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Standard Compressor Performance Map

The Standard Compressor Map is described by polytropic head, Hp, versus actual volumetric suction flow, Qa, and compressor speed, N, You can see in following figure. Depending upon the compressor configuration and instrumentation, changes in molecular weight, temperature, and compressibility are compensated for accurate representation of the compressor operation.

Standard Compressor Performance Map

Standard Operating Point

The definition of an operating point is necessary for any digital controller. It is easy for a person to identify the current operating parameters and relate these parameters to a surge control line on a compressor map. However, this is a difficult task for a controller to perform in varying process applications. Therefore it is necessary to define an operating point as a single number that can be handled easily.To make easy more these calculations.
The Standard compressor map can be defined in polytropic head, Hp, versus volumetric inlet flow, Qa. The operating point is also defined using these parameters. Concluded We can say, the operating point is described as volumetric inlet flow squared divided by polytropic head.

The result is a single number that identifies the operating point that can easily be manipulated by the controller and compared to a corresponding point on the Surge Control Line.
This calculation can be expanded to show that the operating point is invariant of the gas composition. All of the critical parameters in this equation can be measured, and the others can be estimated or assumed constant. A detailed explanation of the necessary equations can be found in the Operating Point Calculations section in our Website. For a simplified view of the measurements necessary to determine the operating point, refer to the process control diagram in Figure

Here it can be seen that the volumetric flow calculation is carried out using three measurements. (The instrument tags are given as an example).

Overview of Compressor Control Functional Block diagram

Standard Surge Control Line

We normally program our compressor map in anti surge controller and data points from surge line are collected from the compressor map. Now question is What is surge control line SCL, The combination of surge line and safety margin line(user configuration as  % of flow of surge).On SCL Line our limitation ends and we modulate anti surge control valve.
The Boost Line, or Backup Line, give more anti-surge protection. When the operating point (OP) reaches this line, a fixed response is triggered to prevent a surge.We can define here Boost Line is percentage of flow behind the Surge Control Line (to the left of).
The Surge Limit Line has series of maximum twelve operating X-Y (with maximum of twelve) points and programmed in controller. Compressor maps can be described in different units. The CCS supports the following compressor map unit entries:
• Discharge pressure versus actual flow, P2=F(flow)
• Pressure ratio versus actual flow, P2/P1=F(flow)
• Polytropic head versus actual flow, H=F(flow)
• Operating point versus reduced head, Q2/H=F(red head)
Occasionally the given compressor map is described in a different unit and will need to be converted.
Additionally, surge limits might be unproven or unknown, so it is sometimes desirable to determine the values used for the surge points by field mapping the compressor.
It is recommended that at least six points are entered. Points must be entered in ascending order, meaning that the lowest flow will be entered at point 1. Compressors typically have higher flow requirements with higher head values.




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1 Response

  1. September 23, 2018

    […] COMPRESSOR MAP OF COMPRESSOR CONTROL SYSTEMWhat is Surge in Compressor […]

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