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Control and Safeguarding Philosophy

National Gas Company of Trinidad and Tobago Waukesha 16V275GL

Ariel JGD/6-1 Reciprocating Compressor Package

Unit #: E002453-01/02/03

Enerflex Document #: G01-E002453-01-001

Control and Safeguarding Philosophy National Gas Company of Trinidad and Tobago

Project #: E002453-01/02/03 Customer / Description: National Gas Company of Trinidad and Tobago, JGD/6-1, Reciprocating Compressor Created by: Yelena Snider Date: August 24, 2017 Revision #: A of 19

Table of Contents

1 Alarm and Shutdown Conditions ................................................................ 4

Alarms ..................................................................................................................... 4

Shutdowns .............................................................................................................. 4

Transmitter Failure and Controller Fail Position ........................................................ 5

Shutdown Bypass..................................................................................................... 5

2 Unit Startup and Shutdown Sequences ...................................................... 6

Start Sequence ......................................................................................................... 6

Common Shutdown Sequence ............................................................................... 13

Emergency Shutdown (ESD) Sequence ................................................................... 13

3 Analog Equipment Control (PID) .............................................................. 14

Controller Modes ................................................................................................... 14

Compressor Capacity Control ................................................................................. 14

4 Discrete Equipment Control ..................................................................... 17

Automatic Blowdown Valves Control Modes .......................................................... 17

Suction scrubber drain ESD valve solenoids SDY-2016A (A/B) ................................. 17

Discharge ESD valve solenoids SDY-2014A (A/B) ..................................................... 17

Inlet fuel ESD valve solenoids SDY-2015A (A/B) ...................................................... 17

Startup bypass valve solenoids SDY-2020A (A/B) .................................................... 17

Compressor pre/post-lube oil pump solenoid UY-5104A ......................................... 18

Process cooler fan motors (EM-2101A/2102A/2201A/2202A) ................................. 18

5 Signals to Plant ........................................................................................ 18

Compressor run status ........................................................................................... 18

Compressor common shutdown alarm ................................................................... 18

Compressor common trouble alarm ....................................................................... 18

6 PID Tuning ............................................................................................... 19




List of Tables

Table 1: Shutdown Action Classes ................................................................................................................................. 4

Table 2: Permissives Required to Move to Step 1 – Ready to Start .............................................................................. 6

Table 3: Controller States While Evaluating Active Shutdowns .................................................................................... 6

Table 4: Discrete Output States while Evaluating Active Shutdowns ............................................................................ 6

Table 5: Permissives required to move to Step 2 – Skid Pressure Adjustment ............................................................. 7

Table 6: Discrete Outputs Energized/De-Energized in Ready to Start step ................................................................... 7

Table 7: Permissives Required to Move to Step 3 – Pre-Lube ....................................................................................... 8

Table 8: Discrete Outputs Energized/De-Energized in Step 2 – Skid Pressure Adjustment .......................................... 8

Table 9: Permissives Required to Move to Step 4 – Start Driver ................................................................................... 9

Table 10: Discrete Outputs Energized/De-Energized in Step 3 – Pre-lube .................................................................... 9

Table 11: Permissives Required to Move to Step 5 – Warm-up .................................................................................... 9

Table 12: Discrete Outputs Energized/De-Energized in Step 4 – Start Driver ............................................................... 9

Table 13: Permissives Required to Move to Step 6 – Online ....................................................................................... 10

Table 14: Discrete Output States while in Step 5 – Warm-up ..................................................................................... 10

Table 15: Events which will cause the package to go offline ....................................................................................... 10

Table 16: Controllers Enabled/Disabled in Step 6 – Compressor Online ..................................................................... 10

Table 17: Discrete Outputs Energized/De-Energized in Step 6 – Compressor Online ................................................. 11

Table 18: Events Which Will Initiate a Cool Down ....................................................................................................... 11

Table 19: Permissives Required to Move to Post-Lube Step ....................................................................................... 11

Table 20: Controllers Enabled/Disabled in Step 7 – Cool Down .................................................................................. 11

