Author: Joseph Veres

Performance Testing of Axial Compressors

Posted by Joseph Veres

Performance testing is a key part of the design and development process of advanced axial compressors.  These are widely used in the modern world and can be found in nearly every industry, and include the core compressor for aeropropulsion turbofan engines, as well as aeroderivative gas turbine engines for power generation.  An example of this are the turbine engines shown in Figure 1 and 2, which feature an industrial gas turbine and a high bypass ratio turbofan engine with a multistage high-pressure core compressor. The development time of these machines can involve numerous expensive design-build-test iterations before they can become an efficient and competitive product. This places a great importance on the accuracy of the data taken during the performance tests during the development of the compressor since the test data taken is often used to anchor the loss models within the design tools. Modern axial compressors typically have high aerodynamic loadings per stage for improved system efficiency and requires precise aerodynamic matching of the stages to achieve the required pressure ratio with high efficiency. Variable geometry inlet guide vanes and stators in the first few stages are typically required to provide acceptable operability while maintaining high efficiency and adequate stall margin.

Industrial gas turbine for power generation.
Figure 1. Industrial gas turbine for power generation. Source
Figure 2. Turbofan engine for aeropropulsion.
Figure 2. Turbofan engine for aeropropulsion. Source

Performance Testing of Axial Compressors

Axial compressors all undergo a thorough design and development phase in which performance testing is vital to their ultimate success as a product. Performance testing during the development phase of these high-power density machines can ensure that the design meets the specified requirements or can identify a component within the turbomachine which falls short of its expected performance, and may require further development, and possible redesign. Performance testing can also ensure that the unit can meet all the conditions specified and not merely the guaranteed condition. Aerodynamic performance testing multistage axial compressors during the early part of development is often done in phases. The development test program is planned and executed with a design of experiments approach and includes varying the air flow and shaft rotational speed as well as the variable geometry schedule in order to fully characterize the compressor. In the first phase, the front block of the compressor is built and tested at corrected (referenced) air flow rate, inlet pressure, temperature and shaft rotational speed. Instrumentation includes utilizing traditional rakes and surveys at the exit, to obtain spanwise distributions of pressure, temperature, and flow angles. Testing in phases is typically done for two reasons. Read More

Vertical Pumps: What Are They, Where Are They Used and How To Design Them?

Posted by Joseph Veres

Introduction

Vertical pump designs are similar to conventional pumps, with some unique differences in their applications.  Pumps use centrifugal force to convert mechanical energy into kinetic energy and increase the pressure of the liquid. Vertical pumps move liquids in the vertical direction upwards through a pipe. All pumps pressurize liquids, which are mostly incompressible. Unlike compressible gases, it is impossible to compress liquids, therefore the volumetric flow rate can not be reduced. Therefore liquids are transported by pumping and the inlet volume flow rate is equal to the exit volume flow rate.

Vertical centrifugal pumps are simply designed machines, and have similarities to their horizontal counterparts. A casing called a volute contains an impeller mounted perpendicularly on an upright (vertical) rotating shaft. The electric drive motor uses its mechanical energy to turn the pump impeller with blades, and imparts kinetic energy to the liquid as it begins to rotate. These pumps can be single stage or multistage with several in-line stages mounted in series.

The centrifugal force through the impeller rotor causes the liquid and any particulates within the liquid to move radially outward, away from the impeller center of rotation at high tangential velocity. The swirling flow at the exit of the impeller is then channeled into a diffusion system which can be a volute or collector, which diffuses the high velocity flow and converts the velocity into high pressure. In vertical pumps, the high exit pressure enables the liquid to be pumped to high vertical locations. Thus the pump exit pressure force is utilized to lift the liquid to high levels, and usually at high residual pressure even at the pipe discharge.

Applications of Vertical Pumps

An “in line” vertical pump is illustrated in Figure 1 (Reference 1), where the flow enters horizontally and exits horizontally and can be mounted such that the center line of the inlet and discharge pipes are in line with each other.  This is a centrifugal pump with a tangential scroll at the inlet that redirects the flow by 90 degrees and distributes it circumferentially and in the axial direction into the impeller eye. The discharge is a simple volute that collects the tangential flow from the impeller exit, and redirects it into the radial direction.

in line Pump - Figure 1
An “in line” Vertical Pump. Source

Figure 2 shows a vertical pump that has a vertical intake that directs the flow straight into the eye of the pump rotor. At the impeller exit, the tangential flow is collected by a volute and diffused in an exit cone. An elbow after the exit cone redirects the flow into the vertical direction to lift the liquid to the desired altitude. (Reference 2). Read More

Pump Design to Feed an Elevated Water Storage Tank – Reduce NPSHr and Improve Efficiency

Posted by Joseph Veres

This blog post will show an example of a pump design task for a specific application, using the AxSTREAM® pump design and analysis code. Centrifugal pumps are designed to meet the requirements of head rise at the discharge, while at the same time the suction performance at the pump inlet must be free of cavitation over the entire operating range.  This requirement places an additional constraint on a successful pump design and a good example of AxSTREAM® capabilities.

Pump Installation and Performance Requirements

The pump installation is illustrated in Figure 1. The pump will suck water from the bottom of a reservoir and discharge into a raised tank that is 145 feet above the pump. The pump should be designed for optimum efficiency and will be driven by a variable speed electric motor. The design flow rate is 2,000 gallons per minute (GPM) and it must operate free of cavitation at all operating points.

pump installation
Figure 1. The pump installation showing the water reservoir feeding the pump inlet with a 15 feet total suction head, and the discharge tank that is at a height of 145 feet above the pump.

The key performance goals and requirements for the pump are summarized below:

Pumped liquid: water
Density, ρ: 62.3 lbm/ft3
Volume Flow rate: 2,000 GPM
Inlet Temperature, Tt1: 527.7 Rankine
Vapor Pressure, Pv: 0.46011 psi
Static Head Rise: 145 feet

Design and analysis approach

Using AxSTREAM Preliminary Design Solver, thousands of flow path geometries can be generated that satisfy the user defined boundary conditions and geometric parameters within given constraints. By determining key parameters such as suction cavitation performance early at the beginning of the design process, users can minimize development cost while maximizing the pump efficiency. In addition to being able to generate the optimum flow path and pump blades to meet the design point goals, users can also analyze off-design operating conditions for the pump in a system environment that can have changing boundary conditions, thus placing different requirements on the pump.
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