AIChE Paper Number 90b
OBJECTIVES OF ANTI-SURGE CONTROL
W. Brian Piercy
Turbomachinery Control Consultant Invensys Operations Management
Prepared for Presentation at the Year 2011 Spring National Meeting, Ethylene Producers’ Conference, Chicago, IL March 13 – 17, 2011
AIChE and EPC shall not be responsible for statements or opinions contained in papers or printed in its publications.
OBJECTIVES OF ANTI-SURGE CONTROL
W. Brian Piercy
Turbomachinery Control Consultant Invensys Operations Management
Abstract: Preventing a compressor surge is the primary purpose of an anti-surge controller. However, there are additional requirements that must be met for an anti-surge control
package to be effective. For example, it must be stable and reliable, while also keeping energy costs to a minimum.
For an anti-surge control project to be judged successful, performance control must be smooth and stable while the recycle valve is open. The compressor must be able to start and shut down without surging or excessively upsetting the process.
Control valves must be fast-acting, yet able to move in small increments. Transmitters must report the correct process conditions to the anti-surge controller over the full range of startup and running conditions. Flow meter sensing lines must be impervious to condensation resulting from temperature changes.
Depending on the application, recycling or venting must be kept to a minimum to reduce energy consumption.
This paper addresses some of the challenges that must be met when designing an anti-surge system.
Forward
This paper primarily is written for surge controls for a charge gas compressor. Many of the points can be applied to other processes.
Compressor Control
When discussing the goals of anti-surge, the general goals of compressor control must first be explained. Although the goal of anti-surge control is primarily to protect the compressor from surge, it must coexist with the other control aspects of the compressor. When formulating a successful anti-surge control strategy, it must work in-step with the overall performance goals of the compressor. Therefore, the two control strategies should be developed together.
In many cases, the recycle valve is an integral part of the performance control. For instance, a turbine-driven compressor running at minimum governor on the surge control line might still provide more flow than the process can handle. The only alternative available is opening the anti-surge valve further than is required for surge protection to reduce flow. In some cases, performance control must be modified to prevent a surge condition. For instance, lowering the speed too much on a turbine-driven compressor will cause a surge condition if the compressor pressure ratio remains constant.
Both performance control and surge control impact the flow through the compressor. Therefore, the stability of the performance control affects the surge controller. As an example, a poorly tuned performance controller causing cycling diminishes the controllability of the surge controller. This requires for the surge controller to have a greater safety margin, which increases energy cost.
The stability of the surge controller affects performance control. For instance, a sticky anti-surge valve will result in step changes to the process flow causing overall performance control to suffer.
Compressor Surge
Compressor surge occurs when the internal head produced by the compressor is insufficient to meet the external head exerted by the process. As a result, the compressor is not able to overcome the pressure ratio and the process flow reverses backward through the compressor. After the flow reverses, the discharge and suction pressure tend to equalize and the external head decreases. When the external head decreases sufficiently, the process will again flow forward. In many cases, the pressure ratio must drop significantly before the flow changes from the reverse to a forward direction.
It is important to avoid compressor surge because it can cause compressor damage. When a flow reversal occurs, compressor load is suddenly lost. When forward flow resumes, the load is suddenly increased. These strong load swings stress the bearings. If the load swing is strong enough, it can result in an axial shift of the rotor wiping seals. In severe cases, the thrust bearing can be damaged and the compressor rotating parts can hit the compressor case. Continual surging also results in severe overheating, which can cause damage.
In addition to causing mechanical damage, compressor surge also causes a major process upset. As mentioned previously, the pressure ratio must be dropped significantly before forward flow can be achieved. During this time, there will be zero flow to the downstream separations unit and pressure will be accumulating in the compressor suction. Typically, flaring occurs.
Another nasty side effect can occur during a flow reversal. A turbine-driven compressor will reach the overspeed trip if the governor cannot cut the steam back fast enough.
