Process Equipment Update
Hot or cold – don’t forget to top up
Andy Burns, technical business manager at Global Heat Transfer, explains the best practice for restarting thermal fluid systems after a shutdown.
Andy Burns, Technical Business Manager bei Global Heat Transfer, erläutert die beste Vorgehensweise für den Neustart von Thermofluidanlagen nach einer Abschaltung.
Andy Burns, directeur des affaires techniques chez Global Heat Transfer, explique les meilleures pratiques lorsqu’il s’agit de redémarrer les systèmes de fluide thermique après un arrêt.
H
Fig. 1. The header tank on a thermal fluid system.
ave you ever wondered where the hottest place on earth is? It would seem that the record belongs to the Mponeng gold mine in South Africa.
Two miles below the surface, rock temperatures can reach 150°F (65.5°C). Workers can only survive there if slurry ice is pumped into the mine and the walls are insulated with concrete. As impressive as this temperature
is, it does not come close to the ones in which thermal fluid operates in industrial environments. For example, the Globaltherm range includes oils which are designed to operate in temperatures of up to 752° (400°C). As chemically stable and secure as these oils are, they will suffer deterioration caused by heat. This means regular maintenance and topping up as often as necessary, are essential actions for any operation. The volume of thermal fluid can
expand by 25 to 30% when the fluid is hot, compared to its volume when it’s cold. Industrial plants normally operate incessantly, which is why thermal oil is kept at a constantly high temperature.
Because of this, the volume of thermal fluid is always expanded. However, periodic shutdowns for planned maintenance result in a decrease in temperature, which makes the oil volume shrink, often below the low level switch. As a rule of thumb, the expansion tank should be one third full when the heat transfer fluid is cold, and half full when the fluid is hot. Volume reduction caused by
problems with system venting or leaks can result in very low thermal fluid levels in the header tank. When this happens, there is a good chance the system will not restart without a top up, because it has ‘tripped out on low level’ or fallen below the low-level switch indicator. In this situation, the best advice is not to panic. There are several ways to resolve the problem, depending on the type of heat transfer fluid your company is using, the size and capacity of your system and the amount of fluid you require.
Is volume loss inevitable? In a nutshell, yes. Assuming the system vents adequately, if it generates more light ends than it breathes out, the volume will not change significantly. However, this will cause low flash points, fire points and auto ignition temperatures, which is another, more serious issue all together. Heat transfer fluids degrade over time, because of extreme temperatures and continuous temperature fluctuation. The two main chemical processes during which thermal fluids degrade or break down are thermal cracking and thermal oxidation. Thermal cracking is the process
that takes place when heat is applied under pressure to the thermal liquid. It results in the breaking of the fluid’s molecules into smaller molecules. This chemical reaction releases several types
10
www.engineerlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68