Controlling Surge & Manual Tuning

April 25, 2017April 25, 2017Ken Bean

I was recently on a service job and watched the tech surge test the compressor. I’ve seen it done a number of times but for some reason a question came into my mind.

I know the tech is manually manipulating the inlet & bypass valves to increase the unit pressure thus forcing the compressor into a surge condition. He is listening for the whooshing sound a compressor makes when the max pressure is reached and the air slips into a reverse flow.

The question that hit was how exactly does the controller know when a surge condition has been experienced. A quick call to my service manager got me the explanation I wanted and I thought you too might benefit from knowing the answer.

The controller is always looking at the unit pressure. Not just the final discharge pressure but also the pressure at each stage of compression. The controller registers a surge when it see a simultaneous drop in pressure and a drop in motor amps. The controller can register the surge regardless of which stage the pressure drop occurred. Meaning the surge could have occurred in any stage of the compressor, not just the final stage.

Once the controller reaches the pre-set surge count, which is usually programmed by the technician setting up the unit, it pushes the surge line out so the compressor should not reach this point again. By limiting how far the compressor can go the controller keeps surge from occurring again.

Now that the controller has automatically compensated for the compressor surging we need to further consider: Why did the compressor have a high number of surges?

This could be caused by a number of reasons including compressor component wear. Perhaps the impeller or diffuser has been worn by particulate or water erosion which now limits the ability of the compressor due to a change in tolerances. More likely, the cause can be attributed to a change in ambient conditions. Very likely a temperature change in the air inlet temperature or a change in humidity has occurred or a combination of both.

Further consider that while the controller has modified settings to protect the unit from further surges it has also taken away a potentially useful area of operation. You see the controller can push the surge line out to prevent further operation within the effected surge area but it will not reverse the line in the opposite direction. So when ambient conditions again allow compressor operation within the previous zone the controller will not allow operation back within this area without being forced.

To force the controller to allow operation within this zone again a manual tuning must be done to override the controllers automatic pushing of the surge control line. Think of this as a reset of the controller’s parameters so to speak.

To accomplish this the unit is manually tuned to the proper control set points by a certified technician or knowledgeable operator.

For this reason we recommend manual control tuning be performed at the beginning of each seasonal weather changes. So as the temperatures begin to rise in the late spring or the temperatures begin to drop in late fall the compressor should be manually tuned by your authorized service company.

When we are selected to provide a preventative maintenance or closed cost service contract we actually retune the compressor each quarter. If your company is without a contract we recommend at least retuning the compressor twice per year.

As always, if you have any questions, feel free to contact me to discuss further.

Compressor Performance Comparison

April 17, 2017April 17, 2017Ken Bean

Last week we discussed how air compressors are rated in performance using variations of CFM (Cubic Feet per Minute).

The article discussed keeping the performance stats consistent for comparison. However, even if various air compressors are listed in the same specification units – how do you know for certain the unit performance you’re being given is an accurate number?

For those of you unfamiliar – enter CAGI. CAGI stands for compressed air & gas institute. Reputable manufacturers that offer products within CAGI’s area are members of CAGI. A great service that CAGI started many years ago is having compressor manufacturers submit compressor performance data to CAGI which is then verified.

With this tool available you can be assured the data you receive is accurate.

You can find the starting point for verification at the following link:

http://www.cagi.org

If you have any question feel free to contact CAGI or myself to discuss.

Understand The Basics: CFM

April 11, 2017Ken Bean

Anyone that’s been near an air compressor is familiar with the term CFM. It’s quite simple and is an acronym for “cubic feet per minute”. This is a flow measurement based on volume, not weight.

Any cubic foot of air occupies the same space but will have different weights depending on:

Temperature

Humidity

Absolute Pressure

Why is this important?

It’s important to understand to properly match your plant load requirements to your air compressor purchase. Different manufacturers display their compressor flow data with various terminology.

ICFM – Inlet Cubic Feet per Minute

ICFM is a measurement of the air flow prior to any component of the compression equipment such as an inlet filter (which will cause a drop in pressure)

SCFM – Standard Cubic Feet per Minute

SCFM is a measurement of air flow at an industry standard condition. That specific condition is normally stated in the U.S. as 14.696 PSIA (pounds per square inch – absolute), 60 degree F (520 degree R) and 0% relative humidity (RH).

ACFM – Actual Cubic Feet per Minute

ACFM is the actual cubic feet of air that is being delivered from the compression equipment. If the exact conditions at the compressor location are equal to the SCFM standard conditions then the ACFM would equal the SCFM. However, this almost never happens!

FAD – Free Air Delivered

FAD is the actual quantity of compressed air converted back to the inlet conditions of the compressor.

