Greetings,

we had two lube oil pumps (vertical centrifugal pump, 7FA turbine) running @ the same time because if only one is working the pressure would decrease.

we have changed the pump with a new one, and the problem was solved. They didn't see any sign of damage or something suspicious when they opened old the pump.

I am not sure if they opened the part that is inside the red circle (see picture in the link), and I think the problem is inside this part.

https://postimg.org/image/wtjkkf9t3/

Any ideas?

 

Check your wiring diagram. You should have a single pressure switch in the discharge header and a off-on-auto switch for each pump.

For example, in a simple circuit "On" designates the primary run pump, "Auto" the backup that kicks in on low pressure.

There are more complicated schemes involving alternator logic so that the pump order is routinely cycled and there are more sophisticated schemes.

 

Aptx4869,

The part in the red circle is the shaft "extension" which connects the AC motor output shaft to the pump impeller shaft. If the pump didn't show any sign of damage or wear it could be that one of the couplings on either end of the shaft extension was loose and spinning.

But rather than looking at wiring diagrams you should be looking at the P&ID for the L.O. system. There should be two check valves, one on each of the pump discharges and if one of them wasn't working properly (the one on the pump which WASN'T replaced!) then that could cause a problem. One which wasn't resolved and which could occur again.

P&IDs are the MOST important drawings supplied with the unit--especially for operators and technicians. They are critical to understanding how systems work. One can be a control system expert--but if one doesn't know how systems work and the various components in the system work which aren't connected to the control system they aren't really going to know how the UNIT works. The OEM has really done a major disservice to operators and technicians by not making displays with animated P&IDS on the HMIs--it makes people unaware of the existence of P&IDs and minimizes the importance of P&IDs to operating and troubleshooting turbines and generators and auxiliary systems.

If you want to really understand how a turbine-generator and it's auxiliaries work, you FIRST need to get your own A3 (or B-size) copy of all the system P&IDs and learn how to read them. It's not hard, and it's well worth the effort. Each system has a separate section in the manuals with generic descriptions of operations and components which are helpful to read when learning the units. By having your own copies of the P&IDs you can make notes and highlight components as you develop your understanding and knowledge of the unit and it's operation.

So, find the manuals, find the L.O. system section, make copies of the P&ID and the system description and start reading and learning. Do this with each system over time and you will become the resident expert--a true expert who knows how the systems and auxiliaries and components work together and are controlled by the control system. That's what makes a good operator and a good technician, as well as a good supervisor in later years as experience and knowledge increase over time.

Hope this helps!!!

 

Thank you for taking time to reply. Apparently, the problem is within the pump. (pump + shaft + discharge) as shown in the link.

Thank you again

 

Dear CSA,

I really like when you elaborate in your reply. As you said, it is essential to understand the P&ID to fully understand the system. Actually usually when I begin studying a new system, I will not skip the P&ID. Thank you for this advice.

Actually we have removed the pump along with the check valve because the check valve is on the discharged pipe very near to the pump, so we removed the pump, discharge pipe and the shaft(extension).

I will check the points you mentioned, do you think it could be from the check valve (spring not as it should) ?

 

Great stuff CSA. When I run training courses, I tell trainees exactly the same. It is amazing the number of people who think that they know their system but you then find that they haven't even looked at their P&IDs.

Your reply should be a sticky for anyone who is learning trouble-shooting!!

 

Can we get a little more detail on exactly where the problem was? It would be useful for us all.