October 10, 2008

Gas Compression Engineering Guidelines To Consider

By Law, Charles W Jr

After gathering and treatment, natural gas iust be transported from its source to its destination via pipeline. Of all the oil and gas engineering exercises involved in moving product from the wellhead to market, gas compression engineering can be among the most challenging. There can be numerous pitfalls and details to be taken into account. If done diligently, however, this discipline can be very interesting and rewarding as well. Because of its low density in its natural state, gas needs to be compressed. In doing so, the volume of gas may decrease some 600 times, allowing for a buildup of pressure. During the transport of the gas, there is a decrease in pressure and flow rate due to frictional loss within the pipeline. A solution to this condition is the placement of an intermediate compressor station at intervals varying between 40 and 100 miles, removing water and other impediments and employing a compressor to boost the gas to the required delivery pressure and flow rates.

Station Design

There are numerous applications for natural gas compression, but this article will focus principally on the factors to consider in design of a compressor facility.

Compressor Types. There are many types of compressors, each with its own advantages and disadvantages. Because there is a significant selection of qualified manufacturers with competitively priced, reliable and field-proven compression equipment, the decision on choosing compression lies mainly with the engineering task that aims at achieving the project's overall objective. The most commonly used compressor types for gas transmission fall into two primary categories - centrifugal or reciprocating.

Centrifugal. The centrifugal compressor type relies on a single or multiple impellers as its main element. Its spinning creates velocity or kinetic energy which it then converts into pressure to accelerate the gas through the pipeline. Its pressure can be dictated by the impeller's speed and diameter. When engineering and specifying for this compressor, surge control and torsional and lateral characteristics are important subjects to be considered.

The centrifugal compressor is preferred for high flow applications with constant capacity and pressures, or feet of head. Its relatively small footprint allows for higher horsepower in a smaller package, translating to a lower installed cost and less weight per horsepower. The unit has a high efficiency and requires minimal maintenance with a dependable life.

The centrifugal unit can be driven by electric motor, gas engine or, more frequently, a gas turbine as its prime mover, utilizing line gas for fuel. When a turbine or large electric motor is to be the driver, careful consideration should be given to the layout in order to account for inlet and exhaust ducting as well as lubricating and cooling equipment. There must also be sufficient space to provide access for driver and compressor removal during overhauls. These details are particularly important for multi-unit installations.

Reciprocating. The reciprocating compressor (recip) is perhaps the most commonly used at pipeline stations. It uses positive displacement to mechanically decrease the volume of gas by compressing it to increase its pressure. Its valves on the suction side of the compressor open to allow gas intake. A piston forces the gas out through the discharge side, increasing its pressure as it does so.

The recip has the advantage of being flexible and of having high efficiency across a broad pressure range. It accomplishes its capacity control through the use of internal volume pockets in the cylinder, valve lifters for varying gas surges, external speed controls and recycle valves. At lower horsepower, it has a lower cost and is a dependable unit that is field repairable. The vast majority of installed packages are driven by natural-gas engines. If driven by an electric engine, consideration should be given to torsional forces and their coupling to the compressor unit, especially with a variable speed drive.

Critical design factors are the vibration and pulsation control necessary because of the reap's operating principle. A pulsation study should be incorporated into the specifications for reciprocating compressor packages above 500 hp. Similarly, attention should be given to vibration problems that can occur with increased run speeds. A skid and foundation study could provide the needed data to address this potential problem.

Sizing Considerations

Whether planning a grassroots compressor station, an upgrade, or an addition to increase capacity, in order to properly size the compressor, the originator needs to provide as much pertinent information as possible to the person doing the sizing and equipment selection. API data sheets or similar are perhaps the best references as they are widely used, easy to understand and readily accepted. Design information, at minimum, should include the following information:

Design Conditions including suction/inlet and discharge pressures; suction/inlet and discharge temperatures; desired design capacity and expected flow rate fluctuations; design tolerances, if allowed; and station manpower requirements.

Engineered Equipment including desired driver (electric motor, gas engine or turbine); client approved specifications; utilities available; and control requirements.

Quantity Required - unit capacity; sparing philosophy; and potential growth and expansion requirements.

Gas Properties - specific gravity, mole weight and components; higher and lower heating values; H2O, CO2 and H2S content; and liquid-handling facilities, tankage and pumping needs.

