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Compressor ControlTechnical Product GuideSingle Stage

Inter-cooled

Refrigeration

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Compressor Control Technical Product Guide

COPYRIGHTby Triconex Systems, Inc. La Marque, Texas, U.S.A.

All rights reserved. No part of this work covered by the copyright hereon may be reproduced or copied in any form or by any meansgraphic, electronic, or mechanicalwithout first receiving the written permission of Triconex Systems, Inc., La Marque, Texas, U.S.A.

Printed in U.S.A. 1999

NOTE: Triconex Systems, Inc. reserves the right to make improvements in the design, construction, and appearance of its products without prior notice.

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Table of ContentsWhat is Surge? ....................................................................................... 7 Performance Curves ............................................................................. 7 Curve .................................................................................................. 7 Compressor Map ................................................................................... 8 A Universal Surge Curve .................................................................. 13 Determining HP/A02 .................................................................................................................................. 13 Determining (Q/A0)2 .................................................................................................................................. 14 Plotting the Surge Line ...................................................................... 14 Summary of Features ......................................................................... 21 Choice of Pressure Rise or Pressure Ratio Algorithms ............... 22 Safety Margin Recalibration ............................................................ 22 Setpoint Hover .................................................................................... 22 Dynamic Adaptive TuningTM ...................................................................................................... 23 Non-symmetrical Valve Response .................................................. 23 Purge and Start-up Logic................................................................... 24 Manual Control ................................................................................... 24 Proportional Function ........................................................................ 24 Valve Prep ............................................................................................ 25 Dump Output ...................................................................................... 25 Valve Linearization and Reversal ................................................... 25 Compensated Recycle Temperature Controller ........................... 25 Process Pressure Control ................................................................... 29

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Compressor SurgeThis document describes the implementation of a centrifugal compressor surge control in the TRISENTM digital governor or control system.in work done per unit mass of the fluid. Foot pounds per pound (ft-lbf/ lb) and kilojoules per kilogram (kj/kg) are the common English and Metric units. For a given impeller design, diameter, and rotational speed, the amount of work energy expended is dependent on the volume flow at the suction. A0; that is, for a given compressor speed, there is only one valid set of gas properties (MW, Z and k) and suction temperature (Ts). If the gas composition and/or suction temperature is variable, multiple curves must be provided to show the performance of the stage. See the previous figure.

What is Surge?Surge occurs in a turbo compressor when discharge head cannot be sustained at the available suction flow. Surge occurs at specific combinations of head and flow, as defined by the compressor manufacturer's performance curves. One or more of the following can result from surge: Unstable operation Partial or total flow reversal through the compressor Disrupted process Mechanical damage to the compressor

CurveCompressor manufacturers use "" curves to define the performance of

The polytropic head developed for a given fluid (gas) and inlet conditions is proportional to the pressure coefficient "," as defined by equation (1).

1.4

1.22 3 1

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1.0

Surge is usually accompanied by the following: Increase in discharge temperature Reduction in discharge pressure Increase in vibration Sharp rise in inlet temperature

Pressure coefficient " " .60.40

.80

80

p2 1 1 2 3 3

Polytropic Efficiency 60 p40

.20

20

0

10

20

30

40

50

60

70

80

% Q / N

Performance CurvesA turbo compressor imparts energy to the gas by accelerating it through a rotating impeller to increase its velocity and pressure. This kinetic energy is then converted to a higher pressure in a diffuser. The amount of energy imparted to the gas is defined

Figure 1individual impeller stages. A typical "" curve is shown in Figure 1. "" indicates the polytropic head provided by the stage for a given volume flow at the suction. Each "" curve is specific for a given N/ 7

Impeller

Equation (1)

Hp = where:

U2 g

Compressor SurgeHp = Polytropic head = Pressure coefficient of the impeller for the specific gas U = Impeller peripheral velocity ft/sec g = Gravitational constant 32.17 ft/sec2

Compressor MapWhile the "" curves are a tool used in the design stage, the final compressor characteristic is defined by a performance curve, or compressor map. This is a series of impeller "" curves combined as a multistage compressor. An example is shown in the next figure. NOTE: The Hp versus Q curve is good for one set of gas conditions (A0) only.

significant deviations from this ideal curve. As the speed increases, surge moves to later impeller stages, due to volume reduction by the initial stages, and consequent lower suction volume flow available to the later impeller inlets. The next figure is an example. Compressor performance maps are presented in various forms. Most provide a basic performance plot, as shown in Figure 3, but with additional curves, relating inlet temperature, molecular weight, pressure ratio, suction pressure, and driver horse-

In the Figure 1 example, the two sets of three curves indicate the performance of the impeller for three different gas conditions. The term A0 is used to define the sonic velocity of the gas at the suction conditions and is determined by equation (2).

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Surge Line

Equation (2)

60 Hp Polytropic Head 40 (1000 Ft lbf/lb) 20 80% 0 0 500 1000 1500 2000 2500 Q - Inlet Flow (acfm) 105% 100% 90% Constant Speed Lines

A0 =where:

1545 k Z s Ts g MW

A0 = Sonic velocity of the gas at the inlet conditions (ft/sec) k = Ratio of specific heats Cp/Cv Zs = Compressibility factor Ts = Inlet temperature R g = Gravitational Constant, 32.17 ft/sec2The upper set of curves, in conjunction with equation (1), relate Hp to Q/ N (inlet flow (acfm)/speed (RPM)). The lower curves relate Q/N to polytropic efficiency hp. Curves 1 through 3 show performance for heavy, medium, and light gases, respectively. The left extreme of each line indicates the surge point.

Figure 2 Compressor Performance Map The Surge Line is not linear with respect to flow. In a low head, single impeller machine, compressing a light gas such as air, the surge line tends to follow the fan law. This law states the following: Capacity is proportional to rotating speed. Head is proportional to the inlet flow squared. Power required is proportional to the speed, cubed. Multi-impeller high head machines can have surge lines which have 8 power. Design gas conditions, such as compressibility factor (Z), and specific heat (k) are usually noted. If Ts and/or MW are variable, several surge curves may be plotted, showing surge limits for different gas compositions and suction conditions. From a surge control standpoint, the challenge is to keep the compressor out of surge without wasting energy on excessive recycling. This requires that the surge point be precisely computed from measurable, compres-

Compressor Surgesor-operating conditions. This goal will be addressed in the construction of the Surge Line. First, we will describe how the TRISENTM system uses the recycle valve to avoid surge.

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60 Hp Polytropic Head 40 (1000 Ft lbf/lb) Surge Line 105% Speed 100% Speed 90% Speed 20 80% Speed

0 0 500 1000 1500 2000 2500 Q - Inlet Flow (acfm)

Figure 3 Multi-Impeller, High Head Performance Curve

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Compressor Surge

NOTES ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________