Link to SUNCENTER
(Chemical)
(OP)
13 Feb 07 06:28Dear all!
I am facing a propane and butane bulk transfer system from horizontal bullets vessels towards rail tank wagons with no vapor return line. As far as I know in steady state the pressure inside the LPG vessels will depend only on their temperature and will be coincident with the LPG vapor pressure but during the bullet discharge there is a transient condition during which the pressure decreases below vapor pressure. Is it right? How can I calculate that transient pressure decrease?
Moreover if pressure decreases below vapor pressure we will have a problem with pump NPSH. Which are the usual solutions for this problem?
Thank you all for any help
Replies continue below
(Petroleum)
15 Feb 07 15:07Pumping liquid from the bottom of the tank, rather than tanking butane gas from the top, the pressure in the vapor space remains equal to the vapor pressure of the butane at the (avg?) tank temperature. Taking liquid will tend to reduce the tank temperature due to a combination of the Joules-Thompson expansion of the butane gas above and some evaporation of the liquid butane as the increased vapor space is filled. The proportion of liquid and vapor at any given time can be modeled by vapor-liquid equilibrum diagram. The vapor pressure reduction will correspond to the rate of the liquid drawoff and the roughly equal rate of vapor space increase, the temperature reduction and corresponding decrease in vapor pressure associated with the temperature reduction.
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
(Chemical)
15 Feb 07 16:04see thread798- too
(Chemical)
17 Feb 07 02:49
BigInch, I fail to see any J-T expansion effect. Can you clarify ?
(Petroleum)
17 Feb 07 03:54You don't smoke then.Is it not the J-T cooling effect on the vapor expanding to fill up the increasing space above the liquid as liquid is drawn off the bottom? I thought that was why there is condensation (often freezing) that can appear on the outside of a butane lighter refilling can, but only on the portions of the can's surfaces that are above the liquid level. Of course the JT effect would be more pronounced on butane that actually vaporizes as it exits the tank.
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
(Chemical)
17 Feb 07 13:34
This is evaporation by pressure reduction. The J-T effect -mostly cooling but not always- is a throttling (flow) process. Right ?
(Petroleum)
17 Feb 07 18:35,I think evaporative cooling is part of the total cooling effect, but a significant portion must be due to J-T. My logic being that lowering pressure at constant enthalpy, whether it is done by taking the cap off the top, removing liquid from the bottom, or by running it through a throttle valve should make no difference. Would not the heat of vaporization be removed from the liquid, cooling that, but actually adding heat to the vapor? But, now you do have me wondering if I remembered correctly. Is it the can's surface above the liquid line that frosts, or the can's surface below the liquid level that frosts??? I'm out of butane right now, so I can't test it myself tonight.
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
(Chemical)
18 Feb 07 05:47I still think there are no JT effects inside the bullets, and that all of the cooling comes from the flashing saturated liquid at reduced pressures.
(Petroleum)
18 Feb 07 08:02Vale. Perhaps was thinking in reverse mode.... or perhaps not at all.
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
(Petroleum)
18 Feb 07 08:10Back to the original post,All the answers seem to be in here,
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
(Chemical)
22 Feb 07 16:19BigIncher:
Although you are usually right on the money, this time I have to concur with 's questioning the existance of a Joule-Thomson effect on a liquid.
In the pure sense of the Joule-Thomson Effect definition, it involves the expansion of gas or gas mixtures - not compressed liquids. The effect of cooling inside an LPG tank (or "bullet") is purely the result of liquid vaporization - similar (but not the same) to the effect seen in an atmospheric cooling water tower. This is the same process that a refrigerant liquid undergoes when it is expanded across the usual expansion valve. I still correct many engineers who think that this, too, is an example of the Joule-Thomson Effect. I don't believe it is. Now, I can believe that I could be mistaken and I welcome any corrections, but all the J-T values I have ever collected are on gases/gas mixtures.
