Gravity flow problem (long line) from water distiller outlet 1

[erbru]

Hi,
I'm new to this forum and would really appreciate some help.

I've been asked to design a line in which hot water (90-95 degC) must fall under gravity from an existing water distillation unit / water storage tank combination (a few meters apart, slight vertical drop from the last condenser of the distiller to the tank inlet) to a further storage tank. The line I must install will tie in a tee just above the inlet valve into the original tank.

The distance between the distiller and the new tank is considerable, a piping run of 130m including 18m of vertical drop (the original tank is on the 4th floor and the new tank on the ground floor). Both the distiller and the tanks operate at atmospheric pressure and it is critical not to back-pressure the water distiller in anyway. The current line is 2 inch OD and the water flow is 3 m3/h, (which has some variation on it possible depending on the distiller output). The situation will be controlled in a fully manual way and is only required in emergencies.

I studied the paper from PD Hills (Designing Piping for Gravity Flow 1983)

Initally I wanted to design for a flooded flow using an orifice plate to choke flow just before the inlet to the new tank to calibrate this for the flow produced by the distiller. However I was advised very strongly against flooding the line from the distiller by the manufacturer and told to do the output flow purely by self-venting flow. As this is a manually operated situation I feel a control valve would be impractical as the operators will have difficulty adjusting to the constant variation in output.

Therefore I am now aiming for self venting flow but I am concerned about the unpredictability of the configuration. There will be sloped horizontal sections (2% or more) interpersed by short vertical sections (3-4 metres at a time to drop a building level). The pipe must be run through an existing building which leaves little choices for choosing the exact pipe run.

PD Hills paper mentions that "little information is available on unflooded flow in systems that include bends, especially for flow changes from vertical to nearly horizontal and vice versa ...entrainment and surging may still occur due to the effect of bends in the system". He goes on to give advice on how to run the slopes and bends (which I may or may not be able to apply due to building restrictions).

My initial thought was to size the system continuing in the 2inch OD, continuing the established self venting flow regime. However I could also push to do this tank filling via a pump in a more controlled way if the project if the risk of deregulation is too great via gravity. I just don't want to accept defeat too easily (and the pump system would have other issues).

Can any of you offer any advice? Surges could have a very serious effect of deregulating the distiller and this ia a process critical fluid so shutting down the distiller would be a serious issue. I understood that there is even risk of damage to the distiller.

Thanks

 

[katmar]

Welcome to the forum erbru. Let me ask a few questions before I reply so that I don't give too much irrelevant advice.

When your emergency situation arises how do you divert the flow towards the new tank? Is the line to the original tank simply closed and now the distilled water flows via the tee to the new line?

Must the new line be self draining - i.e. is it a problem if there are low points which would remain full of water (which would become cold) between instances of using the line?

Does the 2" line already exist, or is this just the proposed design at this stage? I guess you have worked out that 2" pipe is not self venting at 3 m³/h.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[erbru]

Hi
Thanks for replying!

In the emergency situation the distiller feeds either the original tank (via a 2" line and a manual valve) OR the remote tank 4 floors down via the new pipe tee/valve. The valves & piping would allow the operator to switch between the two tanks. The distilled water would always pass by the original tank using the original 2" piping, then flow via a straight tee into the new line via a new valve, which is normally closed.

The line MUST be self draining and low points are to be avoided.
The line will be insulated and it is not desired that the water get colder than 85 C during transfer. The tanks are kept hot with steam jackets.
It is in fact preferable in fact that the line be flooded over this long length (or Fr > 1.15) from the point of view of keeping it sanitised, but it is difficult to control the flow as I mentioned.
Air inlets directly in the line are to be avoided for sanitary reasons.

There is a also drain valve teeing off just before the inlet valve to the second remote tank to rinse the line before first use/between uses.

The 2" line to the first tank does exist now (about 6m long, horizontal, gentle slope with a few bends), but long part (130m) is just a plan.

At 3m3/h in 2 inch the flow is not self-venting (JL = 0.69) but seems in some sort of transitional regime?
The problem is that I don't know the level of water retained in the top of the condenser of the distiller so I can't evaluate the equations 1 & 2 of the PD Hill paper.

