The lull of the year has arrived. Hopefully for you that means a peaceful time between Christmas and New Years where we celebrate, spend time with loved one’s and finally get a little downtime.
I just want to take this opportunity to thank all of my clients who have allowed me to work with them, side by side throughout this year to achieve their goals.
We’ve covered several topics on preparing your compressed air system for cold weather and just wanted to leave you with a final thought on the topic today.
A great way to save money during the winter months is to capture the heat that your compressor is generating with a heat recovery system. Up to 90% of the heat generated by the heat of compression can be captured and utilized elsewhere in the plant with just a little planning and investment. These can include ambient heat for other areas of the plant, pre-heating boiler combustion air and other additional process heating requirements. Even if the temperature’s can’t be brought up to full operating requirements, utilizing the waste heat from the compressor can cut down on alternate heating costs.
System Preparation
It’s never too early or too late to plan and prepare for temperature changes. Especially the extremes of winter and summer. Put the maintenance and planning items on your calendar so you’re not caught off guard by rapidly changing conditions. Just to be sure you’re aware – follow me on twitter where we always announce when high and low temperature events are expected.
We previously discussed winterizing your compressed air system.. Now we continuing with additional cold weather tips.
Be cautious when working in cold weather. It really can be dangerous!
Weatherizing your compressed air system and facility is an important step when preparing for colder months to make sure your compressor continues operating efficiently.
Take the following steps to ensure your compressed air system and plant are prepared for the cold temperatures.
Repair weather stripping
Check weather stripping and replace areas that are worn. This includes outdoor piping or any equipment in poorly temperature controlled areas.
Inspect drains and air intake filters
Openings that are exposed to outdoor conditions can be problematic. Check your drains to assure proper operation and be sure to insulate an lines that carry condensation to area’s exposed to potential freezing conditions. A frozen line does not carry condensate very far and when it backs up into the compressor, catastrophic failures can occur.
Also, be sure that intake filters pulling air from the outdoors has proper rain / snow hoods in place. If they are left unprotected, snow or freezing rain can be drawn into the intake of the compressor and this will create a less than ideal situation.
Plan weatherization
Prepare your system biannually. Prepare your system for the high temperatures of summer and the low temperatures of winter each spring and fall. Regular planned maintenance for your compressed air (and other systems) ensures that nothing is left to chance.
If preparing and maintaining your system seems overwhelming, talk to your compressed air service company. They will be glad to assist in outlining your specific requirements or can develop a maintenance plan to be performed for you with contract specific requirements.
I have a notification system that lets me know when the temperatures are expected to drop below freezing. Yes, it’s great to know ahead of time so I can plan an indoor day but the real reason for this is to notify my clients through twitter (www.twitter.com/fe_airsystems)
As most already know, compressed air systems can be devastated by freezing temps if your not ready.
When I received my first notice of the season last week I was shocked at far into the year we were. I guess it was the extended summer temperatures that caught me off guard and then but I noticed this weekend my area is predicted to see a low temp of 25 degrees.
I hope you’re keeping track better than me but if not, It’s time to get your compressed air system ready for the cold weather.
This is the first of several posts to help you be cold weather prepared.
Of course maintenance takes place all year long, whether by your own maintenance staff or bringing in your preferred compressed air service specialist but there are a list of additional checks and corrections that need to be made ahead of freezing temperatures.
Check all drains for proper operation
Check drains for obstructions or inefficiencies. If a drain is malfunctioning it can cause serious issues when the temperatures drop. In compressor rooms that are not heated, improperly drained condensate could freeze. If the drain trap is left open, it blows expensive compressed air to the atmosphere and let’s face it – efficiency means everything to companies today. If not already installed consider adding no air loss type auto drains. Be sure to inspect drains in the air system equipment, including dyers, receivers and filters.
Maintain air filters
Filters should be checked regularly for clogs or other problems indicated by a high differential pressure. This unnecessary pressure drop can decrease end-use pressure, causing the entire system to work harder and use more energy. Again, efficiency is king! Coalescing filter housings partially filled with water are at risk for freeze and rupture event and nobody wants to be around that time bomb. .
