Diesel Sets vs Gas Reciprocating Engines 3

[scientif]

Hi everyone,

I wish to thank you for your time. Please I need your help to solve this question.

I have a standard distribution system (11kV) making use of an islanded power plant (generator output 11kV) (See attached). The loads for each section are: 0.7MW, 1.1MW, 1.1MW, 0.9MW. Ahead of these loads are transformers of type DYg (11kV/433V, 1500kVA). The other loads are directly connected to the 11kV distribution system and are: 0.6MW (combined load), 3.2MW (Sag Mill), and 5.6MW(Ball Mill). When the ball mill starts, it uses a liquid resistance starter that demands 1.8 times normal load.

I need to determine the number of sets required to start the Ball Mill with a fully loaded plant in two cases: using diesel sets or using gas reciprocating engines.

Diesel sets: 1250 kVA, 11 kV, 50 hertz, power factor 0.8 and inertia constant 50.2 kg-m², damping ratio = 0.13.

Reciprocating engines: 5175 kVA, 11 kV, 50 Hertz, power factor 0.8 and inertia constant 1300 kg-m², damping ration = 0.13.

What should I calculate to find the number of sets?

Thanks a lot for your help.

 

[waross]

Connected load = 7 MW
Ball mill = 5.6 MW x 1.8 = 10.08 MW

Diesel sets = 1250 KVA @P0.8 = 1 MW = 7 sets online.
An additional 10 sets to start the ball mill.
7 sets online plus 10 sets to start the mill.
Total 17 sets.

Gas sets = 5175 KVA = 4.14 MW = 2 sets online.
17 MW needed total.
4 gas sets = 16.56 MW
Close enough for a variety of reasons.

Notes: We are mixing KVA and MW.
MW loading determines the load on the engines. Too much MW loading and the sets will start to stall and drop frequency and RPM.
KVA loading determines the current, internal voltage drop and generator heating.
The short term KVA limit may be determined by the AVR capability to increase the excitation enough to maintain terminal voltage as overloading increases the internal voltage drop.

With diesel sets:
Base load on 17 sets = 7/17 = 0.41 MW per set.
Block loading when the ball mill starts = 0.59 MW per set.
Consult the set specs or contact the manufacturer to determine the block loading capability of the sets.

You can do the numbers for the gas sets.

Is this for school??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scientif]

Thanks a lot for your help Waross! I really appreciate it.

Actually that's for a client with whom I work as a Freelancer.

So if we decide to use the reciprocating engines, we will only need 4 engines, and have less maintenance costs, compared to 17 diesel sets, right?

However, I think we will have more spinning reserve using diesel sets. Plus, the ball mill process doesn't use steam or whatever that could relate to gas being burnt.

Do you think I should suggest using gas reciprocating engines?

Thanks.

 

[catserveng]

I think before you make a recommendation to your customer you need to look at a lot more things.

In general, large medium speed gas fueled engines typically have poor transient response, especially engines designed for lower emissions outputs. May have lower operating cost but you haven't defined term of project. A major overhaul at 40,000 hours can be vey expensive, in some cases you can overhaul a lot of smaller diesel units for the cost of one major on a big gas engine. If this is a five year project maybe not such a great idea, on a 20 year project probably worth considering.

The diesel units will likely have much better transient capability. Again without details hard to say exactly how they compare. If you are trying to keep initial cost down for a relatively short term then multiple "power in a box" units might be the best choice.

Likely your best solution would be a mixed plant, however you would need to work out costs of consumables and maintenance. You would need to take a much closer look at overall load profiles, things like how many times a day or week will you start the ball mill and SAG mill? You defined the ball mill start pretty well, but the SAG mill and it associated equipment may have a higher start demand.

Where are you at in the world, what is the availability of fuel sources and costs? Do you have to meet any regulatory requirements, like for air pollution. Is this a short or long term project? How much reserve capacity do you need?

Lots of things to look at and consider for a power plant to support a project like this. At least in my experience.

Mike L.

