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Selection of Compressor in Booster Compressor Application

When gas production is declining due to decrease in the reservoir then in certain pressure it needs booster compressor to compress the gas at the required pressure in order maintain the gas delivery to the Plant destination. Based on the production profile then in can be estimated what is the production life until the end of production. It can be 5 years, 10 years, 15 years or more. This kind of booster compressor will have, by the time (of production life), variation on suction pressure, variation on flowrate (capacity), and also may be variation on discharge pressure. 

The question then what is the most suitable compressor type from the both technical and commercial point of view. The following we will see some of parameter that give effect to the study and their conclusion at the end of study. Particular project and Client normally have their own procedure and guidance to perform this kind of study including what parameter need to be included in specific, therefore this article is considered as general knowledge and not intended as specific guidance. 

The following parameter are major consideration to start the study: site parameters, process parameters, driver consideration (electric/gas driver), machinery/equipment parameters

Each of above parameter will be detailed below:

A. Site parameters

This parameter is including whether onshore or offshore site, plant elevation, ambient temperature, fuel gas availability, utility available (water, nitrogen, instrument air), installed in new Plant or existing plant with space limitation.   

B. Process parameters

Compressor operating condition will come from process engineer and it will be kind of production profile during the production life of the gas well.  It will state, year by year or in certain period, for suction/discharge pressure, flowrate, operating temperature, and estimated gas horse power. Gas properties is also one of the important process parameters. Suction pressure and capacity will be varied year by year following the reservoir condition, this will be the most challenging since we shall select compressor type and its driver that suitable and can be used during the entire production life with minimum changes on the compressor design.

With this process parameter we can pre-select compressor type which are suitable to use. As guidance compressor map can be found in compressor handbook which is useful as starting point to examine and expect what compressor type is possible to use. It can be that several compressor type is matching to our application for example reciprocating type and centrifugal type are possible as per compressor map. This will be starting point to the market survey (request quotation from vendor) to confirm that actual compressor type is available and feasible. 

       Fig-1 Compressor Map (Source: PIP REEC001-Compressor Selection Guidelines)

D. Driver (Electric/Gas Driver)

Compressor driver is selected mainly based on availability of electricity or fuel gas where the compressor will be installed. For example, if spare electric power from existing plant is enough, it will be the best option to choose electric motor as compressor driver. On the other hand, if the plant location is new and remote area then fuel gas can be the option.

E. Machinery parameters

This parameter is directly impacted by the equipment itself including the driver option. Except for weight/foot print requirement and similarity to existing which non-cost parameter, we should bring all the parameter to be in cost parameter so we can compare all the compressor configuration in the of cost basis. Normally it will be presented as NPV (Net Present Value) since we talked about future cost. 

The following is considered as machinery parameters:

1. CAPEX (Initial Cost)

CAPEX cost may include the following item:

- Equipment base price

- Spare part cost (start-up/commissioning, 2 years, Capital Spare)

- Compressor modification cost, if any, it shall be considered at the beginning of purchase including the year the modification will be done.

All above cost should come from vendor quotation as part of market survey as explained previously. We may include installation cost (including commissioning cost) if it is considered necessary (as we may have more than one compressor type which we considered installation cost may be different)

2. Fuel Gas Cost

When the driver is gas turbine or gas engine then fuel cost can be calculated with the following formula. If the unit is different (i.e. SI unit instead of US customary) then formula need to be adjusted accordingly.  

- Heat rate in Btu/bhp-hr

- LHV in Btu/scf

- Gas price in $/mSCF

- Power Required in HP

- mSCFD (Fuel) is Fuel consumption 

3. Power Cost

When the driver is electric motor the basis calculation is from grid power price. It can be calculated with the following formula

- Power Required in KW

- Grid power price in $/kWh

4. Process Gas Vent Cost

When the compressor is centrifugal type and use dry gas seal, there is always process gas that leakage to the dry gas seal system and vent to flare. Reciprocating type has also venting of process gas through distance piece. It can be calculated with the following formula

- mmSCFD (Vent) is process gas vent from dry gas seal or distance piece

- Gas price in $/mSCF

5. Maintenance Cost

Maintenance Annual Average Costs i.e., US$/year per kW installed need to be considered. The cost is normally based on data base which Company should have internally as they record during operation of the plant. 

For example, reciprocating compressor 35 US$/KW Installed/year, centrifugal Compressor 7 US$/KW Installed/year, gas engine 55 US$/KW Installed/year, gas turbine 25 US$/KW Installed/year, electric Motor 3 US$/KW Installed/year. This value is only sample and it will be varied depend on the location of the Plant. 

6. Production Lost Cost

Production lost cost is due to failure of the equipment so the compressor stop running. So it will be varied based on compressor configuration i.e., 3X50%, 2X 100% or 2X50% etc. Un-availability value can refer to OREDA Handbook or if the Company has reliability and availability data then it can be also as reference. 

Target availability shall be set at the time of compressor selection for example 97% target availability, so the compressor configuration option that does not meet this target will be out from further selection.

mmSCFD is production flowrate

- Gas price in $/mSCF

- UA= Un-availability is (1- Availability). 

7. Weight/Foot Print Consideration 

Depend on the location of compressor to be installed this consideration can be one of the important point. New Plant at onshore location which has sufficient space allocated for the compressor will be different if the compressor is installed in existing location or offshore location.

8. Similar Existing Compressor

From the point of operation and maintenance person who has already familiar with the same model and spare part commonality, this can be also one of the consideration.

Conclusion and Recommendation 

In principle the compressor selection will be based CAPEX and OPEX in general, CAPEX is mainly initial cost of purchased equipment. OPEX will consider fuel cost, power cost, gas vent cost and production lost cost. During the compressor selection we may have several possible configuration of compressor type and its driver. For example it can be as follows:

- Centrifugal Compressor + Gas turbine Driver (3 X 50%)

- Centrifugal Compressor + Gas turbine Driver (2 X 100%)

- Reciprocating Compressor + Gas Engine Driver (3 X 50%)

- Reciprocating Compressor + Gas Engine Driver (4 X 33%)

All this configuration will vary on initial cost, fuel cost, vent cost and their availability. Some arrangement may need certain modification in certain year to suit the new suction pressure or flowrate and the other is not or just small adjustment without major hardware changes. Longer production life will direct impact to maintenance cost and equipment with high maintenance cost will tend to have higher cost. When all the data has been established and the cost for the total life cycle is presented, it can show which configuration is the best to be selected as conclusion and recommendation.