Compressor System Control Logic

FERMILABTechnicalDivisionf MTF Helium Refrigerator

Compressor System Control Logic (Phase I Upgrade)

Doc. No. RCU-98-031Rev. No. 5Date: July 19, 2006Page 1 of 26

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FERMILABTechnical Division

MTF Helium RefrigeratorCompressor System Control Logic

(Phase I Upgrade)




Author: Ruben Carcagno Date: 12/10/98Reviewed by: Date:Approved by: Date:




TABLE OF CONTENTS

INTRODUCTION..........................................................................................................................................3

EQUIPMENT DESCRIPTION.....................................................................................................................4

COMPRESSOR START/STOP LOGIC......................................................................................................5

COMPRESSOR SHUTDWON INTERLOCK LOGIC.............................................................................8

MOTOR CONTACTOR PROTECTION..................................................................................................11

COMPRESSOR LOAD AND UNLOAD LOGIC.....................................................................................12

GAS MANAGEMENT SYSTEM...............................................................................................................15

SUB-ATMOSPHERIC SUCTION PROTECTION..................................................................................................16

SOLENOID VALVES..................................................................................................................................20

BROKEN TRANSMITTER ALARMS......................................................................................................23

APPENDIX A: SYMBOLS DEFINITION FOR LOGIC DIAGRAMS.................................................24

APPENDIX B: COMPRESSOR SYSTEM – COLD BOX SYSTEM CONTROL INTERFACE.......25




INTRODUCTION

This document describes the control logic for the compressor system controls upgrade (Phase I) of the MTF helium refrigerator. Phase I is intended to duplicate the existing functionality of the control console in the new PLC-based control system. For details about Phase I scope, please refer to RCU-98-022.

The basis for this document is the original compressor system control logic documented in RCU-98-10, plus discussions with operators to identify obsolete aspects of the original logic. These obsolete aspects have been eliminated for the upgrade logic. There are a few changes described in this document intended to enhance safety and troubleshooting. Deviations from the original logic and other comments can be found in the “Upgrade Notes” section.

In addition, we contacted the compressor manufacturer (Sullair) to obtain information about current compressor control strategies. We were referred to their supplier (Logix Industrial Refrigeration Controls) and we consulted with them. Logix provided us with their Compressor Control System Operations Manual so we could verify that our controls upgrade basis is consistent with their approach.

For additional details, please refer to the following documents:

“Operator’s Manual – 1500 Watt Helium Refrigerator for Fermi/LBL,” from Helix Process Systems dated March 1978. Available in hardcopies only.

Upgrade compressor system P&ID drawing (1670-ME-304962)




EQUIPMENT DESCRIPTION

The process compressor is a two-stage system on a separate skid approximately 35’ by 8’ by 10’ high and weighs approximately 45,000 pounds. The skid consists of two open drive Sullair screw compressors, oil cooling systems, oil separation system, gas management valving, motor controls, and a local instrument cabinet.

TABLE 1: General Specifications

(This table is copied from the CTI Operator’s Manual)Motor Power Supply 460V, 3 phase, 60 HertzControl Power Supply 115V, 1 phase, 60 HertzControl Power Requirement 600VAOil Capacity – First Stage Compressor package 100 gallonsOil Capacity – Second Stage Compressor package 150 gallonsOil Type UCON LB165Compressor Package Weight 48,000 lbs.First Stage Compressor Motor 200 hpFirst Stage Compressor Motor – Full Load Amps 225 AmpsFirst Stage Compressor Motor – Locked Rotor Amperage 1339 AmpsSecond Stage Compressor Motor 1000 hpSecond Stage Compressor Motor – Full Load Amps 1135 AmpsSecond Stage Compressor Motor – Locked Rotor Amperage 5400 ampsOil Cooler and Aftercooler Coolant 50% ethylene glycolCoolant Inlet Temperature 85 F

Model and Serial Number

First-stage Model C25LB704-26Second-stage Model C25LA704-37Serial No. 29247 FGG

TABLE 2: Operating Limitations

(This table is copied from the CTI Operator’s Manual)Description First Stage

CompressorSecond Stage Compressor

Design Discharge Pressure (Maximum) 150 psig 300 psigOperating Differential Pressure (Maximum) 164 psig (typo?) 314 psiSuction Pressure (Maximum) 80 psig 80 psigOil Pressure (Maximum) 80 psi 80 psiOil Pressure (Minimum) 40 psi 40 psiOil Temperature (Maximum) 125 F 125 FDischarge Temperature (Maximum) 225 F 225 FCompressor Speed (Maximum) 3550 rpm 3550 rpmOil Filter Pressure Drop (Maximum) 35 psi 35 psi




COMPRESSOR START/STOP LOGIC

The commands to start and stop the first and second stage compressor motors are software pushbuttons accessible from operator display screens. There are no local or hardwired start/stop pushbuttons.

