Reverse flow / vortexing question
Reverse flow / vortexing question
(OP)
I have a system that has two 4" gasoline truck loading-arm metering assemblies fed by a single 6" pump header. Each arm may or may not be operating simultaneously for truck loading. In the 4" branch from the header for each, we are injecting ethanol. This injection is downstream of the common header, and upstream of the metering assembly (meter, control valve, arm). Since we need to make sure the correct amount of ethanol is injected in each, regardless of how many arms are operating and when, the conceern was raised if we have any way to make sure that we would get the "right amount" of ethanol in each stream -- for example, we want two E10 streams, not am E5 and an E15.
THe concern is that injected fluid from one of the two 4" pipes could migrate to or affect the flow of the other. The injection of ethanol would be from a higher pressure source (to ensure it's able to inject), but since it's injecting into an "open pipe", the pressure should equalize instantly with the primary fluid (gasoline).
I have no concern about this myself, but can't assemble the right technical reasons or math to prove it won't be an issue. Energy required to backflow against an active fluid stream, potential of vortexing at one tee and "sucking" up fluid, etc. None of this seems possible especially since each arm is metered, but it's only experience and gut, not mathematical equations. Of course, I could be wrong too.
Any thoughts. Am I off-base? Is there a way to calculate minimum distance from common header to the injection point to avoid vortex, or "injection plume", etc?
THe concern is that injected fluid from one of the two 4" pipes could migrate to or affect the flow of the other. The injection of ethanol would be from a higher pressure source (to ensure it's able to inject), but since it's injecting into an "open pipe", the pressure should equalize instantly with the primary fluid (gasoline).
I have no concern about this myself, but can't assemble the right technical reasons or math to prove it won't be an issue. Energy required to backflow against an active fluid stream, potential of vortexing at one tee and "sucking" up fluid, etc. None of this seems possible especially since each arm is metered, but it's only experience and gut, not mathematical equations. Of course, I could be wrong too.
Any thoughts. Am I off-base? Is there a way to calculate minimum distance from common header to the injection point to avoid vortex, or "injection plume", etc?





RE: Reverse flow / vortexing question
As to the back mixing or vortex, I don't see it is a problem. The flow will always go to the lower pressure direction - the tank provided the valve to the tank is open
RE: Reverse flow / vortexing question
- 60gpm of ethanol into injection A for meter A
- 600gpm at meter A of combined gasoline/ethanol
- 60gpm of ethanol into injection B for meter B
- 600gpm at meter B if combined gasoline/ethanol
But since the gasoline itself isn't metered separately prior to ethanol introduction, we may have 600gpm of gas and 0gpm of ethanol on meter A, and 480gpm of gas and 120gpm of ethanol on meter B. Meters won't tell us how much otherwise...
But that isn't a concern (that we can't meter it separately), as long as we know that physically the ethanol has no way of flowing "upstream" to the common header and into the wrong meter. Looking at injection point A - is there a way the ethanol from A (which is only injecting if that 4" meter run is operating) to get into the B meter run?
RE: Reverse flow / vortexing question
You may have problems if the head loss downstream of the injection point causes flow to back up.
One can envision a scenario where one of the discharge valves is not fully open which then causes flow to back up in the other one.
RE: Reverse flow / vortexing question
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Reverse flow / vortexing question
RE: Reverse flow / vortexing question
Bimr, sorry for the confusion but when I said "gas" it's short for gasoline, not gas/vapor. inject downstream is not an option due to process requirements to meter the combined flow with temp correction.
With primary flow always moving "forward", I think BigInch is right that start/stop may be an opportunity for "reverse" flow to travel upstream against primary. To avoid this we can have the injection start after and stop before primary (gasoline) flow starts and stops.
The only other I can think of is if flow thru the tee could siphon flow from injection A if it's "too close" to the tee, and relative flow rates between primary A and primary B differ enough. What I don't know is how to calculate that potential, or to figure out how far from the tee the injection point needs to be in order to ensure we are avoiding that.
RE: Reverse flow / vortexing question
I did a design for a terminal similar to this but I'm pretty sure we metered the gasoline and the ethanol separately and then mixed the two streams. I'll have to see if I have any old P&IDs to be sure.
RE: Reverse flow / vortexing question
If you had the ethanol injectors 5 diameters downstream of tees, possibly 3, I would think vortex mixing would have no chance to occur. My only worries here would be for the transient cases and the lack of positively assured mixture control. Those two check valves would go a long way to enhancing that. Normally you would have another check directly on each sales meter as well to absolutely-positively prevent backflow there. Although you could say that you have a reasonably good idea of what is in the mix, your QA/QC Dept., or your client's laywers, may argue with you forever about that.
Centrifugals do not always flow forward; when stopped, leakage through to suction can occur. Is there a check valve at the pump. Does the header have a relief valve, or another outlet that might cause backflow on depressuring, or some other backflow on depressurizing scenario. In my start-up/shut-down scenario I did not assume that you would have high enough flow velocities on shut-down to worry about. Is there an emergency stop valve somewhere that might cause "reverse" waterhammer, vaporization and backflow when refilling the vapor pockets.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Reverse flow / vortexing question
RE: Reverse flow / vortexing question
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Reverse flow / vortexing question
"sorry for the confusion but when I said "gas" it's short for gasoline, not gas/vapor. inject downstream is not an option due to process requirements to meter the combined flow with temp correction."
There is no confusion. You said in your first post "injecting into an "open pipe" which was an incorrect statement and would imply some sort of drop pipe. Your sketch clarified the matter.
Not sure there is a requirement to meter a combined gasoline and ethanol mixture. Surely your control system can add up the fuel totals if the fluids are metered separately. Since you already have separate meters, why wouldn't you use separate meters for better accuracy?
Both of the attached articles indicate that the ratio method of blending is more common as well as more accurate than the sequential method of blending that you are doing.
http://www.ethanolproducer.com/articles/8340/termi...
http://help.intellisitesuite.com/Hydrocarbon/paper...
Also note the piping diagram in Fig. 4.
RE: Reverse flow / vortexing question
RE: Reverse flow / vortexing question
C <> A+B. A common problem with blending HC liquids, but an advantage if you meter it.
I like that doc.
Note they did show one check valve on the skid, Fig 8.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Reverse flow / vortexing question
One would think that you should have some concerned then if you are trying for that accuracy. Every one of the Typical Ethanol Blending Arm Configurations are shown with check valves on each of the product lines where the lines meet.
RE: Reverse flow / vortexing question
If it ain't broke, don't fix it. If it's not safe ... make it that way.