The 2 most typical stated flow rates normally seen are ICFM and SCFM. As long as all the performance data is kept in one type of calculation then comparison of various compressors should work.

However, to assure you are getting a compressor that will actually meet the plant requirements I always ask the client to specify the worst possible operating conditions so compressor performance can be calculated at that point.

If you select a compressor based on SCFM alone then you will not get the stated flow when the summer conditions are 98 degree F ambient and the cooling water is 110 degree F and humidity is 52%.

The flow rates of air compressors is a widely changing variable and many white papers have been written on the subject so end users truly understand what performance they will receive from a specific unit.

We are available to present an entire session on this topic if you would like to completely understand air compressor flow rates. As a bonus for our readers that are in the engineering discipline, PDH credits can be received for this seminar.

Centrifugal Compressor Controls Tuning

March 21, 2017Ken Bean

A simple concept of air density is cool air is more dense than warm air. In a centrifugal compressor application where the flows and pressure are created dynamically the inlet conditions play a major role in the performance of the compressor.

Several factors play a contributing role in the performance including inlet pressure which should remain constant at your physical location. However, the variable factors include inlet air temperature, humidity and cooling water temperature.

Lower intake air temperatures as well as cooling water temperatures usually go hand in hand. The lower temperature air will be more dense and will result in higher free air delivery (ACFM) and also higher power consumption of the compressor.

Another factor in air density changes is the available turndown of the compressor. Meaning the effective flow range through the use of a throttle valve or inlet guide vanes is possible. A lower temperature inlet air will allow for a wider range of operation in the compressor thus offering a larger amount of turndown.

The lower inlet temperature air also increase the surge pressure!

During the heat of summer the alternate is true where less volume is produced by the compressor and subsequently, less horsepower is used but there may exist an circumstance that the unit will not be able to produce the required plant volume if the unit was not sized appropriately at the onset.

All of this information is critical in initially sizing a dynamic compressors but the point we’re looking to make is the ambient conditions do change the performance of the machine.

Modern controls are designed to keep the compressor out of surge or choke conditions by moving the surge line to an acceptable position. However, by moving the surge line the performance of the unit is also altered.

This is the primary reason we suggest re-tuning each centrifugal compressor based on the season. At least twice per year (as the ambient temperatures increase for summer and decrease for winter) is an ideal time to tune the compressor to achieve peak performance and maximize energy savings.

If your plant has a maintenance contract with a service provider, make sure that tuning the compressor is included twice per year as recommended. If you perform you own service I would advise contacting a local service company to perform these tuning parameters for your as a one off service. Typically, tuning controls is outside the expertise of plant maintenance personnel.

What’s Wrong With My Centrifugal Compressor

March 7, 2017Ken Bean

Like any piece of mechanical equipment, your centrifugal compressor can experience problems.

On any given day things are running great and suddenly there is an increase in vibration, The discharge pressure falls or the outlet temperature is too hot.

The question is: What’s wrong.

The simplest solution is to contact your local service professional but sometimes you just want to check it out yourself. Whether for personal satisfaction or you really need your compressor running in the next few minutes there are always a few simple items you can check and perhaps correct on your own.

We’ve put together a simple list of troubleshooting tips based on potential problems. Feel free to download the guide which may save you some time in your search.

Click here to reach the troubleshooting guide.

Inspections & Checklists–Centrifugal Compressor

January 31, 2017January 31, 2017Ken Bean

After learning about all the components in your centrifugal compressor you should now have a comfort level knowing about the parts and operation of your machine.

To keep your compressor running like new takes time for regular inspections and will also save money on operational cost. Many of the suggested inspections can be performed by your plant maintenance personnel and keeping detailed records will also help to understand problems as they arise. At the very least it will be helpful and therefore less costly when your compressed air professional needs to assist with any problems.

Below is a list of typical items you should be reviewing and the suggested frequency that items should be checked.

Scheduled Maintenance Procedures

Daily:

Operating data logged during loaded operation and reviewed

Monthly:

Inlet air filter elements inspected, (replaced if required)
Oil reservoir venting system filter elements inspected, (replaced if required)
Compressor bypass valve air supply filter checked (if applicable)
Intercooler / aftercooler condensate removal system checked
Control system operation checked

Quarterly:

Compressor operations data analyzed
Main drive motor bearings lubricated per motor manufacturers instructions
Condensate traps cleaned and inspected
Intercooler, aftercooler, and oil cooler performance verified
Lubrication system oil analyzed