Surge Line Development
The primary requirement of a surge controller is to accurately predict where surge occurs. Most modern surge control systems rely on a reduction of the compressor head versus volume flow performance map. If a head producing flow meter is used (such as an orifice or venturi) the not-so-easily-measured values like molecular weight and compressibility cancel out as seen in Figure 1. This type of surge line prediction is very accurate under a wide range of in inlet and outlet conditions.
Head
σ σ ⋅ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ⋅ ⋅ W M 1 P P Z T R s d avg s(Volume Flow)
2 s s s P W M Z T R h ⋅ ⋅ ⋅ ⋅ 100% N 90% N 80% NFigure 1.
Anti-Surge Control Response
An anti-surge controller is typically tuned for slow response when the operating point is not close to the surge line. This is required for stable performance control. However, as the operating point gets closer to the surge line, more aggressive action is required to prevent a surge. A variety of techniques can be applied. While some control systems employ high gain, others step the control valve to a safe position.
Additional features of an anti-surge controller include a Maximum Valve Closing Rate and Adaptive PID Tuning. Both of these features aid in the recovery after a recycle valve “opening event,” restoring operating conditions to normal as quickly as possible without leading to another “opening event.”
Identify Interactions between the Surge and Performance Controllers
When designing a compressor control system, it is essential that both performance and anti-surge considerations are taken into account. It is also helpful to look at the interactions required between the two controllers, especially if the controllers are supplied by separate vendors.
Questions to ask are:
• Is the recycle valve needed to assist the performance controller for turndown? • Is the recycle valve needed to protect a section from over pressure (typically to
avoid lifting a relief valve)?
• Is the recycle valve needed to protect the compressor from low suction pressure?
• Are performance controller limits required to avoid surge?
There are too many possible permutations to investigate to determine the best solution based on the answers to the above questions. In most cases, though, a simple override to the anti-surge surge controller is all that is required to solve the problem. However, if the answer to the last question is yes, see the discussion below on decoupling for more details.
Decoupling Anti-Surge and Performance Controls
A charge gas compressor generally operates with constant suction and discharge pressure. The discharge pressure is set by the downstream separations unit. The suction pressure is controlled by the charge gas compressor’s performance control and is maintained for optimal furnace yield. These remain relatively constant whether the unit is running at full capacity or is partially loaded.
As furnaces are taken offline, the performance controller will lower the speed setpoint to maintain suction pressure. Per the example in Figure 2, if the speed setpoint is reduced below 90 percent, the compressor will surge at the rated pressure ratio. To prevent compressor surge, the speed must be not be lowered enough to cause surge and a recycle valve will have to be opened to maintain suction pressure.
One method of decoupling is for the performance controller to take control of the recycle valve to maintain suction pressure. Speed can be slowly reduced to bring the operating point to the control line.
Minimizing Recycle Flow
It should be obvious that the greater the recycle flow the greater the energy cost. Therefore, it is best to run with as little recycle flow as possible without subjecting the compressor to an unwanted surge event.
While a lower recycle flow saves energy, there simply may not be enough horsepower in the driver to sustain much recycle. In such cases, the safety margin must be kept to a minimum. If the turbine driver becomes overloaded, the speed will drop, causing the recycle valves to open further. This will result in additional load and will lead to a death spiral. The only way to stop this is for the operator to put the recycle valve in manual and close it.
When it is necessary to run continually near the surge line, it is better to have a conservative surge line. A conservative surge line is subtly different than a conservative safety margin. The control system with a conservative surge line, as shown in Figure 3, will apply its aggressive action earlier to prevent a surge than one with a conservative control line.
There usually are natural cycles in the flow even after the process has reached steady state. The safety margin must at least be as large these cycles; otherwise the surge controls will be fighting the performance controls and will upset the process.
It can be beneficial for the operator to have control over the surge margin (within limits). With this control, the operator has the benefit of knowing when the process is stable and when it is possible to reduce the margin. If there is a process upset and the operating point gets too close to the surge line, the margin can be automatically moved out. When operation is steady again, the margin can be reduced.
