11 replies to this topic

Hi all

I am fresh engineer and I have been given assignment to design surge drum for nitrogen witch is generated by a small nitrogen generator in our plant .I highly appreciate if someone can help me how to start.

[viral desai]

first of all, find out the offtake quantity from surge drum in normal operation. This will form the basis of residence time of surge drum. Also you have to take into consideration for orientation (vertical/horizontal)based on the following:

-The effect of inventory fluctuations on downstream section of surge drum
-The footprint

Based on which find out the Length and Diameter of surge drum.

Typical guideline is residence time of 10 min at NLL and L/D ratio is 3.0

[gvdlans]

Viral Desai:
What you seem to be describing is a method for designing a liquid surge drum. Gunair seems to be asking for a Nitrogen Receiver vessel for nitrogen gas. From NORSOK P-100, section 64:

"A nitrogen receiver vessel may be installed downstream the nitrogen generator as a buffer in order to dampen nitrogen pressure and purity variations. A receiver vessel may also be used as a storage device in case of limited supply of nitrogen after generator shut-down."

Gunair:
Please explain your question in more detail. We cannot read minds.

[ankur2061]

Hi,

You can apply the same principles for nitrogen surge vesssel as are described in the thread for instrument air receiver provided you know the following conditions:

1. Surge Pressure for Nitrogen (P₁), psig
2. Allowable minimum N2 pressure (P₂), psig
3. Surge Volume flow out of surge vessel, q_out, cfm
4. Normal Volume flow from N2 source to surge vessel, q_in, cfm
5. Surge Vessel hold-up time, t, minutes (rule of thumb is 5 minutes)

See the thread for calculation:

http://www.cheresour...?showtopic=5836

Regards,
Ankur

Thank you all. It really helps me a lot.

But I want to know how this formula is derived.

Is it mass balance?

Why is atmospheric pressure appearing in the formula?

If any one has an excel sheet regarding that. I appreciate if I can have it.

[gvdlans]

Gunair,

The derivation you can do yourself. However, I made a few corrections to the original post to get you on the right track. It is basically a mass balance, with the mass unit being a "standard cubic feet of air".

The instrument air receiver volume considering a surge requirement time period is calculated as follows:

V= t*(qout - qin)*Pa/(P1-P2)

where
V = volume of the receiver, ft3
t = time for the receiver to charge or discharge, min
qout = air flow out of the receiver (surge flow to consumer), scfm so at atmospheric pressure
qin = air flow into the receiver from compressor (compressor duty), scfm so at atmospheric pressure
Pa = atmospheric pressure (14.7 psia)
P1 = Upper or maximum operating pressure, psia or psig
P2 = Lower or minimum operating pressure, psia or psig
Note: P1 and P2 should both be either in absolute or gage units
As a general guideline a surge time period of 5 minutes is commonly used for air receiver calculations.

[ankur2061]

Gunair,

The derivation you can do yourself. However, I made a few corrections to the original post to get you on the right track. It is basically a mass balance, with the mass unit being a "standard cubic feet of air".

The instrument air receiver volume considering a surge requirement time period is calculated as follows:

V= t*(qout - qin)*Pa/(P1-P2)

where
V = volume of the receiver, ft3
t = time for the receiver to charge or discharge, min
qout = air flow out of the receiver (surge flow to consumer), scfm so at atmospheric pressure
qin = air flow into the receiver from compressor (compressor duty), scfm so at atmospheric pressure
Pa = atmospheric pressure (14.7 psia)
P1 = Upper or maximum operating pressure, psia or psig
P2 = Lower or minimum operating pressure, psia or psig
Note: P1 and P2 should both be either in absolute or gage units
As a general guideline a surge time period of 5 minutes is commonly used for air receiver calculations.

Hi,

First of all I would request you not to hijack my post.

Secondly your interpretation for flow rate is not correct. When I meant cfm, it meant the compressor discharge flow rate at the actual temperature and pressure conditions. Inlet flow to receiver is from compressor at a given pressure and temperature. The outlet flow from receiver is also at a given temperature and pressure. There is a lot of confusion regarding the interpretation of scfm. cfm and acfm. I am posting a web link for the readers of this thread to clear their concept of scfm. acfm and cfm.

http://www.about-air...s.com/SCFM.html

Now as far as the question asked by gunair regarding the role of atmospheric pressure then it is simply based on ideal gas law where the reference pressure is always the atmospheric pressure.

