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# time to fill air bottle 4

## time to fill air bottle

(OP)
Dear All,
there are air compressor, and recharging station. in recharging station, 6 of 9 litre air bottle will be recharged (from 0 to 200 bar) at the same time. I need to calculate time to recharge these bottles. Generally, compressor data sheet gives average flow rate to charge one of 6 (not 9 litre) litre bottle from 0 to 300 (not 200 bar) bar. but i couldn't know how to change this number to my case. could you please help me and show the way how to do that

P.S compressor is not here. So I cant measure time by stopwatch. need calculation

### RE: time to fill air bottle

That's a better post.

Still not much information but here goes.

More volume at the same pressure is basically pro rata. So assuming the pipework isn't a big factor the time for a 9l bottle compared to a 6 is times 1.5..

But 200 bar compared to 300 needs more data about the compressor and how the system works. Might be 50 % less time, might be 60.

Describe the components and operation better then we might get somewhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
LittleInch,

Thank you for response.
I have only one data which shows relationship between compressor pressure and its flow rate. below graph, it stated that charging rate is subjected to tolerance of 15%. performance figures relate to maximum speed 1750 rev/min and ambient normal temperature and pressure of 15C and 1015 m bar.
I need to set compressor discharge pressure and need to prove that with "X" discharge pressure this 9 litre bottle fill in "Y" minutes

### RE: time to fill air bottle

That data seems odd to me. Basically for the same presumably inlet air volume the power hardly changes regardless of pressure.

Also how does this system work? Via a large receiver or start from an empty bottle and then start the compressor?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
LittleInch,
That is actually my problem that I don't understand how system works.
There are 2 compressors which works lead/lag. There are 6 accumulator bottles and each of them 61litr. In line which comes to bottle, there is pressure regulator which reduce pressure to 207 barg.
first question: what do you think what is purpose of pressure maintaining valve (set 260 barg upstream)?
second question: will pressure regulator downstream pressure (207 barg) decrease if I start to fill bottle?
third question: will pressure regulator have impact on mass flowrate through line?
fourth question: how I can calculate time to fill air bottle by using large air storage bottles only?
fifth question: how I can calculate time to fill air bottle by using compressor only?
final one: how I can calculate time to fill air bottle by using both compressor and bottles (which is normal operation) ?

I know there are a lot of question. but I am very unhappy that it is already 2 weeks and I cant find answer to my questions. please help me if anyone knows.

### RE: time to fill air bottle

After a quick look at your P&ID, I'm not convinced that the thing you've labelled as a pressure maintaining valve is doing the things I would expect a PMV to do. I wonder if it's been drawn the wrong way round.

To me, it looks like it should be a reducing valve to allow you to limit the charge pressure in your bank (the 366 l worth of accumulator bottles) to 260 bar, in much the same way as the other PCV is there to limit the charge in the 9 l bottles to 207 bar. You need a bit of extra pressure in the bank if you want it to be able to fill the bottles completely (and at a reasonable rate) when the compressor isn't running.

The thing that does look like a PMV is the 124 bar valve immediately downstream of the compressors. You need that to make the water separators (and any driers) on the compressors work effectively.

"Will pressure regulator downstream pressure (207 barg) decrease if I start to fill bottle?" Yes. By how much depends on the flow, on the droop characteristic of the PCV and realistically on the flow capacity of the system upstream.

"Will pressure regulator have impact on mass flowrate through line?" To some extent, that's what it's there for. Once the downstream pressure reaches the setpoint, that impact will become very significant. When the downstream pressure is lower, the PCV ought to be wide open - but will inevitably offer more resistance than a bit of bare pipe would.

"How I can calculate time to fill air bottle by using large air storage bottles only?" This might be the wrong question. How fast do you actually want to fill the bottles? Bottles filled in haste have a nasty habit of cooling off so fast that they deny you the leisure for repentance. Once you have an idea of the flow rate you want, you can start looking at how much pressure you would lose along the system and whether there is enough extra pressure available at the source to cover the flow losses.

