Seal Systems

Pumps with mechanical seals that seal rotating shafts are used in many industries

, starting from pumps in home heating and food industry to petrochemical and nuclear plants. Since a mechanical seal is a device with wearing parts, unless it is serviced from time to time, it will ultimately fail.

A failure of a mechanical seal will most often mean leakage of the pumped fluid outside. The consequences may vary. Sometimes it is OK that a pump will leak until someone notices it. But sometimes it will mean a possible fire for example if a pump is leaking gasoline.

This is why seals are very often used together with the so called seal systems to alert personnel about a problem and probably shut down the pump for repair, and to prevent or minimize leakage to the atmosphere even if a seal has failed. Now, how is it possible?

There are several possible flush plans as they are called in the API 682 Standard.

Plan 65 seal system for a single mechanical seal will collect leakage in a small reservoir which is connected with a specialized drain. The reservoir will have an orifice in the outlet. Small acceptable leakage will pass through the orifice and go to drain. If leakage rate is big enough, the orifice will create an obstruction now, and the level in the reservoir will start to go up, sending an alarm to a control panel.

Plan 52 seal system for a tandem mechanical seal will provide a bigger reservoir, somewhere between 2 and 5 gallons, which is filled with buffer fluid. The buffer fluid is often mineral oil or alcohol solution. A tandem seal has a stand by outboard pair of seal faces which take the pressure if the primary inboard seal starts to leak. But before that, buffer fluid is needed to lubricate the stand by outboard seal. The buffer fluid is filled up to a certain level is usually under atmospheric pressure. If the primary seal fails the pumped product will leak into the barrier fluid loop, raising pressure and level in the reservoir. (This will happen of course if pressure in seal chamber is greater than atmospheric pressure.) A change in pressure or level is easily detected visually or by transmitters. And what is very important, unless the outboard seal fails, too, the product will never leak into the atmosphere.

by: Jai Prakash Srivastava 1

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