Confusing problem with a Non-clog centrifugal pump

[Thaer Alashqar]

Hello
We have recently installed a centrifugal pump with screw open type impeller (curve attached) to deliver waste water (sludge) from thickener to drying beds (around 300m distance), and we are unfortunately facing confusing problem since the first start. The best way to describe the problem is as follows:

when the pump is turned on it doesn’t deliver any flow to the other side and the reading on the amperage is very low (half the rated). When the pump starts this way it stays like this until we turn it off.

When the pump is behaving that way, the only way to fix it is while the pump is running we open a valve on the discharge to divert the flow to a nearby tank (10m distance) for a short period (less than a minute) as soon we open the valve the amperage would increase (up to 70% of rated) and the pump sound changes. Then after the minute ends we close the valve and divert back the flow to its desired long path and the amperage reaches the rated and flow would be achieved at the other end and the pump could operate like this for many hours without any problems. But when turned off for an hour or more we face the same startup problem again.

Notes:
1-We installed air release valves on the discharge with no luck
2-we have two pumps in duty- standby configuration and the two behaves in the same way!.
3-the operator is refusing the option of opening the nearby tank valve on each startup.

I would like to hear your thoughts on this matter with many thanks.

 

[LittleInch]

Can you send us sections A nd B of your plan drawing above?

Looking at this again from the start it seems to be that the issues is either:

There is no flow because the discharge line to your ponds has become solidified and the pressure required to get movement is more than the static discharge head of the pump

There is no flow because the inlet line has become blocked and the head of the tank isn't enough to force it through.

I'm going for the first one as you haven't told us what this fluid is or what happens when it settles in a bucket or a pipe.

Flowing through tot he first tank could just be enough to get some flow through the pump and then on opening the GV1 valve as you close the GV2 valve there is enough momentum or disturbance that the line becomes liquid again.

Difficult to really tell what's happening without some pressure readings or knowledge of what the flid is.

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

 

[bimr]

For those not familiar with wastewater treatment.

Wastewater thickening typically produces a thickened solids product of 2-8% solids concentration.

Link

If you look at the pump curve, the pump is starting out with a very low discharge head.

 

[Thaer Alashqar]

@Tugboateng

We suspected the check valve before and tried running the pump without it and nothing changed.

@Littleinch

The fluid is thickened sludge with 4-6% solid concentration, when settled in a pipe it becomes thick and looks like mud or ketchup.

After trying many solutions I think that your first explanation is correct and also Bimr concerns on velocity not reaching the required for this type of fluid is also correct. After the sludge is settled in discharge pipes it becomes muddy and generate gases that gets trapped in the pipe. And therefore the pump doesn’t generate enough momentum nor velocity yo push all of these and thats why opening the nearby tank helps with moving the sludge and transferring it to a newotainian fluid that doesn’t require much speed or momentum.

Today i tried to remove one of the pumps check valve and turned on the other to make the pump recirculate the fluid for a minute and then closing the valve. The results were good.

Right now we’re thinking of a configuration that is acceptable to the operator since pumping to the nearby tank is not an option.

Do you think running the pumps without foot valves and manually close the discharge after pump operation is a good operational procedure ? And you’re welcome to give any other ideas

 

[LittleInch]

As said above as part of the shutdown sequence I would try opening the treated water line then closing the inlet valve and running the pump for a further minute or two then shut down.

Worth a try.

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

 

[bimr]

Definitely should not have a "foot valve" if the foot valve is on the suction side.