T here are many variables to be taken into consideration when sizing and selecting compression solutions for downstream applications. From the specific duty, to

power consumption, to gas composition, every process is unique and therefore should be individually evaluated to minimise compressor lifecycle costs and ensure that operator and facility requirements are met in regard to availability, maintenance, efficiency, footprint, etc.

As is often seen in other industries and disciplines, there are certain ‘rules of thumb’ or broadly accurate guidelines that are sometimes used to aid in the compressor selection process. One, which will be the focus of this article, relates to polytropic head, and more specifically the level of head (or pressure ratio) that can be achieved in a single compressor casing and even a single stage.

When specifying compressors for service with common gases, such as air, CO2, or methane,

Harry Miller and James Sorokes, Siemens, USA, explain why generalised rules of thumb should be avoided when selecting equipment to compress lower molecular weight gases.

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generally accepted figures for the maximum level of head that can be achieved per stage fall in the range of 7000 – 14 000 foot-pounds of work per pound of gas (ft-lbf/lbm). It is important for operators to understand, however, that these numbers are highly dependent on the specific composition of the gas being compressed. As a result, universally applying them across compressor applications can potentially lead to the selection and installation of less than optimal solutions.

As will be discussed in this article, when compressing lower molecular weight gases, such as

Figure 1. The DATUM compressor for this petroleum refiner features nine stages contained within a single casing fitted with a single inlet nozzle and a single discharge nozzle to accommodate the hydrogen gas flow.

Figure 2. This DATUM D16R9S rotor shows all nine impellers resting in the lower half of the stationary flow path. When completed, the entire assembly is mounted inside the outer pressure containing casing shown in Figure 1.

hydrogen, much higher head levels per stage and casing can be achieved, enabling end-users to reduce compressor package footprint, installation requirements, and associated costs.

Compression basicsBefore delving into the technical aspects of compressor design and selection, it is important to first define certain terms. Three terms used throughout this article that are key to understanding compression basics are ‘casing’, ‘stage’, and ‘polytropic head’.

A compressor casing (sometimes referred to as a compressor ‘body’) is defined as a pressure-containing vessel that encases one or more compressor section. The casing includes multiple main process piping nozzles, along with shaft connections for power supply. A single compressor casing typically contains multiple stages, each of which consists of an inlet section, impeller and discharge section, return passage, and discharge volute.

Polytropic head is defined as the work or energy imparted to raise the gas from its suction pressure to the desired discharge pressure. Head is expressed as ft-lbf/lbm.

When designing a compressor solution for a given application, a multitude of variables dictate the number of casings and stages required to achieve the desired pressure ratio. These include, but are not limited to, parameters such as the molecular weight of the gas, suction pressure, temperature, discharge pressure, volumetric flowrate, and operating speed.

Operating speed is an important consideration because the head level and pressure ratio that a compressor or stage produces are proportional to the speed squared. Two of the factors that limit operating speed are the impeller stress levels and the Machine Mach number. These two are the primary factors that limit the head level that a machine can attain.

Molecular weight is important because the gas sonic velocity is inversely proportional to the molecular weight and the Mach numbers are inversely proportional to the sonic velocity. Therefore, as the molecular weight of the compressed gas decreases, the sonic velocity increases and the Mach number decreases. This enables impellers to run at a higher speed without exceeding accepted Mach number limits, thereby allowing the compressor to generate a higher head.

As previously mentioned, many published pieces of compression-related literature state general rules of thumb for determining the number of casings and stages required for a given compressor application. A range of 7000 – 14 000 ft-lbf/lbm is often provided as a rough estimation for the amount of work that can

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be achieved in a single stage. It is important to note that these figures generally correlate with commonly compressed gases, such as air, methane, and CO2. When using a compressor for service with a low molecular weight gas, such as hydrogen, the general rules of thumb do not apply. In such cases, the design of the compressor is not dictated by aerodynamic limits because of the extremely high gas sonic velocities associated with such gases. Instead, the speed and, therefore, the head level is constrained by the mechanical strength limits of the impellers.

Mechanical strength limits of the impellers are directly correlated with tip speed. The maximum allowable impeller tip speed varies depending on the specific material used and the geometry of the impeller. These material strength limitations typically are not a concern when designing compressors for service with higher molecular weight gases because the Mach numbers limit the operating speed. However, in the case of low weight gas compositions with very high sonic velocities, the mechanical strength and impeller stress levels become the limiting factors.

