Reciprocating compressors serve many different petroleum, chemical, and gas industries around the world. These large industrial compressors are commonly used in the upstream production of underground oil and gas fields (LNG, Shale, Oil), midstream transmission and storage of these oils and gases on floating production facilities (FPSO or FLNG), and downstream processing of the petrochemicals at refineries. This downstream refining and processing coverts oil and gases into the products that are used in our every-day lives such as fuel, lubricants, fertilizers, rubbers, polymers and many others.

To keep these reciprocating compressors running at their best and prevent unexpected breakdowns many companies choose to implement a maintenance plan. There are many different approaches to compressor maintenance from the hands-off approach of run-to-failure, to performing the standard maintenance recommended by the compressor OEM, to the most advanced predictive maintenance programs driven by continuously monitoring of the compressor components to predict failures before they occur. In my opinion, a well thought out maintenance program can provide many obvious benefits such as improved reliability, efficiency, and safety but it can also add value to the product being produced by reducing the overall operating cost of the compressor.  

No matter what your companies’ approach is to maintenance, at some point over the life of the compressor you will be required to perform service the machine. When the time comes, having the proper tools in place to ensure your maintenance team is ready to tackle the job effectively and safely can be the difference between a successful maintenance outage and an outage that does not meet the planned schedule or budget.

The area of focus for this article is going to be around the installation and removal of the crosshead to piston rod connection. One of the most common crosshead to piston rod configurations is the direct connection of the piston rod to the crosshead, see figure 1 above. In this type of crosshead arrangement, the piston rod is threaded into the crosshead and secured using a single jam nut.  The resulting bolted joint geometry has a very short effective bolt length with little bolt stretch and is not well suited for the dynamic loads generated in the piston rod during compressor operation. The short joint geometry results in a connection that is sensitive to having the proper preload to function reliably over the service life of the compressor. Additionally, if this joint fails during compressor operation, the damage to the compressor can be extensive and resulting in very costly repairs, extended machine downtime, and lost production.

An alternate method used in modern reciprocating compressors is the indirect connection of the piston rod to crosshead. In this type of crosshead arrangement, the piston rod is not threaded directly into the crosshead but is instead secured using a flanged connection and a series of bolts. The bolts used in this type of configuration are longer and better suited to handle alternating loads during compressor operation. Multiple bolts with an improved stiffness ratio share the dynamic load making the connection less sensitive to preload errors. The indirect connection method is most commonly used when piston rod diameter gets larger. Some compressor owners choose to upgrade their direct connection to an indirect connection crosshead configuration when replacement of the piston rod and crosshead is required. This type of change is expensive and cost-prohibitive when your crosshead and piston rod are in good condition. Hydraulic bolt tensioning is commonly used on both direct and indirect crosshead connections, but the focus of remainder of this article will be on the more challenging crossheads with single jam nuts. See Figure 2 below for 3D rendering of an indirect crosshead connection.

Hydraulically tensioned nuts have been around for many years. The technology has proven itself in many demanding bolted joint applications across the oil & gas industries. When looking to apply hydraulic nuts to the crosshead connection it became clear that a standard hydraulic nut would not meet the demanding requirements of this application. A few of the things we considered when designing hydraulic crosshead nuts: 

Dynamic Loading:

This connection experiences dynamic loading, if the preload in the crosshead connection is not sufficient the piston rod will be subjected to fatigue loading which can result in premature piston rod failures. Preload must be applied accurately and repeatable during installation and maintained throughout its service life.  

Very Limited Access through Compressor Housing:

Users have to work through a limited access window in the compressor distance piece which makes it very difficult to apply the correct load using traditional torque tooling.   

Large Piston Rod Thread Diameters:

Rod diameters in these compressors commonly range from 1.50” up to 6.00” in diameter with minimum preload requirements equal to 1.50 times the maximum allowable continuous rod load.   For a 4.00” diameter piston rod it’s not uncommon for preload force requirements that exceed 300,000 pounds in the crosshead to piston rod connection. Factor in limited access and the challenge of applying proper preload increase significantly. Due to these challenges, API 618 recommends that the torqued (slugging methods) are not used on larger diameter piston rods.

Piston Rod Runout:

Alignment between the crosshead and piston rod is critical to proper compressor operation. The installation procedure must evenly load the crosshead to piston rod connection to ensure good runout checks. The method used to tighten this connection cannot apply any additional bending loads into the piston rod, which could result in poor rod runout checks and higher combined rod stress which can lead to reduce fatigue life.

