Page 103 - Compression Machinery for Oil and Gas

92 SECTION II Types of Equipment

In case of primary seal failure, the secondary seal is designed to operate at
primary seal conditions, which prevents uncontrolled leakage to atmosphere in
the event of a failure and should allow for a safe shutdown of the compressor. In
case of a total seal failure where both the primary and the secondary seal are
broken, the intermediate labyrinth provides a restriction between the primary
and secondary seal helping to limit process gas leakage. As a result, the pressure
increase in the secondary vent is reduced and the separation seal is less
heavily loaded.
In addition to the function of preventing migration of process gas into the
bearing cavity in the event of a total failure, the task of the separation seal is
to keep away the lubricating oil from the secondary seal during normal opera-
tion. In Fig. 3.59, a set of floating carbon rings is used as a separation seal. Typ-
ically, nitrogen or air is supplied in the middle of the two carbon rings,
generating a gap velocity of 10–20m/s to prevent oil mist and splash oil
from entry.
To ensure proper function of a DGS, an adequate support system is essential.
Accordingly, the support system is designed to provide:
l clean and sufficiently dry seal gas/separation seal gas at the required pres-
sure, flow rate, and temperature,
l safe venting of leakages, and
l condition monitoring of the DGS and the separation seal.
The most complex support system is given for the case of a tandem seal with
intermediate labyrinth. This arrangement has five connections, all connected to
the support system. An example P&ID is shown in Fig. 3.60.
Typically, process gas is used as the primary seal gas. In order to generate a
positive flow, the pressure of the supply source needs to be higher than the pro-
cess gas pressure at the process seal in any operating condition. Most often, the
seal gas is taken from the discharge or a designated location in the compressor.
As during transient or standstill conditions, the supply pressure can equal or
even drop below the pressure at the process seal, a seal gas booster might be
required. Less often an external source is used. The disadvantage here is that
the settle out pressure increases continuously when the compressor is isolated
and in standstill/idling condition.
In order to achieve the required flow velocity at the process seal either a flow
control or a differential pressure control device is used. Flow control is achieved
with a needle valve as the most simplistic option, or the use of a self-adjusting
flow control valve. The maintained flow is constant and not affected by the
clearance of the process seal. This can lead to higher flow velocities as needed
and represent a recycle loss. A differential control valve regulates the differen-
tial pressure acting on the process seal. The flow rate varies with the clearance
of the process seal but the flow velocity is constant.
To ensure the seal gas is clean a coalescing filter shall be included in the
supply line. The filter should be a duplex design to ensure an online exchange