Fundamental plant design for acid gas compression in cold climates

In North America, the continued depletion of conventional sweet light crude and gas sources is pushing local producers to increase their ability to process an increasing proportion of sour crude and gas for the transport sector.

This increase in sour production, coupled with ever increasing environmental and public scrutiny, is affecting the profit margins of most small, and many large, producers, forcing them to seek cost-effective acid disposal methods that are both environmentally responsible and safe for their operational teams. Typically, the most commercially-viable method, is proving to be re-injection into the formation for sequestration or EOR.

Burckhardt Compression, a Swiss-based company, along with its partners and suppliers, has been working with several large oil producers in northern Canada to develop and execute safe acid gas disposal solutions for their operations.

The most important aspect of acid gas compression is the provision of complete and continued gas containment. This article will discuss three major aspects that have allowed Burckhardt Compression to achieve safe and reliable operation for its clients.

Overall, continued long-term gas containment is the result of careful and detailed process and compressor design. Catastrophic loss of containment and poor decontamination prior to service are the primary causes of workplace related incidents involving acid gas compressor systems. In order to mitigate the risk of such events, Burckhardt Compression has worked, with others, to develop process and piping principles that are proving to be very effective in the field.

The most significant factor is an effective process/compressor design to evenly balance the compressor system; minimize energy and utility consumption; provide automated gas purge systems; and prevent excess pulsation/vibration in the entire plant.

A large component of an acid gas compression system is the dehydration of the gas, to a safe moisture content level, prior to the pipeline entrance, to prevent long-term corrosion and loss of containment. This is most effectively achieved by selecting the appropriate interstage pressure for the selected dehydration process to be applied with minimal energy input. This interstage pressure should correspond closely, on the gas phase envelope, with the natural minimum moisture content of the specific gas stream.

Once this point has been selected, the remaining stages of compression and interstage pressures should be selected to optimally balance the free forces and moments of the compressor. This, along with the following specific plant design practices, will help mitigate the probability of piping experiencing fatigue failure:

• Minimization of compressor rpm
• Minimization of piping penetrations without sacrificing plant monitoring
• Minimization of the use of small bore piping
• Minimization of vibrating free-swinging “pendulums” in the instrumentation and piping practices through the use of studded outlet flanges.

The last, and most critical, portion of the process design is the proper implementation of an automated sweet gas plant purge system to be used to sweep the compressor plant, and the associated piping, free of acid gas. It must purge all separator vessels, of all produced water/condensate, and reduce acid gas concentration levels, within the entire isolatable compression system, to below hazardous exposure limits. Burckhardt Compression typically recommends 30 min. of continuous acid gas (H2S & CO2 combined) concentration levels of less than 1 ppm. The effective implementation of these process design practices aids in the ergonomic design that allows for continuous, safe and long-term operation and maintenance of the plant.

Burckhardt Compression has developed modularized plant layouts for all of its clients’ operations, Fig. 1. However, due to the hazardous environment that acid gas can potentially create, overall plant layout is extremely important to minimize periods of potential exposure for operational and maintenance staff. This is accomplished using a multi-module design that allows for easy access/regress and adequate clearances to complete maintenance. As a general rule, all walkways must be 1.5 m x 2.2 m with a minimum of two, or, preferably, three access/regress options to minimize confined spaces.

Fig. 1. A sample plant design.

Care must also be taken to place all operational valves and instrumentation in such a way that they do not hang into planned access/regress paths and that they are easily accessible for operation and service. Safety requirements, typically, limit personal access to acid gas compressor buildings during operation, and, as such, instrumentation needs to accommodate this requirement.

It is becoming industry standard that all instrumentation have transmission functions and local displays located on a single indicator board at, or near, the entrance to the compressor building. This allows monitoring to be completed very near to the entrance, to minimize personal residence time in the compressor plant. Some operational teams have even elected to have this local indicator board located near to, but outside, the compressor building so that operators can still hear the compressor but are not within the confined HVAC space of the compressor. These special requirements are defined on a plant-specific basis.

The climatic conditions of Northern Canada increase the special process design requirements that must be applied to provide continuous, safe operation of acid gas compressors. Annual temperatures swings can be in excess of 80°C, which must be accounted for when designing cooling and condensate systems.

The standard ambient design temperatures, for the interstage and compressor cooling systems, are between -45°C and +40°C. Due to the location remoteness and ambient temperature swings, air coolers are, typically, selected, as they can, if designed properly, handle the entire temperature range.

Therefore, coils are sized for +40°C and the coolers are outfitted with 100% recirculation plenums, insulation and heaters to accommodate the -45°C winter design constraint. The controls that handle this are carefully arranged to provide adequate ambient temperature compensation without set-point hunting.

Combined with the cooling system, the condensate knockout system requires special attention due to the ambient conditions and specific properties of acid gas.

To prevent gas condensation and hydrate formation, all piping downstream of the coolers must be heat traced and insulated. The most critical portion of this is the condensate drainage lines, which, due to the pressure drop of the control valves, can potentially reach temperatures of -100°C, or lower. To prevent these extreme temperature drops, a cascade-type drainage system must be used. It functions by limiting the pressure drop in each drainage line and allows for re-vaporization of any entrained gases into the compression stream. This has the added effect of minimizing the acid concentration, in the condensate stream, that needs to be processed and disposed of.

These system designs—as well as a specialized distance piece sealing arrangement, which sends all leak gases back to the plants overall gas sweetening process—has allowed Burckhardt Compression to supply its clients with a transient emission free acid gas compression system.