Constant Purge of vessels 1

[cougarfan]

What is the norm for purging vessels while in operation? We typically keep a constant purge of 50 scfh ( 70 acfh) of nitrogen on at all times no matter what size of reactor we have and they vary from 500 gallon to 4000 gallon.

We still purge the vessels with a presure purge before chemicals are added to make the atmosphere inert.

I am wondering if I could reduce the purge to 25 scfh instead and save on some of the nitrogen costs....would this still be safe?

John

 

[StoneCold]

cougarfan
We have inerted vessels as well. However we due not purge the vessel all the time. We close the vent and put 5psig of pressure on the tanks to keep air from entering. We do not purge through all the time, in my opinion you are not gaining anything, just pressure up and hold the inert atmosphere.

 

[CHD01]

My understanding is that the required purge rate is a function of the vent line diameter, vent height, Min O2 Conc rquired to prevent flammabiliy concerns, and the MW of the purge gas. Also remember to include additional flow capability for pump-out.

What I do now is to compare the calculated O2 concentrations per Husa's methods (Method A) versus a recommended vent purge velocity of 0.1 fps (Method B) versus a minimum purge rate of 10 cfh (Method C). I then compare these purge rates against the calculated purge rate to achieve the required O2 level (Method D), and select best method in my engineering judgement so long as calculated O2 level for the purge rate I select is < min O2 level. I have no safety factor included in these calculations - so that is part of the engineering judgement.

See following formulas:

Formulas:

Fb = exp{(0.065*(28.96-MW)} Single Component
Gas Purge
Fb = 6.25*{1-0.75*((MW/28.96)^1.5 Multiple
Component Gas
Purge (MW = avg
MW)

O2 % = 21 * exp{-U * L / (0.0036 * Fb * (D^1.46)}
U is the purge gas velocity, D is the vent diameter, L is the vertical length of pipe.

I'm still looking for more info on this subject; including what is considered to be the best overall purge design; since I'm beginning a project to review plant purge rates.

The more you learn, the less you are certain of.

 

[mawells]

CHD01, how did you get on with your plant purge rate review? I am looking at the purge rate of fuel gas in our vent header. I have used the Shell DEP to calculate the required min purge rate, which is a function of diameter of the vent tip and the Mr of the purge gas. The purge gas is injected at the far end of the vent header.

It appears the same method is used to calculate the min purge rate for both flared and non-flared systems. Without going outside the DEP, I am interested to know if there is justification for reducing the purge rate for a non-flared system.

Thanks,
Mark

 

[Flareman]

Cougarfan
The formulae you are using are based on air penetration into the atmospheric end of the vent line and, although that is part of your concern, is it the reason you are purging the vessel itself ? Most approaches to stack purge make the stack &quot;safe&quot; within some distance of the end (25 ft in Husa's case). Your vessel is probably a distance away from that so you need to review its specific need relative to purge. That's also true for other parts of the relief system where, perhaps, purges are specified for sweetening or sweeping (RE >2000 for example).
Once you have accumulated all these sources in the relief header, then check it against the Husa purge and augment if necessary.
BradStone's recommendation to just hold the pressure should work fine for the vessel itself. Your approach would suggest that you have a wide open discharge with no valve and that you are holding a slight positive by using the back pressure of the relief line, which doesn't seem quite right.


David

 

[CHD01]

mawells: Just got back to work after 6 weeks away - so no progress on review of purge rates. I would like to make it clear I am talking about continuous purge rates in a continuous process to achieve safe operation (preventing fire, etc.).

Couple questions, by DEP do you mean Design Pressure? Also, what is Mr (mach number ratio, molecular weight or?)?

The preferred method is to flare hazardous gases; non-flared vents have to have the proper diameter and vent height to disperse the gases to an acceptable ground level concentration for personnel exposure. You can design a flared vent to safely disperse gass if the flare goes out if not excessively expensive.

I expect to find significant savings in reducing excess purge rates of nitrogen for storage tank vents and for non-flared vents. We shall see, but this was proven on another of our plants at the same site.





The more you learn, the less you are certain of.

 

[MJCronin]

cougarfan,

Are you aware of the AiChe Article &quot;Properly Purge and Inert Storage Vessels &quot; ?

It was in Chemical Engineering Progress magazine (2/01 issue) and is available in .pdf format.....

See:

http://www.cepmagazine.org/pdf/020157.pdf

How are the methods and assumptions in this article different from what that is proposed above ?

MJC

&quot;There comes a time in the affairs of man when he must take the bull by the tail and face the situation.&quot; W.C. Fields

 

[mawells]

CHD01, DEP is Design and Engineering Practice (a Shell standard) and by Mr I mean molecular weight of the gas. I used Reduced Husa to work out the min purge rate in the header, this is permitted by another Shell standard (an Expro standard). The DEP gives results similar to the basic Husa method so it appears the DEP hasn't been updated for Reduced Husa. As Expro standards get priority, I used the Reduced Husa method.