Table 21: Discrete Outputs Energized/De-Energized in Step 7 – Post Lube ................................................................ 12

Table 22: Events Which Will Initiate a Post Lube ........................................................................................................ 12

Table 23: Permissives Required to Complete Post-Lube Step ..................................................................................... 12

Table 24: Discrete Outputs Energized/De-Energized in Step 8 – Post Lube ................................................................ 12

Table 25: Capacity Control - Suction / Discharge Pressure PID Controllers................................................................. 16

Table 26: Auto Blowdown Valve - Fail States .............................................................................................................. 17

Table 25: Auto Blow Down Valves Control Modes ...................................................................................................... 17

List of Figures Figure 1: Compressor Capacity Control ....................................................................................................................... 15




1 Alarm and Shutdown Conditions A key function of the control system is to protect the compressor, engine, and other equipment by stopping them if conditions are outside safe operating limits. Discrete switches, analog values from transmitters, or analog values calculated in the PLC can initiate an alarm or shutdown.

When a shutdown or alarm occurs, the cause of the condition is displayed on the HMI. Any active alarms or shutdowns are displayed on the active alarms annunciation display. The alarm or shutdown will disappear if the condition is cleared and the reset button on the local HMI is pushed. An alarm history page is also provided; it lists all alarms and shutdowns that have occurred.

Alarms

Alarms are events that are not severe enough to cause a shutdown; however, the information may be of interest to an operator so the triggered alarm is displayed on the HMI. Input values are compared against engineering limits to determine if an alarm condition is present.

Shutdowns Shutdown conditions are determined by monitoring analog and discrete input values to determine if a shutdown condition is present. Some or all the equipment will be disabled on the package depending on the severity of the shutdown condition.

The control panel is equipped with an emergency shutdown (ESD) push button. When pressed, the ESD button will disable all equipment on the package and force the equipment to its fail-safe state. All digital outputs will be de-energized via a hardwired disconnect of power to the digital output module. All analogs outputs will be forced to 4 mA via the controller logic to reach their fail state.

The cause and effect relationship for each shutdown is listed in the control system Shutdown Key documentation.

Shutdowns are classified into three classes as follows:

Table 1: Shutdown Action Classes

Class Description

Class A Shutdown must be clear prior to startup of the unit. There is no start-up bypass time delay associated with this shutdown.

Class B/x

Shutdown will be bypassed for x seconds following start-up. The compressor can start without the B shutdown clear or healthy. The compressor will shut down if any B shutdown is active after the time delay. This allows time for operating conditions to stabilize before arming the shutdown. Note: Individual equipment failure to run shutdowns are considered to be class B shutdowns; however, the time delay is generally 10 seconds from the start command for the affected equipment.




Transmitter Failure and Controller Fail Position All transmitter errors such as loss of wire or out of range are to be treated with the highest-level action

associated with the device as follows:

Shutdown transmitter - The above error would cause a trip and drive the input into a shutdown state

Control or alarm transmitter - An alarm would be indicated. In general, a PID loop controller will hold

the last output value upon transmitter or PV signal failure.

Shutdown Bypass The control system is equipped with screens, which allow the operator to put analog and digital input devices

into bypass mode. This allows for maintenance or testing on devices without triggering related shut downs in

the package. Only one bypass at a time is allowed at a time. Critical items like ESD, valve and motor status

failures, oil circuit protection cannot be bypassed. Once a device is put in bypass mode a timer (default 5

minutes) will begin counting down. An alarm will flag as soon as a bypass is enabled to notify and record that a

bypass was used. When there is one-minute left on the bypass timer a warning will appear, which indicates that

the bypass is about to expire. Once the timer expires all bypasses will be reset. If necessary to extend the timer

a reset button can be pressed which will restart the bypass timer. Login credentials are required to use the

shutdown bypass feature. The unit cannot be started while bypasses are active.

Note: All tags in this document are based on E002453-01 unit.




2 Unit Startup and Shutdown Sequences

Start Sequence The compressor control system has been designed to provide an auto start sequence that must be initiated by

the operator once all pre-start checks and startup permissives have been validated. The “Startup” screen in the

HMI indicates the current start sequence step dynamically.

