1. Check valve for incompressible liquid
The check valve used for incompressible liquids is mainly selected based on its ability to not produce unacceptably high shock pressure due to sudden closing due to backflow when it is closed. Migrating this type of check valve to a low pressure drop valve is usually only the second consideration.
For this kind of check valve, the first step is to evaluate the required closing speed.
Rukovsky determined that the static pressure in the pipeline rises when the valve is closed quickly:
In the formula △р- the pressure rise relative to the normal pressure (MPa);
υ-the speed of the interrupted stream (m/s);
α-pressure wave transmission speed (m/s);
ρ- liquid density (kg/m3);
K- liquid modulus of elasticity (MPa);
E- modulus of elasticity of pipe wall material (MPa);
D- pipe inner diameter (m);
e-tube wall thickness (m);
C- pipeline current limiting coefficient, 1.0 for non-restricted pipeline;
B-constant.
When using steel pipe and water medium with a D/e ratio of 35, the pressure wave velocity is about 1200m/s. When the instantaneous velocity changes to 1m/s, the static pressure increment is △р=1.2MPa.
The second step is to select the type of check valve that may meet the required closing speed.
2. Check valve for compressible liquid
Although the purpose of selecting a check valve for a compressible fluid pipeline is to minimize the impact of the valve flap, it can be selected according to a similar selection method for a check valve for incompressible fluid. However, for very large pipelines, the impact of the compressive medium may also become very considerable.
If the medium flow fluctuates in a large range, a deceleration device can be used for the check valve for compressive liquid. This device works during the entire valve movement of the closing member to prevent rapid and continuous hammering to its end.
If the medium flow is stopped and started quickly and continuously, as at the outlet of the compressor, use a lift check valve. The lift check valve uses a lightweight disc that can withstand spring load, and the lift of the disc is not high.
3. Determining the size of the check valve
The check valve should be sized accordingly, so that normal liquid can keep the closing member open stably. In order to obtain the maximum closing time, the check valve should be closed as soon as possible after the speed of the downstream medium starts to slow down. In order to be able to determine the valve size in this case, the valve manufacturer must provide data on the selected size. Figure 3-517 shows an example of such data. The pressure drop is given for the liquid; the fully open position of the valve is marked on the liquid indexing curve. Here is the liquid index W/A, where W is the flow rate (m/s), V is the specific volume (m3), and A is the flow area (m2). There is also a through hole area table showing a specific valve size in Figure 3-517. In this way, it should be possible to find the size of the valve when the valve is fully open at a given flow rate.
4. Selection of check valve type
(1) For high and medium pressure check valves below DN50mm, vertical lift check valves and straight-through lift check valves should be used.
(2) For low pressure check valves below DN50mm, butterfly check valves, vertical lift check valves and diaphragm check valves should be used.
(3) For high and medium pressure check valves with DN greater than 50mm and less than 600mm, swing check valves should be used.
(4) For medium and low pressure check valves with DN greater than 200mm and less than 1200mm, wear-free spherical check valves should be used.
(5) For low pressure check valves with DN greater than 50mm and less than 2000mm, butterfly check valves and diaphragm check valves should be used.
(6) For pipelines that require relatively small impact or no water impact, slow-close swing check valves and slow-close butterfly check valves should be used.
(7) For the inlet pipeline of the water pump, a bottom valve should be selected.