Bump test

AshutoshMehndiratta,

This is Control.com. Occasionally we answer questions that are more mechanical in nature--because controls uses or controls mechanical devices (in many instances).

This is pretty much strictly a mechanical question.

Having said that, when I think of or am asked about a "bump test" in relation to an uprate and MI, I think of a rotor bump check (another name for the same thing--isn't the English language a wonderful thing.?.?.?!!!). That where the turbine rotor, usually uncoupled from the accessory gear and the load gear (for a single-shaft Frame 5 GE-deisgn heavy duty gas turbine) is moved axially to measure the total axial movement. Usually, when the bump check is done it is done with a hydraulic ram pushing on one end of the shaft or the other to move the shaft against the active thrust face, and then against the inactive thrust face. Usually, the shaft is thrust in one direction (say, the active thrust fact, and a dial indicator is set to 0.000. Then the rotor is thrust against the inactive face (in the other direction), and the dial indicator reading is recorded, then the shaft is thrust back in the original direction and another reading is taken to ensure the shaft returns to the same position (0.000) or another set of readings of taken.

From an electrical or commissioning or re-commissioning start-up point of view, there are electric motor "bump tests" (or "bump checks"). In this test, usually performed after an electric motor has been replaced and reconnected, the motor's motor starter is quickly started to observe the direction of rotation of the electric motor output shaft to be sure it is running in the proper direction. This can and should be done for 3-phase AC motors, and DC motors, as well.

Or, if the 3-phase AC mains power to the MCC (Motor Control Center) line-up is disconnected for some kind of maintenance or refurbishment or replacement then the same thing should be done to at least one 3-phase AC motor after the MCC line-up is re-powered to make sure the AC mains was properly re-connected.

So, there are bump tests and bump tests (sometimes called bump checks and bump checks). You just have to love the English language....

Hope this helps!
 
AshutoshMehndiratta,

This is Control.com. Occasionally we answer questions that are more mechanical in nature--because controls uses or controls mechanical devices (in many instances).

This is pretty much strictly a mechanical question.

Having said that, when I think of or am asked about a "bump test" in relation to an uprate and MI, I think of a rotor bump check (another name for the same thing--isn't the English language a wonderful thing.?.?.?!!!).

Sir, I do agree with you. Its the beauty of English language.

What I actually meant is, the start up procedure and precautions, testings that are to be carried out before machine is loaded to base load??
Kindly explain them???


Thanks and Regards

Ashutosh Mehndiratta
 
AshutoshMehndiratta,

[I am completely joking when I say the English language is so beautiful with respect to words and terms such as 'bump test.']

OOHHHHH! You were referring to THAT bump test, the one where after the unit is reassembled and prior to trying to start the unit and accelerate it to rated speed and then load it the unit is given a START command and as soon as the rotor starts to accelerate from zero speed the unit is either tripped or given a STOP. People who had been stationed in various compartments then listen for any unusual sounds as the unit coasts back down (decelerates) to zero speed.

The purpose of the (this particular version of ) bump test is make sure, one, the shaft will rotate after reassembly, and, two, to make sure the internal clearances are ”good” and do not result in rubs (squeaks) which might cause failure of internal rotating and/or stationary parts or cause excessive wear thereby eventually reducing efficiency.

It's a very unscientific and potentially destructive test--but very necessary as a final check of reassembly technique and good workmanship.

An example of bad workmanship would be like the time a 15/16-ths combination end wrench was left in the axial compressor section when the upper casing half was put on and the bolts used to close the gap between the two casing halves were tightened and the reassembly of the rest of the unit was completed. When the unit was bump tested (this particular version of bump test) there was a terrible noise heard coming from the area of the axial compressor in the turbine compartment as the unit started to rotate and it didn't accelerate very quickly from zero speed and it very quickly returned to zero speed.

Several stages of rotating and stationary axial compressor blades were damaged requiring the unit to be disassembled and repaired.

Now, if a bump test had not been performed and the unit had been started normally the resulting damage would have been much worse, so the bump test was very useful from that perspective. But, still, it is no substitute for proper tests (of internal clearances) and inspections (to look for tools or bolts or nuts or washers, etc.) left inside the unit during reassembly. It does happen that internal clearances are too tight sometimes, and that things do get left inside the machine that shouldn't have been left inside the machine. EVERONE, from the mechanics to the electrician's to the instrumentation & controls technicians to the site management to the service provider supervisors are responsible for properly performing all clearance checks/ tests and for ensuring cleanliness of reassembly work and techniques prevent leaving undesirable materials inside the unit during reassembly. A bump test (this particular type of bump test) is NO substitute for proper workmanship and cleanliness. It can reduce, in some cases, the damage from poor workmanship or shoddy reassembly practices--but that's all; it's not a preventative measure in any way.

Let this be a lesson to everyone reading this thread--don't use terms and words you are not familiar with. Make sure you are being clear and concise when asking a question. Try to be sure you are explaining as best you can the situation you are asking about.

Those are all the 'bump tests' I can think of when it comes to a heavy duty gas turbine outage restart or new unit commissioning. There may be others (the quite often imprecise English language can be twisted and perverted in many ways).

As for trying to detail or explain other restart procedures after a maintenance outage--my thumbs need a rest. If this is your first outage restart or the first time you've heard that English language term used on an outage restart, just be patient. Many times well-meaning people use terms others aren't familiar with; those well-meaning people will usually be happy to explain what they meant if you ask them at a good time. Others use such terms in order to make themselves seem more knowledgeable and important than they really are, quite often meaning something slightly or even very different than what they actually meant. You can still learn from such people--but not so much in a technical way; more like how to say what you mean and to be sure you know what you're talking about and that the people listening to know what you are saying or referring to.

If you want to know the tests and procedures that are going to be performed during the restart, there should be a list or schedule of restart tests and activities which will be used to guide everyone during a very busy time (restart). Get that schedule and ask questions of the people you work with who have been through a restart before. A good supervisor or manager should be happy to sit down with you at a good time and help you understand what is going to happen and why and how.

Hope this helps!
 
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