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Exceeding vacuum ratings on unquenched mechanical seals

Exceeding vacuum ratings on unquenched mechanical seals

(OP)
The purpose of this query is to request practical feedback regarding any the detriments associated with running a progressing cavity positive displacement pump’s suction at a vacuum that exceeds what the pump’s mechanical seal is rated for. I understand that under a vacuum (in excess of the mechanical seal’s rating) an unquenched mechanical seal will admit air into the pumped process fluid. I am curious if damage or shortened equipment life (either to the pump or to the mechanical seal) can also result.

The following is some approximate application information that might provide insights:
Manufacturer’s published pump NPSHr ~ -22 in Hg (vacuum) (including a safety factor). This is where the pump manufacture recommends the pump’s permissive pressure switch low set point be set.
Mechanical Seal vacuum rating ~ -4.5 in Hg (vacuum). Mechanical seal is (and needs to remain as) the unquenched type.
Pump permissive low pressure switch set point: This switch is currently set at ~-6 in Hg (vacuum). It would be nice to set this at -22 in Hg (vacuum) to match the pump manufacturer’s recommended setting.
Process fluid: Glycerin, which has enough frictional losses in the suction piping to trip the pump’s low pressure switch (set at ~-6 in Hg) even when a significant static head of fluid (~3-feet) exists.

Thank you for any and all feedback that you may be able to offer.

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