A.

### RE: time to fill air bottle

(OP)
zeusfaber ,
You are great person I am looking for. thanks a lot for clear and wide response.

Thing about pressure maintaining valve is that it is exactly pressure maintaining valve because of documentation. Moreover, when I visited offshore platform I looked at pressure gauges in accumulators they showed 300 barg pressure. So if it is pressure reducing valve, how accumulators can show larger pressure than set one? (maybe they are used in order to charge storage bottles more than 260 barg pressure, more air)

why am I asking about pressure regulator, it is because of pressure difference. For example I have 310 barg pressure in accumulators and I plugged air bottle to line. what will happen? air will flow from 310 barg to 0 barg? or it will from 207 bar to 0 barg? that is point I want to understand

real problem is that, before me, compressor pressure was set 310 barg. Now, it is needed to decrease the pressure of compressor for some reasons ( it is very close pressure to PSV set points). what i want is to calculate amount of time that will be needed to fill air bottle with new pressure for example 290 barg. because before for example, it took 3 minutes (it is number just i gave from mind )to fill bottle with 310 barg, now new 290 barg pressure will take more time than it, right? I need to see the mathematics theory between pressure, flow rate and time.
(by the way only thing I know about compressor flow rate is that it is average charging flow rate is 268litr/min if it fill 6 litre bottle from 0 to 300 barg)

I understood that you are very clever, can you suggest me any way to solve this problem? to see what will new pressure have impact on system?

### RE: time to fill air bottle

OK. If there's supporting documentation to confirm that it is a PMV, then here's what I think is going on. This is a different story from the one in my earlier post.

If there's lots of air in the bank, air will flow from the bank into the charging panels. During this phase of operations, the pressure just upstream of your 207 bar PCV (let's call this manifold pressure) is roughly the same as your bank pressure, and drops as you discharge the bank. The pressure immediately downstream of the PCV is 207 bar The difference between 207 bar and the pressure in the empty bottles plugged into the panel is dropped across whatever device it is that limits the charging flow - on manually controlled charging panels, this is often one of the isolating valves on the panel which is kept cracked just open - or if you're lucky, a needle valve provided for the purpose. In your system, it may well be those orifices drawn just downstream of the panel isolation valves. Upstream of the orifice: 207 bar. Downstream: Whatever is in the bottles you're charging. (This probably answers one of your questions).

As the bank pressure drops below 285 bar (if I read the fuzzy figures right), the first compressor kicks in - followed by the second one a bit later. Assuming that you're still drawing air out the charging panel faster than the compressors can put it in, the bank pressure will continue to fall, with both bank and compressors putting air into the panel. Although it makes no difference at this point, the PMV closes as the manifold pressure drops below 260 bar.

For as long as the bank pressure stays above 207 bar (and if you're only charging two or three sets of bottles, there should be enough air in a 366 litre bank starting at 310 bar to make sure it does), then the charge time depends only on the size of the orifice, the 207 bar above the orifice, the volume of the bottles you're charging and the amount of air that was already in them when you started. It doesn't depend on the compressor setpoint.

As the manifold/bank pressure drops below 207 bar, the PCV opens fully and the top of the orifice sees manifold/bank pressure instead of a steady 207 bar. This is a key moment as it's only at this point that the bottles begin to fill more slowly than before.

The PMV really comes into its own when the bank/manifold pressure equalises with the bottles you're charging. Now you're relying on the compressors alone to finish the fill. With no PMV, the compressor would try to put air into both bank and charging panel - a total volume of 420 litres - and it would all get very slow indeed. Instead of this, the closed PMV (coupled with the NRV between bank and manifold) isolates the bank and ensures the entire compressor flow is dedicated to the charging panel. Manifold and bottle pressures rise together, but the bank pressure does not.

Once the little bottles reach 207 bar, the PCV closes and, with no outflow, manifold pressure rises quickly until the 260 bar PMV opens - at which point, the compressors start to recharge the bank - continuing until the last compressor trips out with 310 bar at compressor, manifold and bank.

