Answer / www.islamqa.com

#"Cv" for valve is known as: valve sizing coefficient or
valve flow coefficient

#It refers to " how many gallons per minute can pass
through a fully open valve under diffrential pressure of 1
PSI "

#Cv is a unitless value and by getting it's value we can
get the valve size in inch from tables.

#Cv is got from calculations using s/w provided by
manufacturer as we have many parameters affect its value,
and Cv value is critical as it affects valve size which
leads us - if wrong - to oversized valve or undersized
valve, which harm the process.

#Cv - as mentioned - is got from calculations using s/w,
but it has a basic formula for gas service and liquid
service.

#For liquid service:

Q = Cv (DP/ROH)^.5, ROH IS SPECIFIC GRAVITY

#For gas service:

Q = 963 Cv (DP(P1+P2)/ROH*T)^.5, T is gas temperature in
Rankin

Regards,

Ahmed Sabry
Control and Instrumentation Engineer
GPC - General Petroleum Company
ahmedsabry.online@gmail.com
CELL.: 00 2 0121367310

Answer / sam

Flow coefficient is the proportional constant between
pressure drop and flow rate and it is determined
experimentally by valve manufactures. It is expressed as the
flow rate of water in m3/h for a pressure drop of 1 bar
across a flow passage.

Flow coefficient = Cv or Kv
Cv = Imperial
Kv = Metric

Selecting the correct size control valve:
When the flow coefficient is calculated for the required
flow rate and known pressure drop, the selection of a proper
control valve is normally done by selecting a control valve
with the next bigger flow coefficient(cv)

So in your case here are some info that might help,

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 2973m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 743m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 330m/h.

So the volumetric flow rate stays the same regardless of
pipe dia if the DP stays the same. The only thing that will
change is the flow velocity.

So,

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 2973m/h

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 4205m/h

With a control valve with a cv of 27 in a clean water line
with a 100mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 5151m/h

And,

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 743m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 1051m/h

With a control valve with a cv of 27 in a clean water line
with a 200mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 1288m/h

And,

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 1Bar
will give a volumetric flow rate of 23m3/h and a fluid
velocity of 330m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 2Bar
will give a volumetric flow rate of 33m3/h and a fluid
velocity of 467m/h

With a control valve with a cv of 27 in a clean water line
with a 300mm pipe dia size, a DP across the valve of 3Bar
will give a volumetric flow rate of 40m3/h and a fluid
velocity of 572m/h

So to summarize if you have a control valve with a cv of 27
you need to have a look at the maximum volumetric flow rate
that will be required in the line that this control valve
will be installed in, in order to see if this valve is big
enough to control this flow.

Keep in mind the perfect control position for any control
valve is between 40 to 60% valve opening. You need to select
a valve with a cv that will allow control to be in that area
under normal operating conditions, so it is important to
consider minimum, normal and maximum flow conditions during
the selection process. To be safe select the next bigger cv
than what your calculations says the correct size will be.
Good luck

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