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Model Selection

1. Size calculation of control valve
In different automatic control systems, parameters like flow, medium, pressure and temperature differ in thousands of ways, besides, flow coefficient of control valve is measured under differential pressure of 100KPa while the medium is normal temperature water. So, how to confirm the control valve's size according to the actual working conditions?
Apparently, we can't make it out by comparing the actual flow with the valve flow coefficient (as the differential pressure and medium differ), thus, we must make CV calculation, taking various actual parameters into corresponding CV formula and making out CV value, that's, translating flows under different working conditions into CV value under the same condition. Comparing the calculated CV value with the valve's CV value to confirm the the size of valve.
Finally, make concerning checking calculations to check whether the selected valve meets the working requirements.
2. Steps to calculate the size of control valve
There are 8 steps to finish confirmation of the valve size:
(1) Confirmation of flow calculation, to confirm the maximum working flow Qmax and the minimum flow Qmin based on the current production capacity, equipment load and medium.
(2) Confirmation of differential pressure calculation, to determine S value based on the system characteristics and then confirm the differential pressure.
(3) Calculation of CV value, to make out CV value in the maximum working flow CVmax on the base of the determined calculation flow, calculation differential pressure and other related parameters.
(4) Preliminary confirmation of the size of control valve, Select the CV value based on the calculated CVmax that is approximate but larger than the CVmax, and make out the size.
(5) Checking opening ratio.
(6) Checking calculation of actual adjustable ratio, as usual the actual adjustable ratio should be larger than 10.
(7) Calibration and checking of differential pressure, checking calculation based on open and adjustable ratio is not enough because it perhaps cause valve loosen or the valve perhaps can not be started, thus we add this step to ensure perfect operation.
(8) If the above checking calculation is qualified, then the valve size is qualified, if not, you need to confirm the valve size and CV value again, or you can select other valves and repeat the above steps until to be qualified. Generally speaking, you can adjust the flow by changing the orifice size between the control valve core and valve seat. However, due to influences from various factors, orifice size changes, the flow changes, causing the whole resistance changes in the system, as well as the front and back differential pressure of the control valve changes. In order to make analysis easy and convenient, we presume the front and back differential pressure of the control valve are invariant firstly, and then discuss the situation in actual condition. The former is called ideal flow characteristics and the latter is called working flow characteristics. Ideal characteristic is also called inherent flow characteristic which has two types, they are straight line type and logarithm type.
3. Selection of valve type
We had better to select light weight control valves
with complete functions to replace other control
valves, which can reduce problems caused by
improper selections like block troubles of sliding
stem control valves, relation between cut off and
size of pressure difference, etc.

4. Selection of actuators
(1) Select imported electronic actuators to avoid a
series of unreliable problems;
(2) Select fine small series for diaphragm type
actuator;
(3) Select gear strip type for piston actuator;
(4) Offer the close differential pressure of the valve to the manufacturer and we will determine sizes of the actuators.
5. Selection of the material
Selection of materials is complicated, because many corrosion-resistant medium don't corrode Four PVF, so, it's better to select full four fluorine corrosion-resistant valves to simplify selections in corrosion-resistant occasions. Only when temperature is >180℃ and <-40℃,PN≥2.5KPa, there is a need to consider corrosive alloy.

6. Selection of spring range
At present, most diaphragm control valves are also equipped positioners. You can make a full use of the 250KPa air source to select a medium-rigidity and quite large output force spring, 60~180KPa; for piston actuator you can select 15000KPa.
7. Selection of flow
characteristics


(1) When you are not sure the
parameters, select logarithm
characteristic;
(2) When response speed of the
modulated system is quite fast,
such as flow adjustment and
liquid level pressure adjustment,
select logarithm characteristic;
(3) When response speed of the
modulated system is quite slow,
such as liquid level system and
temperature adjustment system,
select straight line characteristic;
(4) When the valve works in small
open ratio, select logarithm
characteristic.

8. Selection of flow direction
For single-seal type sliding stem
control valve, it's better to select
flow open type; and when required
cut-off valve and washout resistant,
it's better to select flow closed type
(flow closed type is poor at stability,
thus you should make corresponding
stability measures).
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