Automation of Reciprocating Gas Engine Compressor Packages Using

AUTOMATION OF RECIPROCATING GAS ENGINE COMPRESSOR PACKAGES USING PROGRAMMABLE LOGIC CONTROLLERS

Copyright Material IEEE Paper No. PCIC-89-35

W. H. Goff, Member IEEE ARC0 Oil and Gas Co.

Dallas, Texas

Abstract

An approach taken to automate gas/engine compressor skid mounted packages is described. The approach includes design criteria with s cia1 considerations relating to the application of Programmare Lo ic Controllers (PLCs). Using typical industry standard specikations along with I/O lists, sequence narratives, and logic diagrams the PLCs may be pro rammed to perform the desired operation. The added capatilities of the PLC enhance simulation. checkout and startup.

Introduction

Reciprocating engine/compressor packages have long been a mainstay of the oil and gas production industry. Typical applications may be to increase oil production by a technique called gas lift or increase as production by com ressing gas to pipeline pressure recfucing backpressure on ge gas wells. The remote location of this equipment normally means using self contained control systems to o erate the process equipment, engine and compressor. TEe control systems have been slow to chan e from the well established pneumatic ( as or air) controy systems. As automation expanded, t i e older pneumatic systems were partially replaced by relay panels and later by PLCs.

Most applications of PLCs to compressor packages have been to interface the pneumatic shutdown/control system to other automation based control systems. Sometimes PLCs have been used to add alarm or shutdown controls in addition to the existing pneumatic controls. Little information has been available on using PLCs to accomplish the actual sequence control of the engine, compressor, and process.

Using PLCs to automate production facilities including offshore platforms is increasing 1. The reliability and reduction in complex wiring and tubing has resulted in reduced downtime and faster repair. The expansion of automation has reduced the operating personnel requirements for many facilities.

This pa er is based on experience gained by the installation or three reciprocating engine/compressor packages on Gulf of Mexico remote unmanned platforms. All three packages were installed in 1988 in anticipation of reduced gas reservoir pressures. The successful operation of the packages supports continued development of automation projects with PLCs.

Desicln Criteria

The engine compressor package usually includes process equipment such as scrubbers, filters, and exchangers. Special design is provided for purging the system with gas before startup. For unmanned facilities the degree of purging is important to assure safe operation. Control equipment must be rovided to allow the equipment to shutdown proper? &ring a problem while runnin or starting up. The esign al ows for blowing the system iown to prevent dangerous buildup of natural gas pressure. The yard valves are those valves which control the flow of gas through the process and compression system. The yard valves become critical and must be designed for fail-safe operation. These valves are opened and closed according to

a prescribed operation to purge, start, load, and shutdown the compressor and engine.

Figure 1. is a simplified flow diagram for a typical two-stage reciprocating compressor package. The yard valves include the suction, loading (purge), blowdown, discharge, and recycle valves. Solenoid valves are connected to the compressor package PLC to open and close the valves. Limit switches are added to show the open and close positions of the valves. The compressor package PLC monitors the position of the valves for correct operation of the compressor process equipment.

Pressure and level switches on the process scrubbers and fuel gas filters are inputs to the PLC to provide safety shutdowns and alarms. Local pneumatic control from a level controller dumps the scrubber liquids to the produced water system. Temperature switches are installed on the exchangers and compressor discharge.

The process equipment switches and solenoid valves are the primary shutdown and sequence control for the process automation of the compressor package. The engine and compressor mechanical control must be incorporated in the automation to verify proper mechanical equipment starting and running conditions. Typical inputs to the PLC for the engine/compressor control are ressure, level, and temperature switches. These are general6 established by the vendor and must protect the mechanical equipment. Lubrication, fuel, and cooling systems are monitored as discrete input switches to the PLC. In addition engine and turbocharger speed must be sensed to startup and operate the engine properly. Other sensors are used for monitoring inlet air, make-u water and lubrication tanks, cylinder temperatures, anfvibration.

SDecial Considerations

The design and installation requirements must reflect the needs of the existing operations and facilities. One consideration may be to use separate PLCs for other process equipment or packages essential to the compression equipment. Other criteria may be important to the designer such as the following:

1.

2.

3.

4.

5.

The PLC typically needs special environmental considerations such as air conditionin The PLCs for each of the compressors instalyed were located in air conditioned control buildings.

The PLC may need to be installed in a manner to allow it to be moved with the compressor to a different site.

Inter-posing relays may be required to interface with starters or heater contactors.

SCADA input or other electronic devices may re uire special inter-posing devices. The leaage current through some PLC output devices ma be reater than the turn-on current of the interrace 3evice.

Special training or hiring of skilled technicians may be required to maintain the equipment.

