Function and selection of outlet check valve for centrifugal pump
The function of the check valve at the outlet of a centrifugal pump is to prevent the backflow of liquid in the outlet pipeline, which can cause water hammer and damage to the impeller inside the pump.
The installation of check valves is not related to the number of pumps. Check valves are mainly used to prevent liquid backflow after the pump stops running, causing the pump to reverse and causing damage to the pump. They are also used to prevent liquid backflow from damaging other storage tanks (backflow from high-level tanks can cause tank replenishment and pumping damage) Therefore, add a check valve If it can be ensured that the outlet valve is closed before stopping the pump, and the outlet valve is not leaking or damaged You can also worry about the check valve
1. Selection of pump outlet check valve and check valve
In general pump station engineering situations, the selection of pump outlet check valves should follow the following principles:;
When the diameter of the pump outlet pipeline is small, i.e. DN ≤ 50mm, a lift check valve should be selected.
For pipelines with a diameter DN>50mm at the pump outlet, a swing check valve should be selected. Swing check valves can be installed horizontally or vertically. The rotary check valve disc rotates around the axis of rotation, and its fluid resistance is generally lower than that of the lift check valve. It is suitable for larger diameter applications, and its installation position is not limited. It is usually installed in horizontal pipelines and is suitable for larger diameter applications. According to the number of valve discs, it can be divided into three types: single disc rotary opening, double disc rotary opening, and multi disc rotary opening. Single disc swing check valves are generally used in medium caliber applications. When using single disc swing check valves for large-diameter pipelines, to reduce water hammer pressure, slow closing check valves that can reduce water hammer pressure are used. Double flap swing check valve is suitable for large and medium diameter pipelines. The double disc swing check valve has a small structure and light weight, making it a faster check valve; Multi flap swing check valve is suitable for large-diameter pipelines
2. Installation of pump outlet check valve
① Swing check valves are generally installed on horizontal pipelines. For check valves with a diameter of DN~80mm, they can also be installed on vertical or upward inclined pipelines.
② Direct lift check valves should be installed on horizontal pipelines; Vertical lift check valves must be installed on vertical pipelines, with the medium flowing from bottom to top.
③ Due to the easy damage of check valves, they should be installed near the pump outlet. A shut-off valve (usually a ball valve or gate valve) should be installed on the upper part of the check valve for easy maintenance.
④ For the convenience of pressure relief before disassembling the check valve, if the check valve itself does not have a drain valve, a pressure relief drain valve should be installed between the check valve and the shut-off valve.
3. Protective measures to prevent damage to the sealing surface of check valves caused by hydraulic shock
The setting of the check valve plays a role in preventing material backflow and avoiding liquid hammer on the pump impeller. However, as for the check valve itself, improper setting can cause sudden impact of the valve disc on the valve seat caused by water hammer, damaging the sealing surface of the check valve. Therefore, several protective measures should be taken.
1. For swing check valves with a diameter greater than 80mm at the pump outlet, a horizontal installation method should be used to reduce valve disc recoil and impact.
2. For pipelines with a pump outlet check valve diameter>=150mm, a bypass should be installed before and after the check valve to prevent excessive pressure rise in front of the valve and damage to the sealing surface of the check valve.
3. For pipelines with high starting and ending pressures in the pump discharge pipeline, it is not allowed to set up a bypass. The following measures can be taken.
Install a safety valve instead of a bypass valve. Its function is similar to that of a bypass valve. It is located near the outlet of the check valve, and when water hammer increases the pressure, a portion of the water can be released to prevent damage caused by further pressure increase
Set up air or inert gas buffer tanks. Set it near the outlet of the check valve, and when water hammer occurs, the pressurized gas in the buffer tank is compressed to alleviate the pressure increase caused by water hammer.