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N2 Unit - Cost Eliminatioby In-House Logic

DevelopmentA Presentation by Asadullah Qureshi/AQ (I&E ROP)

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Current Arrangement at the N2 Unit

•

Nitrogen extraction at the N2 Unit is carried out using Sieme200 family of PLCs – CPU 222 at the Old N2 Unit and CPU 22New N2 Unit.

• However, the PLCs come pre-programmed and the programprotected using a Level 4 Lock.

•A Level 4 Lock means the program cannot be accessed for emodification or back-up purposes.

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Current Arrangement at the N2 Unit

•

Since modification is not an option, the usefulness of the covery limited.

• The control algorithm cannot be refined or modified as peroperational requirement.

• A new program must be commissioned every time a modific

required.• The program costs almost the same as the PLC and its assoc

modules.

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The Proposal

•

The nitrogen extraction algorithm is fairly simple and the prbe developed in-house.

• This would allow us access to the program for further modifand eliminate the cost of having to commission the program

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The Proposal

•

The nitrogen extraction algorithm uses a total of ten digital open and close SOVs as per a programmed timing sequence

• The timing sequence may be replicated by recording how thstatus LEDs light up on the DO card as nitrogen extraction isout.

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Logic Development

•

The timing sequence was first studied by making a recordingDO card while nitrogen extraction continued in the backgro

• The timing sequence is shown in the table below:

Valve 57 sec 0.5 sec 0.5 sec 1 sec 0.5 sec 57 sec 0.5 sec 0.5 sec

SOV1 1 1 1 1

SOV2 1 1 1 1

SOV3 1 1 1 1 1

SOV4 1 1 1 1 1 1 1 1

SOV5 1 1 1 1 1 1 1 1 1

SOV6 1 1 1 1 1 1 1 1 1

SOV7 1 1 1

SOV8 1 1

SOV9

SOV10 1 1 1 1 1

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1

3

7

2

4

9 10

5 6

8

Inlet Air

Outlet Air

Exhaust

Tank A Tank B

Nitrogen Extraction via

Pressure Swing Adsorption

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Logic Development

•

A timer runs for a total 1 minute and 59 seconds.• It is used to regulate the entire process.

• It provides a reference point against which each SOV is turned on o

• It restarts the sequence after the 1 minute and 59 seconds mark.

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Logic Development

•

The algorithm treats each SOV separately.• Comparison is made with the timer’s count to come up with

for each SOV.

• This can be seen by using SOV0 as an example.

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Logic Development

•

The timing sequence only continues as long as the level switthe two CMS tanks are HIGH.

• This indicates that the CMS level in the tanks is appropriate.

• Each SOV either turns on once or it turns on twice in one giviteration of the sequence.

• The logic is presented in the next slide.

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Start Scan

Cycle

T >= x

AND

T = x AND

T = y AND

T

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Cost Benefit Analysis

•

The cost of the program factors into the overall cost of procspare PLCs for the N2 Unit.

• By programming the PLC in house, we get to save money.

• It also allows us to incorporate various interlocks into the looperational requirement.

PLC + Program Rs. 235000

PLC - Rs. 119000

Money Saved Rs. 116000

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Cost Benefit Analysis

•

We now have access to the program and we understand thais very simple from a control point of view.

• We can therefore even go as far as replacing Siemens’ S7-20with a cheaper option.

• Before we look at alternatives that may replace the S7-200,

to list out the variables or parameters that determine their operformance. This is discussed in the appendix.

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Appendix  – Room for Expansion

Parameter Siemens’ S7-200

CPU 222

Siemens’ S7-200

CPU 224

Fatek FBs-24MA Allen

Micro

1761

Program size 2048 words 4096 words 20K words 737 ins

437 da

Execution speed 0.37 μs 0.37 μs 0.33 μs

No. of on-board digital

outputs

6 10 10 12

No. of on-board digital

inputs

8 14 14 20

Input current 4 mA nominal 4 mA nominal 7.6 mA nominal 8 mA n

Output current 0.75 A 0.75 A 0.5 A 8 A ma

Communication

protocols supported

RS-485 (on-board),

Ethernet (expansion

module)

RS-485 (on-board),

Ethernet (expansion

module)

RS-232 (on-board), USB

(on-board) and Ethernet

(expansion module)

RS-232

Ethern

modul

Cost Over 100k Over 100k Between 20k to 80k Betwe