Safety operation of slow closing check valve and centrifugal pump

The pump is the core device of the fluid industry Its function is like the heart of human body It provides power for the fluid in each device of the system and ensures the normal operation of the process flow Commonly used pumps are divided into vane pump, positive displacement pump, special purpose pump, etc Among them, centrifugal pump is widely used in vane pump There are two problems to be avoided in the operation of centrifugal pump: the water hammer to stop the pump and the low flow rate There are many ways to solve these two problems, but the basic idea is how to configure a suitable system around the centrifugal pump In these systems, valves play a very important role, and with the deepening understanding of these two kinds of problems, many special valves have been born Stop pump water hammer protection and valve water hammer are a series of sharp water hammer phenomena caused by the sharp change of medium velocity in pressure pipeline The harm of water hammer is very big, it can destroy the pump, pipeline and other equipment There are many reasons for the water hammer in the pressure pipeline, such as the quick closing of the valve, abnormal stop of the pump, etc Mechanism of water hammer in pump stop: when the pump stops working suddenly due to abnormal reasons, such as power failure: in the initial stage, the medium in the pipeline continues to move forward by inertia, but the speed gradually reduces to zero; at this time, if there is a height drop in the pipeline layout, the medium will flow backward to the pump under the action of gravity; When the backflow medium reaches a certain speed, the check valve at the pump outlet will close quickly, so that the speed of a large number of medium arriving here suddenly becomes zero, resulting in the sharp rise of medium pressure here - water hammer of pump stop A large number of documents have pointed out that the main cause of the water hammer is the sudden closure of the check valve at the outlet of the pump However, some studies have shown that although the check valve at the pump outlet can be cancelled in some cases, in most cases, in order to prevent a large amount of media from flowing back into the centrifugal pump, the anti backflow setting at the pump outlet is necessary There are many measures to avoid the harm of water hammer, such as the installation of water hammer eliminator, pressure relief valve, pressure regulating tank, etc the following only introduces two kinds of measures which are widely used and related to general valves 1 Set slow closing check valve Slow closing check valve is a check valve that can be closed slowly by adding actuator and damper The above figure is the schematic diagram of water hammer protection system for centrifugal pump with slow closing check valve installed At this time, slow closing check valve needs to be used with gate valve (on-off valve) When the medium flows backward under the action of gravity, the check valve closes slowly, effectively avoiding the water hammer caused by the sudden closing of the common check valve Its disadvantage is that because of the slow shut-down speed, part of the medium inevitably flows back into the centrifugal pump, so the pump may have mechanical failure 2 Setting slow closing check butterfly valve is a common and effective method in large-scale water conservancy system Slow closing check butterfly valve is composed of butterfly valve, executive structure and hydraulic control system The above figure is the schematic diagram of water hammer protection system for centrifugal pump with slow closing check butterfly valve The system only needs to install slow closing check butterfly valve, which can play the role of check valve and on-off valve When the centrifugal pump is started, it is started in the slow and fast steps to ensure that the pump is started at low load; when the pump suddenly stops working, it is closed in the fast and slow steps, which can not only avoid the generation of water hammer, but also avoid too much medium backflow through the pump, causing mechanical failure of the pump To avoid small flow condition and valve small flow is to ensure that the pump can work normally If the pump works under the condition of low flow, it will cause noise and vibration, the performance of the pump will become unstable, and even cause abnormal cavitation of the pump, reducing the service life of the pump Therefore, measures must be taken to prevent the centrifugal pump from working under the condition of small flow At present, the common practice is to feed the centrifugal pump with a small flow circuit But in terms of the value of the pump itself, it is only reasonable to set up a small flow circuit for those centrifugal pumps with large flow, high head and high power The allowable small flow value of centrifugal pump shall be determined by the pump manufacturer's calculation or experiment For a simple small flow circuit, only one on-off valve, such as gate valve, needs to be installed in the circuit When it is considered that the pump is about to be in a small flow condition, open the valve and connect the small flow circuit to avoid the pump working under a small flow condition It is concluded that the basic structure of the valve described above is the same as that of the general valve However, due to the corresponding improvement according to the application conditions, these valves become the special valves to solve these two kinds of problems.