It is also useful to know how much flow will be recycled while operating on the control line. The engineering units of the safety margin will vary with the control system. Therefore, it is desirable for the surge controller to convert the safety margin into units of flow. The decision for setting the amount of the safety margin can be made more easily if it is based in units of flow.
Transmitters – Fault Tolerant Fallback Strategy
Quite often simplex transmitters are used to instrument a control system. Therefore, it is advisable to plan for the inevitable failure because; even with redundant transmitters there can be common mode failures such as plugged taps. If a failure is detected, it is common to either fallback to a design value or to the last known good value.
Most anti-surge control systems can handle either a direct or square rooted DP transmitter. However, one of the most common problems found during commissioning is that the square root function of the flow transmitter is out of sync with the control system.
Another common problem is condensation in the transmitter tubing upsetting the flow reading. It is best to locate the transmitter as close to the meter as possible. Also, it is best to
Pressure Ratio
Flow Surge
Figure 3 Conservative Surge Line Actual
Surge
Surge Line used for Controller
Safety Margin
avoid vertical tubing runs (think of liquid trapped in a straw with your finger on top of it); keeping the tubing off vertical will allow condensation to drain. Heat tracing the tubing also helps avoid condensation problems.
Recycle Valves
In most cases, two recycle valves are sufficient for a charge gas compressor. If there are too many recycle valves, the pressure ratio across each valve will be too small, resulting in very large and expensive valves. As long as each stage can handle the flow requirement for a downstream stage without choking, the stages can be grouped together under one recycle valve.
It is best to avoid a recycle valve across the entire compressor. The pressure ratio across a charge gas compressor is very high and can result in premature valve failure.
If there is a CO2 removal system integral with the compressor train, it is recommended
that a check valve is installed upstream to keep the system pressurized after a unit trip. This is another reason to avoid a recycle valve across the entire compressor since it will de-pressurize the CO2 removal system on a shut down.
Anti-surge valves need to be fast enough to respond to process upsets. In an Olefins unit, several seconds is typically acceptable for full open travel. Closing speed is not that important. Deadband should be no greater than .5 percent to avoid noticeable step changes to the process. However, this should not be hard to achieve if a digital valve is used.
Anti-surge valves should be sized with at least a 30 percent flow margin above the flow required to prevent surge at all operating conditions.
The governor valve and/or suction throttling valve should also be included in the evaluation since each one impacts the compressor performance and surge control. Typically .5 percent or less deadband provides adequate control.
Commissioning the Anti-Surge Control System
Commissioning the Anti-Surge Controls for a Charge Gas Compressor can be quite a juggling act. The compressor is typically started when there are several furnaces running and the separations unit is ready to accept process flow. Most of the process at this time is typically being flared and the last thing operations wants is to commission the surge controls. After all the planning for a successful control system, it would be a shame not to fully commission it. If planning is sufficient and is implemented with input from operations, it should be possible to get the commissioning completed.
Fortunately, the process takes on flow slowly and there is typically plenty of time to tune and test the surge controls while recycle is still required. However, this window will close once the flow to the process is higher than the flow required for the surge line.
There are no hard rules regarding the sequence of steps that should be taken to commission a surge controller. My preference is to start by tuning dynamics of the surge controller in a safe zone away from surge. This can be done by making small step changes that go unnoticed in the process but provide enough information to set and test the tuning constants.
After tuning the surge controls, the next step is testing the surge line. As a minimum, the operating point should be walked back to the surge line. If horsepower is a problem or there is a probability that the compressor will operate on the control line for long periods, it is
justifiable to reduce the flow until surge is detected. My preference is to leave the controller in automatic and reduce the safety margin. If a surge is detected, the surge controller will open the valve faster than an operator could if it was in manual.
