Questions regarding the normally open setting of the backup pump outlet valve with automatic pump start-up

No problem; if the outlet valve of the standby pump is closed, then what’s the point of having a standby pump? The standby pump only starts automatically in emergency situations; it isn’t turned on regularly, so there’s no problem.

It depends on the size of your pump. Small pumps aren’t worth mentioning in this context; there are no issues with them, and their starting current is certainly higher, but it remains within acceptable limits. For larger pumps, it’s recommended that you install an inverter

Since there is one in use and one as a backup, and it operates automatically, a check valve should be installed for each unit during design and installation; this way, the backup pump will not remain under pressure at all times, without affecting its ability to start up.

Those with lower power have little impact, while those with higher power generally come equipped with solenoid valve switches at the outlet.

Large pump, shaft power of 698 kW, motor power of 1000 kW.

There is no problem with this; it is generally done to prevent excessive motor current during startup from causing overload and tripping. However, pumps that come with interlock-based automatic startup are usually those with low flow rates but high head pressure. The outlet valve is open, and there is also a check valve in place. When the pump is operating normally, there is a certain backpressure at the outlet check valve. In abnormal situations, when the backup pump starts automatically, there is still some backpressure at the outlet check valve. As a result, after the motor current reaches its peak value during startup, it gradually decreases, and this peak value remains within the rated current range. For high-power pump motors, self-starting motors are typically equipped with solenoid valves at the pump outlet; in abnormal situations, these solenoid valves open after power is applied to start the motor, thereby preventing the motor from tripping. Generally, those equipped with interlocked automatic start functions use screw pumps or other types of metering pumps; the operation of such pumps requires that the outlet valve be open!

But when it comes to a high-power centrifugal pump with a power rating of 800 kW, there is no solenoid valve at the pump’s outlet; it’s not clear whether the interlocked automatic start-up function could cause the motor to overload and trip

Young man, don’t talk nonsense about things you’re not sure about

The purpose of closing the outlet valve when starting a centrifugal pump is to prevent the motor from overloading. Doing this frequently can be harmful to the motor, as the current is very high at that time (resulting in significant heat generation), which is damaging to the motor; however, it’s not absolutely impossible to do so. Moreover, this self-starting mechanism differs from normal pump operation: there is a certain back pressure at the pump outlet, which results in a relatively lower flow rate of the pump; as a result, the pump does not draw excessive current. Be aware of the issue of overcurrent in pumps: an overcurrent does not necessarily cause an immediate shutdown. It is usually gradual; I’ve forgotten the exact figure. Let’s take an example: if the current exceeds 5% for half an hour, an interlock will trigger a shutdown ； If the current exceeds 10% for 10 minutes, interlock shutdown will occur.

Contact you to add a frequency converter; by gradually increasing the frequency when starting the pump, excessive starting current can be avoided