Kelair Pumps supplied two Viking pumps for a process oil service. Shortly after commissioning, the pumps started to leak at the mechanical seal. The following investigations uncovered an important lesson in maximising pump life. When we investigated the installation, we noticed that sections of the suction line were steam jacketed (but not all sections). There was a flexible pipe joint just prior to the pump suction port over 0.5m long without any insulation fitted. The pump was not jacketed nor was it heat traced.The steam jacketing on the suction alerted us to a potential issue. For any heat traced, steam or hot oil jacketed line, the pump should also be jacketed or heat traced. Jacketing should also cover every part of the suction line; that includes jacketing flexible pipe joints.
The problem with inadequate pump jacketing is that once the flow stops, the duty fluid within the pump and within the unheated segment of pipe begins to cool. Over time that fluid becomes more viscous. If the pump was selected at a speed for hot, easily flowing duty fluid, that same rotating speed will not be suitable for the increased viscosity once the liquid has cooled off within the pump. The pump may encounter cavitation (NPSH) problems and seal problems upon start-up.
In this specific application, when the pump was switched on the seal faces were immediately twisted open as the “slug” of cooled viscous fluid pried them apart –somewhat like unscrewing the lid on a bottle. After the cold fluid had passed through, the spring loading on the seal faces was insufficient to cause those faces to close. The seal faces remain open and leaking until the seal was removed, cleaned, and if serviceable, re-fitted. Often, a mechanical seal with pinned seat is selected to handle viscous fluids (viscosity dependent).
During our investigation, we were advised that the process oil recipe was to be changed and that it would include abrasive materials. The installed gear gear pumps were operating at 1450rpm. We now needed to handle a viscous liquid with abrasive particles.
Whenever a gear pump is applied to liquids with abrasives abrasive solids, the speed should be significantly reduced to 350 rpm if not lower. Wear rate is not a linear relationship with speed. The wear rate relationship to shaft rpm is exponential!
The pump was changed to a Viking KK4224B steam jacketed pump selected at an ideal 190rpm. Also fitted was the high viscosity handling, abrasive liquid seal. These modifications overcame the seal problems and improved the wear rate on the pump’s internal components, thereby increasing the service life of the pump.