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Compressors/drivers

Topic last reviewed: 10 April 2013

Sectors: Downstream, Midstream, Upstream

Category: Efficient use of power - Compressors/Drivers

Compressors are an integral part of the production process; they are used to increase the line pressure from the gathering system so that gas can be delivered to the processing plants and/or sales lines.

Gas compression is one of the most energy-intensive production processes. For this reason, it is important to investigate the most efficient and appropriate options.

Many types of compressors are avalable, and each has its advantages and limitations. The main types of compressors are:

  • Centrifugal (horizontal/vertical split), also known as radial compressors; these compressors use an impeller to increase the velocity of the fluid and turn this energy into pressure energy, thereby increasing the pressure of the fluid.
  • Axial: these compressors use a continuously rotating airfoil to progressively compress the fluid.
  • Screw: rotary screw compressors use two rotating helical screws to compress the gas into a smaller space.
  • Reciprocating: these compressors utilize pistons driven by a crankshaft.

A summary of the advantages and limitations of each is shown in Table 1, below.

Table 1: Comparison of different compressor types (Reference10)

Compressor Type Advantages Limitations
Centrifugal High efficiency
Reach pressures up to 1,200 psi
No need for special foundations
High initial cost
Complicated monitoring and control systems
Limited capacity control modulation
Axial High peak efficiency
Small frontal area for given gas flow
Increased pressure rise due to more stages with negligible losses
Difficult to manufacture
High starting power requirements
Relatively high weight
Screw Simple design, few moving parts
Low to medium initial and maintenance costs, Easy to install
High rotational speed
Shortest expected useful life
Not designed for dirty environments
Reciprocating Simple design, easy to install
Lower initial cost
Two stage models offer highest efficiency
Higher maintenance costs
Foundation considerations due to vibrations and size

More detailed information on the different types of gas compressors can be found in the ‘Compressors’ chapter of the CAPTProcess Technology Equipment document (Reference 1).

Compressors may be driven by reciprocating engines, gas turbines or electric motors. Reciprocating engines are commonly fired by natural gas; essentially internal combustion engines, they comprise a chamber filled with natural gas which is ignited to drive a piston. Low-speed and high-speed engines are matched with compressors of corresponding speed. Gas turbines rely on the hot exhaust gas discharged by a gas generator to drive a power turbine. The output power from the turbine shaft is used to drive the pipeline gas compressor. Finally, electric motors use an electromagnet to produce movement. Reciprocating compressors are typically driven by natural gas-powered reciprocating engines or electric motors, while centrifugal compressors are usually driven by gas turbines or electric motors (Reference 2).

Technology maturity

Commercially available?:  Yes
Offshore viability: Yes
Brownfield retrofit?: Yes
Years experience in the industry: 21+

Key metrics

Range of application:   Engines used to drive compressors can have power ratings from <100 hp to >1,000 hp.
Efficiency:  30–40 % for reciprocating engines; up to 50% for centrifugal compressors.
Guideline capital costs:   Driver and compressor, skid or foundation, and the systems required for operation (e.g. filters, coolers, instruments, valves, pulsation bottles for reciprocating compressors).
Guideline operational costs:   For electrically-driven motors, the cost is dependent on electricity costs.
Typical scope of work description:   Compressors are used in a variety of gas handling operations:
  • Compression of gas for delivery to market
  • Re-injection for reservoir support
  • Re-injection to improve recovery
  • Re-injection into separate formations
  • Recovery of valuable natural gas liquids (NGLs).

Decision drivers

Technical:   Availability of electrical power;
driver selection
Operational:   Pressure considerations;
design and maintenance
Commercial:   Number of compressors
Environmental:   Improving efficiency of existing compressors
Economic rule-of-thumb Consider carbon cost (if applicable) and $/hp for compression

Additional comments

Considerations in the selection of compression equipment should include:

  • Energy demand/load—one of the most vital steps in ensuring that the selected compression train be as efficient as possible is to ensure that the energy demand and the load are matched. This is an important parameter because it will dictate both the size and the number (see discussion on number of stages below) of compressors used. Incorrectly-sized compressors will likely result in inefficiencies and increased emissions since the engines will be running below optimum load.

