Regarding valve selection issues

Based on the flow rate and pressure difference specified in the process requirements, the valve manufacturer’s calculation indicates a DN25 size; this diameter is sufficient to ensure proper regulation capabilities. The maximum flow rate corresponds to an opening degree of 79%, while the normal flow rate corresponds to an opening degree of 59%. For valves with linear characteristics, this ensures good regulatory performance. If a larger valve is selected, the opening degree will be too small and the adjustability will be poor. Reduced diameter of control valves is a relatively common occurrence. Another point is that the medium for this valve is acidic gas H2S, so NACE treatment is required.

Thank you. Does the reduction at the control valve occur at the flange? And how is it connected to the pipeline ahead? I’m a newcomer and not very familiar with this

Is there only 1 value for calculating CV? Can such a good opening angle be obtained with this rated CV? It is recommended to recalculate it. Additionally, in recent discussions with the manufacturers, it was found that 316SS and 316L exhibit good resistance to H2S, eliminating the need for NACE treatment; this aspect can be further investigated by looking up more information.

No one said that the valve seat diameter must be equal to the pipe size either. . The diameter of a butterfly valve equals the pipe size; single-seat valves are always smaller than the pipe size

This post was last edited by white-cg on 2019-5-8 at 11:15. 1. For the design documents, the pipe diameter is DN50; the valve can be DN50 (a waste), DN40 (acceptable) or DN25 (the best choice). However, DN20 cannot be used, as the necking should not exceed 2 levels. 2. For the single-seat type of valve internals, the CV value corresponding to DN20 is around 8; the maximum flow rate occurs at an opening degree of less than 80%, while the opening degree at normal flow rates is approximately 60%. The equal percent curve ensures adequate regulation capability, which is sufficient for use. 3. This case involves a standard single-seat pressure-controlled valve of 300CL; the pressure difference is very small. The design documents do not specify a maximum closing pressure difference. A single-seat valve was chosen, with 100% unbalanced force, and the value of 1.5 MPa can be used for calculations regarding the actuator to be selected. 4. CS valve body, for acidic gases with anti-H2S protection; NACE treatment is applicable; MR0103 is suitable. The valve internals should be made of 304L or 316L material, preferably hardened to resist erosion and corrosion.

1. Based on the calculations of the process parameters, selecting a DN25 valve will meet the control requirements. 2. The larger the diameter, the higher the cost; therefore, from a cost-performance perspective, if a DN25 valve is sufficient, there is no need to consider a DN50 valve. 3. The reduction in diameter of a control valve refers to the reduction in the diameter of the valve seat inside the valve, and it has nothing to do with the flanges. If you don’t want to use threaded connections for the valves, you can also consider using a valve body with a diameter of DN50, with the valve seat being reduced in diameter

The medium flow rate is low; only reducing the diameter can satisfy the valve’s adjustment range

One method: use tees on the pipeline and connect them with flanges. It is generally limited to a reduction of grade 2, up to 1/2 of the pipe diameter at most. Using a smaller valve saves money. Another option is to keep the valve body of the same size as the pipe, only reducing the size of the valve core, which is then connected to the pipe flange in a normal manner. This is expensive; the price of the valve depends on its size.

What you have circled at the top is the pipe number, not the diameter; the diameter is in the line below, corresponding to a pipe with DN40 diameter； The valve core of a control valve is slightly smaller than the flow channel