Two Control OIl Pumps with Differerent Capacities on CO Unit 1

[magreen]

Hi,
On a chemical plant, there is a steam turbine generator package with a LO Unit and a CO Unit. On the CO Unit, there are two pumps (variable volume piston pumps) with different capacities. Until now on all the plants that I have seen, all the pumps on LO units and CO units were identical with same capacities. But on this plant, the CO pumps are different. Can anybody tell me is there any reason for that?

Thanks in advance.

 

[byrdj]

would it be possible the post the control oil system schematic.

are the two pumps in a parallel configuration with their discharges in an OR gate (checkvalves)?
Is the pump start/run logic where both can be used as standby/backup to the other?
Is the pump start/run logic establish dominacy of one with the other providing reduced/(increased) function operation following failure/(over capicaty transient) of the dominant pump.
are the pumps used for different flow path circuitry?

 

[magreen]

byrdj:
Thank you for your response.
Please see the attached P&ID of the CO Unit that provides answers to part of your questions.
In reply to your questions regarding logic:
(1) Each pump can be stand-by for the other.
(2) There is no priority for start/run for pump A or B.
(3) It seems there is no dominace of one pump to the other in case of failure to provide reduced or increased flow. All conditions for the two pumps on the ligic are parallel.
(4) As shown on the P&ID the two pumps discharge to the same header for the same purpose.

Does this information provide any clues to answer my question?

 

[byrdj]

I see no clues as to why the two pumps would be different capacity. Your arrangement looks like all the CO pumping systems I have seen that had dual identical pumps. Since I can not provide you with an answer, would you still provide some more information to satisfy my curiousity?

what size turbine
what are the two pumps outputs? pressure, flow, power
How do you know they are differnt?
are they physically differnent?
is there any thing about the location on the pumping skid that would make a smaller pump desired, like having reduced access to remove.
Would the connections allow the pumps to exchange position
Is this arrangement from construction/purchase or is it the result of past mainatance like there was an availability problem or obsolecance requiring a substitution.
is there a construction master parts list that states the supplied pumps and does it aggree
In operation, does both provide adaquate supply, then is one cheaper to run than the other.
Do you know the required system flows for steady state and transient as a percent of pump rated output


I can understand if you prefer not to supply information, but these questions might suggest something for you to investigate to find the answer.

Very interseting

 

[tbuelna]

Maybe I misread the schematic, but is the No.2 CO pump not connected to the circuit with the accumulator?

Also, it is much easier to accurately control the output of a pump having a lower flow rate than one having a higher flow rate. So if there is a need for a very precise control of flow, the pump with a lower output range may be helpful.

 

[byrdj]

I hope magreen does not mind, but I colored the drawing for my benifet and attached to show that the two pump discharged are connected in on OR arrangement (check valves). this common discharge header pressure is monitored by a standby auto start PS, protected from overpressurization by the relief valve and also has a small continous bleed back to the tank. the common discharge header is then connected to the supply header to the unit which has the accumulator. a check valve prevents the accumlator maintained pressure in the supply header to the unit from delaying the standyby auto start should the running pump stops.

 

[JJPellin]

We have two systems in our plant where the main and auxiliary oil pumps were not identical. In both cases, one pump was shaft driven and the other was motor driven. For one such installation, the two pumps ran at different speeds. For the other system, they ran with opposite direction of rotation. Based on your diagram, both pumps are motor driven. Is there any chance they were designed to run at different rpm?

Johnny Pellin

 

[magreen]

byrdj:
Your colored lines are all correct. The question mark is a level transmitter. The data come from the mfr's data sheet. Physically, Pump No. 2 is bigger and a bit heavier (20%), with higher flow rate (30%).

JJ Pellin:
The two pumps run with the same rpm.

 

[byrdj]

The OEM I worked (GE) provided an "Instruction Book" with thier units that included some basic discriptions of systems. Do you have such a referance that might indicate why. You might have to "read between the lines".

Your design does have differances from the GE design that I think is a good enhancement. that is the seperation of the accumulator from the pump discharge and the continous discharge flow. One differnace that I am still thinking about is the single relief on the header instead of seperate reliefs directly at each pump's discharge, but it should be just as effective.

If the unit can respond to hydraulic transients with either, then it just becomes a maintance issue in the need to maintain parts/procedures for the two. Maybe you can propose during the next pump inspection to make them identical and provide a cost benifit for inventory of spares?