The barrier fluid selected should be non-hazardous and compatible with the process. Often, the barrier fluid itself can be problematic, as it may not have ideal lubricating properties.
Magnetic and canned pumps
Magnetic and canned pumps are used to attempt to alleviate the issues with wet lubricated single and dual seals. However, plants have seen that a new set of concerns arises with the use of this equipment. Maintenance on this equipment is highly specialized and usually involves using outside repair shops. The bearings supporting the radial and axial thrust in these pumps are lubricated by the process fluid, exacerbating the lubricating needs found in single seals. Other problems including a poor tolerance for intermittent operation, sensitivity for variations in fluid properties or condition, have limited their use.
What can gas seals offer?
Gas seals operate on a gas fluid film and dont generate significant frictional heat. The process fluid isnt the lubricant as with single seals. Unlike dual seals that require compatible barrier fluid, gas seals often use inert nitrogen that isnt a compatibility concern and isnt generally hazardous. Barrier fluid tank maintenance costs, specialized refilling procedures and their impact on reliability are eliminated with gas seals.
Gas seals can be easily applied to standard chemical process equipment such as centrifugal pumps and mixers unlike magnetic and canned driver technology. Gas seals are now available in compact cartridge designs allowing existing equipment to easily gain the advantages of gas sealing (Figure 2). Specialized maintenance procedures do not need to be established and operators do not need to refill pressure tanks.
Figure 2. Even at high pressures (above 150 psig), the make-up gas requirements are small. (Click to enlarge.)
Unique concerns for gas seals
As with all equipment, gas seals do pose a new set of questions. Most gas seals operate using hydrodynamic lift-off for seal face. Separation requires a minimum circumferential speed, which is a function of both the equipment RPM and shaft diameter. Circumferential speeds of 250 ft /minute (1.3 m/s) and higher are typically required in standard gas seal designs. However, special gas seal designs are available for slow speed equipment such as mixers.
Gas seals are lightly loaded, axial face seals; theyre more sensitive to loading changes than liquid seals. Face loading is accomplished by both hydraulic forces and spring forces. Fluids that are film-building, such as paint, can limit the movement of mechanical seals. Slurries, containing more than 5% solids, also present problems. These fluid properties can be managed by using a seal flush and/or anti-particulate stuffing box. Process fluids that chemically attack the o-rings in a mechanical seal may increase seal drag, decreasing gas seal performance. While gas bellow seals can be used to eliminate o-ring sensitivity, they cannot easily attenuate vibration at the seal faces introducing another set of concerns.
Fluids with high freezing points may solidify when ambient temperature gas is introduced into the equipment (auto-refrigeration). Gas seals are more sensitive to high viscosity process fluids. Evaluate process viscosities at normal operating temperature and cold start-up. If these are concerns, the supply gas lines can be heat-traced.
For a successful gas seal installation, clean, dry barrier gas should be supplied to the gas seal. Pressure must be regulated within two limits: above the maximum stuffing box pressure and below the maximum system design pressure. Proper pressure regulation ensures that the seal faces are lubricated with a gas barrier, while maintaining a clean environment between the faces.
Pressurized gas seal should be from a reliable source and set at a pressure thats regulated to at least 15 psig (1 Bar) above the maximum sealing pressure in the equipment. While older designs use large control panels to regulate pressure at maximum pressure levels, newer designs are available that actively regulate gas pressure with respect to changing stuffing box pressure to ensure low gas consumption and maximize reliability. Typical gas usage is shown in Figure 3 and is often well below 1.0 SCFH of consumption.
Figure 3. Plan 74 isolates process fluid, thereby preventing emissions.
The compressed gas should be dry and filtered to ensure reliable operation. Design of the gas panel should allow for routine filter maintenance with minimum intrusion into production. A coarse and fine filter system is the accepted practice as it allows easy filtration down to two microns (Figure 4). Upon seal installation, blow out the gas supply lines downstream from the filter system to eliminate any tramp metal in the lines.
Figure 4. Options are shown for venting gas that may accumulate during downtime.
Managing gas entrainment
Seals will ingest small amounts of compressed gas during operation. Barrier gas will migrate past the inboard seal faces into a process system by design. Unfortunately, seals are often continuously pressurized. During a shutdown gas flows into the piping system. Unless this gas is vented properly, a condition may occur where the amount of gas is in excess of gas entrainment capability of the pump, process piping, or system design. As a result, the pump can become vapor locked or can experience severe cavitation. The solution is common sense. Vent the excess gas to a lower pressure area than that of the pump stuffing box, making certain to by-pass the inlet piping to the pump.