Well Pump Problem
Well Pump Problem
I am an engineer for a small island in the southern West Indies (about 140 miles north of Venezuela). We have been having a continual problem pumping brackish water to our watermaker.
The splines on the spline shaft shear off, allowing the motor to run at no-load speed until either the thermal protection kicks in or the motor burns out. This problem has happened three times, and the pump/motor combination has lasted between 3 months and 3 weeks. Our pump supplier is telling us it is an installation problem, however he also does not recognize the splines are shearing off…so I don’t trust him (meaning he only agknowledges the motor is burned out, even though the splines are obviously sheared).
The motor is a Grundfos (this has also happened with a Franklin motor) 2.00 hp, 60 Hz, MS 4000. The pump is rated for 25 GPM and 218 feet of head (according to the plate on the side of the pump. It is a 4 inch submersible model. The pump is made of 304 SS, as the pump supplier suggested it would work for our brackish water. However I feel the next pump should be made of 904L SS, to better handle the salt content.
The well is a 6 inch diameter 120 foot deep well with a 10 inch bullnose at the bottom. The pump must pull the water up the 120 vertical feet, then along a 1800 foot horizontal run through 2.5 inch PVC to an open atmosphere tank. As I stated before the water is brackish, I don’t have a salinity meter (yet) but I am currently assuming the salt content is about 75% that of seawater (could be more, could be less). The average temperature of the water is 89 deg. F.
I have spoken with the tech support at Grundfos, and they listed three potential problems for the repeating failure, frequent start-stop cycles (which we don’t have, as the pump constantly runs), pump overloading (possible, but the pump should be big enough), and misalignment between the pump and the motor (manufacturing defect occurring 3 times in a pump with an excellent reputation?). So my question after this lengthy problem statement:
1.) What is the cause of this problem? Is it simply the salinity content of the water, and we need to use the 904L ss? Could I drop a zinc down with the pump (we have a lot of boats here, so we have a lot of extra zincs)? Is the pump undersized for the application? I have been through the length of the pipe and there is no blockage.
2.) Are there any quick solutions? This is becoming more important, as our water supply is dropping and, well, that’s a bad thing. More importantly, long term solutions – is this a common problem with Grundfos MS4000? Should we look into other manufactures, or suppliers?
Thank you all for your help. I have I have given enough information.