Rod guides

Sucker Rod Guides and variants sucker rod body wear protection devices have been around for more than 60 years. Originally starting as a rod scraper or paddle, early devices applied to the rod body as a flat metal section, tac-welded to the sucker rod body, in effort to scrape paraffin from the tubing and distribute rod side-loading across the tubing wall. Fast forward 70 years, and guides have evolved primarily from thin metal sheets to plastics, over-molded directly to the sucker rod body.

Wear of plastics

Polymeric materials are non-linear, meaning their performance and mechanical properties vary with relative humidity and temperature. Material wear rates are inversely related to mechanical properties. As a polymer's mechanical properties and strengths decline, material wear rates accelerate. Countless sources of plastics and polymer experts and public works are available which correlate this very thing. A few citations are shown below:

1. A. Abdelbary. Wear of Polymers and Composites, 2014.
2. J.K. Lancaster. Abrasive Wear of Polymers*, 1969.
3. S.B. Ratner et al. Connection between wear resistance of plastics and other mechanical properties. 1964.
4. M.M. Reznikovskii. Relation between the abrasion resistance and other mechanical properties of rubber, 1960.

Therefore, it is well respected and understood that as plastics for rod guides are exposed to elevated temperatures their properties and resistance to wear decreases. Further understanding of plastic material properties and behaviors are explained below.

Plastic material

Resins

Challenging circumstances which combat plastics in the down-hole environment include the following:

• Fluid Submersion
• Temperature
• Pressure
• Compressive Loading (Side-Load)
• Chemicals & Hydrocarbon Fluids
• Linear Reciprocating Wear (Beam Lift Pumping)
• Rotational Wear (Progressive-Cavity Pumping)

Plastics are non-linear materials, meaning they behave differently in different circumstances. A wear rate at room temperature, lubricated environment is different than high-temperature lubricated environment, due to the plastics material integrity. The strength of the plastic material changes with temperature, fluid absorption, and in chemical environments, due to the molecular bonds being weakened by heat or fluid.

Hygroscopy, or the plastic’s ability to collect and hold water molecules, is a phenomenon which can greatly affect a plastic’s performance. Typically this is observed in PA and PPA products, which feature an -amine. This -amine in the molecular chemistry bonds with hydrogen from the water molecules. 

At the macrolevel in polymer chemistry, there are two primary categories of plastics, THERMOPLASTICS and THERMOSETS. Inside of these categories, further chemistry differentiators exist.

Thermoplastics

Thermoplastics are materials which soften under heat, can be re-molded, re-shaped, and re-processed. Nylon, Polyketone, PPA, PPS, PAEK and PEEK are all thermoplastic materials. Apply heat and pressure to the pellets, they soften and melt to be reshaped into a products geometry and design, and then cool. To manufacture rod guides, a process called Injection Molding is used. This includes a material dryer (to remove atmospheric moisture trapped in the material pellets), a mold tool (the "negative" of the guide profile), and a molding machine (electric or hydraulic press to melt, pressurize, and inject the material into the guide mold cavities),. 

Within the THERMOPLASTIC category, there are two kinds of materials, amorphous and semi-crystalline. All rod guide resins currently on the market are semi-crystalline materials. Looking at semi-crystalline material integrity throughout an elevated temperature band, three plateau's exist:

1. The Glassy State - the material is as stiff and stable as it can be. This is often the same condition to which the material is in a dry, room temperature condition. Nylon and Amodel feature Glass Transition Temperatures (Tg, the temperature at which the material transitions from glassy state to rubber state) which ARE affected by fluid and water absorption. Solvay’s  Amodel PPA Design and Processing Guide, Figure 1.3, shows the effects of moisture on Tg. PA and PPA are inappropriate resins for continuous submersion environments, such as an oil-well (oil/water mix), as their Tg reduces at a minimum of 64%, from ~250⁰ F to ~90⁰ F for A-1000 Amodel PPA Resin. Mechanical properties suffer dramatically, therefore the parts are subject to cracking, accelerated wear, and pre-mature failure.
2. The Rubbery State – the region in which most rod guides operate. Down-hole conditions are at elevated temperature, and due to the reduction in Tg from hygroscopic nature, or the inherit integrity of the material itself (PPS Tg: ~100⁰ C, PAEK Tg: ~150⁰ C), often the polymers are not performing in the glassy state.
3. The Melt State – the ‘liquid’ form of the plastic, which is required for injection molding.

In summary, as thermoplastics are subjected to elevated temperature and/or saturated environments, material integrity diminishes. This is due to the nature of thermoplastics being able to be re-processed. Semi-crystalline materials must be processed correctly in order to extrapolate reliable temperature and dimensional stability. This includes proper resin melt temperature, mold temperatures, insert (sucker rod) temperature, and cycle times. Longer molding cycles in heated tools raise the Tg of the semi-crystalline part, providing a technically accurate component in comparison to material property data-sheets from resin suppliers.