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6. Operating requirements ma need to be established to avoid radio inte Arence caused by hand held radio transceivers.

A local panel may need to be provided for use by maintenance personnel. The addition of a local panel for the compressor installations offshore provided a limited degree of local monitoring along with manual startup at the compressor skid.

Decisions on whether certain devices should be converted to electrical or electronic may need to be made based on experience and the degree of automation required. A decision was made to use the manufacturer's pneumatic control for ramping the fuel gas valves, fuel air ratio system and ovemor on our compressor skids. The decision was base1 on the most practical degree of conversion for the equipment available. Other pneumatichydraulic equi ment that was not converted to electrical or electronic &vices included the pneumatic PID controllers for the suction and recycle control valves which loaded the compressor.

The conversion from pneumatic control devices to electric/electronic sensors is a normal result expected when converting to the PLC based system. Special attention needs to be given to device selection when makin such conversions. Significant problems were encounterer? in the installation of the new compressors offshore because of incorrect a plication of certain electric/electronic sensors. Prior instalPations used only pneumatic to electric pressure

7.

pilots. The conversion to all electric/electronic end devices was a significant change. Problems occurred with the application of level and pressure switches which had not been considered. The level switches were not designed for the characteristic bridle installations and the compressor skid vibration. The pressure switch dead bands played havoc with establishing proper setpoints. Once these problems were resolved, the switch o eration was reliable. Careful selection and application of tEese end devices will prevent such problems.

Although PLCs have the flexibility to monitor many types of inputs, certain monitoring functions which could be included in PLCs may still best be served b specialized control units. An example of tks=rG installation of a temperature scanner to monitor the en ine cylinder and turbocharger tem eratures. Although the F&s could have been programme1 and analog inputs provided, the design team decided that the added program complexity and significant changes which may later be needed could disrupt the development of the compressor control system.

The desi n requires that process safety shutdowns required by sa&y standards for offshore latforms be included. Although two levels of safety siutdown are normally required, three levels of shutdown were included. Electronic shutdowns from process equipment sensors (i.e. ,pressure switches, level switches, etc.) are considered the first level of safety shutdown. Pressure relief safety valves and rupture discs provide the 2nd and 3rd levels of mechanical safety shutdown backup.

RECYCLE VALVE 0 - -

2 - STAGE r-3 COMPRESSOR

SUCTION VALVE

LOADING VALVE

EXCHANGER

DISCHAR G E VALVE

SCRUBBER (TY P)

SCRUBBER (TY P)

3 - b

TO SALES

BLOWDOWN VALVE

2 TO

H. P. RELIEF

1 . 4 TO PRODUCED WATER

Figure 1. Typical 2-Stage Compressor Flow Diagram

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The degree of safety shutdown provided by the PLC is de endent on the electrical system reliability. The stem voiage was selected to be 24 volts DC. The availabxty of DC power supplies for PLCs increases the reliability by providing battery backup support during utility or power outage.

Additional criteria used to control the remote operation of the compressor were:

1. The engine was locked out from starting remotely after a shutdown was initiated from a shutdown device.

Only three successive restarts were allowed after the compressor was shutdown remotely and a restart was required.

3. A normal remote shutdown would begin a cooldown cycle for the compressor.

4. For normal maintenance, a local/remote switch was provided at the compressor panel for local start-up.

2.

SDecifications A reciprocating gas engine compressor package

specifics on package typically incorporates references to API-11P 9 , and data sheets developed for previous packages.

8

9

Unfortunately, this information is of little help in the development of a new PLC based control system where the purchaser must use his own criteria as the basis for the PLC and control system.

GAS ENGINE UNIT

Crank Case Oil Level X

TAG#: LG-34 High Jacket Water Temperature X A

For reciprocating compressors API 11P includes a table to define the shutdown, indicating, control and remote devices. Table 1 is typical of the format which may be modified to communicate the purchaser's requirements. Shutdown/alarm function class designations have been established for compressor packages with the following definitions:

Class "A' - Shutdown functions which are always active. Class "A' shutdowns prevent starting or initiate a shutdown when the trip condition is sensed.

Class " B - Shutdown functions which become active after a predetermined time delay. Class "B' logic prevents the shutdown from becoming active until a specific time delay after the engine starts to crank.

Class "C" - Shutdown functions which become active once the initial setpoint is reached. Class "C" logic prevents the shutdown from becomi; active until the compressor is loaded. Once the Class 8'' device clears the shutdown becomes active. Class "C' logic is usually described as logic to "cock the shutdown once the device is initially cleared as the compressor is loaded.

