Control Valve Sizing is known as the order of obtaining by min and max flow and finding Cv,
If you look at the contents described in the attached photo, it is said that dp is obtained as follows.
(Eg, can be determined from the pressure head curve), reducing the pressure loss at each stationary element. When you go to the valve inlet, you can see the pressure p1, which is just upstream of the valve. At this point, you can not directly calculate the pressure drop across the valve. Because we have not yet determined the size and open ratio of the valve.
The next step is to move the pressure downstream of the known control valve (eg, the head to a known tank) and then operate upstream to the control valve to add pressure loss to each of the stationary elements. (The pressure loss is added because you work in the opposite direction of the flow.) If you go to the valve outlet, you can see the pressure p2 just downstream of the valve.
And this work is shown in three to four steps in the order that Stefano wrote in previous post.
( The order of the control valve size selection is as follows: ① Flow rate → ② Diameter → ③ Differential pressure → ④ (Valve differential pressure) → ⑤ Valve opening rate → ⑥ Valve size determination " )
- If you want to set the dp to 0.5 bar,
If it comes out, do you design the system by modifying the system (by adding a valve to raise the back pressure to lower dp)
I wonder if I can select a valve that holds 2 bar (there may be side effects such as noise).
- Simply set dp to 0.5 bar and check that the back pressure of the valve is higher than the back pressure.
I wonder if I can get the Cv value and select the size.
ANSWER
(1) In the case of liquid flowing through the piping, the flow rate is controlled by the valve in the middle. The pressure before and after the valve greatly varies depending on the valve opening rate.
In such a case, the flow characteristics of the valve are such that the differential pressure of the valve is constant and does not follow a single flow characteristic curve that depends on the valve opening ratio. Therefore, even though the valve has a linear characteristic, . This is the case when you expect a linear characteristic and a valve does not emit the expected performance.
This is the reason why the flow control can not be adjusted even if the flow control is adjusted to the valve opening ratio especially when the differential pressure is large.
Therefore, it is necessary to keep the pressure difference between the control valve and the control valve constant so that the flow rate can be adjusted smoothly by using the control valve. If the pressure difference is too high, it is easier to adjust the flow rate if a manual valve or an Orifice is placed in the middle and a control valve is opened by about 70 ~ 75% so that 100%
In the first question, I have a question about putting a valve on the back of an automatic valve, and this method has the same effect. If the pressure before and after the control valve becomes too large. If you operate the valve with a manual valve or an orifice both before and after the control valve, the valve life will increase and the adjustment will be easier. If the differential pressure is very severe, a maze type orifice should be installed to reduce the differential pressure before and after the valve.
(2) If the pressure before and after the system is fixed, it is recommended to put Pressure Regulator or Orifice or Valve in between. If the actual differential pressure is greater than the assumed value at the time of valve selection, it will be operated almost closed. If it is small, it will be selected as a small valve that can not flow the target flow even if the valve is opened even though the maximum differential pressure is maintained.
If you want to set dp to 0.5, make sure that the pressure of the system at the rear end of the shear can be maintained. If the pressure difference is too large, set the valve or the Orifice to two.