1.1 Pressure Vessels
1.1.1 Separators/Scrubbers
The following liquid levels and
residence times are recommended for 2 phase separator design:
Table 1
2-Phase Separator Design Parameters
|
Parameters
|
Vertical Vessel / Column
|
Horizontal Vessel
|
|
Vessel lower tangent or Vessel bottom to LLLL
|
6” (subject to mechanical constraints on distance to weld
lines).
(For thick walled vessels, 12”)
|
(min 12”)
|
|
Between HHLL and HLL
|
1 minute (min 6”)
|
1 minute (min 6”)
|
|
Between HLL and LLL
|
3 minutes (min 12”)
|
3 minutes (min 12”)
|
|
Between LLL and LLLL
|
1 minute (min 6”)
|
1 minutes (min 6”)
|
|
Normal Liquid Level (NLL
|
1 x Vessel I.D.
(min 12”)
|
(min 12”)
|
Surge volumes between NLL and LLL should based on the
following:
|
2 minutes
|
|
|
Product to a downstream heat exchanger
|
2 minutes
|
The requirement volume to handle liquid surges should
be determined on a vessel by vessel basis. The surge volume shall be
accommodated between NLL and HLL.
For 3-phases vessels allowance is made for separation
of the two liquids. For preliminary sizing:
|
Between bottom of vessel and NIL (water volume)
|
5 mins (min)
|
|
Between NIL and NLL (oil volume)
|
5 mins (min)
|
If water volume is small a boot may be used instead of
a separate settling compartment. Preliminary sizing of vapour / liquid
separators and scrubbers should be performed using standard gas load factors
defined as follow:
The following values of K should be used for
sizing:
|
VESSEL TYPE AND INTERNALS
|
K-ft/s
|
|
Vertical Knockout Drum (no internals)
|
0.262
|
|
Horizontal Knockout Drum (no internals)
|
0.328 (bulk separation)
|
|
Vertical with Mist Pad
|
0.361
|
|
Horizontal with Mist Pad
|
0.427
|
|
Vertical with Vane Pack
|
0.656
|
|
Horizontal with Vane Pack
|
0.656
|
|
Multicyclone
|
0.656
|
The criteria should be applied for preliminary sizing
of separators and for cross checking vendor supplied designs. For proprietary
vendor internals vendor supplied K values should be considered. For design
purposes the maximum allowable operating velocity shall be no greater than 80%
of the calculated critical entrainment velocity.
1.1.2 Accumulator / Reflux drums / fractionation columns.
Hold-up time is 5 minutes for
half-full accumulators/reflux drums. For a product feeding another tower
hold-up is 5-10 minutes.
1.1.3 Surge Vessels
Surge vessel such as Instrument Air
Receiver will be sized for 10 minutes hold up based on continuous instrument
air consumption and the instrument air header pressure to fall from 110 psig to
80 psig.
Hot oil Expansion Vessel will be
sized based on the following:
·
Expansion of the entire inventory of the hot oil
circuit from the minimum ambient to the high temperature alarm (TAH) set point.
·
At least one-fourth full by volume at cold
conditions and not more than three-fourth full by volume when operating.
·
Capable of providing at least two minutes of
retention time below the normal operating (hot) level.
·
Capable of providing minimum liquid head
requirements for the Hot Oil Circulation Pumps during start-up and normal
operating conditions.
·
Situated so that the cold level of the tank is
the highest point in the hot oil circuit to serve as the main venting point of
the system.
1.1.4 Pumps for Hydrocarbon & Non-Hydrocarbon Services
A 10% design margin shall be
applied in setting the motor capacity (design as rated flow) for a pump in
general service.
No margin shall be added to the
differential head.
The procedure for pump calculations
shall be:
·
Calculate the rated flow as design flowrate
·
Determine the size of the pump discharge piping
based on pump rated flowrate. The line pressure drop should be calculated for
the design flowrate.
·
Calculate the pump differential head based on
the line size determined. Calculate DP
at the pump rated flow.
·
Maximum size impellers should not be specified
for pumps. Maximum allowable impeller size should be specified as per API Std
610.
·
Calculate the NPSHA at rated flow. A 1 m margin
should be added to the calculated available NPSH for suction system design and
suction vessel elevation.
Where flow conditions necessitate turndown to less
than 30% of rated flow, minimum flow provisions shall be made. For centrifugal
pumps this shall consist of a line from the discharge to the suction source.
Minimum flow requirements shall be based on the pump vendors recommendations,
however in the absence of this data 30% of normal flow shall be used for bypass
line sizing. Pumps shall be provided with individual minimum flow recycle lines
back to the suction source.
1.1.5 Compressors for Hydrocarbon Service
No design margins shall be applied
to the flow-rate or head of compressors. The design flow-rate for compressor shall
is based on design production rate.
