F&E vent exhaust on pump start up

[benpekarek]

I designed some modifications to an existing LPHW heating system for an industrial application a few years ago, when the customer came to commission the system they have noted a strange anomaly.

When the LPHW circulation pumps are started or stopped, the vent line will exhaust a few litres of water into the F&E tank. However during normal pump operation, the system operated correctly.

A bit of background. The system is an Open circuit with F&E tank with steam plate heat exchanger providing heat. The basic configuration of the system is as follows…
(Return) → Duty / Standby Pump → Heat exchanger → cold feed → open vent → (Flow)

Based on the above, the vent should be in negative pressure and I’m unsure how it seems be under positive pressure for short periods during start and stop of the circulation pumps.

This is not an issue but I need an explanation of why this might be happening for my own peace of mind. Has anyone experienced anything similar and/or has an explanation???

[Freon]

My best theory as to what's happening involves the concept of an "impulse". When the water flow stops and starts there is a pressure wave the will cause an brief increase in the water pressure as the wave moves along the loop and dissipates. The physics can be expressed as F delta * delta V. Not knowing the mass of the water volume I can't say this is what's happening but you may want to explore it further as a possible explanation.