The Riverhawk Hydraulic Crosshead Nut is a hydraulic nut designed for long-service life.  A hydraulic nut provides a quick, efficient method of producing and retaining high clamping forces (preload) in the crosshead joint.  This is accomplished by connecting a hose from a high-pressure pump kit to the port in the hydraulic nut piston.  The pressurized oil fills the annular area inside the hydraulic nut, generating a large, purely axial load in the piston rod.  This load stretches the piston rod which creates a gap between the cylinder and retainer.  A wrench is provided to tighten down the retaining collar against the cylinder.  Once the hydraulic pressure or pressurized oil is released, the load is held back by the hydraulic nut.

A common question we often receive is, where is a Crosshead Nut used?  The Riverhawk Hydraulic Crosshead Nut is used on reciprocating compressors for petroleum chemical and gas industries.  The crosshead nut gets its name from the location it is used within the reciprocating compressor.  This is located on the crosshead slide.  The hydraulic crosshead nut connects the crosshead slide, which connects to the piston rod.  This is a very challenging joint to tighten due to space limitations.

• Significantly reduce installation and removal times.
• Increased preload accuracy and repeatability- allows OEM’s to potentially uprate their machine providing better efficiency.
• Reduced likelihood of piston rod failures.
• Uniform, pure tension loading eliminates bending and twisting moments that can be induced into the piston rod by traditional torque methods.
• No additional bending loads in piston rod due to off axis nut loading.
• Reduces the likelihood of fretting between the bottom of the nut and crosshead face.
• Eliminates high pressure sliding surface on threads of piston rods, extending rod life.
• Improved operator safety- eliminates slugging wrench so no more hammers or cranes.
• Hydraulic nut is optimized for alternating loads with high safety factor against fatigue failures.
• Counter-weighted crosshead nuts are custom designed to match the weight of existing nuts and/or counterweights (used for balance).

What is a Coupling Hub?

It is necessary over the course of large-scale turbomachinery’s life to perform maintenance to ensure optimal performance. One such service may be the repair or replacement of a coupling hub that is joined between two pieces of machinery.

The coupling hub serves as a way for a motor to provide torque to a compressor or similar equipment. This makes it essential to the overall performance of the machinery.

Determining the appropriate method to safely disassemble the coupling hub will reduce the risk of damaging the hub and its mating equipment, as well as reducing the amount of time spent servicing the machinery.

Differences in Hub Design

Coupling hubs vary in design depending on the manufacturer, offering a number of ways in which it can be installed or removed.

There are two main designs that an end user can encounter: a straight bore hub and a tapered bore hub.

A tapered bore hub consists of a small internal diameter on one end that gradually increases across the length of the coupling hub. A tapered hub is installed to a mating tapered shaft end such that a pre-determined interference fit between the two surfaces is attained. This style of hub is easily installed and removed through the use of a hydraulic tool such as a Riverhawk hydraulic pusher.

A straight bore hub possesses a consistent internal diameter across the hub length and is typically shrink fit onto a shaft end to generate contact pressure. Unlike the tapered bore coupling hub, a straight bore coupling hub can be removed by a number of different methods depending on the discretion of the end user and the equipment that is available.

Methods of Removal

1. Hydraulic Removal

The most common method of removing a straight bore coupling hub is through the use of a hydraulic removal tool such as the Riverhawk hydraulic puller.

The hydraulic removal tool typically consists of two main components: a cylinder and a piston.

The removal tool is attached to the hub face via tapped holes and utilizes hydraulic fluid to move the piston axially, which in turn will shift the hub axially in the opposite direction (off of the shaft end).

The primary benefits to this method of removal are speed and accuracy; installing and operating the hydraulic tool is simple and the user can accurately measure the amount of fluid injected into the tool and track the movement of the hub as it advances off of the shaft end.

This leads to less time spent on the job, which in turn translates to added cost savings for the end user.

There is a higher purchase cost associated with a hydraulic removal tool as a hydraulic pump and hose kit is required for operation, however this is offset by the decrease in time spent on the job and the repeatability of using said equipment.

2. Mechanical Removal

Another conventional method of hub removal is through the use of a mechanical removal tool.

The mechanical removal tool typically consists of a claw or similar device, and a threaded piston.

The tool itself behaves similar to the hydraulic tool, in that it is secured to the hub face or flange and utilizes a piston to engage with the shaft end and shift the hub axially.

The primary difference is that the tool requires the end user to turn the piston via a wrench or similar mechanical input.

While the purchase cost of the mechanical tool is lower than that of a hydraulic removal tool, the added time and labor costs may prove this to be the less cost-efficient method.

Additionally, the mechanical device is limited by the amount of torque that the user can input when engaging the piston; the tool may not be able to generate sufficient force to fully remove the coupling hub.

    3. Destructive Removal

The third method of hub removal is through the use of a plasma torch or similar device to remove material from the hub barrel with the intent to split the hub and relieve the contact pressure.

Caution should be taken when using this method as it is easy to remove too much material from the hub and begin to damage the shaft end.

Furthermore, the hub will release a substantial amount of force once separated, potentially causing injury to those in proximity.

As this method of removal will render the hub unusable, it should only be used when other removal options are not valid or if the hub is in severe disrepair and requires replacement.

Other Factors to Consider

• Proximity to Equipment – There may be limited space to install and operate a removal tool depending on the location of the coupling hub relative to its surrounding equipment.
• Environment – The coupling hub may be present in a hazardous environment that limits which method of removal to use.
• Dilation Assist – The end user may have access to different methods to dilate the coupling hub and reduce the contact pressure, be it hydraulic fluid or heat.

By reviewing the information above, you can significantly reduce the time spent identifying and selecting the appropriate method of removal for your coupling hub.

