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System Approach & Modeling Lead to a Reduced Impeller Diameter

Figure 1: Process system with one pump operating

EVERY ELEMENT found in a piping system falls into one of three categories: pump, process and control. The secret to understanding any piping system is to have detailed knowledge of the various pieces of equipment and how they work together.

For example, if a pump is not sized properly to meet the system’s process requirements, all the equipment within the system will be affected.

The introduction of the recently released ISO/AMSM 14414 Pump System Energy Assessment Standard states, “In the majority of pumping systems the energy added to the working liquid by the pump is much greater than that required by the process.”

In industries where pumps are widely used, significant savings may be achieved with a better understanding of how the system can be optimized. In my discussions with personnel at process plants, an individual or small group is usually responsible for reducing energy consumption for the plant. This process may result is some savings in electrical power consumption, but additional improvements are overlooked because the focus is not on the total picture.

Just as all elements and equipment found within a system must work together for the system to operate, each person within the plant who is responsible for efficient system operation must work together as a team. This will ensure the equipment for which each of these people is responsible will work with the other equipment within the plant. In this month’s column we will look at a simple process system (see Figure 1) to understand how an oversized pump can adversely affect every piece of equipment within the system. With this knowledge, everyone should have a better understanding of the problems associated with oversized pumps and what can be done to improve system operation.

Looking for Improvements in the System

After reading the ISO Pumps System Energy Assessment Standard, the plant’s utility manager decided to look for ways to reduce electrical power consumption in the plant’s process systems. He developed a presentation on reducing power consumption and shared it with the plant’s operational management team. The response was lukewarm at best—typically “why rock the boat?” The system had been operating for so long that the team feared any changes could cause more problems than they would solve.

The example shows the major equipment comprising the system along with the results of the plant instrumentation under normal operating conditions. The system is operated by one pump, with a second pump on standby. The flow rate through the system is controlled to 600 gallons per minute (gpm) by the installed control valve, and the set points for the tank levels and pressures are listed in Figure 1, above.

Checking with the Operators

The first step is to determine how well the system is operating. It is best to ask the people responsible for operating and maintaining the system.

Plant operators determined that the level in the inlet process tank varied between 5 and 12 feet. The pump element discharge pressure ranged between 86 and 90 pounds per square inch gauge (psig) depending on the tank levels, and there was no indication of pump cavitation. The position of the control element operated between 36 percent and 38 percent, with a comment that occasionally cavitation noise could be heard from the valve. Finally, the outlet process tank level varied between 5 and 12 feet, and the pressure varied between 6 and 12 psig at the tank surface level.

When the operators were asked about the control valve cavitation they all stated that it had happened for so long that everyone considered it normal.

Checking in with Maintenance

The next stop was the maintenance department. When asked about the intermittent cavitation in the control element it was discovered that the valve seat and stem had to be replaced on a regular basis due to erosion. To solve the problem, the valve internals were changed to a severe duty rating with components made from a harder material. This corrected the problem with valve cavitation, but the constant vibration of the control element caused damage to the downstream piping, but that could be corrected and maintained without causing a forced shutdown.

The instrumentation department stated that the flow control loop associated with the control element was difficult to set up and resulted in wider variations in the flow rate through the system. In discussions with the control valve supplier, it was discovered that a valve running at a low percentage of approximately 30 percent open could indicate a high differential pressure, especially when the valve had intermittent cavitation.

When asked if there were any other maintenance concerns associated with the system, the maintenance manager stated that the pump element cost more to maintain that other similarly sized pumps within the plant. After further investigation, it was discovered that the pump had higher instances of seal and bearing replacement.

Determining the System Operation

Now we know that there are multiple problems associated with this system, but as you can see from Figure 1, the availability of installed plant instrumentation was lacking. Without installing additional pressure gauges and flow meters, little more information could be observed from the physical piping system. The lack of installed plant instrumentation usually stops improvement efforts dead in their tracks.

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Mechanical Seals for:

  • Centrifugal Pumps
  • Slurry Pumps
  • Submersible Pumps
  • Mixers & Agitators 
  • Compressors
  • Autoclaves
  • Pulpers