Locate and repair leaks
Leaks can account for up to 25 percent of compressed air use. Identifying and fixing leaks can lead to huge savings. Did I mention efficiency? Some larger leaks can be heard just walking through the area but smaller leaks will require monitoring equipment. You can purchase and utilize an ultrasonic acoustic leak detector or hire a “Quality” compressed air auditing firm. An auditing company can perform a leak audit only or you can go all out and have them perform a supply & demand side audit to determine additional potential savings for your entire system.
These are the first steps to take when winter is just around the corner. Follow the blog to learn more pre-cold weather tips.
In the segment we’ll discuss two critical characteristics of a centrifugal compressor prior to moving into our discussion of the main controller.
What is surge?
•Surge is the reversal of flow within a dynamic compression that takes place when the capacity being handled is reduced to a point where insufficient pressure is being generated to maintain flow.
In layman terms, this means that for the flow through the compressor at a given point, the pressure has reached the maximum limit the impeller of the compressor can push against. Therefore, since the compressor cannot overcome the pressure, the air flow slips backwards rather than being pushed into the system.
•This condition can potentially damage the compressor if it is severe and is allowed to remain in that state for a prolonged period; therefore, control and prevention is required.
The resulting problem with a surge condition is twofold:
The term surge should also be clarified as the term can have multiple meanings.
Surge Terminology in Centrifugal Compressors:
•Throttle Surge – When flow across the compressor drops till the surge line while maintaining constant pressure.
• To prevent such occurrence, the bypass valve is open before reaching the surge point
• Natural Surge – When pressure reach the maximum the compressor can compress (exceed the physical limitation of the compressor).
•Typically 110% of compressor rated pressure
Stonewall is the effect at the opposite side of the curve from the surge point in the chart above. At some point, as the discharge pressure falls and the airflow through increases at full load, the physical limitations will not allow more air through the stages — this point is known as stonewall. Continued operation at or beyond this point can cause such high flow rates with greater pressure differential that the impellers will not totally fill the vane areas and a cavitation-like action will occur, creating another type of surge with damaging vibrations.
How do we keep the air flowing where we need it to go? Control valves!
In a centrifugal compressor the unit is controlled by several valves.
The inlet valve or Inlet Guide Vanes (IGV) controls the amount of air allowed into the 1st stage of compression. The valve can be as simple as a butterfly valve or more commonly an inlet guide vane which functions as the inlet valve. The advantage to utilizing the IGV is the incoming air can be pre-swirled to assist in getting the air moving in the correct orientation for the 1st stage impeller to pick up and compress the air.
The discharge air adjust how much air is allowed to leave the compressor and enter the plant piping system. Personally I prefer the term blow off valve which is simply a valve the blows the compressed air to atmosphere if it is not needed in the plant compressed air piping header. The most efficient compressors will utilize modulating blow off valves rather than an open/closed arrangement which allows for much finer control of the air that blows off to atmosphere. It is important to note that the most inefficient aspect of centrifugal compressors is blowing off air that you have just paid money to compressor!
The discharge check valve is used on the discharge of the compressor to prevent any opportunity for compressed air from the plant header system to backflow into the compressor while it is running unloaded or off. The backwards flow of air into a centrifugal compressor can spin the impellers in the opposite direction causing massive damage to the unit.
A secondary valve on the discharge air line to again prevent any backwards flow of air into the compressor.
All of these valves are controlled by the compressor control system.
The previous discussions have focused on a variety of centrifugal components including bearings and gearing. While a centrifugal compressors meets all of the class requirements for oil free air (just as other types of oil free compressors) there is obviously still a requirement for the bearings and gearing to be lubricated. The centrifugal compressor maintains it’s oil free status by utilizing seals that keep this oil out of the air stream.
The lubrication system is critical to the longevity of the compressor! Most centrifugal compressors utilize 2 oil pumps on each compressors. The primary run oil pump and the auxiliary oil pump.
Prior to compressor startup, the auxiliary oil pump which is electrically driven is started to lubricate the bearings and gears. The control system monitors the oil pressure and temperature to assure proper levels of each before allowing permission for the unit to be started.
Once the compressor is started the auxiliary oil pump continues to run as the compressor ramps up to full speed. As the unit begins turning the primary shaft driven oil pump also begins to add oil pressure to the system. The control system is looking for the additional oil pressure and once this pressure is achieved by both oil pumps running at the same time the control system shuts down the auxiliary oil pump and allows the primary shaft driven oil pump to carry all of the lubrication requirements.
