AUTOMATION ENHANCES COALSEAM GAS-PLANT PERFORMANCE

July 1, 1991
Larry E. Anderson Meridian Oil Inc. Farmington, N.M. George C. Bozant The Foxboro Co. Houston R. Jan Carman The Foxboro Co. Foxboro, Mass. Craig Wentworth T. H. Russell Co. Tulsa Meridian Oil Inc. last year tripled the inlet capacity of its newly purchased Val Verde, N.M., gas plant and automated the three new process trains with an integrated distributed control system (DCS). Results have been reduced energy consumption and lower labor, material, and maintenance costs.
Larry E. Anderson
Meridian Oil Inc.
Farmington, N.M.
George C. Bozant
The Foxboro Co.
Houston
R. Jan Carman
The Foxboro Co.
Foxboro, Mass.
Craig Wentworth
T. H. Russell Co.
Tulsa

Meridian Oil Inc. last year tripled the inlet capacity of its newly purchased Val Verde, N.M., gas plant and automated the three new process trains with an integrated distributed control system (DCS).

Results have been reduced energy consumption and lower labor, material, and maintenance costs.

Specifically, improvements have been seen in ease and cost effectiveness of system expendability, implementation of advanced control techniques to solve difficult control problems, operator efficiency improvements through use of a four-bay cathode ray tube (CRT) graphic display system in the control room, and improved data collection and reporting techniques.

Plans call for automation of the next phase of plant expansion: Processing Train T7 to be installed this year, and automation of the original Process Trains T1, T2, and T3.

COALSEAM GAS FEED

Meridian's Val Verde plant is located in the San Juan basin about 1.5 miles from Bloomfield, N.M.

When Meridian, an oil and gas production subsidiary of Burlington Resources Inc., purchased the plant in May of 1990, three process trains were operating with a capacity of 130 MMscfd of raw gas.

Three new processing trains (T4, T5, and T6) added 300 MMscfd of plant processing capacity when they came on stream during 1990.

Raw gas processed at Val Verde is produced from San Juan basin coalseam gas wells and transported through the Val Verde gathering system (Fig. 1).

The raw gas is dehydrated in the production fields and enters the processing plant with the following typical composition: methane, 87-75%; water, < 7 lb/MMscf; CO2, up to 12%; and ethane, 0.25%.

Total plant processing capacity at the end of 1990 was 430 MMscfd.

Processed gas is pipelined to El Paso Natural Gas Co.'s (EPNG) Blanco plant near Bloomfield. EPNG is a transportation subsidiary of Burlington Resources.

Typical sales gas contains 98.72% methane, 1% CO2, and 0.28% ethane. Approximately 48.3 MMscfd Of CO2 are removed from the processed gas by the six processing trains.

When the Val Verde plant was purchased by Meridian, the existing processing trains (T1, T2, and T3) were controlled by local pneumatic controllers. There was no central control room or control panel arrangement, and the three process operators were required to walk the pipe racks to collect data and set controllers.

Meridian intended to expand the facility by immediately adding three processing trains. An overall scheme of plant automation was therefore considered necessary for reasons of cost reduction, ease of expendability, and operator safety and convenience.

Operation of the additional facilities would be accomplished with a small staff of experienced operators. A central control room was preferred to individual local controllers or local control panels to improve the overall operating strategy of the six processing trains.

Installation costs for a modern distributed control system (DCS) with remote input/output (1/0) modules are lower than those for a comparable pneumatic system. In addition, wide geographical location of the Val Verde plant facilities made central pneumatic control impractical.

A modern, computerbased DCS system was installed for control of Val Verde Processing Trains T4, T5, and T6. The Foxboro Intelligent Automation (I/A) Series DCS system met the requirements of both the engineering contractor, T. H. Russell Co., Tulsa, and Meridian.

Particularly important in the choice of the DCS system was the availability of multiplexed remote 1/0 modules which reduce the cost of field wiring and installation labor. Another important point in DCS selection was the requirement for interfacing with remote control and datamonitoring systems, particularly the gas production radio-telemetry system.

PLANT FACILITIES

The main function of the large Val Verde gas-processing facility is to remove CO2 from the raw gas stream and then dehydrate the gas to pipeline specifications.

The raw gas is dehydrated in the producing-field processing facilities to less than 7 lb of water/MMscf. During the process Of CO2 removal at the Val Verde plant, the gas becomes water saturated and must be dehydrated to 7 lb/MMscf, equivalent to a water dew point of 26 F.

These two basic processes are shown in Fig. 2, which shows the amine process that removes CO2 and the glycol process which dehydrates the processed gas.

The plant has been designed for high reliability with redundancy in critical paths to reduce downtime. Redundant critical components in each train include twin amine reboilers, multiple pumps with spares, and banks of aerial coolers which can be individually isolated.

A single piece of equipment can be quickly taken out of service for maintenance or replacement, resulting in minimum downtime and improved turndown ratios. A single failure has minimal effect on train throughput.