The START request is a momentary signal latched by the motor running feedback coming from the motor contactor’s “closed” auxiliary contact. Once issued, the STOP request remains active until the operator pushes the RESET pushbutton.

There is a single Emergency Stop (E-Stop) hardwired circuit with two pushbuttons: one located in the compressor room and another located in the control room. The E-Stop circuit de-energizes both compressor motor control centers by removing power to the 480V circuit breaker (see electrical schematics for additional details.) Once this breaker is open, a Fermilab electrician has to be called to close it. For this reason, a 110VAC power switch was installed in the compressor control cabinet. This is to allow people to work on equipment without having to worry that someone in the control room will start/stop equipment. This switch is for 110 VAC going to motors contactors, not for 110 VAC for PLC or 24 VDC power supply.

A number of conditions must be satisfied before the START request can actually start the compressor motor. Figures 1 and 2 show the details of the START/STOP logic for the first and second stage compressors. Table 3 and Figures 3 and 4 show the details of the shutdown interlock logic.

UPGRADE NOTES:1. The original system had three E-Stop circuits: one for the first stage motor, one for

the second stage motor, and one for both first- and second-stage motor control centers. We decided to eliminate the individual E-stop circuits, and have only one for both motors (at circuit breaker #3). A 110 VAC switch was also installed in the control cabinet to allow people to work safely on equipment without worrying that motors will start/stop by others or by the PLC.

2. In the original system, the anti-recycle timers had been set to their minimum delay for unknown reasons. We restored these timers to the recommended settings of 20 minutes. The oil pressure differential timer was measured to be 16 seconds. We set this timer to 10 seconds.

3. With the upgrade, the PLC includes the 480V motorized circuit breaker “closed” auxiliary contact signal (1MCB-A and 2MCB-A). The compressor motor START permissive includes now the condition that the corresponding 480V MCB has to be closed.

4. Pressure and temperature switches provided input for interlock logic in the original system. With some exceptions such as the motor overload and the motor winding temperature, we decided to replace these switches by setpoints on the analog signal from a sensor measuring the corresponding process variable. This change significantly reduced the number of field devices and their associated wiring, and




also provides additional information for the operator (e.g., the system now provides continuous gas suction and discharge temperature indication instead of just a shutdown temperature switch that gives no warning before shutdown.) For additional details, see RCU-98-19.

5. The original system was designed to work outdoors and it was equipped with heaters that had been disabled. Since the compressor system is working indoors and the heaters are no longer required, we decided to remove them. This change simplifies the electrical system.

6. Auxiliary oil pumps for the first and second stage compressor were added to the original system and were started manually after a long shutdown and a couple of minutes before starting the compressors. With the upgrade, these pumps can be started remotely from the operator interface.







COMPRESSOR SHUTDWON INTERLOCK LOGIC

A number of conditions must be satisfied for the compressors to start or continue running. If any one of these conditions is not met, the compressor motor will be either prevented from starting, or if it is running already it will be shutdown automatically. If the compressors are allowed to continue running, equipment damage may result.

Table 3 summarizes the shutdown interlock conditions and setpoints. Figures 3 and 4 shows the details of the interlock logic for the first stage and second stage compressors respectively.

TABLE 3: Shutdown interlock conditions

CONDITION FIRST STAGE SECOND STAGELow Suction Pressure PT201 < 14 psia for at least 30

minutesPT206 < 14 psia for at least 2 minutes

High Discharge Pressure PT205 > 75 psia PT210 > 260 psiaHigh Discharge Temperature TT201 > 210 F TT205 > 210 FMotor Overload OLS200 > 480 A OLS201 > 1150 AHigh Motor Winding Temperature TS206 > 248 F TS202 > 248 FHigh Oil Temperature TT203 > 125 F TT208 > 125 FOil Pressure Differential PDS204 < 25 psid (after 20

seconds of motor running signal)

PDS209 < 25 psid (after 20 seconds of motor running signal)

Instrument Air Low Pressure PT209 < 75 psia PT209 < 75 psiaFirst Stage Safety Shutdown - XSecond Stage Safety Shutdown X -Broken Transmitter PT201, PT205, TT201, PT209,

TT203PT206, PT210, TT205, PT209, TT208

High Motor Current IT200 > 250 A IT201 > 1000 A

UPGRADE NOTES:1. PS206, PS209, TS200, PS205, PS210, TS201, TS205, TS203 and TS208 have been

replaced by settings on analog signal provided by PT206, PT209, TT200, PT205, PT210, TT201, TT205, TT203 and TT208 respectively.

