hpi antisurge_1

7/24/2019 HPI Antisurge_1

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Implementation No. 1

Automatic Anti-surge Controller

Because surge can occur very quickly, special control techniques must be used to ensure the

recycle valve opens in time to prevent surge.

The HPI controller is ideally suited to perform surge prevention due to the fast processing time

and the ability to efficiently perform the complex algorithms required. The HPI controller illalso provides interactive coupling beteen the surge and speed control algorithms to improve the

response and stability of the system.

!tandard functionality includes"

• #hoice of Pressure $atio or Head vs %lo.

• #ompensated %lo #alculation for !uction and discharge mounted %lo sensors.

• &utomatic !afety 'argin Increase on detection of surge.• %ast (pening and !lo #losing &uto #ontroller.

• !ynthetic #ontrol )ine for detection of sudden movement toards surge.

• !urge *etection logic, independent of auto control

• 'anual control

+(T" If the application requires it, any of the above features can be enabled. %eatures not

configured and have no effect.

Figure 1 !implified &nti !urge #ontroller Block *iagram



-herePd" *ischarge Pressure

Ps" !uction Pressures" !uction %lo !ensor

d" *ischarge %lo !ensor

Ts" !uction TemperatureTd" *ischarge Temperature&d" &nti/!urge 0alve *emand

System Input/ Output & Units

&s a minimum the system requires the folloing signal inputs

• !uction Pressure

• *ischarge Pressure or #ompressor Head

• %lo !ignal

In addition the system can provide temperature compensation if suction and discharge

temperature signals are provided.

The valve output can be configured for any units but is typically configured in 1 terms

The units for flo measure are customer configurable, and only impact the orifice constant.

The transmitters used to send information to the controller must be fast, repeatable and properlyranged. &bsolute accuracy is not as important since surge tests ill be conducted in the field

using actual instruments.

Surge Point Deriation

The system offers as standard the option to enter the surge line in terms of pressure ratio 0s floor #ompressor head 0s flo. The surge line is recorded into a 23 pint x/y schedule. The

controller determines the actual flo for any given condition by use of the schedule. The !urge

%lo 4s5 is output from this algorithm.



Figure ! surge point derivation

Surge Control "ine

The surge control line is derived by providing 6 means of ad7ustment, namely a percentage offset,

and a fixed flo offset. In addition to these configurable offset parameters, the system

automatically increases the surge control line by a factor knon as the safety margin offset,hich is a factor of the number of surges detected by the system.

Figure # surge point derivation

Flo$ %easurement

The system is designed to read the flo signal from a flo sensor and converts the signal to acompensated volumetric flo value. The system can be configured to calculate the flo value for

either suction or discharge mounted flo sensors. The standard algorithms employed are as

detailed belo"

• Suction Flo$

3.8hs Ts

9 :o ; <Ps

Q

• Discarge Flo$

hd . Pd . Tshs 9

Ps . Td

hd = *ischarge flo element *elta P

hs = !uction flo element *elta P

Pd = *ischarge Pressure Td = *ischarge Temperature



Ps = !uction Pressure

Ts = !uction Temperature

!ubstitute hs into equation for suction flo

Surge Detection

The surge detection algorithm is independent of the other controllers ithin the system, and upon

detection of a surge condition it ill step the valve demand to 2331 open force the valve to open.

In addition the controller ill set a surge detected bit hich can be used by the sequence

controller if required.

& surge is detected hen the error term beteen &ctual %lo and the !urge )ine flo becomes

less than the minimum alloable value 4min5. The system protects against the value of min being set to a minimum value. The surge condition is only cleared hen the flo exceeds the

value of the surge control line. The surge counter is increments, hich in turn increases the safety

margin offset.

& pseudo !urge )ine is also provided hich allos the user to configure the system to respond toa potential surge condition prior to reaching surge. This function ill cause the output of the

integrator to ramp open the valve hen the flo error term falls belo the pseudo surge lineoffset value, thus improving the responsiveness of the system and reducing the impact on the

system from a process standpoint. -hen the pseudo surge condition is cleared the system ill

close back in at the standard rate limited value ithin the valve controller.



Figure ' surge detection

Sa(ety %argin )ecali*ration

If the system detects a transition of the operating point across the !urge )ine, indicating thatsurge has occurred, it automatically read7usts the !urge #ontrol )ine to the right to add additional

safety margin.

ach time a surge transition is detected, the safety margin is incremented 4control line moved to

the right5 by a calibrated amount. ntering a ne safety margin sets the transition counter to >ero,

and sets the recalibrated margin to equal the entered value.

The system can be configured to increment by a fixed amount, or by a progressive amount 42, 6,

?, @ 1, etc.5, on each transition. The maximum number of times that recalibration occurs is also

configurable.

A+aptie ,ain an+ Non-Symmetrical )esponse

The !urge #ontroller is provided ith an adaptive gain feature. This reduces the proportional

action hen operation is to the right of the !urge #ontrol )ine. -hen the operating margin

exceeds a configured distance to the right of the !urge #ontrol )ine, the adaptive gain is invoked.

The recycle valve opens, based on the proportional and integral responses, but a straight ramp

function limits the closing of the recycle valve. This feature allos the valve to respond quickly

to prevent surge. &fter operation is safely to the right of the surge control line, the valve is



ramped closed at a slo 4configurable5 rate to allo the turbine driver and the performance

controller to ad7ust to the ne operating conditions.

