Research Papers: Gas Turbines: Oil and Gas Applications

Single Versus Dual Recycle System Dynamics of High Pressure Ratio, Low Inertia Centrifugal Compressor Stations

[+] Author and Article Information
K. K. Botros

 NOVA Research & Technology Center, 2928 16 Street NE, Calgary, Alberta, Canadabotrosk@novachem.com

J. Eng. Gas Turbines Power 133(12), 122402 (Aug 31, 2011) (12 pages) doi:10.1115/1.4004114 History: Received April 08, 2011; Revised April 10, 2011; Published August 31, 2011; Online August 31, 2011

Compression systems are designed and operated in a manner to eliminate or minimize the potential for surge, which is a dynamic instability that is very detrimental to the integrity of the compressor unit. Compressor surge can occur when compressors are subjected to rapid transients such as those occurring following an emergency shutdown (ESD) or a power failure, which in turn, requires fast reaction. To prevent this from occurring, compressor stations are designed with single or dual recycle systems with recycle valves, which are required to open upon ESD. There has been extensive debate and confusion as to whether a single recycle or a dual recycle system is required and the circumstances and the conditions under which one system or the other must be used. This paper discusses this crucial design issue in detail and highlights the parameters affecting the decision to employ either system, particularly for high pressure ratio, low inertia compressors. Parameters such as gas volume capacitance (V) in the recycle path, compressor power train inertia, compressor performance characteristics, the recycle valve coefficient (Cv), prestroke and stroke time, and check valve dynamic characteristic are crucial in determining the conditions for dynamic instabilities. A simple analytical methodology based on the perturbation theory is developed that provides a first-cut analysis to determine if a single recycle system is adequate for a given compression system. The concept of an inertia number is then introduced with a threshold value that determines which recycle system to use. Techniques to circumvent compressor surge following ESD are discussed and their respective effectiveness are highlighted including when and if a delay in the fuel cutoff will be effective. An example of a case study with actual field data of a high pressure ratio centrifugal compressor employed in a natural gas compressor station is presented to illustrate the fundamental concept of single versus dual recycle systems.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Single versus dual recycle systems

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Figure 2

A schematic of the different phases of head flow through the compressor during an ESD process [11]

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Figure 3

Determination of maximum drop in compressor speed before compressor surging following ESD

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Figure 4

Example of a high head low inertia compressor characteristics

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Figure 5

Schematic of the example station layout showing the axial piping distance between the cold recycle valve and compressor suction and discharge sides of the compressor unit

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Figure 6

Measured data during ESD of the compressor station shown in Fig. 5 using only the cold recycle system

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Figure 7

Dynamic simulation results of the ESD operation of Fig. 6

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Figure 8

Measured data during ESD of the compressor station shown in Fig. 5 using both hot and cold recycle systems

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Figure 9

Dynamic simulation results of the ESD operation of Fig. 8

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Figure 10

Results of dynamic simulation of an ESD operation from slightly higher initial flow conditions, and using both hot and cold recycle systems

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Figure 11

Results of dynamic simulation of an ESD operation using only the cold recycle system with 2 s delay in the fuel shut off



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