Description:

This is an update to the 2019 Calgary Pump Symposium Paper which addresses when and why minimum thermal flow (MTF) is an issue in centrifugal pump. The focus of this update is to provide an additional robust method for calculating a pump’s MTF. In the 2019 paper, the method for calculating MTF is derived from several equations taken directly from Hydraulic Institute (HI) 1.3-2013. This new method uses the vapor pressure curve of the fluid combined with known fluid properties such as the inlet temperature and flow rate to determine the MTF for the pump.

 

Using the principle of a control volume, we know what flow goes in and out of the pump as this is what is stated on the pump’s performance curve. However, the impeller itself can be isolated as its own control volume where the additional flow from the impeller wear ring leakages mixes with the inlet flow. As each of these flows mixes, the bulk flow temperature of the fluid increases. Using the fluid’s vapor pressure curve we use the bulk flow temperature to determine the corresponding fluid vapor pressure. Additionally, we know the fluid’s suction pressure but due to the mixing the bulk flow is higher than the inlet fluid flow, so using the principles of the Continuity equation and Bernoulli’s equation we can calculate the fluid’s pressure.

 

Initially, the pump should be evaluated at the OEM’s specified minimum continuous stable flow (MCSF). If the fluid’s vapor pressure is less than the fluid’s pressure then flashing will be suppressed, and the pump will operate without issue. However, if the pump is unable to operate without issue at MCSF then the OEM can use an iterative approach to determine an acceptable flow rate that will allow the pump to operate without issue.

 

The update will illustrate the use of all applicable equations and assumptions. It should be noted, it is the OEM’s responsibility to ensure that the pump can operate without issue over the course of its life expectancy. Therefore, it is important to evaluate wear ring leakage at two (2) times running clearances. The determination of leakage across an annular seal is derived from Yamada.