What are the selection guidelines for control valves?

It should be something that the other party provided to you; I didn’t understand it

Does the design institute only handle the design and selection of control valves?

If letting others choose the appropriate option, the medium type, the operating temperature of the medium, the pressures before and after the valve, the minimum, normal, and maximum flow rates, the dynamic viscosity, and the differential pressure required to close the valve are all basic parameters.

I mean, how are the pressure before and after the valve determined? Is it the pressure before and after at full opening, or at full closure? I think the pressure before and after the valve is different when it is fully open or fully closed, right?

This post was last edited by HEJIYUER on 2016-1-22 at 22:07. The pressure before the valve and the pressure after the valve are calculated by professionals in process engineering based on the pipe resistance loss. Under normal circumstances, he provides three sets of data: the pressure values before and after at the maximum flow rate, at the normal operating flow rate, and at the minimum flow rate, denoted as F/P1/P2. These correspond to three different operating conditions, and such data serve as the design parameters provided by the process engineering team for the instruments. The instrument calculates the \"required flow capacity\" of the valve – the Cv value – based on three sets of data, and then selects a control valve with an appropriate diameter, with the valve opening ranging from 30% to 85%. For example, with the outlet control valve of a pump, as the flow rate increases, P1 decreases while P2 increases; as a result, the pressure difference across the valve decreases until the valve is fully open and the pressure difference before and after it is at its minimum. Otherwise, the desired flow rate cannot be achieved. If the operating flow rate changes little (by, for example, 10% up or down), the process provides only one set of data—usually that corresponding to normal operating conditions—and the instruments can handle it. If it is not a pump-based system, such as one dealing with gases, where the flow rate at normal operating points varies significantly – by a factor of 3:1, for example – and the pressure drop across the control valve also changes greatly, then the process requirements specify three sets of data; otherwise, at low flow rates, the opening degree of the control valve may be as low as 20%, resulting in poor control performance. Once the valve is in operation, the pressures before and after the valve change along with the flow rate, as described above. @jiaguoyun

This post was last edited by hby1224 on 2016-1-22 09:18. Maximum and normal traffic levels are easy to understand, but what about the minimum traffic level? Isn’t it basically 0 when everything is turned off? Just a little amount of leakage! I see on the design institute’s list that there are three sets of data regarding flow rate, while there is only one set of data for the pressure before and after the valve. That pressure value is calculated under normal flow conditions, right? So what should be the opening degree of the control valve in that case?

1. The minimum flow rate refers to the operating point at which the process load is reduced; it is by no means the flow rate when it is \"almost fully closed\". For example, with household water heaters, the amount of gas consumed is high in winter, but it drops significantly in summer; at that time it can be considered the \"minimum flow rate\". 2. There is only one set of pressure values for the upstream and downstream pressures, which indicates that, according to the process, the pressure changes remain minimal under the three different flow rates; in other words, the differential pressure does not change much. You can use this differential pressure value to calculate the three Cv values. 3. Opening degree for normal flow: 50–75%; the maximum flow should not exceed 90%, and the minimum flow should not be less than 30%. The above is a general overview for straight-acting conventional control valves. @jiaguoyun

1. Pipe dimensions before and after the valve, flange type, and flange surface. 2. Minimum, normal, and maximum flow rates required. 3. Medium name, temperature. 4. Pressure before and after the valve, shut-off pressure difference. 5. Preferred valve type

What information is provided to the design institute for the selection of control valves? Generally, the parameters of control valves should be provided by the design institute to the contractor for procurement and archiving. This includes the diameter of the process pipelines, operating temperature and pressure, density and viscosity of the medium, maximum, normal, and minimum flow rates, pressures before and after the valve, maximum shut-off pressure difference, type of control valve, pressure rating, type of sealing surface, material requirements for components such as the valve body, valve stem, valve assembly, etc., gas supply pressure and diameter, size of electrical interfaces, and so on

When calculating control valves, it is necessary to have the pressure differences before and after the valve at maximum/normal/minimum flow rates; this is required to accurately determine the Cv values corresponding to these three flow conditions. These parameters are provided by the design institute; the control valve manufacturer cannot provide them. If the control valve manufacturer claims to be able to provide them, it’s complete nonsense – they don’t understand the subject.