Automatic recirculation valve escorts safe operation of centrifugal pumps

Automatic recirculation valve escorts safe operation of centrifugal pumps

April 11, 2019

The automatic recirculation valve escorts safe operation of centrifugal pumps. The pump is the core device of the fluid industry, acting like the heart of the human body, powering the fluid in each device in the system to ensure the normal operation of the process. Commonly used pumps are classified into vane pumps, positive displacement pumps, and special purpose pumps. Among them, the centrifugal pump in the vane pump is the most widely used. Two problems need to be avoided during the operation of the centrifugal pump: stop the pump water hammer and below the minimum flow. There are many ways to solve these two kinds of problems, but the basic idea is how to configure a suitable system around the centrifugal pump. Valves play a very important role in these systems, and with the deepening of these two types of problems, many special valves have been born.

Pump water hammer protection and valve

The water hammer is a series of sharp water pressure fluctuations caused by drastic changes in the medium flow rate in the pressure pipe. Water hammers are very hazardous and can destroy pumps, pipes and other equipment. There are many reasons for the water hammer in the pressure pipeline, such as the quick closing of the valve and the abnormal shutdown of the pump.

Pumping water hammer generation mechanism

When the pump is suddenly stopped due to abnormal reasons, such as power failure: in the initial stage, the medium in the pipeline continues to advance by inertia, but the speed gradually decreases to zero; at this time, if there is a high and low drop in the pipeline arrangement, the medium acts as gravity. The pump will flow backwards; when the backflow medium reaches a certain speed, the check valve at the pump outlet will close quickly, so that the speed of the large amount of medium reaching here suddenly becomes zero, causing the medium pressure to rise sharply here - stop The pump water hammer is generated. A large number of documents pointed out that the main reason for the stop pump water hammer is that the check valve at the pump outlet is suddenly closed. However, studies have shown that, although in some cases the check valve at the pump outlet can be eliminated, in most cases, in order to prevent a large amount of media from flowing back into the centrifugal pump, an anti-backflow setting at the pump outlet is necessary.

Pump water hammer avoidance measures

There are many measures to avoid the danger of stopping the pump water hammer, such as installing water hammer eliminator, pressure relief valve, pressure regulating tank, etc. The following only introduces two measures that are more applicable to general valves.

  1. Set the slow closing check valve
    The slow-closing check valve is a check valve that is slowly closed by adding an actuator and a damper. The figure above shows a schematic diagram of the water hammer protection system of the centrifugal pump with the slow-closing check valve installed. At this time, the slow-closing check valve needs to be used with the gate valve (switch valve). When the medium flows back under the force of gravity, the check valve is slowly closed, effectively avoiding the water hammer caused by the sudden closing of the ordinary check valve. The disadvantage is that because of the slower closing speed, a portion of the medium inevitably flows back into the centrifugal pump, which may cause mechanical failure.
  2. Set the slow-closing check butterfly valve
    This is a method that is more common and effective in large-scale water conservancy systems. The slow-closing check butterfly valve is composed of a butterfly valve, an execution structure and a hydraulic control system. The figure above is a simplified diagram of the water hammer protection system of the centrifugal pump with the slow-closing check butterfly valve installed. The system only needs to install the slow-closing check butterfly valve, that is, it can function as a check valve and can function as an on-off valve. When the centrifugal pump is turned on, it is turned on in the first slow and fast step to ensure that the pump is turned on at low load; when the pump suddenly stops working, it is closed in the fast and slow steps to avoid the water hammer, and Avoid excessive media flow back through the pump, causing mechanical failure of the pump.

Avoid minimum flow conditions and valves

The minimum flow valve is the flow that ensures the pump is working properly. If the pump is operated below the minimum flow condition, it will cause noise and vibration, the performance of the pump will become unstable, and even the abnormal cavitation of the pump will be caused, reducing the service life of the pump. Therefore, measures must be taken to avoid the centrifugal pump operating under the minimum flow conditions. It is common practice to feed the centrifugal pump to set the minimum flow loop. However, from the value of the pump itself, it is only reasonable to set a minimum flow loop for those centrifugal pumps with high flow, high lift and high power. The minimum flow rate allowed for the centrifugal pump is calculated or experimentally determined by the pump manufacturer. The simplest minimum flow circuit only needs to install an on-off valve in the circuit, such as a gate valve. When the pump is considered to be at the minimum flow condition, the valve is opened and the minimum flow circuit is turned on to prevent the pump from operating under the minimum flow condition.

The basic structure of the valve described above is no different from the general-purpose valve, but because of the corresponding improvements in the application conditions, these valves have become special valves for solving these two types of problems.