Reference Manual

3−11
where,
SK + K
1
) K
2
+ 1.5
ǒ
1 *
d
2
D
2
Ǔ
2
+ 1.5
ǒ
1 *
16
64
Ǔ
2
+ 0.84
and
F
p
+
ƪ
1 )
SK
N
2
ǒ
C
v
d
2
Ǔ
2
ƫ
*1ń2
+
ƪ
1 )
0.84
890
ǒ
203
4
2
Ǔ
2
ƫ
*1ń2
+ 0.93
and
C
v
+
q
N
1
F
p
P
1
*P
2
G
f
Ǹ
+
800
(
1.0
)(
0.93
)
25
0.5
Ǹ
+ 121.7
This solution indicates only that the NPS 4 valve is
large enough to satisfy the service conditions
given. There may be cases, however, where a
more accurate prediction of the C
v
is required. In
such cases, the required C
v
should be determined
again using a new F
p
value based on the C
v
value
obtained above. In this example, C
v
is 121.7,
which leads to the following result:
F
p
+
ƪ
1 )
SK
N
2
ǒ
C
v
d
2
Ǔ
2
ƫ
*1ń2
+
ƪ
1 )
0.84
890
ǒ
121.7
4
2
Ǔ
2
ƫ
*1ń2
+ 0.97
The required C
v
then becomes:
C
v
+
q
N
1
F
p
P
1
*P
2
G
f
Ǹ
+
800
(
1.0
)(
0.97
)
25
0.5
Ǹ
+ 116.2
Because this newly determined C
v
is close to the
C
v
used initially for this recalculation (116.2 versus
121.7), the valve sizing procedure is complete,
and the conclusion is that a NPS 4 valve opened
to about 75% of total travel should be adequate for
the required specifications.
Sizing for Pulp Stock
The behavior of flowing pulp stock is different from
water or viscous Newtonian fluids. It is necessary
to account for this behavior when determining the
required valve size. Methods have been
developed to aid in determining correct valve size
for these types of applications.
The pulp stock sizing calculation uses the
following modified form of the basic liquid sizing
equation (equation thirteen, above):
Q + C
v
K
p
DP
Ǹ
where,
ΔP = sizing pressure drop, psid
C
v
= valve flow coefficient
K
p
= pulp stock correction factor
Q = volumetric flow rate, gpm
The root of this calculation is the pulp stock
correction factor, K
p
. This factor is the ratio of the
pulp stock flow rate to water flow rate under the
same flowing conditions. It, therefore, modifies the
relationship between Q, C
v
, and ΔP to account for
the effects of the pulp stock relative to that for
water. The value of this parameter, in theory,
depends on many factors such as pulp stock type,
consistency, freeness, fiber length, valve type and
pressure drop. However, in practice, it appears
that the dominant effects are due to three primary
factors: pulp type, consistency and pressure
differential. Values of K
p
for three different pulp
stock types are shown in figure 3-4 through 3-6.