Surge & Choke

November 8, 2016Ken Bean

In the segment we’ll discuss two critical characteristics of a centrifugal compressor prior to moving into our discussion of the main controller.

Surge

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 backward flow of air causes severe vibration within the compressor potentially resulting in damage to internal components.
As air is compressed, heat is generated. When a surge occurs, the air has been compressed (to a certain point) which has heated the air. The air then slips backwards on the impeller and will be grabbed at some point by the impeller to be compressed (moved forward in the compression sequence) at which point it is heated again. As we previously discussed, the temperature of the air moving from stage to stage plays a large part in the design of the entire compressor and at this point we are heating the air above it’s intended design point.

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

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.

Control Valves

October 31, 2016Ken Bean

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.

Inlet Valve

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.

Discharge Valve

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!

Discharge Check Valve

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.

Isolation or Block Valve

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.

Lubrication System

October 18, 2016Ken Bean

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.

The lubrication system should also include the following components:

Oil Cooler:

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.

Oil Filter:

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.

Oil Mist Collection System:

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.

Oil Pressure Regulating Valve:

A simple pressure regulator to assure the proper oil pressure for bearing and gear lubrication.

Temperature Control Valve:

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.

Routine Operation:

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.

Bull Gear: The Driving Force

October 4, 2016October 4, 2016Ken Bean

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.

Keeping Everything In Place: Bearings & Seals

September 27, 2016Ken Bean

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.

Bearings: Radial

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.

Image result for split tilt pad bearing

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?

Bearings: Thrust

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.

Seals

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.

Image result for LABYRINTH SEALS

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.

Pinion

September 13, 2016Ken Bean

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.

Cool The Air

September 6, 2016Ken Bean

We’ve discussed how the air is compressed through each component in a centrifugal compressor. Now that the air/gas has been compressed in the 1st stage, it has an elevated temperature. An example on a 400 HP machine with the air inlet temperature at 75 degrees F. the air temperature as it exits the 1st stage is approximately 242 degrees F.

As previously discussed, the ability of the compressor to meet the required performance is based on inlet pressure, inlet temperature and humidity levels. With cooler air being more dense it is also more easily compressed. Since the air leaving our 1st stage is at 242 degrees, we need to cool the air down before we move it to the 2nd stage of compression.

Enter the cooler, the next component of discussion in our centrifugal compressor. Unless, there is a very specialized application the manufacturer will supply a cooler between each stage of compression and a final cooler (after-cooler) after the final stage of compression to cool the air prior to the next stage of compression or enter the plant distribution system.

Normally on compressors as large as centrifugals the cooling medium will be water although some manufacturers do provide air to air coolers up to certain horsepower machines.

The photo above is a fairly normal centrifugal compressor gearbox and cooler casting. This is the base building block of the compressor. The 3 square holes near the bottom of the casting is where the 2 intercoolers and single aftercooler will be located. the circular holes toward the top are where the impellers and diffusers will reside and the scroll will be mounted to the outside of the circular holes once the impeller and diffuser are mounted.

Remember I stated this is a fairly typical gearbox/cooler casting. Manufacturers will incorporate a single casting design which can be utilized for several HP sizes. This saves the manufacturer money rather then a casting design for each single HP compressors.

Other designs can be found as well. One design is shown below where the cooler or cooling tubes surround the inlet air path. As you can see the air is being drawn down the center toward the impeller. The air exits the impeller/diffuser to each side where it then channels back through the cooler. The main issue with this design is the difficulty in disassembly to make any repairs unlike the casting design shown above where the coolers can be independently & easily removed.

Best in class design can accomplish even more. You can see in the photos below that certain manufacturers create separate gearbox and cooler housing castings. The thought process here is that the gearbox can still be usable for a range of HP sizes but the cooler sizing type can also be customized. By bolting the gearbox casting to the cooler casting you complete the casting assembly. In the event a client needs special large or smaller coolers or perhaps the client needs an API compressor then the gearbox casting and cooler requirements can be totally customized.

Moving on to the actual working of the cooler. The cooler itself contains a series of tubes, the inner portion of the photo below. Best design has the water flowing through the tubes although certain manufacturers reverse this and have air flowing through the tubes. The air enters the shell of the cooler and flows by the tubes containing the water. The air transfers the heat to the cooling water flowing through the tubes and the water then exits the cooler flowing back to a cooling tower, chiller or in rare cases a drain if the plant is using city water with a once through design. (A very expensive alternate) Most often the tubes are also surrounded by fins which aides in the heat transfer process.