Amorphous plastics, feature a glass transition temperature and steady reduction in material properties until melt. These materials, however, are not typically chemically-resistant and therefore, are insufficient as rod guide materials due to the saturated water, hydrocarbon, and corrosives fluid mixture.

Temperature resistant rod guide thermoplastics, in order of highest ideal melt temperature, are shown below. Data is compiled from the associated Solvay Design Guide:

• PAEK (~ 653⁰ F), 
• PPS (~ 620⁰ F), 
• PPA (~577⁰ F), 
• Nylon 6/6 (~ 510⁰ F) 

Coincidentally, material cost is also in the same order of that listed above. As thermoplastics are demanded to be used in higher temperature environments, but retain re-processability, cost increases. For instance:

• PAEK: ~$25/lb
• PPS: ~$7/lb
• PPA: ~$4/lb
• Nylon: ~$2/lb

To gain significant down-well plastic performance, there is an exponential relationship in cost, relative to the linear temperature increase and property requirements.

Thermosets

Thermoset materials include epoxies, urethanes, poly-ester resin, vinyl-ester resin, phenolics, among others. Thermosets are ‘cured’ to a solid through a chemical reaction which is irreversible, and therefore, the 3 states of thermoplastics (Glassy, Rubbery, Melt) do not apply. Thermosets feature long polymer chains, creating stiff, stable materials. Once the material is SET, it cannot be re-processed or re-used. Molding of thermoset materials is a complex, intuitive process featuring significant and precisely controlled times, temperatures, and pressures.

In researching thermoset plastics high temperature performance, Quadra-Tek , a molder of thermoplastic and thermosetting materials, states that “Elevated temperature applications are solid thermoset territory. While many materials claim to be high temperature polymers, few stand up in real life, and those which do tend to be costly. High temperature thermoplastics will deform excessively if load is applied at temperatures approaching their deflection temperature. At lower temperatures, many of these materials will creep substantially. Thermosets on the other hand will often provide dimensional stability and load bearing capability at temperatures exceeding their deflection temperature and over long time periods. In general, thermosets offer high temperature performance equal to or better than other plastics, at a fraction of the cost.

Because cross-linking is irreversible, thermosets do not begin to melt as temperature rises. For this reason, strength and shape are retained at temperatures which cause other plastics to weaken. Thus, a thermoset is usually the best choice in any application where creep, strength, dimensional stability, and reliability at elevated temperatures are primary design considerations.”

RFG Petro Systems and Black Mamba Rod Lift are the only manufacturers of thermoset sucker rod molded products.

Fillers

Common fillers for plastics across all industries include aramid fibers, glass fibers and mineral fillers in flake or powder form. Glass is often used to elevate the tensile strength and stiffness of a product, whereas mineral fillers can help reduce cost, regulate shrinkage, provide wear-resistance, processability improvements, lubrication, or mechanical property adjustments, in effort to tweak the compound for the application.

Glass (silica) is considered abrasive to production tubing beyond ~33% of the guide material composition. Steel is softer than glass, therefore the glass can wear the tubing. 33% has been the threshold for years in the marketplace, from down-hole historical product usage and feedback from operators. PPS 40% Glass Filled, tends to wear into tubing.

A table is reviewing properties of common plastics fillers is published by Machine Design , here.

Base Materials

Common base resin materials include

• Nylon (Polyamide, PA)
• PPA AF or AU (Polyphthalamide, PPA - also known in layman's terms as Performance PA, Performance Nylon)
• PPS (Polyphenylene Sulfide)
• PAEK (Polyaryletherketone, PAEK)
• PEEK (Polyetheretherketone, PEEK)

Other proprietary composites and blends exist on the marketplace, each having manufacturer designated names (TG-100, PPS-X, Martin Polymer (Changed name to Modified Phenolic in 2018), High-Temp, Kalescent, LG-3, SB-1, LG-15). These materials are often blends of the same base resin systems (Nylon, PPA, PPS, PAEK) with various glass and mineral fillers

Relevant material properties

Solvay Amodel A-1133 HS

Solvay Amodel PPA A-1133 HS, a common 30-33% Glass filled PPA material, features a loss in tensile strength during lab testing, of nearly 65% at 350⁰ F in a dry lab environment, though guide manufacturers rate the product for down-hole use to 400⁰ F. This does not include the Hygroscopy effects, which reduce mechanical properties upwards of 80% at near 100⁰ F. In industry, the product is rated for 400⁰ F use.

Solvay Ryton BR42B, Bearing/Wear Grade 40% Glass PPS

Solvay Ryton BR42B, a bearing/wear grade of PPS, with 40% Glass filler, features a loss in tensile strength during lab testing in a dry state, of nearly 72% at 392⁰ F, though guide manufacturers rate PPS products for down-hole use to 500⁰ F.