In sizing and selecting equipment, the life and decline curve of the field should also be taken into consideration. Having multiple stages of compression may prove advantageous when heavy compression is no longer needed in an ageing field. Individual compressor modules can be removed from ageing fields and resited at locations where compression is still needed. Having larger compressors (i.e. less stages) does not allow for this flexibility. In this case, it may be necessary to re-wheel the compressor (i.e. change out the internals for new or optimized parts) at certain points of the field life. This provides an excellent opportunity for a retrofit for existing operations where compression equipment was selected and installed long ago, perhaps without the benefit of energy model results.

In determining the demand required of the compression train, opportunities may arise within the process to decrease this demand. For example, pressure drop allowances for intercoolers and suction lines should be carefully evaluated. If these allowances are unnecessarily large, they can decrease the efficiency of the compression system. Compression savings have also been observed when condensate recycle streams are optimized. If the pressure is reduced in stages (multiple stage separation) rather than flashing high pressure liquids, this minimizes gas re-compression volumes and gives a modest efficiency improvement.

  • Number of stages: In order to determine the optimum number of compression stages, an energy model should be run. Greater thermodynamic efficiency is possible if more stages are introduced, thus increasing the amount of intercooling. However, other parameters, such as limitations on discharge pressure, intermediate system temperature, and footprint also must be considered.
  • Compressor selection: The selection of a compressor should be based on the volume flow, desired pressure rise and change in molar mass as illustrated below.

Figure 0: Compressor selection

 

  • Availability of electrical power: Compressors can be driven by different types of fuels. Electrical motors are well regarded in the industry for having less maintenance requirements than their gas-fired counterparts. In addition, they produce a lot less noise and vibration. However, in some onshore/offshore operations located in isolated areas, electrical power may not be readily available or reliable, and fossil fuelled engines must inevitably be used.
  • Driver selection: Fixed speed drives are limited in the sense that they do not offer much turndown capability. Below the machine’s turndown capability, it will simply run on recycle. To eliminate this energy loss, alternatives such as variable speed drives or multiple compression trains should be evaluated.
  • Pressure considerations: To avoid condensation in the lines, fuel gas is generally taken downstream of the dehydration unit at which point it is at a much higher pressure than needed at the compressor. Options of either drying the fuel gas separately before compression to line pressure or designing the fuel system to avoid condensation should be examined.
  • Number of compressors: Assuming the same configuration and location, one fully-loaded, larger unit will be more fuel efficient and will cost less than two smaller, fully loaded, equivalent-sized units. By contrast, one fully loaded, smaller unit will be more fuel efficient and offer more flexibility than one partially-loaded, larger unit. Therefore, multiple, smaller compressors can achieve better overall fuel efficiency than a single larger compressor if the pipeline operates predominately at less than maximum throughput. Additionally, as explained above, multiple smaller compressors add the flexibility to remove unnecessary modules and use them in other areas where they are needed. The fuel savings, however, may not outweigh the installation costs of additional smaller units (Reference 1).
  • Improving energy efficiency of existing compressors: The quality (temperature, purity, moisture) of the intake gas can have a dramatic effect on the energy efficiency of a compressor. As a general rule, every 4°C rise in inlet gas temperature increases energy consumption by 1% to achieve the same output. One of the easiest ways to reduce the intake temperature is to locate the compressor outside (when possible) so that heat dissipates into the atmosphere, rather than building up indoors. (For engine or turbine driven compressors, the temperature of the intake air entering into the combustion chamber also has an impact on overall efficiency.)

Likewise, intercoolers are generally used to cool the gas between multiple stages of compressions. Impurities in the gas also decrease efficiency so intake gas generally passes through a filter. The pressure drop across the gas filter, however, should not exceed 3 psi, or energy efficiency will decrease significantly. Each 250 mm WC (approximately 3.7 psi) pressure drop across the suction path increases power consumption by about 2% for the same output. Hence it is advisable to clean inlet gas filters regularly and recommended to install manometers or differential pressure gauges across filters to monitor pressure drops (Reference 3).

