In the article, we will discuss 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 compressor 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

Contents

### Differential Pressure Vs Inlet flow rate

In this curve, the differential pressure is on the vertical axis and Inlet Flow rate is on the horizontal axis. With an increase in compressor inlet flow rate, the head developed by the centrifugal compressor start decreases. The compressor develops maximum differential pressure (head ) is called Surge (3A). The flow and pressure corresponding to the surge point are called surge flow and surge pressure.

When the inlet flow rate is increased to the right (Refer below curve), the differential pressure (head) keep on decreases up to a maximum flow rate. It is called as “Stonewall point” or “Stall Point”. This is maximum operation range of a centrifugal compressor recommended by the compressor manufacturer.

The inlet flow rate and differential pressure (head) are related to each other, one cannot be changed without varying the other parameter. The relationship between inlet flow rate and differential pressure (head) is locked until wear or increased shaft seal clearance of the centrifugal compressor.

### Brake Power Vs Inlet Flow Rate

In this curve, the Break power is on the vertical axis and Inlet Flow rate is on the horizontal axis (Refer below performance curve). As we seen from the curve, the brake power does not start from zero, the minimum power represents in the curve is to overcome the frictional losses and centrifugal compressor inertia.

From this curve, the required brake power is obtained from the corresponding inlet flow rate.

### Efficiency Vs Inlet Flow Rate

As we seen from the performance curve, the efficiency curve is raising as the flow rate increase. Then the curve reaches the maximum point and starts decreasing. The maximum point is called Best Efficiency Point (BEP). It is 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 centrifugal compressor 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?

Now we will various curves and point so check in the centrifugal compressor performance curve.

Consider a centrifugal compressor with

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 the centrifugal compressor. The correspond flow is called 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 will provide polytropic head or differential pressure instead of the discharge pressure. The below formula is used to calculate the polytropic head.

## Speed

As we 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) represent 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 the centrifugal compressor is called Stall or Stonewall Point (2). Below this point, the compressor is aerodynamically unstable.

## Surge

The **surge** is the minimum flow in the centrifugal compressor, below with momentary reversal of rotor is occur. The in performance curve the surge is represented by the point (3).

In order to avoid the compressor surge, an anti-surge system is used. As per the curve, the Anti-surge line is represented as (3A). Normally the Anti-surge line is considered at 10% higher than the surge flow.

In process compressor, the compressor is operating at various cases. We need to ensure that in all the cases the operating point will never fall in the region of the anti-surge line.

## Efficiency:

The operating flow of the centrifugal compressor intersect at the efficiency curve, the corresponding efficiency in the Y-axis is called “Operating Efficiency” of the compressor. Refer the curve it is represented as the point (4).

As we seen the efficiency curve, the curve starts rising and reach a maximum point and then starts decreasing with increase in flow rate.

The maximum point of the efficiency curve is called “Best Efficiency Point (BEP)”. In the curve, it is represented as the point (4A).

## Brake Power:

The brake power or rated power of the pump is power required run the compressor to deliver the rated/normal operating flow at the rated/normal operating discharge pressure or head.

As per the curve, the normal operating flow of the centrifugal compressor intersect at the Brake power curve and corresponding power in the Y-axis is called “Normal brake power of the compressor”. In the curve, it is represented as the point (5).

In the current example, the brake power is 3000 kW.

The power corresponding to the “End of curve” in the brake power curve is represented as 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 steam turbine is sized for the 110% of the maximum power required for the complete compressor train.

## Turndown:

It is 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:

As discussed, please remember the below-mentioned points to be check in centrifugal compressor performance curve for the proper selection of Centrifugal Compressor.

- 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

Sergio Canedo says

Thanks a lot for sharing this information, greetings

Gregory Martinez says

Very well explained.

Jungyu Park says

Thank you for your kind explanation for compressor performance curve. It’s very easy and interesting. And I have a question about “rated head”. In the paragraph about “Turn-down”, how can I know Rated-head? In this example, rated flow rate is 66000kg/h but Rated-Capacity point is more than that.

admin says

Dear Jungyu Park,

In centrifugal compressor we never use the term “rated head”, either we use the polytropic head or discharge pressure. In this article, I use the curve “Flow Vs Discharge pressure” for illustration purpose. Normally Compressor vendor will provide “polytropic head or discharge pressure Vs Flow” curve. From that curve for rated flow, the corresponding discharge pressure will be delivered by the compressor will be determined.

I hope now you will understand. Happy learning…!!

Soon I will post an article for polytropic head calculation.

pp007 says

Thanks for well Explained

ali sayed says

I have a question . are these curves for any inlet temperature

admin says

Dear Ali,

Yes. These curves are correspond to the inlet temperature of 40 deg Celsius.

Ragothaman says

Dear Sir: Why it is not possible to operate the compressor or pump exactly at BEP?

admin says

Hi Ragothaman,

BEP is based on the pump or compressor model, it will differ manufacturer to manufacturer. The rated flow of pump or compressor is based on the operating condition required for purchaser or end user.

Some cases the rated flow is match with BEP of pump or compressor model.

Most of the time the BEP of the compressor or pump are not match with the rated flow. Because of this reason, pump Rated flow shall be with in the range of 80% to 110% of BEP flow of as per API 610.

The same range is also applicable for compressor.

Taha Siddiqui says

Hi,

We have a Integrally geared centrifugal compressor for Instrument Air.

For capacity and pressure control we use Inlet Butterfly valve (IBV) and excess pressure is blown off through Unloading valve.

During normal routine the Unloading valve has opening around 25%, meaning blowing air continuously wasting energy.

The Compressor power/discharge pressure (8.7kg/cm2) is controlled through IBV through density control.

The Density range is 0.990-1.085 cm/lit..

can we further decrease density to decrease the excessive blow off air thus reducing power consumption without surging. ?

antonio says

I never read an article about compressor curves so clearly explained. Congratulations.

Antonio