1 st pump operating delay to shut off after 2 nd pump comes into operation

[Arttech92]

I have a question, anybody could help? We have two pumps 1 main / 1 spare, if we would to switch the 1 st pump to 2 nd pump in operating, and do not want to shut off the 1 st pump for few seconds, then how would be a reasonable delay we can consider to operate the two pumps before the 1 st pump shut of come for not having problem for the pumps and the system? (How many seconds?) If it delays long what would happen? The pump is centrifugal, what if it is PD pump?

 

[BigInch]

Depends on what your system piping is doing. You will try to double flowrate. if the system pressure increases a lot, when both are running together, it may exceed the pump's discharge capacity, in which case your pumps will go to low or zero flow and tend to overheat. You may also create large transient pressures. No way to know without modeling, at least some simple calc, maybe a full transient program is needed, or by testing the system live.

Independent events are seldomly independent.

 

[LittleInch]

The effect will dpeend on your system curve and pump curves, but for a 1 + 1 system for centrifugal pumps with a DOL starter, you're probably looking at 3-4 seconds for full wind up of the spare pump and then no more than 5 to 10 secods before you want to turn your first pump off without seeing any major increase in flow. You won't increase pressure that much, but might bump the flow up briefly, unless you have some sort of flow control downstream of the pumps. If you run two pumps for any lenght of time , say > 10 seconds, then you're operating as a parallel pump set up - see the other posts on this page which will tell you all you need to know about that. If you run two pumps for a while in parallel and then turn one off, the flow through the remaining pump can stop briefly until the downstream pressure and flow decays back to normal. This might be Ok, but if you have any no flow switches or alarms, they may need to be disabled for this period or given a longer time before shutting your pumps down.

PD pumps on the other hand will more or less instantly double the volumetric flow rate and it is unlikely that your downstream system will be able to cope with that without a large increase in pressure, possibly either causing the systme to shut down or lift the bypass or pressure relief systems if run for more than few seconds together. Rapid (5 seconds) turn off of the main pump is recomended unless yur system can cope with two pumps running. Only you can tell.

Transient analysis is the only way to really see what would happen.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[ccfowler]

Arttech92,

Can you explain your system and concerns in much more detail. As originally stated, your posting looks suspiciously like it may be a request for help with homework.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.