Hi, all!

I've been asked a question by a former colleague I don't have the experience to answer: When should the Cooldown Oil Pump of a GE-design Frame 5 Model R/N/T be started during turbine start-up/shutdown?

This particular machine has the normal Accessory Gear-driven Main L.O. Pump, and uses an early version of hydraulic ratcheting for Cooldown operation when the unit is at zero speed. The other L.O. Pump is a "small" motor-driven pump--with a 5-HP AC motor and a 5-HP DC motor coupled together to drive the pump. The DC motor provides the Emergency L.O. Pump functionality (88QE). The AC motor is labeled on the L.O. Piping Schematic as the Cooldown Oil Pump, but other documentation provided with the machine (which was re-packaged and re-located to its current site) refers to the AC motor as <i>both</i> the Cooldown Oil Pump (88QC) and the Auxiliary Oil Pump (88QA). The Piping Schematic lists the motor-driven L.O. Pump output as 20 psig at 170 GPM. The Main L.O. Pump rating (on the Piping Schematic) is 360 GPM at 65 psig.

The present sequencing in the turbine control system (not a GE Speedtronic turbine control system) starts and stops the Cooldown Oil Pump as if it were an Auxiliary L.O. Pump: during starting starts at START and shuts down at 95% speed, and during shutdown starts at 94.5% speed and remains on when zero speed is reached and Cooldown starts, until operator selects Cooldown off.

Because the motor-driven oil pump rating is so much less than the Main Oil pump rating, I wonder if it should be running all the way up to 95% speed, and if it should be started at 94.5% speed during shutdown. If I interpret the AC motor's name properly (<i>Cooldown</i> Oil Pump) it would mean the AC motor would run the pump during Cooldown (which it does), and probably during initial starting (but not all the way up to 95% speed), and start at a much lower speed during shutdown. Because the rated output pressure and flow of the motor-driven oil pump is so much lower than the Main L.O. Pump it just doesn't seem to make sense to run the pump at speeds up to and below approximately 95% speed--but, neither does it seem to hurt anything, except that there is apparently a pressure bauble when the motor-driven oil pump starts at 94.5% speed during a shutdown, which sometimes results in a unit trip which can't be explained at this writing.

Another thing to note is that Auxiliary L.O. Pumps usually have output pressures nearer to the Main L.O. Pump pressure, and that helps to establish Trip Oil ("Control Oil) System pressure during starting. The motor-driven L.O. pump on this unit barely has enough output pressure to actuate the Hydraulic Trip Oil pressure switches without pressure from the Main L.O. Pump. So, this isn't a reason for running the motor-driven L.O. pump during a start--except to provide L.O. to the bearings as the unit breaks away from zero speed and during initial acceleration, after which the Main L.O. Pump (a positive displacement, gear pump) should be more than capable of supplying pressure and flow to the bearings. Also, the Trip/Control Oil pressure is taken off the Main L.O. Pump output ahead of a pressure regulator which controls the Trip/Control Oil pressure, so the motor-driven L.O. pump doesn't contribute to Trip/Control Oil pressure at all.

Unfortunately, I don't have access to any old Fuel Regulator or Mark I/Mark II Speedtronic elementaries or Service Manuals for Model R/N/T units, so I don't know how the Cooldown Oil Pump was operated when the unit was new.

So, my question is: What was the original operational philosophy for Cooldown Oil Pumps on GE-design Frame 5 Model R/N/T units?

Hopefully glenmorangie and/or otised can provide some information.... Or, if someone has a similar machine and is in possession of the original Speedtronic elementary and/or the Original Service Manual Starting L.O. System Description for a unit with a Cooldown L.O. Pump rather than an Auxiliary L.O. Pump can take a few minutes and look up what was originally provided for pump operation/control logic.

Thanks in advance for any help!

 

This is what was normally called a piggy-back pump system and was fairly common on old LA and MKI systems. I've had a check and I don't have any written documentation that I can find but your description of the operation is pretty well correct from memory. Yes, the "cooldown" pump runs to 14HS but really the Main pump and cooldown pump check valves control what is going on.In that respect it pretty well operates with the same logic and flow rates as the Emergency L.O. pump, if the Main pump fails the turbine will trip,but,if AC power is available, the "cooldown" down pump will run at 14HS dropout and keep running at standstill for unit cooldown.(If no AC then the DC motor takes over and does the same)
I know it looks a pretty strange and crappy system, but it worked !!

