Brine Pumping Problems - Hycool 20

[Brian_UK]

Question regarding the pumping of a secondary refrigerant namely Hycool 20, potassium formate for -20°C.

Basis:
We have a brine chiller, Frigabon, with four compressors/evaporators serving gravity coolers in a chilled working area and a +2°C cold room. Set point is -7°C for the brine and is achieved and held without problem.

The pump set is based on two Grundfos inverter driven pumps with a low loss header between the two circuits.

Circuit 1 has the pump at a fixed speed and circulates the brine around the chiller and the low loss header.

Circuit 2 has the pump controlled by a differential pressure sensor, across the pump, to maintain constant pressure. It takes the brine from the low loss header, pumps it around the load circuit and returns it to the header.

The secondary load side has seven separate circuits fed via two port controls valves. There is a permanently open bypass circuit across the secondary header circuit to maintain some circulation when the valves are closed.

This bypass also passes water as the various valves close.

Without taking the insulation off the pipes I am guessing that the secondary header pipe is either 3" or 4" diameter (75 or 100mm) and the bypass is only 3/4" or 1" (20 or 25mm).

When the brine is going through the bypass line there is considerable noise being generated where it passes through the regulating valve orifice.

System static pressure with pumps off is 1.5bar; with pumps running the suction pressure at the low loss header would be around 0.8bar. Discharge pressures are roughly 1.0bar above this. System is a closed system with small expansion vessel.

Problem:
At certain flow conditions such as with one control valve open and the remainder closed, brine rushing through the bypass with a lot of noise, the system suction pressure will fall to around 0.2bar to 0.4bar and hold at that for some time.

This will cause the brine system low pressure switch to trip and shut down the system. Obviously not good for the the end product.

Thoughts:
I don't know enough about brines but from what I have read they can form a slurry if the temperature gets near their lower limits and also that they should not pass through too small restrictions.

Do these brines act like our regular refrigerants when throttled through a small orifice such as my bypass valve and pipe?

If they do then is it possible for my -20°C brine to turned into a partial slurry because the operating temperature of -7°C is having a pressure/temperature drop through the orifice?

If I assume that a slurry is being formed then is this physically reducing the system brine volume and thereby reducing the system static pressure?