Do most plumb-in machines require external pressure?
I was talking to Shawn Thompson from Fluid-O-Tech, and got more clarifications regarding drawing water from a reservoir that is under atmospheric pressure. Shawn mentioned that their pumps can in fact handle just as much lift as the Procons, and went into more details regarding which factors influence the pumps lifting abilities. Thank you Shawn for sharing your knowledge!
Had to read about what NPSH and cavitation is to get some understanding of what's going on in there.Shawn Thompson wrote:The pumps are quite comfortably capable of drawing from a reservoir. The safe lift height is dependent on several factors including: plumbing losses, fluid vapor pressure at the working temperature, atmospheric pressure, and the NPSH required of the particular pump model (see NPSH charts for your particular model). We happen to throw around the figure of 3 feet simply as a safe rule of thumb for a very wide range of pumps. I suppose it has served us well over the years and since most applications often don't call for more, it seemed to stick, and I believe is in the instruction manual.
However, greater lift is very often feasible. Ultimately the factors we generally consider are the vacuum capability of the pump, the dry run time of the seal, and the potential for cavitation. Even the smaller vane pumps pull a vacuum of at least double that, and much more once it is wetted, so vacuum capability generally isn't the limitation. That does reduce with age though as parts wear out. The seal runs truly dry usually only during initial startup of a brand new pump that has had time to dry out. After that, there is at least a thin film of residual liquid which aids in lubricating and cooling the seal faces. The less time running dry or partially wetted the better when it comes to seal life, so as I have seen people mention, a check valve might be beneficial in some cases using a reservoir and high lift or large diameter tubing. It can reduce the time it takes for water to reach the brew area, and also eliminates the partially dry running of the seal.
However, if you go that route, a check valve with a low pressure drop across it is strongly recommended to avoid the potential for cavitation. Cavitation effectively results from lowering the fluid pressure to its vapor pressure at a given temp, which results in "boiling" into vapor bubbles which then violently and destructively collapse back down into tiny liquid drops once it crosses over to the high pressure side. This can result from overly restrictive plumbing, high lifts, increasing the fluid temperature, or working at a particularly high elevation. One sure way to check for cavitation potential is to measure the pressure directly at the inlet of the pump. If using a reservoir, this generally will be under vacuum. In any case, be sure to check while the pump is running with the greatest output it would ever likely encounter. You want to be sure that the pressure at the inlet is greater than the vapor pressure of the fluid plus the NPSH required. Generally speaking, the greater the inlet pressure the better. It helps avoid cavitation and the pump doesn't work quite as hard. To be honest though, I can't recall any cases of cavitation in any espresso pumps that have ever come back here. But then again, most probably weren't lifting high from a reservoir just your typical plumbed in machine.