Equipment II Dynamic Pumps Purpose and Function At any given time in a chemical manufacturing facility, thousands of gallons of fluids are flowing through pipes, vessels, and process equipment. Driving these fluids are mechanical machines that transfer energy to the fluids that generate pressure and movement. In this course, you will learn the basics of these machines, how they work, how to monitor, maintain, and troubleshoot problems. One of the most common examples would be a pump. Pumps are mostly driven by electric motors but can be driven by steam turbines, gasoline engines, natural gas engines, water turbines, or wind power. Pumps pull fluids from the inlet, or suction, and force the fluid out the discharge side. In most cases, the fluid at the discharge point is at a higher pressure than when the fluid entered the pump. Pump Categories Pumps are selected for specific applications based on the pressure and flow rate needed for the process, the type of fluid, the type of power that will drive the pump, and, the costs to pay for the pump and to maintain the pump. Pumps are categorized and named based on their function and how they operate. They are divided into two broad categories: dynamic and positive displacement. This categorization is based on how the pump transfers energy to move fluid. Dynamic pumps have an impeller that spins around the pump’s axis to move the fluid through the pump. A positive displacement pump uses a reciprocating or rotary action to move a fixed volume of fluid from the suction side through to the discharge. Within these two broad categories, pumps can be further divided into subcategories. Starting with dynamic pumps, we will look at the design and purpose of pumps within each of these subcategories. Axial Pumps Axial pumps are a specific category of dynamic pumps that drive fluid along a straight line, or axis. An axial pump works similar to a floor or window fan. As the impeller spins, fluid is driven along its axis to the discharge side of the pump. The pump casing prevents the fluid from back flowing. In some designs, the pipe acts as the pump casing. Often a 90-degree elbow allows an external motor to be connected to the pump shaft that turns the impeller or rotor inside the fluid area. Axial flow pumps are very efficient in moving large volumes of fluid with a small pressure rise and are commonly used in drainage and irrigation applications. If a greater pressure rise is needed, a centrifugal pump may be used. Centrifugul Pumps Centrifugal pumps are widely used within the chemical industry because they can move large amounts of fluids in an efficient manner and their design is relatively simple. Centrifugal pumps have a reasonable tolerance for debris and are compatible with high speed turbines and motors. As the name suggests, a centrifugal pump uses centrifugal force created by the impeller to accelerate the fluid as it enters the pump. To work properly, a centrifugal pump must be primed before starting. Pump priming is when the reservoir of the pump is filled with process fluid in order to draw more fluid from its suction. If the pump is not primed before operation, the impeller will just spin and not be able to draw suction. Impeller Designs Impeller designs in centrifugal pumps vary widely. In broad terms, impeller design can be classified as open, closed, semi-open and double suction. In an open impeller, the vanes of the impeller are attached to a small backing plate and the pump casing serves as the surfaces on either side of the impeller. The clearances between the impeller vanes and the pump casing must be very tight. As the impeller vanes wear, the clearances open up, resulting in reduced efficiency. Open impellers have the advantage of being easy to clean and are less prone to clogging, but are often less efficient than closed impellers. In a closed impeller design, the vanes are designed with integral shrouds, which enclose the vanes. The pressure increase is generated by the impeller itself, so a tight-tolerance volute is not required. However, wear rings are needed to prevent the process fluid from flowing back to the eye of the impeller. This design is somewhat more efficient than an open impeller and requires fewer adjustments. However, closed impeller pumps are more difficult to clean than open impeller designs and are subject to clogging. Also, these pumps must be disassembled to inspect the wear rings. A semi-open impeller design consists of a shroud on the back of the impeller, with the side facing the suction pipe being open. This semi-open design is more efficient than an open impeller due to its shroud, and is more advantageous than a closed impeller because it can be adjusted to compensate for vane and casing wear. Semi-open impellers are commonly used with abrasive slurries when clogging is a concern. This is because the impeller vanes are reinforced by the shroud and are not as vulnerable to damage and distortion as in an open impeller design. At the same time, the semi-open impeller design can pass debris almost as well as an open impeller design. If the clearances between the open side and the pump casing increase, adjustments can be made to regain efficiency. The biggest drawback of the semi-open design is that the single shroud causes high axial thrust. The eye of the impeller is at low pressure while the outer circumference is at full pressure. Therefore the back side of the shroud is at full discharge pressure across the entire surface area. So far, we have looked at single-suction impeller designs, in which the inlet of the process fluid is on one side of the impeller. A double-suction pump has dual inlets which results in suction on both sides of the impeller. This double-suction design has two primary advantages. First, since suction occurs from both sides, the fluid pressure loss is less than that of a single-suction impeller design. Second, there is virtually no axial thrust when compared to the single-suction design. This makes the double-suction design a preferred choice for high flow and high head water supply and process service. Types of Centrifugal Pumps This is a typical design for a single stage centrifugal pump. A shaft connects a motor or driver that turns an impeller. As fluid enters the pump, it is forced away from the center due to the centrifugal force created by the impeller. The pump body, or casing, directs the flow of the fluid toward the volute and into the discharge pipe. Centrifugal force causes the fluid to accelerate, creating more pressure or head as the fluid exits the pump. A single-stage centrifugal pump is used when a smooth flow of fluid and a uniform discharge pressure is required. If more pressure is needed, multiple stage centrifugal pumps may be used. The components of a multi-stage pump are the same as a single-stage except there are multiple impellers. Each stage has an impeller added to the pump shaft, all driven by the same motor. The process fluid goes from the discharge of one stage to the suction of another, resulting in a pressure increase during each stage. In a multi-stage centrifugal pump, most of the force is transmitted to the process fluid. However, some axial thrust is exerted by each impeller since the suction side of the impeller is at a lower pressure than the discharge side. Axial thrust can be minimized by arranging the stages so that the suction is to the right of some impellers and to the left on others. This balances the axial thrust of the multiple stages. Another way to overcome axial thrust is to use a “balance drum” which is a disk mounted on the shaft that is exposed to the discharge pressure on one side and the suction pressure on the other side thus canceling out the thrust produced. The axis of a vertical pump is oriented up and down so the suction is drawn from the bottom of the pump and discharged out the side. The motor driving the pump is mounted above the pump preventing fluids from entering the motor. As a result, the motor is better protected from the process fluid