(Article published in Oilfield Technology, July/Aug 2020)
Arun Chandrasekaran, Adam Avey and Corey Philipp, GD Energy Products, highlight the hidden cost of neglecting preventative maintenance of a pump’s power end.
Anyone with a vested interest in pressure pumping spends considerable time ensuring their pumps run efficiently with no unexpected downtime. A pump that is sitting in the repair shop is a harbinger of non-productive time, spiralling costs and operational headaches all-round. Since the repairs that are typically necessary on the power end component of the pump cannot usually be completed overnight, once a failure of a power end has occurred it can take several weeks to get a pump up and running again.
Anatomy of a pump
A frac pump is composed of a power end and a fluid end. The fluid end is the high pressure component of the pump that takes fluid into a chamber at low pressure and discharges it at higher pressure. As such, the fluid end is usually the component that receives frequent services in the field in-between frac stages. During preventive maintenance, the fluid end is periodically opened up to change wear components in order to keep it running efficiently. Conversely, the power end is the component that drives all the moving parts required to create the pressure in the fluid end. The moving components of a power end are contained within an enclosed steel frame and include a crankshaft, crossheads and connecting rods, similar to the rotating and reciprocating components found inside a car engine, but on a much larger scale. Unlike the fluid end, the power end rarely gets opened and serviced in the field. To run the pump, a driveshaft is hooked up to the power end from the transmission, which is turned by a diesel engine. The diesel engine and the transmission, at a certain gear, turn the power end. The power end turns the crankshaft and the connecting rod assembly, which moves the plungers to create the pressure in the fluid end. Ultimately, the goal for a pressure pumping company is to keep turning the power end and to keep the pump running.
The cost of high production
Unfortunately, since the power end is not inspected periodically, the rate of unexpected failures for this component is rising. If a component is only looked at once a strange noise or even smoke is being emitted from it, the chances are that it is already too late to intervene to prevent significant damage to the power end. At this point, the only option would be to take it offline and send it to the repair shop. Typically, when a pressure pumping company goes to a site to perform a fracturing job, they will take some additional pumps with them. These extra pumps could be sitting idle, to serve as backup in case any of the major components, including the power end, breaks down unexpectedly. If customers are able to keep a good maintenance record, or perform preventative maintenance on the power end, they will only need to carry the pumps they require, resulting in lower operating costs and increased efficiency.
The proactive approach
GD Energy Products helps pressure pumping customers actively maintain their equipment. By developing preventative maintenance plans for power ends, customers can better understand what to inspect for in the power end after set time intervals and take a proactive approach in replacing wear components as needed to prevent any unexpected downtime. This will also help to prevent catastrophic failures, which often results in very expensive repairs.
Lessons in preventative maintenance
Pressure pumping companies should consider the following power end characteristics when developing a preventive maintenance plan. The company has helped customers in developing a maintenance plan specific to their operations and duty cycles.
The power end is only rated to perform work within its design specifications. Each power end must be operated within its capabilities when used in the field in order to maximise the total cost of ownership. Each power end model is limited, by design, to operate within the rated maximum rod load (RL). The RL is a function of the discharge pressure and the plunger size.
While designing a power end, each critical load-bearing component is designed to withstand the maximum rated RL. The geometry and the material for the critical components are designed in such a way that the expected lifecycle is reached when operated within the designed RL before fatigue failures occur. The fatigue life is measured in cycles where one cycle equals one complete revolution of the crankshaft. As the power end operates, each cycle adds fatigue to the power end components until the material reaches its ultimate fatigue limit, at which point the component will start to develop a failure mode. Operating the power end above the maximum rated RL will lead to a shorter life span of the power end and also could lead to catastrophic failures.
As explained, the harder and longer a pump is run for, the sooner the bearings will wear out and need to be changed as part of a preventative maintenance cycle. ‘Duty cycle’ is a term used to characterise how hard a pump has been run and for how long. Duty cycle is determined by RL and ‘speed’ – that is how fast the pump is being run, and ‘time,’ meaning the number of pump cycles or run time. Preventative maintenance plans can be developed based on duty cycle. In the US, each different operating area/shale basin has different pressures that pumps are required to pump at. This can drastically change the required preventative maintenance intervals. Customers can provide a record of duty cycles and operating conditions in their area of interest and a preventative maintenance plan can be designed around those.
The company has performed studies to correlate improper mounting to the performance of the load-bearing components. Since the frame houses all the critical components and there are installed running clearances between the frame and the bearing components, stress from any unnecessary twist in the frame is transmitted to the bearing components. Results from studies showed pumps that were not mounted appropriately were subjecting the bearing components to additional stresses and even creating the potential for excess heat generation due to reducing clearances of bearing components below their minimum tolerance. It is recommended to place the pump on the skid and use shims to level the pump before it is bolted to the skid. As part of the preventive maintenance plan, all mounting bolts must be checked periodically for tightness and replaced as necessary.
