The Fundamentals: Shaft Seals On Rotary Valves & Airlocks
Kathy | July 1, 2007
When it comes to sealing solutions for your bulk handling systems, you may have more options than you thought.
Rotary valves and airlocks are fundamental components in bulk material handling systems. They serve to accurately meter product from storage into a process, as well as to isolate pressure differentials between storage and conveying systems.
The rotary valve is essentially a revolving door, on a horizontal instead of a vertical axis. Situated in the discharge opening of a silo, day bin or other vessel, the upward-facing, wedge-shaped “door” section is filled with process material by gravity. As the section rotates into the circular housing, excess material is scraped away, so that the wedge contains a controlled amount of material.
When the filled wedge section rotates to the opening at the bottom of the valve, a controlled amount of process material will drop into the receiving vessel (which could be a screw feeder or pneumatic conveying or packaging system). Varying the rotational speed of the valve, therefore, alters the rate of flow of material into the downstream system.
A rotary valve consistently delivers a specific amount of material at its discharge.
An airlock is a rotary valve whose vanes fit closely enough to its circular housing to maintain an airtight seal. The airlock then can maintain one pressure on its inlet side and a different pressure or vacuum on its discharge side. Thus, for example, pressure from a pneumatic conveying system on the inlet side of the airlock does not affect a loss-in-weight bagging system on the discharge side. Maintaining pressure differential and a precise quantity of material in each of a rotary valve’s wedge-shaped sections depends upon a tight, accurate fit between the vanes of the valve and the circular housing, and upon properlyfunctioning shaft seals. Both factors exert even greater influence over performance of an airlock.
Several different sealing options are available as standard equipment from different rotary valve and airlock OEMS. Some OEMS take the conventional, 3000-year-old packing and gland follower approach. Some use mid-20th century lip seals. Others incorporate latter-20th century quad seals. Several OEMs are beginning to investigate and use contemporary, contacting face seals. The selected sealing option is fitted into a shallow stuffing box, cast into the endplate of the valve, along with a bearing support.
Where packing is used, a simple gland follower is employed to compress the packing. Many different packing styles from many manufacturers are available. Such options include: ptfebased braiding for food grade and chemical environments; ex-foliated graphite yarns for high temperatures; various synthetic fibers braided with ptfe-yarns for abrasion resistance; and graphite braided with various synthetic fibers and or metal filaments to resist corrosion, extrusion and thermal degradation. Packing seals (see Fig. 1) can fail if not regularly attended, and the out-of-the-way locations of airlocks often make packing adjustment diffi- cult or impossible. Once dry product begins leaking between packing and the shaft, abrasive damage can quickly occur, eventually necessitating overhaul of the airlock. By the time leakage is noted, significant damage may have occurred, and any pressure differential across the airlock may have been compromised for a considerable time, sometimes allowing undesired variations in package fill levels.
Lip and quad seals…
Lip seals (see Fig. 2) can lead to the same problems as packing. Lip seals and quad seals use the backup approach for maximizing runtime. By stacking several quad or lip seals in layers, leakage is delayed until the last row is compromised. The components function because the profile of the seal against a rotating shaft generates a vacuum condition at the internal seal location (process side) and a positive pressure on the external side of the lip. The vacuum keeps the process material in.
As the lip wears from contacting the shaft and interfacing with the process material, the vacuum condition degrades, requiring the next ring to seal. This continues through each lip or quad seal until the last seal point fails. All materials and lip configurations are not created equal and some degrade more quickly than others.
Abrasive process material, fine particles and high temperatures are particularly challenging for both lip and quad seal types. The quad seal provides twice the number of contact points per ring (see Fig. 3). This also requires less compressive force against the shaft, resulting in less friction and better wear life. Also, various other lip seal profiles are available with different sealing features.
Sometimes air or inert gas is introduced in an attempt to blow process material away from the seal to prolong service life.
Specialized mechanical seals (see Fig. 4) are available for retrofit in airlocks. These compact, unbalanced, double-faced components can be fitted into the airlock’s packing area and adjusted with the gland follower. They are typically purged with air or inert gas as a barrier fluid for closing seal faces. Careful monitoring of the air/gas pressures can predict product leakage, allowing maintenance personnel with an opportunity to adjust the seals before leakage (and possible shaft damage) can occur.
