Compressor station design criteria

Pipeline & Gas Journal, June, 2007 by Saeid Mokhatab, Sidney P. Santos, Tony Cleveland

A principal component of any gas transmission system is the compressor station. Figure 1 shows a typical compressor station. A given system may have anywhere from a few stations up to well over 50. These stations add enough energy to the gas to overcome frictional losses and maintain required delivery pressures and flows. Compressor station design has been attracting attention because it plays a crucial role on the feasibility of gas pipeline projects. This article covers the general requirements to be implemented in design of compressor stations.

[FIGURE 1 OMITTED]

Process Description

A typical compressor station design, as shown in Figure 2, may consist of an inlet scrubber to collect liquids and slugs that may have formed in the gas gathering system pipeline. The scrubber consists of a primary section where liquids and solid parts are separated from the gas stream and a secondary section where oil mist is removed.

[FIGURE 2 OMITTED]

From the scrubber, the gas is taken to the compressor unit(s) where it is compressed. At the discharge point, or between compressor units in case of a series arrangement, the gas is cooled down, typically with an air cooler, and then it passes through a coalescer filter, in case of the reciprocating compressor, to remove lub oil mist prior to discharging the gas to the pipeline.

The liquids collected from the suction scrubber are handled a number of ways. A typical simplified approach is to route the liquids from the scrubber level control valves to a low pressure (LP) tank. The LP tank can be a pressure vessel operated at a relatively low pressure (atmospheric to ~25 psig) or it can be a simple industry standard "210 tank" (atmospheric tank with 210 bbls of capacity). In either case, the vapors produced from the flashing liquids are vented to the atmosphere or to a flare. The low pressure condensate is periodically trucked out and sold.

Compressor Station Facilities

Compressor stations may be small, situated on gathering lines or laterals, or large facilities on major trunkline transmission systems. All, however, are built up from the same functional blocks of equipment. Each functional element plays a role in the work of the station so the design and sizing of each is essential to the efficient and sale operation of the plant. The elements include gas scrubbing and liquid removal, compressor and driver units, after-coolers, pipes and valves, controls, data acquisition and recording equipment, venting, compressor buildings and weather protection, environmental controls and safety equipment.

Scrubbers

The gas in a main transmission line is nominally clean and dry while that in minor lines may contain loadings of liquids prior to processing, but in all cases there can be entrained liquids and particulates which have to be removed before compression. Efficient and safe handling of the liquids collected from the scrubbers in a compressor station is one of the keys to a good design. Poor handling of these liquids can be the major source of operating and maintenance problems and have a significant impact on the station economics.

The suction scrubber shall be equipped with a mesh-type mist elimination section to avoid liquid entrainment into the compressor. Scrubbers can take several forms, inertial with or without demister pads or the horizontal cyclonic type. The latter are commonly used on mainline transmission stations.

There are three main concerns that should be addressed in the liquid-handling design for any compressor station: safety, environmental impact, and economics. Another point of consideration should be operability, which includes hydrate formation, failure consequences, etc.

Compressors

In gas transmission, two basic types of compressors are used: reciprocating and centrifugal compressors. Reciprocating compressors are usually driven by either electric motors or gas engines, whereas centrifugal compressors use gas turbines or electric motors as drivers. The design philosophy for choosing a compressor should include the following considerations:

* Good efficiency over a wide range of operating conditions

* Maximum flexibility of configuration

* Low maintenance cost

* Low lifecycle cost

* Acceptable capital cost

* High availability

However, additional requirements and features will depend on each project and the specific experiences of the pipeline operator. In fact, compressor selection consists of the purchaser defining the operating parameters for which the machine will be designed. The "process design parameters" that specify a selection are: flow rate, gas composition, inlet pressure and temperature, outlet pressure, train arrangement, for centrifugal compressors: series, parallel, multiple bodies, multiple sections, intercooling, etc., for reciprocating compressors: number of cylinders, cooling, and, flow control strategy; and number of units (Akhtar, 2002).

In many cases, the decision whether to use a reciprocating or centrifugal compressor, as well as the type of driver, will already have been made based on operator strategy, emissions requirements, general lifecycle cost assumptions, etc. However, a hydraulic analysis should be made for each compressor selection to ensure the best choice. In fact, compressor selection can be made for an operating point that will be the most likely or most frequent operating point of the machine.