Have a look at http://control.com/thread/1286918605

Why don't you use the "search" facility, there is a small box.......... !!

 

CSA,

First off, I don't recognize a 5001T. I am familiar with 5001M, N, P & R. The 5001M (also known as the Mary) had Mark I Speedtronic, and fewer compressor stages, fixed IGV's. The R was an upgrade of this, and had Mark II Speedtronic control. The 5001N (also known as the Nancy) had more compressor stages, variable IGV's (usually open-close) and Mark I Speedtronic control. The P was an upgrade of this, and had Mark II Speedtronic control. The last fuel regulator control 5001 was the LA.

My knowledge of the cooldown L.O. pump is limited. I worked on the industrial versions of these machines which always had full capacity AC motor driven auxiliary pumps. (Our industrial customers really did not like unscheduled shutdowns so all vital pumps had full size backups.)

But I did talk with the engineers on for utility customers, and I remember something about the combined cooldown/emergency lube oil pump. My recollection is that on shutdown the cooldown pump's AC motor would start when the oil pressure dropped to the alarm level (63QA), and would continue to run during the cooldown period. It had sufficient capacity to handle the ratchet and to keep the bearings cool.

As far as startup operation, I believe it was started along with any other auxiliaries; the ratchet was used to break the turbine away (the diesels and electric motors used for cranking needed the help). I don't recall when it was shut down. As long as the pump had a recirculation line between its discharge and any check valve leading to the L.O. header, they could keep it running up until 14HS speed relay. But it's also possible it was shut down after the L.O. pressure exceeded 63QA setting (the pressure switch usually had built in hysteresis so the contacts would open at a higher pressure than where they closed on decreasing pressure) and maybe with 14HA as a minimum allowable speed.

I would not take any of this as gospel, I don't have access to any elementary drawings or piping schematics, device summaries, control specifications, etc.

 

glenmorangie,

>Have a look at http://control.com/thread/1286918605

>Why don't you use the "search" facility, there is a small
>box.......... !!

Actually, I did a search for piggy-back pumps and did find this thread, but it was inconclusive.

I have some more information; the unit was an old Thomassen package, with some unique features added--like an Aux. Hyd. Pump instead of a gear-driven Main Hyd. Pump. The L.O. Piping Schematic was a GE drawing; all of the other Schematics were Thomassen, including the Device Summary. They all had 'Model R/N/T' in the drawing name box in the lower right corner.

I had heard of Model M's and N's, but never a Model T; perhaps a Model Thomassen?

This particular unit has a motor-driven oil pump discharge pressure (when running on the AC motor) of just over 12 psig, with a pressure at the collector end of the generator of about 11 psig, and this through the cooler and the filters. I'm now also hearing that the pump is very noisy--mechanically noisy. So, there's some thought that there's something mechanically wrong with the pump. I've asked for a photo of the pump nameplate.

Thanks, glenmorangie; I'm very grateful for the information!

 

otised,

Thanks very much for the information!

I was wondering if the Cooldown Oil Pump needed to run above approximately 30-40% speed, since it's output will be effectively blocked by the discharge pressure of the Main L.O. Pump acting against the motor-drive oil pump's check valve.

As I wrote in my reply to glenmorangie the pump is now reported to be mechanically noisy, possibly indicating some kind of problem with the pump. It's pressure is less than noted on the L.O. Piping Schematic, and this may explain why.

Again, thanks for the reply and the information. When I hear more I will write back. Unfortunately, with the holidays I think it's going to be a couple of weeks at the earliest--but I have passed along the information.

 

otised,

I had another question about this statement:

> As long as the pump had a recirculation line between
> its discharge and any check valve leading to the L.O.
> header, they could keep it running up until 14HS speed relay.

I don't see such a recirculation line between the pump discharge or a bearing header check valve on the unit Piping Schematic. The Main L.O. Pump has a relief valve on its discharge, and the output is directed to a pressure regulator (VPR1-1) to maintain 60 psig (for the Control/Trip Oil system), and the output of the motor-driven oil pump is connected downstream of VPR1-1 through an orificed check valve (to keep the motor-drive oil pump flooded during normal running), and then to VPR2-1 which controls the bearing header pressure. The only check valve is the orificed one on the discharge of the motor-driven oil pump.

