Select advantageous API seal flush plans

Become familiar with merits of each.

By Heinz P. Bloch, P.E., Process Machinery Consulting

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Many hundreds of centrifugal pumps are in service all over the world. Virtually all of these pumps include impellers mounted on a shaft, and, a miniscule percentage excepted, the shaft nearly always protrudes through the pump casing. Because the fluid being pumped is usually expected to stay in the pump casing, a seal assembly is provided at the shaft protrusion.

There are multitudes of different styles of pumps and many dozens of sealing configurations. It stands to reason that their respective construction features are influenced by many parameters. Fluid pressure and temperature, fluid purity and lubrication properties are among them, as are such criteria as product quality and failure avoidance. This is where pump owners often invoke the standards of the American Petroleum Institute (API). Current API standards have enabled the hydrocarbon processing and other industries to take quantum leaps in reliability improvement and downtime avoidance. For decades, these standards have alluded to seal flush plans that allow users to specify, and manufacturers to offer, seal support systems that suit the specific requirements of a particular pumping service.

It’s important for reliability engineers to become familiar with the merits of one flush plan or another. In this instance, the value of changing from API Plan 21 (Figure 1) to Plan 23 (Figure 2) is briefly highlighted.

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Figure 1. API Flush Plan 21, product recirculation from discharge through flow-control orifice and heat exchanger to seal chamber.
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Figure 2. API Flush Plan 23, product recirculation from seal chamber to heat exchanger discharge and back to seal chamber.
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Figure 3. Modern cartridge seal with Plan 23 cooling arrangement. (Source: AESSEAL)

Visualize how, in fluid machinery operating with higher temperature media, single seals often require cooling for reliable long-term service. In many services, this cooling may be needed to improve the temperature margin to vapor formation or to meet the temperature limitations of certain secondary sealing elements — say, O-rings — that were chosen based on lifecycle, polymerization, coke formation, or even chemical resistance considerations. In services where the seal temperature environment must be controlled, both piping plans, Plan 21 and/or Plan 23, are commonly used. However, one plan may save energy, while the other plan will not.

This example illustrates the efficiency difference between two “process side” flush piping plans. In a process side flushing plan, process fluid (pumpage) will come in contact with the seal faces. This example involved a closely monitored boiler feed water application in a modern combined heat and power (CHP) plant at a paper recycling facility. Operating at 160 °C and a seal chamber pressure of 8 barg, the example examines pumps originally fitted with a traditional 85 mm diameter seal. The seal faces received some cooling from the original API Plan 21 configuration (Figure 1). However, the service life of the seal was less than 12 months and this — for water — disappointing seal life was attributed to heat exchanger fouling. Computer simulations indicated that the seal, with Plan 21, would be operating with a seal chamber temperature of 108 °C (226 °F) and with an exchanger heat load in excess of 14 kW. Note that all pumpage (flush liquid) entering at flush port F1 must escape through the throat bushing that separates the seal chamber from the impeller region.

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Figure 4. Bi-directional seal fluid circulation device. (Source: AESSEAL)

Using the same basic operating parameters, a modern mechanical seal (Figure 3), equipped with a bi-directional pumping device (Figure 4) and arranged per API Plan 23 (Figure 2) yielded significant efficiency improvements. Here, the pumping device moves fluid from port F0 to port F1. With this seal geometry and a flow loop per API Plan 23, the seal chamber temperature was lowered from formerly 108 °C to now only 47 °C (116 °F). The heat exchanger load fell to 1.9 kW — less than 14% of the original Plan 21 system. The yearly power savings proved sizable, and their value can be readily calculated. Moreover, switching to Plan 23 and not having to contend with heat exchanger fouling did away with considerable and recurring maintenance expense.

Heinz P. Bloch is a practicing consulting engineer with more than 52 years of applicable experience. He continues to advise process plants worldwide on failure analysis, reliability improvement, and maintenance cost avoidance topics. He has authored or co-authored more than 600 papers or articles dealing with related subjects, among them are 18 textbooks on machinery reliability improvement. A portion of this article is based on the Bloch-Budris 4th Edition text, “Pump User’s Handbook.” Contact him at heinzpbloch@gmail.com.
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