Modular Mechanical Seals Protect HVAC Piping & Duct Systems
These devices are installed around the external circumference of the pipe, sealing it more quickly and effectively than lead-oakum joints or hand-fitted flashings, mastics, or casing boots (Figure 1). The seals expand when tightened to fill the annular space between the pipe and the cored or formed penetration, which is defined as half the difference between the inside diameter of the penetration and the outside diameter of the pipe.
Modular mechanical seals consist of a series of links with five components: an elastomeric sealing element, two pressure plates, and a bolt/nut combination. The sealing element (Figure 2) is usually made of virgin ethylene propylene diene monomer (EPDM), a synthetic rubber with a typical Shore A hardness of 50 +5%. Seals for thin-wall piping and ductwork are made of softer EPDM with a Shore A hardness of 40 +5%. This lower-durometer rubber makes it possible to effect a seal at lower torque, thereby avoiding deformation of the piping or ducts. Some seals are made of virgin nitrile rubber for resistance to oils, gasoline, and solvents, as well as synthetic silicon rubber for steam applications.
The elastomer formulation and dimensions of the sealing element are designed to provide even, uniform volumetric expansion. Virgin rubber is used to ensure the sealing element is evenly vulcanized into a consistent, expandable unit. Recycled rubber and synthetic fillers are sometimes used to control costs, but at the expense of consistent, uniform expansion.
Because the seals are supplied on belts, it is imperative that they be as close to the same hardness or durometer as possible. For example, if a belt of seals with different hardness values goes into the same penetration and the same torque is applied to each link, they will not expand uniformly and the seals can be compromised.
The seals have a bolt hole cavity on each side of which are molded cones, which are forced inward as torque is applied to the bolt, increasing air pressure inside the cavity.
A molded compression-assist boss on each side of the pressure plate permits increased compression loading, preventing liquids from entering the cavity for better corrosion control (Figure 3). It should be noted that the shoulder volume or the thickness of the rubber surrounding the bolt-hole cavity must be sufficient to prevent the bolt from tearing through the seal when it is tightened.
Modular mechanical seals used to have pressure plates made of a combination of steel plate and PVC (Figure 4). The bolt was threaded through the steel plates and the torque applied to it was distributed through the plates and PVC housings. After extensive service in the field, it was determined the PVC material was not only prone to cracking but also exhibited cold flow around the steel portion of the pressure plates causing the seals to loosen over time.
Finite element analysis of the seals by the University of Michigan resulted in replacement of the steel pressure plates with plates made of a reinforced nylon composite. Injection molded with ribs, the redesigned pressure plates distribute torque more evenly, while providing higher dielectric strength and improved corrosion resistance.
Various bolt/nut combinations have been used with modular seals, ranging from vanilla cadmium-plated carbon steel to today’s more exotic metric two-part zinc dichromate sub-coatings per ASTM B-663 under a propriety organic outer coating for superior corrosion resistance. To benchmark the corrosion resistance of these different combinations, four bolts were continuously immersed in tap water for four years, the results of which are shown in Figure 5.
The bolt labeled A is a cadmium-plated carbon steel, B is cadmium-plated carbon steel with an anti-corrosion coating, C is two-part zinc dichromate coated carbon steel with an organic corrosion-resistant coating, and D is Type 316 stainless steel. Note the increasing corrosion-resistance from bolt A to bolt D. In addition, modular seals using bolts with organic corrosion-resistant coatings over zinc dichromate sub-coatings were subjected to 1,470 hours of salt spray per ASTM B117-97 without exhibiting significant corrosion.
The diameters of both the bolt and bolt cavity provide for only for minimal clearance, so the sealing element expands to fill the annular space between the pipe and penetration rather than the space between the bolt and bolt cavity. The bolts are lubricated, those made of Type 316 stainless with a PTFE-based lubricant to prevent galling or seizing during tightening. On most modular seals the nut applies torque to the pressure plates through two planes of distribution (Figure 6).
Modular mechanical seals must be installed so as not to create a path for liquids within the penetration.
As noted, the seals are supplied in belts of the appropriate size and number of links for a specific application. These belts are tightened to hold the hardware in place so it arrives at the job site without missing pieces. The belt containing the links should be loosened so all crevasses, the areas where the sealing elements come together, are closed (Figure 7). Otherwise they can create a liquid path through the seal itself.
The horizontal center of each pressure plate in the seal should be aligned with the tangent of the pipe, so the torque from tightening the bolt is uniformly distributed to the sealing element. The axis of the pipe should be parallel to the axis of the penetration and centered in the opening. The pipe should be adequately supported on both ends, since modular seals are not designed to support the weight of the pipe.
