Steam Accumulator Sizing 3

[TBP]

I'm on the fringes of a project gone bad. The customer has had a steam accumulator sized and installed (by others), that I think is grossly undersized. Having said that, I've never actually sized an accumulator before.

The service conditions are: operating steam conditions are 220 PSIG (sat), dropping to 190 PSIG (sat), to generate 3,000 #/hr of steam. His existing vessel is 6 ft in diameter, and 9 ft 4 inches long. This has been installed for a year now, and is not living up to it's expectations. At best, the owner would be described as "unhappy".

I've got my "Handbook of Mechanical Engineering Calculations" by Hicks out, and have worked thru the example on accumulators. My calculations show that this vessel should be about the size of a boxcar.

Any opinions?

 

[jproj]

TBP:

I think I might be able to help, but I need some additional information.

1) What is your customers required steam discharge time (e.g. 3000#/hr for 5 minutes).

2) What is the recharge (down) time (e.g. after 5 min. of discharge, the unit is allowed to recharge (no outlet flow)for 20 min.)

Reply to this post and I'll see what I can do.

jproj

 

[TBP]

Thanks for your response jproj.

For now, the 5 minutes in serice, and 20 minutes recharge time will be fine.

The Hicks book doesn't seem to have a factor for time, other than to say that "Since most accumlators function for only short periods, say 5 or 10 minutes, it is more important that the vessel be capable of delivering the desired flow than it deliver the last pound of steam in it's #/hr rating."

I'm just trying to get a feel for this procedure, as I'm always concerned that I've overlooked something when in unfamiliar territory.

Thanks again.

 

[jproj]

TBP:

For 5 minutes in service (at 3000 #/hr), and 20 minutes recharge time, I'm calculating a required inlet flow of approx. 600 #/hr and a required liquid storage of approx. 2400 gal.

This does not include the required overhead space to reduce liquid / vapor entrainment or space for a mist separator. I would agree that for the above conditions, the existing tank is undersized. I would recommend (again for the above conditions) a tank size around 6'-6" OD x 13' 0" straight.

By my calculations, the current tank is sized for about 3.9 minutes discharge.

Another thing you might want to investigate is how the liquid level in the tank is maintained. The accumulator won't work well of there's no liquid in it!

Let me know if you need any additional info, I'd be glad to help!

jproj

 

[TBP]

Thanks again jproj. Please bear with me and help me see what I'm missing. Some aspect of this is sailing right over my head. The last thing I need is to get involved in the current procedings and just muddy the water even more.

The example that's worked through in the book requires 10,000 #/hr of steam at 10 PSIG, flashed from an accumulator at 60 PSIG. For these conditions (no time factor is given), the water content of the accumulator is calculated to be 138,000 lbs, or 2,420 cubic feet. The calculated surface area of the accumlator in this example is 445 sq ft.

Two totally separate publications show that dropping from 60 PSIG to 10 PSIG will flash 7.3% of the water. This would appear to validate the 138,000 lbs of storage. (7.3% of 138,000 would be 10,074 lbs.)

As I mentioned, the Hicks book has no factor for time. If you could indicate how I can plug a time factor into this calculation, reducing the required size of the accumulator, I'd really appreciate it.

 

[jproj]

You're on the right track, you just need to tie it together! (believe I have your reference for the above example... hopefully I can explain it to you)

The calculated storage content (138,000 #) is your liquid requirement over a one hour time period.

As you stated, when dropping from 60 to 10 psig, there is a 7.3% flash. If liquid in the tank is required to produce 10,000 #/hr of steam then:

10,000 #/hr (flashed steam) = 7.3% * Total Liquid

Thus your total liquid requirement is:

Total Liquid = [10,000 #/hr / 0.073] or 138,000 #/hr

For one hour of discharge time (10,000 # total steam), the required liquid storage is 138,000 # (10,000 #/hr * 1 hr).

Make sense?

For the previous example, 3000 #/hr is required. Dropping from 220 to 190 psig gives us a 1.49% flash. Thus the liquid required is ~201,000 #/hr or 201,000 # for a one hour discharge.

If only 5 minutes is required then the liquid required is 1/12 (5 min / 60 min) of the 200,000 #/hr or 16,800 #/hr (~2,300 gal) which is roughly close to what I calculated above.

As I'm sure you've figured out by now, it is all just a material & energy balance. You're on the right track though! If you need more help, let me know, I'll respond tomorrow.

Good Luck!

jproj

 

[TBP]

Thanks for your help, jproj!

 

[twjag]

The # of flash steam from a wet steam accumulator (e.g., as per Hicks) is the total # available for one discharge cycle, regardless of time of discharge. The maximum discharge rate #/hr is governed by water surface area flashing to steam: the higher the ratio of water surface area to total # discharged, the faster the discharge rate per unit time.

There is a phyical limit to #/hr/sqft. As I recall Hicks procedure for accumulator sizing provides guidance on sizing water surface area (#/hr/sqft). Spirax-Sarco's excellent steam handbook, as well as many texts/design guides on boiler sizing provide guidance/rules for determining required water surface area for accumulators & steam drums.

A wet accumulator is just a steam drum without the fired energy input: think of it as an extension of the steam drum.

Questions:
Does the pressure in your existing accumulator drop to the desired 190 psig during the 5 min discharge cycle?
What is the available boiler #/hr for recharging accumulator, & a what pressure?
How well is accumulator insulated?
What happens with boiler steam supply during accumulator discharge?

A 30 psig letdown (220psig initial vs 190 psig final) is low for an accumulator, resulting in larger required acumulator.

If properly installed, you can combine boiler & accumulator steam flows during discharge cycle. This requires valving out the recharge steam from boiler to accumulator during accumulator discharge, & directing the boiler steam flow to the process served by the accumulator.

 

[TBP]

The plant where this is installed has pretty much abandoned this mess. It's just a bulge in the steam line at this point. Fortunately, I was not involved, other than to bid on installing somebody else's botched design. I indicated at the time I thought the plant would be better off just installing a proper fire tube boiler. It wouldn't have cost more - in fact at the end of the day, it would have been cheaper. And would have worked, too.