In the event of a shutdown situation, such as a high vibration alarm, high temperature alarm, low oil pressure alarm or any other trip alarm condition the auxiliary oil pump immediately turns back on to assure the unit maintains proper lubrication.
The auxiliary oil pump will also be turned back on when the compressor is put through the shut down sequence. As the compressor is coasting to a stop, the shaft driven oil pump is not sufficient to provide all of the lubrication so the auxiliary oil pump is used to maintain this lubrication. Also, once the compressor has stopped it is standard procedure to continue to run the auxiliary oil pump for a period of time to dissipate the heat and cool the gears and bearings.
To dissipate the heat from the oil after the oil flow has left the bearing and gear spray area. The oil is not only used to lubricate the components but also to carry the heat away. Oil leaving bearings and gear spray areas can be in the 150 degree F range and depending on the manufacturers oil weight requirements, the oil must be cooled to approximately 120 degrees F. Normally the oil cooler will be a tube in shell design where cooling water flows through the tubes.
To filter any metal contamination from the oil. This is the most critical component (IMO) of the lubrication system and I suggest always using an OEM element for the oil filter. These filters can come in either a spin on design or a cartridge in housing design. It is frequently recommend to utilize a dual oil filter design such that a filter element can be changed while the compressor remains on line.
In all centrifugal compressors, a positive pressure oil mist is created within the gearbox by the meshing gears. The positive pressure will result in oil leaks through the oil seals if not addressed. Typically the appropriate action is a simple vacuum venturi which pulls a vacuum on the oil reservoir to collect the oil mist. This oil laden air is then passed through a mist eliminator filter to remove the oil mist prior to the air venting to atmosphere. Most manufacturers also offer an optional electric motor driven vacuum system to eliminate the use of compressed air for this process.
A simple pressure regulator to assure the proper oil pressure for bearing and gear lubrication.
As previously stated, the returning oil (from bearings and gears) is heated and must be cooled via the oil cooler. To assure oil flowing to bearing and gear spray areas the cool oil is mixed with hot oil to assure the proper temperature.
During routine operation, normally a check valve will be used to prevent oil from being pumped back into the reservoir through the auxiliary oil pump. The pressure regulator is used to maintain proper oil pressure to bearings and gearing and returns any excess oil to the reservoir.
The flow can vary between manufacturers but a possible route is oil being returned from the gears and bearings is directed to the oil cooler and then passes through the oil filter. The cool, filtered oil is then directed back to the oil reservoir where it is then picked up by the oil pump and directed to the gear sprays, pinion and bull gear bearings. On some models this same oil is used to lubricate the drive motor bearings.
Our focus has been on the integral geared centrifugal compressor. This arrangement calls for several pinions shafts (previously discussed) to mounted around a central drive gear, commonly called the Bull Gear.
The exploded view of a bull gear (left) shows the helical gearing that will mate with the pinion gear located on the pinion shaft.
The advantage of the integral geared centrifugal is where the bull gear drives multiple pinion shafts, each shaft can be speed altered with simple gear changes.
This arrangement is unlike a barrel type centrifugal compressor where all the stages are driven at the same speed. This allows each pinion to run at different speeds thus allowing every impeller pair (or single) to operate at it’s optimum aerodynamic speed. This results in higher efficiencies along with the ability to package the unit in a smaller space.
The bull gear is typically driven by an electric motor at 3600 RPM although various manufacturers may use motors with a rotational speed of 1800 RPM depending on the compressor application. It should also be noted that in many cases the bull gear can be driven by a steam turbine. This is a particularly advantageous situation when a large facility has excess steam or the unit can be used as a steam pressure reduction valve (PRV). In this instance the operational cost of the compressor can be lowered or completely eliminated.
We previously discussed the pinion. The shaft where the impeller, bearings and seals are mounted prior to the assembly being placed in the gearbox. As with any rotating equipment, there are mechanical forces seeking to wreak havoc within your centrifugal compressor as well as air and oil flowing throughout the machine. Today we discuss how everything is held in place with bearings and seals.
There are a couple bearing designs that carry the pinion with the best & most prevalent option being a split, tilt pad bearing. The multiple pads are free to move about a pivot. The number of pads & preload can be varied to achieve the desired performance. The split design allows the bearing to be dis-assembled in the field for inspection and repair.
