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Floating Roof Seal Selection Guidelines

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Floating Roof
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  FLOATING ROOF SEAL SELECTION GUIDELINES   Conference Paper Presented at NISTM, Orlando 2012 With numerous seal options available and limited guidance from existing standards, selecting and configuring the right floating roof seal can be a difficult task, one that when done incorrectly, can lead to problems ranging from regulatory compliance issues to premature maintenance outages to more significant failures. In addition, as new employees enter the work force and experienced employees retire, it is important to have an internal list of guidelines apart from the standards that will ensure asset longevity and optimal operational efficiencies for your tanks. In this paper, we summarize the standards and guidelines available, explore supplementary design and selection criteria, give examples of problems that can occur when these criteria are not addressed and provide a checklist for tank owners and operators to use moving forward. In the tank industry, most tank owners are familiar with current resources such as API 650, Appendix H & C; EPA Air Rules; EPA AP-42; API MPMS, Chapter 19; API RP 545; NFPA 11; as well as a few international resources such as EEMUA 159, BS EN 14015:2004 and the European Parliament and Council Directive 94/63/EC 1994. Though these existing guidelines provide some specifics such as types of seals required by regulation, shoe dimensions relative to liquid and in some areas, materials requirements, a lot is left open to interpretation. There are numerous crucial purposes for floating roof seals including: emissions mitigation, centering the floating roof to allow safe operation, dampening floating roof movement and protection against rim fires. Whether building new tanks or taking a tank out of service for regular inspection and maintenance, effective planning with accurate data resources is key to ensuring the floating roof seal accomplishes these critical objectives. Following are some key considerations for selecting and designing floating roof seals (above and beyond the existing standards) that will help ensure asset longevity, engineered safety and optimal working efficiencies. The first consideration is chemical compatibility . Of utmost importance is to work with your seal provider to ensure that the stored product will be compatible with the seal materials. Elastomers and fabrics as well as metals must be considered. Seal materials can warp and deteriorate when improper materials are used, creating gaps, holes and in the end an ineffective or dangerous seal. In addition to the current stored product, it is important to consider future potential change in services in order to ensure the seal will be compatible both now and in the future. UV and weather resistance  is another key consideration to seal selection and design. Materials exposed to weather should be resistant to the effects of sun, wind, and depending on where the tank is located, extreme cold and/or heat. Reputable seal providers should be able to provide information on materials offered and how each performs when subject to various environmental factors. In addition, the seal should maintain product purity by mitigating the ingress of water. Secondary seals should keep water and snow out of the rim space and foam dams should be equipped with weep holes and properly maintained to prevent clogging which can lead to topside corrosion, pontoon damage and ultimately, the potential to sink the floating roof. A seal’s resistance to abrasion  is another consideration. Factors such as cycle frequency, service life goals and shell condition should be taken into account. Seal construction including plate width and pressure application can determine seal effectiveness. For example, secondary seal plates should be narrow in order to distribute pressure and prevent accelerated wear points on the wiper tip. Similarly, mechanical shoe seals should have pressure mechanisms that avoid applying point-loads that can wear holes in the shoes over time. In well-designed seals, pressure points are distributed across the shoe in multiple points. This distribution of pressure helps to maintain a tight seal and allows the shoe to conform to shell anomalies. Reputable seal providers can supply cycle test data. Seals are not ‘one size fits all’ and good seals are designed to ensure adequate working range for rim space variations . Every tank has a unique shape, making it imperative to conduct rim-space and verticality surveys prior to fitting a seal to a floating roof. API 650 calls for seals to be able to adjust +/- 4” from the nominal rim space. However, f  or external floating roofs where tank ovality is more prevalent and dynamic forces such as wind and turbulence can move the floating roof, that range may not be adequate. Tank shells can be scored by poorly fit seals with improper pressure distribution; this can damage coatings and increase the probability of expensive shell repairs during the life of the tank. Similarly, designing the seal to operate effectively across varying rim spaces  is extremely important. When rim space varies greatly, support arms in hinged shoe seals should be long enough relative to the rim space to minimize shoe drop. Additionally, the secondary seal  plate length, gauge and tip type should be determined only after careful consideration of the operating conditions including cycle frequency, temperature and shell condition. This can all be determined with accurate survey data and a good understanding of the tank’s intended use. Closely related to ensuring flexibility for rim-space variations is the ability for a seal to have circumferential flexibility . Seal designs and construction should allow expansion and contraction without compromising seal integrity. Using modular vapor barriers