Why is the pressure difference before and after the control valve in many design institutes set at 100 KPa for liquids and 50 KPa for gases?
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As the title suggests, I’ve noticed on many drawings from design firms that, in the data sheets for control valves, the difference between the absolute pressure before the valve and the absolute pressure after the valve is set at 100 KPa for liquids, and 50 KPa for steam and gases (with exceptions for special conditions, such as pressure relief control valves). Is this a requirement specified by the design firms? Is there any scientific basis for specifying these values in terms of process engineering? Shouldn’t it be calculated based on the systems before and after the control valve? I feel very lost; I hope experts can give me some advice.
To save energy and reduce emissions, control valves should not consume too much energy
When transporting media through pipelines, power is required, such as from pumps or level controls, to provide the pressure necessary for transmission. If too much static head is consumed by the control valve, it will create significant resistance in the pipeline transmission, which in turn requires more energy and leads to increased production costs and reduced economic efficiency. Not only the control valve, but any resistance factors in the pipeline should also be minimized; resistive elements should only be installed when necessary.
If they knew the appropriate pressure drop, they wouldn’t write 100, 50 KPA.
The most important parameter for control valves is the pressure difference before and after the valve; this parameter directly influences the suitability of the control valve chosen. If selections are made solely based on this pressure difference, it is very likely that the Kv value of the selected control valve will be too high. In many operating conditions, the valve operates at a low opening degree, which is detrimental to its regulatory performance (especially for control valves that are to be used in automatic mode). Isn’t there a more accurate way to calculate it?
I also want to know how to consider the pressure values before and after the valve
I also want to know how to consider the pressure values before and after the valve
It should be provided by the process engineering team based on calculations.
I also saw a pressure difference of 0.05 MPa in the design documents; it took me a long time to figure out what that meant after looking at it for quite some time. For example, when steam is introduced into the jacket of a reaction vessel, the designed pressure for the piping is 0.4 MPa, while that for the jacket is 0.2 MPa; these values do not match.
This post was last edited by qugd on 2016-11-28 at 10:01. When selecting a control valve, the valve manufacturer uses specialized software to carry out calculations before making the choice. The designer puts forward a basic requirement based on design principles, with the calculation results of the valve being the final determinant. In many projects, discussing the technical specifications of control valves is quite troublesome, as the selection calculation software used by different manufacturers varies, which requires the buyer and seller to check each one individually. For valves that are often operated at high or low opening degrees during operation, this frequent condition should be taken into account during selection. This requires the technical staff of valve manufacturers to carry out accurate and reasonable calculations and selections for each control valve in a responsible manner. For some business agents that handle control valves, if they lack sufficient technical support capabilities, it is better to invite the manufacturer to provide such support.