In this article, we will discuss How to read a centrifugal pump performance Curve. Before we start first we will understand the basics of Centrifugal pump performance curve.
Understanding of centrifugal pump performance curve is very important to select a proper pump for the required application. The centrifugal pump performance curve consists of four sets of curves.
- Head Vs Flow Curve
- Brake Power Vs Flow Curve
- Efficiency Vs Flow Curve
- NPSHr Vs Flow Curve
Centrifugal pump performance is represented by multiple curves indicating either:
- Various impeller diameters at a constant speed.
- Various speeds with a constant impeller diameter.
In this article, we will see only various impeller diameters at a constant speed.
Head Vs Flow Curve (H/Q)
In this curve, the Head is in the vertical axis and Flow rate in the horizontal axis. With the increase in pump flow rate, the head developed by the pump start decreases. The pump develops maximum head at zero flow it is called shutoff point or shutoff head and the corresponding pressure is called shutoff pressure. Normally the shutoff pressure is 1.25 time of pump discharge pressure.
The line continues to the right, with head reducing and flow increasing until the “end of curve” is reached, (this is often outside the recommended operating range of the pump). Flow and head are linked, one cannot be changed without varying the other. The relationship between them is locked until wear or blockages change the pump characteristics
Brake Power Vs Flow Curve (P/Q)
This curve the Break power is in the vertical axis and Flow rate in the horizontal axis. As we have seen from the curve, some amount of power is required at zero flow rate, this power is required to overcome the pump inertia and friction losses. From this curve, for the required pump flow, the corresponding brake power is obtained.
Based on the brake power, the required motor rating is obtained by multiplying the service factor.
Refer to the below table for service factor value
Efficiency Vs Flow Curve (e/Q)
The efficiency curve is raising as flow rate increase (refer figure). 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 pump at the best efficiency point(BEP), but in a practical case, we are not able to match the required pump operating point with pump BEP.
If the pump run at or near to BEP, not only we get higher efficiency and also the vibration of the pump is minimum.
NPSH(r) Vs Flow Curve(N/Q)
NPSH needs two different sets of axes to describe it fully. The lower curve (ref. figure ) shows the NPSH(r) curve rises with increasing flow rate. Note, however, that this curve is not obtained directly from the H/Q test.
NPSH(r) value at pump rated point should be a minimum of 1m less than NPSH ‘available’. It is called the NPSH margin.
NPSHmargin = NPSH(a) – NPSH(r)
Normally NPSH test is carried out when the NPSH’margin’ less than 1meter.
It is made up of four to five points, each point is obtained from a separate NPSH test at a different constant flow. This is normally carried out after the above-mentioned tests.
How to Read a Centrifugal Pump Performance Curve
Now consider a centrifugal pump is required to pump water from one vessel to another vessel, the pump parameters are as follows
Rated Flow = 40m3/hr
Rated Head = 157 m
Liquid Specific Gravity = 1
NPSH available = 4m
1. Pump Rated Flow:
Point 1, represent the rated flow(40m3/hr) of the pump.
2. Rated Head:
Point 2 represent the Rated head (157m). Now the intersection section point of Rated Flow and Rated head in the impeller curve is called “Rated Impeller”. In our example, the “Rated Impeller” diameter is 370mm.
3.Shut off head:
The maximum head produced by the pump at rated impeller is called “Shut-off head”. The corresponding pressure at shut-off head is called “Shut off Pressure”. The shut-off pressure is very important for pump downstream system design pressure. Always Downstream design pressure is greater than Shut off Pressure for safe design.
Downstream design pressure>Shut off Pressure.
4.Minimum Continuous Stable Flow:
The Minimum Continuous Stable Flow(MCSF) is the lowest flow at which the pump can operate without exceeding the vibration limits specified for the pump model. If the pump operates at minimum flow, the pump is heated up due to recirculation of pumping liquid and also witness severe vibration.
MCSF is the minimum flow developed by the pump at the rated impeller. Below MSCF it is not advisable to run the pump.
As a thumb rule, Minimum Continuous Stable Flow is 25% of “BEP Flow”.
5. Efficiency
The rated flow of pump intersects at the efficiency curve, the corresponding efficiency in the Y-axis is called pump “Rated Efficiency”. Refer “5b” in the curve represented the rated efficiency.
As we have seen the efficiency cure, from pump zero flow the curve starts raising and reach a maximum point and start decreasing with an increase in flow rate.
The maximum point of the efficiency curve is called “Best Efficiency Point (BEP)”. Refer “5a” in the curve represented the Best Efficiency Point. The flow corresponding to the BEP is called “Best Efficiency Flow”. The pump vibration levels are low at BEP. It is always preferred to select a pump whose rated flow is near to Best Efficiency Flow.
As per API 610, the rated flow shall be within the region of 80 to 110% of BEP flow. Pumps shall have a preferred operating region of 70 % to 120 % of the best efficiency flow rate (Refer figure is highlighted in “light blue colour”)
6. Brake Power
The brake power or rated power of the pump is the power required to run the pump to deliver the rated flow at the rated head.
The rated flow of the pump intersect at the Brake power curve and corresponding power in the Y-axis is called “Rated power of the pump” (Refer figure “6a”). For our example, the rated power is 35kW.
If the pump is required to start at open discharge valve or auto-start condition. In that case, the power is required for the pump equal to the “End of Curve” of brake power curve. (Refer figure “6b”)
Based on the required brake power, motor kilo-watt or hp will be selected.
NPSH(r):
At rated flow line intersect with NPSH(r) and the corresponding NPSH(r) for the rated flow condition is find the Y-axis. In our case, the NPSH(r) is 2.5m.
In this example, the NPSH(a) is 4m.
NPSH margin = NPSH(a) – NPSH(r)
NPSHm = 4 – 2.5 m
NPSHm = 1.5 m
Conclusion:
As discussed, for proper pump selection the below-mentioned points shall be check-in centrifugal pump performance curve
- Pump Rated Flow
- Pump Rated Head
- Rated impeller Diameter
- BEP flow
- Shut-off head
- Efficiency
- Rated power
- Pump End of Curve power
- NPSH required
- Preferred Operation Range
- Allowable Operation Range