So what does this mean for your problem? I understand you are proposing to wind back the trip pressure on the lead compressor by 20 bar to step back a little from the final stage relief pressures. This will almost certainly entail adjustment to the cut-in pressure and also to the lag compressor settings. The main effect will be to reduce the amount of air stored in your bank when the system has finished topping it up and this in turn will affect the time it takes for the system to switch from (fast) filling by decanting from the bank to (slower) filling direct from the compressors. With the bank and client cylinder sizes you've described and a change from 310 to 290 bar, that switch won't happen until maybe the very end of the charging cycle of your third set of six bottles or (more likely) partway through the fourth set. For the first couple of sets, you shouldn't see any difference at all in charging time.

A.

### RE: time to fill air bottle

(OP)

You are very helpful. really thanks a lot. you saved me. But Again I have questions, I will be grateful if you answer.

From your first paragraph, I understood that pressure (207 barg) after PCV will be constant till needle valve just above of air bottle. Am I right? which means that in ALL cases, there will be air supply from 207 bar ( between PCV and needle valve) to X bar (in bottle) which X will go from 0 bar to 207 bar (at the end). Am I right?

From your third paragraph, I understood that charging time has no relationship with compressor set point ( doesn't matter it is 280 or 400 bar) till bank pressure (manifold pressure) is higher 207 bar. am I right? from there, I understood that, in this case, higher compressor pressure will be helpful to charge not air bottle quickly , but compensate air in bank more quickly. Am I right?)

From your fifth paragraph, now I have a question here. now you see lead compressor pressure 310 and 285 barg, while lag one is 290 and 265 barg. PMV set to 260 barg. If I change compressor pressure for lead 290 and 265, while lad 270 and 245. and setting PMV to 240 barg. will this action have any impact on theory you stated in fifth paragraph? I mean, changing PMV set value can have any bad consequence?

And finally in your last paragraph, you said
" With the bank and client cylinder sizes you've described and a change from 310 to 290 bar, that switch won't happen until maybe the very end of the charging cycle of your third set of six bottles or (more likely) partway through the fourth set"
now, if third set of six bottles will charge till manifold pressure reduces to 207 bar, it means that pressure after pcv will be 207 barg and therefore charging time will be same in these 3 sets. but in last sentence you said " For the first couple of sets, you shouldn't see any difference at all in charging time" it means in the second set charging time will be different than first set? why? shouldn't it be same? if manifold pressure is more than 207?

### RE: time to fill air bottle

I did not study the diagram is great detail, and some of the piping appears to run off of the drawing you supplied. It seems to me that the compressors are designed to keep your storage bottles full to 260 bar. The pressure supplied to the fill bottles is regulated to maximum of 207 bar.

If the supply pressure from the compressors drops below 260 bar, then all of the compressor output is directed to the fill bottles only, and is not used to fill the storage cylinders.

The logic appears to be that the compressors are optimized to run against 310 bar of pressure at all times. This makes sense since multi-stage piston compressors will have much smaller high pressure pistons than low pressure pistons and the piston size ratios can only be optimized for one pressure.

When filling empty bottles, at first almost all of the flow is from the storage bottles. If your usage draws the storage bottles below 260 then all of the compressor flow will be used only for topping off the fill bottles. When the bottles are full, the compressors will work to top-off the storage bottles. If you fill a lot of bottles, the storage cylinder pressures could drop below 207 bar but they will not refill until after your fill bottles are filled.

### RE: time to fill air bottle

(OP)
Thanks Compositepro for your response. That is exactly what I am searching for. Why pressure maintaining valve set to 260 barg? not 250 or 240? there should be story of this value. Moreover, as I fill bottle with air, will pressure after PCV will decrease? or not? ( which is 207 barg)

### RE: time to fill air bottle

I don't think there is much I can add as to the operation of the system as shown in your P&ID from the quite excellent posts above.

clearly changing the set points will impact the operation and if you set the second compressor below the magic 260psi as you propose then the likelihood is that the bottles will reduce in pressure before the second compressor cuts in.

however you posts before have all been about time.

