Page 102 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 91

values for the gap velocity are 5–10m/s. Furthermore, all arrangements have a
separation seal in common which separates the DGS from the bearing cavity.
A single seal arrangement consists of a single set of seal faces separating
process gas from the atmosphere. Single arrangements are mainly considered
for nonflammable and nontoxic gases. The seal gas leakage is typically vented
to the atmosphere.
A double seal arrangement consists of two single DGSs arranged in a mir-
rored configuration. Double seal arrangements are usually used only for process
gas pressures smaller than 5barg. As the common supplied seal gas is typically
nitrogen, there is no process leakage to atmosphere but nitrogen will migrate
into the process. The vent is typically open to atmosphere.
A tandem seal consists of two single DGSs arranged in series. Furthermore,
the tandem arrangement differs in one without and one with intermediate
labyrinth. In the latter, a labyrinth sits between the two single seals. As the com-
pressed gases in the oil and gas industry are usually flammable or toxic, the tan-
dem seal arrangement is the most considered. In the case that leakage of process
gas to the atmosphere is unacceptable the tandem seal arrangement with interme-
diate labyrinth is the right choice. Fig. 3.59 shows this arrangement in detail.
The seal gas pressure is throttled across the primary seal to primary vent
pressure. Secondary seal gas, typically nitrogen, is supplied between the inter-
mediate labyrinth and the secondary seal to prevent primary seal leakage from
reaching the secondary seal. For this purpose, the amount of secondary seal gas
need to be sufficient to maintain a gap velocity at the intermediate labyrinth of
about 3–5m/s. Leakage from the primary seal and the majority of the secondary
seal gas flow is routed to the primary vent. Secondary seal leakage is throttled
across the secondary seal and routed to the secondary vent, which is typically
open to atmosphere.

FIG. 3.59 Tandem arrangement with intermediate labyrinth.