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Kv for flow through valves
- Thread starter Edouard
- Start date
Hi Edouard,
First of all Kv is an dimensionless number and it is a measure of pressure loss across the valve. Kv can be 0,8 but can not be 0,8 m3/h. When you say m3/h then it means that you are talking about flow rate, but not the Kv.
The relation between the pressure loss and Kv is;
dP=Kv*V2/2g where
V(m/s) average flow velocity,
g gravitational acc. 9,81 m/s2
then dP is the pressure loss across the valve
in meter liquid column.
As you can see from the relation above you can not say anything about the flow rate. But, if you know the flow velocity and Kv value than you are able to say the amount of pressure loss across the valve.
The relation between the flow rate and the speed is
Q=V*A where
Q (m3/s) flow rate
V (m/s) average flow speed
A (m2) crossectional area.
So if you know your pump delivery and your pipe size, then you can calculate the flow speed and hence you can find the pressure drop across the valve as long as you know the Kv value.
Hope this helps.
First of all Kv is an dimensionless number and it is a measure of pressure loss across the valve. Kv can be 0,8 but can not be 0,8 m3/h. When you say m3/h then it means that you are talking about flow rate, but not the Kv.
The relation between the pressure loss and Kv is;
dP=Kv*V2/2g where
V(m/s) average flow velocity,
g gravitational acc. 9,81 m/s2
then dP is the pressure loss across the valve
in meter liquid column.
As you can see from the relation above you can not say anything about the flow rate. But, if you know the flow velocity and Kv value than you are able to say the amount of pressure loss across the valve.
The relation between the flow rate and the speed is
Q=V*A where
Q (m3/s) flow rate
V (m/s) average flow speed
A (m2) crossectional area.
So if you know your pump delivery and your pipe size, then you can calculate the flow speed and hence you can find the pressure drop across the valve as long as you know the Kv value.
Hope this helps.
Correction!
Sorry Edouard,
I misunderstood the meaning of Kv. I confused it with loss coefficiant factor (K) which is a dimensionless number.
Kv value of a valve is;
the flow in l/s through a valve with a 1 psi differential across the valve (US costumary units)
or
the flow in m3/h through a valve with a 1 bar differential across the valve (Metric units).
But this does not tell you the flow for a pressure drop different than 1 psi or 1 bar. Kv values, generally, are used in valve sizing.
I think you have to refer to manufacturer's documents to determine the flow through the valve for a specific pressure differential.
Besides, the valve characteristics based on a constant pressure drop is very important.
It may be a quick opening type or a linear type or an equal percentage type. For instance, if it is a quick opening type, then the flow will be higher than the other types for the same pressure differential across the valve.
Regards
Sorry Edouard,
I misunderstood the meaning of Kv. I confused it with loss coefficiant factor (K) which is a dimensionless number.
Kv value of a valve is;
the flow in l/s through a valve with a 1 psi differential across the valve (US costumary units)
or
the flow in m3/h through a valve with a 1 bar differential across the valve (Metric units).
But this does not tell you the flow for a pressure drop different than 1 psi or 1 bar. Kv values, generally, are used in valve sizing.
I think you have to refer to manufacturer's documents to determine the flow through the valve for a specific pressure differential.
Besides, the valve characteristics based on a constant pressure drop is very important.
It may be a quick opening type or a linear type or an equal percentage type. For instance, if it is a quick opening type, then the flow will be higher than the other types for the same pressure differential across the valve.
Regards
Hi! Trying to put the things more clear:
1. Kv, if I remember, gives you an idea how the flow goes through the valve itself. It is indeed a valve capacity parameter.
2. So, we talk about a flowrate which is measured for a certain pressure loss, a differencial pressure taken before and after the valve points. So, if we impose this pressure loss as reference pressure, shall we say, a psi, or a bar, or a torrichelli, we may ask, how much flowrate do I get, for such reference pressure loss? The answer is a flowrate value, which is called exactly: - the Kv of that valve, and it's numerical value depends on the unities system, you are using. Therefore, Kv being a valve parameter, is not a dimensionless one! Can be measured in m3/h, gallon/minute, ounce/sec. This parameter identifies that valve in particulary and you can realize that, in a simple flowrate/pressure loss diagram, which gives you all work-conditions of that valve. This is the answer to the Edouard's question, I suppose.
3. From the Fluids-Mechanics, we know there is a very clear relation between the flowrate and concerning pressure (Bernoulli's). If we define such balance in a system like a valve, we come out with a very simple formule: Kv=Q*(ró/(1000*delta P))^1/2 in SI
with:
- Kv, the flow constant charateristic of the valve
[Note: Sometimes, the Kv value can be different of that declared in technical literature, if the inlet and the outlet conditions are not the normal ones. So you should read in which conditions the Kv of the valve is measured!]
- Q, the flowrate in m3/h (and not m3/sec)
- ró, specific mass (density) kg/m3
- delta P (diff. pressure in bar, as mentioned above)
4. If you want to go deeper about the meaning of Kv, please refer, for exemple, to the standard, DIN EN 60 534, or the equivalent in the States.
Hoping this can help. Goog luck.
235zzzo
1. Kv, if I remember, gives you an idea how the flow goes through the valve itself. It is indeed a valve capacity parameter.
2. So, we talk about a flowrate which is measured for a certain pressure loss, a differencial pressure taken before and after the valve points. So, if we impose this pressure loss as reference pressure, shall we say, a psi, or a bar, or a torrichelli, we may ask, how much flowrate do I get, for such reference pressure loss? The answer is a flowrate value, which is called exactly: - the Kv of that valve, and it's numerical value depends on the unities system, you are using. Therefore, Kv being a valve parameter, is not a dimensionless one! Can be measured in m3/h, gallon/minute, ounce/sec. This parameter identifies that valve in particulary and you can realize that, in a simple flowrate/pressure loss diagram, which gives you all work-conditions of that valve. This is the answer to the Edouard's question, I suppose.
3. From the Fluids-Mechanics, we know there is a very clear relation between the flowrate and concerning pressure (Bernoulli's). If we define such balance in a system like a valve, we come out with a very simple formule: Kv=Q*(ró/(1000*delta P))^1/2 in SI
with:
- Kv, the flow constant charateristic of the valve
[Note: Sometimes, the Kv value can be different of that declared in technical literature, if the inlet and the outlet conditions are not the normal ones. So you should read in which conditions the Kv of the valve is measured!]
- Q, the flowrate in m3/h (and not m3/sec)
- ró, specific mass (density) kg/m3
- delta P (diff. pressure in bar, as mentioned above)
4. If you want to go deeper about the meaning of Kv, please refer, for exemple, to the standard, DIN EN 60 534, or the equivalent in the States.
Hoping this can help. Goog luck.
235zzzo
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