Taiwan Compressor Seminar Turbomachinery Controls Confidential Property of Schneider Electric | Page 3 Type of Compressor Positive Displacement Dynamic Reciprocating Rotary Single

Turbomachinery Controls (TMC) Compressor Steam Turbine Controls Introduction - Theory and Process Basics

Confidential Property of Schneider Electric

Taiwan Compressor Seminar

Page 2 Confidential Property of Schneider Electric |

Contents

Schneider Electric – Turbomachinery Controls (TMC) • Compressor Type Generalities

• Compressor Surge and Performance Map

• Equations and Control Strategy

• Instrumentation Considerations

• Control Features Overview

• Performance Control Overview


Type of Compressor

Positive Displacement Dynamic

Reciprocating Rotary

Single Acting

Double Acting

Diaphragm

Lobe

Screw

Vane

Scroll

Centrifugal Axial


Compressor Performance Map

Surge C

ontrol L

ine

N3

HP Surge L

imit L

ine

N2N1

Q2 ICFM

Variable Speed Compressors

Process Control Loop provides a speed setpoint to speed governor to meet process demand

Constant Speed Compressors

Process Control Loop provides output to throttle valve (suction or discharge) which throttles compressor flow to meet process demand

Surge C

ontrol L

ine

N

HP Surge L

imit L

ine

Q2 ICFM


Surge Cycle

1

23

4

Pressure

Flow

5

Compre

ssor

Surge L

ine

1. System resistance increases, more discharge pressure required and operating point moves up the curve.2. Operating point nears the Surge Limit.3. Operating point goes into Surge Region.4. Flow reverses as discharge pressure drops.5. Drop in discharge pressure re-establishes forward flow. Compressor resumes full flow.

Since resistance has not changed, surge will continue until cycle is broken.

• Operating Point moves from Stability into Surge at the Speed of Sound at the Gas Conditions in the Compressor.

• A complete surge cycle takes from ½ to 3 seconds, depending on the size of the compressor, piping volume.


Compressor Performance Equations


Readily Measurable Variables

• Following variables can be measured from field instruments

MWTRZ

kk

PPH

kk

P1

1

1

2

11 ⋅⋅

⋅

−⋅

⋅

−

=

⋅−

ηη

1

11 PMW

TRZhQ⋅⋅⋅⋅

=

h ,T ,PP

11

2


Compressor Performance

• The Compressor Performance Map can be redrawn as the polytrophic head vs. the inlet volume flow squared.

• The relationship between head and flow is not changed by this modification.

HP

Q2 (ICFM)


Simplification of Head Factor

• Rewrite flow equation squared and remove term for both axis

• Y-axis results

• X-axis results

MWTRZ 1⋅⋅

MWTRZ

kk

PPH

kk

P1

1

1

2

11 ⋅⋅

⋅

−⋅

⋅

−

=

⋅−

ηη

1

121 PMW

TRZhQ⋅⋅⋅⋅

=

−⋅

⋅

−

⋅−

11

1

1

2

kk

PP k

k

ηη

1Ph


Polytrophic Head vs. Efficiency Relationship

• An analysis of equation of X-axis with actual compressor data shows polytrophic head is minimally affected by minor variations in k and hp, and is predominantly affected by the Pressure ratio.

• Recall that Rc=Pd/Ps and the challenge has been met. The variables Pd and Ps are readily measured by conventional pressure transducers.

PolytropicHeadHp

0 2010 30 40 50 60 70 80

Flow, Q

0 2010 30 40 50 60 70 80

20

40

60

80

100

PolytropicEfficiency

ηp

7678

8081

8078

7673

67


Head vs. Flow Part 4

If changes in specific heat ratio and polytrophic efficiency are small then σ can be treated as a constant and removed.


