When there is a catastrophic pumping unit failure it is natural to wonder what happened and how this failure could have been prevented. However, the right answer may or may not be simple to find. The following list shows the most common reasons for a failure and how to prevent or minimize its occurrence in the future:
1) Causes of pumping unit failures:
Overloaded pumping unit structure or gearbox, damage due to improper maintenance (gearbox and bearing lubrication), etc. A chronically overloaded pumping unit structure can be due to selection of a pumping unit whose structure rating is below the required rating based on peak polished rod load. This can be checked using a rod pumping system design software program such as RODSTAR or SROD. When simulating the system always use fluid level at the pump (zero fluid level over the pump) to predict the highest possible peak polished rod load which is used in the calculation of structural loading. Peak structural loading is only a function of peak polished rod loading. Pumping unit balancing has no effect on structural loading.
A short term increase if structural loading can also cause a failure especially in cases where the pump may be sticking due to sand or scale, a seized pump, etc.
2) If there is a gearbox failure:
the reason may be more difficult to find as compared to structural loading. This is because even if a predictive program (such as RODSTAR) shows the gearbox not overloaded when the well is pumped off, this is typically for a balanced pumping unit (a common assumption when designing new rod pumping systems). The pumping unit may have been designed with the correct size gearbox, but if the existing maximum counterbalance moment is not ideal to keep the unit balanced (which minimizes gearbox loading) then the gearbox can still be overloaded. To find out if this is the cause of the gearbox overload you need to determine the existing max. counterbalance moment either by running a software program such as XBAL, using counterbalance charts from the pumping unit manufacturer, or by measuring the existing Counterbalance Effect (CBE). It is also recommended to use a measured dynamometer card and a diagnostic wave equation program such as XDIAG. This program will allow you to enter both the measured dyno and the measured max. counterbalance moment (from XDIAG) or the measured CBE. If XDIAG shows the gearbox to be overloaded but it’ll be okay if it’s balanced, then you can use the calculated max. balanced counterbalance moment in XBAL to find out where you need to move the counterweights to balance the pumping unit and minimize gearbox loading. However, if gearbox loading is over 100% and the unit is in good balance then the problem is that the gearbox is too small. In this case, if the well is pumped off, you may be able to reduce system capacity to better match available production by using a smaller stroke length which reduces gearbox loading. You can use RODSTAR to figure this out. Enter a target production that is a bit higher than the maximum expected production per day and have the program calculate the pumping speed you need. Then, at the pumping unit input window try using the next smaller stroke length. Keep the same rod string as the one in the well (do not ask RODSTAR to design the rod string) and then run the program to see what the gearbox loading will be when the unit is balanced. Also, check if the rods will be okay. Then, export the balanced max. counterbalance moment you need to XBAL and see when the counterweights need to be moved to balance the pumping unit after you change the pumping unit stroke length. You may also need to change the pumping speed to maintain the target production (RODSTAR will calculate the SPM you need).
Reference: By John Svinos, President E8 LLC (e8llc.com or rodpumping.com).