Typically a charge gas compressor has two or three recycle valves. The surge line can be tested in sections defined by the recycle loops. When testing a section for surge be sure that none of the other stages are close to their surge point. If the compressor does surge, there will be minimal impact as long as only one section surges.
Normally, it is desired to maintain constant speed while surge testing. However, as the operating point is moved toward the surge line, the process flow will increase. To maintain constant flow to the process (and minimize operator stress), another recycle valve should be opened further or the speed reduced.
After setting the surge line and normalizing the surge controller, the performance controller should be placed in automatic and tested for stability with the surge controller. Step changes should be made to both controllers in order to finalize tuning.
Conclusion
A comprehensive surge controller that incorporates an accurate surge line and stable control features while complementing the performance control is key for success. In addition, transmitters must be properly installed, valves must be strategically placed and have
responsive actuators. With planning, the surge controls can be properly commissioned. The objectives to protect the compressor using minimal recycle flow and providing stable control can be met.
Glossary
Polytropic Head (Hp) – The head produced by a compressor in a polytropic process.
(1)
Where: R = Gas Constant, Zavg = average compressibility across compressor,
Ts = Absolute Suction Temperature, Pd = Discharge Pressure, Ps = Suction Pressure, σ = Polytropic Exponent
(2) Where: k = Specific Heat Ratio, np = polytropic efficiency
Control Line – The line plotted on the surge control map that is the sum of the Surge Line plus the Safety Margin. It is used to generate the setpoint for the surge controller.
Performance Control – The control that is responsible for maintaining the forward flow to the process. Typical compressor performance control is based on pressure (either suction or discharge) or flow (either suction or discharge).
Safety Margin – The horizontal distance between the Control Line and the Surge Line. The safety margin is the range that the surge controller
Surge Line – Line that defines where surge occurs on a compressor.
Operating Point – Expressed as X-Y coordinates on a surge control map. The vertical axis represents head and the horizontal axis represents flow.
Valve Deadband – The amount the valve demand may be varied without any change in valve position. σ σ ⋅ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎝ ⎛ − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ⋅ ⋅ = MW 1 s P d P s T avg Z R p H
Volume Flow – The volume flow as measured by a head producing flow meter is a function of the differential pressure across the meter and density.
(3) Where h = DP across the meter
C’ is the flow meter constant
(4) Where: MW = Molecular Weight, P = Pressure,
R = Gas Constant, T = Absolute Temperature, Z = Compressibility
Substituting the terms for density into equation 3:
OBJECTIVES OF ANTI-SURGE
CONTROL
CONTROL
Brian Piercy
Turbomachinery Control Consultant
Invensys Operations Management
Year 2011 Spring National Meeting
Ethylene Producers’ Conference IL
h
Objectives of Anti-Surge Control
Charge gas compressors are typically turbine-driven centrifugal
compressors
The objectives of anti-surge control covered in this presentation are:
The objectives of anti surge control covered in this presentation are:
• Protect Compressor from Surge
• Maintain Stable Process Control
• Minimize Energy Cost
• Reliable Operation
Centrifugal Compressor
Diffuser
Return Bend
Impeller
R t
Rotor
Gas is drawn into the impeller and exhausts at
high velocity into the diffuser where it makes
a bend into the next impeller.
Conversion of Kinetic Energy to
P t
ti l E
Potential Energy
Velocity converted to Pressure
Velocity converted to Pressure
If the pressure generated
is less than the process
fl
ill
Flow
pressure, flow will
reverse.
