Low-flow, high-head pump applications can be complicated to specify. Matching a pump to a required flow rate and pressure is only the starting point. The pump also has to operate within its designed range, handle the actual fluid conditions of the application, as well as support the needed flow and pressure for the process.

Jose Gutierrez, executive director of engineered products at ITT Goulds Pumps, has spent more than four decades working with pump applications. He cautions engineers to begin by understanding the operating conditions, rather than simply selecting equipment based on the required duty point. Guitierez sat down with Anna Townshend, Plant Services chief editor, to discuss pump options and how to select the right pump for a low-flow, high-head application.

Q: What defines a low-flow, high-head pump application and which pump technologies can be used?

Gutierrez: “We classify low-flow, where pumps are as many as a few gallons per minute up to 200 gallons per minute. That is a low-flow. And high head is when you have pressure starting at 43 psi equivalent to 100 feet of head.

“For example, in the power industry, you have boiling feed injection pumps, which requires an injection to the turbine, which is a centrifugal pump and can be a low-flow, high-head injection, whereas in the food industry you have many positive displacement pumps handing food like yogurt, or pharmaceuticals, pastes, but they also use centrifugal pumps just for water injection, depending on the system.”

Q: What is the most important consideration when selecting a pump for a low-flow, high-head application?

Gutierrez: “It’s more important to understand the parameters of what the operating region of each of the pumps is, to ensure that the system will operate those pumps within that range.

“Going outside the range, let's say, if the system requires a pump that would be operating out of its normal range, that will prompt the pump to wear more than it should be and then certainly becomes a burden later on for maintenance and repair.”

Q: How should fluid properties affect the initial pump selection?

Viscosity is one of the first screening factors for pump selection. Guitierez points to a substantial difference between applications involving very high-viscosity and relatively low-viscosity fluids.

Gutierrez: “When you have high viscosity fluids, for example, 70,000 centipoise like toothpaste or mustard, to larger around 100,000 centipoise, definitely, the choice is positive displacement pump. When you have very low viscosity fluids, from one centipoise to 50 centipoise, for example, which is engine oil, a centrifugal pump might be the best choice.”

Those ranges should be treated as application guidance rather than universal dividing lines, as actual pump selection also depends on the fluid temperature, pressure, shear sensitivity and other physical characteristics.

The condition of the fluid matters, too. A pump selected for a clean, well-characterized fluid can behave very differently if the process introduces contamination or if the actual viscosity differs significantly from the design specification.

Gutierrez: “If the fluid is not as clean as we think it is, or the viscosity as we defined it to select the right pump is different from what it was specified for, then of course, you have the risk of clogging or having an awful output.”

Q: What other application requirements should engineers consider before selecting a pump?

Viscosity is an important starting point, but it is not the only fluid or process characteristic that should determine the pump. Engineers should also consider:

• shear sensitivity
• fluid cleanliness
• flow precision or pressure control
• operating hours
• consequences of pump failure.

Gutierrez: “Sometimes one of the most common problems is the consequences of using one type of pump to another. It’s very common when companies are selecting pumps that would like a low shear in their fluid, meaning that they don’t want to create too much turbulence or bubbles forming from the pumping action into the fluid. In that case, they should keep in mind that they should select a low shear positive displacement pump.”

Q: Does a low-flow, high-head application favor positive displacement or centrifugal pumps?

Positive-displacement pumps can be advantageous, when maintaining a certain pressure across changing flow rates is the primary requirement for an application. Centrifugal pumps, meanwhile, can be better suited to applications where flow control is the priority and pressure is allowed to vary. The two pump types respond to changes in operating conditions differently.

Gutierrez: “When you want to control pressure, certainly, the positive displacement pump, it will control the pressure better amongst different flow rates, rather than the centrifugal pump.

“The centrifugal pump will change the pressure as the flow rate changes. So that's not a good selection if the purpose is to maintain the pressure among several flow rates. So positive displacement will be the answer. On the other side, if you want to control the flow, and then change different pressure, then certainly a centrifugal pump will be the choice.”

Q: What happens when a pump operates outside its preferred range?

A centrifugal pump can continue operating even when it is operating far from its most efficient point, but that does not mean the application is healthy. 