At any point in time during the start sequence, the operator can set the auto loading of the compressor to “on”

or “off”. If auto loading is turned on the capacity control output limit will be gradually ramped to 100% and

automated PID loops will be enabled. If auto loading is turned off, the capacity control output limit will be

ramped to 0% and the automated PID control loops will be disabled.

Note: The compressor will not begin to load until warm-up step is complete.

Below is the sequence of events for the startup of the compressor and engine:

Step 0 – Evaluate Active Shutdowns

The start sequence will remain in Step 0 until all active shutdowns are cleared.

Table 2: Permissives Required to Move to Step 1 – Ready to Start

Permissives

All active shutdowns must be cleared. For this to occur all analog instruments must be within operating setpoints, all digital shutdowns must be satisfied, and reset button must be pressed.

Compressor oil temperature must be above 0 °C before proceeding

All shutdown bypasses must be clear

Table 3: Controller States While Evaluating Active Shutdowns Controller Enabled /

Disabled Notes

PIC-2002A Master Suction Pressure Control PID

Disabled Set to Software Manual with 0% output

PIC-1301A Master Discharge Pressure Control PID

Disabled Set to Software Manual with 0% output

Table 4: Discrete Output States while Evaluating Active Shutdowns

Output Energized / De-Energized

Notes

BDY-2017A Blowdown Valve Solenoids

Energized (Typically)

Refer to Section Automatic Blowdown Valves Control Modes for full description of blowdown operation.

SDY-2016A Suction Scrubber Drain ESD Valve Solenoids


These solenoids are de-activated only during ESD. See section 4.

SDY-2014A Discharge ESD Valve Solenoids

De-Energized

N/A




SDY-2015A Inlet Fuel ESD Valve Solenoids


These solenoids are de-activated only during ESD. See section 4.

SDY-2020A Startup Bypass Valve Solenoids

De-Energized

N/A

UY-5104A Compressor Pre/Post-Lube Pump Solenoid

De-Energized N/A

YS-2101A Process Cooler Motor EM-2101A Run/Stop Signal

De-Energized N/A


De-Energized N/A


De-Energized N/A


De-Energized N/A

ESM-RUN Engine Driven Equipment Ready

De-Energized N/A

ESM-STR Engine Start Signal

De-Energized N/A

COM-ALM Compressor Common Trouble Alarm Signal to Plant PLC

See Note Energized if no ESD and no alarms present.

COM-SD Compressor Common Shutdown Alarm Signal to Plant PLC

De-Energized N/A

YX-2101A Compressor Run Status Signal to Plant PLC

De-Energized N/A

Step 1 – Ready to Start

The start sequence will remain in Step 1 until a start command is issued by the operator. If a process

shutdown comes in while ready to start the sequence will return to step 0. When the start command is

pressed all controllers and HOA will be set to auto.

Table 5: Permissives required to move to Step 2 – Skid Pressure Adjustment

Permissives

Start button is pressed on HMI

Table 6: Discrete Outputs Energized/De-Energized in Ready to Start step


Notes


Energized




SDY-2016A Suction Scrubber Drain ESD Valve Solenoids

Energized

SDY-2015A Inlet Fuel ESD Valve Solenoids

Energized

COM-SD Compressor Common Shutdown Alarm Signal to Plant PLC

Energized

Step 2 – Skid Pressure Adjustment

In order for the compressor to start properly, the suction pressure of the package must be within the

minimum and maximum start-up setpoints (adjustable from HMI). If the suction pressure is above the

maximum start-up pressure, then the compressor will blowdown until it reaches the maximum start-up

pressure setpoint. If the suction pressure is below the minimum start-up pressure, the operator must

check/adjust upstream inlet valve to ensure the minimum start-up pressure setpoint is reached.

Table 7: Permissives Required to Move to Step 3 – Pre-Lube

Permissives

Gas inlet pressure PIT-2002A is greater than minimum start-up pressure setpoint.