To compensate for the potential impact upon the capacity 4flo5 of the system a unique algorithm

is provided that raises speed as a function of the rate of opening of the recycle valve. &fter the

recycle valve stabili>es or starts to close, speed is then loered at a fixed, slo ramp rate. Thisaction has the effect of suspending capacity control hile recycle control is being established,

then gently restoring capacity control to give the speed control and !urge #ontroller time to

ad7ust.

Anticipation Control

In many applications, the compressor ill not operate continuously, or for extended periods onthe surge control line. -hen operation is to the right of the control line 4safe area5 the system ill

not respond to a movement toards surge until the operating point crosses the control line. This

could result in a large disturbance to the process. To prevent this condition occurring, the system

is design to provide a synthetic surge control line.

The synthetic control line is ramped at a configurable rate from the control line to ithin aconfigurable percentage of the operating point. In the event the operating point should move to

the left of the synthetic control line, then the anticipation control ill become active and open the

valve until the operating point returns to the right of the synthetic control line.

The user can configure the rate at hich the synthetic line is alloed to move toards the surge

line, the position of the synthetic line is calculated dependent upon this factor.

Figure synthetic surge control line



%anual Anti-Surge ale Control

The manual control function is provided to allo the user to manually open or close the valve

either from the sequence controller 4compressor purge5 or hen running in a manual condition.

The automatic controller and surge detection algorithms remain active at all times, and prevent

the user from operating the compressor in an unsafe condition.

The user can provide a manual setpoint to the controller if it is required that the valve be

maintained at a fixed position.

Figure manual control

ale "ineari0ation an+ )eersal

The surge #ontroller provides for lineari>ing the output for an equal percentage trim recycle

valve. This tends to produce a more linear overall system gain. Possible instability is avoidedhen the system operates at a different point than at hich it as tuned.

'ost applications have a recycle valve ith an air/fail open action 4&%(5, requiring reversal ofthe recycle valve output. The system is configurable for either direct or reverse output operation.

Dum* Output

-hen a specified excursion to the left of the !urge #ontrol )ine occurs, a contact output can beconfigured to open a high/volume solenoid to quickly open the recycle valve.

This feature is useful on large valves hich have sloer stroking times. &s the system moves

back from surge, the solenoid ill close. This restores normal proportional control of the valve

the !urge #ontroller.

igest 2ins Selector



The output of the individual controllers are fed into a highest ins gate 42331 9 valve fully

open5. The inning signal is fed through to a close rate limiter. The output of the rate limiter being controlled by 6 configurable close rates. The rate limited value is then inverted 431 9 fully

open5 and fed to the valve.

The rate limited output is fed back to the individual controllers to reset the integrators.

Figure 3 highest ins and valve control logic

Capacity Control

#ontrol &lgorithms

The capacity controller is designed ith a number of possible operating situations in mind. The

controller therefore has control loops for the folloing" /

• !tation %lo #ontrol

• !tation *ischarge Pressure #ontrol

• !tation !uction Pressure #ontrol

• !tation Inter/!tage Pressure #ontrol

The flo, discharge pressure, suction pressure and inter/stage pressure controllers require the

(perator to input the value the controllers are to maintain. The station controller then converts

this to a speed control set point.



%igure @ loest ins gate

The various P2*s feed into a loest ins gate, as shon above. This provides the facility for

override controllers 4e.g. discharge pressure5 if required.

Pressure Setpoint Control

The !tation #ontroller requires a user defined pressure setpoint as one of its inputs. This setpointis the desired pressure that is to be maintained by the turbo/compressor sets in service. The user

can configure the system for station suction, discharge or inter/stage pressure setpoint control.

The !tation #ontroller softare determines at hat speed the turbo/compressors should be run toachieve this pressure.

Pressure Oerri+e Control

It is possible that under adverse, unexpected or transient operating conditions that the !tation

pressures may reach unacceptable limits i.e. the suction may drop too lo or the discharge may

become too high. *uring normal operation the !tation #ontroller ill compensate for gentle orminor fluctuations in conditions by regulating the speed of the turbo/compressors to maintain the

desired station suction pressure.

Hoever, speed changes alone may not be sufficient and it is under these conditions that the

!tation #ontroller ill activate the Pressure (verride #ontroller output. This can in turn be used

to bias the speed demand signal or control a station re/cycle valve depending on the needs of theapplication etc..



Decoupling

In addition, decoupling and feed forard terms are utili>ed to reduce interaction beteencompressor anti/surge controllers and the turbine speed demand.

The purpose of the decoupling term is to reduce the gain of the !tation Pressure !et Point#ontroller PI* hen surge occurs. This has the consequential effect that hen normal station

controller PI* control is resumed, this PI* ill be far from its set point and ith a small gain it

ill take a long time to recover. Therefore, to aid the PI* recovery process a feed forard term isused.

Fee+ For$ar+ 4erm

/The purpose of the feed forard term is to further de/couple the interaction beteen gas turbine

speed governing and closed loop operation of the anti/surge valve. This is achieved by nullifying

the effect that the opening of the anti/surge valve has on turbine speed ithout recourse to the

!tation controller setpoint.

The feed forard function supplements 4if required5 the !tation #ontroller PI* by predicting thefinal PI* output and Aboosting it to close to this value. This feed forard boosting may be

required because hen the &!0 opens the gain of the !# PI* is de coupled making it very slo

to react to changes in the station pressures and, hence, possibly leading to instability.

Decoupling 4erm

The *ecoupling term is the main method of decoupling the interaction beteen gas turbine speedgoverning and closed loop operation of the anti/surge valve. This is achieved by reducing the

overall gain of the station control PI* function hen the anti surge valve is open under automatic

control.

hpi antisurge_1

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