API Std 617 power margins shall be
applied.
1.1.6 Compressors for Air / Nitrogen Service
·
A design margin of 10% will be applied to the
flow-rate calculated for Air and Nitrogen service compressors.
·
No margin to the head is applied.
·
Compression ratio should be the same in each
stage of a multi stage unit.
1.1.7 Turbo-expanders
No
design margin shall be applied to the flow-rate or head of turbo-expanders,
since the design flow-rate is based on the design production rate. API power margins
shall be applied.
1.1.8 Gas Turbines
The
design margin over the driven equipment design output should not be less than
10%. Where the design margin for a standard unit is less than 10% consideration
should be given to future production rates.
1.1.9 Shell and Tube Exchangers
·
A 10% margin will be added to calculated heat
exchange surfaces for shell and tube heat exchangers.
·
No additional margin will be applied to
flow-rates, duties or pressure drop.
·
Minimum approach temperature recommended is 20°F.
1.1.10 Air Cooled Exchangers (Fin-Fan)
·
A 10% margin will be added to calculated air
cooled exchangers.
·
No additional margin will be applied to
flowrates, duties or pressure drop.
·
Minimum approach temperature recommended is 20°F.
·
Air coolers shall be designed for an ambient air
temperature of 90°F.
1.1.11 Plate-Fin Heat Exchangers (brazed aluminum)
·
No additional design margin will be added to
plate-fin heat exchangers.
·
No additional margin will be applied to
flowrates, duties or pressure drop.
·
Minimum approach temperature recommended is 3.5°F.
1.1.12 Plate and Frame Exchangers
·
No additional design margin will be added to
plate and frame exchangers.
·
No additional margin will be applied to
flowrates, duties or pressure drops.
·
Minimum approach temperature by vendor.
1.1.13 Fired Heaters
A design margin of 10% will be
applied to the heat duty of the fired heater.
1.1.14 Dehydration Package Unit
·
Design margin for package-provided equipment
units as discussed above shall apply, unless the vendor is able to justify the
added margin.
·
For molecular sieve vessels, the vendor will
recommend an additional volume margin for the adsorbent bed to prevent
saturation break-through.
1.1.15 Hot Oil Package Unit
·
Design margins for package provided equipment
units as discussed above shall apply, unless vendor is able to justify the
added margin.
1.1.16 Relief Valves
Relief valves shall be designed in
accordance with API RP 520 and API RP 521
The allowable accumulation for
pressure relieving devices shall be:
·
For general relief 10%
·
For fire case relief 21%
The maximum allowable pressure drop
in the inlet line to a relief valve shall be no greater than 3% of the relief
valve set pressure.
When multiple relief valves are
required to achieve the required relief area, the allowable accumulation for
general relief valves shall be 16% with the set pressure for the additional
valves set at 5% above the first valve set pressure.
1.1.17 Control Valves
No margin should be applied to the
rated flowrate or pressure drop for sizing control valves.
For preliminary estimates control
valve size, the following guidelines shall be used. (Instrument discipline is
responsible for final control valve sizing and selection).
At pump rated flow the control
valve pressure drop shall be the greater of:
·
10 psi
·
15 % of the variable system pressure drop at the
rated flow
At pump normal flow, the control
valve pressure drop shall be equal to or greater than:
·
15 psi
·
50% of the variable system pressure drop
(excluding the control valve). This is typically 30% of the total system
pressure drop.
·
5% of the destination pressure.
Control valves in vapour service
should normally be specified for a minimum pressure drop of 10 psi at design
flow unless otherwise dictated by specific process conditions.
Control valves in non-pumped liquid
service shall be specified with due regard to the system hydraulics under all
operating modes. In flashing services particular attention shall be given to
inlet line size with regard to minimizing flashing at the inlet to the control
valve, and to specifying the % liquid flash across the valve.
At maximum flowrate the calculated
Cv shall be 80% - 90% of the valve maximum Cv.
At the minimum process flowrate the
valve Cv should be greater than 10% of the maximum valve Cv subject to vendor
minimum flow requirements.
Acceptable methods for preliminary sizing of control valves
by the Process Discipline are:
·
Masoneilan
·
Willis, Masterflow or equivalent
for high pressure drop choke style control valves
·
Valtek
Bypasses around control
valve sets shall be used on critical services as shown on the P&ID's.
Compressor recycles control valves
shall be sized by the compressor vendor. As a guide, the valve Cv should be 2 -
2.4 times the Cv calculated to pass the flow rate delivered at the surge
control line with the compressor operating at rated speed.
1.2 Storage Tanks
·
Liquids subject to breathing losses may be
stored in tanks with floating or expansion roofs for conservation.
·
Freeboard is 15% below 500 gallons and 10% above
500 gallons.
·
Capacity of product tanks shall depend on the
connecting transportation schedule.
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