Reprinted from January 2016HYDROCARBON ENGINEERING

Gas and steam turbines have long been used in large parts of the world’s oil and gas industry. They drive the compressors and pumps that move oil and gas for hundreds if not thousands of miles through

pipelines. Similarly, they are the workhorses behind the processes in a wide range of refinery and petrochemical operations. While turbines in the field still excel in these types of applications, more and more of them are entering late phases in their lifecycles — with ever rising operating and maintenance costs.

A growing trendIn today’s oil and gas industry, operators of pipelines, as well as refineries and petrochemical plants, are reconsidering the economics and limitations of turbine-driven systems in favour of the many advantages of adjustable speed drive systems. The trend toward electrification is taking place throughout the industry.

In these all-electric systems, an adjustable speed drive, also known as a variable frequency drive or VFD, can be paired with a large electric motor with rated powers of up to 100 MW. This combination can meet most of the primary drive requirements in the oil and gas industry. Not only do variable speed drives

have higher efficiencies than turbines, they also provide a more precise process control along with a faster return on investment. Some applications may use transformers on the front end and gearboxes on the back end that couple the drive train to the compressor or pump. However, this is usually far less auxiliary equipment than a turbine drive system requires.

Assuming the availability of onsite electricity, the following are five reasons1 why adjustable speed drive systems can provide viable alternatives to gas and steam turbines in oil and gas applications:

Good economicsEach year sees a growing number of adjustable speed drive systems being used in the oil and gas industry. This is because the technology continues to prove itself in terms of practicality and economic advantages for compressor and pump applications, as illustrated in the case study.

Except for extremely large systems exceeding 100 MW, an adjustable speed drive system will have many economic advantages. These include lower capital investment costs, depending on the power and technology, and shorter delivery and installation times. Installation costs are also much lower because these systems are far simpler. Furthermore, most importantly, the total cost of ownership (TCO) can be reduced, thanks to lower operating expenses and lower requirements relating to maintenance, repairs and spare parts.

Better performanceAnother reason for the industry’s trend toward electrification is the fact that the performance of the core technologies associated with adjustable speed drive systems are continually

AN ELECTRIFYING ALTERNATIVETroy Salazar, Siemens Industry, Inc., USA, explains how integrated, adjustable speed drive systems can provide economical drive solutions for compressor and pipeline applications in the oil and gas industry.

Reprinted from January 2016 HYDROCARBON ENGINEERING

improving. These advancements continue to improve their business case as viable alternatives to turbines. They include:

n Higher efficiencies. n Smaller footprints. n More flexibility. n More intelligent operations. n Fewer moving parts. n Less maintenance. n Higher availability. n Higher reliability.

With these advancements, the advantages of fully electric adjustable speed drive systems have grown. Reliability, availability and maintenance, often collectively called ‘RAM’, have increased. For adjustable speed drive systems, the mean time between failure (MTBF) can be up to 10 years, while a gas

turbine’s MTBF is typically just five years, due to the higher number of moving parts. Steam turbines can be very reliable, with an MTBF of up to 20 years. However, given the RAM requirements of the technology, its reliability can be compromised by the MTBF ratings of its many auxiliary systems, as will be subsequently explained.

The availability of adjustable speed drive systems is higher than turbines because their maintenance requirements are much lower. They do not require regular overhauls with the associated downtimes. Gas turbines, as mentioned earlier, have more moving parts, increasing their mechanical complexity and in turn the number of spare parts required. In contrast, a fully electric adjustable speed drive system may use active magnetic bearings, which allow oil-free operation and eliminate a regular maintenance task.

A few years ago, a fully electric adjustable speed drive system was installed in a new LNG plant in Norway instead of a gas turbine. This was because the plant operating company wanted to achieve a higher degree of availability. A gas turbine would have meant 25 days of scheduled and unscheduled downtime annually for maintenance and repair. Because fully electric adjustable speed drive systems do not require as much maintenance or repairs, the LNG plant effectively gained a 7% productivity boost by selecting a fully electric, adjustable speed drive system over a gas turbine.

