What API 692 recommends regarding seal gas boosters

The API STD 692 identifies the need for a booster (seal gas booster) per the conditioning module and requirements specified in section 7.4, unless the seal gas supply or an alternate gas can maintain the minimum flow requirements.

Alternate gas supply
An alternate gas is often not the right choice, as it must be a reliable clean and dry source, compatible with the process, and requires managing the added mass injected into the compressor. When using a seal gas booster, compressor process gas is used as the gas source, making it the better choice, as the alternate gas supply issues don’t apply.

Focus is on piston type boosters
API 692 booster requirements focus on piston type seal gas boosters which use gas/air as a means to drive them. The attention was on these boosters, as they were the most common seal gas boosters used when the standard was developed. A key item to note is the API booster requirements are for managing poor reliability and the inability for one unit to deliver adequate flow for a seal gas application.

Regarding poor reliability, the standard suggests a duplicate unit, remote indication to support the unit is operating and an alarm for alerting the operator when a unit is not working. Included is also the requirement for a cycle counter, which assists in managing the low maintenance cycles the users have experienced with these units.

Additional process gas leakage
Piston boosters have rod seals, which produce process gas leakage. A means is required for managing this leakage, as well as monitoring it to identify excessive leakage and the need for a rod seal replacement. There is also a concern with the motive gas, which is typically air, mixing with the process gas and producing an explosive mixture, so the booster design must ensure this does not occur.

Multiple piston boosters required
The standard also indicates more than one unit may be required to deliver the normal seal gas flow, which most users do not adhere to. This leads to low booster and seal reliability.

Along with this the API 692 identifies a concern with lower flow as the unit wears and gas bypasses the piston seal or leaks past the rod seal. In addition to these concerns, pulsation dampeners are mentioned, to remove the pulsating flow produced by a piston booster. This indicates users have experienced lower than acceptable seal gas flow across the process labyrinth, which allows dirty process gas into the seal cavity, due to the pulsating flow.

Deficiencies remain
Piston booster manufacturers have made changes for improving their reliability, such as electronic shuttling valves or driving them with electric motors. These do not eliminate the short falls of low or pulsating seal gas flow, lost seal gas from a worn piston seal and lost process gas due to rod seal leakage.