CENTRIFUGAL PUMP OPERATION
HOW DOES A CENTRIFUGAL PUMP OPERATE?
The pump is the machine that converts the mecahnical power of the rotating shaft to hydraulic power of the flow.
THE ANALOGY WITH THE BUCKET
Let's consider the the analogy with the bucket.
There is a hole in the bucket bottom. The bucket is filled with water
Let’s beging to rotate the bucket. What will happen? The centrifugal force will push the water from the hole. Liquid will flow out of the bucket at an angle.
We can observe the same picture when we ride a bicycle through the puddle. And drops of water flies from the wheel.
A similar process occurs in the centrifugal pump.
Fluid flows to the suction pipeline and then to the inlet of the rotating impeller.
Vanes of the impeller act at the fluid and moves it from the center toward the periferia. The fluid is accelerated and takes velocity or velocity head.
After leaving the impeller fluid enters the volute where the velocity head transforms to the pressure and then goes to the outlet nozzle.
The hydraulic power is proportional to Flow and Head and can be calculated with this formula.
SPECIFIC SPEED OF PUMPS (NQ)
The pump specific speed nq is the coefficient of the similarity of pump impellers depend on their parameters Flow Q Head H and Speed n.
The relationship between these parameters define the geometry of the impeller,
If we need the pump with a higher head and lower flow the nq will be smaller. It means that the pump with bigger impeller diameter and a smaller diameter of the impeller eye is necessary.
If the value of the flow gets bigger relatively with a head the diameter of the impeller gets less in comparison with the impeller eye diameter.
The analogy with bucket helps to understand. If we need more flow we have to make a hole with a bigger diameter. If we need more head we have to rotate the bucket with more speed or at the circle with a bigger diameter.
You see below how the geometry of the impeller changes depend on the nq what means changes of impellers with more flow and less head.
To create a higher pump head we need and impeller with bigger diameter. Lets compare two impellers with same flow but different heads 135 m and 38 m, you see the difference in diameters and 655 and 338 mm. specific speed for the first 11,5 and for the second 29,8.
The electric motor shaft power is the same as the pump shaft power.
The electric motor generates power which is the same as the power consumed by the pump.
Ppump = P motor
These formulas can be helpful for the pump audit and validation of the measured parameters.
LOSSES IN THE PUMP
All losses in pumps can be divided into three groups:
HYDRAULIC LOSSES IN THE PUMP
Vortexes in impeller in the suction, discharge, and other hydraulic channels.
The hydraulic efficiency says how perfect the channels are designed. When the pump operates far away from the BEP hydraulic losses are more and the efficiency is less.
MECHANICAL LOSSES IN THE PUMP
The picture below shows places where mechanical losses happen at an example of the double suction pump.
Friction losses in the bearings
Friction losses in the seals
Friction losses at cylindrical surfaces of shafts. The shaft rotates in the liquid and due to viscosity liquid resists the shaft rotation.
Disk friction of the impeller shrouds. See more detailed explanation below.
VOLUMETRIC LOSSES IN THE PUMP
Every pump has areas with high pressure (discharge pressure) and low pressure (suction pressure), Rotating elements, and stationary elements. The liquid flows from the area with high pressure to the area with low pressure. The element which isolates these two areas is called wear rings. The volume of a liquid that flows through wear rings is defined by the value of clearance and pressure difference.
DISK LOSSES IN THE PUMP
Let’s look again at the impellers for different pump heads with the same flow. It is clearly seen the difference in being of shrouds surfaces and difference in friction losses. Impeller for the bigger head has less specific speed and more friction losses and less efficiency
This is the real illustration of the influence of impeller diameter and nq at efficiency. We see the difference in efficiency between pumps with different heads different impellers diameters (290 and 450 mm) almost similar flows.
The pump with the head of 21 m has an efficiency of 86% the pump with the head of 61 m has an efficiency of 82 %.