or Precision Maintenance?

Have you ever pondered the term of "preventative" maintenance?  Well, I have often done so and must admit that I had to laugh when I pictured how this was supposed to actually work in a plant setting.  It conjured up visions of maintenance people getting a "call" from the machine controls in question and telling them to come in soon as one of the bearing will fail next Sunday night at 4:05am.  While we are closer than ever in diagnosing machinery problems and averting the worst from happening, we are not able to be able to exactly "predicting" when the usable time of a piece of equipment will come to an end.  Precision maintenance is when you identify, prioritize and implement a condition monitoring philosophy that is trending based and is designed provide a review process for any process and incident occurrence.    

Trending is the key to success with precision maintenance.  At Aerzen USA we make good use of state of the art condition monitoring devices that fit any budget.  From low cost vibration sensor based switches to full blown online monitoring systems ranging from IFM Efector's VKS, Octavis and Multiplex devices we have the trending system that will fit anyone's budget. 

 

Any of the devices will at least alarm the user of trouble and also at the same time provide the output of external trending.  The more sophisticated Octavis and Multiplex units have their own built-in internal trending software, which is set up at the factory using our own expert information.  It can eliminate any blowers and compressors from any existing regular vibration analysis routes as the units are constantly monitoring the equipment in real time and are set up with their own alarm and shut down levels.  Customers with real critical pieces of equipment might even choose the Ethernet based connection that allows customer SCADA or DCS integration. 

In any system that you choose, make sure that you do not implement without prior planning session as well as planning to review your results.  Also, keep in mind that results are only as good as the accuracy of the information that goes into the monitoring equipment.  Then always focus on trending and only fix what needs fixing.  That is what I call Precision Maintenance.  One reading alone may not mean anything; or would you only consult one doctor before you undergo open heart surgery because of a single high blood pressure reading? 

Please drop me a note in case you have suggestions or questions.   

Aerzen USA
Ralf Weiser
rweiser@aerzenusa.com

Lubrication mix-up

Is it not amazing how many different lubricants there are in a industrial or even a municipal waste water treatment plant?  At the outset the problem is not all that challenging.  Lubrication or maintenance managers usually start by compiling a complete list of machines and the lubricants they need as per their respective operation manuals.  So far, so good.  Than it starts to get complicated, because maintenance techs, operations people or lubrication crews need to do the actual work and more often than we realize they need to make their own mental notes to remind themselves which machines takes what lubricant.  Sometimes people use permanent markers or other devices and leave themselves scribble notes directly on the machines themselves.  How many times has that caused any machinery trouble at your plant? 

There are many solutions that can help avert such issues.  One effective one comes from the Lean Manufacturing program called 5S in which - among other important aspects - lets you sort and mark all items you need to do your job with.  This is perfect for lubricants.  Visual signals such as color coding bins, drums and cans and using the same colors on markings on the actual equipment will make anyone's life a lot easier and also help you reach the maximum service life of your equipment. 

Kick it up a notch with making a list of the lubricant, where it goes, how often it needs to be changed and checked and how much you need on a label of suitable size and then laminate it.  Post it next or on the machines and voila, you are on the way to using best practices in the industry.  Track your success though as most managers tend to overlook when things are going well. 

Aerzen USA offers a built-in benefit when it ships products to their final destinations.  All standard air and neutral gas machines are shipped with a Polyalphaolefin oil from one of the premium brands in North America.  It is fully synthetic and there is only one brand and a total of one standard viscosity class for each blower and compressor.  The end user does not have to scramble for oil at start up, but the lubrication expert on site has some time to place the machine on his lube oil chart and can even take time to color code it and place a laminated sign on it. 

Does this help you make your working life easier?  As always, I am looking for your feedback. 

Ralf Weiser
Aerzen USA Technical Manager
rweiser@aerzenusa.com

Oil-Free Screw Compressors – An introduction to Dry Gas Sealing Systems

Oil-free compressors are most often used in process gas applications. Hazardous, poisonous or simply dangerous gases are usually being compressed and conveyed by our screw compressors. It is critical therefore, that such conveying media be sealed against leaks to the atmosphere or, in the case of gases incompatible with lubrication oils, against leaks into the compressor's lubrication system. Dry gas sealing systems are often applied for such tasks.

