Page 551 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 9 .11
Be st Practice 9 .12 Dry Gas Seal Best Practices

Best Practice 9.11

Require a primary vent orifice bypass device, for tandem present in the seal chamber and will exit the secondary (outside) seal
seals, whenever the primary vent pressure, during a seal during primary seal failure and/or upset conditions.
failure can rise above the lowest value of N 2 supply
pressure. Lessons Learned
If the seal reference pressure is below 400 kPa (60 psig) Not having a primary seal vent orifice bypass device in high
(Typical minimum plant N 2 header pressure), confirm that the (greater than 400 kPa [60 psig]) seal reference gas appli-
minimum available N 2 pressure at the intermediate seal labyrinth
cations has led to process gas releases in the plant during
will always be greater than the maximum seal reference pressure
a primary seal failure.
to ensure that N 2 gas will fill the seal chamber during a primary
seal failure.
If the seal reference pressure can be greater than the lowest value of Benchmarks
intermediate N 2 pressure, a weight loaded full stem disc valve set to This best practice has been used during new projects since 2000 to
open at a pressure 20 kPa (3 psi) below the lowest value of N 2 at the result in optimum tandem dry gas seal system safety and reliability
seal is recommended. This will ensure that N 2 gas will always be (MTBFs greater than 48 months and counting!).

B.P. 9.11. Supporting Material
vent pressure below the N 2 system pressure. A weight-loaded,
If a complete primary seal failure can cause the pressure in the stem-guided, full-flow disc valve is recommended. A rupture
primary vent cavity to exceed the pressure that can be disc can randomly fail and will require a unit shutdown for
delivered by the N 2 system, the best practice is to relieve the replacement.

Best Practice 9.12

Use an ‘AND trip’ (primary vent high pressure or flow along excessive primary vent flow and detected secondary seal excessive
with indication of secondary seal excessive leakage) to leakage are present).
prevent spurious trips while ensuring plant safety when- A HAZOP review is required for implementation of this best
ever plant daily revenue is greater than $1 MM/day. practice.
Historically, dry gas seal vendors have set primary vent trip levels at Lessons Learned
three times the normal primary seal flow.
However, if the seal gas system is designed with the present (2010) Large (mega) plants have learned that the previous phi-
‘best practice’ available features, increases in primary vent flow can be losophy of primary vent only trips exposes the plant to
safely accommodated. large revenue losses and the present dry gas seal system
Best practice features which will ensure system safety during best practices enable a safe (HAZOP accepted) alternate
increased primary vent flow are: trip arrangement that will optimize plant revenue.
The intermediate N 2 system that is automatically flow controlled.
The use of a primary vent orifice bypass device (see B.P: 9.11). Benchmarks
If the exposure to daily revenue losses is large (greater than $ 1 MM/ The ‘AND trip’ approach has been recommended since 2008 and is
day) and the above best practice features are used in the system, an being used in many new large (over $1 MM/day) revenue plants to
‘AND trip’ philosophy is recommended (trip the unit only when ensure plant safety and optimize plant revenue.

B.P. 9.12. Supporting Material
of a high value product, where availability of the plant is critical,
Monitoring and protection operations may choose to continue running for short periods
after the primary seal fails, while preparing for shutdown. In this
The majority of current DGS installations trip on primary seal case, the recommendation is to trip on indicated primary ‘AND’
failure by measuring primary vent pressure or flow. In the case secondary seal failure (atmospheric side for double seals). Trip

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