Advice wanted on setting up bank configuration

[TN-Steve]

Hey Gang,

The shop I manage is going to be setting up a new workshop / fill station and I'd like some guidence on the best way to set up and configure our air bank.

I'm going to have 12 4500psi tanks available to work with. I believe that they are "T" tanks, they are 52 inches appx from ground to top of neck. My goals are to maximize the number of tanks (normally AL 80's) that we can fill before firing up the compressor, while minimizing the number of line runs to the fill station panel. The tanks are not going to be in easy access from the panel, so manually cascading them from the tank valves isn't an option.

The owners want to go with what I'm referring to a 4x3 configuration, that is 4 "virtual" tanks as far as the control panel is concerned, with each virtual tank being 3 tanks "T" connected together.

My thought is that 4 as far as the number of virtual tanks is good, won't require much additional hardware, and 4 airline runs to the panel isn't going to be a huge factor in cost. (we are doing all the labor inhouse).

I think that a setup of 4-3-3-2 in terms of the number in each array is a better choice, since we usually kill the front of the bank early in terms of useable pressure, and this would sort of 'front load' it. The cascade would run off the 4 first, and the two would only be used for final top-off when needed. (For the record, we currently have 8 tanks in line, and manually work tank valves to cascade)

One other person who's judgement I do trust recommended a bank of 3 virtual tanks, with 4 tanks making up each virtual tank (3x4 config). He says that what we need is VOLUME, to design it to be "Deep" rather than "Wide".

Any help would be appreciated. Any guidance to best practices or general guidelines on designing or setting up this type of system would also be well appreciated.

Make each dive better than the last one.

Steve

 

[Got2Go]

What compressor(s) will be used?
How many scuba cylinders will be filled per day?
Per batch?
How many whips?

 

[TN-Steve]

What compressor(s) will be used?
How many scuba cylinders will be filled per day?
Per batch?
How many whips?

Bauer 18 CFM, Being a dive shop in TN it's going to be mainly batches from classes and quarry weekends, figure 30 AL 80's and about 6-10 HP 100's/ 120's on a Saturday night to turn around, same on a monday morning.
3 whips usually in use.

Hope that helps.

SMW

 

[tbone1004]

few general points

fill the biggest, highest pressure tanks first, starting with the tanks that are at the highest pressure. This is going to maximize the use of the high pressure air in the banks. This is ideal, however requires divers telling you what pressure they're at, and/or a monkey pressure checking. If this isn't the case, then fill the biggest/highest pressures first, and move down the line. Slightly less efficient than knowing all of the pressures, but more so than filling 80's before 120's.

Next. The largest number of banks you can have, the better. I'm not sure what your manifold block setup is, or your whip situation is, but the bigger the better. I.e. do you have a 7 port manifold block that is feeding a pressure regulator into a fill station, or do you just have a board with all of the whips coming off of it? Are their one way valves on all of the whips? etc etc

You have an 18cfm compressor that will be running the whole time you are filling. Deep vs wide is convenient in terms of the effort required for cascading, but to maximize pressure, wide is the only way. Deep is very inefficient in terms of maximizing pressure. If limited to a manifold, the 7 ports are common and limit you to 4 banks so I would run 4-3-3-2. You have 4 big bottles that are going to get low quickly, but when you are swapping tanks over, you shut the low pressure banks off and have the compressor filling the higher pressure banks. The problem with "deep" banks is they fill slowly in terms of pressure.

4500psi cascade bottles are essentially 10cf/100psi. Your compressor fills 18cfm, so 180psi/min for one bottle. If you go 4 deep, it is filling at 45psi/min for that big bank. Your 2x high pressure bank is filling at 90psi/min, and the middle ones at 60psi/min. This allows you to prioritize which bank you want to use and in which order based on what you are doing.
If you have a lot of the tanks at very low pressure, then start with the 4x bank and have the compressor fill the bank of 4 when switching tanks. You'll come up to higher pressures later in the fill cycle.
If you have a lot of tanks at relatively high pressure, you start on the 2x bank and move to the bigger bottles as you go down.
If all of them are at medium ish pressure and you need to get a boat load of them done quickly, you can always open up multiple banks for bulk filling, but by having even banks, you severely limit yourself in terms of the top pressures that you can fill to due to the psi/min that the banks are filling at

 

[2airishuman]

Bauer 18 CFM, Being a dive shop in TN it's going to be mainly batches from classes and quarry weekends, figure 30 AL 80's and about 6-10 HP 100's/ 120's on a Saturday night to turn around, same on a monday morning.
3 whips usually in use.

The cascade comes into its own when filling a few cylinders at a time, since you can do it without starting the compressor. With 12 cascade cylinders, if you're willing to fill them all to 4500 PSI, you can maybe fill 20 cylinders without starting the compressor. But in my experience, most shops don't want the added compressor wear that comes with 4500 PSI operation, so they fill the cascade to 3800 or so, which means you can fill maybe a dozen cylinders without a compressor start.

If you're filling large batches of cylinders at once, the cascade won't make much difference. I would stick with a 3-wide cascade, and use the money saved vs. 4-wide to put in a pressure gauge on each of the three cascade lines, and add an extra fill whip or two. The extra fill whips are helpful for the HP100s/120s because it allows a slower fill and therefore a lower temperature at the conclusion of filling.

