Why Is 1 Bar Important in Cv or Kv Calculations?

[TSN79]

Two questions;

1)
I'm struggeling to understand the concept of the Cv or Kv value, also known as the flow factor. In SI units this is defined as the flow through a valve (m3/h) at a pressure drop of 1 bar, or 100 kPa. Fair, and the formula for calculating it is easy enough. I enter my flow and the desired pressure drop (in this case 5 kPa). So where does this "1 bar" enter the picture?

2)
To measure the flow through a valve one connects a meter to it (called a CBI, made by Tour & Andersson). I always hear people saying that the valve needs to have a certain pressure drop in order for the flow to be measurable. Why is that? I always considered pressure drop as something one wants to avoid...

 

[FredGarvin]

1)The 1 bar is the standard that the valve manufacturer uses to test/measure for the Kv (or 1 psi per gpm to calculate Cv for us in the US). That acts like the equalizer between different sizes and types. That also gives you more information when designing to know what kind of valve has what kind of flow restriction based on type.

2) It does depend on the valve itself, but by stipulating a pressure drop, you are effectively stipulating a minimum flow. I am not familiar with the term you use (CBI), but if it measures flow and requires a minimum pressure drop across it, it means that the instrument is incapable of measuring any smaller delta P than the stated minimum which is needed to calculate the flow rate.

 

[RTW69]

I believe that CBI stands for computerized Balancing instrument. it is a handheld devise used to measure the pressure difference across a valve and the flow rate and temperature of the working fluid.

 

[FredGarvin]

Good to know. Thanks.