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.

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.

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.