FACTORS THAT AFFECT MoSi2 ELEMENT LIFE
MoSi2 elements do not exhibit any change in resistance when they are used continuously in air. While a Silicon Carbide heating element will increase in resistance (age) with use, the resistance of a MoSi2 heating element will not change at all. Therefore, in theory, a MoSi2 heating element should last for many years.
However, in reality, there are a number of other factors that usually cause MoSi2 elements to fail, which makes it nearly impossible to predict element life, even under the ideal conditions. Some of the more common modes of failure for MoSi2 elements include:
• Pest – oxidation/degradation of the cold ends – happens if the ends get too hot.
• Electric contact failure – i.e. arcing due to loose straps
• Erosion and Spalling – the protective glaze layer can flake away when cycling the temperature, eventually eroding the element after many cycles.
• Mechanical Forces – hot elements soften and can distort depending how they are oriented in a furnace.
• Insulation Issues – elements too tight in insulation, insulation shifts and breaks the element, or elements contact the insulation and overheat.
• Product contamination – product or atmosphere reacts with the protective glaze causing deterioration of the element.
• Overheat / Control failures
• Over power – too high a watt loading
Unfortunately there is no way to predict the life of a MoSi2 element in an application because one can not predict how any of the above factors will come into play. Generally, the higher the temperature, the more likely one of the above problems will occur. We have a test element that has been operating at 1400ºC in air with no product load for more than 4.5 years now with no sign of changing. It is doubtful that the same element would last that long at 1775ºC. We would expect problems due to spalling or distortion to shorten the life.
The MoSi2 element are manufactured to the same resistance value as the competitors brand therefore they are interchangeable. We offer very fast delivery and excellent customer service.
QS MOSI2 element is a long lasting heating element due to its low ageing rate. Installed according to our recommendations, their life is to most operator´s satisfaction.
Element life is limited due to several reasons, but the major reason for failure is rarely due to material faults or workmanship.
The design of the element lead through is critical for optimum element life.
The most common reasons for element failure, which we have seen have been due to:
•Handling of element packages by the carriers whilst in transit.
•Handling of elements by the customer after being removed from the package.
•Breakage of elements after being installed in the furnace e.g. bumping into an element with a wrench or elbow when carrying out repair work inside a furnace. Also, breakages when elements are removed before relining a furnace.
•All roof suspended elements should be hung freely to allow unrestricted movement of the element shanks during expansion and contraction due to electromagnetic forces and thermal expansion.
•Any binding-sticking of the elements during thermal cycling can result in mechanical and thermal stresses. If this is the case, the element will typically break around the fusion welded joint between the heating zone and terminals.
The contact straps should be long enough so that no stresses are transferred to the elements. If there is sufficient space available, it is preferable to use busbars or terminal posts for series connecting elements as the risk of damaging adjacent elements will be reduced when replacing individual elements.
•Compounds and gases, which may have a detrimental effect on QS MOSI2 material .
•When the element temperature exceeds the limit for each of the three qualities, the silica in the material starts to boil. The MoSi2 depletes through evaporation of the silica and the hot zone shows signs of surface cracking and is pitted in appearance. The effect is the same if part of the hot zone is restricted to radiate freely. If one side of the hot zone is too close either to the furnace wall or some other restriction, this side can show signs of overheating.
Either one or a combination of the following items can cause too high a temperature on the contacts:
•Terminal length protruding outside the furnace being too short.
•Contacts become loose.
•Poor roof insulation (insufficient thickness, quality, cracking or degradation).
•Poor ventilation over the contacts.
The terminals should be sealed at the cold face with ceramic fiber to prevent convective and radiant heat losses (elements must still be able to move freely).
Terminal length too short
It is quite common to see installations where the contacts are very close to the holders.
Loose contacts Can cause thermal shock breakages due to sparking between contact and terminal end. Symptoms: partly melted contacts, thermal cracking of terminal end due to arcing.
When low temperature oxidation (pest) occurs under- neath the aluminizing, the temperature has been far too high. The effect is the same as with loose contacts.
CHECKLIST OF INFORMATION REQUIRED TO TROUBLESHOOT MOLY-D HEATING ELEMENT PROBLEMS
Production capacity, lb/h:--------------------
Furnace Power Rating, kW:--------------------
Furnace Temperature, °C or °F Firing Cycle:--
Number of Zones Time to temperature:------
Refractory Insulation Composition:____________
Diameters of holes in refractory:----------------
Ceramic Terminal Tube used, size:____________
Width Height Length Chamber/Zone dimension: --
Is atmosphere What is the atmosphere dew point?
contained in muffle:-------
3. Electrical data:
Electrical orientation (Wye/Delta/Single-Phase):______________
Transformer Voltage taps: ______________________
Transformer Rating: ______________________
Method of Control (SCR, Transformer Manufacturer: ______________________
SSR or Contactor):
If SCR, what firing method is employed?(phase-angle, fast cycle with phase-angle start, etc? time base, if known):__________________________________________________________
RMS Current limit setting:
Volt (AC) Phase Hz Main Line Power __________________ __________________ ___________________
volatiles are given off from the load during heating?
Is there any steam?
Type of element & part number: ______________________
Number of elements ___________________
Element loading: ______________________
Power per element, kW:_____________________
Element temperature: ______________________
Hot Zone (Le) Length ___________________
Element dimension: ______________________
Cold End (Lu) Length: ___________________
Element Spacing (Centerline to Centerline)________________
Element spacing from chamber walls:__________
Type of element holders used:
Sealed terminal lead throughs?:
Air cooled lead throughs?
How are the elements supported?
(where applicable): Are passage plugs/ terminal tubes/ lead-in bricks being used?
Are the terminal holes parallel with each other or made in some other way?
Are the terminal holes free of debris?
(any signs of condensates in the holes?)
If yes, do you know/what do you think the residues are?
Are the element terminals being packed around with fiber at the ends where they pass through the refractories?
Are the elements still able to move freely in both linear and radial directions? (important for thermal expansion/contraction) Is there sufficient slack in the length of the aluminum straps so as to not transfer stress to the elements?
Are the element tapers all within the heating chamber and not back inside the insulation?
Are all elements operating in the same environment?
(usually yes, but if some elements are in a gas stream, while others are not, then the conditions between one element and another may be different. If no, please expain) What orientation are the elements installed in?
(Normally vertical but can be horizontal resting on tile/supports, etc.) Are there any signs of residues on the element hot zones?
Are there any signs of residues on the element cold ends