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Column Pressure Relief 1
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Column Pressure Relief
2004 Aspen Technology - All Rights Reserved
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2 Column Pressure Relief
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WorkshopThis model illustrates the application of the pressure
relief unit operation on a column.
Under normal operating conditions the overhead from the
DePropanizer column is condensed in an Air Cooler, collected in an
Accumulator and pumped back to the column (full reflux). The normal
operating pressure on the Column Top tray and the Accumulator are
2000 kPa (290 psia) and 1900 kPa (275 psia) respectively.
A Pressure Relief Valve for the Column will be added and
configured to start opening when the Top stage pressure reaches
2205 kPa (320 psia) and fully open at 2275 kPa (330 psia).
Learning ObjectivesAfter completing this section, you will be
able to install a relief valve.
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Partia
l Proc
ess O
vervi
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4 Column Pressure Relief
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Building the SimulationThe Relief Valve operation is used in
many situations in which there has been excess pressure build up.
Although it is available in Steady State mode, its purpose is to
avert situations that occur in a dynamic environment.
This module continues with the case that was constructed in the
previous module.
Adding the Relief ValveIn this example, install a Relief Valve
off the DePropanizer Column Overhead.
1. Open the case from Module 7.
2. Add a Relief Valve and enter the following information:
3. On the Dynamics tab, Specs page of stream ToFlare, activate
the Pressure Specification. The pressure of this stream should be
atmospheric.
4. The Relief Valve requires a value for the Orifice Area to
initialize. Go to the Sizing page of the Relief Valve Ratings tab
and enter 1300 mm2 (2 in2).
ConnectionsName RV-100 (default)Inlet Bypass to ValveOutlet To
FlareParameters
Set Pressure 2205 kPa (320 psia)Full Open Pressure 2275 kPa (330
psia)
Make sure that your Pressure Flow Specifications are
correct.
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Column Pressure Relief 5
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Check the Dynamic Assistant to ensure that all necessary
information is supplied.
Save the case as PreRelief.hsc.
The easiest way to create an overpressure situation is to close
the Propane Valve. This will cause a build up of propane in the
system and the pressure will rise.
1. On the FacePlate for the Accumulator-PC change the mode to
Man and set the OP to 0%. This immediately closes the valve.
2. Make sure that the Accumulator Pressure is shown on the
active strip chart. You will want to monitor this variable as the
simulation progresses.
3. Watch the Accumulator Pressure rise sharply and the relief
valve open. The relief valve is shown as open on the PFD when a
yellow outline appears around it.
4. Return the Accumulator-PC to Auto and set the SP to 2000 kPa
(290 psia).
Is the Relief Valve big enough to maintain the column pressure
at the set relief pressure? Is it able to prevent the pressure from
rising above the full open pressure?
______________________________________________________
How long does it take for the system to run to its original
operating condition?
__________________________________________________________
Save your case!
Save your case!
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6 Column Pressure Relief
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Notes on the Relief Valve Operation
Once the Valve has lifted, examine the Dynamics - Specs tab of
the Relief Valve. On this page you will observe three
parameters:
Hysterysis Parameters Group
When the Enable Valve Hysterysis check box is activated, the
Hysterysis Parameters group box becomes visible. This group
contains two fields:
Delta Pressure drop across the valveValve Lift This value is
calculated using one of the
two following formulas:If inlet pressure is increasing:
where: P1 = upstream pressure
POPEN = Set Pressure
PFULL= Full Open Pressure
If inlet pressure is decreasing:
where: P1 = upstream pressure
PRESEAT = Resetting Pressure
PCLOSE = Closing Pressure
Percentage Open The Valve Lift in percentage.
Closing Pressure Pressure at which the valve begins to close
after reaching the full lift pressure (i.e. the value entered in
the full pressure cell on the Parameters page of the Design
tab).
Reseating Pressure The pressure at which the valve reseats after
discharge.
LP1 POPEN
PFULL POPEN---------------------------------------=
LP1 PRESEAT
PCLOSE POPEN------------------------------------------=
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Column Pressure Relief 7
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Flow Through the Relief ValveThe mass flowrate through the
relief valve varies depending on the vapour fraction and the
pressure ratio across the valve. For two phase flow, the flows are
proportional to the vapour fraction and can be calculated
separately and then combined for the total flow.
Vapour Flow In Valve
For gases and vapours, flow may be choked or non-choked. If the
pressure ratio is greater than the critical, the flow will NOT be
Choked:
where: P1 = Upstream Pressure
P2 = Downstream Pressure
K = Ratio of Specific Heats
For Choked vapour flow, the mass flowrate is given by the
following relationship:
where: W= Mass flow rate
A = Relief valve orifice area
KL = Capacity correction factor for valve lift
KD = Coefficient of Discharge
KB = Back Pressure Coefficient
V1 = Specific Volume of the upstream fluid
(1)
(2)
P2P1------
2K 1+-------------
KK 1-------------
W AKLKDKBP1K
V1-----------
2K 1+-------------
K 1+K 1-------------
12---
=
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For non-Choked vapour flow, the mass flowrate is given by:
Liquid Flow In Valve
Liquid Flow through the valve is calculated using the following
equation:
where: 1 = Density of upstream fluid
KV = Viscosity correction factor
Capacity Correction Factor (KL)
The Capacity Correction Factor for back pressure is typically
linear with increasing backpressure. The correct value of the
factor should be user-supplied. It may be obtained from the valve
manufacturer. The capacity correction factor for valve lift
compensates for the conditions when the relief valve is not
completely open. Increasing-sensitivity valves have the following
flow characteristics:
(3)
(4)
(5)
W AKLKDP1V1------
2KK 1-------------
P2P1------
2K----
P2P1------
K 1+
K-------------
12---
=
W AKLKDKV 2 P1 P2( )1[ ]12---
=
KLL2
a 1 a( )L4+[ ]1 2
---------------------------------------------------=
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Linear and decreasing-sensitivity valves have the following flow
characteristics:
where:
The valve head differential term allows for customizing of the
flow characteristics with respect to stem travel. Its value can
range between 0 and 1.
(6)
(7)
KLL2
a 1 a( )L2+[ ]1 2
---------------------------------------------------=
a valve head differential a maximum flowvalve head differential
at zero flow
-----------------------------------------------------------------------------------------------=
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10 Column Pressure Relief
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