2. All units are now expressed in psia or psid3. The original system included a shutdown interlock on low suction temperature.

This interlock has been removed because the compressor system has been moved far away from the coldbox and a condition of low suction temperature is not expected to exist.

4. An interlock on cold box vacuum had been added to the system. We decided to remove this interlock because it was intended to prevent cold gas from reaching the first stage suction when the compressor system was installed close to the cold box.

5. A permissive on N2 vent valve open limit switch had been added to the system. We decided to eliminate this interlock because it is no longer relevant due to process equipment changes (this signal had been jumpered)




6. An interlock on sub-atmospheric first stage suction pressure has been added (see Gas Management section)

7. The instrument air low pressure setting has been lowered from 83.7 psia to 75 psia . This is because of the lower pressure differential rating of new 24 VDC solenoids (75 psid) compared to the original 110 VAC solenoids (150 psid). In addition, a transmitter (PT209) to allow trending has replaced the switch PS209. We still need to find out what is the lowest admisible air supply pressure to operate pneumatic valves. After this information is obtained, PT209 shutdown setting may be lowered.

8. Timers for PDS204 and PDS209 were measured to be 16 seconds. We are going to set them at 20 sec.

9. The implementation of the interlock logic includes first-fault indication. This is the initiating event that caused the interlock condition.

10. Once the interlock condition is set, it will remain set until the operator pushes the RESET pushbutton in the operator interface.

11. Broken transmitter interlocks have been added for analog transmitters. See section on Broken Transmitter Alarms. Switches are wired in the Normally Closed (NC) position, so any electrical problems in the loop such as an open circuit will cause a shutdown.

12. The shutdown setpoint on 2nd stage discharge (260 psia) could be too conservative, but operators never run the system that high in discharge pressure. There is a skid relief valve set at 300 psia, so the shutdown setpoint should never exceed this value.

13. To avoid nuisance shutdowns during compressor system startup, we changed the logic to trip the second stage compressor when the suction pressure remains subatmospheric for more than 2 minutes. Initially, shutdown as soon as the suction pressure went subatmospheric.

14. Following the February 2006 fire in IB1A, a current interlock for each helium compressor motor was instituted: 250 A for first stage, 1000 A for second stage. This interlock is software-based, programmed into the PLC. A subsequent startup in July 2006 found that the first stage setpoint was too low. The inrush current is higher than estimated, and the compressor tripped off shortly after startup when the interlock timer expired. The setpoint was then raised to 300 A.

15. The settings for the new overload switches put into service during the May 2006 maintenance shutdown are as follows: OLS200 (first stage) has a 2.5 A setpoint and uses time dial setting 3, OLS201 (second stage) has a 3 A setpoint and uses time dial setting 2.




High MotorCurrent > 300 A

IT200

High MotorCurrent > 1000 A

IT201




Motor Contactor Protection

As part of lesson learned from prior safety –related incidents, we decided to incorporate logic in the upgrade to prevent fires or other damage should the motor contactor become “stuck” between the open and close position. To this end, both motor contactor auxiliary contacts are wired to the PLC. These auxiliary contacts are limit switches that provide indication of contactor in the “open” position and contactor in the “close” position respectively. Both switches are OFF (or OPEN) at the same time only while the contactor is travelling. At all other times, one is OFF (or OPEN) and the other is ON (or CLOSED). Thus, the logic to detect whether the contactor has become “stuck” while traveling from one position to the other is simply to check that both auxiliary contacts do not remain OFF (or OPEN) longer than for a pre-specified amount of time (arbitrarily taken to be 2 seconds). If both contacts remain OFF (or OPEN) after this time has elapsed, the PLC sends a signal to open the 480V motorized circuit breaker, thus removing power from the MCC and avoiding arcs or other conditions that may damage the equipment. Since the 480V motorized circuit breaker “CLOSE” limit switch is part of the motor “START/STOP” logic (see figures 1 and 2), the command to run the compressor motor will also be turned off. Figures 5 and 6 shows the details of this logic.




