6 replies to this topic

[bmicky]

Hi all

I have a practical question that causes me to realize that 2 subject are not fully clear to me.
One is the subject of designing lines for vertical gravity flow, and the other is open channel flow.

Please bear with me as this might be a bit long...

First I will give the raw data:
We have a power plant, which has an air cooler as a condenser.
This air cooler has a collector that collects all the sub-cooled liquid after the condensation to a big header. The header size – usually 24" diameter, and 95 m long .
The header is completely horizontal (no slope at all) and from the header it is being drained from one central pipe ( or 2 smaller, symmetric pipes ) to the pump.


We would like to maintain level in the collector: from one hand not to flood it fully ( it will create backpressure in the condenser ) ,and from the other hand – to have at least enough level to assure: No gas bubbles coming into the pump .

I read the articles ( Simpson ,"sizing piping for process plants" from Chemical Engineering magazine, June 17, 1968 ,and P.D Hills , "designing piping for gravity flow " , 1983 )

I have to say I am still a bit confused and here are my questions :

First, the practical questions :
1) How do I decide the proper diameter of the vertical pipe from the collector? (to assure both level in the collector and gas bubbles not reaching the pump ?
Is it only a matter of Froude calculation or also other considerations (such as open channel hydraulics ) are needed?
2) How will I know weather to preffer one big pipe or 2 smaller symmetric pipe instead?

And to the more general questions:

3) Should I design the vertical pipe to have first a self-venting section, and then reduce it to full-flooded flow? Or full flooded flow from the collector and on?
4) If I have a case of vertical pipe from the collector, then a long horizontal pipe and then another vertical pipe – should I really only care about the last vertical pipe before the pump ,or I need to make sure the first vertical pipe is also fully flooded?
5) Is it indeed correct to relate to this situation as gravity flow, or is it not a gravity flow because the pump in the end is use as some kind of a seal ?

6) What happens in such a long horizontal pipe (with no slope ) ?
Do we have one level at the beginning, and when opening a valve, and the flow starts- what causes the water to flow? Does a slope start to form from the sides to the center? Does it stay the same level, but inertia is dictating the flow anyway?
What will dictate the final level in the collector eventually? (assuming the flow in and out is constant )
What am I missing here and not understanding fully regarding a flow in an open channel?

That is it.

Hope it wasn't too tedious ....

Thanks in advance
Mike

[Art Montemayor]

Mike:

Instead of a long, verbal description of your installation, why not furnish us with a detailed sketch showing the dimensions of the pipes and their locations? That will save a lot of mis-interpretation of your description and simplify the basic data.

[TS1979]

It will not be a gravity flow or open channel flow if you need a pump at the down stream of the collecting header. A pump is a energy addition device. Having a pump downstream of the collection header means that extra energy is required to transfer the condensate to somewhere. Therefore, you will not have a gravity flow or open channel flow. You can have the level controlled with a level control valve at the downstream of the pump discharge. In such a configuration, there will be a level in the collector and the piping to the pump suction will be always full.

If you don't need a pump to drain the condensate, the flow will be gravity flow but you will never reach the channel flow if you don't want uncondensed vapor escape to atmosphere. You have to have a level control valve to control the liquid level in the condenser to make the system works.

If you really don't want to install a level control valve, you probably can install something similar as steam trap to keep the steam not escaping.

For the system, you may need a high point vent to vent the non-condensable gas.

One bigger drain piping will be preferred to two smaller drain pipes.

[bmicky]

Mr.Montemayor,

Thank you .

Based on your recommendation, enclosed is a simplified sketch of the system.

What I am looking for, the design criteria for sizing vertical and horizontal sections of the pipeline in order from one hand not to flood the collector( it will create backpressure in the condenser ) ,and from the other hand to avoid vapor entrainment into the pump's suction line. .

