Understanding Seal Selection – STAMPS

No, this isn’t going to be a discussion on postage stamps, however intriguing an in-depth commentary on the history of the postal service or the stamp collecting scene may be. Instead, we will be evaluating some fundamentals of application engineering in relation to mechanical seals, with a term that can make life easier for all parties involved with the process of seal selection.

It’s very possible that you inferred STAMPS is an acronym, and it’s also possible that you have used it for applications before. It is not a novel method, having seen use in all forms of rotating equipment for decades, whether reduced to an acronym or subconsciously used in the decision-making process. Personally, I saw its use in a manual on packing selection published by the FSA and thought how wonderful it would be for every seal request we saw to have the same basic information.

So, what is STAMPS? Let’s break it down:

Six letters for six words that do a lot of work in specifying the environment a seal is needed for. And in my opinion, a mnemonic device fairly easy to remember, especially compared to some others out there… Dear King Philip Came Over For Good… Soup?

Anyways, while it is good to know the words, it is even more important to understand the meanings. So, let’s take a look at each category and discuss how they impact seal design and application principles.

SIZE

It proves quite difficult to quote a seal without knowing the size needed. For mechanical seals, we are almost always sizing off the equipment shaft, in particular the diameter of the shaft or sleeve through the stuffing box or seal chamber. This dimension forms one of the binding limits of our seal envelope. For most manufacturers, internal seal components are based off this shaft diameter as they are adapted by the seal sleeve.

A bigger shaft size leading to a larger seal seems obvious enough, so what are the additional ramifications?  A big one (no pun intended) is the increase in face contact area. This area is a product of the face cross-section, a value specified by the seal designer, and the mean face diameter. As contact area increases, so too does drag and heat generation. More cooling is required, and more robust drive mechanisms are required to handle the involved forces.

There are other considerations as well, from specifics such as hydraulic thrust on the seal sleeve affecting drive collar design, to generalities such as pricing, weight, and ease of installation. Pump suppliers may consider varying pump models and hydraulics that fit an application and evaluate how the sizing impacts the final seal design and project scope.

TEMPERATURE

Things can get very hot in the world of mechanical seals, but also very cold! From cryogenic transport to boiler feed supply to heat transfer fluid circulation, seals must be designed to overcome the challenges in a wide spectrum of application temperatures. In addition, the same fluid may behave very differently depending on the pumping temperature, so considerations must be made on the specific fluid properties in each application.

It has been stated time and time again that heat is one of the greatest enemies of mechanical seals, and for good reason. Mechanical seals perform best when operated in a steady-state condition for which they were designed for. While that high-temperature metal bellows seal is certainly suitable for your pump rated to 700 °F, the ultra-precise lapped sealing faces are still sensitive to thermal distortion, so steps must be made to ensure excessive heat removal and to prevent rapid temperature fluctuations.

And as touched on earlier, it is also important to understand how a fluid reacts to temperature changes. At elevated temperatures, most aqueous-based solutions become poor lubricants for seal faces, light hydrocarbons will vaporize, salts and caustics will precipitate, and oils will break down and oxidize, forming solid coke residue. Similarly, allowing certain products to become too cool can generate poor sealing environments as well, including hardening and high viscosities that dramatically increase fluid shear forces and face damage.

Piping plans are an essential tool to modify the environment of the product at the seal faces. Use flushes to remove seal-generated heat, coolers to lower temperatures, and steam jackets to keep products hot (or for cooling in some cases!). An accurate understanding of the pumped fluid temperature and any cyclic effects (pump standby, ambient weather, etc.) will lead to the best selection of seal design and the appropriate piping plan for your given situation.

APPLICATION

The category of application can be interpreted quite broadly, but I prefer to use this segment to focus on the equipment setup and operating procedures. The seal and system evaluation for a continuous operation ANSI process pump may be quite different than that of an intermittent service sump pump for the same liquid. Furthermore, fluid hydraulics affect the seal chamber in very different ways when comparing a double suction pump to a vertical turbine pump, and different piping plans will be needed to modify pressure and generate flush flow through the seal.

It should also be considered if the equipment will see regular maintenance or if it is in a remote location. If the latter, utilities needed for certain piping plans may be unavailable, and automated leakage detection systems may be desired. I’ve seen that familiarity with a particular seal system can make a large impact on upkeep and reliability of the seal, and customers may have success and preference for specific setups.

Oftentimes, specific details on equipment condition and operation only come up after repeated seal failures, at which point the solution becomes a joint venture between the seal manufacturer and customer. Perhaps a solvent CIP system is implemented that is incompatible with a certain seal, or operations has been shutting off the barrier fluid pressure source in between batches. Ideally these would be known factors to account for ahead of time, but reality does love to present challenges.

