Confused about liquid seal pressure

[TexasHomeless]

Hi Everyone,
we have a hexane distillation tower, condenser is a very typical configuration: vapor goes through consenser, un condensed vapor goes through knock out and liquid sealing, N2 flow is controled to keep the vapor before the liquid sealing @0.5 PSIG. The strange thing is: the vapor pressure before the liquid sealing is 0.5 PSIG, while down stream of the liquid sealing is 1.0 PSIG. which means siphons inside liquid sealing. It's natural to think there must be a big swing in term of the vapor pressure ( because the vapor must push the liquid out to vent). However, pressure control is very stable. I can not see why?

 

[reena1957]

This appears to be Gap type split range type of control- Nitrogen make-up if pressure is less than 0.5psi. AND vent through the seal if pressure is more than 1psi. No ation between 0.5 psi to 1.0 psi. If the vapor pressure is pressure is less than 0.5psi , you will see no swing as Nitrogen makeup will smoothly maintain the pressure. Only when the pressure goes above 0.5 you will see the swings.
best wishes