25 replies to this topic

[go-fish]

Hi

I have a separator drum which has a vent line open to atmosphere. I have to size the vent pipe. For this, I used the compressible flow equation given in Crane TP410 for calculating blowdown capacity.

Qh=1.0312*Y*D ^2/Sg*(SQRT(Delta P*density/Kt)

where

Y is the expansion factor calculated from appendix A-22 of Crane TP410.
Sg is the specific gravity of vent gas
D is the pipe internal diameter
Delta P is the differential pressure across vent pipe
Kt is the total resistance coefficient of vent piping.

I did the calculations for both an 8 inch and 10 inch pipe assuming same pipe routing and found that the 8 inch pipe is giving higher blowdown capacity which is opposite to the expected behavior. I am attributing this behavior to higher delta P for a 8 inch pipe due to higher line losses and hence,a higher driving force to push out the vent gases.

However, I am not sure if I am correct in my understanding or calculations as I am doing these calculations for first time. Please advice.

[daryon]

Hi,
I don't think your calculations are correct, just consider the terms in the equation when increasing pipe size from 8 inch to 10 inch.
1. Delta P = constant
2. D = pipe diameter increases
3. specific gravity = constant
4. Y = constant. The expansion factor is constant as the upstream pressure is constant and you are discharging to atmosphere
5. For a pipe K=fL/D, where L = length of pipe = constant; f = friction factor which decreases; & D increases, so K will decrease.
6.Therefore Qh increases.

You should be calculating a hight flowrate when discharging to atmosphere through 10 inch pipe than through a 8 inch pipe... it's common sense. Why don't you attach your calculation so it can be checked?

Edited by daryon, 22 December 2010 - 04:51 AM.

[go-fish]

Is the upstream pressure going to be constant? My more experienced colleague said that upstream pressure will be basically the line losses in vent pipe. Since 8 inch pipe has higher line loss, the delta P value for 8 inch case will be higher. Also, the value of Y is a function of del P/ P absolute and Kt from A-22 Crane TP410, so it is not constant.

I have attached the file.

Attached Files

•  Vent Sizing _Spreadsheet.xls   122KB   1089 downloads

[Zauberberg]

If I got your statement correctly, you are discharging from the vessel directly to atmosphere. In such case, the maximum blowdown rate is directly determined by the maximum (choked) flow for given line size, as there is no other source of backpressure in the system. In other words, bigger line = higher flowrate.

Achieving higher flows with smaller size lines can be achieved if there is a backpressure device (e.g. like a tip or orifice) at the end of blowdown line, which builds a backpressure on the system and allows for higher mass flows. This concept is used in sonic flares.

[fallah]

Achieving higher flows with smaller size lines can be achieved if there is a backpressure device (e.g. like a tip or orifice) at the end of blowdown line, which builds a backpressure on the system and allows for higher mass flows. This concept is used in sonic flares.

Dear Zauberberg,

Above mentioned statement dosen't make sense to me.

As far as i know building back pressure restrict the flow and devices such as orifice would be used for flow reduction.

Would you please clarify?

Regards

[Zauberberg]

Hello Fallah,

The idea is to build the backpressure on purpose so that more gas can flow through given line size. This is also the concept of some high-pressure flares I have seen so far. As the backpressure increases, it is possible to obtain higher mass flow for a fixed Mach number.

Similar concept is employed in low-pressure steam strippers (or in atmospheric/vacuum distillation columns) where the sparger itself is designed in such way that it creates sufficient backpressure in order to allow high mass flow of steam through relatively a small line. If it were just a half-open pipe, much larger diameter would be required to handle the same mass flow of steam.

[fallah]

Hello Zauberberg

What about consequent upstream pressure and delta P around backpressure! device (after using it)?

[Zauberberg]

Not sure what do you have in mind, but I'll try to explain what I was referring to through example of steam stripping (there was one interesting thread about it).

- Let's assume that stripping steam (LP) header pressure is 4 barg.
- We need to inject e.g. 5,000 kg/hr steam into Vacuum Column bottoms, with operating pressure of 10 mmHg absolute;
- If we use an open-pipe distributor, obviously there is nothing to create backpressure between the column and the steam flow control valve, i.e. the maximum flow of steam that can be achieved will be defined by choked flow conditions at the distributor discharge, which is deep vacuum. The resultant flow will be quite low.
- If we employ a sparger-type distributor with sufficiently high DP, we can push more flow through the line as the pressure between the column and the steam control valve will be Column P + Sparger DP, which allows for higher mass flow of steam.

