Circulation Systems for Single and Multiple Seal Arrangements (Part Two)


Written by:
Gordon Buck and Ralph Gabriel, John Crane Inc.

Plan 12

Plan 12 is similar to a Plan 11. The flush is taken off of the pump discharge, or and intermediate stage in the case of multiple stage pumps, through a strainer or filter to remove solids, and then through an orifice to control flow, before being introduced into the seal chamber.

API 682/ISO 221049 does not recommend this plan, as problems can arise if not closely monitored where the strainer can become clogged, the flush flow is lost, and the seal is damaged due to overheating. This can be avoided by using a differential pressure indicator or flow indicator to alert the user of impending problems. Some strainers utilize magnets or magnetic strainers to attract metallic particles like magnitite that will be present in most water systems.

Figure 1. Seal Flush Plan 12

Circulation Systems for Single and Multiple Seal Arrangements

Advantages

  • No product contamination from an external source.
  • Relatively simple piping plan.
  • No reprocessing of product.
  • Solids are removed from flush stream keeping the seal chamber clean.

Disadvantages

  • If the product in the pump is not a good face lubricant, or has extremely low or high viscosity, the seal can become damaged depending upon the seal face material combination.
  • Flush is recirculated.
  • Strainer or filter will plug over time.

Plan 13

Plan 13 is a off-shoot of a Plan 11, where the flow comes out of the seal chamber and goes back to the pump suction. This also helps the seal to vent gas out of the seal chamber.

Typically, this plan is used on vertical pumps where the seal chamber is subject to pump discharge pressure at the top of the pump. However, this plan can be used on horizontal pumps, depending upon the type of impeller used and the differential pressures between seal chamber pressure and pump suction pressure. It is useful on high differential pressure applications where the use of a Plan 11 would require the use of multiple orifices.

On the other hand, for small or low speed pumps that have a low differential pressure, no orifice is required. Due to the flow path, Plan 13 is not as effective as a Plan 11 in removing seal generated heat. The path is such that it comes from the back of the seal and rises up to the gland plate outlet port with no direct impingement on the seal faces. In some cases, flow diverters may be incorporated to improve the flow path or flush rates can be increased to make up for the decrease in efficiency.

Figure 2. Seal Flush Plan 13

Circulation Systems for Single and Multiple Seal Arrangements

Advantages

  • No product contamination from an external source.
  • Relatively simple piping plan.
  • No reprocessing of product.
  • Continuous venting of the seal chamber.

Disadvantages

  • If the product in the pump is not a good face lubricant, or has extremely low or high viscosity, the seal can become damaged depending upon the seal face material combination.
  • Flush is recirculated.
  • Less efficient flow pattern.

Plan 14

Plan 14 is a combination of a Plan 11 and a Plan 13. The flush is taken off of the pump discharge and sent to the seal chamber like a Plan 11. A second set of piping takes the flush from the seal chamber, sending it back to pump suction like a Plan 13.

It is often used in vertical pumps to provide adequate flush flow and vapor pressure margin independent of the throat bushing below the seal chamber. It is used on viscous products to provide a flow path out of the box in addition to the throat bushing that can be restrictive. It is also an effective plan when the throat bushing that partially controls flow rates is inadequate for the seals' needs.

Figure 3. Seal Flush Plan 14

Circulation Systems for Single and Multiple Seal Arrangements

Advantages

  • No product contamination.
  • No reprocessing of product.
  • Optimized cooling. The flush flow can be controlled so that the cooling is directed at the faces and adequate flow is maintained.
  • Allows complete automatic venting provided that the "FO" port in the gland is properly located.
  • With a properly sized orifice and throat bushing, the seal chamber pressure remains high, resulting in an adequate vapor pressure margin.

Disadvantages

  • If the product in the pump is not a good face lubricant or is dirty, the seal can become damaged or clogged.
  • Flush is recirculated.

Sizing

For the above three plans, the flush rate should be calculated based on the pumping conditions to maximize efficiency and seal life. For applications above 3600-rpm or box pressures above 500-psig, the flush rate should be calculated to avoid excessive temperature at the seal faces. In lower pressure/speed applications, a "rule of thumb" of 1-gpm per inch size can be used.

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