By Michael Huebner, Flowserve Corporation
We are considering standardizing the design of the mechanical sealing systems in our plant. Are there any special considerations?
Many pump users think of centrifugal pumps in terms of the pump and the mechanical seal. In practice, there can be additional components which improve the reliability of the mechanical seal or perform other functions required by the end user. These solutions are commonly referred to as piping plans and range from simple process fluid flushes to complex systems involving vessels, coolers, and instrumentation.
It is relatively common for end users to standardize the types and models of mechanical seals used throughout their plant. The obvious benefits of this practice are proven experience, operator familiarity, and reduced inventory. It is less common for end users to standardize the design of sealing systems or piping plans. Although there are certainly elements of the systems that are defined in the purchaser’s specifications, it is common for a seal OEM to prepare a proposal which goes back to the purchaser for approval, is modified or revised, and cycles back and forth several times before final approval. This process can lead to multiple systems designs used throughout the plant which can create confusion for operators and maintenance personnel. The overhead involved with the design and purchasing of these systems is reason enough to consider standardizing sealing systems. Standardizing the design of these systems can not only reduce this confusion, but also create a more streamlined process throughout the lifecycle of the pump.
Benefits of Standardizations
Seals systems can range from relatively simple process fluid flushes to complex systems involving components such as coolers, reservoirs, and instrumentation. Due to the scope of disciplines involved, multiple departments are often required for the fabrication of unique system designs, which are often more costly and have long approval processes.
Many end users streamline this process by creating pre-engineered solutions which have been vetted and approved in advance. These standardized designs may be unique to the one purchaser’s company or the end user may select a standardized solution which is already offered by the seal or sealing system OEM and mitigate the risk of production delays.
A second advantage to standardized sealing system designs is that they can create a more uniform interface between the operator and the equipment operators. This can make it easier for the operator to use a piece of equipment and familiarize themselves with how to adjust the system or monitor its performance. Standardized maintenance interfaces can also make it simpler to maintain the equipment or add critical fluids to support the system, while standardized instrumentation can make it easier to interface with data collection systems. All of these factors make it simpler to provide uniform training for the operators and maintenance personnel who use these systems.
As pre-engineered systems reduce the upfront engineering on a specific order, standardized sealing systems can also benefit the procurement process; it can reduce lead times for the purchaser since the OEM can order and stock components in advance. End users who maintain inventory can see reduced stocking levels since the same systems and components are used throughout the plant.
Challenges with Standardizations
While there are many benefits of standardized sealing system designs, there are also some challenges which must be considered. There is not just one mechanical seal piping plan nor is there is just one set of application conditions. Different applications will use different pumps with different fluids at different pressures and temperatures. For example, the sizing of cooler components will depend on the piping plan and heat removal requirements; more aggressive chemical environments may require more exotic materials to control corrosion.
Many standardized systems offer flexibility in their designs to handle these challenges. A system may have a standardized design while providing options for components such as reservoir sizes, seal cooler designs, and material selection. Although the selection of instrumentation will need to be specified to meet the required pressures, temperatures, and flowrates for the application in question, a common design and/or model should be used as much as possible.
Many plants do not have engineers who specialize in mechanical seals. Purchasers will therefore rely on the expertise and experience of the seal OEM to select the correct seals and materials for a specific application. Sealing systems, however, may be comprised of pipes and tubing, instrumentation, pressure vessels, and electrical components; virtually every plant has engineers and technicians who are local subject matter experts in these fields. The challenge with this situation is that each discipline may have established best practices for their plant. This can result in different designs for the same equipment depending upon who places the order. In some cases, these special requirements are necessary but often they are the result of personal preferences or the purchaser’s experience. For an end user to establish more standardized designs for sealing systems, there must be some level of commitment from various experts within the plant. This requires a commitment from all personnel to use a specified standard on all future purchases, and not buy a potentially lower cost, non- compliant offering.
Options in Standardizations
Industry standards
The most obvious place to start the search for sealing system standardizations is within the industry standards for pumps and mechanical seals. Most pump standards have limited requirements for mechanical seals and sealing systems (e.g., ASME B73, API 610). The most complete standard specifically focused on seals and sealing systems is API 682. This standard is the most commonly used reference for the definition of seal piping plans and the design of sealing systems.
API 682 was written with the intent to capture the practices which had a proven record of performance and reliability in the field. The definitions of piping plans are thoroughly documented in this standard.
The standard gives in-depth tutorials on the installation, commissioning, and use for many of these plans. There are also some specific examples for calculations used to select components and estimate system performance.
While the standard does an excellent job of defining the piping plans, it provides fewer designated requirements for the sealing system design and components. Many of the seal system components only have a few requirements which specify their most common attributes. Other components, such as reservoirs, will have a more complete set of requirements, including dimensional requirements for many of the reservoir’s features.
As complete as API 682 is for sealing systems, it falls short of fully defining a standardized design, or offering, for piping plan systems. It does, however, provide the minimum set of requirements for most end users and can help ensure a safe and reliable system. For a plant to create a standardized system design, there must be a more thorough review of the options and intended benefits of the standardization program. This includes P&IDs of the basic layout of the plans, general definitions of the plans and their functions, and minimum requirements for piping and instrumentation.
Specific end user standards
Some end user companies have defined standard system requirements and designs at the plant and/ or corporate level. These standards are most often created in collaboration with the seal or sealing system OEMs. This allows the end user to gain the benefits of the seal OEMs large installed base of applications across industry with their insights on the design and procurement implications of the various options. One of the most mature outcomes of this process is a pre-engineered design that is standardized by the end user. As stated earlier, these pre-approved designs can be used throughout the end user’s company without additional engineering review.
OEM standards
Another option is to rely on the system design expertise of the seal OEM. Seal and systems OEMs have decades of experience in designing and supplying vessels. Many of these designs, were however, heavily influenced by the purchaser’s individual preferences. This has resulted in literally thousands of different designs for equipment which basically serve the same function.
Many seal OEMs are beginning to address this challenge by creating standardized component and system designs which capture many of the best practices used throughout the industry. This can create opportunities for easier procurement along with the other benefits of standardization. This does not imply that the purchaser should simply buy readily available products. It implies that there are standardized systems designs which offer the flexibility of meeting many common applications requirements. This can serve as the foundation for a company’s standardization program. The end user should consult with their seal OEM to identify these opportunities.
Michael Huebner is a Principal Engineer at Flowserve Corporation in Pasadena, Texas. He has over 30 years of experience in the design, testing and application of mechanical seals both in the USA and Europe. He has authored numerous articles and lectured extensively around the world. He has a BS in Engineering Technology from Texas A&M University.