How Integrally Geared Technology Brings Greater Agility to NGL Recovery

About the author: Tushar Patel is a integration manager , CPC Pumps, Atlas Copco Gas and Process, a part of Atlas Copco’s Gas and Process Division. Contact him at tushar.patel@cpcpumps.com

To learn more about integrally geared compressor technology, visit atlascopco-gap.com

Considering how flexible today’s NGL recovery plants have to be to accommodate changing demand, it’s essential for propane refrigeration compressors to share that flexibility. Though oil-flooded screw compressors were once a welcome improvement over inline designs, an even better solution has emerged. Integral gear technology, which has been used successfully in the hydrocarbon and air separation industries for decades, offers maximum flexibility — and minimal CAPEX — due to the modularity, high efficiency and superior reliability that its standardized compressor design provides.

Up until the early 2000s, inline compression technology dominated the gas processing / NGL recovery industry. Typical applications included feed / inlet gas, residue / sales gas and refrigeration compressors, all custom-designed in typical plants. Once shale gas gained popularity in the U.S., however, standardization became the industry watchword.

Smaller plants (30 MMSCFD to 200 MMSCFD) sought to keep up with enormous — and growing — market demand by minimizing construction and delivery time while lowering capital expenditure (CAPEX). Inline technology proved to be a poor fit in this new landscape. It was too large for smaller modules, and long delivery times delayed construction, causing CAPEX to balloon. To accommodate the rapidly evolving shale gas market, a standardized solution for propane refrigeration compression was needed. Oil-flooded screw technology quickly emerged as the most flexible option.

Modular in nature and well suited to a wide range of refrigeration duties, the oil-flooded screw compressor is effective in a variety of situations: ethane recovery or rejection mode; warm or cold climates; and rich or lean gas conditions. The total refrigeration duty can be further served by splitting it between two (50 percent duty), three (33 percent duty) or four (25 percent duty) stages. Depending on the plant’s requirements and operating philosophy, this allows for anywhere from 10–100 percent turndown.

What’s more, oil-flooded screw compressors are easier to operate. And because they are based on the principle of positive displacement, they are less affected by fluctuations in mole weight, suction pressure or temperature. They operate the same, regardless of feed gas density. Yet oil-flooded screw technology has a notable drawback: it is difficult to prevent the refrigeration loop from becoming contaminated. Oil in the compression chamber can come in direct contact with the refrigerant if the oil filtration system downstream isn’t maintained properly. When this happens, the refrigerant fluid must be changed out, and the loop system must be cleaned before operation can resume. Not only is the process itself costly, but so is the loss of availability during remediation.

Though the oil-flooded screw compressor is still the most widely used in the industry, another option has been growing in popularity — and for good reason. Successfully used for decades in the downstream, chemical-petrochemical, fertilizer, power and air separation industries, the integrally geared centrifugal compressor (IGCs) has found a foothold in the upstream and midstream markets due to its high flexibility and low CAPEX. It is completely oil-free, which lowers operating costs by eliminating the need for oil changes and filter maintenance, as well as the risk of cross-contamination. The solution is completely standardized — including its aerodynamics — which speeds deployment and shrinks CAPEX. It also supports precision process control, allowing operators to run their plants more efficiently. The compact size of the IGC also makes it ideal for modular plant design, and its ability to accommodate side streams supports its role in flexible plant operations.

Standardized gas processing plants have a different set of technical and logistical requirements than larger facilities do. Quick deployment, scalability, redeployment capability and a fast response to market demand are always going to be the most important factors. For dew point control and NGL recovery, commercial refrigerant-grade propane is used in all types (and sizes) of refrigeration systems, including JT valves, cryoplants and mechanical refrigeration. The chiller temperature is usually kept at 35 to -42ºF, with the approach temperature of the cold box / heat exchanger at 5 to 7ºF. These parameters can vary based on the plant design and gas conditioning processes used to achieve 99+ percent propane recovery.

Standardized plants up to 200 MMSCFD are usually designed based on two stages of refrigeration cycle, with an economizer in between that operates relative to the pressure being used. Normally, the air-cooled condenser has a pressure drop of 10 psi. The demethanizer pressure and temperature are maintained for 99+ percent of propane recovery. (In some cases, ethane recovery / rejection mode is also offered.) Due to its unusual characteristics, shale gas becomes richer as the well matures. To allow for maximum LPG recovery, the gas processing plant has to be flexible enough to handle inevitable variations in feed gas conditions throughout the well’s lifecycle. One way to do this is to offer additional refrigeration duty via mechanical refrigeration, which allows for standardization of the expander-compressor, cold box, demethanizer, residue gas compressors and other components / modules.