My question is with this unknown flow regime continuing on the long horizontal/vertical pipe either continuing with 2" (or even reducing to 1.5") that we plan to install are we asking for trouble (water hammer, etc)?
It would seem that as far as possible we should steepen the slopes in the horizontal portions and avoid totally vertical sections.
I'm not sure such a pipe run is possible due to geographical constraints (the pipe passes through 3 buildings and it is tough enough avoiding low points)

 

[MikeHalloran]

I'm not clear on why manual intervention must be required in your emergency situation.

I'm also not clear on why you couldn't just rig the line to the new tank as an overflow from the first tank.
... e.g. a new open-topped standpipe extending up from the bottom of the existing tank, cut off just above the normal level, so when the water level rises, the excess flows out the standpipe to the second tank with no intervention required.
Of course, the utility of that arrangement depends on the nature of your emergency, so far not stated.



Mike Halloran
Pembroke Pines, FL, USA

 

[katmar]

You have got a whole bunch of conflicting constraints there, and it is not going to be easy to satisfy them all. In my experience almost all of the self-venting installations have required a vent open to atmosphere, but this is clearly undesirable in your case. An alternative to atmospheric venting is to oversize the pipe and keep the slope as steady as possible so that you have a continuous gas phase above the liquid over the full length of the pipe. This keeps the 2 ends of the pipe at the same pressure. But this would not satisfy your aim to keep the line flooded.

Another conflict seems to be the "emergency" nature of the operation (which in my mind means a frequency of less than once per month) and the need to keep the water hot. If the line is kept full of water it will get cold, and if it is kept empty you could have problems flushing the air out with JL = 0.69.

The only way I can see to satisfy all these requirements is to put the 2 tanks in series rather than in parallel. Whatever process is currently fed with water from the existing tank will need to be fed from the new tank. The line feeding the new tank would have to come from the base of the original tank and an automatic valve at the inlet to the new tank could control the level in the original tank. This would keep the line feeding the new tank hot, flooded and continuously flushed. If you need a certain velocity for flushing (Fr > 1.15?) you could implement a bang-bang level controller so that the transfers would be batchwise rather than continuous. Having the control valve at the new tank will prevent the ingress of air to the line.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[erbru]

Apologies, I have not been very clear.

It is not an emergency situation in a safety sense, but in an operational one.
The intention is to use one distiller to back up another circuit in case its own distiller fails (this is the "emergency")

The system is currently configured:
Distiller 1 + Tank 1 supply to Loop 1
Distiller 2 + Tank 2 supply to Loop 2

There is automated level control in the tank/distiller in each system & the tank and distiller are in each case a few metres apart from each other.

The manual element would come from using Distiller 1 to fill Tank 2 (because Distiller 2 has failed) via this new gravity pipe (and some manual valves to divert to tank 2 or drain, and if necessary control the flow).
It would entail manual surveillance of level in Tank 2 (which is usually in a control loop with Distiller 2)

Between the two different the Tank/Distiller combinations of Loops 1 & 2 are 4 storeys of a building (18m from top Tank 1 to top of Tank 2) + horizontally they are also some distance apart.

We can't overflow simply these tanks, they are not designed to be operated that way and it would not be accepted by the plant.
The idea is simply to use one distiller (in a "controlled" manner) to back up another Loop, in this case via gravity flow (simply because it is available).
The plant had not given any thought to the issues of entrainment in gravitational flow and were planning to simply install a 1.5'' line between the tanks with no control and nothing other than 2% slope (for drainage) in the horizontal sections. They later asked to have it studied further which leads to my investigation.

 

[erbru]

Yes, there are very many constraints, unfortunately.

As for the need to keep the water hot, it doesn't need to stay in the line. The intention is to allow the line to drain out between fillings and then flush it again with clean hot wtaer prior to the next filling (possibly some hours later). A source of clean steam is available to flush the line prior to refilling it if necessary.
But it's true that with JL so low the air may not be flushed out with the water stream alone. I'm not sure that flushing capacity of this water regime has been fully considered.

I fully agree the best solution is to work downstream of Tank 1 (which has a pump for its Loop & it's possible to tee in downstream of it to feed Tank 2).
This is not what the plant wanted as they prefer to keep the loops separate (upstream of the tanks) for cross contamination reasons - hence the demand for gravity filling.

