The CavtProt function block helps to avoid operating the VS and FS pumps in a cavitation situation caused by a low-pressure situation on the suction side.
To achieve this, it constantly verifies the suction pressure in the pumping application. The CavtProt function block starts to adapt the pressure setpoint if the suction pressure value drops below a certain cavitation protection start limit. If this measure does not stop the decrease of the suction pressure, a second limit value, referred to as NPSH+, is used. This value is a value provided by the manufacturer of the pumps. It indicates the minimum pressure that has to be available at the entry point of the pump to prevent the liquid from boiling and thus to avoid a cavitation situation. If the suction pressure drops below the NPSH+ value, the CavtProt function block indicates an alarm. As a result, the pumps are stopped.
Cavitation Protection Procedure
The diagram illustrates the influence of the suction pressure on the pressure setpoint during cavitation protection:
1 Suction pressure curve
2 The suction pressure becomes inferior to the suction pressure alert on-limit (iq_stCavtInit.rNpshAlrtOn), triggering the active cavitation protection.
3 The suction pressure reaches the NPSH+ threshold; alarm state = TRUE (1); pumps are stopped.
4 The suction pressure leaves the NPSH+ alarm area.
5 A user reset command and a user automatic command starts the machine.
6 The suction pressure exceeds the suction pressure alert off-limit (iq_stCavtInit.rNpshAlrtOff), terminating the active cavitation protection.
7 During the active phase of the cavitation protection, the setpoint is adjusted.
8 During an alarm state, the value of the setpoint is set to zero.
9 After a user reset command, the active cavitation protection starts to adjust the setpoint with the iq_stCavtInit.rSpMin as start value.
10 If the active cavitation protection ends, the setpoint is set to 100% (q_stRef_Sp.rVal:=i_rSp).
The cavitation protection process consists of the following stages:
|
Stage |
Description |
|---|---|
|
1 |
The suction pressure in the pumping application is constantly verified during normal operation. |
|
2 |
If the suction pressure drops under the lower suction pressure limit value (position 1: q_stCavtSta_Hmi.rNpshAlrtOn), the cavitation protection function starts and the CavtProt function block executes the following actions: oThe operating mode is set to cavitation margin mode (iq_stOpMode.xCavtMarg). oThe output value q_stCavtSta_Hmi.xProtActv is set to TRUE (1) to indicate that the cavitation protection function is in progress. oThe output value q_stCavtErr.stCavtAlrt.xSuctPresLim is set to TRUE (1) to indicate that a cavitation alert has been detected. oThe pressure setpoint is adjusted according to the suction pressure value in order to reduce the flow and to increase the suction pressure. |
|
3 |
If the suction pressure drops to the NPSH+ limit (iq_stCavtInit.rNpshPlus) (position 2), the CavtProt function block stops the pumps by executing the following actions: oIt indicates a detected cavitation alarm (by setting output q_stCavtErr.stCavtAlrm.xSuctPresNpsh to TRUE (1)). oIt sets the outputs to the default values (0 and FALSE (0)). oIt sets the alarm release bit (iq_stAlrmCtrl.xAlrmRele) to FALSE (0). oIt sets the operating mode to manual mode. |
|
4 |
Once the suction pressure is greater than the NPSH+ limit (iq_stCavtInit.rNpshPlus), a reset must be executed to restart the pumps. |
|
5 |
After the reset has been carried out, the cavitation logic resumes while the suction pressure is below the upper suction pressure alert on-limit (iq_stCavtInit.rNpshAlrtOn). The CavtProt function block executes the following actions: oThe cavitation margin mode flag (iq_stOpMode.xCavtMarg) into the operating mode structure is set to TRUE. oThe output value q_stCavtSta_Hmi.xProtActv is set to TRUE (1) to indicate that the cavitation protection function is in progress. oThe output value q_stCavtErr.stCavtAlrt.xSuctPresLim is set to TRUE (1) to indicate that a cavitation alert has been detected. oThe value configured for the parameter iq_stCavtInit.rSpMin (position 9) is used as the first pressure setpoint during restart after a reset. In the course of time the pressure setpoint is adapted according to the sign of the measured suction pressure gradient. If the suction pressure is rising, the pressure setpoint is incremented, and vice versa. The adjustment is realized with the value of the time constants TN (iq_stCavtInit.rTn). |
|
6 |
As soon as the suction pressure alert off-limit (iq_stCavtInit.rNpshAlrtOff) is reached, the cavitation protection action is stopped, and the CavtProt function block executes the following actions: oThe cavitation margin mode flag (iq_stOpMode.xCavtMarg) of the operating mode is set to FALSE (0). oThe output value q_stCavtSta_Hmi.xProtActv is set to FALSE (0). oThe setpoint is set to 100% (q_stRef_Sp.rVal:=i_rSp). oThe suction pressure in the pumping application is constantly verified. |
The diagram shows the pressure setpoint being adapted according to the suction pressure.
In this example, the reduction of the pressure setpoint and the flow is sufficient to prevent the suction pressure from dropping below the NPSH+ limit:
1 The suction pressure falls under the suction pressure alert on-limit (iq_stCavtInit.rNpshAlrtOn), triggering the starting of the active cavitation protection phase.
2 The suction pressure exceeds the suction pressure alert off-limit (iq_stCavtInit.rNpshAlrtOff), terminating the active cavitation protection phase.
The cavitation protection function adapts the pressure setpoint according to the suction pressure as follows:
|
Stage |
Description |
|---|---|
|
1 |
The suction pressure in the pumping application is constantly verified during normal operation. |
|
2 |
If the suction pressure drops below the suction pressure alert on-limit value (position 1: iq_stCavtInit.rNpshAlrtOn), the cavitation protection function starts and the CavtProt function block executes the following actions: oThe cavitation margin mode flag (iq_stOpMode.xCavtMarg) into the operating mode structure is set to TRUE. oThe output value q_stCavtSta_Hmi.xProtActv is set to TRUE (1) to indicate that the cavitation protection function is in progress. oThe output value q_stCavtErr.stCavtAlrt.xSuctPresLim is set to TRUE (1) to indicate that a cavitation alert has been detected. oThe pressure setpoint is adjusted according to the sign of the measured suction pressure gradient. If the suction pressure is rising, the pressure setpoint is incremented, and vice versa. The adjustment is realized with the value of the time constants TN (rTn). |
|
3 |
As soon as the suction pressure value reaches the suction pressure alert off-limit position 2: iq_stCavtInit.rNpshAlrtOff, the cavitation protection adjustment is stopped and the CavtProt function block executes the following actions: oThe cavitation margin mode flag (iq_stOpMode.xCavtMarg) of the operating mode is set to FALSE (0). oThe output value q_stCavtSta_Hmi.xProtActv is set to FALSE (0). oThe setpoint is set to 100% (q_stRef.rVal:=i_rSp). oThe suction pressure in the pumping application is constantly verified. |
