We talk again about centrifugal pumps, and in particular about the correct start-up procedure of a pump in order to ensure its proper functioning. This is a very common operation during the commissioning of our thermoregulating units.
The first step in the start-up procedure of a pump is to fill the circuit with the working fluid, water, glicol water or oil, using as reference the minimum level, or just above it, or otherwise the pressure level specified in the technical manual. The next step is to give a small power to the pump, in order to verify the right rotation direction of the pump. This is important to avoid damages to the mechanical seal. If the pump rotates in the wrong direction, one of the three power cables must be switched (we’re clearly talking about three stepper motors).
Next step is to power up the pump, throttling the gate valve installed on the pump’s inlet. While powering up the pump we check the value of the pressure indicated on the pressure gauge on the inlet. The value must be stable. During the start-up of a plant, there will be easily the presence of air bubbles within the circuit, and therefore the measurement will be quite wobbly.
We thus have to open partially the gate valve on the pump’s inlet, stop the pump and proceed to discharge the air from the vents located on the pipes. Then the pump can be re-started. The operation has to be repeated until the pressure level on the inlet gets stable.
Now the pump must be set on it characteristic curve. Read the working pressure specified on the technical manual, and let’s measure the amperes absorbed by the pump. What we have to check is in fact that the pressure drops generated when opening the gate valve on the inlet are such as to maintain the pump in absorption mode. In case the absorption parameters of the engine are exceeded, the motor can indeed get damaged.
These operations are quite easy on plants working with water, but they get a little more complicated when using hydraulic oil or diathermic oil. The higher viscosity of oil requires indeed a longer time to fulfill all the spots of the pipes in the circuit. Very likely there will then be presence of air bubbles in the circuit for a few hours still. In this case, the overall operation must be repeated until the pressure value on the pump’s inlet gets finally stable.
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