plant control - iea · 2009-04-10 · control of biological wwt 2002-03 gustaf olsson, iea, lund...

Plant Control

Gustaf OlssonLund University,

SwedenGustaf.Olsson@iea.lth.se

Control of Biological WWT 2002-03

Gustaf Olsson, IEA, Lund University

Content

• Nitrate recirculation control

• Carbon dosage control• Chemical precipitation control

• Phase length control• Plant wide control

• Sampling – measurements• Control computers

• Plant information systems

Nitrate recirculation control

Pre-denitrification plant

Aerobic reactor

Sludge outtakeSludge recirculation

Influent

Internal recirculation

Effluent

Anoxic reactor

Nitrate recirculation control

��

Flow rate (lines 3 and 4)

Effluent total nitrogen

��

Total N concentration (lines 3 and 4

Comparison of ratio and NO3setpoint control

Recirculation rate

Total N

The Nitrifier concentration as function of DO and NO3

Carbon dosage control

Influence of C addition

Carbon Dosage Control

Chemical Precipitation Control

Chemical dosage based on hydraulic load

Effluent PO4-P conc.

Ingildsen

Chemical dosage based on P load

Ingildsen

Chemical dosage based on P sensor feedback

Ingildsen

Detailed Look into the Control

Standard deviation is typically 0.03 mg/l PO4-P

Chemicals Consumption

Strategy Average dosing

(l/min)

Relative consump. (compared to in situ

controller)

Insitu 0.42 100%

Constant 0.66 167%

Flow proportional 0.62 156%

Load proportional 0.54 136%

Källby WWTP: 130 % of the insitu control was used in parallel line using load proportional control

(in compliance during 90 % of time)

Phase length control

One day in an alternating plant (Avedore, Denmark)

Content

Plant Wide Control

• Prepare the plant for large disturbances• Coordinate sludge treatment and WW

treatment• How to return to “normal” operation?• Implementations are gaining momentum

but at a low speed

Uncertainty at the Plant Level

• External disturbances– storms, concentration variations

• Compromise sewer capacity -plant capacity

• New regulations

Plant Wide Control (1)

Key interactions:

• hydraulics• use of resources• recycles within the plant• the operator

Plant Wide Control (2)

• System definition– control handles

– setpoints of concentrations (C, N, P)– disturbances

• Degrees of freedom• Constraints• Objective function

Ex: Plant wide BNR control

• Highly variable load

• Automatic adjustments of setpoints using N and P sensors

• Phase control

• DO setpoint control• Pure oxygen

addition• Chemical dosage• Return sludge flow

Ex: Stormwater Control

Required settler volume:

Vclarifier =

= k * Q * SSto clar* SVI

Aeration tank settling (Krüger A/S)

During high loads:increase the settler volume

Operational Aspects (1)

• Almost universal acceptance of ICA –still a great amount of opportunity to further apply ICA

• Surveys show approximately 50% of control loops are currently run in manual mode

Operational Aspects (2)

• On-line sensors no longer the main limitation for on-line control

• The lack of process flexibility is more troublesome and limiting

• “Software sensors” can be used to further enhance the benefits of ICA

Conclusions

• We have to assume that all the local controllers are working

• Good DO control is crucial for the rest!• Use NH4 sensor to determine DOref

• Nitrate recirculation vs. DO setpoint• Plant wide control: take couplings into

consideration!

Content

Sampling

Sampling (1)

0 1 2 3 4 5 6 7 8 9 10 110

Sampling - alias (2)

0 1 2 3 4 5 6 7 8

Sampling - alias (3)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

Sampling -alias (4)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

Loop sampling interval (1)

• In commercial process control software: 1 – 10 times per second

• Using larger sampling intervals – reduces the load on the computer– reduces wear and tear on the actuator

Loop sampling interval (2)

• Sampling rate 6-10 times the bandwidth• Sample 6-10 times within the rise time

of the step response• Take notice if the instrument analysis

time is long

Measurements

Signal transmissionMeasurement

principle

Instru-ment

SampleTrans-mission

Scree-ning

Sto-rage

Valuedisplayed

App-lica-tion

Sen-sor

Trans-mission

Scree-ning

Sto-rage

Visualisation

• What you measure is considered important => Work will be focused to improve what is measured

• To measure is to know• You can control what you measure• Without information you can not take

responsibility • With information you will take the

responsibility

1. Know the demand2. Measurement principle3. Transmission4. Validation5. Maintenance 6. The application7. Self diagnosis

Measurement Quality

Common errors

• The sensor– Is the measurement

representative?– Is the sensor

mounted properly?– The sensor is never

better than the measurement principle

• Presentation– Do not show

unreasonable accuracy (DO conc = 2,54675 mg/l)

– Show the inaccuracy (DO conc = 2,5 +/-0,1 mg/l)

Content

Control computers

The State Concept

Sequencing circuits

– one state at a time– state transfer

– conditions for each state transfer

Grafcet 11

Close outlet valve

Pump on

Tank empty

Pump off

Heat on

Heat off

Wait_time = tim1

Open outlet valve

Empty ∗ start " True if the lower level sensor Empty = 1 and when a start order is given

" Starts the filling

Full " The level has reached the upper limit

" Stop filling and start heating

Temp " The desired temperature is reached

" Define waiting time = tim 1

Wait_time " The tim1 has expired

Empty " Tank is empty

Grafcet 2• Developed in France 1977• Graf + AFCET (Association Francaise

pour la Cybernetique Economique et Technique)

• French standard 1982• IEC standard 1988 - IEC 848

(=SFC, Sequential Function Chart)• Essential part of IEC 61131-3

PLC programming

New standard 61131-3 implies:• definition of sequential function chart

(SFC), and 4 language options– instruction list– function block– ladder diagram

– structured text

PLC open

• PLC open is an organisation to support the standard IEC 61131-3

• Manufacturer and product independent• Members of the PLC open group are

expected to deliver products that follow the 61131-3 standard

Plant information systems

The Operator

• Makes critical decisions in real time

• Well educated

•Demands improvement

Use the human resources to improve the process

OEE - Overall Equipment Efficiency

• Availability• Plant efficiency• Quality

Process-industri

Plastindustrin Skärandebearbetning

Elektronik,övriga

Elelektronik-industrin

Plåtindustrin Elektronik,ytmontage

OEE in some industries

Functional View

Supervisory Level

• Time minutes to hours• Plant-Wide Control:

– model-based control– optimisation

– scheduling– decision support systems

Management Level

Time - hours to days• Information Systems:

– production reporting– financial reporting

– production planning– decision support systems

Reporting

• Key to successful management• Format reports CAREFULLY:

– apply KISS (Keep It Short & Simple) principle

– keep a high signal-to-noise ratio

– tailor reports by reader, time and place– organise information hierarchically (easy

lookup for custom reporting)

Quality Information System

Quality information

• Confidence limits for measurements• Check that a variable is not constant too

long• Compare measurements at the same

time• Compare measurements with models• Alarm for deviations from expected

Decision Support

• Supervisory:– operational advice (encapsulating process

knowledge)– problem detection & diagnosis

• Management:– production planning constraints

(encapsulating process knowledge)

Implementation

Implementation Issues

• Accessibility - data flows and rights of access

• Reliability - architecture, redundancy, time to repair

• Flexibility - “standard” & common open architecture

• Equipment Constraints - avoid proprietary systems

• Cost - yes always, but compare to the benefits

Summary

• Many different time scales – sampling• Measurement quality• Who needs the information?• Quality in information• Information for decision support

plant control - iea · 2009-04-10 · control of biological wwt 2002-03 gustaf olsson, iea, lund...

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