Woodward has commissioned an integrated turbine-compressor control (ITCC) system featuring a unique advanced control strategy that maximizes compressor availability (preventing trips) and minimizes energy consumption.

The Process
The recovery of natural gas liquids (NGL) from raw natural gas coming directly from the well goes through several stages of processing. During this process, raw natural gas is purified, dehydrated and finally cooled to liquefy the heavier hydrocarbons, yielding lean pipeline grade natural gas and residue gas. Frequently
a part of this residue gas is injected back into the gas wells.

As part of the purification process, propane refrigeration compressors (PRC) pre-cool the stream of raw natural gas before it enters a cryogenic turbo-expander for full NGL separation. Proper operation of the PRC is critical for effective removal of impurities and for maximizing NGL product yield, a major cost indicator for NGL recovery production. At the same time PRC operation offers challenges during the gas processing plant’s startup. Every hour of delays during plant startup may cost operators hundreds of thousands of dollars in lost production

The Control Challenge
A typical closed loop propane refrigerant system includes evaporative chillers, at least one single-case centrifugal compressor with multiple inlets, suction scrubber(s), economizer(s), anti-surge recycle valve(s), liquid refrigerant quench valve(s), desuperheater, condenser, and a liquid refrigerant letdown/level control valve. To protect the compressor from surge, each compressor stage is usually equipped with an anti-surge recycle valve (ASV). To keep the compressor loop from overheating, each compressor stage has a quench valve that injects liquid refrigerant into the stream of hot recycle gas.

Conventional compressor control systems have proven to fall short in providing fully automated control of multiple control elements in a manner that provides safe and energy-optimized operation. These control systems typically suffer from following limitations:

• They do not have adequate speed of response or effective anticipative control action to keep the compressor from entering surge, thus requir­ing larger surge safety operating margins and higher recycling rates.
• They use a fixed setpoint control scheme to con­trol suction temperature during startup or par­tial load operation. This results in control action windup and overflowing suction scrubbers with liquid refrigerant, causing high level trips and allowing the compressor to ingest liquids, which can damage equipment.
• Their general PID dynamic responses lack the ability to react to large prime mover load swings, leading to overload trips.

The Solution

Woodward-patented Rate PID anticipative control response effectively protects compressor from surge during fast transients, minimizes overshoots, and prevents gas flaring

Woodward’s control solution for propane refrigerant compression systems minimizes overall load of the prime mover during startup and prevents trips. This is accomplished by employing the following control strategies.

1. Powerful anti-surge Boost Response and patented “Rate PID” algorithms that facilitate effective predictive surge prevention and safe operation at the lowest pos­sible suction pressure – all with minimal surge safety margins and gas recycling rates.
2. Optimized flow rate of quenching refrigerant on each stage of
   compression by using an adaptive temperature control setpoint.
3. Optimized condenser temperature control reduces the overall requirement for quenching refrigerant.
4. Fully automatic coordinated distributed control of quench valves and anti-surge recycle valves ensures the thermal balance of the entire refrigerant loop.

On-Site Customer Challenges Solved Through Collaboration
In 2012 Woodward custom-engineered a complete NGL recovery compressor controls solutions on a total of 12 compressor trains and a PRC solution for 5 of those trains at two different sites for a customer in the Middle East. All control systems, including the PRC, underwent rigorous application functional testing in a dynamic simulation environment using Woodward’s NetSim™ controls emulation software. This simulation was tightly integrated with dynamic compressor models developed on Simulink® software.

In October 2015 two more PRC systems were commissioned for this customer. One PRC train was started up and ran many days on partial or no load. Its anti-surge and quench system minimized recycling and quenching rates, keeping the machine’s operation within safe limits and allowing the plant operators to focus on other equipment. The second PRC train, equipped with quench control had challenges staying under stable control until plant operators implemented the same control strategy  as on the first train. Woodward engineers collaborated with plant personnel to develop algorithms, which maximized compressor operating efficiency and minimized commissioning time.

New Opportunities Are Still Ahead
A more complex PRC control system is undergoing commissioning at the second site. This one was configured to operate two 3-stage electric motor-driven PRC trains in parallel with load sharing on each compression stage. The prime mover minimum loading control strategy was achieved via fully automatic coordinated modulation of a total of 9 control valves (inlet throttle, anti-surge recycle and quench), each affecting the load of the electric motor. Additionally, the control system provided for position setpoint control of the hydrodynamic coupling, control of startup bypass valves and surge control algorithm using split-range flow transmitters.

This article originally appeared in Woodward’s Industrial Controls News, April 2016.