A commercial refrigerant-grade propane refrigeration loop is designed to achieve anywhere from 5 MMBTU / hr to 60 MMBTU / hr for plants up to 200 MMSCFD. The chiller pressure ranges from atmospheric up to 50 psig, with temperatures between -42ºF and +30ºF. The air-cooled condenser duty is designed for operation between 85 and 120ºF, in cases of warmer conditions. Feed gas can vary in richness from 3.5 to 10 GPM / MSCFD. These variables change based on the NGL recovery process, of which there are several proprietary versions. The process can be any combination of valve expansion, turboexpansion and mechanical refrigeration. The right mix will strike an appropriate balance between the initial CAPEX and long-term efficiency savings. (Ethane recovery or rejection mode of operation might also figure into the decision.)

Though the application of the two-stage (section) propane refrigeration design may vary, as far as the compression technology is concerned, the specific processes are very similar. They employ a commercial refrigerant-grade propane with a molecular mass ranging from 42 to 46 kg/kmol. And for suction and discharge, the compressor pressure requirements vary in a band of 50 psi, with a total pressure ratio of approximately 12:16. Because the range of compression requirements is quite narrow, standardization of the IGC’s aerodynamic design is possible. Inside the IGC’s central gear box, a main bull gear drives several separate pinions. These pinions supply rotational power to compression stages that are paired sequentially. During the design process, the optimal speed for each pair of compression stages is determined by the rotor and the aerodynamic properties of the parts of the machine that compress the gas: the impeller, diffuser and volute.

This speed is then used to assign the gear ratio between the bull gear and the corresponding pinion for its stage group. As a result, impeller blade geometry and other aerodynamics do not need to be customized to each process, which helps reduce manufacturing time. This means delivery is quicker, commissioning is faster, and CAPEX is considerably reduced. As an added benefit, the improved aerodynamics that IGCs obtain at each stage further translate into higher per-stage pressure ratios. Thus, fewer compression stages are required to reach the target outlet pressure, resulting in lower material and manufacturing costs. Considering how inherently capital-intensive gas processing projects are, lower investment costs for the compressor can go a long way toward plant viability.
 

EFFICIENCY AS ENERGY SAVINGS: A CASE STUDY Through aerodynamic enhancements and stage optimizations, IGCs for propane refrigeration can achieve anywhere from 80–86 percent polytropic efficiency. Depending on the process design, this can result in total energy savings around 10–12 percent as compared to oil-flooded screw compressors. In one case, where an 80 percent polytropic efficiency was assumed, a three-stage (impeller / rotor) IGC for 17 MMBTU/Hr (5000 kW) chiller duty running at full availability saw a power savings of approximately 10 percent compared to a base oil-flooded screw compressor design. At the very competitive electricity price of $0.045 / kWh, this resulted in a cost savings of nearly $192,000 per year.

Adjustable process control is another benefit of IGC design. Through the use of variable diffuser guide vanes, precision control is achieved with comparatively lower power consumption, allowing the system to accommodate changing variables with ease. Let’s take a system that uses an air-cooled condenser, for example. When the ambient temperature rises, the refrigerant condensing temperature also rises, and pressure increases in the cycle. Similarly, when gas richness increases or the ethane rejection and recovery mode of operations is employed. In any of these cases, to keep cooling constant, the system requires increased flow at higher pressures. The variable diffuser guide vanes not only offer the flexibility to facilitate this adjustment, they also allow the compressor to start/operate at a higher suction pressure — up to 300 psia — under settle-out conditions on a hot day, all without venting (and wasting) the refrigerant fluid. In addition to operational savings, the benefit here is environmental protection.

The IGC’s small size, convenient skid frame and tight sealing technology builds on the CAPEX, efficiency and flexibility it offers. No compressor is more compact: a single skid measuring just 25’ x 8.5’ can accommodate a three-stage integrally geared compressor, motor, instrumentation and peripherals (such as lube oil, seal support and control systems). Installation is simple, as grouting is not required, and it easily fits into modular plant design for rapid startup and commissioning. In gas processing applications, IGCs are paired with a dry gas seal. The dry gas and floating carbon ring seals minimize process gas leakage and prevent oil from contaminating the compression chamber.

Though a single external-facing dry gas seal comes standard, using tandem dry gas seals can further minimize seal leakage, thus reinforcing machine reliability while reducing refrigerant leakage by up to 50 percent. Over the course of a year, that can equate to $15,000 saved in refrigerant top-ups. Finally, IGCs pair low maintenance requirements with high reliability — the highest, in fact. At 99.8 percent reliability and 99.7 percent availability, IGCs surpass both oil-flooded screw and reciprocating compressors. Their eight-year MTBF also tops the list. But not only do IGCs deliver more uptime; they also simplify servicing. The horizontally split gearbox casing is designed to make it easy to replace bearings and monitor the health of the gears and pistons.

It comes as no surprise that an increasing number of plants are making integrally geared centrifugal compressors part of their growth plan. The ICG is economically attractive from a CAPEX perspective, and it offers remarkable cost savings throughout the product lifecycle. It is quickly solidifying its reputation as a reliable and efficient compression solution that mirrors the agile and flexible nature of the gas processing landscape. As NGL recovery grows, so will the use of IGCs.