Specify a gas compressor package after the sales pressure is known

Operators who want to monetize associated gas, or stop flaring it, often start by looking for a compressor. It is a reasonable instinct. Packages are available new and refurbished, and a unit on the market today looks like a fast route to first gas sales.

The difficulty is that a compressor package cannot be properly specified until one number is known: the pressure at which the gas buyer will accept delivery. Buying before that number is agreed is the most common way to end up with a unit that is too small, too large, or simply the wrong configuration.

Discharge pressure drives the design

A reciprocating compressor raises gas from suction pressure to discharge pressure in one or more stages. The ratio between the two determines how many stages are needed. The number of stages, together with the gas rate, determines the installed horsepower. Horsepower and stage count in turn determine the frame size, the cylinders, the coolers and the driver.

Change the discharge pressure and every one of these can change. A package designed for a low-pressure gathering system may not reach a pipeline delivery point at all. A package designed for high-pressure delivery may be larger, heavier and more expensive than a nearby power plant or local buyer actually requires.

The sales point sets the discharge pressure

The discharge pressure is not a free choice. It is the pressure at the gas buyer’s delivery point, plus the pressure drop in the pipeline and metering between your facility and that point. It is set by the gas sales agreement, the buyer’s network and the route the gas takes.

Until those terms are agreed, any compressor specification is a guess. Work back from the delivery point to the wellhead, not forward from whatever compressor happens to be available.

Gas quality matters too

The buyer will also specify the quality of the gas it accepts: water and hydrocarbon dew point, H2S and CO2 limits, and heating value. Meeting these may require dehydration, dew point control or treating upstream of the compressor or between stages.

Gas composition also affects the compressor directly. Gas gravity changes the horsepower required for a given rate and pressure ratio. H2S affects materials selection, with NACE MR0175 / ISO 15156 as the usual reference. Compressor packages are commonly built to API 618 or API 11P, and the supplier should state which applies.

Rate, turndown and the life of the field

Gas rates from associated gas change over time as oil production rises and falls. A package sized only for today’s rate may be overloaded later or run inefficiently at low rates. Specify the design rate, the expected range, and how much turndown the package must handle. Consider whether two or more smaller units give better coverage than one large unit.

Plan for the unit being down

A single compressor is a single point of failure. When it is down for maintenance or repair, gas either has to be flared, if permitted, or oil production has to be cut back. Decide at specification what happens during downtime, and whether the economics justify a second unit or spare capacity.

Do not forget low-pressure gas

Gas flashed from tanks and low-pressure separator stages is often flared because it is below the suction pressure of the main compressor. Vapor recovery units capture this gas and raise it to a usable pressure. Whether that is economic depends on volumes and on what happens to the gas next, but it should be part of the same specification exercise rather than an afterthought.

Fuel gas comes off the top

A gas-engine-driven compressor burns part of the gas it compresses. That fuel consumption reduces the volume available for sale and should be included in the economics when comparing gas-engine and electric-driven options.

Refurbished compressor packages can work, on the right basis

A refurbished package can be a sound choice if its frame, cylinders and driver suit the agreed pressures and rates, and if the refurbishment scope and certification are documented. The test is the same as for a new unit: does it meet the specification written from the sales point back? If the specification is not written yet, there is nothing to test it against.

A practical order of work

1. Agree, or at least bound, the delivery pressure and gas quality with the buyer.
2. Define rates, turndown and expected decline from your production forecast.
3. Write the compression and conditioning specification.
4. Compare new and refurbished packages against that specification.

If you are working through a gas sales opportunity and need a compression specification, we can help. Start an enquiry.