API Thread Compounds for Casing & Tubing

Introduction

Compared to the body of the tubing and casing, matched threads are not noticeable.However, threads are an essential component that serves as connections and seals.The normal functioning of threads under different working conditions is closely related to a product called thread compound.Indeed,thread compounds canbe used in casing, tubing, line-pipe, or drill string on every threaded connection.Inthis article, we will deal only with the type used on tubing and casing.

Purpose

Threading compounds for tubing and casing were originally developed by the Mellon Institute of Industrial Research and funded by the API Research Program to meet the following objectives:

Have sufficient lubricating qualities to prevent wear of the threaded connection during the make-up process.

At pressures of 68950 kPa (10000 psi), sufficient inert packing must be used to prevent leakage of API sleeves and tube fittings.

There is no tendency for excessive flow at temperatures up to 1500 degrees Celsius.

Can be easily applied to pipe fittings with a brush in cold weather.

Performance of threaded compound will not be altered by instability, drying or oxidation.

Resistance to water absorption.

Composition

The two most common compounds used in tubing make-up are API and API modified. Both of these compounds are mixtures of metal and graphite powders uniformly dispersed in a grease matrix in the ratio of 64% solids to 36% grease matrix.

The solids should consist of the following materials:

Powdered graphite (28%),Lead powder (47.5%),Zinc powder (19.3%),Copper flakes (5.2%).

Modified API and API have the same percent solids and composition. The difference between the two is the grease base. While the API-modified thread compound has a 36% pure grease base, the API’s base consists of 20.5% grease, 12.9% silicone compound and 7.2% silicone fluid.

Silicone inclusions improve low-temperature performance and can improve application to water-wet threads, but do not necessarily improve wear resistance or sealing ability. These properties are a function of the amount and particle size of the particular combination of powdered solids.

Sealing

The types of solids used in threaded compounds are primarily ductile, relatively weak, low melting point materials that are easily deformed when pressure is applied. When the joint is tightened, the metal powder particles fill in all the small tool marks, indentations, and defects in the joint as they compact together.

Typically, in API tubing and casing connections, the thread compound will fill the void left between the top and root or side of the pin and box threads and provide a pressure seal along with the bearing pressure affected by the surface fit. Thus, in API connections, the primary function of the thread compound is to provide sealing capability.

In premium connections, the role of the thread compound as a sealant is secondary because the primary seal is dependent on the fit of at least one set of finely machined metal/metal seals. However, the function of the thread compound as a lubricant remains essential. Correct application to the surfaces of the mating connection is necessary to prevent wear and ensure smooth operation.

Coefficient of Friction

The amount of rotation can be controlled by the coefficient of static and dynamic friction of the compound. Much of the applied torque will be expended in overcoming friction that a highly coefficient compound will produce. Therefore, rotation will be limited. The same torque applied to a low co-efficient compound allows more rotation as less torque is required to overcome friction. In general, lubricating oils, Teflon, lead, graphite, and sulfur, decrease the friction coefficient, and zinc, copper silicates, zinc-oxide, and non-lubricating oils increase the friction coefficient.

Deformation

The solidification and elongation of metal powder particles requires energy. This energy is provided in the form of torque. The greater the particle density, the greater the torque required for deformation and breakage. A table of the relative yield strengths of metal powders is shown in the table below. It can be seen that the torque required to deform copper or zinc particles is greater compared to PTFE, graphite or lead. The rate of deformation significantly affects the angular rotation of the connection. The faster the torque is applied, the greater the stress required to deform the particles, thus reducing the angular rotation for a given torque.

Work hardening

Work hardening is the property of a metal to resist deformation during deformation. Once the yield stress is reached, the additional stress required to deform the particles is significantly higher. In most thread compounds, additives such as lead oxide, zinc oxide and silicates produce dispersion hardening. These secondary particles are embedded in the metal powder during crushing, further increasing the resistance to deformation.

The above properties are related to the importance of thread compounds as lubricants, anti-wear agents and sealants. The rotation limiting properties are related to shoulderless connections, and it must be remembered that the primary function of thread compounds used in quality connections is that of a lubricant and anti-wear agent.

Practical Use

Thread compounds provide lubricating properties and reduce friction between the pin threads and box threads during the make-up process. This is directly related to the torque required to make a proper connection. The more friction that exists, the more torque is required to overcome it. Different thread compounds act as lubricants to varying degrees, and we take into account the different properties of the various thread compounds by using a correction factor or thread compound friction factor. Published torque data is based on a factor of 1, the same as for API-modified coatings.

The correct torque values for the different thread compounds are then derived by multiplying by the appropriate factor. In practice, this means that we need to determine the type of connection and the torque figure, and when using thread compounds other than API-modified thread compounds, we need to determine the type of thread compound and its correction factor (if it is greater or less than 1).

It is also important when using any thread compound to make sure that it is thoroughly stirred after opening a new can and that it is not mixed or adulterated with any other substance such as diesel fuel etc. In cold conditions it may be necessary to heat the thread adhesive to enable it to be brushed onto the threads.