Source: Solvay, Ryton PPS Engineering Properties., BR42B Tensile Strength Properties 

Solvay Ryton Xtel XE5030BL PPS Alloy, Glass + Mineral + Impact Modifier Blend

Solvay Ryton Xtel XE5030BL, an economical impure PPS comprising of glass and mineral filler, features a loss in tensile strength during lab testing of nearly 74% at 392⁰ F, though guide manufacturers rate PPS products for down-hole use to 500⁰ F.

Source: Solvay, Ryton PPS Engineering Properties., Xtel XE5030BL Tensile Strength Properties. 

Solvay Ava-Spire AV750 GF40 PAEK

Solvay Ava-Spire AV750 GF40, an economical variant of PEEK polymer, comprising of 40% glass filler, features a loss in tensile strength during lab testing of nearly 70% at 392⁰ F, though guide manufacturers rate PEEK/PAEK products for down-hole use to 500⁰ F.

Source: Solvay, 2015. AvaSpire PAEK Design & Processing Guide, Figure 3.8 

Molding

Manufacturing

Mold-On Rod Guides feature plastic guides directly molded to the sucker rod body. The sucker rod must pass through the molding machine, and the mold tool must clamp around the rod. Including a foreign object in the molding process which is not part of the mold is called "Insert Molding". Adding an insert to molding can greatly complicate the molding process and the material integrity. The backside of a molded plastic part, approximately 180 degrees around the centroid of the component opposite of the gate (material flow injection location), is called the weld-line or knit-line. This is also associated with the parting-line, the area of the mold tool halves which come together to form the molding cavity of the part. The weld-line is the region where plastic flows and mends back into itself after it is has passed around the insert, and is the weakest region of the rod guide. A video showing injection molding flow from a central gate is shown here.

Page 14 of Dupont's General Design Principles for DuPont Engineering Polymers recites that the weld-line strength of an injection molded part featuring glass or mineral reinforcements may have only 60% of the strength of an unreinforced material. Dupont also states that the inserts, when insert molding, should be pre-heated to the recommended mold-temperature for that resin system and product. This is crucial to providing the best molded part possible, due to the complexities of molding sucker rods and guides around them.

Material wear rates

Many guide manufacturers have their own internal wear testing for their products and competitor products. 3^rd party ASTM or ISO based studies for wear of plastics on metal relevant to guides and tubing are most reliable and honest. RFG Petro Systems' has consulted a 3^rd party lab to perform testing based on the labs recommendation of a modified ASTM G133 test, Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear. The study is publicly available via their website.

Rod guide friction

Rod string design programs, along with The Beam Lift Handbook, authored by Paul M. Bommer and A.L. Podio, cite rod guide friction coefficients to be higher than that of rod-on-tubing friction, which is input as 0.1 – 0.3 in string design software. RFG Petro Systems, a guide manufacturer, has a publicly available study which analyzes guide materials friction coefficients against steel in a lubricated, elevated temperature environment, taken from an ASTM G133 based test, Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear.  The study is publicly available via their website.

Designs

Standard (legacy) rod guides

Typically observed as rod guides with 4-fins for centralization, this sucker rod guide design has been in existence for 70+ years. Most often, the guide manufacturer tries to optimize product life by molding the largest volume of plastic outside of the diameter of the associated coupling. The volume of the plastic which can be worn away prior to a coupling contacting the tubing, inducing tuber wear, is referred to as erodible wear volume. This is an important value in conjunction with wear rate of the plastic material in the down-hole environment, as it can be used to predict rod guide performance from one product to another. Erodible wear volume values alone can be taken out of context.

Opposite of Erodible Wear Volume (volumetric plastic available to wear) is Fluid Flow Area (annular cross-sectional area of allowable fluid flow). Flow area is often looked at in relation to coupling flow area. Rod guides should allow fluid to flow past them similarly to that of a standard sucker rod coupling.

It should be understood as a guide wears, its fluid bypass flow area increases. The plastic fins have become smaller, therefore increasing fluid bypass area. 

Fluid flow analysis

Permian Basin operations often cite rod parts just beyond the sucker rod guide, typically above the mold-on rod guide, where there is drastic evidence of corrosion and erosion along the rod body, approximately 4 - 6" beyond the rod guide. This is due to the turbulent fluid flow patterns as fluid moves up, over and through the guide, and back down to the slick rod.

• Anti-turbulent rod guides exist on the market, and do a great job of controlling fluid flow patterns.
• Asymmetrical rod guide designs, where fins have different geometry about the axis, have proven to create turbulent flow patterns.
• Step-faced rod guides, where the guide does not taper all the way down to the sucker rod body, and there is an abrupt edge, also create galaxy style flow patterns.
• Multi-fin rod guide designs are more obstructive than single fin rod guide designs.

Oil flow patterns have a tendency to maintain turbulent flow beyond the guide for a longer distance than water.