Issues related to the parallel operation of two pumps
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Question 1: As shown in the diagram, the operating condition is that a circulation pump is used for circulation, creating a negative pressure inside the tank. The flow rates of the two pumps are different; when both pumps are running simultaneously, will P01 have an impact on P02? Is it necessary to install a check valve after pump P02?； Question 2: If there is no check valve behind pump P02, could the operation of pump P01 cause the impeller of P02 to rotate in the reverse direction when P02 is shut down, thereby damaging the pump? In my opinion, when both pumps are operating simultaneously, since their head is the same, the lower flow rate of P01 will be affected by P02. However, a check valve is installed after P01, so the operation of both pumps does not interfere with each other ； When the pump is stopped, the impeller should be fine even if it rotates in reverse; this is different from the situation where the pump’s motor rotates in reverse or when the pump is subjected to external forces during operation. I would appreciate some guidance from those of you who are more experienced in this field!
The first thing to consider is the pressure issue; focusing only on the negative pressure inside the tank ignores the inlet pressure of the feed pump. Therefore, a check valve must be installed after the make-up pump. The circulation pump can be omitted; it should not reverse direction. Furthermore, it is important to consider whether there are any safety risks to the entire process system in the event of a power outage and loss of power. If there are two pumps, although their head specifications are the same, the system pressure before the tank needs to be taken into consideration. These are unofficial opinions for reference only.
The circulation pump has stopped running; the inlet and outlet pressures of the pump should be equal to the pressure inside the tank. Even if the pump runs in reverse, its speed is very low, so it’s impossible for the pump to get damaged.
First, a check valve should be installed at the outlet of the centrifugal pump. Question 1: When both pumps operate simultaneously, the suction pressure on the side of the circulation pump is lower than that on the side of the feed pump, which results in the outlet pressure of the circulation pump being lower than that of the feed pump. Under these operating conditions, the motor of the circulation pump becomes overloaded. Question two: In the absence of any power input, the circulation pump will rotate in reverse when the valve is open; when it needs to be started, the excessive load on the motor can easily cause it to burn out. There is a high level of danger when starting the pump on-site.
Do you mean that the safety hazard associated with power outages refers to the situation where, in the absence of readings from instruments after a power outage, negative pressure in the tank continues to draw in material, resulting in an excessively high liquid level in the tank?
I’ve learned something from this. Regarding your second reply, if the circulation pump is operated in reverse with low-frequency starting, it shouldn’t cause excessive load on the motor and lead to its damage, right?
It is recommended to indicate the process pressure values at each stage of the process; this will make things clear at a glance. When power is lost suddenly, it becomes clear as well what the pressure conditions are in various parts. For reference.