Liquid falling in a pipe 4

[Casimo5]

I have a situation where liquid (anywhere between 15 gpm and 52 gpm) is falling about 50' from one vessel to another (both open to atmosphere via 3/4" vents) through a 3" pipe. The liquid is oil with trace amounts of water and methanol entrained. At high rates (52 gpm), the falling liquid pulls air through the vent of the first vessel and out the vent of the second. The receiving vessel vents the air with small amounts of methanol and water vapor. I would like to prevent the second vessel from venting in order to keep the methanol in the system to be recovered.

I placed a restriction orifice at the inlet of the receiving vessel in order to build a liquid column in the pipe, however the velocity in the pipe at high rates is close to 2 ft/s and air still gets sucked down.

Does anyone know how to prevent this from happening? The first vessel is not rated for a vacuum and the receiving vessel is not rated for any pressure, so capping the vents isn't an option.

 

[stanier]

Connect the vent from the receiving vessel to the first vessel. Then any vented product will return to the system or condense on the vent pipe and run back to the receiving vessel.

 

[Casimo5]

That option was considered, but because the line may also have trace amounts of oil that would freeze in the pipe and cause it to plug, it would have to be traced and insulated, raising the cost of the solution.

Is there anything that can be done to prevent the problem?

 

[zdas04]

Soapy73,
At that flow rate, a 3/4" vent is a scary proposition. It only takes a few ounces of pressure to blow the lid off of an API tank (and mm Hg of vacuum to collapse one).

The problem you're trying to solve is a classic Vapor Recovery Unit (VRU). There's thousands of these systems in refineries around the world and many hundreds of them on lease tank batteries. Many of them work with a liquid-ring compressor and some work with ejectors. In either approach the ejector or compressor has a control scheme that allows it to maintain the tank in a slight vacuum (say between 3 inHg and 1 inHg for example) and a one-way valve on the tank vent. In normal operation any vapors are sucked into the compressor or ejector. In an upset condition the tank pressures up to positive numbers and the vapors are lost out the one-way valve.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Montemayor]

Soap:

David Simpson is absolutely correct voicing a concern. There should be a serious concern about the stress being put on your tanks unless you have an accurate rating in hand and don't allow the pressure (or vacuum) to go beyond design.

A 3/4" pipe is, in my opinion similar to a constriction for the venting operation and may create a backpressure that could damage your tank. I would certainly check this out before proceeding any further. You may have good strength in your tank, but that's an awfully smally vent line. I hope you're well within a safe range.

 

[Casimo5]

The vessels are not your conventional cylindrical storage tanks. The first is about 60" ID and about 60' tall, the other is 48"ID and about 10' tall. Having said this, I agree that design conditions should not be exceeded.

We do have a vapor recovery system in place and have estimated the cost of tying the receiver vent into this system. However, I don't believe the design intended for the receiver vessel to vent so much.

We also considered having the vessels re-rated for some positive pressure and a vacuum and closing the vents, however the cost for this is about the same as tying in the vents to the vent system.

It's starting to look like ther is no way to stop the flow of air down a pipe that has liquid falling in it. If that is the case, we will be forced to go with one of the other more costly alternatives. Unless someone can think of something. Anyone? Bueller? Anyone?

 

[zdas04]

Soapy73,
Bottom line is that you just can't fool Mother Nature. A cubic meter of liquid leaving a vessel will either lower the pressure or be replaced with an equivalant mass. Same way with mass entering a vessel, the pressure will either go up or and equivalant mass must leave. That is the (isothermal) waterfront. You have to determine if the vessel can handle the pressure changes from the magnitude and rate of the mass shift or provide a way to add/remove mass to dampen the pressure swings.

David

 

[Casimo5]

That's the whole problem. Both vessels are at steady state. The liquid level in both is constant. There are no pressure swings or level changes in either vessel. This is possible because the first vessel is fed at the bottom and the discharge is about a foot from the top out the side, naturally controlling the level at any feed rate. The receiver has a level control valve at the bottom so it maintains a constant level no matter what the feed rate is.

 

[katmar]

Soapy,

A flow of 52 gpm in a 3" line is almost a perfect design to ensure a syphon. This means that if the liquid from the higher vessel is overflowing from a free surface (and it sounds like this is exactly what you have) then it will entrain a considerable volume of air/gas/vapor with it. This will work like a rather efficient pump, moving the gas from the top vessel to the lower one.

You need to increase the size of the overflow pipe to 5" or 6" to ensure that the pipe will be self-venting. At this size the gas can travel back up the pipe and avoid being drawn down into the lower vessel. You need a length of 10 to 15 feet of single phase liquid at the lower end of the pipe to ensure the bubbles can escape. I would replace the whole line with 5" or 6" pipe and I would size a restriction orifice at the entrance to the lower vessel to ensure that there was a steady head of about 10 or 15 feet driving the liquid through the orifice.

