Pump performance assessment
Frequently asked questions
1 How would you measure the flow by using tracer method?
2 What are the various ways of measuring flow?
3 A pump motor draws 75 A current. The voltage is 415 V. Assuming a power factor of 0.9. Calculate the power drawn?
4 The suction head is 1m below the pump centerline. The discharge pressure shows 3 kg/cm2. The flow is calculated to be 100 m3/hr. Find out the pump efficiency.
5 The pump efficiency is 70%. The hydraulic power is calculated to be 22 kW. Find out the motor power required to drive the pump.
Purpose of the Performance Test
• Determination of the pump efficiency during the operating condition
• Determination of system resistance and the operating duty point of the pump and compare the same with design.
Performance Terms and Definitions
Pump Capacity, Q = Volume of liquid delivered by pump per unit time,m3/hr or m3/sec
Q is proportional to N, where N- rotational speed of the pump
Total developed head, H= The difference of discharge and suction pressure
The pump head represents the net work done on unit weights of a liquid in passing from inlet of the pump to the discharge of the pump.
There are three heads in common use in pumps namely
(i) Static head
(ii) Velocity head
(iii) Friction head.
The frictional head in a system of pipes, valves and fittings varies as a function (roughly as the square) of the capacity flow through the system.
System resistance: The sum of frictional head in resistance & total static head.
Pump Efficiency: Fluid power and useful work done by the pump divided by the power input in the pump shaft.
Field Testing for Determination of Pump Efficiency
To determine the pump efficiency, three key parameters are required: Flow, Head and Power. Of these, flow measurement is the most crucial parameter as normally online flow meters are hardly available, in a majority of pumping system.
The following methods outlined below can be adopted to measure the flow depending on the availability and site conditions.
Flow Measurement, Q
The following are the methods for flow measurements:
• Tracer method BS5857
• Ultrasonic flow measurement
• Tank filling method
• Installation of an on-line flowmeter
Tracer Method
The Tracer method is particularly suitable for cooling water flow measurement because of their sensitivity and accuracy.
This method is based on injecting a tracer into the cooling water for a few minutes at an accurately measured constant rate.
A series of samples is extracted from the system at a point where the tracer has become completely mixed with the cooling water. The mass flow rate is calculated from:
qcw = q1 x C1/C2
where qcw = cooling water mass flow rate, kg/s
q1 = mass flow rate of injected tracer, kg/s
C1 = concentration of injected tracer, kg/kg
C2 = concentration of tracer at downstream position during the ‘plateau’ period
of constant concentration, kg/kg
The tracer normally used is sodium chloride.
Ultrasonic Flow meter
Operating under Doppler effect principle these meters are non-invasive, meaning measurements can be taken without disturbing the system. Scales and rust in the pipes are likely to impact the accuracy.
• Ensure measurements are taken in a sufficiently long length of pipe free from flow disturbance due to bends, tees and other fittings.
• The pipe section where measurement is to be taken should be hammered gently to enable scales and rusts to fall out.
• For better accuracy, a section of the pipe can be replaced with new pipe for flow measurements.
Tank filing method
In open flow systems such as water getting pumped to an overhead tank or a sump, the flow can be measured by noting the difference in tank levels for a specified period during which the outlet flow from the tank is stopped.
The internal tank dimensions should be preferable taken from the design drawings, in the absence of which direct measurements may be resorted to.
Installation of an on-line flowmeter
If the application to be measured is going to be critical and periodic then the best option would be to install an on-line flowmeter which can get rid of the major problems encountered with other types.
Determination of total head, H
Suction head (hs)
This is taken from the pump inlet pressure gauge readings and the value to be converted in to meters (1kg/cm2 = 10. m).
If not the level difference between sump water level to the centerline of the pump is to be measured. This gives the suction head in meters.
Discharge head (hd)
This is taken from the pump discharge side pressure gauge. Installation of the pressure gauge in the discharge side is a must, if not already available.
Determination of hydraulic power (Liquid horse power)
Q = Volume flow rate (m3/s),
ρ = density of the fluid (kg/m3),
g = acceleration due to gravity (m/s2),
(hd - hs) = Total head in metres
Measurement of motor input power
The motor input power Pm can be measured by using a portable power analyser.
Pump shaft power
The pump shaft power Ps is calculated by multiplying the motor input power by motor efficiency at the existing loading.
Ps = Pm x ηMotor
Pump efficiency
This is arrived at by dividing the hydraulic power by pump shaft power
ηPump = Ph / Ps
Example of pump efficiency calculation
Illustration of calculation method outlined
A chemical plant operates a cooling water pump for process cooling and refrigeration applications. During the performance testing the following operating parameters were measured;
Determining the System resistance and Duty point
Determination of the system resistance curve and imposing the pump curve over it will give an idea of the operating efficiency of the pump and also the drop in efficiencies
when the system curve changes from normal / design. The example following from the earlier example outlines the method of constructing a system curve.
Example:
Location of equipments
The Refrigeration plant is located at +0.00 level and the Process plant condensers are located at +15 M level.
One cooler having a design pressure drop of 1.9 kg/cm2 is located at the 0.00 level (ground level). Other relevant data can be inferred from the earlier section. See schematic in Figure
The step-by-step approach for determining system resistance curve is given below.
Step-1
Divide system resistance into Static and dynamic head
Find static head;
Static head (Condenser floor height) ; 15M
Find dynamic head;
Dynamic Head = Total Head – Static Head
Dynamic head = (54-15) = 39 M
Step-2
Check the maximum resistance circuit
Resistance in the different circuits is as under
It can be noted that at full load the condenser and cooler circuits offer the maximum resistance to flow.
Step 3 ;
Draw system resistance curve
Choose the condenser loop as it offers maximum resistance and is also having a static head component
Static head: 15 M
Dynamic head at full load; 39 M
Compute system resistance at different flow rates
Step 4 -
Plot the system resistance against flow in the pump efficiency curves (see Figure 2) provided by the vendor and compare actual operating duty point and see whether it operates at maximum efficiency.
In the example provided it is found that the pump system efficiency is lower by 4 % due to change in operating conditions.