Case studyIn 2016, a major US petroleum refiner selected Siemens to supply a centrifugal compressor for hydrogen recycle service at its Gulf Coast refinery. The project is part of the customer’s goal of integrating two of its refineries to create one of the premiere refining complexes in the world.

The compressor, a Siemens’ Dresser-Rand DATUM D16R9S (Figure 1), will be powered by a 16 000 hp TECO Westinghouse motor in a hydro-treating unit. The compressor package itself was based on a well-proven design, with no special modifications. The unit is used for hydrodesulfurisation, which is a common catalytic chemical process used in refining applications to remove sulfur from natural gas and refined petroleum products. The hydro-treating unit will play a key role in helping the customer meet EPA Tier 3 sulfur regulations, which mandate that refined gasoline contains no more than 10 ppm of sulfur on an annual average basis and no more than 80 ppm on a per-gallon basis.1

The DATUM compressor will be used in a straight-through configuration. Its design features nine stages contained within a single casing fitted with a single inlet nozzle and a single discharge nozzle to accommodate the hydrogen gas flow (Figure 2).

The compressor package and associated hydro-treating unit are set to become operational in 2021. In preliminary performance testing at Siemens’ manufacturing facility in Olean, New York, US, the compressor achieved a polytropic head of 169 954 ft-lbf/lbm (~18 880 ft-lbf/lbm per stage) – the highest level ever recorded with a DATUM unit.

Compressor testingThe D16R9S compressor was performance tested in accordance with the ASME PTC 10-1997 Type 2 test guidelines. The compressor was tested using CO2 for the test gas medium at a target suction temperature of 100°F, 64 psia suction pressure and a volume reduction speed of 2343 RPM. This test corresponded to the Case 3 EOR Rated certified operating condition. Five data points were observed between overload capacity and minimum stable flow, along with two unsettled off-speed surge points. The balance drum seal leakage was measured during the test.

The as-tested overall performance of the compressor was larger in capacity than predicted; however, it closely matched the predicted curve shape (Figure 3). At the Case 3 EOR Rated certified operating capacity, the polytropic head was 2.6% higher than predicted. The efficiency was slightly higher as well. At the inlet pressure, the polytropic head was 169 954 ft-lbf/lbm vs the quoted value of 167 586 – a 1.4% increase, which is within the

American Petroleum Institute (API) limit of +5.0%.The slightly higher polytropic head achieved

during the test was attributable to the constant-speed

Figure 4. A single-casing design, like the one on the far left, requires fewer spare parts, takes up less space and saves on the cost of purchasing two trains.

Figure 3. Comparison of test (symbols) vs predicted performance (solid lines).

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application. A conservative design approach is often required in such cases, as there is no mechanism for adjusting the speed of the compressor. Designing a compressor that could reliably achieve the required head level within a margin of safety ensures that no major modifications have to be made to the compressor on the test stand. In many cases, particularly those in which delivery of the compressor package is part of the schedule’s critical path, modifications can result in loss of time, increased costs, and liquidated damages as a result of project delays.

Advantages of single-casing designsCompressor performance requirements and inlet conditions vary greatly from application to application, and while every project should be approached uniquely, many benefits can be realised by having the capability to generate extremely high head levels in a single-casing compressor design (Figure 4).

One significant advantage is that the overall footprint of the compressor package is reduced, which simplifies logistics and transportation to the site and potentially reduces the amount of manpower and time required for installation. The smaller footprint also allows for increased design flexibility in the plant layout. Additionally, because single casing designs

often contain fewer components than multi-casing designs (i.e. couplings, bearings, seals), operators can reduce their spare parts inventory, which equates to lower operating cost and increased return on investment.

ConclusionIn conclusion, generalised rules of thumb can be used as a guide in the selection of rotating equipment such as gas compressors, but in some instances they can lead to overly conservative and less than optimal solutions.

In many cases, published figures for head levels that are achievable in a single stage are only applicable in instances where the compressor will be used for service with gases, such as air or methane. When compressing lower molecular weight gases, such as hydrogen, much higher head levels per stage can be achieved, enabling end-users to reduce compressor package footprint, installation requirements, and associated costs.

Reference1. ‘Overview of EPA’s Tier 3 Gasoline Sulfur Regulations – 40CFR

Part 80, Subparts D, E, H and O’, US Environmental Protection Agency (EPA), (September 2016).