July 8, 2020 | 5 minute read
API 692 Part 3 discusses and defines the requirements for seal gas systems. Systems have sometimes been considered a commodity item, placing the onus on cost and basic functionality. API 692 attempts to rectify this by implementing industry best practices and codifying them into a comprehensive standard widely available throughout the industry.
Read below as we outline some of the major changes from previous thinking on systems, while simultaneously explaining the key components of the API control schemes outlined.
1. What is a dry gas seal system?
At their core, dry gas seal systems do two main jobs: provide a clean and conditioned environment for the dry gas seals and monitor the health of the dry gas seal. To accomplish these goals the dry gas seal system is usually broken down in five separate independent streams for tandem seals, and four streams for double seals.
2. Seal Gas
(Sometimes referred to as buffer gas.)
Seal gas flows across the primary seal (inboard stage) of the dry gas seal out towards the primary vent, and across the inboard process labyrinth, back into the process. Seal gas prevents dirty, unfiltered process gas from entering the dry gas seal. This gas must be cleaned and conditioned so as to be completely dry and free of particulate contamination. The conditioning is usually accomplished via the addition of filters and a gas conditioning unit, such as separators, boosters and heaters.
3. Secondary Seal Gas
(Only applies to tandem seals; sometimes referred to as buffer gas, intermediate injection or intermediate gas.)
This gas flows across the secondary seal (outboard stage) of a tandem dry gas seal and out the secondary vent, as well as across the intermediate labyrinth and out the primary vent. It is usually nitrogen and must also be cleaned and filtered to ensure the gas is dry and free from particulate matter. Not every seal will feature an intermediate labyrinth or secondary seal gas, but API 692 specifies them as standard. Secondary seal gas offers numerous benefits including vastly improving the ability to monitor the health of the secondary seal.
4. Separation Seal Gas
This gas is injected between the two separation seal elements. If a labyrinth, it is injected between the two separation labyrinth halves, if a carbon ring type seal (T93FR or the T83), then between a pair of carbon rings. Separation seal gas prevents bearing oil ingress into the sealing area. Typically, separation seal gas is nitrogen or another inert gas, but air may also be used. If using air, particular care must be taken to avoid creating explosive mixtures in the secondary vent. These can be pressure or flow controlled dependent on customer preference.
5. Primary Vent
The primary vent isn’t a gas flow per-se, but rather a gas exit. The primary vent consists of the leakage from the primary seal plus any secondary seal gas flow if present. Typically, most monitoring of the seal is performed via the primary vent. The primary vent will usually feature flowmeters and pressure transmitters which monitor seal health via the leakage. Leakage tends to be a major indicator of seal health and most alarm settings are based on a multiple of leakage (e.g., 10x guaranteed leakage).
6. Secondary Vent
The secondary vent is the simplest, and quite possibly the most safety critical stream. The secondary vent flow is made up of the leakage from the secondary (outboard) seal and the leakage from the inboard section of the separation seal. Secondary vents should be unrestricted, meaning that nothing which can create a backpressure should be fitted. Differential pressures across secondary gas seals are generally small, and any backpressure generated in the secondary vent could easily reverse pressurize the secondary seal. API 692 makes provision for, but does not require, pressure monitoring and flow monitoring in the secondary vent. Pressure in the secondary vent should be at approximately atmospheric, and the flowrates through it should also be incredibly low. As such, we do not recommend including monitoring on the secondary vent, as no new useful information is gained.
7. API 692 Enforces New Flowmeter Requirements
While the vast majority of API 692 merely codifies best practices, one significant departure in system design is the requirement to include flowmeters in every supply line. Historically, there might have been limited flow monitoring of the seal gas stream, or none at all with the only flow meters being in the primary vent. The additional flow monitoring vastly improves the fidelity of the system and helps with diagnostics of seal performance anomalies in the field, by providing more information about the flows within the seal.
8. Gas Conditioning Units (GCUs) Have Substantial New Requirements
One of the leading causes of gas seal failure is improperly conditioned gas entering the seals. All gas must be both clean and dry, before it goes across seal faces. GCUs condition the gas protecting the seal. GCUs are typically made up of four items: filters (coalescing and separators), heaters, boosters, and coolers. The most significant change is that now these items should be considered as standard. Meaning that sufficient justification must be made by the system vendor not to include such feature as heaters and boosters.
API 692 imposes a few new requirements on systems as described above, with the major changes involving monitoring requirements, and the requirements surrounding the gas conditioning unit. However, the standard does not require radical departure from what already exists. As with any new project, careful consideration should be taken to understand the real-world value of the design verses the cost of the initial purchase, considerations such as improved reliability and reduction of downtime due to unplanned maintenance should feature in such an assessment.