No one can start to answer this question without knowing:
Are there 6 bottles or 5 as shown in the P &ID
Do they all start off at the same time? - I can only assume as shown that you work in sets of 3 bottles and then 2 bottles otherwise the pressure will equalize very rapidly.
KEY question - What is the restriction device shown just downstream of the two Hand Valves??
What happens with 6l bottles (does the compressor run nearly continuously or what?)

Time then becomes a mass flow equation. If this item is in fact an orifice then you're in critical flow at the start and given upstream pressure stays constant, then mass flow will stay constant until you get to about 70% of the inlet pressure (about 145 bar). The pressure time graph line will be more or less straight up to that point and then start to get parabolic as the mass flow rate starts to be affected by the downstream pressure through the orifice or restriction device more and more. For this you need to either do small step changes to work out the rate, increase the pressure and then revise or get a quadratic equation.

So altering the compressor settings could have a big impact if you're charging hundreds at a time and your compressors are at the limit now or could have no impact on charge time. Understand mass flow rates and you'll understand a lot more about your system.

The KEY is in the flow restrictor. And also what is the mass flow restriction coming from the 207 barg regulator.

PS - the note on the drawing "set to bottle charge pressure" relates to the storage bottles, not the smaller bottles you're filling As noted above, this valve will only open when the pressure upstream your 207 barg regulator is > 260 barg.

To me this implies that this is an intermittent operation whereby a relatively small set of compressors gradually pumps up some big accumulators which then de-pressurize from 300 bar to say 210 whilst filling a set of smaller bottles at a mass flow rate a lot higher than the compressors mass flow rate. Otherwise I can't see why you would have such a significant set of storage bottles. Hence my advise to you is follow the mass flow. It seems at the moment that unless you change that flow restrictor, if you put on 9l bottles instead of 6 l bottles, nothing much will change apart from the fact it will take you longer per bottle and possibly per kg of air.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
LittleInch thanks for response.
you mentioned" The pressure time graph line will be more or less straight up to that point and then start to get parabolic as the mass flow rate starts to be affected by the downstream pressure through the orifice or restriction device more and more. For this you need to either do small step changes to work out the rate, increase the pressure and then revise or get a quadratic equation."

This is what I am looking for equation that will be suitable to find flow rate and plot the graph. please help me if you know

### RE: time to fill air bottle

Before you can do that you need to find out two things.

1 What is the restriction device and it's size. Is it an orifice plate, a venturi, some other type??

2 What is max flow through the regulator?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

To pick up on a couple of questions in the 13/21:59 post.

First "Am I right?": Almost. The only exception is when the bank empties and the manifold pressure drops below 207 bar. Under those circumstances, the pressure downstream of the PCV will also drop (it's a valve, not a compressor after all).

Second "Am I right?": Not quite. Changing the compressor setpoints won't make a lot of difference to how fast the bank fills. It's much more about how much air is stored in the bank before the last compressor cuts out.

As noted previously, winding all the upstream pressure settings down by 20 bar will reduce the end pressure in the bank. Whether or not this is a "bad thing" depends on what you actually require the system to be able to do - important information that you haven't shared with us. How many bottles do you need to be able to fast fill in one session before the fill rate drops to whatever the compressor pushes out? Will you still meet this requirement?

You misinterpret "For the first couple of sets, you shouldn't see any difference at all in charging time". It does mean charging time for set two should be the same as for set one.

A.

### RE: time to fill air bottle

(OP)
LittleInch,
I am sorry what do you mean by restriction device? ( I am just student) By looking at P&IDs , there are valves. I looked at datasheet of valves to air bottles. orifice size is 0.156 inches and Cv value is 0.35.
Max flow through regulator according to datasheet is 3887 l/min when inlet pressure is 310 bar and downstream pressure is 207 barg.