No "automation" is allowed, they want a purely manual system/hardwired system .... The only control I can use is throttle manual valves/an orifice plate at the inlet to the tank which can be calibrated to limit the flow taken off the pump & its loop.

As for using the pump they also have a concern that with the additive pressure of the pump outlet (max head about 7-8 barg, usual operating point about 5 barg) + the height of water (about 2 bar) this could take the pressure above the design pressure of the system (8 barg).
The pressure would be reduced with an orifice or other manual throttling into the tank (the tanks have a 2" bursting disc set at 3 barg, and are kept at atmospheric pressure) but for saftey reasons it might be necessary to put the orifice much closer to the pump outlet (before too much vertical drop) & add pressure switches hardwired to close pneumatic valves (and so on).

 

[bimr]

Gravity flow through vertical drops is designed differently than gravity flow through horizontal pipes.

The recommended maximum permissible flow in a vertical stack is 7/24 of the total cross-sectional area of the stack. This value has been determined empirically.

At the center of the stack is a core of air that is dragged along with the water by friction. A supply source of air must be provided to avoid excessive pressures in the stack. The usual means of supplying this air are through the stack vent or vent stack. The entrained air in the stack causes a pressure reduction inside the stack, which is caused by the frictional effect of the falling sheet of water dragging the core of air with it. Because of the airflow, you should have a conservation vent on the top of the stack.

Maximum velocity in the vertical pipe will be 10-15 ft/sec as discussed in the attached article.

It seems that the 2-Inch diameter system that you have was sized correctly according to the Table 1-3.

Suggest you read the attachment on drainage systems.

You comment about 2-Inch OD pipe is puzzling, pipe is usually sized on ID.

For the 3 m3/h capacity, a 2-Inch vertical drop pipe is adequate and a 2% slope on the horizontal pipe is adequate.

 

[erbru]

Hi bimr
Thanks for the link. I discovered it yesterday morning and was still digesting it.

The pipe in this case is ASME BPE tube, the OD is 2 inches but the ID is somewhat less (47.5 mm ID)

In this case it is not just a stack but many 2% sloped horizontal runs followed by short stacks of a few meters, then more 2% sloped horizontal runs etc.

Adding air vents along the stack or runs is not possible, the only air that can be supplied is via the distiller's air vent (which is at the very beginning of the pipe run feeding a horizontal 2" pipe to Tank 1 which then continues with a much longer (new) initially (approx 40m) horizontal run interspersed with horizontal and vertical runs to Tank 2 over a total of 130m along and 18m drop) plus the air vent on the tank which it feeds into.

Table 1-2 suggests for r=7/24 , 1.45 lps capacity in 2 inch pipe (not stated if OD or ID so it is not very precise) = 5.2 m3/h, so appears at first sight ok, but what about the lack of air vents?

This is further complicated by the fact it is near boiling water (90 degC) so there is water vapour also present.
I'm still left with a degree of uncertainty of what will really take place in the pipe.

 

[bimr]

You only need one vent at the top.

Treat the vertical drops like a stack.

Design the rest like a horizontal sloped gravity drain. 2% slope is adequate.

The article demonstrates the phenomenon that occurs in a vertical pipe with falling liquid.

 

[katmar]

The vertical downflow sections will work as air pumps because of the entrainment and the air will have to be vented from the No.2 storage tank. It shouldn't be much, but it would be worth checking on start up. At the high temperatures you are using this venting air will carry a bit of a plume with it. Probably not dangerous in any way, but people get nervous when they see "smoke" coming out of a process.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Can you run a parralell line above this one and connect at many high points to act as a vent line?

This would vent back at the beginning and may just solve your issues.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Sounds like a serious attempt to embarass the newbie.

 

[racookpe1978]

If done that way (a 2% slope) you MUST be rigorous about verifying the hangers and the "droop" between pipe hangers. They (small sloped lines) are all too easy to get "lazy" during installation or design re-re-redesign stages to create low points or high points that cause the self-vent/self drains to fail at accidental low and high points.

Very long runs between buildings also cause problems on pipe runs and with isolated pipe supports on too-small, badly balanced pipe support foundations that slip, twist, get run into by trucks, etc.

 

[bimr]

Good point. Any horizontal vents would also require slope.