Unlike packing and lip seals, mechanical seals do not contact the rotating shaft with a non-rotating sealing element. The rotating seal faces are fixed to the shaft, and they seal on a plane that is 90° opposed to the shaft’s axis. This effectively eliminates abrasive wear to the shaft—and in so doing, eliminates the need to replace or resurface a prime overhaul component, saving labor, time and materials during periodic overhauls.
At the same time, this configuration also eliminates abrasive wear of packing or lip seals, and the associated risk of product contamination by seal face detritus. Product purity is more easily maintained. Mechanical seals often require more radial and axial space for installation than lip seals and small-section packings. Consequently, the stuffing box space incorporated in the airlock frame may not be large enough to retrofit a mechanical seal. There are several steps you can take if your stuffing box space proves insufficient.
- Most airlock end plates are easily removed. If there is sufficient packing box depth, the plate can be chucked and centered in a lathe and the packing radius enlarged.
- Where there is insufficient packing box depth, the face of the existing box can be faced off in the lathe or milling machine, an extension ring can be centered and welded in place atop the existing box, and the ring and stuffing box i.d. can be re-cut to provide a smooth, consistent bore.
- Where needed, a larger-diameter gland follower can easily be fabricated from a piece of pipe welded to a steel plate flange, then drilled and machined to suit.
In this way, most rotary valves and airlocks can be retrofitted with mechanical seals in your own plant’s maintenance shop (or in a local machine shop) with little effort.
The true cost of maintenance
The sort of proactive maintenance offered by air-purged contacting face seals can extend the operating life of airlocks between overhauls, minimize product waste and can also aid in predicting maintenance shutdowns.
Airlocks are periodically taken out of service to resurface and machine the vane tips. Some configurations have replaceable vane tips as an integral part of the design. Others are repaired by building up the vanes with weld and re-grinding the tip surface. Replaceable vane tips often are used in highly abrasive wear applications. In non-abrasive applications, seal replacement is the primary maintenance performed between overhauls. Vane tip service and seal and bearing replacement are the main reasons for overhauls which necessitate equipment shutdowns.
To avoid unnecessary shutdowns, the minimum goal for any airlock seal should be to last at least as long as the tips of the vanes. This provides the optimum meantime- to-repair. Ideally, shaft seals should be rebuilt or replaced when the vane tips and body are overhauled. Since the overhaul invariably takes place in the workshop, all maintenance can be performed in a clean, controlled environment, with tools and equipment ready at hand.
Trying to perform seal replacements and other maintenance in the field between overhauls is hard on personnel and, in turn, makes quality workmanship difficult to maintain. This makes mean-time-to-rebuild unpredictable.
The true cost of a single production line shutdown includes the cost of workers idled by the shutdown, product lost before the problem was identified, product not manufactured and product lost while bringing the line back to grade on restart, as well as the actual parts and labor costs associated in the maintenance operation. Taking these factors into consideration, an organization may look on mechanical seals not as a more expensive sealing solution, but as a thrifty investment in reliability. More importantly, when properly maintained and monitored, a mechanical seal’s performance is predictable.
The best sealing option
Any of the sealing options discussed in this article will work reasonably well when sealing non-abrasive materials. Difficulties arise when abrasive materials and very fine materials pass through the rotary valve or airlock.
Materials found in the home building products and mining industries, salts and sugars in the food industry and additives such as TiO2 or starch are all extremely abrasive. Such materials will embed into packing and act as a grinding mechanism, cutting into the shaft and making the rotary valve or airlock unreliable.
Lip and quad seal elastomers also can become abraded, lose their sealing characteristics and begin to cut into the shaft. Face seals can stop shaft damage, redirect wear to a sacrificial component and provide a predictive maintenance mechanism—but at a higher price.
What’s best for your application can best be determined by balancing the purchase cost of any given seal type against the maintenance and downtime costs associated with it.
Paul Wehrle is chief engineer with the MECO Seals Division of Woodex Telephone: (207) 371-2210; e-mail: firstname.lastname@example.org