Gas inlet pressure PIT-2002A is less than maximum start-up pressure setpoint.

Note: If the above permissives are not met within the allocated maximum pressure adjustment time, the start sequence will be aborted.

Table 8: Discrete Outputs Energized/De-Energized in Step 2 – Skid Pressure Adjustment


Notes


Energized See Notes

If depressurization is required, then solenoids are de-energized


Energized

N/A

Step 3 – Pre-lube

Prior to running the unit, it is important that the compressor has adequate lubrication. The compressor

pre-lube pump is controlled via solenoid UY-5104A. The sequence will stay in the pre-lube step until the

compressor oil pressure reaches the minimum oil pressure permissive setpoint, and maintains this

pressure for a minimum of 120 seconds. Once the minimum timer has expired, the sequence will move to

the “Start Driver” step. If the compressor pre-lube pump runs for a maximum amount of time (adjustable

from HMI) without reaching the permissive, the start sequence will be aborted. Engine pre-lube is

controlled by ESM.




Table 9: Permissives Required to Move to Step 4 – Start Driver

Permissives

Compressor Oil Pressure meets the required minimum permissive setpoint

Compressor Oil Pressure maintains required minimum permissive setpoint for the set minimum pre-lube time. (Ariel compressors require a minimum of 120 seconds of maintained pre-lube pressure)

Note: If the above permissives are not met within the allocated maximum pre-lube time, the start sequence will be aborted.

Table 10: Discrete Outputs Energized/De-Energized in Step 3 – Pre-lube


Notes


Energized N/A

Step 4 – Start Driver

Once in the “Start Driver” step, the control system will send a signal to the ESM to initiate the engine start. All fuel and ignition sequencing is done internally in the ESM. The “Start Driver” step is considered complete once minimum speed permissive of the engine is complete. If this speed is not reached in a maximum amount of time (Max Crank Time setpoint adjustable from HMI) the “Start Driver” step will retry to start and a crank attempt will be recorded. There will be a pause in between crank attempts (adjustable from HMI). If the maximum number of crank attempts is exceeded the start sequence will be aborted.

Table 11: Permissives Required to Move to Step 5 – Warm-up

Permissives

Minimum engine speed setpoint is reached.

Note: If the maximum number of crank attempts is exceeded then the start sequence will be aborted.

Table 12: Discrete Outputs Energized/De-Energized in Step 4 – Start Driver


Notes


De-Energized De-energized as soon as the engine starts to crank


Energized

ESM-STR Engine Start Signal

Energized The ESM Engine start relay is pulsed for 20 seconds as per ESM requirement




Step 5 –Compressor Warm Up and Minimum Load

While in the Warm-up step, the engine will run at idle speed. It will continue to do so until the engine

jacket water and compressor oil temperatures meet their set permissives (adjustable from HMI). In

addition to these permissives, there is a minimum warm-up timer that must expire prior to going online.

This ensures that the oil and jacket water have a chance to circulate and ensure that there is not an

inaccurate reading on the temperatures. Once the temperature permissives have been met and the

minimum start time has completed, the engine will ramp up to minimum load speed. It will remain

running at minimum load speed until the operator pushes the “Auto Load” button located on the HMI in

case it is disabled. At this point, the package will be considered online and all PID loops will become

active.

Table 13: Permissives Required to Move to Step 6 – Online

Permissives

Compressor oil temperature meets minimum permissive setpoint (adjustable from HMI)

Engine Jacket Water temperature meets minimum permissive setpoint (adjustable from HMI)

Minimum Warm-up timer expires

“Auto Load” is set to on.

Engine speed reaches minimum load speed (an optional dead-band can be added if the min load speed cannot be reached)

Table 14: Discrete Output States while in Step 5 – Warm-up


Notes


Energized N/A

Step 6 – Compressor Online

When online all PID loops are enabled, the compressor will operate to maintain the set operating

conditions. It will continue to run in this manner until either an operator stop request is initiated, a

process shutdown, or an ESD occurs. If at any time when the compressor is online and the “Auto Load” is

set to off, the package will ramp to a fully unloaded state (compressor motor at minimum speed and

bypass valve is fully open). It will remain this way until “Auto Load” is turned back on.