Once the belt of individual seals is connected around the pipe, all bolt lengths protruding from the rear pressure plates should be of equal length so the seal can be tightened uniformly. All surfaces of both the pipe and penetration should be free of dirt and debris. If weld beads are encountered within the footprint of the seal, they should be removed per the piping manufacturer’s recommendations before installing the seal.
After the seal is fastened to itself around the pipe, normal sag or slack may be noticed. Some installers are inclined to remove what appear to be extra links so the seal slides smoothly into the penetration. However, modular seal assemblies are supplied with the correct number of properly sized seals for the penetrations in which they are to be installed. Removing links can alter the volume/void relationship, impairing the effectiveness of the seals. On smaller-diameter pipe the links may have to be stretched. In addition, pressure plates must be aligned so their outermost surfaces are all on the same vertical plane.
While proper seal selection and installation are critical, consideration must also be given to proper hole forming. Holes through poured concrete walls can be saw cut or formed with steel or thermoplastic sleeves. If saw-cut, there must not be any voids in the footprint of the installed seal, and the penetration must be free of dust and dirt. The modular seal can then be installed directly into the penetration.
If the hole is to be formed with a steel wall sleeve, the outside diameter of the sleeve must be fitted with a 2-in. collar around its entire circumference to anchor it into the concrete wall and prevent water from migrating around the outside of the sleeve. This collar must be continuously welded on both sides to ensure that water will not run under it, and the wall sleeve must be round, clean and free of weld slag.
Thermoplastic wall sleeves have molded-in water stops with textured surfaces to better adhere to the concrete. These sleeves must be able to withstand the forces poured concrete exerts on them without going out-of-round. Reinforcing ribs and end caps that fit into the sleeves help position them and maintain their concentricity (Figure 8). Thermoplastic wall sleeves too must be round, clean and free of mold burrs.
Low-durometer EPDM seals can be used to seal round ductwork into foundation walls. However, it is important that equal force be maintained around the ducts as the seals are tightened. The seals should be tightened in a clockwise direction, and no bolt should receive more than two turns, rather than the standard four, during each successive pass.
In addition, heat tracing can be sealed into a poured concrete wall using another penetration parallel to the heating/cooling water piping. The seal is sized for the dimensions of the heat trace and installed in the adjacent penetration.
The expansion and contraction of steam piping present a number of sealing challenges. Variations in temperature and the diameter of the pipe must be taken into account in sizing seals for these applications. The seal is correctly sized if the minimum and maximum pipe diameters are within its published size range. If the seal is not correctly sized, it will have a tendency to creep and eventually “walk” out of the penetration. Installing loosely fitting flange faces into both sides of the penetration wall will prevent this from occurring, and is now accepted industry practice.
All modular seals can accommodate some angular misalignment, which varies with the relationship between the size of annular space to be filled and the size range of the seal. The closer the annular space is to the middle of this range and the thicker the seal, the more misalignment it can tolerate. If misalignment of the pipes through a wall is a design factor, consideration should be given to oversized sleeves or cored penetrations that can accept larger seals. Modular seals can also provide a modicum of vibration dampening if the minimum and maximum movement of the pipe falls within their range.
EPDM sealing elements used in HVAC applications take on a compression set of approximately 15% of their expanded range (based on being subjected to a temperature of 158°F/70°C for 22 hrs)
Take for example a seal with an expanded-state thickness of 1.87 in. and a static-free-state thickness of 1.43 in. The difference between these leaves an expandable range of 0.44 in., 15% of which is 0.064 in.
Subtracting this loss in the seal’s expandable range from the expanded-state thickness yields a value of 1.80 in. If the annular space of the penetration is greater than this value, the seal cannot be reused. If the annular space is between 1.43 in. and 1.79 in. it can be. The smaller the seal, the less expandable range it has, limiting the possibility of reuse.
Modular mechanical seals have evolved substantially since they were first developed 45 yrs ago. Among the enhancements for improved performance is an internet-based calculator to identify which type of seal would provide the optimum sealing pressure, vibration dampening and ease of installation for a given penetration. Bolts and pressure plates have been standardized, with bolts for smaller seals changed from hex head to Allen head to facilitate installation in smaller annular spaces. In addition, pressure plates have been redesigned to provide 15% more strength (Figure 9).
Properly designed and installed modular mechanical seals offer one of the most cost-effective and reliable methods of sealing piping or duct work into floor or wall penetrations. There are numerous suppliers these seals, some of which are still using technology dating to the 1980s. Ultimately, it will fall to the HVAC engineer to review all pipe and duct penetrations and select the seal that best meets the requirements of a particular project.ES