Gutierrez: “If your pump is designed, let's say for 10 gallons per minute at the best efficiency point, and then you operate below the minimum flow, let's say just as an example, one gallon per minute. Then, the pump probably will operate, but it will overheat, and then it will damage the impeller to the point that it may not last long, and then you need to replace the impeller and the casing as well.

“On the other side, if a positive displacement pump is over pressurized, then the mechanism of operating this pump will need to have major maintenance because it’s not maintained properly with the range of operation.

“That’s why I really emphasize that knowing the preferred operating range of the pumps and where the system needs the pump to operate has a lot to with the pump life.”

Q: How should engineers balance footprint, pump speed and total cost of ownership?

Gutierrez: “Users always need to think about the total cost of ownership. It's not only about buying the cheapest pump to get a job done, but also how much it will cost the user to maintain and then to replace parts instead of replacing the complete pump? That’s something that needs to be considered.”

Physical footprint, purchase price and lifecycle cost can all point in different directions. Higher-speed equipment can provide a compact solution for some low-flow applications. A lower-speed pump may occupy more space and cost more initially, but those factors don't necessarily determine the long-term total cost of ownership. For users operating low-flow, high-head pumps at very high speeds, especially centrifugal pumps, maintenance can be costly to replace the entire pump.

Gutierrez: “Whereas, if they would buy a pump with lower speed, maybe the footprint is a little bigger, but understanding that long term the maintenance cost is much lower.”

That tradeoff becomes more significant as operating hours increase and equipment becomes more critical to production.

A lifecycle evaluation can therefore include purchase price, operating hours, component wear, replacement parts, repair frequency, downtime and the availability of replacement equipment.

Q: How should maintenance strategy affect pump selection?

Gutierrez: “There are critical areas in each of the systems where customers may opt to have, a double pump, one on standby, one on operation, and the reason for this, maybe some of these systems are so critical that they choose to have a backup pump to ensure that they have continuous system operation.”

For maintenance on a critical application, engineers may need to consider:

• duty and standby arrangements
• expected operating hours
• wear characteristics of the selected pump
• how easily components can be serviced or replaced.

The skills available at the facility can also influence the practical cost of maintaining different pump designs. For the most critical equipment, end users may choose to keep a full spare pump or important components like pistons or bearings.

Generally, Gutierrez says the pump maintenance program should consider:

• the life expectancy of the pump
• integral wear parts
• the fluid that is pumped
• the hours of operation
• end user experience. 

Q: How does maintenance vary for centrifugal vs. positive displacement pumps?

Gutierrez: “There are centrifugal pumps that, especially on low-flow, high-head, require high technical expertise. Depending on the centrifugal pump type, there are pumps that are so compact, they require a level of expertise perhaps from the OEM to provide the maintenance of those centrifugal pumps, whereas the positive displacement, they do not require very precise tight tolerances as some of the centrifugal pumps have, and that might be easier to maintain.”

Q: Can digital controls and monitoring compensate for changing conditions?

Gutierrez: “Companies are implementing ways to put controls in this equipment, and they’re putting electronics to measure and control pressure or control flow or control the pump itself. Some applications may require a variable frequency drive upfront, and then, these companies are doing an integral package with the driver and the variable frequency drive to help put those controls upfront to monitor and change the conditions based on the system’s demand.”

Instrumentation does not eliminate the need for appropriate pump selection, but pressure and flow are particularly important operating indicators. Monitoring those parameters can help identify whether the pump is operating where the process and equipment were designed to operate.

Q: How much should end users consider efficiency?

Gutierrez: “Some customers may care about efficiency, but because they are very small pumps, low-flow, high-head, efficiencies may become a secondary qualifier. For example, the efficiency on positive displacement, it increases with increasing pressure, which is one of the qualifiers of the positive displacement, whereas the efficiency on centrifugal pumps, it decreases if the pressure increases. Is that a concern for the customer? Well, maybe not because we’re talking about very little kilowatts or horsepower that is used on the motors.”

That doesn't make efficiency irrelevant; rather, the relative importance of efficiency should be evaluated alongside reliability, maintenance, operating hours and the consequences of failure.