Tight Envelope Requirements:

Hydraulic crosshead nut should fit within the same envelope (Outside Diameter & Height) of the existing hex or cylindrical nut. This ensures adequate clearances during machine operation and has little effect on the overall balance of the system.

Improve Safety:

Eliminate dangerous torque method (slugging with hammer or “crane” tightening) used to install and remove traditional jam nuts and provide a higher integrity connection at the crosshead resulting in improved compressor reliability and safety when the compressor is running.  

Hydraulic tensioning has a proven history of being one of the most accurate and repeatable bolt tightening methods. During installation, very high loads can easily be generated using a hydraulic hand pump. The load applied stretches the piston rod and creates a small gap between the hydraulic nut retainer and cylinder. Once the desired load is reached, the retainer is tightened down to mechanically hold the load after the hydraulic pressure is released. Since the load in the piston rod is generated by the pressurized hydraulic oil from the pump, the retaining collar can easily be tightened down using a small spanner wrench without fighting the effects of high frictional resistance. Figure 3 above, shows typical hydraulic crosshead nut components.   

Accuracy and Repeatability of Preload

Knowing the hydraulic area in the nut and the hydraulic pressure applied, the force generated during installation can easily be calculated (Force = Pressure x Area). Similar to other hydraulic tensioners, there will be a load loss (relaxation) as the load shifts from the pressurized fluid to the retaining collar. This loss is predictable based on bolted joint geometry and stiffness. Load verification testing on various piston rod diameters has been performed in our engineering lab to verify the expected amount of relaxation based on typical crosshead geometry. Installation and removal forces can now be controlled by monitoring the hydraulic pressure on the gauge of the pump kit. This helps simplify the installation process and reduce the amount of operator experience (“feel”) required to accurately achieve the desired preload using torque techniques.

Even Loading on the Crosshead and Piston Rod

The annular hydraulic area in the nut naturally produces an evenly distributed load around the crosshead while pressurized during installation. When the pressure is released, the load held by the hydraulic nut retainer does not impart any additional bending loads that can result from the improper assembly of multi-jackbolt nuts. Additionally, the pure axial load generated in the piston rod eliminates piston rod windup (twist) that can occur during torquing and helps to maintain proper rod alignment for improved run-out checks. A spherical washer can be integrated into the hydraulic nut design to further help compensate for perpendicularity misalignment between the piston rod and crosshead nose. 

Ease of Use

Limited access through the window in the distance piece (doghouse) is no longer a concern. A hydraulic pump is positioned outside the compressor and a flexible hose is feed through the access window and connected to the nut. The load is quickly generated by the hydraulic pump and the retaining collar can be tightened down using a small spanner wrench, independent of high friction resistance. Using the power of hydraulics, you can now tighten a 6.0” piston rod just as easily as a 2.0” piston rod.

Thread Optimization

The threaded connections on the hydraulic nut are optimized to promote even loading throughout the entire thread engagement length. This helps reduce the peak stress risers in the thread roots and enhances the fatigue resistance of the nut and piston rod. Additional surface treatment is performed on the hydraulic nut after machining is completed to enhance the fatigue properties of the threads.

Enhanced Seal Design

Compressors can run many years between service intervals. The durability of the seals used is very important to ensure that the disassembly process is as easy as the assembly process.   Elastomeric seals have a tendency to degrade over time resulting in inability to hold pressure on removal. Riverhawk uses special metal to metal seals with wear & galling protection to enhance seal life and durability.

Speed of Installation and Removal

The assembly and alignment of the piston rod into the crosshead will follow the same procedures as your traditional torqued nuts up to the point when you begin to apply preload. Using the power of the hydraulic pump, the load can be generated very quickly with the whole tensioning process being completed in a couple minutes.

In the end, the goal is to achieve accurate preload levels in the crosshead connection and maintain those loads over between machine service intervals. Hydraulic crosshead nuts will significantly improve the likelihood of achieving a repeatable target preload, in addition to improving safety and decreasing installation and removal times when compared to torqued hardware.

Riverhawk has been designing hydraulic nuts specifically for the compressor crosshead to piston rod connection since 2009 and has provide many crosshead solutions to compressor OEMs, compressor service companies, and direct to end users. We have an engineering staff that is highly experienced in hydraulic tensioning technologies and can assist with any bolting applications needs.