Method for identifying if a submersible pump is sucking partly liquid and partly air

1. A method for stopping a submersible pump when the pump is snoring, wherein the pump is operatively connected to a control unit, the method comprising the steps of:

regulating, by way of the control unit, an operational speed of the pump in order to direct an average power of the pump towards a predetermined set level,

determining whether an instantaneous power of the pump is outside of a predetermined range by monitoring at least one of the following parameters: power [P], current [I] and power factor [cos ϕ],

determining whether the operational speed of the pump is increasing by monitoring a trend of change of the operational speed of the pump, wherein the pump is determined to be increasing if the trend of change of the operational speed of the pump is increasing based upon a plurality of measured instantaneous operational speeds of the pump over a predetermined period of time, and

stopping the pump due to snoring, by way of the control unit, when the instantaneous power of the pump is determined as being outside of the predetermined range at the same time that the operational speed of the pump is determined to be increasing.

2. The method according to claim 1 , wherein the step of determining whether the operational speed of the pump is increasing is performed after an affirmative determination that the instantaneous power of the pump is outside of the predetermined range.

3. The method according to claim 1 , wherein the step of determining whether the operational speed of the pump is increasing is performed by monitoring for changes in the operational speed of the pump.

4. The method according to claim 3 , wherein monitoring for changes in the operational speed of the pump is performed by the steps of:

measuring a plurality of instantaneous operational speeds [n1, n2, n3, n4, . . . ] of the pump during a predetermined period of time [t],

comparing a mutual relationship of pairs of adjacent instantaneous operational speeds [n1;n2, n2;n3, n3;n4, . . . ],

monitoring a number of times [m] a latter instantaneous operational speed [n2] of a pair of adjacent instantaneous operational speeds [n1;n2] is greater than a former instantaneous operational speed [n1] of the pair of adjacent instantaneous operational speeds [n1;n2], and

confirming that the operational speed of the pump is increasing when the number of times [m] the latter instantaneous operational speed [n2] is greater than the former instantaneous operational speed [n1] is greater than a predetermined threshold during the predetermined period of time [t].

5. The method according to claim 4 , wherein the plurality of instantaneous pump operational speeds [n1, n2, n3, n4, . . . ] is equal to ten.

6. The method according to claim 4 , wherein the predetermined threshold of the monitored number of times [m] that the latter instantaneous operational speed [n2] is greater than the former instantaneous operational speed [n1] is equal to four.

7. The method according to claim 4 , wherein the predetermined period of time [t] is equal to or greater than two seconds, and equal to or less than five seconds.

8. The method according to claim 1 , wherein an upper limit of the predetermined range of the instantaneous power of the pump is equal to a factor 1.02 times the predetermined set level of the average power of the pump.

9. The method according to claim 1 , wherein a lower limit of the predetermined range of the instantaneous power of the pump is equal to or less than a factor 0.98 times the predetermined set level of the average power of the pump.

10. The method according to claim 1 , wherein the pump is kept inactive for a predetermined pause time after the pump has been stopped due to snoring.

11. The method according to claim 1 , wherein, after the pump has been stopped due to snoring, the pump is kept inactive until the control unit obtains a start-signal from a level sensor.

12. The method according to claim 1 , wherein the control unit is constituted by a variable frequency drive [VFD].