When you are installing a pool pump one of the most important things that gets overlooked is the elevation of the pump in relation to the water in the pool. Specifically it is important to consider if you should install valves on the suction lines or if manual valves will suffice.
If your pump is installed at an elevation lower than the pool water level then you will need to install isolation valves on the suction and return lines. The logic is that with your pump installed lower than the pool water level you will need to manually close these valves prior to opening the pump to clean out the skimmer basket. If you have a pump installed below water and you open the lid of the pump without closing the suction and return valves then the pool will begin to drain out through the open pump.
On a pool pump installation where the pump elevation is higher than that of the water level in the pool requires a different configuration. While manual isolation valves are not required to prevent the pool from draining out through the pump you may want to consider adding check valves (one way valves) into your system.
Imagine a situation where the power goes out in the middle of the night. Your pump turns off and all of the water that was in the plumbing lines leading up to the pump drain back to the pool. In an ideal sense when the pump turns back on it can often regain the prime on its own by drawing water up through the pipe from the pool to the pump. However some pumps installed with a larger elevation lift, or pumps installed on systems with small leaks in the suction lines may have difficulty in priming without someone manually adding a full strainer basket of water to the pump.
In this situation is it quite possible for the pump to run i excess of 10-15 minutes or longer as it struggles to prime. Should the pump be unable to prime itself there will be no one present to notice this and turn the pump off before heat damage happens. Running a pump without water (without prime) for anything more than ten to fifteen minutes is very likely to cause damage to the pump. Though there is a thermal disconnect in every electric motor in the event of overheating, this is more to protect the pump from starting on fire than to protect it from heat damage.
Since it will not always be possible for you to be present when there is a power failure to make sure the pump breaker gets turned off, the solution is check valves.
If a one way valve is installed on the suction line and then another one on the return line to the pool then when the water shuts off it will not be possible for the water to leak back down to the pool. So long as you do not have any leaks in your plumbing system itself which would compromise the ability for the check valves to function, then having these valves in place makes sure that your pump does not have to lift the water all the way from the pool and through the plumbing lines.
It is important to note that check valves to represent a flow restriction similar to how ball valve, gate valves and plumbing fittings are a flow restriction. Do not put check valves (or any other valves or fittings) directly in front of the suction port on the pump as this can interrupt the flow that the pump receives and makes the pump work harder than it is supposed to. fire hose nozzle