### RE: time to fill air bottle

(OP)
zeusfaber,

If accumulator new pressure is 290 barg, then I can fill 12 bottles with the amount of air in air accumulator. Of course, after filling 12 bottles accumulator pressure will be 212 barg approximately. so in terms of amount of air it is not problem. My problem was about charging time with new set pressure of compressor. And you said that it will same till manifold pressure is higher than 207 barg. so I understood that there will be no problem about charging time and amount of air in terms of air bottles. Okay then, what about air accumulators? for example, imagine air bottles is full and it is needed to fill air accumulators with air comes from compressor. for instance, with 310 barg pressure, compressor fill 366 l accumulator to 310 barg in 12 hours. what will be happen if compressor pressure is 290 barg? will 290 barg compressor fill 366 litre accumulator in 12 hours? or more time?

### RE: time to fill air bottle

tarlan,

What I mean is this - see highlighted parts below.

The symbols simply show this as a "restrictor". Could be many things, might be adjustable, but until you know what it is, how it is sized, what equations of flow to use you can't start to do any calculations on mass flow.

I can only assume your 3178 is litres at standard conditions. Your 9 litre vessel at 200 bar will be around 1800 litres air at atmospheric conditions.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
LittleInch,
I found that restrictor is orifice and area of it is 1/4 inch. max flow rate through regulator is 3887 l/min

### RE: time to fill air bottle

(OP)
IRstuff,
no it is not for school, it is just real problem I see

### RE: time to fill air bottle

Tarlan,

There's something not right about that orifice size. Even at 1/4" diameter this seems to provide for much more air flow than the regulator which doesn't make sense.

You can check it using any gas orifice plate calculator you can find online.

however you also rally need to find out what that regulator does at 0 barg, 10 barg, 20 barg .... downstream because at the moment as soon as you connect your bottle(s) to the connection and turn on the tap, the air pressure will drop to close to zero virtually instantly and the only real restriction on flow is your regulator if your orifice plate dimensions are correct.

Your P & ID is very indistinct, but it looks to me like the connecting tube between the compressor skid and the filling bottle connections is 1/4" OD tube?? 6.3mm

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
LittleInch,

but zeusfaber mentioned that pressure after pressure regulator will be constant( 207 barg) and doesn't matter you fill bottle or not. it will be constant unless upstream pressure of pressure regulator is more than 207 barg

### RE: time to fill air bottle

No he didn't. What he said was that it was a maximum of 207. It might remain close to 207 providing the mass flow through the regulator is equal to or more than the mass flow out. But if it is less them the pressure will fall.

Your current data basically has an open ended pipe with no real flow restrictions going into empty vessels.

Your 1/4" area or even diameter is far too big and flow will be limited by the flow through the regulator.

Please check the orifice size and coburg that we're talking about instrument tubing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

Lots of interesting things here.

You asked what I mean by "restriction device". I'm talking about whatever it is that is used to control how fast air is going into those bottles - and to answer that question, you need to spend a forenoon on the shop floor watching somebody filling bottles (or better still, doing it yourself). It could be done with an orifice in the place your diagram has orifices marked, or it could be done by cracking the panel isolation valves, or it could be done by cracking the individual whip isolation valves or even by cracking the individual cylinder isolation valves on the bottles. I've seen all of these done in practice and been guilty of most of them myself.

I share LittleInch's scepticism about the size of your orifices. If they really are 1/4" diameter then, with just two or three bottles downstream of each, they aren't going to be the thing which limits the charging rate. Just for education, try taking the cylinder valve of one of those little bottles apart and have a look at the size of the jet that the seat closes down onto. The air has to get through that hole too. It may be that the orifice is a safety device to try to limit the trajectory of the charging panel should you lose a union somewhere, in which case, the flow rate is being controlled by something else. The 0.156" orifices in the (whip isolation?) valves are too big to limit the flow effectively too.