Dry Gas Seals

Dry gas seals are mechanical face seals. They consist of a stationary (primary) ring and a mating, rotating ring. During operation, grooves in the mating ring generate fluid-induced dynamic forces, causing the primary ring to separate from the mating ring, thus creating a gap between them. A typical value of the running gap between the primary and mating rings is 3 to 4 microns. A sealing gas in injected into the seal, providing the seal between the atmosphere (or the oil system of the compressor) and the compressor's conveying chamber.  Typically, a labyrinth seal separates the gas seal from the process gas, while a barrier seal, also a labyrinth seal, separates the gas seal from the compressor's bearings and their oil lubrication system. Barrier seals are buffered with gas - typically nitrogen, at a fixed pressure, and vented to the same vent as the seal gas exiting the dry gas seals.

The pressure of the seal gas is controlled, such that it is slightly higher (typically 0.3 bar, 5 psi) than the process gas pressure. Thus, part of the seal gas volume will leak into the process gas, while part of the barrier gas volume will leak into the compressor's bearings and then into the lubrication system, eventually reaching the oil tank. A vent is provided, such that the excess seal and barrier gases can be escape form the seal, insuring a continuous flow.

Configuration

Dry gas seals are available in a variety of configurations. The "tandem" style is typically applied in process gas service. Tandem seals consist of a primary and secondary seal contained within a single cartridge. During normal operation, the primary seal assumes the sealing task entirely, while the secondary seal acts as a backup. Separate vents for the primary and the secondary seals are provided, with the barrier seal gas being vented by the secondary seal vent. Double gas seals are also utilized, with the seals mounted in a back-to-back configuration. Seal gas flows equally through both seals, with the seal gas leakage from inboard seal flowing into the process gas, while seal gas leakage from the outboard seal flows into a vented cavity, which also insures the venting of the barrier gas.

Seal Gas Systems

Seal and barrier gas systems are specially designed to supply gas, at the correct pressure and flow, to the dry gas seals. These systems are generally panel-mounted and installed adjacent to the compressor skid. Frequently, seal gas supply systems are designed to regulate the differential pressure between the seal and process gas. Differential pressure control valves are used for this purpose, with flow transmitters providing a seal gas flow indication. An improvement over this system consists of flow transmitters that provide feedback for the differential pressure regulators. Thus, the differential pressure between the seal and process gases is varied, such that the flow of seal gas is tightly controlled.

Important components of seal gas systems are the seal and barrier gas filters. They insure gas cleanliness, which is the most important factor in maintaining the gas free of foreign material. Ingression of such material into the running gap of the seals leads to degrade sealing performance (increased gas leakage) and eventual failure of the seal. Seal gas must be dry and free of particles 3 micron (absolute) or larger.

Gas Seal Contamination

There are three sources of seal gas contamination:

• Gas supply - gas filters, installed as part of seal gas systems, are tasked with removing this type of contamination
• Process gas contamination - incorrect control of the differential pressure between the seal gas and the process gas will lead to process gas coming in contact with the seal ring faces.
• Contamination originating in the bearing lubrication oil - an eventual malfunction of the barrier gas would allow lubrication oil to come in contact with the dry gas seal

Operation and Maintenance Considerations

Generally, experience shows that operation of the compressor under transient conditions greatly increases the opportunity for seal gas contamination. Operation of the compressor at idle conditions should be avoided if the process gas is used a source for seal gas supply.  It is highly recommended that seal gas be supplied from alternate sources.

Good maintenance practices must always be followed. Seal gas filters should be equipped with differential filter transmitters, or switches, that would give a good indication of the filter fouling.  A good seal gas system should always include low point piping drains. The drains have to be monitored and opened on a regularly scheduled basis. Tracking and trending of the differential pressure between the process and seal gases, together with the seal gas flow to the seals are good means to establish performance benchmarks and detect eventual deterioration.

Radu Valasutean
Process Equipment Engineer
rvalasutean@aerzenusa.com

Considerations when specifying and purchasing a screw compressor

Buyers have many alternatives when considering which style of compressor is most appropriate for their process requirements. These can include:

1)  Type of compressor:  positive displacement or dynamic

2)  Type of compression:  oil free or oil injected

3)  Construction of compressor:  gastight, hazardous gas or atmosphere, compressor metallurgies, instrumentation, motor type and manufacturer

4)  Operation of compressor:  variable ambient conditions, variable flow and/or pressure requirements, variable properties of the air or gas

Type of compressor is the first and most important selection criteria.  A dynamic compressor will typically provide for small fluctuations in flow and discharge pressure through the use of guide vanes and/or throttle valves.  These compressors operate in-between choke and surge regions and the vanes/valves help the compressor to run at its most optimal point.  With the use of VFD the flow/pressure curve can be shifted on the performance curve to provide some variability.  