All of you readers are familiar with:

P₁V₁/T₁ = P₂V₂/T₂

Always one of the set of reference values is standard conditions. For air compressors the reference value for P & T is:

P = 14.696 psia (rounded off to 14.7 psia)
T = 68°F

I do feel that most of new engineers would be more familiar with theory than a veteran like me who is getting on in age as well as in remembering engineering formulas.

Regards,
Ankur.

Thank you all, and specially ankur.

Mr. ankur , I did a mass balance but I did not get the same formula. What I did is the following.

DM/dt =M in-M out

M= density*volume, density = (MW*P)/RT

D(VMW/RT)/dt=qin*MW P(pressure coming to the surge drum)/RT-qout *MW*P(pressure out of drum).

Assume T is constant, so MW/RT will be canceled from both side of the equation.

The equation after simplified is

Vdp/dt=qin*P* (pressure coming to the surge drum)-qout*P*(pressure out of drum).

So at the end I got

V*(P2-P1) = qin*P* (pressure coming to the surge drum)-qout*P*(pressure out of drum)*t(time).

Please, advice me what to do?

[ankur2061]

Gunair,

Regretfully I haven't been able to clear the doubts in the minds of many readers about receiver or surge drum sizing.

I have therefore prepared a spreadsheet with an example calculation. I have checked the result against an online calculator provided by a compressor manufacturer "Kaeser" and the result almost tallies. The online calculator can be accessed by this web link:

http://us.kaeser.com...zes/default.asp

While accessing the web link please enter 100 cfm which is also the net surge flow from my example and the respective pressure and surge time I have put in my spreadsheet.

I was expecting a spreadsheet from some of my younger colleagues but I suppose in this age of instant gratification youngsters expect everything to be ready-to-serve.

Guido: Request you to look at the spreadsheet and provide your comments. I apologize for being abrasive, it was probably one of those bad days at office. Your comments would be highly appreciated.

Regards,
Ankur.

Attached Files

•  Air_Receiver_Sizing.xls   21.5KB   448 downloads

[gvdlans]

Ankur,

No problem, I think you did have a point.

On the spreadsheet, it is basically your formulla in "Excel form". Crucial issue is whether flowrates in/out of the receiver should be in cfm or scfm. I noticed that Kaeser indeed indicates cfm in their toolbox, while quite a few others show scfm in similar formulas:

http://www.engineeri...vers-d_846.html
http://www.aeriscorp...c_air_tanks.pdf
http://www.ifps.org/...irReceivers.htm
Encyclopedia of Energy Engineering

So this is definitely something to look into.

I look at it as follows:

The gas storage volume in standard cubic feet of the air receiver vessel can be calculated as follows (with the assumption that temperature is at standard temperature and relative humidity):

Vstorage = Vreceiver * (P1-P2)/Pa (1)

So for example if you have a receiver of 1 ft3 and P1 = 5 atm and P2 =2 atm, you can store 1*(5-2)/1 = 3 scf of air in the vessel.

Now this storage volume should be at least equal to the amount of scfs that need to be buffered, which is the time duration times the difference between flow out and flow into the receiver. So:

Vstorage >= t(qout-qin) (2)

By combining (1) and (2) and solving for Vreceiver that is just large enough we get:

Vreceiver = t*(qout - qin)*Pa/(P1-P2) (3)

Which is the by now well known formula.

Note that if the air receiver is not at standard temperature, following correction should be included:

Vreceiver = t(qout - qin)*Pa/(P1-P2)* Treceiver/Ts

with:
Ts is standard temperature
Treceiver is the air temperature in the receiver

Ts and Treceiver should be both in either K or in degR

Thank you so much Ankur really appreciate what you did. This would help me a lot

Hi all

I already used the formula that you gave me but my supervisor asked me to prove it. I NEED YOUR HELP in that. I try mass balance but I could not prove it please tell me what to do.

formula is

V= t*(qout - qin)*Pa/(P1-P2)