In the article, we learn about how to read a Centrifugal Compressor Performance Curve. If you are new to the compressor, this article will definitely make yourself familiar with the centrifugal compressor performance curve. If you already know about the centrifugal compressor performance curve, definitely this article will refresh your knowledge. Now let we see the performance curve.
The centrifugal pump performance curve consisting of the following three important curves
- Differential Pressure (or some cases Polytrophic Head) Vs Inlet Flow rate
- Brake Power Vs Inlet Flow rate
- Efficiency Vs Inlet Flow rate
Differential Pressure Vs Inlet flow rate
In this curve, the differential pressure indicated in the vertical axis and Inlet Flow rate in the horizontal axis. With an increase in compressor inlet flow rate, the head developed by the compressor start decreases. The compressor develops maximum differential pressure (head) called Surge (3A). The flow and pressure corresponding to the surge point refers to surge flow and surge pressure.
When the inlet flow rate increases (Refer below performance curve), the differential pressure (head) keep on decreases up to a maximum flow rate (right side of the curve) called as “Stonewall point” or “Stall Point”. The Stonewall point or Stall Point refers to the maximum operating range of a centrifugal compressor.
The inlet flow rate and differential pressure (head) are related to each other, one cannot be changed without varying the other parameter.
Brake Power Vs Inlet Flow Rate
In this curve, the Break power represents in the vertical axis and Inlet Flow rate on the horizontal axis (Refer below performance curve). As we have seen from the curve, the brake power does not start from zero, the minimum power represents in the curve to overcome the frictional losses and centrifugal compressor inertia.
Based on the compressor flow rated, the rated power obtain from this curve.
Efficiency Vs Inlet Flow Rate
As we have seen from the performance curve, the efficiency curve is raising as the flow rate increases. But at one point the curve reaches the maximum and starts decreasing. The maximum point is called Best Efficiency Point (BEP). It’s always preferred to run the centrifugal compressor at the best efficiency point (BEP), but in the practical case, we are not able to match the required operating point with compressor BEP.
If the centrifugal compressors run at or near to BEP, not only we get higher efficiency and also the less vibration of the compressor.
How to read a Centrifugal Compressor Performance Curve?
Consider a centrifugal compressor, the required operating points are referred below
Rated Flow rate – 66000 kg/hr
Suction Pressure – 10 bar
Discharge Pressure – 65 bar
Differential Pressure – 55 bar
Operating Point
The Red Colour-mark represents the operating point of a centrifugal compressor. The flow corresponding to the operating point refers to “Operating Flow” and the corresponding discharge pressure is called “Operating Discharge Pressure”. As per our example, the operating flow is 66000kg/hr and discharge pressure is 65 bar.
Sometimes the compressor manufacturer provides a polytropic head or differential pressure instead of the discharge pressure.
Speed
As we have seen from the performance curve, the Flow rate Vs Discharge Pressure curves for corresponding speed. (ie., for various speed of the compressor the flow rate Vs Discharge pressure curves change accordingly)
As per the curve, the point (1) represents the 100% speed (also called rated speed) of the curve and the point (1A) represent the maximum continuous speed of the compressor. In this case, the maximum speed is 105% of the rated speed.
Stall or Stone Wall Point
The maximum flow developed by a centrifugal compressor is called Stall or Stonewall Point (2). Below this point, the compressor is aerodynamically unstable.
Surge
The surge point refers to the minimum flow in the centrifugal compressor, below with momentary reversal of rotor occur. Refer performance curve for the surge represented as a point (3).
To avoid the compressor surge, an anti-surge system installed in the compressor system. As per the curve, the Anti-surge line represented as (Point 3A). Normally the Anti-surge line considered at 10% higher than the surge flow.
In process compressor, the compressor operates at various cases. It’s necessary to prevent the compressor operates in the left side surge line. If the compressor operates in this region, it may lead to catastrophic failure.
Efficiency
The operating flow of the centrifugal compressor intersect at the efficiency curve, the corresponding efficiency in the Y-axis called “Operating Efficiency” of the compressor. This point represented as the point (4) in the curve.
As we have seen the efficiency curve, the curve starts rising and reach a maximum point and then starts decreasing with an increase in flow rate.
The maximum point in the efficiency curve called “Best Efficiency Point (BEP)”, it’s represented as the point (4A).
Brake Power
The “brake power” or “rated power” represents the power required for the compressor to deliver the rated/operating flow at rated discharge pressure or head.
As per the curve, the operating flow of the centrifugal compressor intersect at the Brake power curve and corresponding power in the Y-axis called “brake power of the compressor”. It’s represented as the point (5) in the curve.
In the current example, the brake power is 3000 kW.
The power corresponding to the “End Of Curve” in the brake power curve represented as a point(5A) in the Curve. Based on the brake power required, motor kilowatt or hp will be selected.
If the compressor is driven by a steam turbine, then the steam turbine has sized for 110% of the maximum power required for the complete compressor train.
Turndown
It’s the percentage of change in capacity between the Rated-Capacity and the Surge Point capacity at the Rated-Head when the unit is operating at Rated-Suction Temperature and gas composition. Refer below the figure for turndown representation.
Conclusion
For proper selection of Centrifugal Compressor, the below mentioned points to be check in the centrifugal compressor performance curve.
- Normal Operating / Rated Flow
- Normal Operating / Rated Head
- Speed of the Compressor
- IGV opening (if IGV will use for flow control, then IGV full close represent the minimum flow of compressor and IGV full open (100%) represent the maximum flow to the compressor )
- Surge flow & Anti-surge limit
- Efficiency
- Compressor brake power
- End of Curve power
- Turn down capacity