These compressors are well suited for applications in which the flow/pressure doesn't vary more than 30%, or the ambient conditions or air/gas composition doesn't exhibit swings of more than 30%.  The positive displacement compressor will provide a relatively constant flow regardless of what the pressure requirement is.  With the use of a VFD this machine can provide for many applications with varying flow and pressure requirements.

The positive displacement is typically simpler than a dynamic compressor since it does not require the use of guide vanes, throttle valves, or sophisticated instrumentation.  Finally, the buyer will need to determine if the compressor will need to be 100% oil free or oil injected.  The oil injected machines can include oil separation and filtration to lower the oil content in the gas down to a few ppm.  Some applications can handle this while others cannot, especially if the downstream oil accumulation over time is detrimental to the process.

In addition to the type of compressor, the buyer will need to consider the installed environment and duty cycle of the compressor.  Consideration may need to be given to gastight components or special construction for hazardous or classified areas.  They buyer may also want to specify plant standard instrumentation, motors, etc.  Also, care must be given to changing conditions or the air or gas that is compressed.  Can the air/gas be dirty?  If so, some compressors (usually dynamic types) are very sensitive to dirt or solids in the air or gas stream.  This could include moisture or humidity of the air or gas.  

Is the compressor installed outdoors?  If so, can it handle the extremes from winter to summer?  This not only pertains to the temperature that the compressor is operating in and how it affects the lubrication and metallurgy of the parts, but also to how the air or gas properties will change.  The buyer will need to consider changing environmental conditions as well as changing process requirements to determine if the compressor selected can handle the wide array of performance points.

George Hubbard
ghubbard@aerzenusa.com

Applying the proper flare gas compression technology

 The refinery flare gas can be considered one of the most challenging, but it can be handled with the right compressor.

So how can we compress flare gas from a refinery?

Several types of compressors are being used. 

Centrifugal compressors should not be recommended in refinery flare gas applications because they cannot easily deal with wide swings of mole weight and are sensitive to any solids or liquids entrained by the gas stream. 

Positive displacement machines are best suited for these conditions because they can handle easily wide changes in the composition of the gas.  I know of five types of mechanical compressors being used, more of less successfully in flare gas applications:

• - Sliding vane compressors
• - Liquid ring compressors
• - Reciprocating piston compressors
• - Oil-flooded screw compressors
• - Oil-free screw compressors

Each of these technologies has its place and can be successful when applied correctly. 

A few pros and cons of each of these types of machines:

Sliding vane compressors are simple compressors with a single shaft off-center in a cylindrical housing.  They work best in low pressure clean gas applications.  They are not very reliable when dust or liquid is ingested in the machine and prevent the vanes from sliding back-and-forth in the rotor slots.  Jacket-cooling is required and oil needs to be injected continuously for lubricating the vanes that slide along the housing.  They may require relatively frequent overhauls, but these are relatively easy to handle. Initial costs are low.

Like the sliding vane compressors, the liquid ring compressors are also simple machines. The principle of operation is very similar to that of sliding vane compressors with the exception that a ring of liquid, generally water, is used instead of vanes that slide radially.  Liquid ring compressors are therefore well suited for wet gas, dirty gas applications.  The gas maintains a low temperature during compression as the heat is absorbed by the liquid.  These compressors are available in a variety of materials to handle a wide range of gases.  As compressors, these machines are not very energy efficient, as it takes power to maintain the liquid ring in proper motion under all operating conditions. Moreover, the liquid will scrub and cool the gas.  This requires the liquid to be treated, before being discharged, and also cooled, filtered if it is used in a closed loop.  Of course, large amounts of water are not available everywhere and special measures may have to be taken in cold climate.

Reciprocating Piston compressors are well known and very efficient machines that can handle high compression ratios and high discharge pressures.  Piston compressors exert alternating forces on supports and foundations and produce pulses in the gas stream that need to be attenuated in pulsation dampeners. Valves and piston rings are the components that are most sensitive to solid or liquid contaminants and do nor handle well gases that may polymerize in an accelerated manner.

