Wednesday, February 27, 2013

Beware! Steam Turbine Deposits of 1929 to 1936 have surfaced once again.

At one IPP in India H. P. Heater Safety Valve blew when the machine suddenly touched 103.6% of rated load. Maintenance Engineer thought that the original Safety Valve setting could have been disturbed but on checking the data in DCS it was found that the extraction pressure had actually reached the set value of Safety Valve to blow.

Now this is not a simple matter. The set pressures of Heater Safety Valves are such that the Turbine could never provide steam at that pressure i.e. the set pressure is higher than the extraction pressure under Turbine VWO (Valve Wide Open) condition. The blowing of the Safety Valve indicates restriction in the Turbine Casings.

On further analysis the Turbine First Stage Pressure was also found very high and higher than the value recorded during VWO test on Turbine. In spite of more than 6% capacity over TMCR under VWO condition established during Performance Guarantee Test the load had to be restricted at 98% of TMCR due to high First Stage Pressure. Analysis of all extraction pressures revealed restriction limited to High Pressure Turbine.

Restrictions may be caused by the deposits on Turbine Blading but Power Station Boiler Water Chemistry is very advanced since 1960's and deposits in high pressure zones are unknown.

I had to dig out a paper published in May 1936 by University of Illinois
Engineering Experiment Station. It was possible to get this paper because University of Illinois had undertaken Large-scale Digitization Project in 2007 at at Urbana-Champaign Library.


The title of the paper is The Cause and Prevention of Steam Turbine Blade Deposits and download link is here.


Prior to 1936 the Power Station Water Chemistry was evolving in USA. In fact the author of the paper was Special Research Assistant Professor of Chemical Engineering at University of Illinois.


I am taking liberty to quote from this paper:
"Purpose of Investigation.-Steam electrical generating stations
have encountered difficulty in the form of fouling of turbine blades.
This difficulty has become of major importance in many large stations,
whereas it has only meant annoyance in other stations.
There are several types of deposits which form on the turbine
blading and cause this fouling. One type is that which is apparently
caused by a deposition of solids carried in the steam from the boiler
water, and another is that caused by a chemical reaction between
chemicals in the steam and the material in the turbine blades. The
first type is the most common, and is readily distinguished from the
other in that it is largely soluble in water, and is washed off with comparative ease, whereas the other type of deposit adheres to the blades very tenaciously.
The deposition of solids carried in the steam appears to be the
major cause of difficulty. The efforts of this research have been directed entirely toward a study of this type of deposit, and no study
has been made of the other type.
The purpose of the present investigation has been to assemble
data relative to the occurrence of this type of deposit on steam turbine
blades in order to determine the cause of the difficulty and to devise
methods of preventing it."
Resume of Central Station Experience.-The following extract
from a letter serves to illustrate very clearly the difficulty caused by
this kind of turbine blade fouling.
"The operating records show the machines can only be kept in service for a matter of 3 to 4 weeks before the effective output of the machine drops about 20 per cent. The deposit is easily removed by washing, but of course this necessitates shutting down and leaving machine cool off, with a subsequent loss in the overall station efficiency as well as temporary reduction in the plant availability. The washing process adopted does not involve anything more than allowing the machine to cool down for 36 hours, and then starting up in the normal way, the condensation produced being sufficient to clear the fouling."
Although Power Station Water Chemistry is very advanced in India there is a reason to believe that Boiler Feed Water got contaminated with Cooling Water at this IPP and the conditions similar to pre 1936 in USA got created inadvertently.

Moreover when the unit was down for Annual Overhauling for more than 30 days the restriction due to high First Stage Pressure had vanished after Overhauling and there is no need to get surprised if you read the bold sentences in the above quotation from the research paper.

This post is to caution the new IPPs coming up in our country.



Wednesday, February 13, 2013

L. P. Turbine Bearings in separate pedestals.

The title may surprise new generation of Turbine Engineers because L. P. Turbine Bearings are mounted in separate pedestals since BHEL the indigenous supplier began manufacturing as per KWU (German) technology.

But if you look at 200/210 MW Steam Turbines manufactured by BHEL as per LMW (Russian) technology the bearing housing is just an extension of lower half of L. P. Turbine. Even the Generator front bearing is placed in L. P. Turbine casing.

Condensers are placed on springs which get compressed due to weight of water in hot well, Condenser Tubes and Water Boxes. This may bring the bearings down and there was a theory that it may affect the alignment between I. P./ L. P. Rotor and L. P./Generator Rotor. Although we did not check the rotor alignment by filling water in hot well (normal level plus additional water equivalent to water in tubes and water boxes) this was done at other power projects. When one such expert visited Obra where I was doing the erection of 200 MW Steam Turbines, I showed him that the alignment was getting affected even due to variation in ambient temperature by taking the alignment readings at 4 PM and 6 AM. This was happening because the L. P. Turbine Casing expands and contracts vertically from its foundation and the height of the bearing from foundation is considerable.

The pedestal mounted bearings have more rigidity and they are not affected by movement of L. P. Turbine Casings due to weight of water or thermal expansion.

Recently I was asked whether 4x600 MW Steam Turbines at Jharsuguda manufactured by Dongfang have bearings in separate pedestals or in the extended portion of L. P. Turbine. The answer is very simple the Steam Turbine at Jharsuguda is state of the art and the bearings are mounted in separate pedestals like BHEL KWU design.