MAINTENANCE – Maintenance of electric steam generators


by Gualtiero Beretta – Part 2

The first part of this important technological contribution was presented in the last issue of Detergo, focusing on the frequency of servicing, work on the boiler, pipes and level checks. This second and final part looks at flanges, washers, gaskets, start up and electrical power supply.

Cleaning the flanges
The next maintenance operation consists in cleaning the flanges of the heating elements and that of the float, if applicable. The flanges must be cleaned thoroughly, ensuring that all residue left from old gaskets is removed and the sealing surfaces are brushed by hand or machine.
Any irregularities due to erosion must be evaluated carefully. If there are signs of erosion on the face of the flange that can be removed (the heating element or float side), it may be more convenient to simply replace the flange or schedule its replacement at the next planned maintenance.
Deep irregularities on the faces of the fixed flange on the boiler body may require highly professional servicing in order to restore the sealing surface. Metal fillers and the like rarely succeed in resolving problems related to high-temperature pressure seals.
Assembling the gaskets
P1010024The sealing gasket can be assembled after the flanges have been thoroughly cleaned.
The recommended material is self-adhesive Teflon, in fact many manufacturers already implement this choice in their first equipping.
An adhesive Teflon gasket must have a cross-section suited for the preload imposed by the bolts of the flange, since this type of gasket provides top performance when compressed until becoming a pliable, compact film; thicker gaskets require greater compression of the upper flanges, which is only possible when the bolts are sized appropriately.
You should also avoid circles around the holes of the clamping screw threads, since any leaks in the ring cannot be detected while silently corroding the screw.
A single circle of the sealing material well-adherent to the inner edge of the flange, if correctly applied on clean surfaces, can provide a perfect hydraulic seal.
Preparing the connection elements
P1010038In order to protect the threaded parts, facilitate tightening and above all make future disassembly easier, the connection elements of the flanges (screws or stud bolts) must be coated with an anti-seize paste resistant to the generator’s operating temperatures of 150/180 °C according to the working pressure.
The paste prevents any seizing up between metal parts due to clamping friction, and it also protects the thread surfaces against the formation of intermetallic oxides that can make it impossible to disassemble the fastening element later.
A bolt that breaks during disassembly causes a serious problem which requires significant time and expertise to fix. It’s much wiser to prevent this by dedicating a few minutes to protecting the threaded element during assembly.
Any corroded screws whose resistant cross-section is compromised must be replaced.
Tightening the flanges
Once all the threaded elements have been set by hand, they can be tightened. The P1010040tightening must be done at low speed and following an appropriate crisscross sequence in order to promote uniformity of the flange support.
The final tightening of the threaded elements must be done with the aid of an appropriately set torque wrench.
The threaded element, in fact, behaves mechanically as if it were an elastic spring and requires a preload which, in addition to causing its elastic elongation, allows correct compression of the flange so that the seal is guaranteed once it has been stressed by the internal pressure. The load is actually proportional to the working pressure of the generator and the square of the flange’s size; if the threaded element is not preloaded correctly, under the pressure load it would undergo additional elongation with consequent separation of the flange and loss of the hydraulic seal.
On the other hand, excessive tightening of the threaded element may cause the elastic limit of the screw material to be exceeded, thereby causing irreversible plastic deformation. A screw that has undergone plastic deformation (elongation) can be identified by the modified pitch of part of the threaded profile or the reduction of a non-threaded zone. A deformed element no longer possesses its original strength characteristics and must be replaced.
Controlled tightening allows the correct preload to be applied to the threaded element by adjusting the torsion force applied to the wrench based on the coupling characteristics, i.e., in relation to the diameter of the threaded element, the strength characteristics of the material of which the element is made and the friction coefficient concerning the state of lubrication of the threaded surfaces.
The tightening values can be obtained from appropriate tables. For a thread that has been lubricated well with anti-seize paste, we would choose a table with low coefficient of friction (such as 0.10nU), then we would look for the column regarding the strength class of the screw (usually 8.8) and the row regarding the diameter of the screw, and finally we would choose the value where the two intersect.
