January 2017 – Compressedairducation

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.

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.

Month: January 2017

Inspections & Checklists–Centrifugal Compressor