Pumps in Series Vs. Parallel Graph

[eth0s]

For two pumps in series you should place the weaker pump feeding the stronger pump for maximum performance, otherwise the weaker pump would restrict the flow of the stronger pump. But as for a powerful pump "burning out" or overpowering a weaker second pump, I am not really sure that is possible, because the extra pressure will pass through the second pump (albeit reduced by the restriction of the pump). However, I suppose I can imagine a scenario, where if you had some kind of a super-powerful water-jet type pump, maybe you could do some physical damage to a weaker pump, but even then, I don't think so. To give you a real world analogy of what is happening when pumps are in series, suppose you have three pumps with 10 feet of head pressure each, and you need to get water out of a hole in the ground that is 25 feet deep. Now this hole is so deep that if you try to use any one of your pumps, there will not be enough head pressure to lift the water out of the hole (because each pump will lift the water only 10 feet). So what do you do? Put the pumps in parallel? No way, in parallel you will simply pump the water to a height of 10 feet, but three times faster. You need to add the pumps in series, thereby adding the head pressure of each additional pump, until you get enough pressure to get the water to a height of 25 feet in order to reach the top of the hole. With three pumps with 10 feet of head pressure each, in series, you should be able to pump the water out of a 25 foot deep hole. Imagine the first pump is at the bottom of the hole and, that each additional pump is mounted 10 feet above the other. The same principle will apply inside your computer case, except instead of overcoming gravity, you are overcoming restriction in your water blocks and the friction resistance of your tubing. And as Martin pointed out in his graph, the more restriction in your loop, the more you will benefit from pumps in series.