Correct tightening of the screws is essential for the safety and long service life of a threaded coupling. Uniformity of compression of the gasket, uniformity of distribution of the stresses due to the pressure load (as high as a few tons, not to be underestimated), and uniformity of stress on the coupling elements are all qualities that guarantee reliability, safety and a long service life of the coupling.
A guide and excerpt of the tables for tightening carbon steel screws are available, for example, in the Facom tools catalogue.
Electrical connection and switch-on
P1010048After the electric power supply connections of the heating elements and the level control connections have been restored and the continuity of the earthing connections has been checked, the system can be switched on.
Caution: the operations described below must be carried out exclusively by the electrically skilled person in charge of the electrical installation (with suitable training to perform work on live parts), as these operations involve measurements made at less than 15 cm from live parts.
The first thing to check is the correct operation of the level regulator (the heating elements must remain disconnected until there is a minimum level of water that guarantees complete immersion of the heating elements).
Then we must check the level of water reached at pump switch-off, as the generator body must conserve a space for steam accumulation equal to at least 25% of its total volume.
With the heating elements connected and operating, measure the power supply voltage and absorption of each element; using a clamp for dispersion currents, it is then possible to check the effective degree of insulation of the heating elements under working conditions (subjected to working pressure and temperature and immersed in water). Often heating elements that seem to have good insulation when dry and cold actually trip the plant circuit breakers when placed in working conditions.
Interrupted heating elements and faults of the control contactors can be discovered by measuring the voltage and absorption.
Finally, check the hydraulic seal of the flanges and fittings with the plant at maximum operating pressure. Seeping can usually be identified through the noise generated by the leak.
The check valve of the pump must also be verified. With the boiler at maximum pressure, there must be no return of steam towards the pump, which must remain cool just like the check valve.
Test of electrical connections (Thermography)
Thermography allows us to easily “see” that which is difficult to measure. This case shows that even though apparently in excellent condition and despite the normal measurement results, thermography analysis reveals unmistakable overheating of the wiring of the central and right heating elements.
P1010049This problem can often be found on heating elements terminated with a Faston-type connector. The Faston, even though of excellent quality, becomes annealed by constant high temperatures, consequently losing its elastic characteristics and often oxidizing the limited contact surfaces. When removed and reconnected, the Faston deforms and permanently loses conductive properties; this increased resistance to the passage of current causes the cable to reach critical temperatures (300/400 °C) that damage the insulation which, even though made of heat-resistant silicone rubber, carbonizes when it reaches these temperatures. Even the copper of the wiring takes on a dark brown color due to oxidation that occurs as a result of the high temperatures.
Identifying this problem early makes it possible to repair by simply replacing the Fastons and, if necessary, deoxidizing the male terminal of the heating element. If the connection continues to overheat, the heating element terminal often becomes unusable due to thermal oxidation, causing irreversible damage to the power wiring.
As we have seen, it is essential that electric steam generator maintenance be carried out with the correct frequency in order to guarantee correct and safe operation as well as extend the service life of the equipment.
P1010051We often mistakenly think that we can save money by skipping some maintenance, but the contrary is often true: failure to perform adequate maintenance entails higher costs due to unforeseen machine shutdowns and, even more so, unexpected deterioration of the equipment itself.
The quality of maintenance is also of great importance, as the diagnosis of malfunctions require specific experience and equipment in order to be carried out efficiently. What’s more, failure to apply some important tactics during assembly, whether we’re talking 04 INTERNET 2014about electrical, hydraulic or mechanical parts, subsequently leads to significant problems at the next disassembly. For this reason, it is always recommended that these maintenance operations be carried out by a professional as detailed by the equipment manufacturer’s instructions.