(Petroleum)
22 Feb 07 23:09Not to split hairs, but during the pressure drop of a liquid near its bubble point, some molecules become a gas (flashing), those molecules that are gas next drop in pressure. So, in effect some propane molecules do go through the JT effect. Water on the other hand when sprayed does not readily produce water vapor by the change in pressure like in the cooling tower.
(Chemical)
23 Feb 07 00:33
The JT expansion mentioned by Montemayor refers to fluids, i.e., it includes liquids. The interesting (counterintuitive) fact is, that
all fluids
appear toheat up on expansion
as long as their reduced temperatures (=T/Tc) are lower than 0.75 or greater than 5.0, or reduced pressures (P/Pc) above 12. For mixtures, one can use pseudocritical properties.(Mechanical)
23 Feb 07 02:14I want to know whether there are fluids flowing into the LPG bullets. If so, can they make up for the pressure decrease during the transfer process, or how do you control the operations of the LPG pumps.
If no pressure makeup, a vapor return line is essential, otherwise your pumps may cavitate frequently due to low pressure head in the suction and thus the low NPSHA. Purely relying on evaporization may not be adequate.
(Chemical)
23 Feb 07 06:58
I've been told by experts that these pumps probably always cavitate but due to the low bubble energy severe damage to pump parts is seldom found.
(Petroleum)
25 Feb 07 15:40Well, lowering pressure at constant enthalpy... is J-T, one way or another.
BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com
For more information, please visit lpg motor pump.
(Chemical)
26 Feb 07 02:51
When analysing the process inside an LPG bullet being emptied, or filled, let's not forget that enthalpy is an
extensive macroscopic
property, i.e., depends on quantity of contents.(Chemical)
(OP)
26 Feb 07 03:20Any book o publication explaining how to calculate that transient until the equilibium is reached again during which the pressure inside the LPG vessel being emptied or filled is not coincident with equilibrium pressure?
Best regards
(Chemical)
26 Feb 07 04:47I haven't found such a source. The following threads may be instructive:
(Chemical)
26 Feb 07 09:54From experience only. We had verticle can pumps for loading trucks. We had 3 30,000 gallon bullets to pull from to load the trucks. We loaded through spray bars at 300 GPM. If we pulled from all 3 bullets at the same time our pumps would not cavitate. If we pulled from one bullet at a time, the pump would cavitate when the outside temperature was below about 60F. The fix was to use a steam coil to heat a small amount of recycled propane to make vapors to put back in the bullet tank. (you can do the math to calculate 100% vapor replacement). WE COULD NOT HAVE A VAPOR RETURN because of quality specifications and odorization.
(Chemical)
1 Mar 07 11:27
Probably a return of liquid from the pump to the bullet would induce the turbulence required for mixing and promote vertical convection currents to aid in "turn over" and minimize thermal gradients between the bottom outgoing stream and the liquid surface.
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Pumping LPG: Take A Load Off Loading Readers suggest how to avoid problems in pumping LPG
THIS MONTH'S PUZZLER
Our refinery, which is located in the upper Midwest, runs into problems pumping liquefied petroleum gas (LPG) from storage bullets to tanker trucks during the summer. Except for low tank level, we can pump 600 gpm in the winter without any issues but have trouble pumping at all by late morning in the summer. That's when we divert propane, with some difficulty, to underground pipelines.
The bullet pressure reliefs are set at about 500 psig. The operating pressure starts at about 140 psig when the bullets are full but is only about 105 psig at about 45% full, which is as low as we can go. We feed four bullets at a time to the pump. During filling, pressure can spike at above 200 psig on a hot day.
Suction piping: the bullets have a single 4-in. discharge through a ball valve; flow is through about 150 ft of aboveground 8-in./10-in pipe. Discharge piping: two dryer tanks separate liquids from the LPG; there also are two filters and a strainer. (We haven't kept maintenance records for the dryers or filters.) A 600-ft aboveground run of 6-in. pipe goes from the 8-in. pump discharge to the flow control valve; a 2-in. hose connects the skid to the truck.