This would allow any entrained bubbles to rise up out of the liquid and flow up against the down flow of liquid.

If you have access to a library that carries old journals you will find this problem very well described by Larry Simpson in Chemical Engineering, June 17, 1968, pages 192-214 (see particularly pages 203-205).

regards
katmar

 

[Casimo5]

I will do a search through our library to see if I can find that article. 1968? They had printing presses back then?

The syphon effect you describe is exactly what I'm seeing. Right now I have about 18 feet of liquid in the pipe due to a restriction orifice of about 1" in the 3" pipe. However this liquid is moving at about 2 ft/s (at high rates). A larger pipe would slow it down significantly, but I'm afraid this would also be very costly. It would be cheaper to route the vent to the vent header or tie the two vents together and get a cyclical flow of the vapor.

I think a better solution would be to install a control valve on the line between the two vessels to control the liquid level in the pipe (and indirectly in the first vessel) so it remains liquid full. This way we don't have to route any more piping in the plant that would need to be insulated, traced, inspected, maintained, etc.

There, I've answered my own question.

 

[25362]

If the intention is to avoid the need of additional piping, and preclude air being entrained down, why not eliminate the first vessel altogether ?

 

[Casimo5]

The first is essential to the process. Getting rid of it is not an option, however the receiver is just needed to control the effects caused by changes in the feed rates. if we install a control valve on the outlet of the first vessel, you could question the need for the receiver at all. But getting rid of either vessel would probably cost more than adding any piping.

 

[Montemayor]

Soap:

The topic of gravity flow is discussed in great detail in the following threads:

thread378-81608
thread378-125323

You can try to obtain the following article that has been used as a gravity flow standard by such E&C contractors as Bechtel and others:

“Designing for Gravity Flow”
P. D. Hill, Imperial Chemical Industries PLC
Chemical Engineering Magazine; Sept 05, 1983; p.111

This article is not as old as Larry Simpson’s classic, but it is very detailed in explaining and resolving your problem. I have a copy of the Hill article, complete with illustrations, in an Excel fluid flow workbook that I’ve assembled as a handbook of useful information. It’s that good.

Katmar’s response is, as usual, correct and right on the money. I’ve known and read Larry Simpson, a renowned Union Carbide Fellow who has written extensively on fluid flow. We had very good printing presses back in 1963. We also had what seems to be a far superior and basic knowledge of fluid flow that the current wave of Chemical Engineers. That’s why the simple answers can easily be found in such ancient documents. I wish I could recommend more recent articles, but unfortunately no one today seems to be able to understand fluid flow as well as Simpson or Hill or is able to write about it in intelligible English.

I believe you can confirm Katmar’s recommendation to your basic question by going to the following website:

http:/

http://www.freecalc.com/selffram.htm

This site calculates a self-venting pipe size.

I also think that connecting the lower tank's vent pipe to the top tank's vapor space does, in effect, the same thing. Katmar's method is more direct and keeps any liquids or solids out of the external vent line.

I hope this helps you out.

 

[unclesyd]

I think you will be able to find the above mentioned articles at the website below.

They are extremely helpful in locating articles.

http://www.lindahall.org/

 

[Casimo5]

Thanks Montemayor, those other threads discuss exactly what I am experiencing. The calculations show that a 4.5" pipe is needed for self-venting at my high rates, that's why I don't have any problems with the current 3" pipe when we run lower rates.

 

[Montemayor]

Soap:

That all reconfirms what Katmar predicted. (which only proves that's one sharp Kat) I've never bought a 4-1/2" or a 5" nominal diameter pipe. I was taught by expert pipers (& by the pipe price lists) that these sizes are "bastard" sizes and are to be avoided. The standard 6" seems like the practical answer.

The best way to install the gravity flow by overflow is to install the downcomer pipe straight, vertically out and down through the top tank's bottom head and into the top of the bottom tank. That way you promote "true" and pure overflow by avoiding an elbow. (read the Hill article). The overflowing edge of the downcomer pipe should have "V" notches cut into its edge in order to maximize even irrigation around the internal surface of the downcomer. This stimulates even, consistant flow and ensures an open venting area at the core of the downcomer pipe. This technique was/is used in distillation column downcomers and distributors.

Hope this helps.

 

[katmar]

Soapy,

Your suggested solution of installing a level controller on the upper vessel to ensure that the overflow is flooded at all times is fine. This would totally avoid the vapor being entrained and obviate the need for a means of dealing with the vapor.