Similarly, for sonic flare applications, the sonic tip creates high backpressure on the flare KO Drum (e.g. in offshore platforms here in Qatar, Flare KO drum operates @ 8barg backpressure at maximum relief conditions) thus allowing higher flow through given line size - particularly during early stages of blowdown when flow requirements are at their maximum.

[fallah]

Actually,before you expedite mass flowrate by increasing delta P between sparger and column,you had decreased the delta P around control valve (assuming upstream pressure of control valve to be fixed).
Therefore,seems using sparger causes the flowrate through the line being decreased!

[Zauberberg]

The phenomenon is in accumulation of pressure in between the sparger (or flare tip) and the steam control valve (or BDV in case of high-pressure flare). As the system cannot be relieved sufficiently fast enough due to choked flow, accumulation of inventory causes pressure to increase and thus more steam (gas) can flow.

I have attached a paragraph from the offshore Sonic Flare design basis.

Attached Files

•  Sonic Flare Relief.pdf   113.49KB   393 downloads

[kkala]

Looking into "copy of Vent Sizing_spreadsheet.xls" (reattached here for convenience) indicates that equivalent length of elbows is probably underestimated; besides equivalent length for entry & exit is probably inversed, but this does not affect the result of the calculation.
Commented spreadsheet, limited to the 10" pipe size, is attached as "vent.xls". Velocity heads (K) from Perry (7th edition) have been used for fittings (and inlet / outlet), instead of equivalent lengths. In the specific case equivalent length is L=K*dia/f=K*0.255/0.0141=K*18. K values for fittings may also depend on pipe diameter according to Crane (recognizing there is not geometric similarity for different diameters), but I do not have Crane's book at hand.
I believe "vent.xls" gives a more precise result, although there are still margins for improvement. At any case the original spreadsheet makes a very good useful work that (as usual) should be "debugged" before general use. The company I offer cervices to always "certify" the new software, even if it comes from recognized commercial companies.
Note: It is understood from the posts that the vent is not installed on an API 650 atmospheric tank (venting requirements per API 2000), even though vessel operating pressure is quite close to Atmospheric. More precise (and less conservative) results would be obtained if an average pressure between inlet and outlet of vent were assumed (this would affect density); but sizing of vents should have generous safety margins to cover uncertainties.

Attached Files

•  Copy of Vent Sizing _Spreadsheet.xls   122.5KB   463 downloads
•  vent.xls   117KB   586 downloads

[DB Shah]

The phenomenon is in accumulation of pressure in between the sparger (or flare tip) and the steam control valve (or BDV in case of high-pressure flare). As the system cannot be relieved sufficiently fast enough due to choked flow, accumulation of inventory causes pressure to increase and thus more steam (gas) can flow.

I have attached a paragraph from the offshore Sonic Flare design basis.

Dear Zauberberg,
I am uploading an xls file to campare two cases - one with a restriction at end of pipe & second- venting through open pipe. If you can pl take any hypothetical case and plot the pressure profile for both the cases, I think it will calarify how a tip with restriction will allow for higher flow instead of open pipe. I too, like fallah, think that any addition of restriction in the system will reduce the over all flow capacity.

Thanks.

DB Shah

Attached Files

•  Vent-Effect of restriction at end of pipe.xlsx   13.11KB   450 downloads

[Zauberberg]

I too, like fallah, think that any addition of restriction in the system will reduce the over all flow capacity.

This is true for subsonic flows.

If we have such discharge flow rate that result in reaching sonic conditions at the discharge pipe end (atmospheric pressure), the flow can't get any shigher than that - and that is the choked flow at final discharge conditions. If there is a high-dP device at the end of discharge system, the pressure at which gas flows through the discharge pipe is higher than in the first case, allowing for higher mass flow. This is completely logical to me and I have observed the same phenomenon in two different applications: steam spargers in vacuum columns, and sonic flare in high-pressure offshore platforms.

The key thing is that the tip/sparger creates higher backpressure than it is the sonic pressure, at given gas flow. See attached graph for clarification.

