Frame 5 Aux. Lube Oil Pump Pressure Issue

F

Thread Starter

FSE1988

Below the steps carried out trying to normalize the lube oil pressure GT.

first day.
We noticed that auxiliary L.O. pump was starting with 4-5 bar delivery pressure and Hi-noise and floating behavior of the L.O. pressure.

o We vent the cooler but the noise was still too high and the max reachable pump delivery pressure was 6,5 bar (required 8,5 bar)

o suspected orifice as a possible cause of low delivery pressure,

second day

1. to change the orifice size from 47 to 38 mm

2. vent the system in order to eliminate the cause of the fluctuation of the oil pressure.

3. Install 2 pressure gauges before and after the orifice in order to calculate the system flow rate

Below the result:
1.
a.After orifice change the delivery pressure increased to 7,5 to 8,5 bar,

b.The noise coming from the pump was normalized - pressure Fluctuation still present

2.
a.After we've vented the cooler, floating behavior was still present.

b.Running the pump we noticed that pressure floating behavior was getting better as long as the pump was running.

c.Assuming that air pocket were the reason of the floating phenomena, we decided to keep running.

d.The floating pressure disappeared after some time

3.
Pump started , COOLER LINE SELECTED ON TCV
a. Aux. pump discharge pressure 7.8 bar (not fluctuating)

b. Filter DP 0.25 bar

c. Orifice upstream 1.95 bar
Orifice downstream 1.8 bar

d. Lube oil bearing header pressure 1.6 bar

Pump started, 50% COOLER LINE SELECTED ON TCV
e. Aux. pump discharge pressure 7.15 bar (Not fluctuating)

f. Filter DP 0.25 bar

g. Orifice upstream 2bar
Orifice downstream 1.82 bar

h. Lube oil bearing header pressure 1.64 bar

Pump started, BYPASS LINE SELECTED ON TCV
i. Aux. pump discharge pressure 7.4 bar (Not fluctuating)

j. Filter DP 0.25 bar

k. Orifice upstream 1.95 bar
Orifice downstream 1.79 bar

l. Lube oil bearing header pressure 1.60 bar

Note:
2 lube oil tank heaters in off condition

yesterday:

o Since the max reachable header pressure was not enough,customer wanted to try to reinstate the 47 mm (this assuming that no more air pocket were present in the interconnecting piping)

o Installed the 47 mm orifice in place, the system went back to the previous status: Low pressure, Hi-noise and floating behavior of the L.O. pressure.

o We didn't succeed neither on the removal of air pocket and increase the delivery pressure, which reached max 6 bar. (due to vent line, site condition)

What could be the exact problem?

Unable to reach required header pressure and delivery pressure?
Pump getting noisy few time while adjusting the cooler by pass line?

Give some tips for this CSA.
 
fse1988,

The most curious thing to me is the "Hi-noise" and noise that you keep describing but seemingly haven't investigated.

Have you checked the coupling between the motor and the pump?

Have you tried using a photo tachometer on the coupling between the pump and motor to see if the coupling is slipping?

Have you checked the current being drawn by the motor?

If I recall correctly, there is no pressure regulator on the discharge of the Aux. L.O. Pump motor, but there is a check valve between the discharge of the Main L.O. Pump and the Aux. L.O. Pump. There is also a relief valve on the discharge of the Main L.O. Pump to control the discharge pressure of the Main L.O. Pump. About the only other things I could think of would be a problem with the check valve between the Aux. and Main L.O. Pumps, or a problem with the impeller on the Aux. L.O. Pump (spinning freely when the flow-rate gets high enough).

Please write back to let us know what you find!
 
Hi CSA,

Thank you so much,

Finally we changed orifice (RO) 47mm to 38mm in auxiliary Lube oil pump and we achieved required Discharge & header pressure (8.0 bar and 1.73 bar).

As we faced same issue in EOP higher header pressure (1.65 bar) we used 38mm orifice and we achieved 1.28bar, still under progress.

we have enough header pressure 1.73 bar but discharge pressure is 4 bar only during the main lube oil pump (FSNL & 1 MW). we tried with PCV but not fluctuating.

is that necessary to raise the discharge pressure to 8.5 bar?
 
FSE1988,

I must have not been thinking very hard.... Sorry.

