To be continued - A Few Simple Principles Guide You To The Best Seal Flush Plan

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3. External Flush

The external flush method is used when it's desirable to isolate the seal from the pumped fluid. However, if you do not have a suitable flushing liquid available or if diluting the pumped fluid with the flushing liquid is unacceptable, this is not an optimal solution. This method consists of a minimal flow of a clean, relatively cool liquid (usually water) from an external source injected into the seal gland flush connection at a pressure higher than the pump seal chamber pressure. The external flushing liquid flows past the seal and into the pump, mixing with the pumped fluid. Usually a flushing liquid pressure of about 10-15 psi above the pump seal chamber pressure is sufficient.

Flow rates of about 0.25-0.50 gpm are typical, but any positive flow is probably adequate unless there is a need for a specific flow rate to keep the seal temperature at a desired level. A restriction bushing or lip seal can be installed at the seal chamber throat to minimize the flush water flow. A control valve, gauge, and flow meter should be installed in the flushing liquid supply line to provide the control functions necessary to set and monitor the flushing liquid supply, with the flow meter being the most important so that a positive flow of flushing liquid into the pump can be confirmed.

An external flush can perform several functions:

Flushes damaging solids away from the seal

Isolates the seal from the pumped fluid

Cools the seal

Carries away seal generated heat

But, it also has its disadvantages:

Additional costs

Potential problems may occur when diluting the pumped fluid

Possibility of seal failure when the flushing liquid supply is interrupted by system failure or human error

4. Induced Closed Loop Circulation System

Not common, but may be recommended when it's necessary to control the temperature at the seal to a level different than the pumped fluid. It can only be used with a seal that has special circulating features. The seal must be equipped with a device that generates some circulation flow (basically a small impeller) and piping in and out of the seal chamber through a heat exchanger. In most cases a bypass flush with a heat exchanger would be the more reliable method to obtain the same results, but an induced closed loop circulation system can be used to reduce the heating/cooling load since only a small amount of liquid contained in the seal chamber is circulating through the heat exchanger.

If you are considering an induced closed loop circulation system contact your local seal supplier to come up with the best system design.

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