Pump down plant down. piping is minimal through Tech-Taylor Valve. Have two variable speed drives VSD or switching gear between both..
The pumps are high wear, impellers,liners etc,, not much extra piping is required as an adequate valve system on one pipeline is sufficient, Tech-Taylor valves do a good job on the discharge, be sure to install extra cyclones in the pack as they are high wear as well. You will need to size the whole plant accurately so it works as one, If in doubt, it's cheaper in the long run to consult an expert.
It's quick to change a VSD from one pump to the other.

The answer to your question will depend on grinding circuit capacity, abrasiveness of the ore, risk appetite of the operator and maintenance practices on site.
For example, I would probably make different decisions for 2,000 TPD and 80,000 TPD (single line) circuits.
I have done several trade-off studies on this topic and it is always a contentious issue.
Considering saving on pipes etc and less building space with compact layout, one pump option is often better. Necessary spares may be maintained for treplacement when required for smooth operation.

The pump size you want is the pump size you need, depending on Tph, abrasion, recirc etc.
But from an operational cost, the payback of a duty, standby system is half a dozen shuts for pump rebuild. I like two pumps with individual VSD's, but have used switching gear, the standby then goes DOL. You can get away with no standby, but its the Mills that pay the bills.
This plant has a strange setup, in it is a closed circuit, high aspect SAG, with the p80 at 75um, so we can carry a large recirc depending on the orebody being processed. So our pumps are on the need a lifting device to rebuild size, many hours to rebuild.
But the smaller the pump the smaller the costs the smaller the loss, to scale a loss is a loss.

Hello everyone,

1. a 40 000 tpd plant with a 15 MW primary SAG mill and 2 pcs 10 MW secondary ball mill. If you try to change the H/C feed pump without stopping or just during a short shut down of the grinding line, you may not get a single drop of slurry into the stand by pump because the feed sump is half full of ball scrap and coarse rock, blocking the pump inlet pipe of the standby pump. Therefore the plant has a tactics to change the standby pump to operation during the SAG mill relining shut down. They never even install the standby pump, but keep a fully revised pump on the floor of the pump level waiting for the liner shut down.

1. A 3 000 tpd plant with a 0.7 MW rod mill and a 1.5 MW ball mill. If you try to change the H/C feed pump during continuous run, you will just find out that the chamber of the autoball valve is half - full of rod scats moving nowhere and leaking badly.

The only realiable way of designing H/C feed pumping or any other item inside or outside the plant walls is through tens of years of “on the floor” experience. If that is not available at the moment when the “electrical pen” of the CAD – designer makes the first move, it is a step to 180 degrees wrong direction. And what is more, even if the experience is available, a project team composed of guys from all disciplines (project management, plot plan, architecture, civil, process, piping, electrical, automation / instrumentation, HVAC), without intensive, honest and communicative cooperation, will produce a marvel of poor construction.

Thank you all for your comments. Very good input and discussions.
What I hope to see more are: hours you needed to replace the pump, how well you can predict the pump worn-out, if you actually experience un-expected pump down for a single pump layout, or for a double layout, whether you actually experieced block-off of Tech Taylor valve, block-off at pump suction when you try to switch to the second one.

For small pumps, we allow vertical discharge, so the layout is pretty simple. When pump is relative large (do not have a clear cut-off, I'd say for a plant of 10,000tpd), we like to have horizontal pump discharge, and the layout becomes a difficult job. For further larger system, say 40,000 tpd, when a Tech Taylor is not available for the pipe size, the layout becomes a pain.

Worked and talked to quite a few well experience process engineers who worked on single pump plants, their comments are quite positive on such layout. Worn-out of pump liners or impellers can be predicted very well based on experience and monitoring of the motor power/speed and planned shut-downs actually affect little of the overall plant availability (sometimes impellers/casing liners may be replaced prematuredly when there is a shut-down for mill maintenance - does this happen in your operation?).

While some well experienced engineers, based on their negative experience on single pump operation, strongly oppose such layout. They want double pump option at all cost -- no matter how difficult the layout job is - let alone the extra budget.

Another whole issue is sump design, particularly if you get into the concept of one SAG to two ball mills, with two cyclopacs. Should the sump be split, which then leads to how do you distribute the SAG discharge, or should it be a single sump, which leads to how do you prevent build up on pump inlet if that side is down?

Then you can have a case of two SAG's to three ball mills. Does each ball mill have their own cyclopacs or common cyclopacs?

It's all about ROI. when someone says "the mills pay the bills" please pay attention. If you can afford to have the place sitting while a pump is changed that's nice. Your lucky. Reality is that for most operations the loss for a shutdown (even scheduled) always hurts but it may be necessary. Goal is maximum safe operational hours.