The Main L.O. Pump is usually a positive displaceent pump, and GE and most packagers of GE-design heavy duty gas turbines use relief valves to control the discharge pressure of the Main L.O. Pump. If I didn't suggest to check that before, then I'm suggesting it now--because if it's relieving too much that will cause problems downstream.

I'm not a fan of changing orifices to re-establish normal pressures. If that's necessary, then something else is wrong--like either the Main L.O. Pump clearances are too large (they are usually gear pumps so internal clearances are important) or seals are leaking; or the Main L.O. Pump "pressure regulator" (which is a relief valve--there's no other way to control the discharge pressure of a positive displacement pump), or there's something amiss with the check valves in the L.O. pumps (they're usually just swing-checks, with orifices drilled in to them prevent loss of any flow whatsoever), or there's a leak in the system somewhere causing excessive flow and reduced pressure.

I think you said you've refurbished the Bearing Header Pressure Regulator, but is there a possiblity that it wasn't refurbished correctly?

Again, I'm not a fan of changing and orifice from what the packager specified on the P&ID--if all components are original equipment or known to match original equipment, and there are no leaks or problems with check valves (refer to the L.O. P&ID), then shouldn't be a need to change orifices to re-establish pressures in the system as per specification.

Sorry; I think there's something I've missed or that we don't know about the system or work that's been done recently or in the past. But, it's just not clear why it would be necessary to change orifices to re-establish pressures if all compoenents were working correctly--and there were no leaks anywhere in the system.

Use the L.O. P&ID to verify all components in the system--including suctions strainers, any foot valves or check valves--noting that ckeck valves in the L.O. pumps usually have orifices drilled in them to either prevent staving downstream systems or to ensure a constant pressure against the suction of a non-running pump (sort of like a poor-mans suction pressurization system).

But I just can't understand a situation where it would be necessary to change orifices if everything in the system is working correctly. And that includes the bearings--are the bearing clearances correct? Are the bearing feed openings properly sized? Have the bearings been changed recently? Were the bearing L.O. feed openings compared to those of the original bearings, or to drawings, to ensure they are correct?

Please write back to let us know what you find and how you progress. At a minimum, am very curious because this is something I've never encountered before (and while I've certainly never encountered every possible problem, I've certainly seem a lot!) and I always like to learn as much as I can from the experiences of others.
 
fse1988,

You didn't respond to CSA questions.

> The most curious thing to me is the "Hi-noise" and noise that you
> keep describing but seemingly haven't investigated.
> Have you checked the coupling between the motor and the pump?
> Have you tried using a photo tachometer on the coupling between the
> pump and motor to see if the coupling is slipping?
> Have you checked the current being drawn by the motor?

A deep investigation should be done before to touch machine design and here, if I have correctly understood what you posted, you reduced the size of <b>two orifices and you still under OEM Spec.</b>

Here where I am, to make a change you must go through a whole MOC process (Management Of Change) with several approvals people, and you could believe me no one will agree a design modification without an OEM Approvals (NIC, TILs,..). And if there is any urgency to put the GT back to service, a <b>temporary</b> MOC will be signed time to fix the trouble.

> is that necessary to raise the discharge pressure to 8.5 bar?

If it's a Frame 5 with double shafts I will say YES!
I am familiar with Frame 5 double Shafts driving Centrifugal Compressors (CC). And on those machines, LO discharge pressure is 9,5 bars for both Main and Auxiliary pumps. The LO discharge will be delivered to the <b>Control, Hydraulic & Starting Systems and to the LO Header. </b>

<b>1 Control System</b> require 6 to 8 bars LO pressure (MS5002B/C & D)
<b>2 Hydraulic System:</b> I know there is a hydraulic pump but, the ratchet mechanism (to rotate the HP rotor after trip/Shutdown) uses the LO from the Auxiliary Pump discharge.

<b>3 Starting System:</b> Usually there is a converter and I'm not sure how much it should be the LO pressure.

In your case (power GT), you may not be concerned with all these systems. You have to get a look to your LO machine P&IDs and maybe there are systems which require LO discharge pressure over the 4 bars you've got?

One more thing; Is your GT equipped with a LO mist eliminator? If the case you need to keep an active vacuum to prevent LO fumes (potential fire hazard).

In my opinion; using smaller orifice to catch the lost pressure you have just hide the problem you didn't solved it. It may be a temporary solution to keep running your GT but never a permanent solution.

Hope this will help
Karim
 
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