If you're using an progressing cavity pump and drivehead in electric applications, then braking is something that you should be aware of, as it can have a major impact on the efficiency and safety of your equipment. It's important to make sure that everything is working properly to get the most out of your apparatus—and in this blog post, we will explain what drivehead braking is and how it works so that you can guarantee the smooth running of your oil or gas production operations.
When a Progressive Cavity Pump (PCP) is no longer being driven forward, the torque that was previously applied to the system is released in the reverse direction. The energy that was initially input into the system is then output and without any braking restriction, would result in an extremely high rod speed. Additionally, the produced fluid begins to drain back down the hole, causing the PCP to function as a motor and creating further backspin as the produced fluid drains. This excessive speed has the potential to surpass the maximum speed of the belt sheaves (if used), damage gears, fold over of Polished Rods, or the unthreading of rods.
Drivehead braking plays a vital role in the drivehead system, ensuring safe control of the recoil speed of the PC system. This preventive measure is crucial to avoid potential catastrophic equipment failure, which could pose a serious safety risk. The drivehead brake needs to allow for a complete fluid dump as well as restrict the initial recoil of the rod string. Additionally, it cannot produce any sparking or excessive heat due to the presence of flammable gasses. This creates numerous hurdles to overcome for the drivehead designer.
There are a few styles of brake system available, and some that are no longer used due to potential hazards. Disc brakes were once common in the industry, however have since become relatively non-existent. External disc brakes would create a very high localized heat on the disc that potentially could start fires. Internal disc brakes eliminated the localized heat concern, however both relied on a hydraulic system to activate the brake. Hydraulic systems are prone to leaks and once the hydraulic fluid leaks past the suction point, the brake will no longer function. Another style of brake that has disappeared is the vane brake. The vane brake used the same principal as a vane pump by restricting the flow of a non-compressible fluid. Leaks would again result in the brake not functioning. In addition to that, heat would lower the viscosity of the fluid in the system which resulted in a lower brake torque being generated and vane wear resulted in a relatively short service life. The use of this style of brake has been being phased out since the late 1990s and they are very rare to still find in operation.
A hydraulic braking system that is still available is the restricted orifice system. Typically used in hydraulic driven systems, some manufacturers utilize this concept in electric systems as well. They force a non-compressible fluid either through a fixed orifice or a control valve to generate hydraulic pressure and torque through a hydraulic pump. Again the concern with these braking systems is the loss of fluid results in no braking as well as the viscosity of the oil as it heats results in less braking ability. As with all hydraulic brakes, the functionality of the pump, mating components and orifice washing all also are integral to a functioning brake. On the other side of the spectrum, contamination can "plug" the orifice resulting in torque being stored in the rod-string. Great caution should be used whenever working on a drivehead with a hydraulic braking system to ensure that there is not torque stored in the drivehead.
Another fluid based system available is the hydrodynamic brake. This system uses opposing fins to force a braking fluid to flow through a restriction. Although this system sounds similar to the orifice system above, it does have the advantage of not requiring a hydraulic pump or the mating (driving) system that goes with it. The fins oppose each other in such a manner that there are no mating components, simply tight tolerances. That's where the differences end however, the leaking of fluid and viscosity changes occurring from the heat generation result in the same issues as a hydraulic braking system. Fluid in the hydrodynamic braking system is required to maintain a consistent fluid level for operation. It doesn't require a complete fluid drain to make the system inoperable, a level lower than the top set of fins will result in no brake operation.
The other prominent braking system available is the centrifugal brake. The centrifugal brake uses shoes that throw out against a housing to generate drag similar to a semi trailer. The difference being, that instead of brake fluid activating the shoes, the centrifugal force generated by backspin is used to "throw" the heavy shoes out. The benefit to this system is that it is completely mechanical and does not rely on a hydraulic system or fluid to generate braking. If fluid completely drains, more localized heat will occur, however the brake will continue to function. Current systems on the market also utilize a spring system to retract the shoes once they are under a predetermined speed which ensures a complete fluid dump and no torque stored in the system. There is a requirement to mate to the driveshaft in a manner that prevents the shoes from engaging in the forward direction and different manufacturers use different methods.
In conclusion, the selection of brake systems for drivehead applications necessitates a careful evaluation of several factors such as safety, reliability, and the potential for fluid leakage. While each system has its unique features and operational principles, safety needs to be the primary driver in your drivehead selection. The potential for bodily harm is very real with a system that is not operating correctly or is used in a manner that exceeds its limitations. The ongoing advancements in drivehead technology underscore the importance of continual learning and adaptation in order to enhance operation safety and efficiency.
For more information regarding braking systems and your application, be sure to reach out to your Brightling representative and discover how we can help you pick a system to fit your needs.