Table 15: Events which will cause the package to go offline

Permissives

Local or Remote Stop is initiated

Process Shutdown Occurs

Emergency Shutdown Occurs

Table 16: Controllers Enabled/Disabled in Step 6 – Compressor Online

Controller Enabled / Disabled

Notes


Enabled





Enabled

Table 17: Discrete Outputs Energized/De-Energized in Step 6 – Compressor Online


Notes

SDY-2014A Discharge ESD Valve Solenoids

Energized N/A


De-Energized N/A


Energized N/A


Energized N/A


Energized N/A


Energized N/A

Step 7 – Cool Down

If a “Soft Stop” of the package is initiated, then the start sequence will move to the cool down step. In

this step, the engine is ramped to idle speed and the bypass valve is ramped fully open. The package will

run in this state until the cool down at idle timer expires. If for any reason the engine is unable to ramp to

idle speed, then the cool down will complete when a maximum cool down timer expires. Upon

completion of this step, the start sequence will move to the post lube step.

Table 18: Events Which Will Initiate a Cool Down

Permissives

“Stop” pushbutton is pressed on the local HMI

“Remote Stop” is initiated from the plant PLC

Table 19: Permissives Required to Move to Post-Lube Step

Permissives

Engine at idle speed and cool down at idle timer expires

Maximum cool down timer expires

Process shutdown occurs during Cool Down Step

Table 20: Controllers Enabled/Disabled in Step 7 – Cool Down

Controller Enabled / Disabled

Notes


Disabled Ramped to 0%





Disabled Ramped to 0%

Table 21: Discrete Outputs Energized/De-Energized in Step 7 – Post Lube


Notes


Energized

Step 8 – Post Lube

Upon completion of the cool down, or after any process shutdown the Post Lube step will be initiated. At

this time, the compressor and engine post lube pumps will be turned on in order to prevent damage to

the equipment that can occur from hard stops. The most important aspect of this step is to provide oil to

the engine turbo chargers while they spin to a stop. The post lube will continue until the minimum post

lube timer has expired, at which point the start sequence will be reset. If there are no shutdowns

present, the sequence will be ready to restart. If there are process shutdowns active, they would have to

be cleared prior to restarting the package.

Table 22: Events Which Will Initiate a Post Lube

Permissives

Cool Down step is complete

Process Shutdown occurs

Table 23: Permissives Required to Complete Post-Lube Step

Permissives

Post Lube timer expires

Note: If process shutdowns are present move to “Step 0 – Evaluate Active Shutdowns”

Note: If no process shutdowns are present move to “Step 1 – Ready to Start”

Table 24: Discrete Outputs Energized/De-Energized in Step 8 – Post Lube


Notes


Energized


De-Energized


De-Energized


De-Energized


De-Energized





De-Energized


De-Energized

Common Shutdown Sequence A common shutdown will occur when the unit control panel determines that a protection limit has been reached

per the shutdown key documentation. The following will occur when a common shutdown has been initiated:

The engine is stopped

Engine post lube does occur

PID loops are disabled

Emergency Shutdown (ESD) Sequence If an emergency shutdown has been triggered by the local ESD pushbutton on the unit control panel, the

building ESD pushbuttons, or the remote ESD signal, then the following events occur:

The engine is stopped via ESM ESD contacts

Engine and compressor post lube does not occur

All PID loops are disabled

All auxiliary equipment and control valves are shut down to their fail-safe states

The auto blowdown valves will open immediately and vent the unit piping to the common vent header




3 Analog Equipment Control (PID) The reciprocating compressor package incorporates several control features to allow the unit to safely operate in

an automated fashion. The control system monitors the process variables and adjusts itself to maintain the

predetermined operating conditions. The following sections describe the control loops that have been

programmed into the PLC.