Pressure Rise or Ratio Method

Pressure Rise Method Pressure Ratio Method


Head vs. Flow Part 5

If changes in specific heat ratio and polytrophic efficiency can’t be ignored, use following:


Flow Meter Examples

Flow Tubes Venturi

Annubar Delta Press Transmitter Turbine Meter

Orifice Plate


Anti-Surge Control Features • Pressure Ratio vs. v/Ps surge line

(Invariant to Pressure, Temperature, MW Compressibility, Speed)

• Incremented safety margin if surge occurs

• Setpoint hover function

• Surge PID Controller with Adaptive Gain

• Surge Override Action

• Process Friendly Open / Close Valve Limiting

• Decoupling between Process and Surge Controls

• Fallback strategy for faulty transmitters

• Turbine / Compressor Simulation


Manual Control

Full Authority Manual Control

It allows the valve to be closed, regardless of the action of the Surge Controller. This option is useful for testing and setup, but should not be configured for normal operation. If the system is left in manual operation, the Surge Controller will not be able to open the valve to prevent surge.

Partial or Limited Authority Manual Control

This option sets a minimum recycle valve limit, allowing the operator to open the recycle valve, but not close it if the controller needs to open it to avoid surge.


Actual Operating Margin


Safety Margin


Set point Hover


Set point Hover

In most applications, the compressor will not operate continuously, or for extended periods on the Surge Control Line. When operation is to the right of the control line (safe area), the setpoint to the Surge Controller is ramped (at a configurable rate) to within a configurable percent of the current h value.

The following occurs after a small, quick movement toward surge, past the hover setpoint :

• Immediate Opening of the Recycle Valve

• Hover setpoint is then ramped down (at the same rate) until the recycle valve closes.

• New operating point is established


Safety Margin Recalibration

If the system detects a transition of the operating point across the Surge Line, indicating that surge has occurred, it automatically readjusts the Surge Control Line to the right to add additional safety margin.

Some conditions which can result in surge are: • Shifting of the Surge Line due to compressor wear • Transmitter out of calibration • Insufficient safety margin • Drastic changes in process conditions • Incorrect surge line used • Improper tuning or set up of the surge prevention system

Each time a surge transition is detected, the safety margin is incremented (control line moved to the right) by a calibrated amount.

Entering a new safety margin sets the transition counter to zero, and sets the recalibrated margin to equal the entered value.


Adaptive Tuning


Surge Override


Surge Override

• The system has a proportional-only term, which forces the recycle valve to open, independently from normal P+I controller action. This occurs, due to a severe process upset, if the operating point moves to the left of the Surge Control Line, and the normal controller tuning provides insufficient response.

• This term begins to open the valve at a specified configurable margin (usually 7% from the surge line), to the left of the Surge Control Line, and fully opens the valve as the operating point is moving to the left and reaches the Surge Line.

• In other words, the valve is opened proportionally to the instantaneous operating margin, less the initiation value. The proportional term is applied through a signal selector, and the anti-windup action of the controller forces the controller output to track the proportional term.

• This feature will protect the machine, even if the Surge Controller is poorly tuned.


Proportional Function Map


Process Overrides

• Low Suction Pressure

• Higher Discharge Pressure

• Higher Discharge Temperature

• Motor Amp Limiter (Axial Compressors Only)


Anti-Surge Controller Overview


Transmitter Faults Fallback Strategy

Faulty Pressure or Temperature Transmitter

• Minimum Flow Fallback

• Keep last good three scan Value

Faulty Flow Transmitter (Options)

• Ramp Surge Valve Full Open

• Hold Surge Valve in Last Position plus/minus additional safety offset depending on the last good actual margin

• Full Manual


Performance Control

Performance Control is based one or more of the following:

• Suction Pressure

• Discharge Pressure

• Flow

• Load (Motor Amps)

Performance Control is achieved by one of more of the following:

• Manipulating Turbine Speed

• Manipulating Suction Valve or Inlet Guide Vanes

• Manipulating Recycle or Vent Valve


Dynamic Break Point

In effect, the Triconex Dynamic Process Control Algorithm is a special split-range controller with a variable breakpoint. The point at which the capacity control switches from controlling the speed to controlling the recycle valve opening is called the Dynamic Breakpoint. This point is dynamic because it is variant depending on the process flow and pressure demands.

Questions?


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Taiwan Compressor Seminar Turbomachinery Controls Confidential Property of Schneider Electric | Page 3 Type of Compressor Positive Displacement Dynamic Reciprocating Rotary Single