X

X

Table 1. Shutdown/ A l d Indicator Location List

FUNCTION I LOCATION

X

X x2 x2

TAH-3047 UA-2 UX-2

X x2 x2

PDAL-6001 UA-2 UX-2

X x 2 x 2

LAL-7802 UA-2 UX-2

Item

10

11

I I I I

TAG#:TE-3047 Jacket Water Low Differential Pressure X B

TAG#: PDCL-6001 Jacket Water Tank Low Level X A

TAG#:LCL-7802

NO. IUnit/Service Ilndicator ]Control IAlarn /Shutdown IClass

4. Grouped signal only.

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Input/Output Lists

An input/output list is critical for defining the PLC requirements. Each item of the list needs to be clearly described including the function. The input/output (I/O) list should be established by including the vendor's specific safety requirements needed to protect his equipment. The list is also taken from process flow and com ressor/engine utility diagrams that are normally produced E r process packages. The final list should include the following:

Tag number or label of 1/0 device.

I/O device (i.e., pressure switch, level switch, speed switch, etc.)

1/0 device interface requirements (i.e., motor starter, AC control, etc.)

1.

2.

3.

Sequence Narrative

Although the 1/0 lists to a large degree define the functions of operation, the PLC programmer needs to understand significantly more about the com ressor package. A sequence narrative or description furnisEes a means to communicate the correct operation.

A sequence narrative for a reciprocating compressor provides initial information on the sequence of events required for each ste of the operation. The narrative explains the sequence Eom pre-lubrication through cooldown and postlube cycles.

A sample portion of a sequence narrative is:

5. Engine crank cvcle

A. When 30 second purge timer times out start crank cycle.

B. Energize "Ready to Crank " light in Remote control anel (output #123), "Ready to Crank' light in Local Cont!ol Panel (output #102) and "Ready to Crank signal to SCADA system (output #83).

C. If in "Auto" mode sequence will continue at ste D In the manual start mode the sequence wilfstop at this point. Start a 5 minute timer. If unit has not started within 5 minutes initiate a shutdown on "Sequence Failure" (Outputs #58,66, & 99). Push "crank push-button on local control panel to initiate crank.

D. Open discharge valve (SDV-5010) by energizing solenoid SDY-5010 (output #118).

E. Confirm discharge valve (SDV-5010) is open - limit switch ZCO-5010 (output #118).

From the narrative, the I/O list and Table 1; the information to establish the PLC logic is determined.

Logic Diagrams

Logic diagrams can be an important aid that relates the desired engine and compressor operation to engineering, maintenance, and operating personnel. The use of logic diagrams normally leads to the most basic approach to programming which saves rungs or steps of programming and gives the programmer more freedom in his approach. The programmer may find additional methods for improving the program if he is freed from creating the fundamental logic.

I 'I

Logical AND

Logic state which turns i ts output on when all --d AND f-

Logical OR Logic state which turns its output on when any incoming signal is positive

Logical invert

Logic state to -{NOT 1 change a positive

signal to negative and vice versa

A 4 Logical Latch

Logic state which latches to steady condition output when "A" is applied. Output continues regardless of state of "A" until reset by signal at "R". If both signals exist simultaneously the circled signal overrides.

-

NOTE:

Logic is positive. Signal is on when condition occurs. All discrete input contacts are closed during normal operations. All output solenoids or signals are normally energized during normal operation.

Figure 2. And/Or/Not Logic Symbols

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Figure 3. Logical Latch Symbol


SUCTION PRESSURE LOW

-+

- - D R

. COMPRESSOR

RESET -3 -

DISCHARGE PRESSURE LOW

I COMPRESSOR SHUTDOWN

Figure 4. Typical Class C Logic

The proliferation of logic diagrams, truth tables, sequence charts, flow charting along with different formats (i.e., NEMA, ISA, etc.) requires that lo ic symbology and format be established before the logic %velopment. The logic symbology used for our compressor packages is shown in Figures 2 and 3.

Figure 4 represents a logic diagram for a Class-C shutdown. Applying this logic to relay systems would have required a timer to prevent a relay race as the pressure switches cleared on rising pressure. Because the PLC lo ic must scan each rung of ladder logic prior to the next, $e logic as shown will function properly.

The remaining program logic included the alarms, sequence failure logic, grouping of alarms for indication, and general administrative tasks.

Care must be taken in the logic development when considerin shutdowns from and to existing shutdown systems o t the facility. When considered separately the logic can easily establish a continuous loop such that a shutdown in one system interlocks a shutdown in the other preventing reset of either system. Both systems must be considered during the early stages of logic development.

The most demanding logic devised was the cool-down cycle. Each step of the logic had to be studied to prove the equipment operating requirements for each action of the compressor cool-down. The cool-down cycle logic could not effect the normal startup and running logic. By looking at each step of the sequence logic the cool-down cycle was integrated into the total system logic.