The plant processes are designed to operate in a cold environment, at ambient temperatures down to -20 F.

Each process train employs an amine solution for removal Of CO2 by absorption and chemical reaction. The amine solution consists of 35 wt % diethanolamine (DEA) and 65 wt % distilled water.

The lean-amine solution is fed into the top of the amine contactor and flows down the tower on a series of trays. The inlet gas flows up from the bottom of the tower and contacts the amine solution on the trays.

The trays are mechanically arranged for intimate contact between the inlet gas and the amine solution to provide sufficient time for the amine to absorb the CO2.

The treated gas, with most Of its CO2 removed, leaves the top of the contactor. The amine solution leaving the bottom of the contactor is now saturated with CO2 absorbed from the processed gas.

An amine regeneration process removes CO2 and methane from the rich amine, creating a lean-amine solution which is then air cooled, pumped to pressure, and returned to the contactor to treat more raw gas.

The glycol dehydration system operates in a manner similar to the amine system.

Triethylene glycol (TEG) is used to absorb water from the treated gas to bring the moisture content in the sales gas to a level acceptable for transport through the pipeline system.

Lean TEG enters the top of the glycol contactor and flows down across the trays to absorb water vapor from the treated gas. The rich TEG leaves the bottom of the contactor through a level-control valve. The sales gas leaves the top of the contactor and is delivered to the pipeline.

The rich TEG is passed to the glycol-regeneration process which removes the absorbed water. Lean TEG is pumped to pressure, air cooled, and returned to the glycol contactor to dehydrate more treated gas (Fig. 3).

PLANT AUTOMATION

The plant automation system (Fig. 4) consists of two separate groups of equipment:

  • Control-room equipment including operator interface graphics screens, pick and point devices, and printers.

  • Field-mounted multiplexed 1/O module assemblies.

The control-room area of the DCS system is shown in the upper portion of Fig. 4. The computer processing modules reside physically and electronically on a redundant node which communicates between modules using the IEEE Standard 802.3 communication specification.

These modules include a graphics-keyboard interface, serial device interface, file server, control processor, and foreign device gateway. These processor modules communicate with each other across the IEEE 802.3 node bus, and each module provides an intelligent link between the devices listed with it:

  • Graphics-keyboard interface: Graphics CRT screen, keyboard, alarm panel, and pick-and-point devices

  • Serial device interface: Printers and VT-100 terminals

  • File server. Bulk storage devices (hard disk, floppy disk, streaming tape)

  • Foreign device gateway: Scada systems, remote computer systems, any device with RS-232 physical connection

  • Control processor.- Link to remote field modules through RS-485 field bus.

The field-mounted, multiplexed 1/O assemblies are physically located near the skid-mounted processing trains. These assemblies consist of enclosures suitable for mounting in Class 1, Division 2 process areas which contain the electronic 1/O modules residing on the twoconductor, shielded field bus.

The individual intelligent field-bus modules provide the links to such field instruments and control devices as conventional 4-20 ma transmitters (pressure, differential pressure, temperature, flow), intelligent digital transmitters, process analyzers (chromatographs), switch closure outputs and inputs (live or dry contacts), and I/P converters.

Several important control-system issues affect the performance and profitability of the Val Verde plant.

FIELD WIRING

Conventional DCS systems require that each individual field instrument and control device be interconnected with the control room (or to a marshalling cabinet near the control room) by an individual cable, usually a shielded twisted pair of wires.

Depending on the complexity of the process train and its geographical relationship with the control room, the amount of wiring can reach huge proportions (many miles of cable and conduit). The cost of the wiring systems and installation labor and services generally represents a large portion of the start-up cost of a new distributed process control system.

As an example of cost reduction produced by field installation of multiplexed 1/0 modules, a recently completed gas plant automation project in Wyoming saved approximately 13% of the project budget compared with direct wiring of each instrument to the control room.

This DCS system was installed with five remote 1/0 module enclosures up to 300 ft from the control room. This savings represents the cost of materials and installation labor.

The DCS system chosen for the Val Verde plant automation project combined with the modular, skid-mounted process equipment offered the opportunity significantly to reduce the installation cost when compared with the installation cost of the traditional approach.

Fig. 5 is a simplified illustration of the control-system arrangement showing a fieldmounted intelligent 1/0 module cabinet located near the process skid. The instrumentation shown illustrates some of the instrument types found on a typical processing train in the Val Verde plant.

An orifice flow measuring station along with a gas analyzer is shown along with the locally mounted 1/0 module cabinet. This cabinet is connected to the control room by a redundant field bus.

The installation cost of the redundant field bus between the process skid and the control room is much less than the cost of each individual instrument cable being wired to the control room.

EXPANSION, IMPROVEMENTS

The basic architecture of the DCS system installed at Val Verde makes plant expansion technically simple and economically attractive.

The multiplexed 1/0 system allows easy expansion of the instrumentation on the process train skids. Once the system is installed, additional field instruments can be easily accommodated at minimal installation cost.

