Porosity in Cast Bronze Pump Impeller

Introduction

Cast bronze impeller is by far the most used impeller material in the oil and gas industry. It’s great mechanical and corrosion resistant properties has proven to work in some of the most difficult environments. An impeller is primarily found in a centrifugal pump, in which do to its rotational speed, it transforms the kinetic energy into pressure energy. Porosity in the casting is by far the most common manufacturing defect that can be found in bronze impellers. Since the impeller is the heart of the pump, it’s crucial that this component has the least number of defects possible. In this project, we will present a way how to incorporate ICME methods to further understand the defects that are present in the material and understand the bridging between each stage.

Pump

A pump is a device that moves fluids through mechanical action. It utilizes suction pressure to raise or transfer the liquid. Pumps have been around for many years and have had many improvements to satisfy industry requirements. Normally, the driven part of the pumps can either be a motor or turbine. The most common types of pumps are: 1) Positive Displacement: reciprocating piston, rotary 2) Kinetics: centrifugal, regenerative turbine In the oil and gas industry, pumps tend to be classified as: - American Petroleum Institute (API) 610: this type of pump is more robust, it operates at a higher temperature than the ANSYS pump and its feet are mounted on the horizontal center line of the casing - ANSYS: operates at a lower temperature than the API 610 pump and its feet are mounted on the bottom of the casing

Centrifugal Pump

One of the most commonly used pumps in the oil and gas industry is the centrifugal pump. It relies on the centrifugal force to add energy to the system. It consists of a rotating impeller inside a stationary volute (casing) to move the fluid. The shape of the volute casing is wider at the discharge nozzle and smaller at the suction nozzle. Liquid enters the impeller eye through the suction inlet pipe. The velocity of the fluid increases as it is impacted on the walls of the casing (volute). This is known as Kinetic Energy, which is the energy in motion. This kinetic energy is transformed into pressure energy which forces the liquid out of the discharge nozzle by the speed of the rotating impeller.

Types of Centrifugal Pumps

1) Overhung Impeller: this type of pump has the impeller mounted on the end of the shaft which is overhung from its bearing supports. Normally, this type is classified as: - Closed Coupled: the mounting of the impeller is direct to the motor shaft - Separately Coupled: the mounting of the impeller is on a separate shaft. Both shafts are connected via a coupling 2) Impeller between bearings: both ends of the impeller has bearings. Normally, this type is classified as: - Separately coupled: axial split case (horizontal), radial split case (vertical)

Different Types of Impellers

The types of centrifugal impellers are:

- Open Impeller: The vanes are cast free on both sides - Semi-Open Impeller: The vanes are free on one side and enclosed on the other - Enclosed Impeller: The vanes are located between the two discs (shrouds), all in a single casting

Gas Porosity: Bronze Casting Impeller

Bronze impeller (copper and tin) is one of the materials of choice for pumps in various applications in the oil and gas industry. Some of the reasons for this choice are its mechanical and corrosion resistance properties. Most impellers that are used in the industry are from a casting process. The impeller being the heart of the pump, it’s critical that this part is manufactured with the least number of defects possible. However, the percent rejection rate due to leakage or porosity tends to be high in many foundries. A lot of these defects are found after machining which can result to be very costly. Porosity in most casting impellers is undesirable. The higher the density of porosity in a metal casting, the more likelihood crack nucleation will start. These types of cracks, if left unattended, can lead to a very expensive repair or a catastrophic failure.

The most plausible causes of gas porosities are: - Absorbed gases in the raw material or during the melting process. - Insoluble mold gases may be trapped in the metal during casting. - Trapped carbon monoxide and carbon dioxide gases during solidification from metallic oxides.

The solubility of some gases in molten bronze is temperature dependent; they increase with temperature and decrease during solidification. This makes it possible for the trapping of gases to form gas cavities. These types of gases are to be considered as sources of the problem. However, there are some gases that are soluble at room temperature. Generally, these gases will not cause porosity.

Mold gases in the form of distinct bubbles can originate in several ways such as steam from green sand, the air held in the pores of the sand, etc . The viscosity of the metal plays an important role in porosity formation. If it is too viscous it will not allow the gases to escape. It will remain in the form of blowholes, typically distributed randomly throughout the casting but often confined to the surface. The pouring temperature of bronze casting is important to minimize the number of defects. If the temperature is to low, the molten metal will not flow enough and there will be voids within the mold cavity. It is noted that maximum soundness and physical properties can be obtained within a very narrow pouring temperature range. There are different types of casting processes to form a bronze impeller, such as sand casting, gravity casting, low pressure die-casting, high pressure die casting, and investment casting. Sand casting is normally the process of choice.

