Heat Accumulation in Centrifugal Pumps
For most centrifugal pumps, there is a minimum flow rate at a given set of conditions, where the temperature of the pump case begins to increase continuously to an unacceptable level. Unacceptable temperature levels may be defined by various factors such as heat damage to the pump, pump driver, pumpage, piping, or the controls and sensors.
Below this threshold flow rate, heat added to the pumpage by pump inefficiencies, is not sufficiently removed by flow through the pump, at which point the pump case temperature begins to rise sharply, non-linearly, and continuously, to an unacceptably high temperature. Some conditions affecting this threshold flow rate are: ambient air and/or fluid temperatures, pump construction materials, type of cooling fluid (ambient air, pumpage, other).
Heat added to the pumpage by inefficiencies in the pumping process, is removed by three principle means:
Convection through the pump case.
Convection through the driver shaft.
Heat carried away in the pumpage.
The first two means of heat removal, convection through the pump case and driving shaft, vary depending on ambient temperature, type of cooling medium (air, water, etc.), and construction materials (iron, plastic, stainless steel, etc.). The third means of heat removal from a centrifugal pump (pumpage moving through the pump), depends on: flow rate, fluid type (specific heat), and temperature of the pumpage.
Based on the fact that the greater the temperature differential between any two materials, the faster the two materials will transfer heat between them, the following is therefore true:
The colder the pumpage, the lower the flow rate at which heat begins accumulating.
The colder the ambient temperature (air for air cooled motors, fluid for submersible fluid cooled motors), the lower the flow rate at which heat begins accumulating.
Robert Johnson in Palm Beach, Florida, was the first person we know of in the irrigation industry to implement this concept into his Hot-Stop product to protect pumps. Bob stated at the time that he borrowed the idea from the pressure booster market where heat accumulation within a pump was used to control the pump.
Irrigation Craft adapted the concept to our system by using a volute heat sensor to provide Redundant (back up) protection for all other flow controls on pump stations. This adaptation was made by Irrigation Craft based on the observation that users could purchase a $30,000 dollar pump station, and then if one sensor failed or one sensor was not calibrated properly, a pump motor could be burned out or some other severe type of damage could occur. Irrigation Craft viewed this result as incorrect, and also we believe it to be irresponsible to manufacture a pump station that could be damaged so easily, seeing that the customer is purchasing a pump station for just that reason, to receive a more robust and durable result than if just a pump were installed with little or no controls.
Where appropriate, every Irrigation Craft pump station incorporates this volute heat sensor on each pump as a redundant device to prevent damage when other sensors fail or are adjusted improperly.
