Slow-closing Check Valve And Safe Operation Of Centrifugal Pump

CIP Return Pump is the core device of the fluid industry. Its function is like the heart of the human body, providing power for the fluid in each device in the system to ensure the normal operation of the process. Commonly used pumps are divided into vane pumps, volumetric pumps, special-purpose pumps, etc. Among them, centrifugal pumps are the most widely used in vane pumps.
Two problems need to be avoided during the operation of centrifugal pumps: pump stop water hammer and flow rate below the minimum. There are many ways to solve these two types 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 the understanding of these two types of problems, many special valves have been born.
Pump stop water hammer protection and valves
Water hammer is a series of rapid pressure alternating rise and fall water hammer phenomena caused by the drastic change of medium flow rate in the pressure pipeline. The harm of water hammer is very large and can destroy pumps, pipelines and other equipment. There are many reasons for water hammer in pressure pipelines, such as rapid closure of valves, abnormal pump stop, etc.
Pump-stop water hammer generation mechanism
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 decreases to zero; at this time, if there is a height difference in the pipeline layout, the medium will flow back 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 amount of medium arriving here suddenly becomes zero, causing the medium pressure here to rise sharply - pump-stop water hammer occurs. A large number of literatures point out that the main cause of pump-stop water hammer is the sudden closure of the check valve at the pump outlet. However, 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 medium from flowing back into the centrifugal pump, a backflow prevention device at the pump outlet is necessary.
Measures to avoid pump-stop water hammer
There are many measures to avoid the harm of pump-stop water hammer, such as installing water hammer eliminators, pressure relief valves, pressure regulating tanks, etc. The following only introduces two measures that are more commonly used and related to general valves.
1. Set up a slow-closing check valve

A slow-closing check valve is a check valve that can be closed slowly by adding an actuator and a damper. The above figure is a schematic diagram of a centrifugal pump water hammer protection system with a slow-closing check valve installed. At this time, the slow-closing check valve needs to be used in conjunction with a gate valve (switch valve). When the medium flows back under the action of gravity, the check valve closes slowly, effectively avoiding the water hammer caused by the sudden closing of an ordinary check valve. The disadvantage is that due to the slow closing speed, part of the medium inevitably flows back into the centrifugal pump, and the pump may therefore have mechanical failures.

2. Set up a slow-closing check butterfly valve

This is a method that is commonly used in large-scale water conservancy systems and has a relatively good effect. The slow-closing check butterfly valve consists of a butterfly valve, an actuator structure and a hydraulic control system. The above figure is a schematic diagram of a centrifugal pump water hammer protection system with a slow-closing check butterfly valve installed. The system only needs to install a slow-closing check butterfly valve, which can play the role of both a check valve and a switch valve. When the centrifugal pump is turned on, it is turned on in a slow-first-fast-later-fast order to ensure that the pump is turned on at low load; when the pump stops working suddenly, it is turned off in a fast-first-slow-later-fast order to avoid water hammer and excessive medium backflow through the pump, causing mechanical failure of the pump.
Avoid minimum flow conditions and valves
The minimum flow refers to the flow that ensures the normal operation of the pump. If the pump works under conditions lower than the minimum 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 avoid the centrifugal pump from working under the minimum flow condition. At present, the common practice is to set a minimum flow circuit for the centrifugal pump. However, in terms of the value of the pump itself, it is reasonable to set a minimum flow circuit only for those centrifugal pumps with large flow, high head and high power. The minimum flow value allowed by the centrifugal pump is calculated or determined by the pump manufacturer.
The simplest minimum flow circuit only requires the installation of a switch valve in the circuit, such as a gate valve. When it is believed that the pump is about to be in the minimum flow condition, the valve is opened to connect the minimum flow circuit to avoid the pump working under the minimum flow condition.
Summary
The basic structure of the valves introduced above is the same as that of general valves, but because they have been improved according to the application conditions, these valves have become special valves for solving these two types of problems.