Chemical Engineering April 2012 - 39

Present Op. Point
210
80
200
70
60
50
40
30
60
Flowrate, m3/h
Table 2. ValVe CharaCTerisTiCs
Flow
Characteristic
Valve
Size
Valve Opening, % of Stroke Range
10 20 30 40 50 60 70 80 90 100
Equal
Percentage
in. Cv
1 0.35 0.5 0.7 1.1 1.9 3.6 5.6 7.5 9.1 10
11/2 0.74 1.1 1.5 2.3 4 7.6 12 16 19 22
2 1.9 2.7 3.8 6 10 20 31 41 48 54
3 3.3 4.7 6.6 11 18 35 54 72 84 95
4 5.9 8.5 12 18 32 61 96 128 153 175
6 13 18 25 40 68 130 203 271 324 360
creases in frictional losses. This leaves
us with losses across control valves.
The existing reflux pump P-101 has
two routes, namely the product route
to battery limit, and the reflux route to
Tray No. 1 of the column. The second
route happens to be the controlling
route (that is, this route has the higher
resistance and hence determines the
pump differential head). The prospect
of decreasing the allowable pressure
drop across the control valve FV-101
was examined. It was found that in the
existing unit, an allowable pressure
drop of 1.0 kg/cm2 across the valve was
specified. It could still be brought down
to 0.85 kg/cm2 in the revamp. With all
the above, we arrive at the revamp
conditions of the pump.
Under the revamp conditions, the
pump has the following specifications:
Rated flow:
Head:
67 m3/h
75 m
Liquid density:
Liquid viscosity:
770 kg/m3
0.28 cP
The flow has increased by 40%.
The head has increased marginally.
The other parameters have remained
unchanged.
Referring back to Figure 3 again,
we find that the revamp figures of
flow and head can be met with an impeller
of diameter of 208 mm. Hence
we need to procure a standard impelAt
maximum flow, Cv =
162.9(0.77/14.22)1/2 = 37.9
At minimum flow, Cv =
74.8(0.77/24.17)1/2 = 13.4
The existing valve has a Cv of 54 and
a size of 2 in. It has an opening of
77% of stroke range at maximum flow
and 53% at minimum flow (see Table
2, which is a typical chart similar to
those supplied by manufacturers).
ler of 210-mm dia. and get it trimmed
to 208 mm.
Control valves
Consider the control valve on the reflux
line, that is, FV-101 in Figure
1. For the existing plant, the control
valve parameters are as follows :
Maximum flow: 37 m3/h (162.9 gpm)
Minimum flow: 17 m3/h (74.8 gpm)
Pressure drop at maximum flow:
1.0 kg/cm2 (14.22 psi)
Pressure drop at minimum flow:
1.7 kg/cm2 (24.17 psi)
Specific gravity relative to water: 0.77
The Cv
90
= impeller dia., mm
120
190
170
180
Figure 3.
This pump
characteristic
curve
shows the
original
pump and
that considered
in the
revamp
Now consider the following specifications
of the same control valve
under revamp conditions :
Maximum flow: 52 m3/h (229.0 gpm)
Minimum Flow: 24 m3/h (105.7 gpm)
Pressure drop at maximum flow:
0.7 kg/cm2 (9.95 psi)
Pressure drop at minimum flow:
1.5 kg/cm2 (21.33 psi)
Specific gravity relative to water: 0.77
Note that while the flow has increased,
a reduced pressure drop across the
control valve has been specified in
order to minimize the increase in
pump head.
At maximum flow, Cv =
229.0(0.77/9.95)1/2 = 63.7
At minimum flow, Cv =
105.7(0.77/21.33)1/2 = 20.08
At a Cv of 63.7 (at maximum flow),
is
the existing 2-in. control valve will
not be adequate since its rated Cv
54. Hence we need to go for the next
higher size valve, which is 3 in. (see
Table 2).
For a 3-in. valve, with a rated Cv
of 95, the openings shall be 75% at
maximum flow and 51% at minimum
flow. (In normal practice, we operate
control valves between 10 and 80% of
stroke range). In other words, we need
to replace the existing 2-in. valve with
a new 3-in. valve.
of a control valve in liquid
service is defined as the number of gallons
of water that would pass through a
valve with a 1-lb/in.2 pressure drop, and
is given by the following equation [3]:
(1)
Pressure relief valves
Like all other equipment in a plant,
elements of the pressure relief system
also need to be evaluated for bottlenecks.
The safety equipment generally
consist of pressure relief valves, and in
some cases, rupture discs.
Consider the pressure relief valve
mounted on C-101 in the plant under
discussion (Figure 1). The size of a relief
valve is determined by its orifice size,
which in turn gives the inlet and outlet
flange sizes of the relief valve. API 526
provides details of various standard orifice
sizes of relief valves as well as the
inlet and outlet flange sizes [4].
Per the present rating, the parameters
for this relief valve in the existing
plant are as follows:
Required relieving rate:
Molecular weight:
23,000 kg/h (50,706 lb/h)
78
ChemiCal engineering www.Che.Com april 2012 39
Revamp Point
Head, m
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Chemical Engineering April 2012

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