Boiler Feed Deaerator Doesn’t Work
What is most concerning in this story is that several investigations spread over a period of five years missed the obvious. This was a very early experience in my career; the lesson served me well on several occasions and many years later.
I was initially hired in to my first post-graduation job to develop the company’s cold water deaerator designs for oil field water injection. This was very successful and it made the company the world leader in this small but significant market.
In spite of this achievement I still wasn’t considered a fully-fledged engineer, even though I might have thought I was at the time. My manager and mentor decided I needed more hands on experience so I was despatched to a refinery to assist a field technician to investigate a long running problem with one of our boiler feed deaerators.
The boiler feed deaerator had never functioned properly; it barely reduced the dissolved oxygen concentration requiring copious amounts of scavenging chemicals be added to prevent corrosion. Non-condensables (nitrogen) in the steam also impaired the steam utilisation. The boiler feed pumps also suffered numerous failures.
The deaerator system consisted of a water header tank, simple spray deaerator, steam ejector to reduce the deaerator pressure, an extraction pump and a pressure control valve downstream of the extraction pumps which regulated the supply pressure to the boiler feed pumps. The extraction pump was a suction head controlled pump that uses low NPSH to regulate its flow rate. Incidentally these pumps are very robust but they sound like they already broken as they run with continuous cavitation. They sound even worse when the discharge is over restricted.
With so few components, it seemed odd that this one should be so problematic, whilst there were hundreds of similar units out there that worked just fine. It had perplexed everyone who had looked at it over the past five years.
My only thought was that the pressure control valve was blocked or too small as it was operating fully open. The back pressure was so great it caused the deaerator level to back up over the top of the spray bars. That seemed to be too obvious, after all, several people had looked at this over the past five years. It was explained to me that this had been considered several times and investigated. Eventually, they were persuaded to open the globe valve up one more time. With very limited experience looking at control valve internals, I still thought the globe and disc looked small. Like those before me I looked at the data sheet which matched installed internals.
I phoned the manufacturer, yes we did have phones back then, and asked that they confirm the valve sizing. They phoned back a few minutes later saying the data sheet valve size matched the installed size. But they hadn’t answered the question; I asked they check the valve sizing not the data sheet size. Meanwhile a quick calculation of the flow through an orifice showed the disc orifice to be far too small. The valve vendor phoned back about half an hour later. The valve had been sized incorrectly.
We now had the problem of how quickly we could get a replacement plug and disc for our valve. We conducted a search of the refinery’s spares stock and found the correct size of plug and disc. These were fitted and the deaerator started to work properly, but (there is always a ‘but’), the extraction pumps now kept tripping on high power. It turned out that the extraction pump’s motor high current set points had been set too low.
What about the boiler feed pumps, they had failed numerous times? As the pressure control valve was too small the boiler feed pumps had been running with insufficient NPSH. Unlike the extraction pumps, boiler feed pumps don’t fair well when starved of NPSH. The short operating life of the boiler feed pumps was fixed by the correct pressure control valve trim.
The refinery’s Engineering Manager was very grateful for the zero-cost fix, even though it had taken 5 years to get there.
Something simple and basic, everyone had failed to do. They didn’t go back to the source data, instead they relied on derived information without checking it. It seems they missed the obvious because it was too obvious.
Then there’s the ‘but’. Fixing one problem often exposes another. Don’t walk away without checking the whole unit operation.