COMPRESSOR LOAD AND UNLOAD LOGIC

An electrical actuator replaces the original slide valve actuator in both stages.This electrical actuator contains a motor and a gear reduction mechanism to drive the slide valve. Limit switches actuated by cams establish the maximum and minimum capacity positions for the slide valve. As part of the upgrade, a potentiometer will be added to the actuator to provide continuous indication of slide valve position.

The original system included an option for automatic load/unload of the system based on the refrigerator operating mode and several pressure switches. Operators no longer use this option. Loading and unloading are always performed manually by pushing the corresponding buttons in the control console. However, the original system included logic to automatically unload the compressor in case of overpressure or overcurrent conditions. If this action is not sufficient to lower the current and/or pressure, then the compressor will shutdown when the shutdown setpoints are reached. We decided to preserve this logic to avoid unnecessary compressor system shutdown, particularly when the system is running unattended.

For the upgrade, we decided to eliminate all pressure switches associated with the original automatic load/unload operation. Phase I will only include manual load/unload capabilities from the operator interface plus automatic unload in case of overcurrent or overpressure conditions. Automatic load/unload control strategies will be added later and performed by software as part of the Phase II upgrade.

The control logic drives the slide valve to the minimum position at shutdown. In addition, the control logic prevents the compressor from loading during the initial ten seconds of running.

Figures 7 and 8 show the unload/load logic for the first stage compressor, and figures 9 and 10 show the unload/load logic for the second stage compressor.

UPGRADE NOTES:1. Operators never run the system in AUTO load or unload. They manually load or

unload the system, using the motor current indicator as a feedback signal. Typically, they like to run the first stage motor at 150-180 Amps, and the second stage motor at 480-800 Amps. The upgraded system includes a potentiometer to provide slide valve position feedback.

2. The original system included logic to automatically unload the compressor in case of overcurrent or overpressure conditions (under either MANUAL or AUTO load or unload). We decided to preserve this logic for the Phase I to avoid unnecessary shutdown when the system is running unattended.

3. Automatic load and unload controls will be added as part of the Phase II upgrade. It will be a software upgrade module for the overall gas management control automation.




4. The system had an old style slide valve actuator in the second stage, and a new style actuator in the first stage. We decided to replace the old style by the new style, so the upgraded system will have the same style actuator in both stages.







Gas Management System

The function of the gas management system is to: (a) regulate the compressor discharge pressure, (b) to provide makeup gas to the compressor, and (c) control the capacity of the cold box by by-passing high pressure gas the low side.

At initial startup the second stage compressor suction pressure will drop rapidly, PCV221 will open diverting discharge gas into the suction line. Valve PCV221 is set to control the intermediate pressure at 3.2 atm (PIC203 in the main control panel is set at 47 psia). As the pressure continues to fall valve PCV211 opens to supply make up gas from the buffer tank E1 (PIC202 in the main control panel will be set to approximately 44 psia).

As the intermediate pressure increases PCV211 will start to close and will be fully closed at 3.2 atm.

When the first stage starts the suction pressure will drop, PCV210 will then open to raise the pressure to 1 atm (PIC201 in the main control panel will be set to 15 psia). As the discharge pressure of the second stage exceeds 219 psia or the set point of PIC200, PCV204 opens bleeding high-pressure gas into the tank.

When both compressors are running fully loaded the intermediate pressure will be in excess of 47 psia. If the intermediate pressure starts to fall below this set point, PCV221 will open, and thereby maintain intermediate pressure.

During operation of the liquefier, PCV215 will be open, then PCV214 and then finally PCV200, allowing high-pressure gas to flow into the cold box. Initially during cooldown the high-pressure discharge of the compressor will be maintained by PCV204. When the coldbox reaches steady state conditions, PCV204 will be closed and PCV210 will supply makeup gas required for liquefaction.

The gas management system has two operating modes: makeup and recirculation. A software switch in the operator interface (originally a hardware switch LS200) selects these modes. Figures 12 and 13 show a simplified process flow diagram for both modes of operation of the gas management system. In the recirculation mode, makeup from the gas storage system is disabled so that the discharge pressure and refrigeration capacity fall until the load is matched. During shutdown the output of PIC201 will automatically switch back to control PCV210 when the main refrigerator supply valve (PCV200) is closed.