6 Month Interval

Oil reservoir venting system (air ejector) filter element changed
Oil system filter element changed (if required)
Lubrication system oil tested and changed, (if required)
Oil pump motor lubricated with recommended grease (if required)
Coolant and condensate chemically tested
Compressor inlet and discharge valves inspected
Inlet guide vane assembly drive screw lubricated (if applicable)
Bypass valve lubricated (if Required * Note manual recommendations)
Bypass valve air supply filter replaced (If applicable)
Bypass valve silencer element and gasket replaced
Discharge air check valve inspected and tested for free operation
Main drive coupling inspected (coupling type dependent – see manual for proper maintenance)
Main drive motor maintenance checks completed and correct alignment verified
Perform compressor surge test, recorded result and adjust control setpoints

To further assist you with the checks and inspections above you can download this list at the following link: Maintenance Checklist

To assist with your information collection you can find a checklist to download at the following link:

Operator Inspection Checklist

Of course there will always be items that are beyond the scope of your plant personnel. That’s when its time to call in a professional. We recommend an annual visit by your selected service company for detailed inspections on your equipment and to handle the more difficult maintenance that typically requires the unit to be dis-assembled to some degree.

Of course that doesn’t mean you should be out in the dark. You can download a checklist at the link below that helps you know in advance what items (minimum) that your professional technician should be covering and assist you in making sure all items are covered.

Professional Checklist

I hope the above lists and inspections items are helpful as you work to keep your compressed air system in top operating condition.

Controls

January 17, 2017Ken Bean

We’ve progressed through the components of a centrifugal compressor. While the goal is to supply the plant with oil free compressed air, the ideal conditions of supply and demand rarely, if ever, match up. Meaning the plant use is never matched with the exact output of the compressor.

For that reason a controller is implemented to match the output of the compressor with the needs of the plant. The controller is responsible for monitoring all of the instrumentation on the compressor such as vibration probes, oil pressure, oil temperature, air temperature both entering the compressor as well as the temperature at the different stages. This is to assure proper cooling is taking place in the intercoolers. The controller also typically monitors the pressures at the discharge of each stage to assure the unit is operating at the design point and also measures the motor current.

Turndown

Another important term to understand in the centrifugal compressor world is turndown. Basically, turndown is the operating range of the compressor between the lines of surge and choke.

This gives the end user an understanding of the operating range of the compressor which assists in understanding the efficiency of the unit. Since the requirements of the plant rarely match the exact output of the compressor we need to know how much the compressors can effectively throttle back to match the plant air requirements.

Control Mode

The controller may normally be set up to control in several different scenario’s.

Constant Pressure:

Constant pressure control is frequently used when system air pressure must be held steady at a specific value or in processes where swings in system pressure is not acceptable. The inlet valve is modulated to meet the system pressure set point while holding motor current within the Max/Min set points. In a case of low demand, the compressor will throttle back to the surge control set point. If demand continues to fall below these set points, the discharge (blowoff) valve will open (modulating if capable) to bypass enough flow to keep the compressor from reaching a surge condition.

Constant Flow

Constant flow control provides a constant flow delivered from the compressor to the system in special applications and works much the way constant pressure controls work.

Auto Dual

Auto Dual control provides efficient compressor operations where some pressure swings are acceptable to the plant. In Auto Dual mode the compressor controls operate the same as constant pressure until the compressor throttles back to surge control set point. If demand falls below the throttled condition the controller will unload the compressor. If the plant requirements increase the compressor will reload to supply air to the plant. On some models, if the demand remains below this threshold for a set period of time the compressor can be programmed to shutdown and auto restart when system demand requirements rise.

Efficiency Note

The controller for the compressor has a primary function of assuring the compressor meets the demand of the plant air system requirements. Secondarily, the controller should operate the compressor at peak efficiency in order to keep electrical operating cost to a minimum. Any time the discharge (blowoff) valve is open, expensive compressed air is being blown off to the atmosphere. After paying to compressor the air, blowing it to atmosphere is a tremendous waste of resources.

While most manufacturers offer a variety of interconnected local controllers, Most of these controllers simply do not have the computational power to fully utilize the compressor at peak efficiency. Regardless of control methods, sophisticated algorithms in central control systems offer faster monitoring & control while utilizing more sophisticated programs. An example would be where the master controller monitors the rate the system pressure falls to determine the likely time expectancy that the unit will need to reload.

For maximum efficiency it is always recommended to hire a professional compressed air auditing group that maintain engineering personnel dedicated to control systems.

Prepare Your Compressed Air System For Winter: Final Thoughts

December 20, 2016Ken Bean

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.

Heat Recovery

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.

Prepare Your Compressed Air System For Winter: More Information

December 13, 2016Ken Bean

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.

Winterization

November 15, 2016Ken Bean

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.