You may find other concerns that were not listed in this article. If that is the case, Riverhawk staffs a number of qualified engineers whom can address these concerns and help guide you in selecting which method is right for the job.

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.

THREE FACTORS TO CONSIDER WHEN SELECTING A HYDRAULIC PUMP KIT

Why Use Hydraulic Pump Kits?

To say that there are many moving parts during installation or service of large-scale turbomachinery would be an understatement. Each component plays a critical role, but one piece of equipment that is often overlooked is the hydraulic pump kit. Many variations of pump kits exist, but for the purpose of this article we will focus on manual and air driven pump kits.

Hydraulic pump kits help provide the flow of fluid that is needed to generate power for tooling or equipment processes that require it. In our industry, pump kits are often used in conjunction with the installation or removal of keyless tapered hubs, as well as tensioning studs and bolts on casings, flanges, bearing housings, or connection rods.

Both manual hydraulic pumps and air driven hydraulic pumps offer a number of advantages over mechanical solutions. Some of these include versatility and reliability with a broad range of applications. In addition, the element of repeatability is offered for pressure output.

That’s a lot of information to process for just one component of the installation process. But to take it another (important) step further, selecting the appropriate pump kit is just as crucial as having one in the first place.

When selecting a pump kit for your next application, the three questions outlined below can serve as a guide for your selection. Choosing the right pump for the right job can actually cut down time spent on a job, which can ultimately translate to a savings on cost.

Types Of Hydraulic Pump Kits

Air Driven Hydraulic Pump Kits

An efficient, compact and portable high-pressure system that provides instant and reliable hydraulic power. Used for daisy chaining multiple tensioners or applications that require a high volume of hydraulic fluid.

Manual Hydraulic Pump Kits

Typically used in situations where the required volume of hydraulic fluid is minimal. Available configurations include a single pump design for small tensioning purposes or a dual pump design for hub installation applications.

What is the required pressure output needed for your application?

First and foremost, you should consider the desired pressure rating that you need for your application. You will want to review the job with your engineering team and field service personnel to help determine the appropriate pressure output that is needed for your hydraulic equipment.

 Most hydraulic tooling and equipment will call out a maximum operating pressure, which will give you a good idea of what kind of pump kit you need.

 Manual pumps typically come in two pressure ranges: a low-pressure range and a high-pressure range. Low pressure pumps are rated for 10,000 psi or lower and are used to supply hydraulic pressure to a variety of hydraulic tooling. High pressure hydraulic pumps can reach operational pressures of up to 40,000 psi and are commonly used for dilation during the installation or removal of a keyless tapered coupling hub.

 Air driven pumps can be preset to a desired maximum pressure rating at the factory, and traditionally have a maximum operational pressure of 22,000 – 40,000 psi. Air operated hydraulic pumps are commonly used for tensioning bolts and nuts during the installation of compressors and other machinery.

Is a constant supply of hydraulic pressure needed for your application?

There will be times where you will find yourself in need of a varying supply of hydraulic pressure to your equipment or tooling in order to finish a job. Determining the type of supply that is needed will help you select the appropriate pump for the job.

 One of the main differences between a manual pump and an air driven pump is the quantity of hydraulic pressure that is supplied to your tooling or equipment.

 A manual pump will supply pressure with each stroke from the operator.

 This allows the operator to adjust the pressure going into their tooling or equipment without needing to be concerned about over-pressurizing or over-stroking. For example, the installation of a keyless tapered coupling requires the operator track the amount of travel the hub makes up the tapered shaft end.

 A Riverhawk hydraulic pusher mated with a low-pressure pump allows the operator to accurately install the coupling to the desired position along the tapered shaft while maintaining control of the hydraulic pressure feeding to the pusher tool.

 An air powered hydraulic pump constantly supplies a set amount of hydraulic pressure so long as the pump is actuated. This makes the pump ideal for operating multiple pieces of equipment at once without requiring constant mechanical input from the operator. An example of this would be the installation of a casing on a compressor.

 The casing requires multiple studs and nuts be properly and evenly tightened, which can be difficult to achieve through mechanical torquing methods. By utilizing an air driven pump and multiple Riverhawk hydraulic nuts daisy-chained in succession, hydraulic pressure is applied evenly to each of the nuts. This allows for even load distribution to each of the studs, thus eliminating the concerns of excessive or erratic bolt loads.

Where will the hydraulic pump be used?

One factor that you may not have thought of, but is equally as important, is where the pump is going to be used.

 Not all jobs that require the use of a hydraulic pump will be performed in a repair shop or factory floor. You may be required to service equipment outdoors or in an environment with limited space. Having a pump kit that is compact and easily transportable will cut down on the time required to complete a job off-site.

 Manual pumps are lightweight and often come configured in a protective housing that is ideal for mobility. The hoses and other hardware may require a separate container or housing when transported.

 Air driven pumps come in a variety of styles and configurations that may or may not be suitable for transportation. Typically, higher rated air driven pumps are built into carts with solid casters, making them ideal for a shop setting, whereas lower rated air driven pumps come in aluminum boxes or similar housings that are more compact and easier to transport.

Other Considerations

In addition to the questions above, here are a few other points to consider:

 Volume

– Does the pump that you select have a large enough reservoir to supply hydraulic fluid to your equipment and maintain the desired pressure?

 Accessories

– Does the pump that you need come with all of the necessary hoses and fittings to get the job done?

 Safety

– Do you require safety devices in or on your pump (i.e., burst disc, remote operating switch, etc.) in order to operate it?
By using these questions as a guide, you can significantly reduce the time spent identifying and selecting the appropriate hydraulic pump for your application. In addition, the right hydraulic pump will serve you better regardless if you are in the field or in the shop.

 If you still need help making a selection or have a concern not outlined, our engineering staff are always available to help address or answer any questions you may have.