I have helped commission and have operated in mines that were built both ways (with and without standby pumps)

Some thoughts:

Get good operations input on the design.

Accountants can give you the numbers, engineers can tell you what you need for equipment (and good ones with some on floor experience are invaluable when it comes to what will work in the real world)

1. hours to change out a pump in a 28,000 TPD concentrator would not be acceptable as at the current metal prices this operation produces $750 a minute worth of metals and would result in a production loss of $90,000

And with only a single pump the trend is to "limp" it through until the scheduled down, resulting in other production losses (which will poor separation at cyclones, loss of recovery, lowered throughput, clean up of spilled material etc.)

As production drops the cost per tonne rises and the value of the ore body drops.

At one operation the increase in overall plant availability from 85% to 92% resulted in a very profitable mine life extension of over four years. (waste became ore because it could then be mined at a profit)

Standby pumps have a significant cost, and designing a transfer system that works well based on the potential material in the ball or valves is a challenge, but in the end they pay for themselves.

Blocked standby pump suctions can be a problem in large copper porphyry concentrators where the primary grind is quite coarse. The problem can be mininmised by using short straight suctions and having plenty of air and water available at the sump to lance the blockage. A more elegant solution is to design a deep conical or prismoidal sump where turbulence at the suctions keeps the standby relatively clear. This design however, for a given slurry retention time, has the undesirable effect of raising the centreline of the mills discharging into the sump (adding to capex).

Great comments. Using a standby pump seems to have more flexibility to overcome pump maintenance. Any shut down because of pumping maintenance is not acceptable. The pumps are scheduled equipment (I mean by following P.M based on experience). Using more cyclones in cyclopac also can help. Retrofitting a new pump in high capacity circuits also depends on sump and other pumps layout because of more spaces that will be needed.

Blocked standby pumps and Tech Taylor valves are a real issue.

Identify the potential for blockage.

Build pump boxes to move the largest and heaviest particles to the inlet of the active pump and not leave a standby pump blocked.

you may have to install adequate flush lines to the inlets to back-flush. (On a 16" suction for a SAG discharge a 6" high pressure line was adequate to move the settled material, fill and keep the pump from cavitation and stir up the material until the material from the sump reached the pump).

Flush lines need to be able to back flush the buildup at the suction AND keep the material moving while the pump is coming up to full draw from the sump.

Hard pipe flush lines so op's are not fighting to get flush lines hooked up.

Keep spool pieces short if possible.

Valves should be as used to isolate and drain the spool pieces as close to the sump as possible.

Operations need to isolate and drain pumps as quickly as possible when they go down. I am a big fan of air actuated valves with automatic fail to needed position. I.E. on power failure the suction and discharge valves close, drain valve opens and flush line opens for set time.

It may be necessary to drain the pump discharge line AFTER the pump isolation as well.

Use good valves with seats designed for the material.

If the valves have flush line connections for the glands it helps to have them hard piped

Have the valves and start / stop stations where they are accessible to the op's with the basement flooded.

When it is known the standby is going to be needed the flush for the valves and suctions can be done prior to the switch. A bump test of the pump is recommended.

Tech Taylor valves,

size them properly. when the standby pump is down the discharge line should drain to prevent material from building up on the standby pump side of the ball.

after a bump test of the pump to ensure it is ready to run the suction and pump need to be full prior to the switch (flush water). If the pump cavitates it will not build adequate pressure to move the ball and small amount of buildup on the discharge side.

for some materials flush lines pointing into the valve may be needed to clear the ball and upstream pipe prior to the pump change.

Put valves where they can be worked on by the operators and maintenance!!!!!!

I.E. in one operation a Tech Taylor valve was 10 m up from the pump and under the floor above. All maintenance had to be done by chain fall winches and was a pain for the operations personal to reach if there was an issue. solution was to extend the pump discharge lines by 2 m and put the valve just above the floor. Access by operations and maintenance personnel greatly reduced issue with the valve.

Designing sumps, adequate pump switching procedures and planing for the pump change overs is a challenge, but pays for itself in operational hours.

Probably could write a book on this subject.

Have a good day

There are many great plants where changing pumps on the run is not an issue, there are plants where someone in their wisdom has made a change and created a monster, why reinvent the wheel. Mike'have a look at a plant that has no such issue and copy that design, How many times have we seen a good plant moved to a new location and the engineers have made a couple changes for various reasons and that plant becomes difficult to run and visa versa a bad plant has been relocated and the engineer has made it operator friendly, thus becoming more efficient. I have not had blocked standby pumps or techtaylor valves and how does one block a discharge line. These issues are employees, may it be the engineer, or the operator, dump valves and flush lines are basic, stick to basics it's not that difficult.

Thanks everyone for your great contribution! Hope to see comment from more people especially problems you see in your operations, although I may not be able to respond from now on.