Station Site Conditions - elevation; ambient temperature and prevailing wind direction; environmental, permitting and governing regulations; remoteness and security concerns; noise and emission limitations; soil conditions; equipment spacing guidelines; power and utilities available; and traffic flow, parking and movement of personnel.

When assessing the design parameters that have been provided, it is important to determine their level of accuracy. Design capacities, in particular, need to be verified. In the past, it was common to add an operating margin and to overstate requirements. The result was often the oversizing of compressor capability, resulting in inefficiencies and higher costs. Technology for properly sizing compression and supporting equipment today is far more accurate and, with precise information entered, equipment sizing can more closely parallel actual performance.

It is also necessary to provide and use a full analysis of the natural gas, wherever possible. If production characteristics will vary, it is practical to include a range in the analysis. It is not prudent to totally rely on a specific gravity as it might not be reflective of the gas properties.

Even with specific gravity, horsepower requirements, pressures, discharge temperatures and other factors being known, if gas mole percentages deviate from what is expected those gas compositions can affect filtration, liquids handling equipment and after-cooling equipment sizing and selection. The discrepancies could be further compounded in multistage recip or multiple impeller centrifugal compressor configurations.

Inlet design temperatures can impact sizing, power and cooling requirements. If, for example, there is a wide variation in day and nighttime ambient temperatures, or wide seasonal variations, the gas cleaning and cooler design parameters will need to be carefully considered so that the proper equipment is selected.

Industry Specifications

Industry specifications can be useful in the early definition of a compressor. However, sole reliance on these standards can cause potential problems. They often contain "exceptions" that are noted in these standards. For example, there are certain sections of the API specifications that are specifically noted and require decisions to be made by an engineer to complete or modify the design. Applicability of the standards needs to be observed closely and approved by the owner/operator.

In the case of reciprocating compressors, there are two applicable standards, API Standard 618, "Reciprocating Compressors for Petroleum, Chemical and Gas Industry Services" and API Specification IIP "Specification for Packaged Reciprocating Compressors for oil & Gas Production Services."

API 618 deals with low to moderate speed compressors, typically in the 300-700 rpm range, whereas API IIP covers high speed compressors (900-1,800 rpm typically) used in field compression applications. Relying on the wrong specification can cause conflicts between engineers and vendors, resulting in wasted efforts and added costs. Standards are continually being updated based on new material test results and field data. Using an outdated standard could possibly cause the wrong material or component to be specified.

When issuing specifications to accompany a Request for Quotation (RFQ), care should be taken to include only the applicable criteria and governing standards that are relevant to the project. Pertinent data sheets necessary to define the compressor and its ancillary components are to be included, but non-applicable information can only cause confusion for the vendor and should not be part of the RFQ. Working With All

The three main parties with a stake in compressor station design - the owner/operator, the engineer and the equipment vendors have similar goals. They all want the project to be successful, and the correct equipment selections are integral to that success. Basic ingredients that can be useful to the process are early communication among all these stakeholders, the provision of sufficient and accurate information on which to base sizing decisions, a diligent and complete analysis of the gas properties, and the assembly of applicable data sheets and industry specifications.

Equally important are pre-determined stakeholder meetings along the way to review design progress, conduct process hazard analyses, review equipment and material fabrication schedules and prepare operating manuals. This joint effort, including knowledgeable input from the consuiting engineer, will help provide positive project results.

Turbine driven centrifugal compressor.

3-D model of station with large turbine driven centrifugal compressor

By Charles W. Law, Jr., P.E., Alliance Wood Group Engineering LP, Denver, CO

Author: Charles W. Law, Jr., P.E., is vice president in charge of the Onshore Business Unit for Alliance Engineering, based in the company 's Denver, CO office. He has more than 28 years of experience in the natural gas industry with expertise in project management, program management, asset optimization and field operations. He also works as the company's senior project manager for compression and pipeline projects. He is a graduate of Penn State and is a Registered Professional Engineer in Colorado. He is also a Certified Project Management Professional (PMP) and, since 2001, has been an instructor in the Factors of Compressor Station Design for both the Gas Machinery Research Council (GMRC) and Gas Technology Institute (GTI).

Copyright Oildom Publishing Company of Texas, Inc. Sep 2008

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