Sand Casting In sand casting, different range of impeller sizes from small to very large can be done effectively. Types of patterns used in sand casting include but are not limited to: (a) solid pattern, (b) split pattern, (c) match-plate pattern, (d) cope and drag pattern.

Some of the desirable sand mold properties are strength, permeability, thermal stability, collapsibility, and reusability. One of the most utilized foundry sand is Silica (SiO2) mixed with other minerals. This type of sand has good refractory properties: small grain size for better surface finish and irregular grain shapes strengthen molds due to interlocking. A sand mold is classified as green-sand molds (sand, clay, and water), dry-sand mold (organic binders), and skin-dried mold (drying mold cavity surface utilizing torches or heating lamps).

Since bronze is an alloy between copper and tin, most alloy freezes over a temperature range. Its grain structure is of segregation of allowing components in the center of the casting possessing a dendritic type structure.

Bridges to Model Porosity in Cast Bronze Pump

In order to obtain a good model, we must first start with the end in sight and work backward. Our goal is to model and impeller for a centrifugal pump. The pumping of fluids within a plant and between plants is integral to the economic functioning of any chemical or petrochemical industry. It is thus of utmost importance that premature failure of these pieces of equipment is avoided. We are going to utilize several ICME length scale to obtain an understanding of the material behavior. In today’s designing, most companies utilized some type of software to properly model stresses that material can endure. When a material is subject to different operations scenario it’s important that the correct material is utilized to achieve the desired goal. The idea of developing the ability to predict damage progression is imperative for the design of components that will experience overloads during service due to impacts, rough ground, and crash environment. The end goal for the ICME multiscale modeling for this case study is to have the ability to embed in each element of the finite element mesh the different grain and inclusion/defect sizes, nearest neighbor distances, and volume fractions in order to initialize the constitutive model. Normally, in FEA we assume a homogeneous material state in the material, but with different microstructural features included, a true heterogenous material state can arise, which is a more realistic representation of the engineering materials. One of the reasons we will utilize a different length scale is to help determine the correct behavior of the material under certain operating conditions. Sometimes the microstructural evaluation will quantify the sources of damages in a component differently than an FEA. Bridge 10 connects the single component of the pump to the system with many components. It also connects the macroscale continuum constitutive model that includes the microstructural features with the structural scale simulation. Bridge 9 connects the pore growth and interaction to the macro scale continuum damage model. Bridge 8 connects the pore nucleation arising from particles to the macroscale continuum damage model. Bridge 7 connects the defects produced from the solidification process of the material to the macroscale continuum damage model, we can utilize phase-field modeling. Bridge 6 connects the high rate damage mechanisms to the macroscale continuum model. Bridge 5 connects the elastic moduli to the macroscale continuum equations. Essentially, bridge 5-10 connect the different length scale results directly to the macroscale continuum equations. Bridges 1-4 connect the two length scales for building the upscale methodology. Bridge 1 connects the electronics principle simulations of the energy and elastic moduli to the atomistic level. Bridge 2 connects the fracture and debonding criterion determine from the atomistic simulation to the microscale finite element simulations. Bridge 3 void nucleation from particles that arise from the microscale to the mesoscale 1 where realistic microstructure features are modeled. Bridge 4 connects the pores coalescence from the pores arising from the particles to the casting pores.

Although porosity has been the most examined quantity for void growth, many factors besides void volume fraction are known to play significant roles on influencing void growth and strain localization. The works that have considered the void volume fraction and void growth studies, for the most part, have neglected coalescence. Voids within a casting are generally in-homogenous presented in various shapes and sizes.

- Bridge 10 = finite element analysis (FEA)

- Bridge 9 = void growth analysis

- Bridge 8 = porosity formation analysis

- Bridge 7 = dislocation motion – casting analysis

- Bridge 6 = high rate mechanism analysis

- Bridge 5 = elastic moduli analysis

- Bridge 4 = particle interactions analysis

- Bridge 3 = hardening rules

- Bridge 2 = mobility analysis

- Bridge 1 = interfacial energy analysis