Proaramming

A systems house was selected to furnish and pr the compressor package PLC. Their programmer u s x ? logic diagrams to develop the program and used techniques from his experience to add additional capabilities to the initial logic defined in the rogram. For example, programming a "CLOSE' lamp to &sh was used to show a valve closing from the open position.

Simulatioq

Simulation is a very valuable tool for debugging the PLC program and verifying the desired operation. Indicator lights are test connected to the PLC to repre5ent outputs such as solenoids, motors, and interface signals. Toggle switches are used to represent switch devices. For analog devices trip settings may be established and preset with signal generators.

The number and layout of the devices determines how accurately the simulation board represents the equipment. A simulation board with graphics representing the process along with the compressor and engine utilities creates a more interactive setting for simulating the compressor package operation.

Figure 5 is a block diagram depicting the correlation of the different types of logic used for the compressor packages. Logic was first developed for process shutdowns and alarms by class (Class A, B, or C). As illustrated in Figure 5, the operation of the yard valve solenoids and limit switches was created to correspond to the shutdown logic. For example, the suction valve could not be opened after a shutdown until a compressor reset occurred. The mechanical shutdowns were added and checked against the yard valve

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PROCESS AND MECHANICAL SHUTDOWN

LOGIC .-. - ^^ - - - .

YARD VALVE LOGIC

(SOLENOIDS AND LIMIT SWITCHES)

[CLASS A. tl. C.] L A L

\\ h//

SEQUENCE LOGIC

(INCLUDING COOLDOWN)

I ALARM AND MISC

Figure 5. Correlation of Different Logic

solenoids. The sequence logic was then developed to correspond to the shutdown and yard valve logic. Many iterations were required to verify the correct logic at each step of the sequence. The final sequence logic consisted of the following drawings:

1. Start thru Begin Prelube 2. Prelube thru Begin Purge 3. Purge thru Engine Ignition 4. Engine Ignition thru Compressor Loaded

An actual run test of the engine and compressor then completes the debugging and verifies the sequencing operation of the equipment. The run test of the engine may actually be performed before all the compressor package construction is complete. One feature of most PLCs is to "force" inputs and outputs to different states. Another feature allows for rapid changing of timer settings. These features mean the test can be carried out faster and without all the process inputs and outputs.

Checkout and Startup

Checkout and startu of the engine/compressor was also simplified 9 the use oPPLCS. During the checkout the outputs can be ' forced' to operate solenoids and outputs for field checks. Documentation available from the PLC printout cross references the signals. After startup the ladder

diagrams become the predominant method for trouble shooting. Problems can be analyzed faster because the ladder programs can be printed with cross-reference documentation. "Search functions provide quick access to input and output locations in the ladder diagrams on the CRT displays.

The CRT also allows display of information during start-up for quick analysis of problems. Displaying the various timers used for sequencing the starting and shutting down of the compressor package is another help to the start up team. As the equipment is started, the sequence status may be monitored. This information helps determine which equipment is operating correctiy or whether the system is functioning properly. Because knowledge of the key points of the startup sequence can be valuable for maintenance problem solving, sequence indicating lights were installed at the local compressor panel. Each light would show a certain part of the start up sequence was completed. When all lights were lit the compressor was loaded and the lights were reset.

Observations

Many other steps were taken in the automation development which were found to be impractical. For example, sequence ladder programming appeared to be a practical approach to reduce the logic development. Several different attempts were made to develop sequence function

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charts. Too many variables at each level of the sequence made that method impractical.

During the checkout and startup of the first compressor/package several programming changes were made to debug or enhance the operation of the unit. The original logic diagrams were then revised from the ladder diagram printout of the first unit for development of the other two packages.

Future development may be directed towards eliminating the pneumatic controllers by using electronic PID loops in the PLCs. Various manufacturer engine fuel su ply control systems may be desi ned into the PLCs by worEing with the engine vendors. dectronic governors are already available for engine control systems. As emission control requirements become more stringent more complex control schemes can be implemented using PLCs.

Conclusion

An approach taken to automate skid mounted reciprocating as compressor packa es has been presented using PLCs. %he automation inclujed start/stop and cool- down sequencing for remote unattended operation. The application as described sets the stage for future development such as load and recycle control.

References

111 J. Thibodeaux, “PLC‘s used for offshore production-platform control”; Oil and Gas Journal, July, 1988, PP 38-43.

12) American Petroleum Institute (API) Specification 11P, API SDec ification for Packaaed Rec iprocatina Comixessors for Oil pnd Gas Production Services, June, 1989.

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Automation of Reciprocating Gas Engine Compressor Packages Using

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