Inspection of the Superheater
Types of Superheaters
Superheaters fall into three categories.
1. Radiant Superheating - The superheater is placed directly in the combustion chamber.
2. Convection Superheating - The superheater is placed outside and above the combustion chamber in the path of the hot gases.
3. Separately Fired - The superheater is located in a combustion chamber outwith the boiler that is separately fired to maintain the required temperature of the superheated steam.
We shall use the conventional type of superheater as an example for inspection.
Once an access scaffold is in place, the boiler cleaners can give the superheater nest a good cleaning, both between the tubes and around their surfaces. Particular attention should be paid to the hanger attachment welds, these being wire brushed ready for inspection.
Once the tube nest has been cleaned and blown down a thorough inspection is carried out in the following areas.
- Outer surfaces of the tubes
As we have seen, the superheater is located in the path of the hot combustion gases. These gases contain sodium and vanadium that is deposited onto the surface of the superheater tubes, causing corrosion if not removed by daily operation of the soot blowers.
Note: The soot blowers operate on compressed air motors that rotate and extend a telescopic perforated tube. Once in the right position, the tube is supplied with high pressure steam that is injected through suitably positioned nozzles across the superheater, blowing off the deposits of soot and corrosive scale on. The steam should not come in direct contact with the superheater tubes as this would cause corrosion. Normally the forced draft fan output is increased to assist the blowing of the soot out the funnel.
Many years ago I was sailing as third engineer on an ESSO tanker that had two marine water tube boilers supplying the main turbines. These were pretty old ships, but very reliable – except for the soot blowers. Every day after lunch on the noon-four watch I blew the main boiler tubes soot blowers that were driven by electric motors and they worked fine. But, the superheaters had air driven motors that frequently seized up with the heat in the boiler room, despite being stripped and cleaned countless times. These blowers needed to be operated by hand using a chain block arrangement. This was one of the engineer’s worst jobs, especially when in the Persian Gulf!
Anyway, excuse the ramblings of an old retired Irish Marine Engineer. Continuing with the inspection of the outer surfaces, these normally show signs of corrosion and erosion, but as long as this is not excessive there is not a problem. We used to keep a note in the log book on the tubes' condition as we found them during the drydock inspection. This was so we could refer back to that time, but as everyone has a different perception, the Certifying Authority had the last word on the condition.
Also look for any evidence of thermal creep or cracking both in the tubes and their attachment to top and bottom headers.
The hanger brackets are welded to the structure and are very susceptible to cracking at the attachment welds. It is very important that these welds are thoroughly inspected for high temperature creep fractures and fatigue cracking. The superheater fixing bolts are also prone to wear and should be replaced at the yearly inspection, with high temperature/tensile bolts and locknuts being used.
Some superheaters incorporate screens to divert the excessively hot fumes from the outer tubes. These are also hung from welded brackets and should be inspected as per the main hangers; we replaced them as a matter of course as they were fabricated from thin sheet plate and were usually in tatters.
A sketch of a conventional superheater operating in a water tube boiler is shown below.