Gravity Flow Pipe Surge 1

[Auriferous]

I am pumping a slurry (10% solids) through an 8" steel pipe 19,000' long and on a downgrade of 1%. At the end of the line, we are seeing large surges of fluid followed by lulls. I am guessing that this is being caused by the pipeline being able to carry more via gravity than the pump is producing. According to my calculations, the pipe can handle around 380 GPM at full flow, but the pump is only putting in around 160 GPM.

Question 1: Would this situation alone be causing the surges?

Question 2: Could the solution be as simple as partially closing a valve about 1000' from the outlet? Or is a surge tank a better idea?

Thanks to anyone that may have some experience in this area.

 

[CRG]

Is there an atmospheric vent downstream of the pump to allow stable gravity flow? A vent may solve your problem for your flow rate.

 

[Auriferous]

Thanks. I believe that the system is closed to the atmosphere the whole way downstream of the pump. One of the alternatives we were looking at was putting a gravity feed tank at the upper end of the line, but we are afraid of settling the solids. The vent you suggest may work, since our velocity from the pump is sufficient to maintain suspension.
Thanks again.

 

[stanier]

The situation appears to be a problem of a line being oversized, perhaps for future requirements.

If the pump can handle the correct flow, then there needs to be a device at the outlet to provide a head loss. This would keep the line under pressure and ensure full flow.

In other slurry lines I have seen ceramic chokes used to reduce the velocity, disspate the energy and keep the line full. What you are observing may well be vapour pockets arising from negative transients in the pipeline. It could be air coming out of solution due to low pressure areas in the line.

Other systems I have been involved with dropped 1000m in 8000m through some 200+ vented drop boxes. These were used to dissipate the energy. At Bougainville Copper, before the troubles, they installed Warman slurry pumps to act as turbines to recover energy whilst dissipating energy.

AFT's Impulse will model slurries and downhill pumped pipelines for transient events.Look at

http://www.aft.com.

They also offer consulting services.

 

[c2sco]

My calc suggests similar to yours - the pipe should be able to support a gravity flow of about 100m3/hr or 440 USGPM. The Froude number of the outlet is 0.22 which would allow bubbles to rise even in a flooded section, unless the liquid is very viscous. Throttling the flow will just stop air entering beyond that point by making the local Froude number higher hence preventing air getting into the pipe. So you could put the restriction (say a valve) at the outlet and tweak it until it suits you.
Stuart