Saturday, July 30, 2011

Screwtape Zero, Andy One

This summarizes the features of the fresh water system in M/V Pilgrim and measures taken to repair the problem of the fresh water pump failing to turn on until pressure is non-existent.

The freshwater pump was replaced in July 2010. The water pressure was falling to 0.5 bar before the pump would come on. It would then cut out at 1.7 bar. If you were in the shower you would have to get out of the shower to open the sink tap to trigger the pump to come on so that you could complete the shower.

At the end of the day, the problem was the selection of pressure switches. The original installation had an outboard pressure switch installed, but the new pump had an integrated switch. Loathe to foul things up I installed the new pump without modification, bypassing the outboard switch and using the integrated switch. It turns out that the pressures on the integrated switch are too low for our needs so we needed to reinstall the original outboard switch.

The second problem was determining maximum system pressures. There are at least six different choices, depending on which component you examine. Documentation is poor. We were unable to find documentation for the installed semi-rigid blue piping, but research indicated that most PVC piping can sustain pressures much higher than the accumulator tank (10 bar) or installed pressure gauge (4 bar). So in the end we adopted the maximum of the installed pressure gauge.

The third problem was then adjusting the outboard switch to fall within this range of pressures. We were not able to determine whether the switch had differential setting capabilities so had to experiment. It appears that it does not. Thus we had to settle for a 2.0 bar differential between cut-in/-out. The “green zone” maximum of 2.5 was adequate but this would cause a 0.5 bar cut-in, which is too low. So we accepted 1.2/3.2 bar.

A remaining problem may be final tweaking of the accumulator tank air pressure. It should be 0.15 bar below the pump cut-in switch. We have it somewhat less than that.

And then finally we need to investigate why the hot water side is substantially lower than the cold water. But tomorrow is another day.

It all looks terribly simple in the result, but life is easy when you know how and terribly difficult when you do not.

I do now know how, at least on this bit.

We have cut-in at 1.2 bar, cut-out at 3.2 bar, well within the pressure limits of the installed pressure gauge, accumulator tank, piping, and pump. And we no longer have to get out of the shower to open a tap to trigger the pump. Life is good.

Full report at

No comments: