Flow backwards
That's all your life's about
And it's not right

("Flow Backwards," by Syd Matters, 2006)

 

THE CONCEPT OF DYNAMIC COMPRESSOR "SURGE" DESCRIBED

The most brilliant PTOA Readers and Students … meaning those who have been reading the PTOA Segments in the intended, sequential order … already know that the PV Pressure in a Dynamic Compressor varies widely within the confines of the Compressor Casing.

After all, the purpose of the Compressor is to infuse the gas that flows within its casing with energy in the form of Pressure.

Throughout the process of a Dynamic Compressor Start-Up, the PV Pressure on the discharge side of the Compressor most definitely can be greater than the  PV Pressure on the suction side. That situation describes the final status of the Compressor at the conclusion of a successful Dynamic Compressor Start-Up.

After reading PTOA Segment #165, all PTOA Readers and Students fundamentally recognize that no fluid can help but flow from an area of high PV Pressure to an area of lower PV Pressure.

The reversal of gas flow during the Start-Up of a Dynamic Compressor is called "Surge."

As the gas flows backwards through the Impellers of the Dynamic Multi-Stage Centrifugal Compressor or the Rotor Blades of the Dynamic Axial Compressor, damage can be done to this hardware as well as to the Thrust Bearings and the windings of Motors that may be used to drive the Compressor.

 

 

The symptoms of Surge are:

• A low gas flow rate discharged from the Dynamic Compressor.
• Violent fluctuations in Dynamic Compressor Discharge Pressure.
• Excessive vibration (which damages Thrust Bearings).
• A muffled, banging sound within the Dynamic Compressor.
• Extreme variations in the motor current for motors used to drive Dynamic Compressors.

 

This PTOA Segment focuses on the Dynamic Compressor phenomenon known as Surge and the Compressor design and automatic control scheme which diminish the occurrence and magnitude of Surge.

 

THE MANY EXPRESSIONS USED FOR CAPACITY
(AKA "SUCTION FLOWRATE" AND "INLET FLOWRATE")

The graphs used to describe when Surge will occur during the Start-Up of a Dynamic Compressor are based upon the rate of gas flow into the Suction Line of the Compressor.  The short hand name for "Inlet Gas Flow Rate" is Capacity. Understanding Surge-Point/Surge-Line Graphs begins with understanding the many expressions in use to describe the Capacity of a Dynamic Compressor.

PTOA Readers and Students have probably already noticed that Capacity (aka Inlet Gas Flow Rate) is shown as an increasing value on the X-Axis of every chart or graph used to predict Compressor operations.

Guess what? Capacity is a Process Variable Flowrate expression.  PTOA Readers and Students will very soon complete the PTOA PV Pressure Focus Study Area and begin the PTOA PV Flowrate Focus Study Area.

For now, PTOA Readers and Students can Imagine a real-world Control Board Operator gazing at whatever Human-Machine Interface is employed to keep the Control Board Operator aware of the Dynamic Compressor's operating status.  S/he or they may notice the units of Capacity shown for the soon-to-be-compressed gas represented as one of the following:

CFM means "Cubic Feet per Minute."

Note that "Cubic Feet" is a volume. Therefore "Cubic Feet per Minute" is a "volumetric flowrate" which quantifies how much volume of fluid is flowing by a certain point over a specified time.

The CFM  flowrate expression works fine describing the volumetric flowrate for a fan or an air blower because the gas being moved is air at ambient PV Temperature and PV Pressure.

CFM does not correct for specific gravity (refer to PTOA Segment #145 to refresh on "specific gravity"). Otherwise stated, CFM assumes the gas  being blown through a fan or blower is a lot like air at ambient PV Temperature and ambient PV Pressure.

PTOA Readers and Students learned about the SI units for measurement in PTOA Segment #143. The SI units for Capacity include "Cubic Meters per minute (M³/m) and "Cubic Meters per Hour" (M³/hr) or "Cubic Meters per Day" (M³/day).

 

ACFM means "Actual Cubic Feet per Minute."

PTOA  Readers and Students learned way back in PTOA Segment #3 that a processing facility typically spends a lot of money on energy and personnel specifically to create a different PV Pressure and PV Temperature than would exist at ambient conditions.

Furthermore, in the real-world of process technology the gas that is flowing through process piping and eventually sucked into the Dynamic Compressor may very well not be air, but rather a gas that is lighter or more dense than air.

ACFM is the actual cubic feet per minute flowrate of the gas that flows into the Dynamic Compressor's suction because the ACFM flowrate is corrected to whatever the actual  PV Temperature and PV Pressure and specific gravity of the flowing gas are stated or known to be.

Brilliant PTOA Readers and Students have already learned about how control systems evolved from local pneumatic control to computerized DCS in PTOA Segments #9 through PTOA Segment #13. The pneumatic control boards of yore did not have sufficient inputs to the board, nor was there technology to correct a gas flow rate real-time (aka dynamically). The Outside Operator recorded the Capacity, PV Suction Pressure, and PV Suction Temperature on a logsheet.  Process Engineers used this recorded data to correct the Capacity into a an ACFM volumetric flowrate.  An assumed flowing specific gravity was also used for the conversion calculation to ACFM. Alternatively, perhaps the Lab Tech analyzed a stream sample so the Process Engineer could calculate a specific gravity for a more accurate estimate of Capacity expressed as ACFM.

Modern Distributed Control Systems (DCS) are "smart" because they can use the inputs of dynamically changing PV Temperatures and PV Pressure to correct a Capacity flowrate real-time via "calc blocs" or algorithms within the DCS software.

Ergo, the "ACFM" Capacity is corrected to the actual  PV Temperature and PV Pressure sensed and measured at the Compressor Suction. The specific gravity of the compressed gas is still typically assumed to be a certain value and is not measured and updated real-time.

Beware! Some American Compressor Manufacturers use ACFM to measure the flow of the gas at the Discharge PV Pressure and PV Temperature. The Discharge PV Temperature and PV Pressure will be greater than the Suction PV Temperature and PV Pressure.

 

SCFM means "Standard Cubic Feet per Minute."

OMG, Fred!

The topic of the SCFM volumetric flowrate falls into the category of "Gas Laws that are Always … and Never …Used" which was featured in PTOA Segment #154.

And one more time, Fred …. how do gases behave differently than liquids?

Righteeo! Gases are compressible! The PV Temperature and PV Pressure and density of a gas will significantly change upon being compressed.

A gas flow rate that is corrected to Standard Cubic Feet per Minute (SCFM) represents the gas flow rate adjusted to reflect what the molar volume of gas would be at "Standard PV Pressure" and at "Standard PV Temperature."

The "Imperial and USA" system of measurement defines the "Standard PV Pressure" as 14.7 psia (1 atm). The Standard PV Temperature used in process industry is 60 °F (aka 520 °R, 15.6 °C  288.7 °K ).

Measuring gas flow rate in SCFM is particularly helpful in process operations that involve chemical reactions because the compressed gas is also a reactant.

At Standard Temperature and Pressure, the volume of 1 mole of a gas is 0.870 cubic feet (aka 23.6442 liters).

Since the software in the DCS performs all the math converting the flowing PV Temperature and PV  Pressure  to STP conditions, the Control Board Operator is typically unaware that the Ideal Gas Law is being employed to help control the process.

PTOA Readers and Students who may work in a laboratory setting will be aware that the PV Pressure  "standard" is 1 atm; however the  PV Temperature "standard" is typically 25 °C.

Who amongst the brilliant PTOA Readers and Students has figured out that when the compressed gas is air, at standard conditions, SCFM is equal to ACFM?

Guess what? The inlet Capacity of Dynamic Compressors that has labyrinth seals may be erroneously high by 1-2% as that is the amount of gas dedicated to sealing which does not flow out of the Compressor's Discharge.

 

COMPRESSOR SURGE POINT and SURGE LINE GRAPHS

Surge Happens ... when the Capacity is decreased to a point where insufficient Pressure Head can be generated to maintain the desired flow path from the Suction through to the Discharge of the Dynamic Compressor.

The nearby simplified, colorful chart will serve as a good introduction to interpreting Dynamic Compressor Surge-Point/Surge LIne Graphs.

As expected, the X-Axis represents increasing volumetric flowrate (aka Capacity) at the Compressor's inlet. The units of Capacity might be  ACFM  (English-Imperial system of measurement) or NM³/hour or such (SI system of measurement).

The Y-Axis is a fancy name that simply means Pressure Head.

Pressure Head is an imaginary height of a column of gas that would be at the Compressor Discharge. The height of the imaginary column (aka Head) would be much taller for a high PV Discharge Pressure versus a lower PV Discharge Pressure. Another way to look at it is that Pressure Head is an imaginary visual representation of how much Pressure energy has been infused into the gas that entered the Compressor Suction.

Each  Y-Axis value for Pressure Head was determined by subtracting the PV Suction Pressure from the PV Discharge Pressure as the Capacity on the X-Axis  increased.

The Start-Up dilemma caused by "Surge" is evident once one realizes the Dynamic Compressor Start-Up goal is to get to the desired Operating Point which just happens to be at 100% of the designed inlet Capacity (X) and 100% of the designed Pressure Head. This target Operating Point is labelled on the green arced line.

A successful Dynamic Compressor Start-Up would be one wherein the Capacity and Pressure Head flawlessly increase while always staying on the right side of the red Surge Arc Line and eventually achieve the desired Operating Point.

However, when the Dynamic Compressor is started up, the Pressure Head is significant yet the Capacity is in the zero to low range. This scary Capacity/Pressure Head region is that area on the left side of the graph represented by the red Surge Line and the area to the left of the red Surge Line.

Without automatic controls and compressor design modifications, being on the right side of the red Surge Line is not a given during Start-Up. Adding hardware modifications and Anti-Surge Automatic Instrumentation and Controls increases the probability that the Capacity/Pressure Head relationship never moves to the left of the blue Anti-Surge Line.

Here's another nearby Surge-Point/Surge Line Graph.

Just like the other one, the X -Axis represents increasing Compressor Capacity as the Pressure Head (graphed on the Y-Axis) increases. .

The take-home message of this Surge Point/Surge Line Graph is to illustrate that the Surge Line is composed of Surge Points. Each Surge Point is a specific Capacity/Pressure Head relationship that must be avoided!

Tying the relationship of these points together comprises the red Surge Line.

Take another look at the Surge Point/Surge Line Graph above that has the blue Anti-Surge Line. Observe that:

 

 

The Surge Line is not a linear, straight line. but rather a red, arcing  line.

The lower the Inlet Flow Rate (Capacity),  the lower the Pressure Head that will cause the Dynamic Compressor to Surge … which means the compressed gas flows backwards.

 

ANTI-SURGE TECHNOLOGIES

The Minimum Recycle Gas Flow Line

PTOA Readers and Students are now knowed-up about:

• • The concept and definition of Dynamic Compressor Surge
  • Surge is caused when the gas flow Capacity dips to a point where insufficient Pressure Head is being generated to maintain flow through the Compressor from Suction to Discharge.

 

Doesn't it just make sense to install a Minimum Recycle Gas Flow Line from the Compressor Discharge to the Compressor Suction?

This mechanical fix would help insure that there is always sufficient Capacity to keep from approaching the Surge Point/Surge Line during Start-Up.

The nearby schematic of a Centrifugal or Axial Compressor (driven by a Gas Turbine) features a Minimum Recycle Gas Flow Line that  branches upward from the Discharge Line of the Compressor.

The Recycled Gas flows through a green Anti-Surge Valve (labelled FCV in the graphic) and thence flows to the suction side of the Compressor. The Recycled Gas flows into the Suction Line at a junction point which is located prior to where the Capacity is sensed and measured. The Capacity Flow Transmitter is labelled in the schematic as "FT".

Guess what?

Gases are compressible! The discharged gas has a greater PV Pressure, PV Temperature and is more dense than the gas that flowed into the Compressor.

Therefore, the Recycle Gas needs to be cooled down. Any liquid that may have been created in the compression process must be removed.

The nearby graphic depicts a Dynamic Compressor that is driven by a Turbine.  The compressed gas leaving this Compressor is cooled down via heat exchange. Downstream of the heat exchanger, a slip stream of Minimum Recycle Gas Flow flows through an Anti-Surge Valve and thence to the Compressor's Suction Line at a spot that is located prior to where the Capacity flowrate is sensed and measured.

 

.Anti-Surge Automatic Instrumentation and Control 

Surge prevention and control would not be fixed with just a Minimum Recycle Gas Flow Line.

Anti-Surge Automatic Instrumentation and Control is required!

As the nearby schematic illustrates, the Anti-Surge Controller (located in the DCS) receives inputs from:

• The inlet Capacity Flow Sensor/Measurer (labelled FT)
• The  Suction PV Pressure sensor/measurer (labelled PT and located on the Compressor Suction Line).
• The Discharge PV Pressure sensor/measurer (also labelled PT and located on the Discharge Line).

 

Those three inputs make it possible to calculate the dynamically changing Capacity and  Pressure Head.

The Anti-Surge Controller "does the math" via calc blocs or algorithm (or whatever new technology has taken their place).  The output from the Anti-Surge Controller is sent to the Anti-Surge Flow Control Valve. Note that this FCV is labelled and located on the Minimum Gas Recycle Line.

When the Capacity is determined to be too low to generate sufficient Pressure Head, the Anti-Surge Controller sends an output signal to the Anti-Surge Flow Control Valve which instructs the FCV to open up a bit more. This action allows more Minimum Recycle Gas Flow to be moved from the discharge side of the Compressor to the suction side of the Compressor.

Alternatively, when the Anti-Surge Controller determines that the Capacity/Pressure Head relationship is not in danger of Surge, it sends an output signal to the Anti-Surge Flow Control Valve instructing the FCV to close off a bit more. This action directs more of the compressed gas to exit the Compressor through the check valve on the Discharge Line and thence to be distributed to wherever the compressed gas is needed. .

Final Comments on Surge

The success of the algorithm used to keep the Capacity/Pressure Head relationship on the right side of the Surge Line depends upon how closely the Compressor design criteria aligns with the real-world Dynamic Compressor operating conditions.

The alert Process Operator will notice the "shudder" of a GT's Axial Flow Compressor as the GT is started up and approaches Surge.

There will also be a gaggle of Mechanical Engineers and Mechanical Techs monitoring the PV Temperatures of the Thrust Bearings and the voltage readings of the vibration sensors during Start-Up.

These sensors are part of the Machinery Protection System. The impacts of Surge are so costly and time consuming that the Anti-Surge Controller is not 100% relied upon to get the Dynamic Compressor through a successful Start-Up.

Internet High Five to PTOA Readers and Students! The Dynamic Compressor PTOA Focus Study has been completed!  Onward to finish Positive Displacement Compressors!

TAKE HOME MESSAGES: Surge is a Dynamic Compressor phenomenon.  Surge occurs when there is insufficient Capacity to support generation of the required Pressure Head to maintain gas flow through a Compressor from the Suction to the Discharge.  Hence, Surge means that the gas flows through the Dynamic Compressor backwards.

Surge typically occurs during the Dynamic Compressor Startup. Surge will damage Impellers, Blades, and Vanes. Surge will damage Thrust Bearings and the windings of motors. 

The symptoms of Surge are: 

• A low gas flow rate discharged from the Dynamic Compressor.
• Violent fluctuations in Dynamic Compressor Discharge Pressure.
• Excessive vibration (which damages Thrust Bearings).
• A muffled, banging sound within the Dynamic Compressor.
• Extreme variations in the motor current for motors used to drive Dynamic Compressors.

 

Because Surge is dependent upon the Capacity/Pressure Head relationship, this PTOA Segment was selected to review the various expressions for Capacity which include:

• CFM = Cubic Feet per Minute. CFM is most accurate for air blowers and fans.
• ACFM = Actual Cubic Feet per Minute ACFM corrects the Capacity to the "actual" flowing PV Suctiion Temperature and PV Suction Pressure of the gas. The specific gravity of the gas is typically a presumed value known to be in the ballpark.
• SCFM = Standard Cubic Feet per Minute. SCFM corrects the ACFM to a Standard Temperature and Pressure (STP). Use of SCFM makes it easy to convert the volumetric flowrate to molar flowrate because at STP the volume of 1 mole of a gas is 0.870 cubic feet (aka 23.6442 liters).The values used for STP are worth checking for each service. For the English-Imperial units of measurement in USA process industries, the Standard Temperature is 60 °F (520 °R) and the Standard Pressure is 14.7 psia (1 atm).

 

All of the above expressions express a Volumetric Flowrate, aka the volume of flow that passes by a certain point over a specified time interval.  (Volumetric) Flowrate is a Process Variable. Very soon PTOA Readers and Students will move on the the PTOA PV Flowrate Focus Study Area.

Surge Point/Surge Line Graphs predict where the Capacity/Pressure Head relationship of the gas being compressed will cause Surge during Start-Up. Increasing Capacity is graphed on the X-Axis.  Increasing Pressure Head is graphed on the Y-Axis.  Pressure Head is determined by subtracting the Compressor Suction PV Pressure from the Compressor Discharge PV Pressure as the Capacity is increased. 

The mechanical fix to avert Surge is a Minimum Recycle Gas Flow Line piped from the Compressor Discharge Line to the Compressor Suction Line, prior to where the Capacity is sensed and measured. 

The Automatic Instrumentation and Controls needed to mitigate Surge include:

• An Anti-Surge Controller located in the DCS. The output from this controller drives the position of the Anti=Surge Flow Valve.
• An Anti-Surge Flow Valve located on the Minimum Recycle Gas Flow Line.
• Inputs to the Anti-Surge Control Valve: Capacity, Suction Gas PV Pressure, Discharge Gas PV Pressure.

 

©2021 PTOA Segment 0224

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSOR

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