by Ray Hardee, Engineered Software, Inc.
February 17, 2015

In 2014, this column focused on improving the operation of piping systems by performing pumped system assessments. The columns have covered energy cost balance sheets, the assessment process and the performance of sample assessments. By now, readers are aware that all the equipment found in a piping system falls into one of three categories:

  • Pump elements add all the fluid energy to the piping systems.
  • Process elements are used to make the product or provide the service.
  • Control elements are used to improve the quality of the product or service.

The next series of columns will focus on how these basic elements are used within the piping system and how they work together to achieve system operational objectives. With an established understanding of the equipment used, operators can evaluate the total system, which leads to a better understanding of what can be done to reduce operating, maintenance and capital costs while improving system uptime.

Pump elements include the pump, the drive and the ancillary equipment needed for pump operation. Many types of pumps and drives are available, but for the purpose of this column, the discussion will be limited to centrifugal (or rotodynamic) pumps driven by alternating-current induction motors. These are the most common pump and drive combinations used in industrial piping systems.

Why Pumps Are Typically Oversized

A common scenario often leads to incorrectly sizing pumps. In this scenario, a company decides to release a new product. The results of its marketing study project sales of 50,000 tons of product per year for the first two years. After this, sales would increase to 100,000 tons per year. An annual sale of 200,000 tons per year is projected after five years.

The engineering, procurement and construction firm (EPC) is contracted to design and build a new plant and ultimately allow for the production of the product. During the discussions between the client and the EPC firm, a mood of optimism prevails. The plant is designed to meet the future estimated capacity of 200,000 tons per year. The engineering group determines that the processes used to make the product needs a series of simulation studies to optimize the design. This includes the required mass flow rates, fluid physical properties, and the pressure and temperature requirements for each of the plant’s systems.

The mechanical engineering team uses the process flow diagrams provided by the process engineering group to size the equipment needed. The team determined the following:

The example system requires a pump with a capacity of 800 gallons per minute (gpm) to achieve the plant’s design objectives (see Figure 1).

Figure 1. This single system is one of the many interrelated systems designed for the total process. (Graphics courtesy of the author)

During the preliminary design process, the long lead items are specified early to meet the project schedule. However, the location of the equipment has not been determined. As a result, margins are factored into the design to ensure that the completed pump system is capable of meeting its process requirements.

Next, the total head required must be determined. This is done by arriving at the static and dynamic head values for the process elements.

Calculate the Static Head

In the example, the exact location of the supply tank and reactor have not yet been decided. The designers have an approximate size of the tank and vessel based on the process requirements. Until the civil department is able to complete the design of the tank foundations, the exact elevations and location of the supply tank and pressure vessel are estimated. The bottom of the supply tank is estimated to be at an elevation of 100 feet above sea level, but the bottom elevation of the tank could vary by plus or minus 10 feet. The estimated elevation of the nozzle into the reactor will be 180 feet above sea level, with a margin of plus or minus 10 feet.

Using the estimated elevation of the tanks, the elevation head for this system could be as low as 60 feet (170 feet – 110 feet) or as high as 100 feet (190 feet – 90 feet). The preliminary design will use 100 feet. Last, the operational pressure in the reactor is specified at 10 pounds per square inch gauge (psig) including the static head, but the operational pressure could vary by plus or minus 2.5 pounds per square inch (psi) from the design pressure. This pressure variation creates an additional 6 feet of pressure head. The resulting static head used in the preliminary design is 106 feet.

Determine the Dynamic Head

The next step is to determine the head loss in the pipeline as a function of the flow rate. This calculation incorporates the physical properties of the process fluid, the pipe material, the pipe length, and the valves and fittings.

Figure 1 includes all the isolation and check valves in the system. The design features these items to allow for the isolation of equipment for safe operation and maintenance. Other items, such as elbows and pipe lengths, are not indicated in the piping schematic. This exact information is not available until well after the preliminary design’s completion.

As a result, the pipe length and the number of fittings needed along with the design margin must be estimated. The pipe length between the pump discharge and process equipment is estimated at 150 feet, but a 50 foot margin is added. Further, a need for five elbows is estimated, with a margin of three more.