Plant Capacity

[zdas04]

I was a witness in a case last week that hinged on a definition of "Plant Design Capacity". I searched far and wide and couldn't find a "standard" that described a process for fixing plant design capacity. The definition that I developed was:
[ul]
[li]Determine the design capacity of each major component using a consistent set of assumptions[/li]
[li]Look at the component with the smallest capacity and (for new design) determine if there is an economic way to reduce that bottleneck.[/li]
[li]Repeat (for new designs) until further improvements cost more than they save. For existing systems use the component with the smallest design capacity [/li]
[/ul]

For this case the limiting component was a compressor. So I figured the flow rate at nominal suction and discharge pressure with all cylinders operating and the machine at max rpm. This is a sweetening plant to remove CO₂, and with just Amine and dehydration, the capacity was 10 MMSCF/d. They are currently flowing 3 MMSCF/d so they had to cripple all four cylinders and the new deliverability was 7 MMSCF/d at max rpm. I claimed that since they removed the valves and kept them, and that replacing them was about an hour's work, removing the valves did not impact "Design Capacity". My argument was that in order to change Design Capacity, a modification must:
[ul]
[li]Require significant capital outlay, AND [/li]
[li]Involve significant time to make the change[/li]
[/ul]

One of the partners drilled a new well behind the plant a few years ago and hit heavy hydrocarbon gas that was too rich for the sales line and they installed a J-T skid for this well. The J-T skid takes a significant pressure drop and requires the compressor discharge to be increased by 300 psi. That discharge pressure increase lowered the capacity of the compressor (with all valves installed) to 8.7 MMSCF/d. I claimed that the inclusion of the J-T Skid changed the function of the plant from "Sweetening" to "Sweetening and Liquids Removal" and that if the rich well was running the Design Capacity was 8.7 MMSCF/d and if it was not running then the capacity was 10 MMSCF/d. The lawyer for the other side said "Does returning the J-T skid to service require significant capital outlay?" "no" "Does returning the skid to service require significant time to make the change?". "No". "So the capacity is 8.7 MMSCF/d, right?". I stammered that "the J-T skid changed the function of the plant". He asked "where is plant function in your list of criteria?". I think I lost that one in a major way.

Do any of you have a definition of "Plant Design Capacity" that would have gotten me in less trouble?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Latexman]

I think you needed a raw material specification, and a product specification.

Define the input. Define the process. Define the product. Then, there should be a maximum throughput to convert the raw materials to finished product.

Good luck,
Latexman

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[LittleInch]

I think you had a good definition at the start of your post of which the key factor to me was " using a consistent set of assumptions", i.e. a written design basis. Your error, if there was one, was saying that to change design capacity needed significant money and significant time. In my opinion, if you make significant changes to the design basis or assumptions made in calculating the design capacity, then this will or could have a significant impact on the design capacity of the plant, without necessarily costing a lot or any money or time. For anything physical, such as flow, pressure or composition, changes can have a significant effect, which as far as I can see in your case would have been a higher outlet pressure of the compressor due to the skid (change of composition) , but the same plant outlet pressure to meet grid requirements

It is a pity that lawyers apparently see the world in black and white whereas engineers see it in it's true colour of shades of grey and trying to determine things on the spot is far from easy when confronted with someone who is not really listening to your answers, so you have my sympathy.

Whilst the J-T skid changed the function of the plant, the reason was the change in the design basis by the change of composition. Design basis data can incorporate many things so for instance if the design basis assumed utility power available of say 10MW, but in reality or due to factors outside your control, the power available was only 5MW, then this would impact the plant capacity.

Hope this helps for the next time...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dcasto]

You needed to say "and the plant meets all required product specifications". for example, the amine plant has all the equipment to process 10 MMSCFD at 5% CO2 with 2% CO2 out to meet specifications. No bottlenecks in any equipment, now make the inlet gas 8% CO2, the plant will flow 10 MMSCFD, but will it make specifications!

 

[zdas04]

I wish the hell I had started this thread before the Arbitration hearing. I held my own (we won every specification), but I was not at all impressed with my performance on that point. Clearly the thing that was missing from my definition was the assumption of a certain inlet stream (the outlet specs were clearly defined in the contract). I said it, but then I ignored the "consistent assumptions" part of the definition. It would have been the perfect answer and it just didn't come to me on the stand.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[SNORGY]

I find it absolutely blasphemous that scumbag lawyers using intelligent word smithing and convoluted word twisting can emerge from these kinds of things looking smarter than the engineers.

Lawyers are a close second on my "list" of professions.