But that is not all. The maintenance demands of both gas and steam turbines can be even higher than the turbines themselves. This is because they are typically supported by auxiliaries, such as fuel systems, steam boilers, water treatment systems – all needing their own operational monitoring, including maintenance, repairs and spare parts. In comparison, an adjustable drive system does not need these systems. This lowers maintenance costs even further, and TCO, while boosting the utilisation and output (i.e. productivity) of a facility’s assets.

Energy efficiencyAs fully electric adjustable drive systems are typically more efficient than turbines, power demands can be much lower than turbines, thus reducing energy costs and a facility’s carbon footprint. The efficiency of a fully electric adjustable drive system by itself can exceed 90%, while that of turbines tops out at 35% (or 40% and higher for gas turbine models derived from aero engines).

Of course, if the adjustable drive system is connected to a grid, its overall efficiency depends on the source of the grid’s power. However, if a utility does not use fossil fuels, but instead generates power from hydroelectric, geothermal, wave or solar sources, this high energy efficiency will remain intact.

Should a local utility use fossil fuels, its efficiency combined with an adjustable drive system can drop to as low as 35%, which is comparable to turbines. That is due to the 40 - 45% thermal efficiency typical for these types of power plants. But in combined-cycle utility plants, where exhaust heat from a gas turbine powers a steam turbine, the resulting thermal efficiency of an adjustable drive solution can rise to above 50%.

It is worthwhile to note that one particular drain on the operating efficiency of gas turbines is high ambient temperatures, which does not impact a fully electric variable speed drive system. This is because the density of the air drawn into a gas turbine falls as its temperature rises, meaning that less

Case study: Electrification of a refinery’s alkylation process replaces steam turbineTo replace an existing steam turbine used to drive a large alkylation process compressor, a US refinery commissioned Siemens to design, build, test, install and commission a high speed, 3750 kW motor with an adjustable speed drive.

The refinery’s business case was based on a two year return on investment (ROI). This reflected expected cost savings from higher reliability and resulting availability, fewer disruptions from maintenance downtime, reduced maintenance costs and improved process efficiencies.

The motor was designed according to API 541 specifications, with a speed range of 3800 - 5300 rpm. The refinery’s compression application needed a constant torque between 3800 - 5150 rpm and constant power between 5150 - 5300 rpm. Two duty points were also required at 1000 and 2400 rpm.

A solid rotor was used to minimise vibration levels, along with optimised 4-lobe oil-film bearings, which provided the best rotor dynamics over 3 and 2-lobe bearings as well as tilt-pad bearings. Ultimately the motor required no combined structural resonance points in its specified operating speed ranges.

To further minimise vibration levels, extremely tight tolerances were required, and precise component machining plus near perfect alignment of the stator, frame and bearing housing elements.

The specifications called for a totally enclosed, air to air cooling system, provided by a four-sided ventilation arrangement that routes cool air to the centre part of the stator.

Before shipping the system, factory acceptance testing was carried out as specified by API 541 and all tests were completed within specifications.

Installation and commissioning went smoothly. The refinery successfully replaced its steam turbine with a high speed, induction motor driven by an adjustable speed drive. After the expected two year ROI, the facility will continue to accrue cost savings as described in this article, while achieving much more precise process control and monitoring for high process transparency.

Reprinted from January 2016HYDROCARBON ENGINEERING

oxygen is available in the combustion chamber. Turbine efficiency also declines under partial load conditions.

Precise load matchingAnother important distinction is that adjustable speed drives can be specifically designed to match driven load requirements. Turbines have fixed power and speed values, which force compressor solution designers to use those values as given criteria. As mentioned above, turbine efficiency decreases under partial load conditions.

