There is a misconception that pumps will produce the pressure they are described as-
i.e a "60psi" will produce 60psi when you plug it in, a "100psi" will produce 100psi etc. This is not so.
Pumps will only generate the pressure required to pump the water through the system, up to a point where the pressure switch cuts in and protects the pump by switching it off. Generally the "psi" stated is the pressure switch setting.
All systems have individual characteristics, and such things as hoze bore size, length of hose, number & type of connections, valves, height of brush etc etc all contribute to determine the system pressure. What is ideally needed is a pump that will generate the pressure required, that such pressure is well below the pressure switch setting, and that the amount of water (flow rate) AT THIS PRESSURE is sufficient for the user.
As systems have evolved, we at Williamson Pumps have recently seen the pumps generally used go from 60psi to 75psi, and now 100psi are the norm.
All pumps have a performance curve, and as the pressure it delivers increases, the actual flow of water decreases. The balancing act is to get both the pressure and the flow rate to suit the system.
Another problem to throw into the equation is where one pump feeds 2 poles. A standard 100psi pump may easily cope with the pressure required, but will it deliver the amount of water (flow rate) required to feed both poles?
Also, pump materials and motor duty cycles need to be considered. Many diaphragm pumps available are designed for Agricultural Spraying applications, and as such the materials of construction are not always condusive for pumping pure water for extended periods of time.
Sorry to get a bit techie, but hope this helps people understand pumps a bit more.
Martin