The energy efficiency of a compressor can also be improved by altering the way the compressor is operated. A compressor consumes more energy at higher pressures for the same capacity. A reduction in delivery pressure by 1 bar reduces the power consumption by about 6–10%. Compressors should not be operated above their optimum operating pressures as this wastes energy and leads to excessive wear, which wastes even more energy and causes unnecessary downtime. If low pressure gas is required, it is advisable to generate low and high pressure gas separately, rather than reducing the pressure through pressure reducing valves, which invariably wastes energy (Reference 3). Finally, when operating multiple units, either in parallel or in series, energy efficiency can be improved by load sharing controls. If the units are fairly similar in efficiency and size, they will generally achieve the lowest overall fuel consumption if units are evenly loaded. For example, the overall efficiency will be great if two units operate at 75% than if one operates at 100% and the other at 50%. If the units are dissimilar in size or efficiency, it is generally more efficient for the larger or more efficient unit to carry most of the load, using the smaller or less efficient unit to compensate for load swings (Reference 2).

Finally, the system design and maintenance can influence energy efficiency. Similar to the drop in pressure across the gas filter, the pressure loss from the discharge point to the point of use affects energy efficiency. The typical acceptable pressure drop in industrial practice is 0.3 bar in the mains header at the farthest point and 0.5 bar in the distribution system. The pressure drop can be minimized by using a loop system with a two-way flow, minimizing corrosion, and properly sizing equipment. In addition, a significant amount of energy can be conserved by locating and repairing leaks, by installing controllers to automatically turn compressors on or off based on demand, and by properly maintaining the system. Proper maintenance involves checking oil pressure frequently (daily if possible), changing the oil filter frequently (monthly if possible), checking and replacing gas filters, checking automatic condensate traps for leaks, draining manual condensate traps, and checking and replacing gas dryer filters.

Alternative technologies

The following technologies provide similar benefits and may be considered as alternatives to gas-fired compression:

  • Electric motor, if electricity is available.
  • Turbines

Operational issues/risks

Gas-fired engines require regular maintenance to operate at high efficiency and minimize air emissions, and are usually subject to a rigorous service schedule. Servicing can range from simple preventative maintenance activities to repairs that require the engine to be removed from the site and re-machined. This downtime should also be taken into consideration when sizing the compression train.

The regulatory framework governing the area where a compressor station is located can have a significant effect on the choice of compressor, as well as the way a compressor is operated or the decision to modify an existing station. In the USA, for example, modifying an existing compressor station may trigger the EPA’s New Source Review, requiring the pipeline company to apply for a permit and to implement control technology which can be expensive and result in the compressor running less efficiently. Additionally, in the case of existing compression, more stringent emissions standards may require controls or replacement of the engine if no suitable emissions controls can be located. Because drivers and compressors are sometimes sold as packages, there may also be a need to replace the compressor.

In some countries, CO2 emissions are or may become taxed, whilst avoided CO2 emissions may have an economic value. This may affect the choice of driver. For example, electric motors produce no direct emissions; however, access to the power grid is not always available or may be unreliable in some locations, hence electrically-driven compressors may have limited application in some remote areas.

Opportunities/business case

Opportunities:

  • Design (equipment selection and number of stages) may be optimized, especially for greenfield developments.
  • Opportunities exist to fit existing equipment with variable drives.
  • The system may be subject to additional environmental emission regulations (e.g. in the USA).

Natural Gas Industry, USA (References 7, 8)

Natural Gas STAR Partners in the USA investigated three separate areas to reduce emissions from compressors:

    1. Taking compressors offline
    2. Reciprocating compressor rod packing
    3. Centrifugal compressor seal systems

The approaches analysed here not only reduce emissions, thereby saving costs, but also improve operational efficiency and save energy.