An alternate to the tilt pad bearing is termed a “Hydrostatic squeeze film Babbitt sleeve bearing”. This bearing is used by a single manufacturer and while the bearing does hold up well in the application it does not maintain the ability to field inspect / repair. For this reason, this particular manufacturer supplies what is termed a “rotor cartridge”. This terminology basically means that the pinion is supplied with the bearings, seals and impellers installed and balanced for either placement into a new compressor or to be sent to a client for a repair installation in the field. While this sounds like a time saving feature for the end user it actually just locks down the client requiring all repairs to be returned to the factory. While the virtues of this design might be praised by the manufacturer it can be noted that this is NOT the arrangement the manufacturer uses in their custom designed compressors! If a tilt pad bearing is used on high performance compressors, wouldn’t you prefer this same design to be used on your smaller plant air compressor too?
Thrust within a centrifugal compressor is a given. The question is: How is the thrust limited?
For thrust, think of a box fan. When the fan is running at low speed the box is stable. However, if the fan speed increases the pressure increases with the air movement from the blades pushing against the static air or perhaps a window screen. In this circumstance the fan is likely to fall over backwards from the force.
This same axial force is present in a centrifugal compressor. The impeller moving the air forward pushes the pinion shaft in the opposite direction. Additionally, the helical gearing used for integrally geared compressors create their own axial thrust issues and the force also changes as the load requirements change thus creating a change in the amount of air being pushed through the stage. As the clearances are extremely tight within the compressor, pinion movement could cause an impact within the compressor and thrust bearings are therefore used to limit this movement.
Some manufacturers install bearings on the pinion itself to limit this movement while others use thrust collars to transfer the movement force to the larger bullgear where the thrust load can be taken by much larger thrust bearings. In my opinion, the larger the surface taking any load, thrust or otherwise, is a better, more reliable option.
As previously discussed there will be 2 seals on the pinion at each impeller placement. One being the air seal to eliminate any compressed air leakage and the other to eliminate any oil migration into the air stream.
There are 2 basic types of seals used for these applications. One being a carbon ring seal and the other a labyrinth seal.
The carbon ring type seal physically contacts the shaft to seal either the air in or the oil out. A drawback to this type seal is the wearing component that contacts the shaft. As the rings inner bore wears the clearance of the seal opens allowing leakage. This requires shut down of the compressor for maintenance (replacement) of the seal.
The alternate seal type is a labyrinth seal which provides a tortuous path to prevent air or oil migration. The biggest advantage of a labyrinth seal is that this type of seal is non-contacting and will not need maintenance (replacement) under normal operating conditions.
While every component within a centrifugal compressor is crucial, bearings and seals are extremely critical items and care should be taken to make sure you understand how your compressor or a newly proposed compressor operates. Obviously, the manufacturer’s choice of bearing and seal arrangements are determined by the cost of the arrangement as well as the desired life of the component. Always have an in depth conversation with your compressed air professional to assure your understanding of your air compressor.
The next component to discuss in the centrifugal compressor is the pinion. Basically a shaft that carries several key components of the compressor. Below you can see a photo of a pinion that is set in a lathe for repair but this gives a great view of the bare pinion only.
As you can see in the photo, there is a gearing cut in the center of the pinion (shaft) which will be used to rotate the assembly and then on each end of the pinion there are some raised areas and some smooth areas.
The pinion carries several other key components of the compressor including the impeller or impellers if the pinion is a double hung design as the way pictured. Below is a photo of a pinion with two impellers mounted.
The photo below demonstrates a layout of the various other components mounted on the pinion.
As the photo shows, in addition to the impeller on the left end of the shaft; moving to the right you can next see the air seal is shown followed by the oil seal and finally the high speed bearing that carries the weight of the impeller. Moving down the pinion you can see where the bullgear meshes with the gearing on the pinion. This gearing will be discussed at a later time as well as the seals and bearings.
Not shown, there would be at a minimum another bearing to the right of the bullgear so that the 2 bearings are completely supporting the entire pinion assembly.
FYI, the term pinion assembly or rotating assembly or cartridge assembly are all terms used which simply mean the entire rotating assembly which would include the pinion, impeller, bearings and seals.
If this happens to be a double pinion design, then the entire arrangement would be duplicated on the right side of the pinion shaft past the bullgear which would include another bearing (already mentioned), another oil seal, air seal and impeller.