I was trying to get to a min purge rate in the vent header to keep the O2 conc below the LEL. As I understand, Reduced Husa allows a flammable mixture to exist for some distance in the vent stack from the tip. I guess the assumption is that, if an ignition were to occur then the flame would only propagate for a short distance. I am used to flared rather than non-flared systems, however I don't know if it is good practice to use Reduced Husa on a flared system as the likelihood of a flame existing in the stack at low flow rates must be great?

I received some useful articles from Flareman on the subject (thanks Flareman). I am trying now to evaluate the effect of adding nitrogen to the purge gas stream. Part of the purge gas is fuel gas, and part is N2 (approx. 70% by vol. of required purge rate is made up of fuel gas). There are a couple of things which I would like to clear up and would appreciate feedback on:

1) It seems that N2 is better than fuel gas (Mr=18) at keeping air from entering the stack but I haven't seen a method for quantifying this. I have included the N2 as an equivalent quantity of fuel gas and assumed it to be conservative.

2) Small amounts of O2 are present in the N2 (it comes from a PSA unit). Can you use Reduced Husa directly in this case, or will the presence of O2 increase the purge rate? What is the latest thinking on this?

Thanks
Mark

 

[mawells]

I've started a new thread in Chemical plant design & ops as flare & vent purge isn't really relevant to the original question!

Mark

 

[Flareman]

Mark
A couple of points about your procedure and questions.

Nitrogen is better at reducing the air infiltration because the infiltration ocurrs mainly by buoyant decanting which is a function of the difference in density between inside and out, so (28.96 - 28) is better than (28.96 - 18).

Nitrogen also works better for the flammability because the inert gas dilutes the flammable and adjusts both the LEL and UEL such that a UEL of 3.7% oxygen in a flammable gas MW 18 (say) becomes a UEL of 9.3% oxygen (about) in a mixture of 4 Nitrogen to 1 gas (these numbers just to give the idea).
You can also get the idea by playing with an interactive graph at

http://www.geocities.com/flareman_xs/Gases/Flammability_calc.html.

Because you are trying to control the oxygen percent, you should reduce the desired oxygen number in your calc by the percent oxygen in the upcoming gas.


David

 

[mawells]

David,

I am having problems with understanding your point about nitrogen dilution:

&quot;a UEL of 3.7% oxygen in a flammable gas MW 18 (say) becomes a UEL of 9.3% oxygen (about) in a mixture of 4 Nitrogen to 1 gas..&quot;

Do you mean LEL? that is, the addition of N2 to methane (say) pushes the LFL upwards?

If the statement is correct, could you please explain qualitatively the argument?

Thanks

 

[Flareman]

Yes, sort of......

Example Methane
LEL (LFL) = 5.6% in air
= Gas 5.6 : Oxygen 3.54 : Nitrogen 13.3

Stoic = 9.5%
= Gas 9.5 : Oxygen 19 : Nitrogen 71.5

UEL = 14.3% in air
= Gas 14.3 : Oxygen 18 : Nitrogen 67.7

When you dilute the flammable with inert the LEL stays roughly at the same total dilution (ie. 5.6 in 94.4) but if the some of the dilution is provided by the mixture itself, (say 1 gas: 4 nitrogen) then that nitrogen becomes part of the overall dilution number (94.4)
This gives the following ratios
Gas 5.6 : Diluent N2 22.4 :Oxygen 15.1: Atmos N2 56.9

The stoichiometric ratio of oxygen to gas doesn't change so the stoic mixture is
Gas 6.9 : Diluent N2 27.6 : Oxygen 13.8 : Atmos N2 51.7

The UEL also changes (here it's a bit more iffy and this is just an guide for estimations) in roughly the same proportion as the original excess air LEL/stoic/UEL ratio which was 1.77:11/1.59) Applying that proportioning (1.771/1.59) to the diluted stream becomes 1.249 : (1/1.122) which gives
Gas 7.7 : Diluent N2 30.8 : Oxygen 12.9 : Atmos N2 48.6

I know this is all a bit convoluted. It works a lot better and is easier to see graphically but it means that the oxygen at UEL for the gas only is normally 18% in air and the total oxygen at UEL for the diluted stream in air is 12.9%. For purging, you are concerned with the oxygen concentration, not the gas concentration (which is the LEL or UEL value)

Added to this is the fact that all diluents affect the LEL in proportion to the specific heat of the diluent so the values for CO2 or Water as diluents are different. This is all (in brief) in my paper (I think you have it).

The flame speed, like the LEL, depends largely on the radiative heat transfer from the burning flame to the non-burning gas and will also slow down as the LEL/UEL envelope narrows.

I hope that it's clear. Remember that these are rules of thumb, not a scientific presentation .. which would need to delve into heats of formation and free radical formation .. so it's an engineer's solution not a chemist's.


David