Thanks in advance, Mike

Attached Files

•  Description.pdf   714.48KB   95 downloads

[Art Montemayor]

Mike:

I’ll follow what you request, but be advised that, in my opinion, you are dealing with a badly designed condensate pumping system. I am basing this opinion on what you have furnished as basic data and how you have described the situation. Please refer to the attached Excel Workbook where I try to depict a more accurate visual description of what you describe and how it should be configured – if you are dealing with an existing, designed, and installed condenser-pump unit. You have not stated that it is something existing that has been handed down to you, so I have to assume something in order to offer comments.

I also assume you are located somewhere in the Canadian country side; otherwise, I don’t see how you could rely on an air-cooled exchanger to subcool your saturated Pentane condensate by 12 ^oC. Again, you haven’t furnished any background in your personal profile, so I have to assume that in order for the design to exist, the location must be a cold one.

Basically, you started this thread by going off in a totally different direction. You have no “channel” or gravity flow to account for. Yours is a simple, sub-cooled condensate pumping operation with a badly designed collection and pumping system. You are pumping one heck of a lot of liquid pentane and why you are using a header to feed a pump is something I don’t understand. This is a fairly large flow rate of pentane that should easily justify a serious, robust-designed pumping system that does away with any possibility of vapor binding the product pump. With a hydrostatic head of 7 meters, you are OK. However, you have little or no room to effectively control the positive pentane level that is needed to ensure the NPSHa, the pump-back flow rate required for minimum pump flow control, and the residence time normally given to this type of operation to secure adequate instrument responses. The size of collection header you mention is almost the same size as the suction line I come up with for the pump!

This system surely has a professional P&ID – and that is what is required in order to discuss the system and its operability and control. There is no Froude Number or gravity flow involved here that I can detect. What is involved is a good, safe, and operable pumping installation that allows you to start it up and be able to turn your back on it and walk away feeling secure that it will operate without any problems.

My comments are not intended to be just critical and negative regarding your installation. I truly believe you are being candid and honest in seeking comments and possible help to assist you in gaining a good handle on this very large pumping operation. Please furnish us with all the basic data and scope of work if you have need to continue this discussion and gather further comments from our expert Forum members who, I hope, will all join in with their valued comments.

Attached Files

•  Pumping Sub-cooled Pentane Condensate.xls   376KB   68 downloads

[katmar]

See http://www.eng-tips.....cfm?qid=301909

I still believe you have a control problem and not an hydraulics problem.

[kkala]

1. Pump max flow is about 600 m3/h. Net volume of upstream surge drum should be about 600/60*15=150 m3 (residence time = 15 min) as per local refinery practice (we would specify 100 m3 in fertilizers, ~ 1979). This surge volume is much bigger than that of 24" suction pipe, even if supposed to run full of liquid.
2. A usual design would include such a drum, also suggested in the web reference of post No 6, with vortex breaker at outlet and level control acting e.g. on a control valve at pump discharge or on its RPM.
3. But mickey points out: "in our company they did it years without a surge vessel, and bought a very low NPSH pump ( 2 ft , although the air condenser height is sometime 6 meter ) . before employing anything new, a proper justification will be needed.." (web reference of post No 6).
4. It is difficult to assume stable operation in the above mentioned case of para 3, where NPSHr=2 ft and EL understood as 6 m (versus ~ 7 m in the present case). No controls are mentioned, either. Is the flow of the past case of para 3 variable?
5. In the present case flow varies 59 - 100 kg C3H8 / s. Well, suction level could vary between 7 m and ~ 1 m, but this is expected to be almost negligible compared to the total head developed by the multistage propane pump. There must be controls not yet explained, though this would not change the need of a surge drum in a "usual" design (para 2).
6. How does the past case (NPSHr=2ft, EL=6 m) operate satisfactorily? A sketch with data like "Description.pdf" referring to the past case, along with controls and operating curve of that pump, could help a bit in clarifying the case. If bmicky can supply them.

Edited by kkala, 22 June 2012 - 10:50 AM.