With over 40 years of experience in the pump industry we have seen a few instances where a pump has seemingly run dry but the customer has insisted this could never have happened.
When a mechanically sealed or magnetic drive pump runs dry (i.e. with no liquid in the pump) the damage is usually very obvious as the contact areas (seal face and seat or internal bearings) will quickly overheat causing obvious melting of the surrounding areas. But there are a few instances where the same damage may occur.
1. The pump has actually run dry, which most often occurs due to operator error.
2. Air lock within the system. If a non-return valve on the discharge side doesn’t have an air bleed it may not release the air when the system is initially primed, so the liquid can not run freely into the pump causing it to, in effect, run dry. The solution is to always incorporate an air bleed on the pump discharge line or to crack open the discharge vale to physical check liquid has flowed through the pump prior to initial start up. Please note that this is a potential problem that would only normally occur at initial installation of the pump.
3. Running beyond end of curve conditions. A pump is supplied against a specification of capacity and discharge pressure supplied by the customer but we have occasionally seen installations where the discharge pressure is actually much lower than originally specified. This means that the pump is running much further to the right hand side of it’s performance curve and, in extreme circumstances, this can lead to the pump cavitating. In effect this means the liquid entering the pump can evaporate due to the pressure change that can cause damage similar to running dry.
A simple and cost effective way to avoid these problems is to install an electrical device such as our PSP1, this works by detecting the power drop from the motor when the pump runs dry and automatically tripping the pump.
The Crest PSP1 monitor is connectedly directly to the drive motor cable and constantly monitors the true power consumption of the motor drive and displays this as a percentage (%kW). When the pump runs dry, the motor load decreases, the relay operates and the PSP1 initiates an alarm and/or stops the pump.
Three phase electric motors of any current rating can be monitored. Currents greater than 8 Amps also require an external current transformer.