Reduction in diameter before the flow control valve

For any control valve, the valve diameter calculated based on the process conditions is generally smaller than the pipe diameter, so a diameter reduction is required.

Steps for selecting a control valve: 1. Based on the known process parameters—pressure before and after the valve, flow rate, specific gravity, etc.—calculate the valve’s Cv value； II. Based on the calculated Cv value, select valves from the manufacturer’s selection catalog that have a Cv value similar to, but greater than, the calculated Cv value ； III. After deciding on CV, consult the table to select the valve diameter ； IV. After all models have been selected, a check is performed on the opening degree; under normal conditions, it operates at around 60% to 80%.

Changing the diameter can alter the temperature as well as the pressure

Variable diameter is a requirement of the design specifications, but I forgot which specification it is!

Whether it’s a requirement for a change in diameter in the design or a requirement based on the calculation results, I just want to know why there is a need for such a reduction in diameter in the design In other words, why can’t a control valve with the same diameter be used? Because our previous devices did not have such a reduction in diameter. There must be a purpose behind such a design.

Generally, the diameter of the control valve after calculation is smaller than that of the pipe; how can it be installed if the diameter doesn’t change? There are parameters on the control valve; check the pipe diameter.

The fundamental purpose of the diameter changes before and after the control valve is to increase the pressure upstream of the valve in order to achieve more sensitive control, while the diameter change downstream is intended to reduce the pressure in the outlet pipe.

In other words, is the purpose of changing the diameter to make the adjustment more sensitive? And a smaller caliber can save costs?

The calculation of the diameter of control valves is something that automation design engineers must master; the accuracy of this calculation determines the quality of regulation provided by the control valves. You are familiar with the characteristic curve of control valves, right? 1. Fast opening 2. Straight line 3. Equal percentage. Inaccurate calculation of the valve diameter can lead to fast opening; specifically, there is a large change at low opening degrees while the change is small at high opening degrees. Example: For a control valve with a fast-opening characteristic, when the opening degree is increased from 15% to 16%, the flow rate may rise from 5 m3/h to 7 m3/h ； By adjusting the opening from 70% to 90%, the flow rate may increase from 15 liters to 15.5 liters. It is sensitive at low opening degrees but sluggish at high opening degrees; this makes it difficult to tune the PID parameters, as the proportional gain has to be set very high, resulting in slow automatic control and delayed responses. The KV value of the single-seat control valve with DN65 size is 63; that is, when there is a pressure of 1 kilogram across the valve, the DN65 control valve can handle a flow rate of 63 m3/h. If there is a pressure of 1 kilogram on both sides of the control valve, and the required process flow rate is 50 m3/h with a flow velocity of 2 m/s, then a DN100 pipeline is appropriate for the process. For a control valve of DN65, this will be sufficient. In the case of disc valves or ball valves, whose KV values are higher, a DN50 valve will also be adequate. The valve is smaller than the pipe; do you think a reducer is needed? Suggestions: 1. Find some materials on the calculation of control valves and study them carefully. 2. Find some materials on pipeline pressure drop and the calculation of centrifugal pumps, and learn how to perform such calculations on your own when the process does not provide information regarding the pressure drop across the valves.

I remember asking someone else before, and the answer was that the flow capacity of a valve depends only on its valve core. If the size of the valve core calculated is smaller than the diameter of the process pipe, it doesn’t matter whether the diameter is reduced or not; however, reducing the diameter can save money, so that’s why it’s done. It seems there are other reasons as well

I’m not sure about everyone’s answers either, but I hope those who know can outline the specific reasons so that future people can understand clearly! ! !

Quoting Floor 12: “I remember asking someone else before; the answer was that the flow capacity of a valve depends only on the valve core. If the diameter of the valve core calculated is smaller than that of the process pipe, it makes no difference whether to use a reduced-diameter fitting or not. However, using a reduced-diameter fitting can save money, so that’s why it’s used. It seems there are other reasons as well.” 1) Reduced-diameter fittings save money: This is problematic. If a flange with the same diameter as the valve core can be used, it’s almost certainly cheaper than a reduced-diameter flange. 2) Someone else suggested that reduced diameters are used to improve control conditions, and in principle this is correct. But one should be careful – just because a reduced diameter is used doesn’t necessarily mean that the control conditions will improve. Think about it: what would happen if a reduced diameter fitting were used at a high pressure of 50 MPa? Haha, think about it yourselves. I’m just making logical deductions here; please point out any mistakes if there are any.

The reduction in diameter before and after the control valve is merely a superficial phenomenon; the underlying reason is that the diameter (DN) of the control valve, calculated based on the process requirements, is smaller than the diameter of the process pipeline, which is why there is such a reduction in diameter. Calculating the parameters for a control valve involves complex procedures: first, the flow capacity value Kv or Cv of the valve is determined based on the process conditions, and a valve is selected using a value that is about 1.5 times this figure. Then, the opening degree of the valve is calculated so that it operates within a range of 60% to 80% most of the time; this process requires repeated selections and calculations until an appropriate value is found. Finally, the actual operating conditions are taken into account to determine the appropriate diameter for the valve. As a result, the diameter of the valve chosen in this way is usually smaller than that of the process pipeline. The approach described above is based on theoretical principles from textbooks; its accuracy in practice cannot be guaranteed.

In practical applications, there are those without diameter reduction; the valves used for water distribution in the heavy-duty and Atlantic continuous casting machines all have the same diameter. Go and check out small and medium-sized steel mills!

It cannot be said universally whether diameter reduction occurs or not; the selection of control valves is based on calculations of relevant process parameters of the controlled system, and this calculation process is quite complex, relying on fluid mechanics. It’s hard to explain clearly here, but nowadays many manufacturers of control valves have developed specialized calculation software based on the manufacturing characteristics of their valves; we just need to input some relevant process parameters. For controlling low pressure differences and low flow rates, the use of reduced-diameter control valves can increase the pressure difference and improve control accuracy; however, in cases of high pressure differences, it is more important to prevent cavitation and erosion of the valve. In some applications, expanded-diameter valves are used to reduce the pressure difference. Therefore, we cannot understand things in a one-sided manner, as the control objects are constantly changing. All choices made for control are aimed at achieving an optimized control process in a simple and reliable way; it is the control objects that serve as the basis for our selections. :loveliness: :loveliness:

It is calculated based on the process conditions, so a diameter change is required.

The calculation for control valves begins with determining the flow capacity value of Kv or Cv based on the process conditions; a valve is selected using a value that is 1.2 to 1.5 times this figure. Afterwards, the opening degree of the valve is calculated so that it remains within the range of 60% to 80% throughout operation. This process of selection and calculation needs to be repeated until an appropriate value is found.

This is determined by calculating the flow capacity of the control valve based on the control parameter requirements specified in the process, and by taking into account the performance parameters of the control valve

You say it’s calculated based on the process conditions, but are there any that are larger than the process pipelines? If so, should the pipelines be enlarged? Adding a variable diameter wouldn’t be appropriate in this case