Long Suction Pipe in Pumping Station

Contact the pump vendor independently and ask questions. He probably has experience with similar poorly designed installations. Your consultant is correct, you simply cannot place the pumps at any distance from the reservoir and expect them to work properly.

Cavitation and flow instabilities can occur with these kind of pumps if the intake stream is poorly configured or if the pump is started too quickly. Investigate how fast that the pumps will start and if a very slow start is possible.

Both the venerable "Pump Handbook" by Kassarik and Hydraulic Institute guidelines contain limitations on suction channel configuration design.

It would not be unusual to have a pump vendor perform scale model studies to verify flow. This scale model testing is expensive. I have personal experience with this kind of testing that was done for a nuclear power plant in Sweden.

Calculate the average velocity of the fluid in the pipe under all conditions of pump operation intended, ready to discuss with the pump manufacturer. Note that doubling the number of pumps in use does not double the flow, as the operating point moves to a different place on the pumps' pressure-flow characteristic curves on the basis of what the piping system presents to them.

Using the Darcy-Weisbach equation for the process fluid at the temperatures experienced, calculate the pressure drop in the pipeline feeding the pump set(s). Subtract it from the lowest head condition in the upstream reservoir. If the pump inlet pressure under any operating condition is below the fluid boiling pressure at the ambient temperature, the fluid will cavitate, placing the pump impellers at great risk. If so, then a redesign or reselection may be necessary. Once these calculations are to hand, challenge the other parties involved to come up with their calculations to justify their statements.

And go from there.

There is not enough information provided to say whom is right. Diameter of the 500m run to the diameter of the pump input(suction side). These could be 2m runs to the pumps with 150mm suction inlet to the pump. With a 1m line connecting the two. Where it's being pump to and at what pressure. You give a large variance in pump volume. Is that due to motor controls installed or head pressure. There are a lot of variables involved isn't that why you choose some one to design it. And not the senior reviewer. We have no way of knowing their knowledge of the subject.

"Our senior reviewer is arguing"

Go back to your client and tell them that your organization is incapable of performing a technical review of this project, due to the incompetence of your staff.

Or, hire a mechanical engineer with the proper education to perform the technical analysis.

lol I've considered it many times and still do

but our senior consultant already told'em straight - "your project falls into category of BS"

Good for him!

Tornado's suggestion of slow ramped pump start may solve the cavitation problem.

Depending on the lift and flow required, your power may, or may not, be enough. If your pumps are properly designed and adequately powered, you won't have any problems.

Can you set this up with VFD motor control and a fairly generous ramp-up time?

Yes, the motors shall be equipped with VFDs, but the opinion is that the power won't be sufficient, and that the pumps will not be able to deliver water.

Power is a different problem altogether. Why do they think the power will be insufficient?

With 5-10 meters of positive head, you don't need a pump.

You also don't indicate if there is any lift required, or the quantity of water you want to move per unit of time.

You still need someone on staff who has something other than an opinion.

He said "The water column in reservoirs is approx. 5 - 10m above pump center line." He hasn't given the level in the downstream tank, but I think we can assume it's higher than the suction tank.

Maybe.

The pump delivers to downstream reservoirs which are approx 20m higher than the suction reservoir, through pumping main of some 40km length (depending on the operation scenario). Pumping station output can go as high as 5.2 m3/s.

However, the start up and initial operation is questionable here.

Wow.

That's 25 miles.

That changes everything.

Getting 25 miles worth of water moving is not going to be easy.

That's 36 tons.

Or, did I miss something?

Yeah, you're absolutely right.

Ditto this. You must have slow spin up, and have back-pressure build up at low flow condition. Water has some inertia.

I don't pretend any expertise in this, but shouldn't that have been based on the 200m distance??

If these cells really are as huge as claimed (no figures given) and the size and shape of the header connections are scaled accordingly it seems to me that the initial surge would draw somewhat evenly between the first cell and the remaining header to the other 2 cells, then splitting again for the remainder. Once the draw was established the effects of the water column height would take over; providing the header connections are not restrictive, the draw from the further cells would then increase until the flow was somewhat equalized. Have I missed something in this?

200 vs 500m - I wasn't sure about the layout, so I took worst case.

.

Will apply this to the particulars of the project. Thanks!

Is any of this built yet?

Earthworks going on.

Has any piping or other apparatus been ordered and paid for yet?

No, no purchasing yet. Technical specs are also under reivew.

At least this project is being done in the correct sequence.