My first guess is insufficient suction pipe and fitting size for handling the increased flow with more pumps in operation. That implies that the NPSH req is not met.

A closed valve startup and controlled switch over or restrictive out-flow regulation may help.

A priming return flow bypass for the delivery reflux vale to the startup pump with valves may also help.

Are you pumping water?

I'm pumping water.

I think i'll try your 2nd suggestion..is it ok if the priming return flow valve kept open during operation?

I cannot think of a reason to close it during operating that pump.

If not running the pump will be under pressure and may leak at the standing shaft You also would need a valve during maintenance.

If you don't have an isolating / reflux valve on the suction side the vale should also be closed when not running. (otherwise some water will just circulate)

I would however suggest a standard procedure of closing the bypass after startup but that will depend on your situation and personnel.

Is your suction side adequate?

Is this setup further or higher away from the source than the other 2?

hi

first - the resistance of pipe geting higher versus high capcity.

i had such a problem & relized that when 2 pumps working together the resistanance of the pipe is becoming higher & probably your pump is not strong enough for it- so it fails.

is that sound possibile in your case?

Can you please provide us with the drawing of the suction line indicating pipe sizing?

Because I think the dividing of the mass of water is sufficient to 1^st and 2^nd pumps and insufficient to 3^rd one. In these cases we have to add a subheader, I mean the suction line shall be branched to 2 branches and those 2 branches shall be collected in one header and this one header shall be branched to 3 inlets, one inlet per pump.

I have to wonder if you are just losing your prime when the pump switches? It could be a faulty foot valve. I would think that you would lose the prime when any pump was started if this was the case though. It is strange to me that it only does it when switching from number one to number two pump and not all of them. I am assuming that all three pumps have separate discharges?

pipewelder

Here is the diagram of the duplex pumps i installed. It has a single suction pipe and single discharge pipe. Z = 1.5 to 2 meters.

pump specs: 9.6 cbm/hr at 36.5 m max. TDH 1.1 kW motor 380V 60 Hz 3-phase

operating pressure: 30-35 PSI

pump delivers water to a dual media filter and activated carbon filter connected in series then to a 10 cbm storage tank

actually the pump capacity is 3 cbm/hr at the max. TDH which is 36.5 m.

Hi joozmax6

What is the difference between the faulty one and the other 2 systems?

Can you prime and start pump B on its own?

What happens when you switch from B to A?

Do you have a vacuum breaker on the discharge? I have a couple of large water/ash pumps that pump several hundred feet head on the discharge side and about 2000 ft distance to the ash pond. We have had problems with the vacuum generated by the water rushing back to the pump when we shut down to switch pumps. You may just need to shut down let things settle down and then start the other pump. It still doesn't make since to me if you have the exact same system running in 2 places but not there. Your positive that everything is exactly the same? I will talk to some of our maint. guys and mention your problem and see if they have ever run to this.

pipewelder

There are no problems when a pump starts. problem occurs when the pumps switches. I've already set the time delay to 20 seconds during switching to stabilize the system before the alternate pump starts. I forgot to add the by pass line from discharge pipe back to the cistern tank in the diagram. Only after i've opened the by pass valve and drain the discharge pipe will the alternate pump operate normally.

Thank you for all your comments. I really want this problem solved so I can proceed with the other similar projects.