Exceeding vacuum ratings on unquenched mechanical seals

[hondashadow1100vt]

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.