Controller Modes

Each control loop in the system is programmed to operate in either “Auto” or “Manual” mode. In “Auto” mode,

the PID output is controlled according the automated control scheme. “Manual” mode allows the operator to

set the control output manually to any value within the valid range of 0-100%.

Compressor Capacity Control

Compressor capacity is controlled by two master PID loops. The two master controllers are suction and

discharge pressure. The lower controller output becomes the capacity control setpoint. The higher controller

output tracks the lower controller by 2% above. The capacity control setpoint is split between speed control and

bypass valve. The lower portion of the capacity setpoint controls the bypass valve and the upper portion controls

the engine speed. The split is adjustable, so that speed and bypass valve control can overlap to allow the engine

to start to speed up before the bypass valve is fully closed. The user enters the limits (‘Load Min’ and ‘Load Max’)

from the HMI. Capacity control is active when the unit run state is “Online” and “Auto-LOAD” is ON.

Both the engine speed and the bypass control valve position can be controlled automatically or manually by the

operator depending on the position of the “Auto” / “Manual” soft selector hand switch located in the HMI.

“Auto” Mode

The bypass valve position and engine speed are determined via capacity control PID controllers

(as shown in Figure 1: Compressor Capacity Control).

The output of the low signal select is then split as follows:

0% - A% Load (A = typically 50%)

Output to the bypass control valve, which controls the valve 0-100% closed

B% - 100% Load (B = typically 50%)

Output to the engine speed, which manipulates the speed set point between the minimum and maximum speed set points.




“Manual” Mode

This mode can be activated via the HMI

In this mode, the operator can enter a desired output percentage for the bypass valve position or

a desired engine speed in RPM. This mode is primarily for maintenance purposes.

The following Figure 1: Compressor Capacity Control depicts the compressor capacity control configuration:

Figure 1: Compressor Capacity Control

PIC-2002A

Master suction

pressure control

PIC-1301A

Master discharge

pressure control

PY-2018A

Recycle Valve

Manual

Setpoint

Se

Low Select

Se

Split Range

Se

SY-2018A

Driver Speed

Manual

Setpoint

Se

[Recycle Min %] to [Recycle Max %]

[Driver Min %] to [Driver Max %]




The following Table 25 provides a brief description of the suction pressure and discharge pressure PID

controllers used in the capacity control configuration:

Table 25: Capacity Control - Suction / Discharge Pressure PID Controllers

Controller Details

Master Suction PID Controller PIC-2002A

Type: Direct Acting

Feedback Input: Gas Inlet Pressure (PIT-2002A)

Output: 0-100% (Scaled)

Parameters: Control Set Point

PID tuning parameters (Gain, Reset, Rate)

Manual Control (Operator may set the controller output)

Enabled: Enabled in Step 6 when compressor is online Notes: The controller output will decrease in low suction pressure

conditions and increase during high suction pressure conditions.

Master Discharge PID Controller PIC-1301A

Type: Reverse Acting

Feedback Input: Final Discharge Pressure (PT-1301A)

Output: 0-100% (Scaled)

Parameters: Control Set Point

PID tuning parameters (Gain, Reset, Rate)

Manual Control (Operator may set the controller output)

Enabled: Enabled in Step 6 when compressor is online Notes: The controller output will increase in low discharge pressure

conditions and decrease during high discharge pressure conditions.




4 Discrete Equipment Control

Automatic Blowdown Valves Control Modes

The automatic blow down valve is used to relieve pressure within the compressor package during different shut down situations, apart from an Emergency Shutdown (Local or Plant initiated). The package will only blowdown to the maximum allowable startup pressure (based on the suction pressure). The blowdown valves on this package are listed below.

Table 26: Auto Blowdown Valve - Fail States

Device Control Tag Fail State

Blowdown Valve BDV-2017A Open

There are 3 situations which will trigger a blowdown sequence, these are described below in table below:

Table 27: Auto Blow Down Valves Control Modes

Trigger Trigger Description Blowdown Operation

Emergency Shutdown

A unit or plant emergency shutdown or loss of power to the control panel immediately stops the driver and does not execute any post-lube.

Upon this shutdown, the blowdown valve will immediately open and remain open until the Emergency Shutdown is cleared and reset.