 

[tbone1004]

@2airishuman why gauges on each bank? Just have a gauge on the manifold block and based on which bank is open, it will tell you the pressure in that bank. That's the whole point of a manifold block.
Money wise for 3 wide vs 4 wide is going to only be one shutoff valve since everything else is still the same.

The short fills from the compressor also require more banks if you want to get up to pressure. You don't get very far with a 3800psi fill trying to get bottles up to 3500psi.

The compressors are usually rated to 5000psi, having them cut off at 4500 psi is not going to bother them at all. What is ideal for the compressors is that when filling, you leave them filling the low pressure banks while filling, and then when you go to top the whole system off, you do it bank by bank. That lets the compressor go down to the back pressure valve or close to it which helps it to cool off. What can hurt them is by asking them to fill all 12 bottles to working pressure at the same time. Far better to let it go up and down. Also remember that compressors need at least 30 minutes of run time to be happy and cycling them on and off is not good for them.

 

[2airishuman]

@2airishuman why gauges on each bank? Just have a gauge on the manifold block and based on which bank is open, it will tell you the pressure in that bank. That's the whole point of a manifold block.

Either way will work. The place I get my air has a gauge for each bank, and it saves some time and motion because then the operator doesn't have to manipulate valves to read the pressure and then memorize or record the pressure in each bank.

Money wise for 3 wide vs 4 wide is going to only be one shutoff valve since everything else is still the same.

And another manifold port, and another line from the fill station to the cascade room. And another gauge <bg>

The short fills from the compressor also require more banks if you want to get up to pressure. You don't get very far with a 3800psi fill trying to get bottles up to 3500psi.

The setups I've seen are plumbed so that the compressor can be connected to the whips without being connected to any of the cascade bottles.

The compressors are usually rated to 5000psi, having them cut off at 4500 psi is not going to bother them at all. What is ideal for the compressors is that when filling, you leave them filling the low pressure banks while filling, and then when you go to top the whole system off, you do it bank by bank. That lets the compressor go down to the back pressure valve or close to it which helps it to cool off. What can hurt them is by asking them to fill all 12 bottles to working pressure at the same time. Far better to let it go up and down. Also remember that compressors need at least 30 minutes of run time to be happy and cycling them on and off is not good for them.

Depends who you ask, I guess. Maybe it's just folklore. The people have to foot the bill for 3rd (or 4th as the case may be) stage rebuilds tend to want to run the banks at lower pressures, IME.

 

[tbone1004]

@2airishuman only thing is the line from the cascade room. The gauge isn't required and any panel operator that needs gauges on 4 banks doesn't have much of a memory. That said they don't really need to know what each bank is.
With a manifold, the block would have to be bigger, but a 6 port vs a 7 port block isn't that big of a deal. Most of the ones you see are 7 port anyway.

Assuming 1 way valves on the whips-which they should have to minimize risk of contamination into your banks. You connect all three tanks and open their valves fully. Crack the fill valves with the first bank. Those that are lower than the first banks won't do anything and you wait until the first bank equalizes. Compressor is running the whole time.
Shut the first bank off, and open the second bank. Wait until everything equalizes. Lather, rinse, repeat.

Compressor is running the whole time, and when you are done filling, shut the whips off, and leave the compressor filling the high pressure banks.

Compressor is likely a K15, which is a 5000psi compressor, so running up to 4500psi is a non issue. You don't have to run them all that high, but I would at least have one or two of the banks up that high in order to ensure you actually get full fills on the HP steels. The K15 assuming no banks are open, will fill the 120's at about 450psi/min for a single, 150psi/min for the three. You can run the compressor for the high pressure fills if you want. 3800psi banks means you are almost guaranteed to need at least 5 minutes to top off the 120's, so having a pair of cascade bottles at their working pressure will make save a lot of time.

Remember that the pressure maintaining valves are usually set at 60% of the working pressure of the compressor, so the compressor is always running at at least that pressure. Higher pressures are going to yield better filtration due to longer dwell times which is part of the use of the PMV's.

The K15's are rated at 3000 hours for their rings, and the intent is that they are run to their working pressures. That is 3.2m cubic feet of gas. That is 31000 Al80's filled from empty. For conservatism, let's assume that the OP is filling 30x 80's and 10x 120's, from empty, 2x/week. That is equivalent of 90 al80's/week. 344 weeks=7 years between ring jobs. If the shop is following the compressor service intervals based on recommended hours *which they should since it is a commercial breathing gas filling station*, then you shouldn't be extending that service interval. If you are following that, then you may as well run it up to the working pressure of the banks. Do it intelligently and don't equalize all of the banks and then let the compressor try to fill them up together, but it's not going to really hurt something that was designed to work at those high pressures.

 

[2airishuman]

Interesting. None of the shops around here have check valves in their whips. I've asked. I don't like the answer.

 

[tbone1004]

Interesting. None of the shops around here have check valves in their whips. I've asked. I don't like the answer.

if they have a fill station, it may have them built in, or it may have them elsewhere. Really risky allowing tanks to backfill into the banks. You really have no idea what was inside of the tanks that you are filling, so you don't want anything coming back