Slack Flow Versus Siphon

[oilman11]

Hello,

First time posting here, apologies if question is unclear.

I have spent hours searching the difference between operating in a slack flow vs in a siphon. Numerous sources say that when Hydraulic grade line (HGL) falls below the pipeline elevation, slack flow occurs. My question is: What if the HGL falls below the pipeline elevation but still remains above fluid's vapor pressure? Slack flow occurs when the absolute pressure in the pipeline falls below the fluid's vapor pressure. What happens in a condition where absolute pressure is negative but still above fluid's vapor pressure?

The above mentioned condition is occurring on a slurry line going into a tailings pond 100m below the start of the pipeline. I want to analyze the velocities in the entire piping run. How would I calculate velocities in sections where there is slack flow versus where there is siphon? My concern is increased pipeline wear in sections where velocities are very high.

Thanks

 

[LittleInch]

"What if the HGL falls below the pipeline elevation but still remains above fluid's vapor pressure? "

Essentially nothing changes. As you say this turns into something approximating a syphon where pressure in the pipe falls below atmospheric pressure, but the liquid stays as a liquid, not a mixture of liquid and gas.


The only issue you get is if the pipe cannot handle negative pressure, e g. a flexible pipe collapses or somehow lets air in.

If you have slack flow then velocities increase in the liquid phase and become very difficult to calculate or analyse as you end up with "glugging" or two phase slugging.

That's one good reason to avoid it.

If possible I would aim for a larger vertical pipe but even a slurry line is controllable with e.g. pinch valve or knife valve. I normally avoid slack flow like the plague. IMHO.



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

 

[georgeverghese]

There will be significant cavitation in the top sections of this line where absolute dynamic pressure < fluid vapor pressure. And bubble collapse together with vibration and erosion corrosion of the pipe walls at the lower end of this line - see page 6-44 in Perry Chem Eng Handbook in the subsection " Cavitation".

Suggest installing an automatic control valve at the bottom of this line sensing pressure at the top of the line. Controller and control valve to keep the line pressure at a few psi above vapor pressure (or to simplify, at 0psig) of this liquid.
This is a major problem in coolant seawater gravity flow dump headers in offshore platforms which plunge 20-30metres from the top down to dump caisson at sea level - they resorted to an air fill line to "break the vacuum" at the top of the line. Air inflow was very large and in actual operation on this platform in the North Sea, and there was significant vibration in the downpipe with 2phase flow.
With a control valve at the bottom of this line, you wont need to admit air a the top through a vacuum break line. Also would suggest asking for stronger supports for this downpipe and extra wall thickness to withstand high vibration and erosion in case of control valve / controller failure.

 

[1503-44]

"What happens in a condition where absolute pressure is negative ..."

That cannot happen.

Absolute pressure is always +

Vapor pressure is always +

Atmospheric pressure is always +




A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

I think the OP meant when absolute pressure is less than atmospheric pressure, but agree as written it is incorrect.

Gravity flow down vertical headers is indeed a different thing and if you don't get it right it will vibrate a hell of a lot.

I have somewhere a report from the Alyska line where they for some strange reason started running in slack flow mode and there were reports of noise and ground vibrations. I've never understood why anyone wants to run a pipeline in this manner as the increase in flow versus maintaining say 1 barg at the relevant point is usually incredibly small.

I think because you often have to maintain a significant back pressure at the end point, people who don't understand hydraulics think that it is somehow a "waste" or that the upstream pump station is being used inefficiently. Total rubbish, but sometimes you need to educate them.

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

 

[1503-44]

Alyska has experienced a decrease in supplies. It hasn't been operating at design capacity for many years.

BTC Pipeline is designed to operate in slack flow for the last segment before Cayhan terminal, due to the steep loss of elevation in that segment and actually changes diameter from 42" to 30" to burn off excess head. Critical velocity is avoided, thus vibration does not occur. The alternative,would have caused extreme pressures at the backpressure valve at the terminal. There is one there, but operating in slack mode allows the backpressure setting to be greatly reduced.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[oilman11]

Thank you for your input.

This application is for a highly abrasive slurry and putting any sort of valve for creating back pressure will require significant maintenance. As mentioned above, I think putting in a smaller diameter pipe to create back pressure would be a better idea. One other option I was thinking of was running an open channel flow with a bigger pipe diameter since the line only goes downhill (maintaining velocity above slurry’s settling velocity).

Some of you mentioned some pipelines operating in slack flow. In these cases, how would introducing air into the pipeline helps? How is that better than operating without the air ingress?

 

[LittleInch]

Introducing air helps to keep it as open channel flow and not slugging or glugging.

But you do need a continuous slope to make that idea work and ideally a constant slope.

Pinch valves will handle a lot of abuse.

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

 

[1503-44]

Slack flow is mostly the same as open channel flow except the pressure at every surface is the product's vapor pressure, whereas in open channel flow it is atmospheric pressure and the interface is product /air. The biggest difference is if the pressure rises above vapor pressure in a slack flow line, the vapor condenses back to liquid, the line flows full (slack flow stops). The vapor collapse can be accompanied by significant shock pressures as the liquid columns on either side slam together. The pipeline flow level should be controlled to gradually rise until flowing full again. It should not be flowing at say 50% level then experience a rapid increase in hydraulic pressure that collapses the vapor very quickly.

Air pockets will not totally collapse and disappear, but they do experience great changes of volume with chances in pressure, so air can cause effects similar to crossing a vapor pressure threshold, just usually not as severe. You have to guard against rapid pressure changes, generally caused by rapid changes in slope.



A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[oilman11]

Appreciate the help everyone! This deifintely helped me clarify a lot of the confusions I had regarding slack flow and open channel flow.