Yes, I did say that the outlet pressure of the PCV would be a constant 207 bar as long as the upstream pressure was high enough - and that's close enough to being true for most purposes provided you don't abuse the device. If you look more closely, the outlet pressure varies a little with inlet pressure (by how much and in what direction varies depending on whether it's a spring regulator or dome loaded, balanced or unbalanced and on whether the seat is upstream or downstream of the jet). Under normal operation, it also varies a bit with flow (I mentioned droop in my first post). That all changes changes dramatically once you exceed the design flow for the regulator. As you draw more flow, the valve opens wider and wider to compensate for the falling outlet pressure until, once the valve is as open as it can go, the outlet pressure starts to collapse quite quickly. What LittleInch is telling you (quite rightly) is that if you parallel up two or three 1/4" orifices with no other restriction, the initial flow will vastly exceed the 3887 l/m limit of the regulator and you must expect to collapse the outlet pressure.

So does this mean the regulator is too small for the job? Actually, I don't think it does. You haven't told us how fast you want to charge the 9 litre bottles - but if you blow six of them at a time at 3887 l/m, they will be full in under three minutes, will get scorchy, scorchy hot - and will subsequently cool down to some pathetic pressure (probably in the 150-170 bar range - again, a bit of time on the shop floor would be instructive) by the time you go to use them for anything. If instead you aim for a fill time of ten to fifteen minutes, the regulator capacity is ample (but a 1/4" orifice isn't going to give you a ten minute fill time).

Finally, what will changing the compressor settings do to the time needed to recharge the bank. This is where you need to think a bit more carefully about what it is you are changing on the compressor. If, as I suspect is the case, you are just winding down the settings on the pressure switch that turns the machine on and off, then turning the compressor pressure down will reduce the time it takes to fill the bank. Why? In rough terms, a recip compressor behaves as a constant mass-flow device. A small compressor charging into a large receiver will cause a (very) approximately linear rise in receiver pressure. The slope of that pressure rise depends on the capacity of the compressor to suck air out of the atmosphere, not on the setting on the cutoff switch. Of course, if the switch is set to cut out earlier, then you are declaring the bank to be full earlier and have reduced your refill time by the simple expedient of moving the goalposts.

Don't know how how far down your studies you are, but this is a really good example of real world engineering - where you have a moderately complex system made of lots of interacting bits, none of which behaves exactly like a simplistic model says it is going to and some of which behave really differently once you step beyond normal operating conditions. It highlights the importance of knowing really clearly what it is that you are required to achieve (in terms of charging times, number of bottles you need to fill in one session and recovery time for the bank) and shows the benefit of both a bit of practical experience of what size things usually are and also how system operators routinely work. You'll notice that, apart from a little bit of multiplying pressures and volumes together, there's been almost no recourse to flow formulas so far (you need to know how to do that as well, but it often isn't the answer). Hope you manage to learn from it (and do try to get some hands-on time using the system).

A.

### RE: time to fill air bottle

LittleInch posted while I was still typing - benefit of saying in just four lines what took me a couple of dozen. We're both on the same page.

A.

### RE: time to fill air bottle

What post. Well done. Agree with it all. LI.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

Tarlan,

You have received a number of very extensive replies to your posts, yet appear to have gone to ground. This is disrespectful to those who have responded in their own time free of charge.

If you can supply more information as requested then we might be able to actually try and answer your initial question, but some sort of response to finalise this would be the proper way to respond.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: time to fill air bottle

(OP)
Thanks al lot to all of u! I wasn't here, therefore couldn't reply. Thanks again! you are very helpful!

### RE: time to fill air bottle

Something does not smell quite right here , we are talking about a bottle refill station . A lot of charging lines that have multiple storage banks, close off on or two cylinders in the storage bank at full pressure, then decant for final fill so that they do not have to wait for the compressors. Then when the fill is done , they open the storage tanks back to the compressors saving time. Unless of course these compressors are so powerful they can bring the tanks to line pressure without the associated pressure drop and lag.
B.E.

You are judged not by what you know, but by what you can do.

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