For more details, please see the technical note “Compressor Skid Gas Management System,” RCU-98-032, August 19, 1998 by R. Rabehl. This note describes in detail the as-built gas management system and the upgraded gas management system.

For details about PID tuning, please see the technical note “MTF Compressor Skid PID Loop Tuning Procedure,” November 17, 1998 by R. Rabehl.




Sub-atmospheric suction protection

In the original CTI system, only the second stage suction was protected against sub-atmospheric suction. If the second stage suction is below 1 atm, pressure switch PS206 was made and the compressor would trip. However, there was no equivalent protection for first stage sub-atmospheric suction. This could be a problem, because air can be pulled into the system and introduce contamination.

Protection against sub-atmospheric suction is particularly important for unattended operation, where large amounts of contamination can be introduced into the system. Eventually, the refrigerator will shutdown due to contamination-induced failure. Recovery from this condition can be time-consuming and equipment damage may result. Therefore, we would like to include as part of this upgrade a condition to protect against sub-atmospheric compressor suction conditions.

One scenario leading to potential sub-atmospheric first stage suction is depletion of gas inventory while the system is running in “Makeup” mode. The buffer tank pressure decreases, and after a point PIC201 will no longer able to maintain first stage suction pressure at its setpoint of 1 psi above atmosphere (15.7 psia). To avoid this condition, we are going to include PLC logic to automatically switch to “Recirculation” mode when the first stage suction pressure remains below 14 psia for at least 15 minutes. The timer is included to prevent switchover during normal short-term compressor suction oscillations. Figure 11 shows the details of this logic.

If the condition persists after switching over, there will be a shutdown condition for the first stage compressor suction running below atmospheric (see Figure 3), just like there is for the second stage (see Figure 4). However, switching first to “Recirculation” mode should normally prevent reaching the shutdown condition.




UPGRADE NOTES:1. Operators run the gas management system normally in the “Makeup” mode

(LS200 ON).2. A switch had been added to the system to use PIC202 to control PCV1406 instead

of PCV211. PCV1406 brings gas to the medium pressure line from HX-1400. Operators normally run the system with this switch ON, so PCV211 is normally closed, PIC202 is put in MANUAL and its output is used to control PCV1406. This valve is usually controlled to be 100% open or closed, so the PID functionality and variable output is not really required. The upgraded system eliminates the switch, PIC202 is used exclusively to control PCV211, and an OPEN/CLOSE command will control PCV1406.

3. Operators like to run the system with PIC203 controlling PCV221 at 20 to 25% open.

4. Operators run the system with a setpoint range for PIC200 between 150 psi and 220 psi, about 38 psia for PIC203, and 1 psi above atmosphere (15.7 psia) for PIC201.

5. The original system had been modified to add a manual valve in the pneumatic line joining PCV226 and PCV207. The original intent of this seems to be to allow PCV206 to open to bleed gas into the first stage suction if valve PCV204 is open fully and the discharge side pressure continues to climb. Since the manual valve is always closed, this logic has been disabled. The upgraded system will remove this pneumatic line and install a standard solenoid for valves PCV206 and PCV204. If a two-tier control stragegy for discharge pressure is needed, it will be done in software as part of the Phase II upgrade.

6. The original system was designed so LS200 will automatically switch from makeup to recirculation mode when the dewar liquid level reaches a predetermined setpoint. This logic did not work very well because of noisy dewar liquid level signal, so it had been disabled. LS200 was only actuated manually by the operator. The Phase I upgrade will not include any automatic switching between makeup and recirculation mode, but Phase II may include this automation.




FIGURE 12:




FIGURE 13:




Solenoid ValvesThe compressor system isolation valves PCV215, PCV 214, and PCV200 are N.C. valves. These valves open by energizing their corresponding 3-way solenoid valves PCV212, PCV224, and PCV201. When the solenoid valves open, instrument air is supplied to the isolation valves and these valves will open. When the solenoid valves close, air is exhausted from the isolation valves and these valves will close. There are certain permissives that must be satisfied before the solenoid valves can open. Figure 14 shows this logic.

