Condensate flow through the slopped down line

[Mike4chemic]

Hi,
I am currently involved in the process design of the geothermal condensate pipelines that convey the geothermal condensate flow rate-2,200 t/h at 10 bara @ 187deg. C to the power plant.
The piping sizing, eq.length and elevations are presented in the attached sketch.
I would like to ask you the following questions:
1. If I can assume that the vertical down line will be full of water ? Otherwise,what I can do in order to ensure that
the vertical down line will be full of water?
2. Which condensate pressure would be expected at the plant,upstream of heat exchanger?
The pressure drop (friction losses) via the 3000m, 24" pipe is about 20 m.

Thanks in advance, Mike

 

[katmar]

It's not clear from your sketch or description which is the vertical down-flow section that your question (1) refers to. Does the 3000m section slope downwards evenly over its length, or is there a vertical section in this pipe that you are concerned about?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mike4chemic]

I am concerned about 3000m section that slope downwards evenly over its length.

 

[katmar]

1. If the pressure at the pump discharge is 12 Bara as shown on the sketch then you will have boiling in the 120m line that slopes up from the pump to the highest point at 1100m. The static head from the pump to the highest point is -30m, making the pressure at the 1100m elevation about 9 Bara. At this pressure water boils at 175C. So, if the pump discharge pressure is 12 Bara then the 3000m section will not be full of liquid.

2. The condensate pressure upstream of the HE must be calculated by working from the eventual discharge point back to point where you have shown the PT. You cannot work forward from the pump. Always start from the point of known conditions.

However, here you have the added complication of wanting to avoid boiling in the pipes. To avoid boiling you must specify that the pressure at the highest point is at least 12 Bara. This makes the pump discharge pressure at least 15 Bara and the pressure upstream of the HE at least 34 Bara (=12 Bara plus 278 m of water at a density of 880 kg/m3 less friction of 2 Bar).

I say all these pressures are "at least" because you may need higher pressures to satisfy the conditions downstream of the HE. If the pressure drops are low downstream of the HE then you would use your PV to ensure that the pressures are at the "at least" values given above.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mike4chemic]

Katmar,

Thank you very much.

1.Since we deal with condensate at saturation conditions and in order to avoid flashing in the 120m line, the pressure at highest point must be min. 12 bar a. This is a reason why we design a pump.

2. You wrote "the pressure upstream of the HE at least 34 Bara (=12 Bara plus 278 m of water at a density of 880 kg/m3 less friction of 2 Bar)".

As far as I understand, this calculation based on the assumption that the 3000m line is full of water. Is it correct?
However, in order to have a downward line full, the frictional pressure loss in the line would need to be equal or greater then 278 m. In order to meet this condition, the flow via the 3000 m pipe should be about 8,000t/hr !!!!, when the nominal flow is 2,200 t/h only.

May be I am missing something ?

 

[katmar]

1. Well, we agree on this point, so that is a good start ;-)

2. Yes - the pressures I calculated were based on the assumption that the line is full of liquid. Your calculation of 8,000 t/h is correct if you want to use up the full static head of 278 m as friction. In this case the pressure at the base of the sloped line would be the same 12 Bara that you had at the top because all the static gain is consumed.

Consider the case when there is no flow and the pipe is full. You do not need to consume any of the static head - it is simply there as potential energy.

I agree that your 24" pipe could run part-full with the flow of 2,200 t/h if this were an atmospheric application. But in your case the line is not open to the atmosphere, so what fluid would be above the liquid in the part-full pipe? It can't be air, so presumably it is steam. At the HE you would have to install a gooseneck (or some other device) to prevent the steam flowing through the HE.

The prospect of having two-phase gravity flow of a saturated fluid down 3,000 m of 24" pipe would fill me with dread and I can't imagine anyone doing it this way. But I must admit I have never worked with a geothermal installation like this.

I think the way I would do it would be to locate the PV on the upstream side of the HE and use it to control the pressure at the peak elevation to 12 Bara. This way your HE would not see the full pressure - provided the downstream pressure drop is relatively low.

You probably need advice from someone who has been there, done that and worn out the tee shirt. But looking at it from a first principles point of view I would not design for part-full flow in the 3,000m section.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mike4chemic]

Katmar,

Thanks.

Our design was based on the assumption that the 3,000m section is full. The question is, if keeping on 12 bara at the highest point of the pipe by control valve located downstream of the HE will ensure that the pipe will be full??

If I can assume, that in order to ensure that 3,000m section will be full at 2,200 t/h, the friction losses via the all system,that includes 3000 m pipe + HE + control valve should be equal to full static head of 278 m ?
If yes, the pressure immediately downstream of the control valve will be 12 bara. If is it correct?

 

[katmar]

You cannot say that the pressure downstream of the valve is 12 bara on this basis. See my answer to point 2 in my second post in this thread. The pressure immediately downstream of the valve is determined by the piping between the valve and the eventual discharge point.

The pressure at the highest point (12 bara) plus the static head of 278 m must be offset by the friction losses in the 3000 m section plus the HE plus the valve and the downstream piping.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mike4chemic]

Katmar, Thank you for your valuable input.

1.If my understanding is correct,in order to the 3000 m section will be full, the static head of 278 m plus 12 bar must be offset by the friction losses in the 3000 m section plus the HE plus the valve and plus downstream piping.

2.Keeping on 12 bara at the highest point of the pipe will ensure that the pipe will be full.

 

[katmar]

Mike, Sorry - this seems to be going on and on. Just one final clarification. Yes, you need to keep the highest point at 12 bara or more, but the way to do it might not be to use that pressure as your control point. I have not thought this through completely, but it might be better to measure the pressure at the bottom of the sloped line and control that. It may be possible to have 12 bara at the top without the line being full. You need to consider if that could happen.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Mike4chemic]

Katmar, Thank you very much again.