Ariel provides suitable materials options and configuration options for a successful packing vent system. Much of the success of a packing vent system falls on the End User and Packager for design of the packing vent system and the End User for operations and maintenance of the system.
Options
Materials options are available for key components in the gas stream for sour and corrosive gasses. Materials options for components such as piston rods, valves, cylinder bolting, bearings, tubing and tube fittings are discussed in the Ariel Sour Gas topic.
Configuration options are available including purged pressure packing cases, short and long two compartment distance piece arrangements, flushing oil system and separate lube oil arrangements for the cylinders. These are discussed in the Ariel Sour Gas topic.
When a long two compartment distance piece and purge packing is purchased the intermediate and wiper packing cases come with purge rings and purge gas connections (except for the JGU:Z where the intermediate packing is purged, but not the wiper packing). These are available for incorporation into the system design.
Limitations
Purge pressures should be 5-15 psi above the disposal system pressure.
Distance piece compartment pressure should be minimized. Compartment pressure to be limited to 25 psig maximum.
Maintenance of the system and the wearing parts is key in minimizing the vent gas. As the packing wears, the leakage rate increases and can overcome the application of the purge gas.
Detection of a failing system can include measuring:
Purge gas flow into the packing case
Packing vent gas flow out of the packing case
Packing vent gas temperature out of the packing case
Packing case temperature
When using vent gas temperature to detect failure the temperature measurement of packing vent / drain line should be provided as close to the guide as possible. One temperature element per pressure packing case is recommended for the greatest measurement sensitivity.
Shutdown logic to protect personnel and equipment should be considered.
References
There are many ways to design a packing vent system. The guidelines for these systems are best outlined in API-618, fourth edition, Appendix I. The systems described here are only some of the many approaches used in designing a successful system.
Reference System Schematics and Discussion
The schematics for the long two compartment guide system and the short two compartment guide system are one of the many successful systems used in the field today. These show the "push and pull" system utilizing a purge on the pressure packing and distance piece and a vacuum pump on the disposal line.
The goal is to control the transfer of packing vent gas from the cylinder into the frame and into the atmosphere. The frames are open to the atmosphere through breathers on the top cover. Any gasses that migrate into the frame from the cylinder will escape out of these breathers. The primary packing case breaks down the pressure of the gas along the piston rod, but does not create a gas tight seal. This gas leakage must be disposed of prior to entering the frame. Additionally, hydrogen sulfide and many of the corrosives are soluble in the cylinder oil and will tend to migrate along the piston rod and into the frame.
Oil collected from the pressure packing case vent / drain and possibly from the distance pieces will be contaminated with hydrogen sulfides and / or corrosives. This contamination must be considered in the design of the oil disposal system.
In order for the packing vent system to be successful design, operations and maintenance must be considered. If the packing wears and is not kept in proper maintenance most purge systems will fail to segregate the sour gas. Monitoring and maintaining this system and predicting the packing wear is essential to the success of any fugitive emissions control system.
Ariel
Corporation Application Manual
7 Sept 2004