Typical loading of the rack, with 600 gpm, is at about 225 psig, immediately upstream of a 4-in. equal-percentage globe valve (CV = 220) with a final pressure of about 150 psig at the tanker truck. A turbine meter immediately upstream of the valve measures flow erratically. During startup, the flow is 100 gpm at 30 psig, with a pressure of 270 psig into the valve.
We need to address vapor lock of the valve during startup and pump cavitation during low level. We're seeing a one-year life on the seals in these multistage inline vertical LPG pumps. During the summer we always have trouble starting a pump once it has stopped. A manual vent line at the pump bowl connected to a flare is used to bleed off the gas. We always see a surge when we start the pump, hot or cold. There are secondary 2-in. lines at the top of each bullet for manually venting them to flare.
What is the cause of our pump problems? Would using a smaller valve help? What can we do to improve this operation?
CHANGE THE VENT
The tank pressure varies with the ambient temperature, 140 psig in winter when ambient temperature is lower and up to 200 psig in summer. It also appears that the tank vapors are not vented back to either the process or the trucks. Therefore, when filling with liquid, the rising level has to compress the vapors until they can condense and this takes time. Keep in mind that the liquid propane is always at bubble point in the tanks. Any heating or pressure drop will cause flashing. Also, many of these tanks make-up and pump-out from the same end, resulting in stagnant product. Putting the make-up into the tanks at the opposite end and in the vapor space is better.
Then, there are the pump issues. The pump does not have enough net positive suction head available (NPSHA) to run properly. When the winter tank level gets down to low level, there isn't enough head to feed the pump and the suction vaporizes and cavitates in the pump. The pump is probably always cavitating as evidenced by the erratic flow in the turbine meter. Suction strainers are notorious for causing cavitation. You should have a 2-in. line from the pump discharge (before the check valve) back to the tank vapor space. When starting the pump, or when it's vapor-locked, open the line and recycle the LPG back to the tank ' this can be automated or manual. Each pump will require a startup line.
The pump suction line is also important. Typically, the suction line should be at least 12-in. ' one size larger than the suction flange ' right up to a reducer on the pump suction flange. The 4-in. discharge valves on the tanks must be full port and should increase to at least 6-in. immediately after the valve. Everything you can do to reduce pressure drop will help. Also, the piping must not have any pockets that would trap vapors. Furthermore, the suction pipe is long; move the pump closer to the bullets.
Consider the pump discharge. The 6-in. discharge line looks okay, 0.5 psi/100 ft and 7 ft/sec velocity. The rack control valve maintains pressure so that the turbine meter can operate properly. Size is not critical. If the valve is open less than 30%, consider installing a reduced trim in the valve. When the pump stops, the LPG trapped in the discharge line should be vented back to the bullets. A small valve and tubing back to the vapor return line will ensure that some of the LPG is always returned to the bullets and will keep the line cool when flow is low.
So, what's missing? A vapor return line. A truck is required to have a vapor equalization line that connects back to a common header on top of the bullets. The header also allows the tanks to maintain an even level.
Climate is also important. In winter, the tanks are about 83°F at 140 psig, and in summer about 108°F at 200 psig. In winter, the ambient temperature is usually colder than 83°F and so Mother Nature is helping cool the LPG in the suction line and thereby adding NPSH. However, in summer, the sunlight on the tanks and piping can heat the LPG enough to cause vaporization. It is best to keep the suction piping in the shade provided by the tanks. Also try painting the lines white or consider insulating the lines, but be careful about corrosion under insulation. Do not assume this will solve all of the problems. You need vents on all high points and any length of horizontal piping with the vent valve piped or tubed back to the bullet vapor space.
Larry Tarkington, project engineer
Company undisclosed, San Antonio, Texas
MAINTAIN THE BULLET PRESSURE
Let's summarize the symptoms. First, the flow from the bullets is difficult below 45% level. The pressure drops in the bullets from about 140 psig to 105 psig at the lowest level. A surge is affecting the pump seal life. It's very difficult to start the pump once it has stopped. The pressure spikes in the bullets when they are filled on a hot day. The pumps have little trouble delivering propane to an underground line, except when the bullets are below 45% level. And, the suction line for each bullet is restricted by a 4-in. valve. Clearly, the problem is pumping from a bullet where neither the level nor pressure is maintained, causing the propane to flash.
