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Dear All Following are details o

Dear All Following are details o

Dear All Following are details o

(OP)
Dear All

Following are details of steam turbine used to drive synthesis gas compressor train at our plant

Make: Nuovo Pignone

Type : Extraction & Back Pressure

No of Stages: 5

Extraction after 2 stages

Inlet steam flow through turbine is regulated by means of five valves mounted on a lifting bar and designed to open one after another in a predefined sequence. Recently turbine was overhauled in presence of OEM representative. Following components were replaced during overhauling

1st Nozzle Ring (New Nozzle Ring Installed)

Rotor (Refurbished rotor previously remained in operation from 2004 to 2013 installed)

All five inlet valves along with lifting bar and spacers (New parts installed)

After overhauling maximum steam flow through HP section with inlet valves wide open has reduced from 345 Ton/Hr to 320 Ton/Hr. Another strange observation is that increasing inlet pressure from 101 Kg-f/cm2 g to 103Kg-f/cm2 g had no effect on mass flow of steam. This is unexplainable for me atleast

Following checks have been done

Maximum Total Lift of lifting bar (and actuator piston travel) has been achieved

Inlet Steam Pressure, Temperature and Extraction Pressure are comparable with previous values
Cross Section of HP Control Valves is attached.

Normal Inlet Steam Conditions 105 Kg/cm2g and 500 0C

Normal Extraction Pressure = 40 Kg/cm2g

Any expert opinion would be a great favor as this problem is limiting plant load.

RE: Dear All Following are details o

Our largest steam turbine has a similar valve rack arrangement with seven valves on a lifting bar. On two occasions in the past 30 years, the turbine failed to perform following an overhaul. Both of these overhauls were supervised by manufacturer’s field engineers and rebuilt with OEM parts to OEM specs. I would not assume your problem is the same as we experienced. But, this demonstrates the sorts of errors that can occur.

In 1991, our turbine was overhauled and rerated. The valve sizes and lift settings were modified along with a rotor redesign and a new nozzle ring in order to increase horsepower. On start-up, the turbine was immediately underperforming and a supplemental compressor had to be brought on site. We verified all of the external parameters, as you did. Four years later, we removed the cover on the valve rack and found that they had reversed the valve rack (end for end) as compared to the print. The number seven valve was in the number one location and the number one valve was in the number seven location. Since the valves were different sizes and supplied a different number of nozzles, the velocities were all wrong and this was the problem. Unfortunately, we did not have the parts on hand to correct the error. We had to install the valve rack again and run for four more years before we could correct it.

At the overhaul when we resolved this first problem (1999), we also started up and had poor performance from the turbine. We were power limited and not able to achieve full speed on the compressor. The manufacturer insisted that the linkage was installed correctly and that we were getting full lift on the rack. However, measurements I was able to take externally showed that additional lift was possible. They insisted that lifting the rack further would have no effect. I tested the theory by slipping a slender aluminum wedge under the cam follower to “trick” the rack into lifting higher. The turbine sped up and the performance improved. We were able to make a permanent change to the external linkage, on the run, to allow the additional lift.

If you have pictures from the overhaul, review all of them to look for errors. Review all of the assembly specs to check for anything that could be limiting steam flow. Verify every part number installed. Read all of the field notes. Assume nothing. Verify everything.

Johnny Pellin

RE: Dear All Following are details o

(OP)
JJ Pellin

Thanks for useful reply. I am actually much confused because increasing inlet pressure seems to have no effect on mass flow. One of my colleague is saying this may be because of choke flow through nozzle. But I believe even at choked flow increase in upstream pressure should cause increase in mass flow through nozzle. Because at choked flow reducing downstream pressure does not increase mass flow

RE: Dear All Following are details o

Once had a problem with a binding vane operating mechanism on a CentraVac HVAC centrifugal compressor; stinking hot summer day, indoor temperatures in the office building steadily rising, unit would only load part way, could not lower chilled water temp down to where it should have been, circulating water inlet & outlet temps almost identical, all pumps operating correctly and water flows normal, refrigerant surging due to being outside operating envelope, air pressure to pneumatics up to normal levels...but actuator simply would not drive the vanes open beyond a certain point.

I decided to give it a little help, and used a handy eight-foot-long 2" x 4" piece of wood [a construction material in ubiquitous North American use] to give it some assistance.

BAM! Gates promptly went wide open, and the unit tripped on overload; not quite what I had in mind...

I restarted the unit, and to prevent overshoot, this time I placed the vanes on manual control and dialled the setpoint to just above the position at where the gates would bind. When I applied the 2 x 4 once again, I was able to stroke the gates past the sticky section, subsequently easing the unit up to full load. Once the temps were down to within normal range, I was able to successfully place it on auto.

When I reported the defect is when my troubles began...

The maintenance department examined the unit, and reported nothing wrong with it.

A fellow operator, senior to me, said, "Don't do that again; you'll break something."

The shift superintendent said, "You may think you know what you did, but clearly you didn't; don't do that again."

Next heat wave, offices got very, very warm during the day, and nobody could figure out why. I came in for my afternoon shift, and the operating supervisor came up to me privately and requested me to "see if there was anything I could do." I did, and had myself a task for every heat wave after that until I transferred out of that division a few years later.

I still couldn't get anybody to believe me; I guess if you've already made your mind up, no evidence in the world will get you to change it...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

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