Oil-flooded screw compressors are compact rotary positive displacement machines. They do not require heavy foundations.  One rotor drives the other and oil is injected in the rotor chamber to lubricate the rotors, remove the heat of compression and seal the gap between the rotors and between rotors and housing.  These machines are very simple, can reach in a single stage high compression ratios and achieve very good efficiency levels. The challenge is that the oil will scrub the gas of its contaminants and the oil may react with the gas.  As a result, the oil may loose its lubricating properties, acid could form, and in some cases a gel like substances may develop.  Therefore, precautions must be taken to avoid frequent oil changes, maintenance and repairs. An oil-flooded compressor therefore cannot be recommended in all applications.

The one machine that finds itself "at the intersection" of all the previously mentioned ones is the oil-free dry screw process gas compressor. Unfortunately, its initial costs may be the highest probably in the range of reciprocating compressor solution.  However, the higher investment may be worth it because it combines the advantages of the above mentioned compressors without combining their individual disadvantages.

• The oil-free compressor is a rotary positive displacement machine that handles wide swings of mole weight and gas composition
• It does not require special foundations
• The process gas side of the machine is completely separated from the oil side by 4 internal seals. Therefore, the oil cannot be contaminated by the gas.
• The compressor can handle contaminants: solids and liquids and water or solvents can be injected intermittently or continually for cleaning and for cooling if required.
• The rotors are almost naturally self-cleaning, however, liquid injection may be required to remove substances that may stick to the rotors or the casing and accumulate in the inlet or discharge channels.

What is Aerzen's experience in this field?

Aerzen began supplying compressors for flare gas recovery to clients in Europe already some forty years ago. The machines for this application have been in the range between 500 cfm and 5000 cfm (15 to 150 m3/min) and discharge pressures from 30 to 190 psi (2 to 13 bar g) with the bulk of them around 90 to 100 psig (6 to 7 bar g).  These machines are either single-stage or two-stage compressors with inter-cooling to achieve best efficiency.  However, single stage machines are also used and may require constant water injection.  The water flow requires varies of course with the gas, temperature, pressure and materials used.  The amount of water is very small: in this particular application, typically less than 1 GPM / 1000 cfm of gas.

The oldest installations in North America are in Canada.  In the early 1980s, several refineries, were equipped with Aerzen dry screw compressors for the compression of flare gas.  These have been compressing 4000 cfm of gas to 95 psia.

Pierre Noack - President
Aerzen USA
pnoack@aerzenusa.com

Things to consider when making blower purchasing decisions

Sizing - Proper sizing of the blower begins when the buyer provides performance requirements to a blower manufacturer. The buyer should understand the differences between scfm (volumes at standard conditions) and icfm/acfm (volumes at intake or actual conditions).  Knowing the ambient conditions (elevations and temperature ranges) and the required operating pressures which the blower must produce will also effect the manufacturer's selection.

Performance - Only buy Aerzen's blower packages... just kidding, but there is a serious point here.  Any manufacturer's claims of performance need to be verified to insure the correct amount of pressure will be generated for the application.  Insist on knowing the performance and efficiencies (power requirements) of the complete blower package you buy not just the performance of the bare stage.  An undersized drive motor will limit performance.

Package design - Buy a complete package from the manufacturer when possible.  A package assembled by others can consist of mismatched components that could have a negative impact on the blower's performance, and ultimately, its warranty.

Pricing - I put this last, because it is the least important component in the blower selection process.  A decision based solely on a lower up-front capital cost can end up costing much more in the long run when cost of ownership is evaluated and the inappropriate, and possibly, less reliable equipment was selected.

Steve Lungstrum
slungstrum@aerzenusa.com

Top questions to ask when buying a blower for a Pneumatic Conveying System

Are you ready to buy a blower for a pneumatic conveying system? Look out for the following:

• Is the blower sized for the correct amount of air? Too much air will lead to higher pressure drops and increased power consumption. Too little air will result in line plugging.
• Is the blower and motor sized with enough safety to overcome upset conditions?
• When comparing blowers from various manufacturers, did you look at:
• Reliability
• Efficiency
• Noise
• Maintenance requirements
• If you are conveying food products, you also want to make sure that no particles of the silencer absorption material and no lubrication oil can enter the compressed air stream and thus mix with the conveyed product.

Feel free to comment or contact me to discuss.

Karl Mueller - Industrial Specialist
Aerzen USA
(770) 951-7035
kmueller@aerzenusa.com