Attached Files

•  Flare.JPG   89.02KB   147 downloads

[fallah]

Refer to attached thumbnail,considering sonic pressure of 1.844 bara how about the upstream pressure value?

With the back pressure even lower than 1.844 bara we can see the flowrate decreasing with pressure reduction.

Edited by fallah, 29 December 2010 - 07:57 AM.

[Zauberberg]

Exactly. My understanding is that, without sonic tip, the relief flow could not be higher than the calculated ~3.6 MMSCFD as it is equal to the sonic flow for given line size and final discharge ("sonic") pressure of ~1.8 bara. When high-dP device is used, the backpressure created on the discharge line allows for higher mass flow.

[fallah]

My first question ( Refer to attached thumbnail,considering sonic pressure of 1.844 bara how about the upstream pressure value?) hasn't been replied.

Indeed,what is the value of back pressure corresponding to 3.6 MMSCFD?

[DB Shah]

Dear Zauberberg,
This is indeed getting interesting. If I try to summarize your view-
In open pipe for a given upstream pressure of end pipe, flow will be restricted to choked value. You try to create back pressure by providing an orifice. As back pressure increases flow through the end pipe will increase.

What I percieve- In open pipe for a given upstream pressure of end pipe, flow will be restricted to choked value, now what I visualize is - the end pipe will restrict the flow but the source valve (which can be a safety valve or a control valve) will open and tend to push more flow, end pipe being at choked condition the fluid will start accumulating in the valve downstream and start building pressure and thus you will have higher pressure upstream of vent end.

Kindly correct if I am wrong somewhere.

I again reuqest you to give a hypothetical pressure profile for both the cases (I had uploaded an excel file in earlier post), this will help to clarify a lot.

[Zauberberg]

Gents,

I believe that the graph I have uploaded in my previous post shows the maximum flows at any given backpressure - that's the concept itself of sonic flare system. Or, in other words, for discharging from e.g. 100 barg into atmosphere (flare tip discharge), one can put more flow as the backpressure increases. Fallah, I think that answers your question?

Shah: pressure values (those which you would like to be entered in the spreadsheet) will be different by time. However, for the sake of simplicity, if we consider the exact time when maximum/design relief flow is achieved, pressure upstream of the tip will be 130 psia. Please read through the paragraph uploaded earlier: "The sonic flare design flow cannot be achieved until the design backpressure of 8barg is achieved." As the sonic/shock wave travels through the discharge line, gas accumulates inside the line and increases the backpressure since it cannot be relieved/discharged into atmosphere due to choked flow conditions. As the backpressure increases, more gas can flow through the line and, once when 8 barg backpressure is reached - according to the flare system calculations - full relief flow can be achieved.

[fallah]

Gents,

I believe that the graph I have uploaded in my previous post shows the maximum flows at any given backpressure - that's the concept itself of sonic flare system. Or, in other words, for discharging from e.g. 100 barg into atmosphere (flare tip discharge), one can put more flow as the backpressure increases. Fallah, I think that answers your question?

Shah: pressure values (those which you would like to be entered in the spreadsheet) will be different by time. However, for the sake of simplicity, if we consider the exact time when maximum/design relief flow is achieved, pressure upstream of the tip will be 130 psia. Please read through the paragraph uploaded earlier: "The sonic flare design flow cannot be achieved until the design backpressure of 8barg is achieved." As the sonic/shock wave travels through the discharge line, gas accumulates inside the line and increases the backpressure since it cannot be relieved/discharged into atmosphere due to choked flow conditions. As the backpressure increases, more gas can flow through the line and, once when 8 barg backpressure is reached - according to the flare system calculations - full relief flow can be achieved.

That paragraph continued following the above highlighted red color as below:

"While the pressure at the inlet of the flare tip rises to 130 psia,the flow leaving the flare system through the tip (around 51,000 kg/hr) is less than the flow entering the flare system through the BDV."

Above statement indicates we have restricted the flow through flare system by flare tip!

Actually,i think sonic flare tip would be used for huge overall flare load to transfer created choke-flow point from flare system to flare tip and also achieving better air assistance due to higher turbulency.

Thus,IMO using sonic flare tip wouldn't increase outgoing flowrate from flare system!

Edited by fallah, 31 December 2010 - 01:58 PM.

