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5. FLOW RATE AND Kv VALUE

Flow rate of any given valve depends on valve size, type of fluid and the pressure which forces the fluid through the valve. Kv value is given for each valve type in the table. With known parameters the flow rate can be calculated according to the following formula:

Q......flow rate [l/min]

Kv.....value given in the table for each valve type

p....difference between inlet and outlet pressure [bar]

p1.....pressure measured at valve inlet [bar]

p2.....pressure measured at valve outlet [bar]

......specific mass of fluid [kg/dm3] (equals 1 for tap water)

Kv value is approximately the same for tap water and other fluids (e.g. oil) of viscosity not exceeding 21mm2/s. At higher viscosity a correction of Kv value is required.

1 CFM = 28.328 l / min

Kv=L/m

Example 1/2" Pilot operated diaphragm is 20KV at Water 1bar or 14.7PSI

The flow coefficient - Cv - and the flow factor - Kv - are commonly used to specify the capacit of control valves.

The Flow Coefficient - Cv

It is often convenient to express the capacities and flow characteristics of control valves in terms of the

* Flow Coefficient - Cv

The flow coefficient - Cv - is based on the imperial units system and is defined as:

* the flow of water through a valve at 60 oF in US gallon/minute at a pressure drop of 1 lb/in2

The flow coefficient is commonly used in the U.S.

The Flow Factor - Kv

The metric equivalent of the flow coefficient - Cv - is based on the SI-system and is called the

* Flow Factor - Kv

The flow factor is defined as

* the flow of water through a valve at 20 oC in cubic meters per hour with a pressure drop of 1 kg/cm2 (1 bar)

The flow factor is commonly used outside U.S.

Converting between Flow Coefficient Cv and Flow Factor Kv

The connection between Cv and Kv can be expressed as:

* Cv = 1.16 Kv (1)

* Kv = 0.853 Cv (2)

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6. PRESSURE RATING

a) Maximum operating pressure differential is the maximum difference between valve inlet and outlet pressures at which the valve will still reliably operate. Values in tables (with tolerance +/- 10%) are given at ambient and coil temperature 22°C.

b) Minimum operating pressure differential is the minimum difference between valve inlet and outlet pressures at which the valve will still reliably OPEN. If required pressure difference is not achieved the valve will not open when activated.

For example a 3/8" SMC valve will not open at 250PSI because the differential of pressure is greater then the 200 It is ratted for. It simply takes more power to open then the coil can supply so your valve is now 'stuck'

c) Maximum static pressure is the highest fluid pressure endured by valve body and internal parts without damage.

I find if you exceed this value the valve plate (weakest point) will flex and pass air around the seal and worst case deform or even explode violently.

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