Stroke length is a pump characteristic that is specific to each pump model and cannot be altered. It denotes the distance the plunger strokes for each revolution of the crankshaft. For each stroke of the plunger, a specific volume of fluid is displaced and the output volume can be increased or decreased by varying pump speed. This is a key piece of information for customers, as there are many pumps available with varying stroke lengths. When looking for a specific flow rate out of these pumps, the pumps must be operated at varying speeds to provide a uniform flow output. A longer stroke pump needs to be run slower than a short stroke pump in order to output the same flow rate.
However, it is important to remember that when pumps are run faster, they are accumulating fatigue cycles at a more rapid pace. Pump components are constructed with steel. By its very nature, at some point steel will reach its fatigue limit and start to develop a failure mode. How quickly it takes to reach the ultimate fatigue limit determines the life of the component. Ideally, pressure pumping companies operate their frac units at an optimal speed that does not result in too much load for the engine, transmission or pump. Overall, fatigue plays a major role in pump failure. Overall usable life of any component can be increased by managing accumulation of fatigue cycles more efficiently.
Because power ends generate a significant amount of heat, approximately 60 – 70% of serious power end failures occur as a result of a lack of adequate filtered lubrication. When the requirement for clean lubrication is not sufficiently met, the moving components of the power end quickly start to break down. The company does not design lubrication systems, but has partnered with an industry-recognised lubrication provider to offer recommendations on appropriate lube types based on customers’ operating conditions.
All power ends manufactured by the company are run through an extensive factory acceptance test where they are tested at their operating limits and have the temperatures of their critical components monitored prior to shipping to the customer to ensure the pump is receiving proper lubrication and ready for operation.
In April 2018, a pressure pumping company using GD Energy Products C-2500 pumps experienced several power end failures across their fleets due to overheating of internal power end components. In an effort to understand and further investigate the actual root cause of the failures, a frac unit from the same fleet was tested at GD Energy Products’ Fort Worth facility in Texas, US. Pressure gauges and flow meters were systematically installed at various locations in the lube system to better understand the system dynamics at various pump load conditions.
The unit was first operated as received, with the exception of the installation of the instrumentation. The power end oil was replaced with clean ISO 220 oil, as per the equipment manufacturer’s specification, and the oil filter was changed before operation. Significant issues were found with the amount of oil shown to be flowing through the gear pump relief line, as well as in the gear pump suction line. Additionally, the pressure relief valve at the power end was not adjusted correctly to maintain adequate (≥100 psi) pressure throughout the installed oil’s temperature/viscosity range. The decision was made to reroute the gear pump pressure relief valve relief line to the lube reservoir to improve the gear pump’s suction conditions. The unit was operated and the gear pump pressure relief valve was adjusted to increase flow to the power end. The power end pressure relief valve was also adjusted to maintain 145 psi at the power end. System performance was greatly improved with the modifications made.
It was discovered that at some point during testing, the thermostatic bypass valve was no longer functioning as designed. The valve was allowing pressure communication to the oil cooler loop prior to its designed thermal set point. A new valve was delivered and installed. Additionally, due to the component issues found system-wide, it was determined that lower viscosity oil could be a better fit for the system. The ISO 220 oil was removed and the system was filled with synthetic 75W/90 for an additional round of experimental testing, as well as for the factory acceptance test.
Specific factory acceptance test parameters were calculated for the unit in question, based on the transmission gear ratios. It was observed that the system was only able to maintain 90 psi at the power end pressure relief valve. An attempt was made to adjust the pressure relief valve, but, due to the low oil viscosity, the pressure was unable to be increased further. The equipment manufacturer was advised that slightly more viscous oil could be a better fit for the system as installed. Oil such as 80W/140 could provide enough viscosity to maintain the required pressure at the power end.
The unit from the customer was not supplying the required lubrication pressure and flow to the power end. As a result of the testing performed, GD Energy Products recommended making a comprehensive array of best practice recommendations to all affected units as soon as possible. This resulted in substantial reduction of heat-related failures and overall improved productivity across the customer’s entire fleet.
A pump could cost tens of thousands or even hundreds of thousands of dollars to repair if one of its critical bearings fails and causes a power end failure on the jobsite. These unexpected expenses can be avoided, and total cost of ownership reduced by simply ensuring the bearing is changed at the right time. Fortunately, the cost of committing to a proactive maintenance schedule is modest, and can be planned and forecast for. A long-term outlook centered on preventative maintenance is essential in safeguarding a pressure pumping company’s ability to fulfill their customers’ demands and take on new work.