08_ColumnPressureRelief.pdf

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Column Pressure Relief 1

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Column Pressure Relief

2004Aspen Technology - All Rights Reserved

Instructor:

Eng. Ahmed Deyab Fares

Mobile: 0127549943


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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 tostart opening when the Top stage pressure reaches 2205 kPa (320 psia)

and fully open at 2275 kPa (330 psia).

Learning Objectives

After completing this section, you will be able to install a relief valve.


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PartialProc

essOverview


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Building the SimulationThe Relief Valveoperation 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 ReliefValveoff the DePropanizer Column

Overhead.

1. Open the case from Module 7.

2. Add a Relief Valveand enter the following information:

3. On the Dynamics tab, Specspage of stream ToFlare, activate thePressure Specification. The pressure of this stream should beatmospheric.

4. The ReliefValverequires a value for the Orifice Areato initialize. Goto the Sizingpage of the Relief Valve Ratingstab and enter 1300mm2(2 in2).

Connections

Name RV-100 (default)

Inlet Bypass to Valve

Outlet To Flare

Parameters

Set Pressure 2205 kPa (320 psia)

Full Open Pressure 2275 kPa (330 psia)

Make sure that yourPressure FlowSpecifications are correct.


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Check the Dynamic Assistantto ensure that all necessary information issupplied.

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 theAccumulator-PCchange the mode to Manand set the OP to 0%. This immediately closes the valve.

2. Make sure that the Accumulator Pressure is shown on the activestrip chart. You will want to monitor this variable as the simulationprogresses.

3. Watch the Accumulator Pressure rise sharply and the relief valveopen. The relief valve is shown as open on the PFD when a yellowoutline appears around it.

4. Return theAccumulator-PCtoAutoand set the SP to 2000 kPa (290psia).

Is the Relief Valve big enough to maintain the column pressure at the setrelief pressure? Is it able to prevent the pressure from rising above the fullopen pressure? ______________________________________________________

How long does it take for the system to run to its original operatingcondition? __________________________________________________________

Save your case!

Save your case!


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Notes on the Relief Valve OperationOnce the Valve has lifted, examine the Dynamics - Specstab 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

Parametersgroup box becomes visible. This group contains two fields:

Delta Pressure drop across the valve

Valve Lift This value is calculated using one of thetwo following formulas:

If inlet pressure is increasing:

where: P 1 = upstream pressure

POPEN= Set Pressure

PFULL= Full Open

Pressure

If inlet pressure is decreasing:

where: P 1 = upstream pressure

PRESEAT= Resetting

Pressure

PCLOSE = Closing Pressure

Percentage Open The Valve Li ft in percentage.

Closing Pressure Pressure at which the valve begins to close afterreaching the full lift pressure (i.e. the value entered inthe full pressure cell on theParameters page of the

Design tab).

Reseating Pressure The pressure at which the valve reseatsafterdischarge.

LP

1POPEN

PFU LL POPEN---------------------------------------=

LP

1PRE SE AT

PCLOSE POPEN------------------------------------------=


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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 NOTbe Choked:

where: P 1= 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)

P2

P1

------ 2

K 1+-------------

K

K 1-------------

W AKL

KD

KB

P1K

V1

----------- 2

K 1+-------------

K 1+

K 1-------------

1

2---

=


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For non-Choked vapour flow, the mass flowrate is given by:

Liquid Flow In Valve

Liquid Flowthrough the valve is calculated using the followingequation:

where: 1 = Density of upstream fluid

KV= Viscosity correction factor

Capacity Correction Factor (KL)

The Capacity Correction Factorfor 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 AKL

KD

P1

V1

------ 2K

K 1-------------

P

2

P1

------

2

K----

P2

P1

------

K 1+

K-------------

1

2---

=

W AKL

KD

KV

2 P1

P2

( )1

[ ]

1

2---

=

K

L

L2

a 1 a( )L4+[ ]1 2

---------------------------------------------------=


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Linear and decreasing-sensitivity valves have the following flowcharacteristics:

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)

KL

L2

a 1 a( )L2

+[ ]1 2

---------------------------------------------------=

avalve head differential a maximum flow

valve head differential at zero flow

-----------------------------------------------------------------------------------------------=


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08_ColumnPressureRelief.pdf