The steam turbine lube oil system I refer has a LP discharge from MOP (around 3bar), then there is a lube oil cooler with 30 degC set during the first period of the speed up (high viscosity) and then the set changes to 40 degC around 1400 rpm (low viscosity).

After the cooler there is a relief valve with set pressure of around 1.5 bar, later there is a filter, accumulators and finally the bearings. The lube oil pressure indicator is between the accumulators and the bearings.

There is a important lube oil pressure fall while speeding up from 1900 rpm (heat soak ) to 3000 rpm. My first question is anyone know why?

I was wondering about a change of the MOP point of operation due to change in the oil film on the bearing while speeding up. This would mean the pump is working in a point with more less pressure head and more flow, pressure at the relief valve is the same (1.5 bar) but because of the higher flow, there is higher pressure drop trough the filter up to the pressure indicator.

But this theory vanishes when sometimes because of the lube oil pressure reduction, the AOP starts up and pressure in the pressure indicator rise to 2.5 bar.

How can it reach 2.5 bar if the relief valve was adjusted to 1.5 bar???

Does anyone have an idea about this?

Thanks

 

Hey Handsome,

Is the main oil pump turbine driven? At our plant, the steam turbine has an auxiliary oil pump that will supply oil to the jacking pump (lift oil for turning gear) and it will supply lube oil once the turbine is rolling up. At 90% speed (~3250rpm) the main lube oil pump that is driven by the turbine shaft takes over and the aux pump turns itself off.

If your plant has this configuration could it be that this transition is not happening seamlessly?

Just an idea.

 

would it be possible to provide a legeble scan of the oil system schematic on a file share site?

anyway,
is the MOP an AC driven pump?

does the MOP have two discharges? if so is the low and high pressure interconnected either in the pump casing or with external piping and valves?

is the AOP a mirror image of the MOP including outputs and connections?

when the AOP starts running (and pressure is at 2.5) did you secure the MOP or does it continue to run?

is there a shaft driven oil pump?

how much of a drop in pressure occurs between 1900 and 3000 rpm?
when you say "important" do you mean it is a significant drop in pressure that is unexplained OR do you mean this is a design feature that must occur?

 

The MOP is AC driven, with two different discharges, one is HP for auto stop oil and the other LP for bearing lubrication. AOP is exactly the same kind of pump as MOP and mirror.

The drop of pressure is not too much, around 0.3 bar, but enough for the pressure switch to auto start the AOP.

There is not any unsafe operation and I was just wondering about the physics behind the drop of pressure during speed up.

It is surely related with the speed change because if you see the trend, lube oil press is constant while lube oil temp is being increased at 1900 rpm, and suddenly when the ST speeds up till 3000 rpm, pressure starts to decrease, reaching the bottom near 3000 rpm and after sped stabilizes at 3000 rpm, lube oil pressure recovers.
This is one of my doubts.

The other one is as I said, when the AOP starts, pressure raises to 2.5 bar, but I didnt expect the pressure to rise more than 1.5-1.7 bar, which is the set pressure for relief valve.

Sorry I cannot share the PID, but as I explained, it is made up of two same mirror pumps, AOP and MOP, only one is supposed to be working during normal operation. the LP discharge goes to the lub oil cooler (temp control valve is on CCW side), then there is the relief valve, filter, accumulators and finally bearings, The pressure indicator I take the readings from is between accumulator and bearings.

 

> Sorry I cannot share the PID

so I guess we can play "twenty questions" as the answer would be differ for any variances in the system design.

Is the device that sets the bearing header pressure to 1.5 a relief of a reducing valve?

--with a relief dumping excessive pressure to drain to maintain the pump's low pressure discharge near 1.5bar. this arrangement is normally used with a positive displacement pump, but is not common used with a centrifugal pump.

--with a reducing valve regulating a pressure drop between the pumps low pressure discharge and the bearing header, thus it doesn't really control the 1.5, but the delta between across so the exhaust header is 1.5.

Is the low pressure section of the pump also providing supply flow to the HP section of the pump?

--thus the demand of the HP oil will effect the LP pressure.

do you have pressure indication for the header from each pumps
discharge?

