Abstract

Centrifugal compressors, especially variable speed machines, offer the user considerable advantages resulting from the availability of a wide range of operating flow and head. The performance map with multiple speed lines, for example, can increase from two (2) to three (3) dimensions by simply adding changes to suction conditions. The convenience afforded by the wide range of operating points, however, presents a challenge in modeling the performance. The traditional methods of modeling - table lookups and fan laws fail to sufficiently reduce and eliminate inaccuracies due to their tedious nature, the shape of the performance curve and interpolation method. They work, but not well enough.

This paper proposes a new method of modeling compressor performance using non-dimensional performance parameters apart from lookup tables and fan laws. The case is made for utilizing work input coefficient, polytropic head coefficient and total flow coefficient as non-dimensional parameters that can not only model the compressor with all the variability mentioned above, but also indicate when there is a performance problem. This new approach proposes the use of real time input data and response capabilities. Through the methods put forward in this paper, a user will have a realistic performance model, and a performance monitoring and alert system that can be used as a real-time feedback mechanism into the gathering system model.

Introduction

In today's highly competitive low margin natural gas market gaining an edge by keeping production costs low is an imperative. Identifying and realizing any and all efficiencies in natural gas production and transmission could significantly improve the profit margin over long term operation. The centrifugal compressor, as a critical component in this process, provides the flexibility in capacity range and head as required by the process. Despite these benefits, it has been a challenge to monitor the aerodynamic health of the machine considering the number of variables affecting the operating condition of the compressor; especially the inlet conditions, compressor speed, head, and inlet mass flow. A common evaluation, for example, utilizes the compressor's OEM (Original Equipment Manufacturer) performance curves. While the OEM provides a variety of performance curves based on different sets of operating conditions, it leaves a lot of room for interpretation and, consequently, error. With all the variability mentioned above, finding the machine's current operating point on a predefined set of curves can be difficult. This generally leaves the engineer to make an educated guess regarding machine performance against the original design.

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