Here's a crude drawing:<pre> To
Control
Oil
System
^
|
| Main | | | | | | 25 psig
---| L.O. |--------| VPR1-1 |-----------| VPR2-1 |----> Bearing Header
| | Pump | | | 60 | | | 25 | (thru filter
| | psig | psig and
| | --- cooler)
| | Relief | /
| | Valve | - Orificed
| | - Check
| | / Valve
| | ---
| To |
| Tank |
| | Motor- | Centrifugal Pump
| | Driven | Rated at 20 psig
| | L.O. | (Currently operating
^ | Pump | at ~12 psig)
From |
Tank |
^
From
Tank</pre>Other than the sensing lines for the pressure regulators (omitted for brevity), there are no other lines in the system.

I'm not quite clear what effect a recirculation line on the discharge of the oil-driven pump would have in keeping the pump running up to 14 HS (~95% speed).

If you can provide some enlightenment on the purpose of a recirculation line and why it would make operation up to 14HS possible/safer/easier on the system, it would be much appreciated!

 

CSA,

I had assumed that a recirculation line back to the tank would be needed to avoid dead-heading the small pump once the shaft-driven pump raised the line pressure high enough to close the check valve between the cooldown pump and the header. For the past 25 years or so I've been dealing with much larger pumps that would not survive being dead-headed. I did say that I would need a piping schematic (P&ID) to provide a more accurate analysis.

By the way, your "crude" drawing is pretty readable.

 

I have been trying to understand your query. On hand I have the P&ID for that setup, and a schematic for a MS1002 which are both similar.

The recirculation line was not visible on my print, but that could be the result of a poor quality copy.

Wouldn't that type of line facilitate a bump-less transfer? when flow restrictions are present on startup.

> I'm not quite clear what effect a recirculation line on the
> discharge of the oil-driven pump would have in keeping the
> pump running up to 14 HS (~95% speed).
>
> If you can provide some enlightenment on the purpose of a
> recirculation line and why it would make operation up to
> 14HS possible/safer/easier on the system, it would be much appreciated!

 

otised,

The motor-driven L.O. pump is a centrifugal pump, and in my experience and understanding running a centrifugal pump dead-headed (with little or no flow) does not generally cause damage to a centrifugal pump. I would have to temper that remark by saying some pumps, and some fluids in some pumps, would be subject to problems if deadheaded for long periods of time. A recirculation line of some size would likely be needed, and would somehow need to be blanked off (say, with a check-/non-return valve--and that's not evident on the L.O. Piping Schematic drawing for this machine. And, perhaps, this particular pump--making the noises it is reported to be making--has been damaged over time because it is or has been dead-headed, possibly for long periods of time (unintentionally).

I still tend to agree with your description of the operation of a Cooldown oil pump--and that is, given that the pressure is less than the bearing header pressure and regulated Main L.O. pump pressure it should not be run much above 40% speed if the Main L.O. pump discharge pressure is above 63QA's actuation setpoint. Convincing the owner/operator of that is going to be difficult, at best. I just wish I could find a System Description from the Operation & Service Manual for a system with a Cooldown oil pump, or a set of Mark I/Mark II elementaries for a system with a Cooldown oil pump to review and be able to say with certainty, "This is the way it was designed to operate."

Anyway, otised, thanks very much for your help with this; as always, your knowledge and experience are very helpful and much appreciated!

 

Your sketch is just about correct, missing was [VR-1] back pressure relief valve for main driven shaft pump, set a 65psi.

[VPR-1] HYDR TRIP SET @ 30PSI
[VRP-2} HEAD PRESS RES 25PSIG

Oil Cooler in return line.

I have never uploaded any type of document especially of this type; for public disclosure, so I don't know how to.

I just wish I could find a System Description from the Operation & Service Manual.

> system with a Cooldown oil pump, or a set of Mark I/Mark II
> elementaries for a system with a Cooldown oil pump to review
> and be able to say with certainty, "This is the way it was
> designed to operate."
>
> Anyway, otised, thanks very much for your help with this; as
> always, your knowledge and experience are very helpful and
> much appreciated!