[Zauberberg]

Above statement indicates we have restricted the flow through flare system by flare tip!

The flow is constrained because of sonic/choked conditions, not because of the flare tip. This is followed by the shock wave due to accumulation of inventory, and as pressure builds up in the system the flow of gas increases.

[fallah]

The flow is constrained because of sonic/choked conditions, not because of the flare tip. This is followed by the shock wave due to accumulation of inventory, and as pressure builds up in the system the flow of gas increases.

The flow is constrained because of sonic/choked conditions around the flare tip!

We have three pressure region: Upstream of BDV (named regoin A),Between downstream of BDV and upstream of Flare Tip (named region B or back pressure) and Downstream of Flare Tip (named regoin C neer atmospheric pressure)......

Anyway,as long as build up pressure in region B would be below the relevant sonic pressure (with respect to pressure at upstream of BDV or region A),we have no change in flow of gas leaving flare system.

On the other hand,if build up pressure in region B would be greater than the relevant sonic pressure,the flow of gas leaving the flare system even could be decreased.

[go-fish]

If I got your statement correctly, you are discharging from the vessel directly to atmosphere. In such case, the maximum blowdown rate is directly determined by the maximum (choked) flow for given line size, as there is no other source of backpressure in the system. In other words, bigger line = higher flowrate.

Achieving higher flows with smaller size lines can be achieved if there is a backpressure device (e.g. like a tip or orifice) at the end of blowdown line, which builds a backpressure on the system and allows for higher mass flows. This concept is used in sonic flares.

The line is open to atmosphere without any orifice. So what upstream pressure should I use? If backpressure is the only source for upstream pressure, then a small pipe size will result in higher backpressure due to higher line losses. And based on Crane equation, this is giving higher flow.

[Zauberberg]

The line is open to atmosphere without any orifice. So what upstream pressure should I use? If backpressure is the only source for upstream pressure, then a small pipe size will result in higher backpressure due to higher line losses. And based on Crane equation, this is giving higher flow.

Go-Fish,

Let's see if I understood your spreadsheet data in a correct way:

- You are discharging from the vessel operating at 0.14barg to atmosphere;
- Three different sizes for the vent line were evaluated, 8", 10", and 12", with corresponding pressure drop 0.394bar, 0.137bar, and 0.062bar, respectively (for the maximum blowdown flow).

Is this correct? Please confirm.

If this interpretation is correct, then the 8" line case is an impossible one - it would result in creating the backpressure that is higher than the vessel operating pressure. In other words, the vessel would pressurize itself during blowdown which certainly cannot happen in reality. This should be interpreted in the sense that, for the 8" line size, it is not possible to achieve the calculated blowdown capacity as there is no sufficient backpressure (= 1.013 bar + 0.394bar) available to push that much quantity of gas, since the vessel operates at 0.14barg only. So the resultant flow will be less - you can iterate the flow rate and see how much gas you can push through the 8" line without exceeding 0.14bar pressure difference between the vessel and atmosphere, being the driving force for flow.

10" and 12" line cases are realistic ones and you can notice that there is almost no difference between them, as they both could handle the maximum relief flow with given (maximum) possible backpressure, or - if it is easier to observe it that way - the available pressure drop that can be "consumed" for blowdown. Therefore, if you have done the fluid hydraulic calculations correctly, 10" line should be your choice.

[Zauberberg]

Or,

Looking from the reverse perspective: what is your required blowdown flow? If it is less than those calculated in the spreadsheet, you can re-iterate the calculation work but this time with two parameters being fixed:

- Available pressure drop (= 0.14 bara);
- Required relief flow.

By knowing these two, you can easily obtain the optimum/minimum vent line size.

[kkala]

The line is open to atmosphere without any orifice. So what upstream pressure should I use? If backpressure is the only source for upstream pressure, then a small pipe size will result in higher backpressure due to higher line losses. And based on Crane equation, this is giving higher flow

I would suggest you debug the calculation first (e.g. see post on Dec 28th); then repeat calculation, assuming drum operating pressure at the inlet and atmospheric pressure at the outlet.
What is the role of this vent? As understood it is not a relief device. If it discharges drum content to atmosphere when "unit" does not operate, line size affects duration only. Or it may do some purge. In both cases a valve on it is indicated (note that hydrocarbon discharge to atmosphere is usually not permitted by law.)