THROTTLE CONTROL OF PUMPS
This is the most ineffective method but is the most popular.
Most pump users know this but apply this method of pump control.
Why pump users apply throttle control?
When can a throttle control save a pump?
Changing of a system curve in dependence of the valve position.
During the closing of the valve characteristic of the system is moving from position "System curve with an open valve" to the position "System curve with half closed valve".
Duty point moves along the pump Q-H curve and parameters of the pump changes from values Q1, H1 to values Q2, H2.
How much energy is lost during throttle pump control?
The pressure drop in the valve makes the friction losses larger and the system curve steeper. The system curve gets steeper and steeper as the valve closes. 2 valve-positions shown
The simple way of power loss calculation during the valve control
p2, p1 - pressure in bar;
Q - flow in m3/h;
Ploss - power in kW
H2, H1 - head in m
Q - flow in m3/h;
Ploss - power in kW
PUMP IS OVERSIZED
The main reason why pump users are forced to apply the throttling or valve control is the oversizing of pumps.
What are the common reasons that lead to pump oversizing?
For any case. Yes, I know that I need the pump with the head 90 m but for any case, I will take the pump with head 125 m.
Errors that were made in projects. The calculations of pumping systems characteristics were not correct. Sometimes designers of pumping systems follow the same practice of increasing safety factors.
System characteristics can change during the operation. Among the main reasons pipes corrosion, changing of system demand.
Let's consider the typical situation with pump oversizing.
The flow demand is Q1. In accordance with the system curve, the necessary head for this flow is H1.
If we know the required parameters, we have to select a pump so that the point Q1, H1 lay within the operating range of the pump.
In our case, it will be pump #1.
But pump user decide to select a pump with the necessary flow but with more head - Pump 2. (for any case)
The duty point will be not #1 but #2 on the curve of Pump 2.
If a user selected Pump1 the duty point would lay within the operating range.
Selected pump 2 will operate in duty point 2.
We see in the graph that in this case duty point # 2 will lay beyond the operating range and pump will consume the excessive power and to overload an electric motor.
Sometimes pumps users don't believe that the pump with more head and more powerful motor can overload the motor.
Selecting a pump with excessive head and more powerful motor a user thinks that he increases a safety factor. But the result is the opposite. An electric motor is overloaded and can burn or the control panel turns off the pump. Selection of a pump with more head and more powerful electric motor might be the next wrong step. The result is the same but happens sooner.
In what cases the throttle control can be applied?
Usually, the throttle control of pumps applied when the user understands that the pump is oversized and in this case, throttle control allows to user to shift operating point to the recommended operating pump range.
Throttle control is applied for the exact adjustment of flow. For example, for the balancing of heating systems.
It is necessary to take into account the wearing of a pump during the operation in the future.
What pumps do suit for throttle control?
Pumps with the "flat" H-Q curve. A flat curve allows user change flow with less pressure drop on the valve.
On the contrary, pumps with a steep H-Q curve when controlled by a throttle valve will cause great power losses on the valve.
Alternatives to Pump Throttle Control
There are a number of alternatives that can be examined as an alternative to throttling.
- Speed control
- Trimming of an Impeller
- Parallel operation of Multiple pumps of same or different sizes
- Combinations of the above