Of course, volumes have been written as to why pumps fail. Also, it is no stretch to assert that, when pump fires result from such failures, a mechanical seal is usually involved (Fig. 1).
Mechanical seals often fail as a consequence of prior bearing distress. In some instances, seal manufacturers supply products with close clearances between the periphery of rotating seal parts and the diameter (bore) of stationary seal parts. We believe that users should specify seals for hydrocarbon products to comply with API (American Petroleum Industry) Standards. We also believe that deviations from the user's specification or applicable guidelines (such as API-682) should be brought to the purchaser's attention.
There are, however, other ways to avoid seal failures. One of these would be to specify, whenever applicable, canned motor centrifugal pumps. Also called "hermetically sealed" pumps, they do not incorporate mechanical seals.
History of canned motor pumps A canned motor pump cross-section is shown in Fig. 2. The development of these centrifugal pumps is closely associated with the expansion of nuclear power generation technology. As of the early 1950s, safety considerations led to the development of hermetically sealed operating loops. It was then that the design principle of the canned motor— known since 1914—found practical application.
The chemical industry's recognition of the advantages of these pumps followed soon thereafter. Indeed, the additional demand created a widespread economic base for the manufacture of canned motor pumps.
By the 1960s, canned motor pumps had evolved to the point of standardization. Expanding populations and rising standards of living in both the industrial and developing countries required innovative technologies to solve the demanding and progressively urgent problems of protecting the environment. Increasingly, canned motor pumps became part of the answer.
We readily acknowledge that, in the last three decades, much progress has been made in the field of mechanical shaft sealing. Yet, beginning with the mid-1970s, environmental considerations, industry consolidation and automation of processes have become increasingly important. It is in this context—in a wide range of fluid movement tasks—that the more traditional types of sealing either provide inadequate safety or present pollution and loss concerns that can no longer be tolerated.
In some instances, the cost of seal support and monitoring systems is disproportionate to the potential success. It is only fair to point out the existence of services that simply cannot be performed using "open" or conventionally sealed centrifugal pumps. Absolutely hermetic transport of fluids using centrifugal pumps is only possible where the torque applied to the pump rotor is generated externally. To satisfy this requirement, a rigid external stator system using either electromagnets or permanent magnets is needed. In some industries, conventionally sealed pumps can endanger human life and physical assets. Thus, traditionally sealed pumps are not always "best available technology." Accordingly, wherever the state-of-the-art makes this protection feasible, the means employed must be aligned with the results achieved.
And so, keeping in mind the limitations of "open design" pumps (those with packing or mechanical shaft seals) that can contribute to air and water pollution, we should endeavor to be thoroughly acquainted with state-of-the-art of hermetic drive techniques for centrifugal pumps. The facts may surprise us.
Design and functional description As extremely environmentally sound machines, canned motor "hermetic" pumps are now very widely used in Europe and Japan. While making inroads in the United States, lost ground needs to be recovered in our quest for competitiveness. That said, wherever it is necessary to move dangerous, toxic, polluting, expensive, caustic, potentially explosive, high-temperature or low-temperature fluids, canned motor pumps deserve very close consideration.
The canned motor combines the well-understood hydraulics of centrifugal pumps with the equally wellproven three-phase induction motor. The hydraulic section is directly connected to the drive motor. A pipelike sleeve or "can" is inserted in the magnetically-bridged gap between rotor and stator. The "can" absolutely and hermetically separates the rotor chamber from the pressurized fluid pumping environment. In other words, the "can" is the boundary between the liquid-enveloped pump rotor and the non-wetted stator chamber (Fig. 2). The "can" thus separates the motor into two functional areas; it represents the hermetic sealing element of the pump assembly. In essence, the torque required for shaft rotation is transferred via the can, which consists of a non-magnetic material, by electromagnetic means. This type of drive does not require a shaft aperture through the fluid-containing (usually pressurized) housing; there is thus no need for dynamic gaskets or mechanical seals. The necessary static gaskets are generally problem-free but, in special cases, may be replaced by welded connections.
Canned motor pumps, therefore, are fully hermetic pump units. The pump section can be of single or multistage design. The pump impeller (or impellers on multistage pumps) is mounted at the overhung end of a shared pump-and-motor shaft. The performance parameters of these pumps now correspond to the stipulations of their main areas of application—the chemical and refining industries. At present (2008), the upper power limit is the vicinity of 600 kW. In order to approach as closely as possible the dimensional and performance-related envelopes of large numbers of standard centrifugal pumps (DIN 24256 or, respectively, ISO 2858 used in the chemical industry), many thousands of canned motor pumps in service today are available with the standard hydraulics of this pump range. The identical external dimensions of DIN and ISO hermetic and "traditional" centrifugal pumps allow rapid conversion from conventional to hermetic pumps. Needless to say, this allows reducing the spare parts inventory requirements of any modern facility.
There are, however, substantial additional advantages. These advantages, as well as the impressive efficiency and MTBF statistics of hermetically sealed API-compliant pumps, are discussed in the following Sidebar section, co-authored with George Dierssen, of IndustryUptime.
As equipment reliability consultants, we clearly can see at least five (and more likely 10) definable benefits of canned motor pumps over traditional API-610 pumps. These benefits belong to one or more categories that ultimately translate to safe, reliable, low-maintenance, low-installed-cost, as well as environmentally compatible service. To enumerate some benefits:
At present, each of the five refineries in the San Francisco Bay Area has one or more canned motor pumps in highly satisfactory operation. While representing from near-zero to perhaps 2% of the pump population, we now believe canned motor pumps (CMPs) are probably applicable to 50% of the pumps at a typical refinery site. There are, of course, restrictions (i.e. thermal shock, dry running, slurries, etc.), which, in most cases, can be handled by engineering controls. Nevertheless, it seems that CMPs are the ideal choice for many HP (hydrocarbon processing) services. This is especially so since today's plants have high expectations regarding plant uptime and operational safety. Although these expectations are reflected in a solid standard—API-685—the meager representation of CMPs in U.S. plants is puzzling.
Barring any unusual mathematics, one of the co-authors (of this Sidebar section) was surprised to discover that the average mean time between repairs (MTBR) for CMPs—factoring in every one of the many thousands made by two separate major manufacturers—is 7.5 years. This leads us to wonder as to refining industry data overall. Our understanding is that 80-90% of Japanese plants use CMPs (one manufacturer apparently ships 1700 pumps per month!) and 60-70% of European plants use either magnetic-drive pumps or CMPs. Thus, we followed up on the question. Our evaluation and substantial input from a respected CMP manufacturer can be summarized in a number of important points.
As always, we invite your comments.