Simple relation between Viscosity , Temperature & Clogging of impulse lines
The principle is simple
Temperature and Viscosity of fluid are inversely proportional
Examples when this principle works for us
1.Honey processing industry
Honey processing industry uses this principle as honey is very viscous it is difficult to make it flow through pipes so they raise the temperature through heat tracing and that reduces its viscosity thus making it easy to handle honey
2.Glass manufacturing industry
Molten glass is extremely viscous and approaches infinite viscosity as it solidifies .
So we heat glass at high temperature put in mold we desire and then when cooled the viscosity increases and TADAAA!!! you have your solid glass back
Examples when this principle works “against” us
1.Relief valve clogging Problem
We had a relief valve controlled via pilot valve .
We were using the configuration above
a)sensing line b)Heat Exchanger c)pilot valve d) main valve
This pilot valve had a max temperature of 262.8°C but our process had max temperature of 300 °C .so before sensing line a heat exchanger was added to reduce the temperature reaching the pilot
The heat exchanger option used here is a mini version of the big heat-exchanger
This instrument instead of dissipating with another fluid (hot or cold) used the air to dissipate heat .this was provided by vendor .
Refer snap below .
So as you see the heat exchanger would reduce the heat and the liquid would be cooled and brought below 262°C !!!
And that over time the temperature of liquid at output of heat-exchanger will reach to ambient temperature .EVEN GOOD FOR US !!!!!
But there is another thing to be considered .
What will happen to viscosity when it is reduced from 300°C to ambient ?
As temperature reduces, will the viscosity of fluid be so high that it would clog impulse lines and the valve would never operate
IMAGINE IF THE LAST RESORT IN HAZARD “RELIEF VALVE” DOES NOT OPERATE WHEN REQUIRED WHAT WILL HAPPEN !!!
Solution used in our case :-
In our case yes the temperature could not be brought to ambient (-20°C to 30°C) as the fluid had to be maintained at 45°C as minimum else it would clog
So good that this simple principle was considered and the solution was to heat trace the sensing line to maintain a temperature of 45°C as minimum .
2.Impulse line problem
Impulse line dissipate heat .
Just as thumb of rule it could be considered around a feet of impulse lines dissipates 100°F or a meter of impulse line around 100°C (depends on lot of factors like ambient temperature , material etc)
So ,especially when DP level is measured using wet leg configuration The impulse lines are very long and it has high chances that over time the temperature could reduce drastically so heat tracing could be considered as solution or there are also other methods but it depends on the size of your pocket !
Also ,
Remember the law above is for liquids only
This definition below is by Tony Rudolf (Lessons in industrial instrumentation) which would make the difference clear
The mechanism of viscosity in liquids is inter-molecular cohesion. Since this cohesive force is overcome with increasing temperature, most liquids tend to become “thinner” (less viscous) as they heat up. The mechanism of viscosity in gases, however, is inter-molecular collisions. Since these collisions increase in frequency and intensity with increasing temperature, gases tend to become“thicker” (more viscous)
PS: This is as per best of my current understanding .
Hope this was helpful !!