Regarding the start-up and shutdown operations of multi-stage pumps
Thread Content
Multi-stage centrifugal pumps are generally equipped with a minimum flow line; it is necessary to open this line before starting the pump. Are there any requirements regarding shutting down the pump? After the pump starts operating normally, the minimum flow line is closed or switched to automatic mode. When shutting down the pump, should the pump’s outlet valve be closed directly, or needs the minimum flow line to be opened first before closing the outlet valve?
Could the foam moderator explain how to operate the minimum flow line?
A orifice plate is generally installed on the minimum flow line; its purpose is to ensure that fluid flows through the pump, thereby carrying away heat and preventing the fluid from vaporizing as well as the pump from overheating, especially with saturated fluids. Therefore, when stopping the pump, the minimum flow line should be opened first. Of course, except in special cases.
Due to the fact that the performance curve of multi-stage centrifugal pumps differs from that of ordinary centrifugal pumps, these pumps generally exhibit low flow rates but high head pressures; therefore, it is necessary to establish a minimum flow rate limit to prevent pressure buildup that could damage the pump! Before starting the pump, it is necessary to open the valve for the minimum flow rate; theoretically, it can be closed during operation, but I personally recommend keeping it open. There are three advantages to this: 1. It prevents the rear control valve from reaching its limit position, that is, from being fully closed. 2. Prevent incorrect operation of stopping the pump by directly closing the outlet valve without opening the minimum flow line. 3. It can be used as a heat pump line or pre-heating line in cold weather or when the oil is heavy.
Well, I’ve learned about it too; I think the minimum flow line should be turned on while it’s in operation. But currently, the minimum flow line on this device is turned off, so I’m asking! !
Also, my supervisor and the others stopped the pump without turning on the minimum flow line first, which is why I’m a bit confused! !
Your master is being lazy and trying to cut corners, because as soon as the motor stops, the pressure drops as well! Lower pressure is safer than higher pressure!
An automatic recirculation valve (minimum flow valve) can be installed at the pump outlet; once the pump flow falls below the set value, the bypass opens fully to ensure the minimum flow required by the pump. Even if the main route is completely closed, that is, the traffic on the main route is zero, the minimum flow can still be automatically circulated through the bypass. Stop the pump and check if there is a check valve at the outlet; if there is a check valve, simply cut off the power supply, and water hammer will not occur. If there is no check valve, the valve needs to be closed before stopping the pump.
The minimum flow line can be equipped with a flow-limiting orifice plate, or it can be controlled directly by a gate valve. When starting up, enable the line with the minimum flow rate first. You need to understand the function of this line: it protects the pump by preventing overpressure inside it. In those cases where lubrication is not required and the pump relies on its own medium for lubrication and cooling, this line must remain open as long as the pump is running. You need to stop the pump; if you don’t open this line, pressure will build up inside the pump once all the outlet valves are closed. Therefore, when stopping the pump, it is necessary to open this line with minimum flow first. After the pump has come to a complete stop, close this minimum flow line; there is also an incorrect term for it, namely minimum return line.