Fluid Handling: Rethink Your Seal Support System

Consider how its cleanliness impacts your process

By Oliver Pogmore, AESSEAL plc.

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Contamination can result in the destruction of multiple product batches, which can cause substantial losses in revenue. Worse yet, contaminated product reaching the market can significantly affect a company’s profile and lead to potential legal and financial ramifications. So, maintaining a high level of cleanliness during production is an imperative.

Chemical manufacturing relies heavily on rotating equipment such as pumps and mixers. Their operation requires some form of mechanical seal to enable rotation while ensuring minimal loss of product. Typically, this consists of a double mechanical seal along with a seal support system. The seal support system allows for circulation of a barrier (or buffer) fluid through the internal cavity of the seal.

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A seal support system can be configured in a number of ways — generally following designs suggested by the American Petroleum Institute (API). When making a selection, the user should consider its potential effect on hygiene standards, the cost to operate and the level of maintenance expected.

Choosing the wrong plan can undermine a processor’s efforts to maintain a high level of hygiene. The conventional seal support system frequently harbors dirt and bacteria on the interior of its main vessel; the inherently enclosed design hampers inspection, cleaning and maintaining high levels of hygiene. Therefore, for all hygiene-sensitive operations, a processor should select an alternative system designed to ease proper and effective cleaning procedures.

Common Choices

In many applications, following an API plan is an efficient and effective method of ensuring optimal operating conditions for the seal. One of the simplest forms of seal support system, API Plan 62, runs water through the seal and discharges it directly to a drain (Figure 1). This plan can provide a more hygienic method of maintaining the mechanical seal and often is found in processes that require a higher level of cleanliness. However, it is inherently expensive to operate due to the continuous high volume of water required (as much as 700,000 U.S. gal or 2.6 million L per year), potentially resulting in a lower mean-time-between-failure (MTBF) rate. Obviously, this has significant implications both on the running costs of the process and environmental concerns about water wastage.

An alternative solution is API Plan 53a (Figure 2). This layout utilizes a vessel to provide the seal with a constantly recirculated flow of a fixed volume of water. It can operate using a thermosyphon effect, whereby convection causes hot fluid to rise up out of the seal and draws in cooler fluid.

API Plan 53a can reduce water usage by more than 99% compared to an API plan 62 design, and has been shown to improve the MTBF of mechanical seals. When used to replace an API Plan 62 system, API Plan 53a can provide a return on investment of less than 6 months.

However, the design of current vessels makes them almost impossible to inspect or effectively clean their internal cavities. In most instances of contamination, operators aren’t aware there’s a problem (or even the possibility of a problem) until it is too late and, as a result, the consequences can be expensive and far-reaching.

Such closed systems typically have a number of cavities or “bug traps” inherent within the design, especially within the vessel. These can’t drain under natural conditions and aren’t specifically included in any cleaning regimes. Over time, these bug traps can cause issues and lead to bacterial growth and contamination. Plus, in the event of a seal failure, contaminated barrier fluid then can find its way into the process.

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