--do have any data from them to correspond

I'll end this series of questions intended to determine what type of system you have to throw out some observances I have seen on what may be similar configuration.

--one one design I do set the bearing header to about 0.3 bar greater when on turning gear than the desired pressure at speed. I rarely then need to trim adjust the pressure reducing valves once at speed. the volume of oil flow from TG to full speed is normally about 5 times more for pressure feed bearings (liner of pad design)

--on a retrofit pump modification once, the multiple implear pump's first stage OD was excessive and resulted in the pressure reducing valve set beyond its operating range

--on the units I seen the AOP starting switch is on the pump's discharge headers, not the bearing header. Only the DC EBOP was started from bearing header pressure

 

>Is the device that sets the bearing
>header pressure to 1.5 a relief of a
>reducing valve?

-Relief valve adjusted around 1.5 to 1.7 bar

>--with a relief dumping excessive
>pressure to drain to maintain the pump's
>low pressure discharge near 1.5bar.
>this arrangement is normally used with a
>positive displacement pump, but is not
>common used with a centrifugal pump.


-It is centrifugal pump, it is commonly used in this OEM. I have seen displacement pumps managing flow by using bypass line back to pump inlet.

>--with a reducing valve regulating a
>pressure drop between the pumps low
>pressure discharge and the bearing
>header, thus it doesn't really control
>the 1.5, but the delta between across so
>the exhaust header is 1.5.

-It is not reducing valve, but pressure relief valve.

>Is the low pressure section of the pump
>also providing supply flow to the HP
>section of the pump?

-They are independent

>--thus the demand of the HP oil will
>effect the LP pressure.
>
>do you have pressure indication for the
>header from each pumps
>discharge?
>
>--do have any data from them to
>correspond

-As I said, I dont have since there is pressure gauge on site, but I was not on site/

>I'll end this series of questions
>intended to determine what type of
>system you have to throw out some
>observances I have seen on what may be
>similar configuration.
>
>--one one design I do set the bearing
>header to about 0.3 bar greater when on
>turning gear than the desired pressure
>at speed. I rarely then need to trim
>adjust the pressure reducing valves once
>at speed. the volume of oil flow from
>TG to full speed is normally about 5
>times more for pressure feed bearings
>(liner of pad design)
>
>--on a retrofit pump modification once,
>the multiple implear pump's first stage
>OD was excessive and resulted in the
>pressure reducing valve set beyond its
>operating range
>
>--on the units I seen the AOP starting
>switch is on the pump's discharge
>headers, not the bearing header. Only
>the DC EBOP was started from bearing
>header pressure

-Today I had a talk with a colleague involved in this project. He told me the relief valve had not enough capacity to relief with two pumps working at the same time.

And the drop of pressure was due to foaming when speeding up. The lub oil quality had some problems.

 

>-Relief valve adjusted around 1.5 to 1.7 bar
>centrifugal pump, it is commonly used in this OEM.
>relief valve had not enough capacity to relief with two pumps working at the same time.

I will guess that both MOP and AOP have discharge check valves. so with this design, when the both oil pumps are running the discharge check valves remain open and double flow is provided to the relief valve and overload its dumping capacity.

the dual motor driven pump configurations I have seen have the common discharge header at pump discharge pressure so that in dual pump run, one pump would be checked. But this design uses a pressure reducing valve (not a relief).

the most common lube oil system I worked does use a relief valve with a single variable speed centrifugal pump. the setting procedure is a balance between speed and setting the relief valves discharge swing pipe to have about a 1/3 pipe full discharge flow to assure the relief is in operating range and not overloaded.

I would guess with your design, with single pump running on turning gear, you would have near full relief valve discharge due to reduce oil flow to the bearings. then once at speed when bearing flow increases the relief would be at minimum discharge. do you know if the jump in pressure is as great with both pumps running when at speed as it is when on turning gear?

>drop of pressure was due to foaming when speeding up.

having foaming oil during roll up was one of the strangest problems I encountered. we actually would loss pressure and trip during roll, but when the tank level was inspected a few minutes latter the level was OK. Only by watching inside the tank during roll was the EXCESSIVE foaming observed. the oil quality problem was traced to new oil supplied was shipped in fuel oil tanker