Taking compressors offline
When compressors are taken offline for maintenance, emissions can occur from different sources depending on the pressure of the system. For depressurized systems, emissions occur from the blowdown of the gas left in the compressor and continued leakage from unit isolation valves. For fully pressurized system, leaks occur from the closed blowdown valve and compressor rod packings.

Figure 1: Offline compressor scenarios

The main strategy for reducing emissions when taking compressors offline is to keep the unit pressurized. Additional strategies include routing the blowdown gas to the fuel gas system and installation of a static seal on the compressor rods to eliminate rod packing leaks during shutdown. The table below summarizes the benefits of each of these strategies.

Table 2: Benefits of emission reduction strategies when taking compressors offline

Strategy Net volume of gas saved (Mcf/yr) Net value of gas saved ($/yr)1 Implementation cost4 ($) Payback2
Keep compressors pressurized 4,400 13,200 0 Immediate
Keep compressors pressurized +
route gas to fuel
+1,3453 +4,0353 1,250 4 months
Keep compressors pressurized +
static seal installation
+1,2003 +3,6001 3,000 10 months

1 Value of gas = $3.00/Mcf
2 10% discount rate
3 Incremental over base
4 2003 cost basis.

Centrifugal compressor seal systems
Centrifugal compressor seals can also be a significant source of emissions. Traditionally, the seals on the rotating shafts use high-pressure oil to prevent the gas from escaping the casing. Methane emissions from these ‘wet’ seals can range from 40 to 200 standard cubic feet per minute (scfm) and occur when the circulating oil is stripped of the gas absorbed at the seal face. 

Figure 2: Wet seal system

 

Replacement of wet seals with dry seals, which use high-pressure gas instead of oil, reduces emissions to rates of up to 6 scfm. Other benefits include lower power requirements, improved operating efficiency and performance, better reliability, and less maintenance.

 

Figure 3: Dry seal system

Although the conversion to dry seal is not always possible due to housing design or operational requirements, the selection of a dry seal system for new or replacement compressors can pay for itself in as little as 14 months. The table below summarizes the benefits of replacing wet seals with dry seals.

 

Table 3: Benefits of replacing wet seals with dry seals

Strategy Net volume of gas saved (Mcf/yr) Net value of gas saved ($/yr)1 Implementation cost4 ($) Payback2
Replacing wet seals with dry seals 45,1201 240,0002 135,360 14 months3

1 Based on the difference between typical vent rates of wet and dry seals (i.e. 100 scfm versus 6 scfm) on a ‘beam’ type compressor operating 8,000 hr/yr
2 Value of gas = $3.00/Mcf
3 Based on replacement of a fully function wet seal with additional $73,000 in operating and maintenance cost reductions.


References:

    1. Center for the Advancement of Process Technology (CAPT) (2009). ‘Process Technology Equipment’.
    2. INGAA (2010). ‘Interstate Natural Gas Pipeline Efficiency’. Interstate Natural Gas Association of America, Washington D.C., October 2010.
    3. Kurz, R., Lubomirsky, M. and Brun, K. (2011). ‘Gas Compressor Station Economic Optimization’. In: International Journal of Rotating Machinery, Vol. 2012, Article 715017.
    4. UNEP (2006). ‘Compressors and Compressed Air Systems’. Energy Efficiency Guide for Industry in Asia.
    5. BP Energy Efficiency CI Practice
    6. NORSOK Standard S-003 (2005). ‘Environmental Care’. Rev. 3, December 2005.
    7. U.S. EPA. (2004). ‘Lessons Learned From Natural Gas STAR Partners: Reducing Emissions When Taking Compressors Off-Line’. EPA430-B-04-001, February 2004.
    8. U.S. EPA. (2006). ‘Lessons Learned From Natural Gas STAR Partners: Replacing Wet Seals with Dry Seals in Centrifugal Compressors’. EPA430-B-03-012, November 2003
    9. Solar® Turbines. (2009). Turbomachinery Package Specifications: Saturn 20 Compressor Set and Mechanical Driver.
    10. Ling, A. L. and Mulyandasari, V. (2011). ‘Compressor Selection and Sizing (Engineering Design Guideline). KLM Technology Group, January 2011.