A few other items that are important to note:

The tubes in the cooler can be either straight or U-Tubes. A straight tube design has the water entering one end of the tube, flow straight through to the other end where it exits the cooler assembly. A U-tube design (cheaper) has the water enter one end of the tube, flows to the opposite end of the cooler where the tubes bends 90 degrees and flows the water back to the originating end of the cooler where it exits. A U-tube cooler can only be chemically cleaned! A straight tube design offers the ability to remove both end caps and be mechanically rodded for cleaning. Please allocate the extra money (if required) and specify straight tube coolers!

The tubes and fins can be made of special materials depending on the service of the machine. Normal material of construction is copper tubes and aluminum fins.

You will normally hear of coolers being rated in approach temperature such as a 10 degree approach or a 15 degree approach. This simply means that the air leaving the cooler will be X degrees higher than the cooling medium – in our case normally water. Where X is the approach rating. So if a cooler is rated at a 10 degree approach and we have 80 degree water, the air leaving the cooler will be 90 degrees.

So now we’ve cooled the air down to an acceptable temperature for use in the next phase – either the next stage of compression or plant use.

If you read my dryer series you might remember when you have hot air and then cool it down you also condense water. The same thing just happened in the inter or after cooler. We had hot air (which is capable of holding large amounts of water vapor) and cooled it down so the water vapor condenses changing it to a liquid state. Now we certainly do not want liquid water going downstream to our plant and we REALLY do not want liquid water going into our next impeller! Therefore the design of the cooler casting must be such that the liquid water drops out of the air stream to a low collection point where an automatic drain valve will disperse the water to a drain outside the cooler casting.

A final note on condensate. We’re discussing water & metals which equals corrosion over a period of time. Air passages can be sprayed with an anti-corrosion coating for additional protection. Some manufacturers charge extra for this item and some provide it as standard. Yes, coating also wear away but any additional protection from corrosion is a good thing! Remember, your impellers are spinning at high speeds (up to 100, 000 RPM) with very close tolerances. A piece of rust slag hitting an impeller is not a good idea!

Rise & Collect

August 30, 2016August 30, 2016Ken Bean

We’ve previously discussed that high velocity air leaving the impeller impacts the blades on the diffuser which slows the air which causes the air pressure to rise. The next component in a centrifugal compressor is the scroll. Sometimes referred to as the volute.

Once the air passes through the diffuser blades it enters the scroll. As you can see in the above picture, the air passes around the scroll and exits at the top right discharge port. The air passing through the scroll is further reduced in velocity in which again causes a rise in pressure.

At this point the first stage of compression is finalized resulting in the final pressure from the 1st stage of compression which typically results in a discharge pressure of approximately 14 PSIG on a 100 pound compressor design. Remember, the aerodynamic engineer can alter the various aspects of the impeller, diffuser and scroll to achieve various outcomes.

It’s also important to note that we likely started with an inlet pressure to the compressor at approximately 14.3 PSIA. The absolute atmospheric pressure at the location of the compressor. This location and subsequent absolute pressure are critical considerations for the compressor. A machine designed for installation at the beach (sea level) with an absolute pressure of 14.7 will not have the same performance if it is moved to a mountain in Denver with an atmospheric pressure of 12 PSIA.

Always assure you’re using the correct readings, whether PSIG or PSIA. Gauge pressure vs. absolute pressure makes a huge difference. It’s also interesting to note that most of the work related to pressure increase on a centrifugal compressor is done in subsequent stages.

The secondary function of the scroll is to provide a smooth collection of the air where it will be passed to the next section of the compressor. The next section could be discharge to the plant, discharge to the next stage of compression or most commonly to a cooler.

Slow The Air Down–Raise The Pressure

August 23, 2016Ken Bean

We previously discussed how to get the air moving and the role of the impeller in a centrifugal air compressor. Now that we have air moving at a high rate of speed we need to slow it down. Slow it down? Speed up, Slow down – sounds like old people driving on Sunday morning!