In contrast, an adjustable speed drive system can be designed and engineered to match specific drive loads. This gives EPC and OEM solution providers more design freedom in addressing speed and process control challenges, ultimately giving operating companies a higher degree of precision in managing these parameters.

Environment, health and safetyFully electric adjustable speed drive systems inherently provide several environmental, health and safety advantages that are worth noting.

First, an adjustable speed drive system by itself has no CO2 or NOX emissions. Its overall carbon footprint is limited to the power it draws from the grid and the type of fuel the utility uses.

Second, a fully electric adjustable speed drive system is far quieter than a turbine drive. This improves the work environment for workers in the vicinity as well as a pump station’s compliance with noise abatement regulations in urban areas. In some cases, suburban sprawl may have enveloped a pump station in recent years, bringing on noise abatement requirements that did not exist when the facility was first built decades ago.

Third, fully electric adjustable speed drive systems also lack any fuel-fired equipment and, in the case of steam turbines, boilers. The latter has inherent safety hazards, including the fuel itself, whether stored onsite or piped in, if natural gas. With an adjustable speed drive system, operating companies can reduce their risk profile and potentially save on insurance costs.

Table 1 lists the many advantages of using an adjustable speed drive system in compressor and pumping applications, compared with gas and steam turbines.

ConclusionThe trend in the oil and gas industry to use electric drives for compressor and pump applications will keep growing, as adjustable speed drive solutions are increasingly

replacing gas and steam turbines. The momentum behind this trend is the long list of economic advantages of adjustable speed drive systems. Simply put, the business case has become too compelling to ignore. Developers and operating companies using adjustable speed drive-based solutions are also finding that sole sourcing drive train components can provide additional advantages. These include shorter project execution time, costs and risks. They also can have greater confidence that when the solution components are installed, they all work as designed and engineered.

References1. 'Variable Speed Drive Systems: Economical Turbine Alternatives for Oil

& Gas Applications'. Siemens White Paper. 2014.2. IBID.

Table 1. Summary comparisons of gas/steam turbines versus fully electric adjustable speed drive systems2

Comparative attribute

All-electric adjustable speed drive system

Gas turbine (GT) Steam turbine (ST)

Capex Lower (but similar in very large systems)

Higher (but similar in very large systems)

Higher (but similar in very large systems)

Delivery time Shorter, <1 year Longer, >1.5 years Longer, >1.5 years

Installation Lower cost, faster Higher cost, longer Higher cost, longer

Opex Less More Much more

Reliability (MTBF hours)

Up to 10 years Up to 5 years Up to 20 years

Availability  Higher than GTs and STs, as overhauls are not needed

Lower, as frequent overhauls are needed

Lower (but less frequent overhauls needed than GTs)

Applicability (power)

Available up to 100 MW

Limited availability in small powers, <5 MW

Available up to 100+ MW

Applicability (speed)

Flexible, as high as 15 000 rpm; easy to match load speeds

Some GTs lack alignment with driven-load speeds

Some STs lack alignment with driven-load speeds

Speed control Faster response than GTs and STs

Slower response Slower response

Maintenance costs

Less periodic maintenance required

Much higher than VSDS

Much higher than VSDS

Adjustable speed capability

Broad speed range, <50 - 105%

Small speed range, 70 - 105% (aero derivatives)

Small speed range, 80 - 105%

Footprint and weight

Comparable to GTs Comparable to VSDS Much larger than GTs and adjustable drive systems, due to auxiliary systems

Emissions No emissions High emissions, with levels depending on fuel types

High emissions, with levels depending on fuel used in steam boiler

Noise Less (but similar to GT/STs in gearbox systems)

Higher Higher

Starting No extra equipment needed and fast to start

Heavy duty units need large starter motor or adjustable speed drive

No additional equipment required, but slow to start

Efficiency High, >90% (but overall system efficiency depends on power source, 35 - 55%)

Low - Heavy duty, 28 - 35%- Aero derivative, 35 - 42%

Low, 28 - 35%