Rotor
The Surge Cycle
Flow reversal - Load is immediately lost causing a thrust
Pressure ratio decreases while flow is reversed
Forward flow resumes - Load immediately returns causing a thrust
Cycle repeats, if corrective action is not taken
Consequences of Surge
Axial shift can wipe seals and damage bearings
Consequences of Surge
Production of upset and downstream processes is disrupted
Off spec product
Environmental impact due to flaring
Loss of load can result in overspeed trip
Compressor Performance Map
The performance map defines the flow produced by the compressor at
various speeds and head
Constant speed lines
Surge
Polytropic Head
100%
90%
Choke
Sonic velocity
80%
So c e oc ty
reached
Volume Flow
Slide 8Polytropic Head and Flow Equations
Polytropic Head
S
y
p
σ
⎟
⎟
⎟
⎞
⎜
⎜
⎜
⎛
−
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⋅
⋅
1
s
P
d
P
s
T
avg
Z
R
Surge
σ
⋅
⎟
⎠
⎜
⎝
⎝
⎠
MW
s
P
Volume Flow
s
s
T
Z
R
h
C
⋅
⋅
⋅
Where:
Head flow meter
s
s
s
P
MW
C
⋅
Where:
(orifice or venturi)
h = Flow Meter DP
Slide 9Cancel Variables and Remove
C
t
t
Constants
Polytropic Head
σ
⎟
⎞
⎜
⎛
⎟
⎞
⎜
⎛ d
P
σ
⋅
⎟
⎟
⎟
⎠
⎜
⎜
⎜
⎝
−
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⋅
W
M
1
s
P
d
P
s
T
s
Z
(Volume Flow)
2
h Z T
M W P
s
s
s
⋅
⋅
⋅
Slide 10Predicting Surge
Remove Constants and Cancel Variables
Head Term
P
P
d⎛
⎝
⎜
⎞
⎠
⎟ −
⎛
⎝
⎜⎜
σ1
⎞
⎠
⎟⎟
P
s⎝ ⎠
⎝⎜
⎠⎟
σ
⎞
⎛
σ
⎞
⎛
P
T
Where:
Flow Term
h
P
s
s
Td
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
1
2
1
2
T
T
P
P
P
s
Ps
Pd
log
Ts
Td
log
=
σ
Slide 11Predicting Surge
If the polytropic exponent is constant, the map can be further reduced
Head Term
Head Term
P
P
d
s
Flow Term
h
P
s
s
Slide 12Surge Control Line
d
P
P
s
P
h
s
P
h
Slide 13Anti-Surge Control Features
Recalibration of safety margin if surge is detected
PID Action with Adaptive Gain and Integral
Aggressive action as operating point reaches surge line
Fast Opening / Slow Closing of Recycle Valve
Decoupling with Suction Pressure Control
Decoupling
A ti S
/S
ti
P
C
t l
Anti-Surge /Suction Pressure Control
A charge gas compressor typically operates at constant pressure ratio
Traditionally a suction pressure controller is cascaded to a speed
Traditionally, a suction pressure controller is cascaded to a speed
controller
Discharge pressure is maintained by downstream separations unit
When furnaces are brought off line the suction pressure controller
When furnaces are brought off line, the suction pressure controller
reduces speed to maintain pressure
Why Decoupling Is Required
In the example, if speed is reduced less than 90%, the compressor will
surge regardless of how fast the recycle valve is opened
Surge Mitigation – Decreasing Load
When making large reductions in load:
Slowly Reduce Speed
Open a recycle valve to maintain suction pressure
Interaction with Suction Pressure
C
t ll
Controller
The recycle valve may be required to:
• Maintain suction pressure controller if the turbine reaches
minimum governor
• Protect an interstage pressure from lifting a relief valve
Charge Gas Compressor Train
Separations Caustic
Scrubber
Minimize Recycle Flow
Less recycle flow results in lower energy cost
Too much recycle flow can overload the turbine and cause loss of
speed control
speed control
Display safety margin in units of flow
Recycle Valves
Actuator < .5% deadband and < 3 seconds opening time
Digital valve positioner is recommended
Size > 30% additional capacity above surge flow
Valve Sizing
Although more flow is required,
higher Delta P may result in smaller valve size
P
P
d
P
s
Although less flow is required,
lo er Delta P ma res lt in larger al e si e
Flow Term
h
P
s
lower Delta P may result in larger valve size
Slide 23