Gas compressors play an important role in Solar Turbines’ effort to provide solutions to the oil and gas industry. Solar Turbines has made significant advances in the use of modular design in modern multi-stage compressors.

Solar’s compressors are optimised for hydrocarbon compression under the conditions encountered in offshore or onshore upstream compression scenarios, or for transmission pipelines. These compressors are also specifically designed to match the power output and power turbine speed of Solar’s gas turbines, but they also are well suited for electric motor drives.

Solar’s compressors use rotors of modular design. Rather than using a solid shaft with interference fit impellers, a modular rotor assembly consists of stub shafts, impellers and, if required, rotor spacers with the assembly held together by a centre bolt. This easy-to-assemble modular design is successfully used in most gas turbines. The API 617-compliant design yields a stiff shaft, which results in favourable rotordynamic characteristics, and allows for solid diaphragms, avoiding interstage leakage problems. In addition, pre-determined rotor spans eliminate most of the concerns regarding the rotordynamic performance, because the rotordynamic behavior is known from previously built and tested, mechanically identical compressors.

The primary design goal in rotordynamics is to ensure that the compressor is stable under the operating conditions the customer requires. A benchmark is defined by meeting or exceeding API 617 requirements for centrifugal gas compressors. The following requirements were established in the conceptual design phase in order to achieve this goal:

• Insensitive to unbalance;
• Operating below bending mode;
• Critically damped lower modes; and,
• High stability margin.

In particular the capability to provide a rotor with critically damped lower modes can be of great importance if the compressor has to be operated at far lower speeds than originally specified.

The aerodynamic design of the compressor makes use of pre-designed, pre-engineered, and pre-tested components, rather than customised stages. Modern computational tools, together with expanded test capabilities, have been used to design compressor stages with high efficiency and a wide operating range by carefully optimising each individual impeller and diffuser, together with the inlet system and the discharge volute.

Pressure capability

Solar’s compressors continue to gain acceptance at higher discharge pressure levels with compressors operating successfully at discharge pressures in the 3,000–4,000 psi range. This increase in pressure capability was a conscious process to gain increased operating experience under increasingly challenging operating conditions. The process was complemented by continuous improvement in dry gas seal technology, and improvements in the rotordynamic modelling capability. The primary rotordynamic considerations are the vibration level within the operating speed range and the location of the resonant frequencies. The compressor rotor should be insensitive to unbalance throughout the operating speed range. Inevitably, rotors will have residual unbalance that appears as vibration during operation. The rotor system should be able to accommodate the API residual unbalance limit without exceeding the vibration limit.

Major advancements in recent years come from improvements in the areas of bearing/rotor interaction, seal/rotor interaction, and modelling of bearings. The amplitude at the suction side X-Y probes for the speed range of a compressor is the result of an unbalance sensitivity test. The different modelling techniques of a journal bearing can affect bearing stiffness and damping characteristics and, thus, the overall rotor vibration. An 80/20 mixing model (i.e., 80 per cent hot oil from the previous pad mixed with cooler fresh oil at the inlet of the next pad) leads to better correlation of vibration level at higher speeds.

In addition to seeking better correlation in rotor response, Solar has also worked closely with a bearing manufacturer to develop the deflection pad bearings that helped to improve the stability of several older compressors, enhancing their operational capabilities. These deflection pad bearings are available for several older models and can be retrofitted if necessary.

Proven tools and a large number of tested configurations combined with pre-engineered rotor systems result in high confidence in predictions even during the project inquiry stage. For example, the rotor of each compressor with a common designator has identical seals, bearings and bearing spans. Therefore, all the experience data accumulated with a compressor of that designator applies to all new projects.

Advancing technology

The tools described in the previous section are a means to modify existing compressors or generate new products that add value to customer operations. One challenge in the aerodynamic design of multi-stage compressors is to have impellers that generate as much head as possible without sacrificing efficiency and operating range, while keeping a short axial length. Meeting these requirements allows the development of a compressor with high head-making capability for a given rotor length. For a given impeller tip velocity, the head per stage can be increased by reducing the impeller back sweep. Unfortunately, reducing the back sweep can lead to a reduction in operating range and efficiency. By using the models provided in the 1D design codes, and further improving the design using computational fluid dynamics (CFD) codes, it was possible to design a compressor stage that met the requirements.

Solar has avoided the use of vaned diffusers in either pipeline compressors or multi-stage machines. This decision is mainly driven by the desire to create machines with the widest possible operating range. Field experience shows that most compressors in the field are operated under changing conditions, often far away from the original design points and speeds.

Another key point is the attention to detail. Not only do the impeller and the diffuser contribute to the performance of a compressor, carefully designed and well-integrated inlet systems, exit systems and volutes do as well. Detailed test data is compared with CFD results, which constantly improves the accuracy of the computational predictions.

Using state-of-the-art compressor design tools and aerodynamic testing has given Solar the opportunity to review its existing compressor designs. As a result, existing designs for compressors, designed in the 1970s, can be improved regarding their structural strength as well as their aerodynamic performance. The new impellers for these compressors are interchangeable with the existing impellers and provide the same head.

Compressor restage

Compressor operating conditions often change from year-to-year for a variety of reasons. When the conditions change enough from the original design point and move close to choke, surge, reduced speed, or maximum speed, replacing the aerodynamic components of a compressor can restore efficiency, increase surge margin, and increase the flow or head capability. The design of Solar’s compressors, with pre-engineered components and a modular rotor, lends itself to easier restaging either in an overhaul facility or in the field.

Lean manufacturing

A key requirement is the capability to produce compressor packages within a short lead time to meet project timelines, while providing products of the highest quality.

In addition to the investments in people, technology, and facilities, Solar has also implemented lean manufacturing processes in the factory. Implementing lean methodologies has enabled Solar to respond to customers’ special requirements in less time. Lean manufacturing methods focus on replenishment of product as it is consumed from buffers, sometimes called ‘supermarkets’, strategically placed within the process. This is sometimes called a ‘pull system’. The consequence is greater efficiency, less work in process, and ultimately higher velocity, resulting in a more satisfied customer. These benefits help Solar to be the supplier of choice when the customer’s schedule is a concern.