such as gaskets can create gaps. Seals should use a continuous vapor barrier to ensure a proper seal. A good seal should be flexible and apply continuous pressure in order to handle shell surface anomalies . Specific shell conditions such as ovality or local shell deviations should be disclosed to seal providers prior to seal selection. For shoe seals, conforming to shell deviations means using a flexible shoe with a pressure system that applies pressure consistently in multiple points across the shoe, distributed both horizontally and vertically. For wiper-type seals, the tip should be flexible, durable and continuous pressure should be applied to prevent gapping. Shell surface variations can present quite a challenge to maintaining a tight seal and seal wiper tips should be thoughtfully selected to ensure gap-sealing integrity and longevity. The floating roof seal should apply enough pressure to keep the floating roof centered . When the floating roof isn’t properly centered, it can drift from wind, turbulence or drag force on columns and gauge poles. In extreme cases, the result can be catastrophic. In order to keep floating roofs centered, the pressure mechanisms in floating roof seals should be substantial and made from materials that do not yield or degrade over time. The seal should maintain a tight seal  to prevent volatile mixtures of vapors and oxygen. Gapping seals and/or torn vapor-barrier fabric can result in excessive vapors above the floating roof. This is especially dangerous during electrical storms. When a lightning strike occurs, the momentary difference in electrical potential between the tank shell and the floating roof will likely create sparks in the seal vapor space. Even in tanks which follow API RP 545, there is no way to guarantee that sparks will not occur in the vapor space. A tight seal becomes an added layer of security, keeping the vapor space saturated and safely above the Upper Explosive Limit (UEL). On the other hand, a gapping seal or torn fabric will allow oxygen into the vapor space, creating a volatile environment. Maintaining a tight seal is done through a combination of several factors discussed above –  proper working range, quality non-yielding materials, flexibility to conform to the shell, durable and chemically compatible fabrics, and an excellent pressure distribution system that keeps the roof centered and the seal tight. A secondary seal height should maximize tank capacity . A lower-profile secondary seal allows the floating roof to travel higher, increasing working capacity. It is important to have good verticality and rim-space data to ensure a low-profile seal has the proper working range. If the upper rings of the tank shell are able to handle additional capacity, gains due to proper seal selection can mean the difference in thousands of additional barrels per turnover. Seals should be easy to clean and gas free . Traditional foam log, bag and tube seals have the potential to trap hydrocarbons, which creates an unsafe environment for maintenance workers and also poses environmental disposal problems. In order to avoid those issues, make sure you talk with your seal provider to discuss options that are safe and environmentally friendly. Additional considerations include seal fabrics and wax scrapers. There are multiple options for seal fabrics. Seal fabrics  should be chosen to ensure chemical compatibility as well as durability under specific environmental and operating conditions. In addition, fabrics should be fire retardant. Wax scrapers  may be a consideration if the product creates waxy buildup on the tank shell. If this buildup is allowed to occur, the secondary seal can partially scrape some the wax off as the floating roof travels upward, resulting in hydrocarbons on the top of the roof; this product is not only a fire hazard and large source of emissions, but can lead to plugged foam dam weep holes and topside corrosion of the floating roof. The solution is effective wax scrapers located below the liquid level. These scrapers should be made of hardened stainless steel (to prevent yielding) and be designed with additional pressure application located just above the wax scraper. Good seal selection and design starts with good data and careful planning. Accurate tank data including rim space and verticality surveys will ensure the seal system is designed to effectively negotiate the rim space. Other important considerations include the stored product (now and in the future), cycle frequency, shell anomalies, operating conditions and any additional local regulatory requirements. Proactive collaboration with a reputable seal provider will help ensure you are getting the right solution for your specific application. Seal Design and Selection Considerations:      Chemical compatibility     Fatigue resistance    Ability to prevent water ingress    Ability to conform to shell roundness anomalies    Ability to conform to shell surface anomalies    Low profile to optimize working capacity    Ability to prevent a volatile mixture of vapor and oxygen    Ability to dampen lateral roof movements    Ability to handle waxy shell residue    Secondary must provide access to primary for inspection    Abrasion resistance    Flame retardancy    Resistant to heat    Adequate working range    Adequate circumferential flexibility    Ability to handle drag forces and torque from roof travel    Ability to handle potential gas upsets    Ability to maintain pressure over time for seal integrity    UV resistance    Ability to handle freeze-thaw cycles    Ability to handle snow loads    Ability to maintain seal integrity with wide-ranging rim spaces    Ability to keep the floating roof centered to allow for smooth, safe travel    Ability to clean and gas-free    Does not pose a risk of trapping vapors    Safe and easy to install; even in-service when possible    Secondary must be mounted independent of primary
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