I went in search of a great description for today’s topic, the diffuser. Below is an excerpt from Wikipedia:

As the flow continues into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster. According to a form of Euler‘s fluid dynamics equation, known as pump and turbine equation, the energy input to the fluid is proportional to the flow’s local spinning velocity multiplied by the local impeller tangential velocity.

In many cases the flow leaving centrifugal impeller is near the speed of sound (340 metres/second). The flow then typically flows through a stationary compressor causing it to decelerate. These stationary compressors are actually static guide vanes where energy transformation takes place. As described in Bernoulli’s principle, this reduction in velocity causes the pressure to rise leading to a compressed fluid.

What?

I’m glad you’ve made it to this point. I’m sure you’re thinking, “Principles & Equations, I just want to know how a centrifugal compressor works?”

Here goes: The next piece of the centrifugal compressor is the diffuser.

The air leaving the tips of the spinning impeller at high speed now impacts on the stationary diffuser to slow the air down. By slowing the velocity of the air, a rise in pressure is created.

Imagine a car hitting a wall. Until reaching the wall the car moving with only slight resistance from atmospheric air. Note that there is some slight pressure against the car at this point which will be important for later discussion’s. But once it hits the wall the pressure is increased and the increase in pressure collapses the metal of the car.

Same thing when the high speed air hits the diffuser. An increase in pressure, which is what we’re really looking for – compressed air.

Notice the diffuser below is not just a plain wall. It has blades on the surface as well. We don’t really want the air to completely stop (like a car hitting a wall). We just want to slow it down a bit so the pressure will increase.

If you recall discussing the impeller, aerodynamic engineers determine the speed of the impeller along with the length and depth of the impeller blades so the air is moving at the desired velocity. The same is true with the diffuser. The engineer determines the number, length and depth of the blades on the diffuser to slow the high speed air to the pre-determined amount to get just the right amount of pressure rise while maintaining the desired flow of air to the next component of the compressor.

Centrifugal Compressor: Moving The Air

August 16, 2016Ken Bean

A recent blog post discussed a presentation to a valued client on the topic of centrifugal compressor components, operation, maintenance and troubleshooting. This client owns and operates 20+ centrifugal compressors ranging from 1000 to 5000 horsepower. We were surprised to see the number of attendees to the class. Our thought process was with the number of compressors at this facility, everyone knew everything about centrifugal compressors. We were wrong. So you don’t misunderstand, there are lots of capable personnel at the facility but with the constant turnover of position changes, promotions and general churn, there were plenty that were looking for education.

I surmised that if this plant could use some refresher, then a lot of other people would likely be looking for information as well. For the next few weeks I’ll be covering some of the items from that presentation.

The photo below is the basic component of a centrifugal compressor: The impeller or often called the wheel.

The wheel is the primary rotating component that moves the air.

The wheel is mounted on the pinion which lies horizontally in the compressor. You’ll note that the pinion below has two impellers, one on each end. This is a common configuration although it is equally common to have only one impeller on a pinion.

The air enters the impeller at the small end and the vanes grab the air and accelerate it through the vanes to the larger end where it is basically thrown from the fins. The increasing velocity of the air is the beginning point of how the pressure is increased in a centrifugal compressor.

Notice the variation of the blades of the impeller from the two photos above. The bottom picture shows a much shallower blade as well as the blade being shorter. The depth, length and angle of the impeller blades is one of the keys to how the aerodynamic engineers achieve various performance from the unit. The material strength and amount of material is also critical as these impellers can turn up to 70,000 RPM and higher. Obviously, at these speeds you certainly do not want a blade breaking off which would complete wreck the compressor.

In todays modern engineering and machining world, these impellers are typically cut from a blank stock piece of material using a 5 axis milling machine to achieve the precise characteristics the engineer has determined is required to meet the performance requested by the end user. ie: a given flow (ICFM) at a given pressure. Some manufacturers will create impellers by a casting or forging process although these will result in a less sophisticated component. The design of the impeller is two-fold, in that it achieves the flow & pressure characteristics required while offer the optimum efficiency for the compressor’s energy requirements. Again, this is where the best of engineering design creates the most reliable and efficient compressor.

The stock material can be of carbon steel, stainless steel or in certain cases, exotic materials depending on the particular gas the unit will be compressing. Top rated centrifugal compressors for service in compressing air will use stainless steel material rather than carbon steel to achieve longer component life .