PCV215 and PCV215 are equipped with limit switches to signal that these valves are fully open.PCV214 (MP Valve) limit switch is used to energize a 5TI relay (CR290), which is used as one of several permissives to open T1 and T2 turbines inlet valves in the cold box. This limit switch is also used to activate a pilot lamp to indicate that PCV214 is fully open. PCV215 (LP Valve) limit switch is used to activate a pilot lamp to indicate that this valve is fully open.

UPGRADE NOTES:1. The PCV214 solenoid (PCV224) had been disabled, and the pneumatic line

connected directly to PCV214 for unkown reasons. As a result, PCV214 no longer closed but operators still needed to push the OPEN button to satisfy permissives to open PCV200. A consequence of bypassing the solenoid is that PCV214 open permissives based on pressure differential (PDS200 and PDS201) were no longer effective. The upgrade will restore the functionality of the PCV214 solenoid and its open permissives, as it was originally intended. According to R. Barger, this is important to prevent damage to turbines.

2. The permisive on cold box instrument air supply OK has been removed. This seems to be an unnecessary interface between the cold box and the compressor system.







UPGRADE NOTES:1. Solenoid PCV227 (air to FCV204, the interstage isolation valve) has been

hardwired to CR-204 (the second stage motor start relay) according to hand-written notes. In the original CTI system, it was wired to the 5TI output Y22. However, the original 5TI program does not contain logic for Y22. R. Barger explained that this valve was originally included by CTI to prevent second stage compressor backspin after shutdown, so they wanted to make sure it closed immediately after shutdown. It was still not effective to prevent backspin, so a check valve was added to the system later. With the check valve addition, FCV204 is not as critical, so we decided to wire it to the PLC and replace the hardwired logic by PLC logic. The current 5TI program version does include logic for Y22, but it does not seem to be related to PCV227. We need to verify where is Y22 wired to during the disassembly.

2. Solenoid PCV226 was added to the system and solenoid PCV207 was changed to a standard 3-way solenoid. See RCU-98-032 for details.

3. Manual valves have replaced the three solenoids for the buffer tank (EV-240 to 242) because they are used for maintenance only.




Broken Transmitter AlarmsAnalog transmitters are used to shutdown equipment when the corresponding process variables reach a critical alarm setpoint. However, if the transmitter is broken the equipment may continue running unprotected. Therefore, it is important to identify conditions indicative of a broken transmitter and shutdown equipment before an undetected dangerous condition exists.

All analog transmitters operate in the range of 4 to 20 mA. A transmitter signal between 0 an 4 mA is therefore indicative of either a broken transmitter or a problem in the electrical loop. The PLC analog input cards convert the analog input to a digital value between 0 and 32,000. Therefore, any digital value in the range 0 to 6400 (corresponding to the 0 to 4 mA range) is indicative of a broken transmitter. The strategy to detect a broken transmitter is to check the raw digital value, and if it is between 0 and 6400 issue an alarm. When the alarm is for transmitters associated with shutdown conditions, the equipment will be automatically shutdown because it is not acceptable to run with broken critical transmitters.




APPENDIX A: Symbols Definition for Logic Diagrams

Logic diagrams were produces according to the ANSI/ISA-S5.2 standard, “Binary Logic Diagrams for Process Operations.” Please refer to the standard for additional details.




APPENDIX B: Compressor System – Cold Box System Control Interface

Control Signals from Compressor System to Cold Box System:

1. First Stage “ON” (wire 101M-A2 signal to X5 5TI input)2. Second Stage “ON” (wire 201M-A2 signal to X21 5TI input)3. PCV214 Fully Open (wire PCV214 limit switch ZSH214 signal to X26 5TI input )4. PCV200 Open Command (Output to solenoid valve PCV201)5. PCV215 Fully Open (wire PCV215 limit switch ZSH215 signal to X27 5TI input)

Control Signals from Cold Box System to Compressor System:

None

UPGRADE NOTES:1. The 5TI signals are 110 VAC, so auxiliary control relays to switch 110 VAC may be

needed to provide these signals.2. We decided to eliminate the cold box vacuum signal to the compressor system. This

signal (wired to X49) would trip the compressor if the cold box vacuum is bad. The 5TI program was modified to accommodate this logic. The purpose of this seems to be to prevent cold gas from running through the compressors, which was more critical when the compressors were close to the cold box.

3. We also decided to eliminate the N2 vent valve limit switch signal. The compressors were prevented from starting if this valve was closed (again, this logic was added to the 5TI system). This condition is no longer relevant, and the signal had been jumpered.

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Compressor System Control Logic