Extra space for additional field-bus modules is provided in the nearby field enclosures. The only wiring required is between the individual process instrument and the nearest field-module enclosure.

Changes to control schemes can be made quickly, on-line, without modification to existing hardware. Control schemes are implemented in system software, and software changes can be made quickly and efficiently.

Process changes and improvements can be made with minimal disruption and at low cost.

Meridian has implemented a simple statistical process control (SPC) scheme to compare the actual plant residue (tailgate) gas volume with the targeted gas volume to calculate the required plant throughput to meet the daily target.

The DCS is capable of running SPC software prepared by a programmer familiar with the inherent capabilities of the system. Meridian also plans to implement a fanmanagement system to control the 14 single and twospeed fans in the aerial coolers in each train.

INTERFACE; FLOW CALCULATIONS

Prior to the automation of the new process trains (T4, T5, and T6) at the Val Verde plant, the process operators were primarily accustomed to pneumatic single-loop controllers mounted locally at the process. Process-data recording and setpoint control were done at the local field instrument or control room.

The installation of the new DCS system in the control room presented these operators with a new approach to controlling the plant processes. The intuitive nature of the CRT graphics and the trackball interface, however, eased the transition to the new world of computer control.

The operator can run the new process units from any one of the four CRT screen consoles in the control room using a trackball to initiate commands (actuate push buttons), read data, or change setpoints.

This windowing environment with multiple-screen overlay capability presents an operator interface that was easily learned and accepted by these plant operators.

Critical gas-flow calculations at the Val Verde plant are made directly in the DCS system-control processor modules. The critical gas-flow orifice meters are:

  • Inlet gas flow-T1, T2, T3 (grouped together), T4, T5, and T6

  • Fuel gas flow-T1, T2, T3 (grouped together), T4, T5, and T6

  • Bypass gas flow (bypasses all process trains to trim CO2 content of sales gas)

  • Sales gas flow meters.

The flow rates are calculated from measurements of orifice-plate differential pressure, static pressure, and temperature with the calculation procedures described in the AGA-3 standard.

Flow rates are also adjusted for CO2 composition by chromatographic input being supplied to the AGA NX-19 calculation procedure. These calculations take place in the control processor modules located in the control room DCS system enclosures.

The calculated flow rates are displayed locally on the operator consoles and are used as both measurements and controlled variables in the various control schemes which operate the plant.

In addition, the calculated flow rates are transmitted by radio link to Meridian's Farmington regional office through the foreign device gateway processor and its associated modem and radio system.

This link is shown in Figs. 4 and 5.

There are several advantages in performing the flow calculations in the DCS system. The most obvious advantage is one of lower cost, in that there is no requirement for a field-mounted flow computer because the calculations are done in the control room.

The need for additional wiring to bring the three measurements (differential pressure, static pressure, and temperature) back to the control room is eliminated because of the multiplexed field 1/0 module enclosures located near the processing-train skid assemblies. This represents the lowest-cost solution for performance of the flow calculations.

Additional benefits include the reliability of performing flow calculations in the DCS system where the results are ultimately stored and presented to the operators.

INTERCONNECTIONS; CONTROL

Meridian has installed a remote monitoring system throughout the San Juan basin gas-production fields which transmits field-production data by radio telemetry to the Farmington regional office (Fig. 6). A complete description of this system can be found in Raybon, et al.1

Meridian determined that it would be an administrative advantage to have access to on-line process data from the Val Verde plant in the Farmington regional office.

With the radio telemetry link already in place and operating, a method was needed to interconnect the process DCS system with the remote telemetry system. In particular, transmitting the gas flow rates from each process train to the regional office was required.

The foreign-device gateway processor module which resides in the control room DCS system enclosure provides a convenient and economical interface to the remote telemetry system.

The gateway processor allows the remote telemetry system to transmit live process variables directly from the DCS system to the regional office for further analysis by a computer system. Although currently this is a one-way system, transmitting process data from the plant to the regional office, the DCS system gateway will accommodate bidirectional data transmission.

This will permit plant target throughput rates, which are determined by market demand, ultimately to be transmitted from a remote company office directly to the DCS system which will control the plant processes accordingly.

Several advanced control techniques have been implemented which have enhanced process performance compared with conventional control schemes.

In one instance, a feedforward control scheme was developed and implemented on the DCS system to control the flow of amine solution to the amine contactors based on the process-gas flow rates and compositions.

A second feed-forward control scheme provides improved energy conservation by adjusting the amine reboiler-fuel gas flow automatically with changes in the flow of amine through the contactor.

These techniques improve the fuel efficiency of the process.

A special algorithm is being developed on the DCS system to control the fans on the aerial coolers. Both high-low fan speeds and on-off cycles are controlled to maximize energy efficiency.

REFERENCE

  1. Raybon, Kenneth F., Flowers, Bill, and Johnson, Jim, "Telemetry and process instruments control coal gas production," OGJ, Nov. 12, 1990, pp. 98-103.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.