Chemical Engineering March 2017 - 81

TABLE 4. (CONTINUED)
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Nozzle Schedule
Mark
A1
B1
B2
H1
H2
J1
L1
L2
M1
M2
M3
N1
Notes:
1. Data by the mechanical-vessels group.
2. Material grade by the vessels group.
3. Internal floating-roof details by storage-tank vendor.
4. For details, see let mixer datasheet (Ref. Doc.: J-1001-PDS, Rev. A).
5. Nozzle A1 and B1 to be located on opposite sides of shell.
6. Nozzle N1 and H1 to be located on opposite sides of roof.
7. Suitable vacuum breaker (breather valve on rim vent) to be provided on roof when it rests at minimum.
8. The roof supports should be adjustable for minimum operating level from bottom and minimum level for manual cleaning.
9. Nozzles H2, L1 and L2 to be provided with stilling wells.
Holds
1. To be confirmed during detailed engineering.
2. Instrumentation group to confirm all instrument nozzle sizes.
for cost-estimation at a very early
stage of the project. However, the
dimensions of the tank need to be
firmed out as the project progresses
in design phases. Firming up a tank
dimension or tank sizing
involves
checking the following three steps:
1. Accommodate process volume or
the working volume in the tank.
2. Set tank overfill protection level
requirement (to permit operator
response).
3. Set minimum operating volume in
the tank.
Setting alarms
The overfill-protection volume and
the minimum-volume allocation can
be best understood in terms of level
alarm (LA) values stated in the datasheet.
Typically, four types of alarms
are set at the following levels (see
Figure 2 and Table 3):
* LLLL - low low liquid level
* LLL - low liquid level
* HLL - high liquid level
* HHLL - high high liquid level
Usually, levels are set above some
point of reference in the tank. First,
LLLL is set. It is the lowest liquid
level
below
which
the
operation
and safety may be affected; for example,
to provide sufficient NPSHA
(net positive suction head available)
for the pump, or to avoid surface
dry-out of the tank's internal heating
coils. In most cases, the tangent
to the top of the tank-outlet nozzle
is considered as the LLLL alarm.
Above the LLLL, some buffer volume
is provided until LLL, to avoid disturbing
the process volume due to
draw-out by the pump. Above LLL,
the height equivalent to process volume
is then accommodated to reach
HLL. To prevent overfill of the tank,
an operator-intervention time of 20
minutes is considered and a height
corresponding to this volume, or a
minimum of 3 in., is added above
HLL to attain HHLL. As a minimum,
HHLL should be set at least 500 mm
below top of the tank.
For a fixed-roof tank, as explained,
we consider LLLL = 205.15 mm (at
the tangent of 2 in. pump out nozzle)
and then set the remaining alarms
starting from this point.
However, for an IFRT that also has
an internal jet-mixer nozzle, we have
an additional approach to fix the levels.
We evaluate tangent to the top of
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ATM = Atmosphere
N.A. = Not applicable
the jet mixer nozzle as 390.15 mm.
As a good engineering practice, LLLL
is set such that: 1) there is a minimum
clearance of at least 4 in. between
the internal floating roof and any internal
parts, such as jet mixer nozzle;
and 2) the roof remains floating with
its supports at least 4 in. above the
tank bottom. Also, based on experience,
it is assumed that the elevation
at the tip of the mixer nozzle inside
the tank is 4 ft. Thus, the LLLL is set
at an elevation at the tip of the mixer
nozzle plus the minimum clearance
between the internal floating roof and
the jet mixer nozzle at 1,319 mm. LLL
is then set 3 in. above LLLL.
Preparing the tank datasheet
Once the sizing is done, we move to
preparation of the tank datasheet.
The datasheet may be considered as
the owner's permanent record for describing
a tank, and it is used to make
proposals and place subsequent
contracts for fabrication and erection
of the tank. This section explains the
information to be placed in the datasheet
by the process engineer.
General instructions. This set of instructions
are of a basic nature, but
81
Size
4
2
2
4
6
6
6
Flange rating/face
RF/150#
RF/150#
RF/150#
20 (Hold 1) RF/150#
RF/150#
24
20
24
4
RF/150#
RF/150#
RF/150#
RF/150#
RF/150#
RF/150#
RF/150#
Service
Feed
Outlet
Sump outlet
Emergency vent
Gage hatch
Jet mixer
Level transmitter
Level transmitter
Shell manway
Roof manway
IFR manway
Nitrogen
Mark No. Size
R1
P1
T1
T2
1
1
1
1
2
1.5
1.5
Hold 1 RF/150#
RF/150#
RF/150#
RF/150#
Flange rating/face Service
RVVB
Pressure tap
Temperature element
Temperature indicator

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Chemical Engineering March 2017

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