Tues 13.30 Misbehaving PID R Hughes

8/10/2019 Tues 13.30 Misbehaving PID R Hughes

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Compressor Seal Oil Control

Compressor Seal Oil system A case study

Richard Hughes


2/23

I have a short case study to present. With hindsight the solution is pretty obvious but Ithink its interesting as a demonstration that simple problem can be hidden by a complexsimulation.

A PID pressure controller on a seal oil system would periodically oscillate and the causewas not understood. It was not clear if it was the PID loop causing the problem or if itwas reacting to outside disturbances.

Life is really simple, but we insist on making it complicated

(Confucius)

The business schools reward difficult complex behaviour more than simplebehaviour, but simple behaviour is more effective.

(Warren Buffett)

Introduction


3/23

Compressor seal oil

dP


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Flare


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Compressor Seal/Control Oil system

Seal oil system looks very complicated

5 high pressure and 5 low pressure oil users

3 oil pumps ( 2 on line 1 spare ) ,

32 pipe sections

Two sets of filters and coolers

Many issues

Can not survive transition from two to one pumps [expensive!]

System would work for a few months then pressure swings would start

and the relief valves would feather open. Relief valves looked at , pumps looked at

Usual fix was to work on the control valve and its positioner

Had been a problem for > 20 years


7/23

History

Dynamic simulation carried out when plant owned by ICI ( 1983 )

Some system changes made as a result of this

Valve characteristics changed

Some recommendations to change pipe lengths

Some recent concerns about 250ms scan time on DCS PID controller

Recent question if we should have positioner or not

Re-visited in 2009 as many plant trips due to seal oil system

We will see later the dynamic model is very complicated but fails to model

the key features of the system.

Conclusions drawn from a highly accurate model that modelled a lot of un-necessary detail but did not capture what was important

The simulation gave no insight into the problem


8/23

Oil Capacity 1

PC

Oil Capacity 2

SEAL

DPC

210 psi

Header pressure usually held at 430 psi

( Relief valves prevent us running higher )

Five users but

limiting one is highpressure process

gas compressor .

(Casing at 210psi)

Trip


9/23

SIMPLE MODEL OF OIL COMPRESSIBILITY

dtVC

P ..1

B

VoC

P = pressure (psi)

V = volume flow into system ( m3/sec)

C = system capacitance

V0 = total volume of system (m3)

B = bulk modulus (psi) =

D

EBpipe

Bpipe

1

Boil

1

B

1d = wall thickness

D = pipe diameter

E = modulus of elasticity = 29500000 psi (cold rolled steel )

000,701

Cpsi/m3

1 m3 oil with B = 72,500 psi

50m of 6 pipe with 10mm wall.

Rough Guesses but not critical !


10/23

SIMPLE VALVE MODELS

sg

dPVKCVF ...

F = flow (m3/hr)

dP = pressure drop across valve (psi)

CV = valve CV

K = 0.227 , conversion for CV in US gallons per minute

Sg = 0.83 = specific gravity

V = fractional valve opening ( 0 to 1 )

Kickback valve CV is 34.2 ; other valve modelled as sum of 5 valve CVs .

Seal resistance modelled as valve and CV chosen to match plant oil flow


11/23

Validation looks good

plantdata

Simulation

Recovery timefrom pump trip

120 secs 140 secs

Minimumpressurereached

160 psi 160 psi

Valve

movement 33 % 38 %


12/23

But is it

430 PSI

250 PSI

160 PSI

Remo e inte polation


13/23

Remove interpolation

430 PSI

250 PSI

160 PSI

Pressure falls in 1 scan period of historian

Tells us nothing about the response time of the system except its < 10secs

Low pressure ( and valve travel ) function of halving flow

And valve CVs only

Recovery time

Function of controller tuning

But is it


14/23

In the simulation the PID tuning can be made much faster


15/23

So what have we failed to model

The header pressure PID loop parameters tell us a lot

Gain Kp = 0.04 % per %, Integral time Ti = 0.03 min


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If the actual plant tuning numbers matched ZN open loop methodthen

Ti = 3.3.Td = 0.03 mins which tells us our dead time isprobably of the order of 0.6s

Kp = 0.9/K * ( T/Td ) = 0.04 % / %

So low proportional gain tells us either process gain K is veryhigh or Td >> T

Back of an envelope shows the process gain is of the order of 2

Which implies a process time constant of 0.1 sec and a deadtime of about 0.6 s


17/23




In fact for dominant dead time processes they are usually tuned withmuch more integral and lower gain.

Typically Ti = Td/3 Kp = 0.18 / K

This would imply dead time of about 5 seconds and time constantof about 0.1 seconds.

This is pretty reasonable the hydraulic oil is virtually

incompressible and we can easily get to a few seconds or so of deadtime

Adding two seconds of dead time to the simulation was enough toprevent us raising the controller gain beyond that seen on the plant


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19/23

Simulation

Added a ramp rate limit to the simulation and a couple ofseconds of dead time

Tuning values from the plant now limiting values forsimulation

Some comments in the original simulation report

Transients are not accurately simulated but the behaviourof the system can be qualitatively predicted

It is not proposed to give controller settings here pastresults would suggest they do not pass well from model toplant with a great deal of success


20/23

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

dead time / time constant

%R

eductioninp

eakbycontroller

Quick simulation

To avoid dropping below the trip setting we would need to reduce

Dead time/time constant to about 1 i.e. dead time about 1/10 sec


21/23

Simulation is below the trip setting for 70 seconds. To reduce this to the trip

delay period of 10secs requires us to reduce our dead time/time constant to

About 2 .

Quick simulation

0

10

20

30

40

50

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70

80

90

100

0 2 4 6 8 10 12 14 16 18 20

dead time / time constant

timebelow

tripsetting

(%o

fratio=20)


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Tues 13.30 Misbehaving PID R Hughes