If the selected valve is suitable for the task except from Cv:
1. On/off valve with too high Cv: small or insignificant effect on flow characteristic.
2. On/off valve with too small Cv: flow conditions could in worst case give turbulent flow, or higher resistance and in absolute worst case increased wear or even cavitation conditions.
3. Throtteling or regulating valve with too high Cv: unsatisfactory throtteling curve (too small percentage of operning actually regulating), else as point 2. Fear of cavitation.
4. Throtteling valve with too low Cv: Unsatisfactory regulation (especially for high flow ?) Else as point 2, Fear of cavitation.
All flow controlling devices must be analyzed with all data for flow and devices avaailable to give an answer containing any figures.
First of all thank you for your soon reply.
What do you mean by too high cv and too small cv?
I have seen some manufacturers use oversizing factor=1.2 to select cv.
for example when calculated cv=290 then selected cv will be=348.
what if we select a higher cv?
You would have to calculate and consider the consequences for the flow, using the selected Cv, pressure before, pressure after the valve, flow capacity of the upstream pipeline, increased loss by increased flow etc. for each situation. If varying flow you should control minimum, normal and max flow for above.
Example: For 'normal' flow velocities for 'normal' fluids, for instance drinking water, mains are often dimensioned for a flow from say 0,3 to 1,5 m/s. This could be more or less, but from (say) 2-3 m/s you will have a 'cleaning/washing out' velocity of the mains. Obviously, if you then have a low flow velocity (0,3 m/s) you could use a suitable Cv within a larger range than if you have a normal velocity around 2m/s.
For gases (for instance steam at 15-40m/s) or other fluids( hydraulic oil, petrochemical) the velocity and consequences will be different.
You pick a larger valve so it won't run 100% open at the required flow. Lots of companies want their control valves to run 70% to 80% open at the maximum sizing case.
What happens if you select a larger valve? The valve obviously runs at a lower % open. You'll need to consider what happens if the valve goes fully open and how it operates at the other expected operating conditions, a larger valve could mean the valve isn't open very much at turndown conditions.
If the control valve is on the discharge of the pump there may not be much change depending on the the system curve and the pump curve. If you are letting down from a high pressure source to a low pressure system, it will increase your overpressure requirements for the low pressure system, maybe it fits in the same relief valve, maybe you have to upsize your relief system.