Anyway, I am glad you came to a solution.

Art,

I was interested to hear that 5" pipe is regarded as non-standard in the USA. Here in South Africa it is certainly not a stock item, but I once had some light gauge stainless pipe made to the 5" standard by spiral welding from coil for a very specific duty. It is a pity it is not more widely used because the jump from 4" to 6" is actually rather large in terms of pressure drop.

regards to all
katmar

 

[Casimo5]

I have worked in and around refineries and plants for a long time, and I can't recall ever seeing a 5" pipe. Usually after 4", it's pretty much even numbered sizes unless it's a very custom job.

Thanks for all your input guys. This thread has been a lot of help.

 

[Montemayor]

Harvey:

During my initial engineering job in Jamaica, my mentor Alf Newton was an ex-British ship engineer and he was a great one for practicality. He would take my fluid flow pressure drop calculations and say: “It’s a flawless, correct calculation that shows that we need a 3.675” ID pipe. Now, tell me what is the RIGHT size of pipe we need!” He then proceeded to use his fountain pen and scratch out the listings of 2-1/2, 3-1/2, and 5” in my Crane Tech Paper 410 and labeled them “Bastards”. I still have a copy of that Crane. Later, over a beer, he further dictated that only certain pipe sizes were to be used in our plants. He further confided that those sizes were based on two things: practicality (they were readily available and reasonably priced) and standardization to reduce plant spare inventory. The latter reason established a rule-of-thumb that had the pipe sizes up to 10” increasing in roughly twice the cross-sectional flow area. Anything above 10” was designated “big-bore” and was a “wild card” in that it called for custom design and special attention.

In my later years, through many jobs and countries, I confirmed what Alf had taught. Even the major contractors like Bechtel, Fluor, etc. as well as the major Chemical companies all rigidly establish their “standard” sizes – like Alf did. This “popularity” listing affects the pipe fabricators production supply and consequently increases the prices of the unpopular sizes. In the USA I’ve always found an aversion to using 5” pipe. Like Alf, I also went on to establish my “dictated” sizes whenever I ran or operated plants. My preferred sizes are: ¾, 1, 1-1/2, 2, 3, 4, 6, 8, and 10”. Below ¾”, I always use SS tubing where applicable and ¾” is the only stuff I’ll thread. I was able to reduce a lot of pipe inventory and maintenance costs that way and, as you well know, when you establish a successful custom or routine it’s very difficult to get away from it. And with different engineers all over the world using different criteria or needs, this poses a perplexing and confusing situation for many young graduate engineers today. They don’t know the basis for piping decisions, have never been taught those things in university, and what’s worse is that today they have been deprived of engineering mentors. I’ve met far too many young engineers today who don’t know what a pipe schedule is and how it is determined.

If we take the cross-sectional flow area increases for pipe sizes we find: from 3/4” to 1” = 1.62; from 1” to 1-1/2” = 2.36; from 1-1/2” to 2” = 1.65; from 2” to 3” = 2.2; from 3” to 4” = 1.72; from 4” to 5” = 1.57; from 4” to 6” = 2.27; from 6” to 8” = 1.73; and from 8” to 10” = 1.58. I guess most persons have wanted to establish a capacity factor of approximately 2X in the jump between 4 and 6”.

I consider this subject relevant to this thread because it is related to what we identify as THE answer: which pipe size to install (not necessarily what size does the calculation yield)? None of this stuff is taught in university – a deficiency that I deplore and for which I always criticize every ChE prof I meet. Some of the recent ChE profs in the USA are changing this outlook and I applaud them for their foresight and extra work in teaching the subject more closely to what the student will actually find in industry when he steps out of the academic campus. This is what makes this piping Forum more interesting and stimulating to me – and, unfortunately, one of the reasons why a lot of potentially bright ChE graduates stay away from the forum. They probably just don't understand a lot of the words and reasoning we employ when we deal with transporting fluids through pipes.

There’s a lot more to Fluid Flow and Fluid Mechanics than just the Darcy-Weisbach equation.

 

[zdas04]

Art,
In my mind I package the lack that you're talking about into the category "good enough", and I'm confident that that category is simply not taught in any university. The great bridge master Charles Goren said it best when he said "It is incumbent on [you] to try for the best result possible, not the best possible result".

It drives me crazy when I see a 10-yr engineer specify 3.427 inch ID pipe and expect anyone to be able to build it within any rational budget (to say nothing of buying valves and fittings). These guys see everything in the world as either "Perfect" or "Perfectly wrong" and they don't accept shades of gray. Again, I blame both the pedantic professors at universities and a near total lack of mentoring in industry.

David