MEDIA

In the vein of seal selection, media is the product, process, or fluid that is in contact with the seal. For most cases, this is defined by the liquid listed on the pump data sheet. However, evaluating the media in an application is more than just looking at compatibility charts.

It is critical to account for all constituents of the fluid, as well as understanding the nature of the fluid itself.

• Are there any solids in the pumped stream?
• Are there any corrosive contaminants, such as H₂S or chlorides?
• If the product is a solution, what is the concentration?
• Does the product set up or solidify in any encountered conditions?

These are just a few questions that may be asked to gather as much information as possible on the product.

If the product is hazardous, or does not provide suitable lubrication with any other reasonable piping plan, an external flush or double pressurized seal will likely be recommended. In these cases, it is also important to analyze compatibility of the product with other fluids that may be used with the seal.

• Is dilution of the product with an external flush acceptable?
• If a double seal is selected, will the product react poorly with the seal barrier fluid?

Outside of this, most seal manufacturers (including PPC) have general compatibility charts that will indicate what metals, face materials, and elastomers are suitable for specific chemicals and temperatures. Just keep in mind that the scopes of these guides lend themselves to more general decisions, whereas industry-specific or even service-specific guides will narrow down into more desirable and proven solutions. Or take the best route and ask an experienced seal specialist (we have those too!)

PRESSURE

Pressure and speed, a fundamental combo in pumping and Bernoulli’s Principle, are predictably important in seal design as well. A product of the two, oft-referred as PV (pressure-velocity) in the sealing world, is a common term in the performance ratings of seals. While PV is outside the scope of this particular article (stayed tuned for future articles on seal limitations!), understanding pressure and how it functions in a more general sense with applications is just as important.

When working with pump applications, it is common to see suction pressure and the required differential pressure (or the head equivalents) on the data sheet. From this, discharge pressure can be estimated, or is sometimes listed. For the seal, we are most concerned with the pressure in the seal chamber, that is, the pressure acting on our seal faces. This value can be estimated based on pump design/hydraulics and is also calculated by some pump OEMs.

This seal chamber pressure is evaluated for many purposes:

• Many seal designs are limited by a static pressure limit that should not be exceeded.
• If the static limit is satisfied, then the dynamic limit (PV) can be evaluated based on seal materials and fluid properties.
• For many fluids, a suitable vapor pressure margin should be included to prevent the vaporization of the fluid due to seal face generated heat.

For most applications, a value of 50 psi for the vapor margin is targeted. This indicates that seal chamber pressure is 50 psi greater than the vapor pressure of the fluid at the pumping temperature. Achieving this may require flow and pressure management with the primary flush plan.

Beyond seal limitations, pressure also has an impact on the implementation of seal piping plans. Orifices and pump throat bushings may be used to raise or lower the seal chamber pressure as needed (the aforementioned vapor margin must still be maintained!). Plan 53A systems are also limited in pressure capability due to gas entrainment of the pressure source into the barrier fluid. Depending on the fluid and application, a limit of 150 to 300 psi is recommended before switching to a Plan 53B that uses a bladder accumulator to eliminate gas entrainment.

SPEED

Working with rotating equipment, it shouldn’t be much of a surprise to see speed on the list. Often less weighted than other application attributes, speed can certainly make an impact at its extremes. While it is most common to see 60 Hz electric motors at 1800 or 3600 RPM on process pumps, gearbox-driven mixers and VFDs allow for a wide range of values.

The effect on the seal faces is realized as the rotational speed is converted into a surface speed, accounting for the diameter of the shaft or sealing faces. A greater surface speed leads to more frictional heat, and a greater need for cooling. At high velocities, the centrifugal forces on springs will push the need for a stationary spring design, which also includes the benefit of less spring fatigue from equipment misalignment. Industry guidelines recommend a stationary spring design when exceeding 4500 feet per minute.

Once velocities reach very high values, as in the case of some compressors and integrally geared pumps, considerations for tolerances and rotating mass imbalances in the seal assembly must be more carefully evaluated. These applications often require an engineered design, as viscous drag and churning also increase at a high rate, leading to high heat generation in the seal area.

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SEND-OFF

Complex at a glance, simplified under scrutiny is a phrase that is suitable for many things but can especially be applied to mechanical seal selection. There are often a multitude of potential solutions, but a full understanding of the specific needs and requirements of the application will typically lead to a best fit.

While this is not a comprehensive seal selection guide for any and all applications, hopefully it can streamline the thought process to identify key aspects when working to specify a sealing solution. Just remember your STAMPS, and you will be a step closer to reliable seal performance.