 

[waross]

Those figures are quick and dirty. If this was my project I would consider the following;
Loading; Don't trust the customers load figures. There is a big difference between connected load and actual load.
Eg; I was tasked to total up the connected HP for each department of a large sawmill. The figures were used to charge each department a portion of the power charges. Due to diversity, duty cycles and in some instances over-sized motors, the actual load was about 1/3 of the connected load.
On the other hand the load figures may be years out of date and a lot of new equipment may have been added.
If the mill is running now you can often get a lot of useful information from the billing meter. The meters should show the peak demand since the last time that the demand indication was reset.
The sum of several demand meters may be high as the peak demands may not occur at the same time. But the sum of the various demands should be a safe figure.
Beware of seasonal variations. When I go to the power bills for information, I often request copies of the bills going back at least two years.
Consider the instance when you have to make a "black start". It is best to stagger the loads if possible when picking up the loads after an outage.
You may want to consider designing the switch gear to allow he easy addition of more generating capacity. Consider bus current rating and short circuit ratings. This is cheap insurance for the unexpected.
Consider an N+1 scheme. (For 17 diesels maybe an N+3 or +4)
Cost: The cost of running will be much more than the cost of the sets.By far the largest expense of running large, loaded gen-sets may be the fuel. I would carefully evaluate the cost of the various fuel options. When evaluating various sets, compare the fuel economy. Extra cost for a more efficient set may have a short payback period.
Eg; Our little island power plant was replacing two old 600 KW Cats with a 1400 KW Cat. The Cat representative claimed that the greater fuel economy of the newer sets would save enough to cover the monthly payments. (I wasn't in the financial loop and don't know if this proved to be true but I met with the manager daily and never heard a complaint.)
Heads up! When comparing fuel consumption read the fine print. Particularly the specific gravity of the fuel used for fuel consumption testing. Years ago I was comparing fuel consumption for about 5 or 6 different sets. the fuel consumption of all but one set was very close. To close to be a deciding factor. One set claimed about 10% better fuel economy than any other. Fine print; They were testing with fuel about 10% heavier than anyone else.
The first disappointment would be higher than anticipated fuel costs.
The second and more serious disappointment may be when the heavy fuel is not available and the set will only produce 90% of rated power.
Prime power sets have a 10% cushion built into the rating but standby sets don't and standby sets are often sized as close to the load as possible. I had some challenges when a salesman sold my customer a set rated at the wrong voltage and as a result it was 14% undersized. I had to arrange switching to leave some loads off the standby set as it was not capable of pulling the full load.
Diesel or Gas: Check the block loading capability of each against your mill starting load before making a choice.
Multiple sets: Why are you limiting your diesel choices to 1250 KVA? 17 governors tracking together to pick up large block loading sounds interesting. Doable but maybe challenging. The challenge will be greatest when a governor has to be re-adjusted after an overhaul or replacement. An electronic load sharing panel may be the way to go.
Cat will sell you a diesel set that is capable of over 7 MVA
[link]http://www.cat.com/en_US/products/new/power-systems/electric-power-generation/diesel-generator-sets/18492095.html[/url]
By the way, catserveng is our Cat guru and has more large set experience than I. If he posts in, pay attention to his advice.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scientif]

Thank you all for your replies. Just wanted to share with you some conclusions I came up with.

Based on your answers and on some web search, I found that Diesel sets are more suitable for our case. The goal is to keep O&M costs low (not necessarily initial costs). Diesel sets are most widely applicable for small and medium power (non-utility) generation up to 15MW, which is the case of the studied distribution system. The main reason is the higher efficiency of the diesel engines (43 -45 %) when compared with gas turbines in the output range considered. Moreover, the ball mill and the SAG mill require low captive power, without any requirement of process steam (steam is more likely to be available when gas is used in the process). That’s why I think using gas sets wouldn’t be of a great benefit for our case.

The US Energy Information Administration (EIA) has historical data on the average annual operation, maintenance, and fuel costs for existing power plants by major fuelor energy source types (

http://www.eia.gov/electricity/annual/html/epa_08_04.html).

The diesel O&M costs are low (2012).

- Spinning reserve:
Reciprocating engines: have more spinning reserve (5*5.175 @ PF0.8 - 17.68 load = 3.02 MW)
Diesel sets: have less spinning reserve (18*1.250 @ PF0.8 - 17.68 load = 0.32 MW)

Thanks a lot.I'd like to know what you think.

Regards.