Calculation method and selection of regulating valve diameter

To calculate the diameter of the control valve, it is necessary to determine the calculation flow rate, determine the differential pressure, calculate the flow capacity, select the flow capacity, check and determine the diameter of the control valve. Today, I would like to share with you the relevant knowledge of the diameter selection of the control valve.

The flow capacity is the main basis for selecting the diameter of regulating valve. In order to calculate the flow capacity correctly, it is not necessary to determine the flow rate and pressure difference of the control valve. Generally, the flow rate and pressure difference substituted into the calculation formula of flow capacity are called calculated flow rate and calculated pressure difference.

Selection procedure of regulating valve diameter

Determination of calculated flow rate

The calculated flow is the maximum flow through the regulating valve. The flow value should be determined according to the production capacity of process equipment, the change of object load, the change of operation conditions and the control quality of the system. However, there are two tendencies that should be avoided: one is to over consider the allowance, so that the valve diameter is too large, which not only causes economic waste, but also makes the valve often work at a small opening, so that the controllable ratio is reduced, the control performance is deteriorated, and even oscillation may be caused in serious cases, thus greatly reducing the service life of the regulating valve; the other is to only consider the immediate production and emphasize the quality control unilaterally As a result, when the productivity increases slightly, the control valve can not adapt and is forced to be replaced.
The calculated flow rate can also be determined by referring to the capacity of fluid conveying machinery such as pumps and compressors. Sometimes, a variety of methods are combined to determine.

Determination of calculated pressure difference

The calculated pressure difference refers to the pressure difference on the control valve when the valve is fully open and the flow rate is maximum. It is necessary to consider both power consumption and pressure difference control. The larger the ratio of the differential pressure on the valve to the pressure difference of the whole system, the smaller the distortion of the flow characteristics of the control valve, the more guaranteed the control performance. However, the greater the pressure difference between the front and rear of the valve, the more power is consumed.
The calculation of pressure difference is mainly based on the size and change of the system pressure difference composed of process pipeline and equipment, and the steps are as follows:

• ① The two equipments with the closest distance between the front and back of the control valve and the basically stable pressure are taken as the calculation range of the system.
• ② Under the condition of maximum flow rate, the pressure loss △ pf caused by various local resistances (except the control valve) in the system is calculated respectively, and then their sum ∑△ PF is calculated.
• ③ Select the s value. S value should be the ratio of the pressure difference △ PV on the control valve and the total pressure loss of the system when the control valve is fully open, i.e. s = △ PV △ (△ PV + ∑△ PF), and S = 0.3-0.5 is usually selected. However, some systems can still meet the requirements of control performance even when s value is less than 0.3. For high pressure system, in order to reduce power consumption, it can also be reduced to s = 0.15. For gas medium, because the resistance loss is small, the pressure difference on the control valve accounts for a large component, so the s value is generally greater than 0.5. However, in the low pressure and vacuum system, due to the small allowable pressure loss, s is still 0.3-0.5.
• ④ According to the ∑△ PF and the selected s value, the pressure difference of the control valve can be calculated by using the formula s = △ PV △ (△ PV + Σ△ PF), and the calculated pressure difference △ PV = s ∑△ PF △ (1-s).

In view of the frequent fluctuation of static pressure in the system equipment, the change of pressure difference on the valve will be affected and s value will be further reduced. For example, in the boiler feed water control system, the calculated pressure difference should be increased by 5% – 10% of the system equipment static pressure (set the boiler rated static pressure as P), that is, △ PV = s ∑△ PF △ (1-s) + 0.0521p.
The increase of pressure difference on the control valve is beneficial to the control, but excessive pressure difference may cause cavitation phenomenon of the control valve. Cavitation should also be considered when determining the calculated pressure difference.

Checking calculation of regulating opening and controllable ratio of regulating valve

After the calculation of flow rate and pressure difference, the adjustment opening and controllable ratio of the control valve should be checked.

Checking calculation of regulating valve opening

Generally, the opening of the control valve should be about 90% under the maximum flow rate and not less than 10% under the minimum flow rate. The ideal flow characteristics and working conditions of the control valve must be considered when checking the opening. The following gives two kinds of commonly used flow characteristics of the control valve under the working conditions (series pipeline) opening checking formula.
Opening of linear characteristic regulating valve:

Equal percentage control valve opening:

Where k is the valve opening at the flow rate Qi; Qi is the flow rate at the checked opening, m3/h; and R is the medium density, kg/m3.

Checking calculation of controllable ratio

At present, the ideal controllable ratio r of the regulating valve designed in China is generally 30. However, due to the limitation of maximum opening and minimum opening, the controllable ratio generally drops to about 10. In the case of series pipeline, the actual controllable ratio RC = R √ s. Therefore, the controllable ratio is checked according to the following formula RC = 10 √ S. if RC > Qmax / qmin, the selected regulating valve meets the requirements. Otherwise, the s value of the control valve must be changed, and the method of increasing the system pressure or adopting two control valves (reducing s value) for split range control can meet the requirements of controllable ratio.
To sum up, according to the data provided by the process, the steps to determine the diameter of the regulating valve are as follows:

• ① Determine the calculation flow: according to the production capacity, equipment load and medium condition, determine the calculation flow Qmax and qmin.
• ② Determine the calculated differential pressure: select s value according to the selected flow characteristics and system characteristics, and then decide to calculate the differential pressure.
• ③ Calculation of flow capacity: according to the determined calculated flow rate and calculated pressure difference, the flow capacity Cmax at the maximum flow rate is calculated.
• ④ Selection of circulation capacity C: according to the obtained Cmax, in the standard series of the selected product type, the C value greater than Cmax and closest to Cmax shall be selected.
• ⑤ Checking calculation: checking the opening degree and controllable ratio of control valve.
• ⑥ Determine the diameter of control valve: after checking, determine the nominal diameter and valve seat diameter of the control valve according to the flow capacity C value.

Source: China Valve Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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