Note: During an emergency shutdown all equipment will be in its fail state.

Start Sequence Initiated

If in Step 2 (Skid Pressure Adjustment) and suction pressure is above the startup setpoint.

The blowdown valve will open and remain open until the relevant suction pressure is below its maximum startup setpoint. Once this occurs, the blowdown valve will close and the start sequence will proceed.

Manual Blowdown

The manual blowdown pushbutton on the local HMI is pressed.

At any time when the package is stopped and the start sequence has not been initiated, the operator can manually open the automatic blowdown valve using the local HMI.

Suction scrubber drain ESD valve solenoids SDY-2016A (A/B)

This drain ESD valve is closed only during Emergency Shutdown. Otherwise, it is always open.

Discharge ESD valve solenoids SDY-2014A (A/B) This discharge ESD valve is open once the warmup is complete and the compressor is online. This valve will close

during ESD or once the post-lube is complete.

Inlet fuel ESD valve solenoids SDY-2015A (A/B) This inlet fuel ESD valve is closed only during Emergency Shutdown. Otherwise, it is always open.

Startup bypass valve solenoids SDY-2020A (A/B)

This startup bypass valve is open once the unit is started and closes once warm up is complete.




Compressor pre/post-lube oil pump solenoid UY-5104A Compressor pre-lube is enabled in step 3 Pre-lube and remains on until engine begins to crank. Also, this pump is

enabled in post-lube step 8.

Process cooler fan motors (EM-2101A/2102A/2201A/2202A)

These are MCC controlled motors to cool down discharge gas from compressor discharge. The motors are

started as part of the start sequence.

5 Signals to Plant

Compressor run status Compressor run status is enabled when the unit starts running in Warm-up step and stays on until the unit stops

for shutdown (stays on while the compressor is rotating).

Compressor common shutdown alarm Compressor common fault is disabled if there are any shutdowns present.

Compressor common trouble alarm

Compressor common trouble alarm is disabled if there are any alarms present.




6 PID Tuning The proportional/integral/derivative (PID) equation is a popular method to control a process with varying external

conditions. The compressor application uses PID control to maintain process conditions at desirable levels. The PID

terms and abbreviations are as follows:

Set Point (SP): The desired pressure or temperature or level or flow or speed

Process Variable (PV): The actual pressure or temperature or level or flow or speed being measured

Error (E): The difference between the set point and the process variable

Control Variable (CV): The output of the PID equation, which is used to manipulate a valve to affect the

process conditions

The calculation is performed every 100 ms (dt). The CV is calculated using three constants in the PID equation:

Proportional (KC): A value directly proportional to the error in the system. KC is the controller gain and

its value affects all three terms. This is the dependent form of the equation (ISA Standard).

Integral (TI): The integral time in minutes and works to reduce the error in the system. The longer the

error exists in the system and the greater the error, the greater the effect of the integral term.

Derivative (TD): The derivative time in minutes and it looks at how quickly process conditions are

changing. The faster the conditions change, the greater the effect on the derivative term.

The three factors KC, TI, and TD are adjusted to vary the respective effect of the proportional, integral and

derivative terms and are therefore used to tune the PID equation.

Tuning Tips

When changing the tuning parameters make small changes (such as 0.1 for gain and integral) and monitor the

effect on the process. Simulate changes to the process by changing the set point by a realistic amount and make

changes to the following three parameters as follows:

Proportional (KC): When the gain is increased, the controller will react more quickly when an error is

introduced to the system. If the gain is too high, however, the controller will oscillate and it will not be

able to maintain the process at the desired set point. If the gain is gradually increased until the controller

begins to oscillate, reduce the gain by ½ before continuing with tuning. Note: A change to the gain affects

all three terms.

Integral (TI): The integral time should be set close to the natural period of the system. This is the time

between peaks when the controller is oscillating. Increase the integral time constant to reduce the speed

at which the controller works away at the system error.

Derivative (TD): The derivative time can be set at 0 and increased by small increments (0.01) if necessary


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