A quick calculation with a simulator shows that at 140 psig propane remains a liquid up to about 82°F. At 105 psig the maximum temperature is only 63°F. Add to that the radiation effects and the true temperature of the pipe surface won't be ambient. The heat transfer is more complicated. Flowing propane auto-refrigerates. If it's moving, flow through several hundred feet of 8-in. pipe shows little increase in temperature. So, if it's liquid in the bullets, it will flow. If it's a gas, it won't. This describes why there's a surge in flow and also why weather affects the pump. Perhaps operating with a higher pressure drop would increase the auto-refrigeration?
Another problem is what happens to the pump when vapor forms. When it goes through a pump a 5'10% mixture of gas with liquid will start at 70°F and be 110°F when it exits the pump. Gas typically has one-tenth the heat capacity of liquid on a per-pound basis. Although it's now a liquid, when it goes into the control valve it will become gas again because of the higher temperature.
Leaving 45% in the tank bothers me. Noncondensables may be building up in the bullets. This will only get worse.
The biggest problem with the pumps is the pressure control of the bullets. Some of this could be relieved by moving the vent line from the pump suction to the discharge. Since the mid-s, manufacturers have indicated that as the correct placement of the vent. The main problem with this action is that the only good vent is the manual vents on top of the bullets; bottom delivery would be more suitable, at the other end of the bullets, away from the suction. A new pump may be required.
Another option is to increase flow to the bullets to maintain pressure during pump-out. If quality control is a problem the refinery could go to continuous monitoring or a set of isolation tanks to prevent contaminated product from being shipped.
Replacing the valves, either the 4-in. suction or the flow control valve, may reduce the surge but it won't make it easier to load a tanker truck. The flow in the 4-in. suction valve is fine; using the 25N2 rule [where N is nominal pipe size] for maximum design flow, 25×42 = 400 gpm; this is greater than the 150 gpm flowing if we assume equal splits between the bullets ' but splits may not be equal. The velocity in a schedule-80 4-in. pipe is 4.2 ft/s assuming equal splits; the velocity in 8-in. is 4.2 ft/s; API RP 14E recommends 3ft/s for suction lines and 9 ft/s for discharge piping (for light hydrocarbons). This means that the suction velocity may be high if API is to be believed.
Avoid changing the rack control valve to a smaller one. Studies have shown that a larger body allows gas to expand more and reduces valve damage. Going with a linear valve will likely require a bigger opening compared to an equal-% valve thus reducing the velocity through the valve and lessening the damage from gas bubbles. There also may be two-phase problems in the flow measurement. Perhaps a vortex shedding or an annubar-type flow meter would be a better choice than the turbine. For more on this sort of problem, check: www.corken.com/media/training/cp226.pdf and www.eng-tips.com/viewthread.cfm?qid=.
Dirk Willard, consultant
Wooster, Ohio
MAY'S PUZZLER
We're suffering wild yeast problems in our ethanol fermenters. Our clean-in-place system had run flawlessly for 15 years with little attention. The cycle is a rinse with steam/water at 160°F, followed by a weak caustic wash at 210°F, a hot water rinse, a rinse with an organic iodine solution with phosphoric acid, and finally another rinse with hot water. A few months back during the winter we increased the number of times we reused the caustic from three to six before sending the stream to waste treatment. Now, it's spring and we're seeing wild yeast in our cream tanks. We went back to using the caustic three times and then to just once ' but that only modestly affected the wild yeast we're seeing in the product. Is the change in the number of times the caustic is used to blame? Or is it something else? How can we solve this problem?
Send us your comments, suggestions or solutions for this question by April 12, . We'll include as many of them as possible in the May issue and all on ChemicalProcessing.com. Send visuals ' a sketch is fine. us at [ protected] or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL . Fax: (630) 467-. Please include your name, title, location and company affiliation in the response.
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