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floater ball valve pressure drop and flow velocity calculations

floater ball valve pressure drop and flow velocity calculations

(OP)
I would like to have some assistance in verifying the calculations i came up with on this floater ball valve, based on the attached sheet.

ΔP = G (Q/CV)2 ....eq (1) in attached sheet. this is the most simplified method of calculating the pressure drop across a ball valve

my questions is regarding the flow coefficient, is it dimensionless ? it was provided to me in si units m3/hr, so do i have to convert it to gpm ? the units in this formula is not adding up!!! if the pressure in psi and the flow in gpm but both the G and Cv are dimensionless ?

now in order to know the flow velocity so Q= AV where A is the smallest cross section of the valve, is that a right approach?

if the flow is water do i still need to do viscosity correction?

is it as simple as that to calculate the pressure drop and flow velocity? that attached sheet gets very complicated and i am not sure if i should go into that depth

RE: floater ball valve pressure drop and flow velocity calculations

Sheet doesn't seem to be attached.

RE: floater ball valve pressure drop and flow velocity calculations

You have two calculation philosophies to choose from. So if it were my first time doing this, I'd follow both methods for liquid sizing and then see if there's a large difference between the two methods for this situation and then discuss with appropriate people.

In the "viscosity correction" section, it says that the correction is meant for situations when water is not the flowing medium.

Q = A*v will give you the average velocity at the point you choose. If you pick the smallest cross sectional area within the valve, the velocity you calculate is the highest velocity the fluid attains while still maintaining continuity.

Other valve literature I've seen with this equation usually list some additional constant that is meant to correct the units. In your case, I think if you use the US customary units (e.g. GPM) throughout the equation the additional constant is just 1. Hopefully someone else will back me up on this.

RE: floater ball valve pressure drop and flow velocity calculations

(OP)
Jari001,

Thanks for your input, the situation i am dealing with was the information i was provided by the vendor. he said Cv = 96 m3/hr based on water flow rate of 7200 m3/hr

so this statement is completely wrong because the definition of Cv : numerically equal to the number of US gallons of water at 60 F THAT WILL FLOW THROUGH THE VALVE IN ONE MINUTE WHEN THE pressure drop across the valve is 1 psi

therefore i am not sure how to make use of the numbers he provided to calculate the actual pressure drop when my flow rate is 400 gpm. that's my dilemma.

RE: floater ball valve pressure drop and flow velocity calculations

Your definition of Cv is correct. Either the value, units, or both are wrong for the number given you by the supplier.

Correct equation for Cv is Q (gpm)/square root (DP, psi). Note the Delta P in the denominator is in a square root. The definition above is correct since square root of 1 psi is 1 psi, but people often forget the square root.

To do flow calculations for valves, google "Metso Flow Control Manual". It is a pretty bid PDF file, but has all the flow equations and unit conversions and corrections for calculating flow rates, pressure drops, etc. for liquids and gas.

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