Your questions answered: Choose the right pump for the application

Questions that were not answered in the July 28 webcast are answered to help you understand which type of pump to specify for nonresidential HVAC applications.

By Jim Swetye, Technical Training Expert, Grundfos Pumps Corp., Ohio August 3, 2015

Question: Are there pumps that do not work with a variable frequency drive (VFD)?

Jim Swetye: The pump is not the issue. If a pump has an impeller, it will work with a VFD. The problem is that the motor must be compatible for use with the drive. Not all of motors are. However, if the pump has an extremely small diameter, it may not be suitable for VFD use. Having said that, I know of some impellers that are only about 1.25 in. in diameter, and they are used on variable speed.

Question: Does the pump minimum flow vary by pump type?

Swetye: The minimum flow by volume will certainly vary between many pump types. A small circulator can go to a much lower gpm than a small split case can. However, as a PERCENTAGE OF FLOW at best efficiency point, they all tend to be in the same general range. You absolutely MUST review the pump manufacturer’s curve to be certain.

Question: Does a vertical turbine pump always flooded in a cooling tower sump (therefore always externally cooled) have a minimum flow?

Swetye: Yes, the minimum flow is either stated on the curve by the manufacturer, or can be had from the manufacturer.

Question: How do I calculate the pump demand?

Swetye: It is all about the flow rate. How you determine flow rate varies from one market segment to the next, and then from one application to the next within a segment. For example, in a hydronic heating application there is an equation used that is based on Btus per hour and delta T, or supply and return water temperature differential. In the cattle industry it’s about how many cows you have and their daily water consumption each. In a school plumbing system, it is about the number and type of fixtures. There are tables for this stuff. So there is no one answer to the question. It is all about determining flow needed. Then you can figure out what the head is based on how you design piping and other parts of the system.

Question: Is there a consideration to the selection if the water is salty water like the ocean water or hard water like the wells?

Swetye: Seawater generally has a specific gravity of about 1.03, so the pump will have a slightly higher horsepower load than when pumping fresh water. Seawater may also impact the psi output of the pump (but not the head). It will also probably require corrosion-resistant materials. Whether you can use nodular iron, or all-bronze, or some sort of stainless steel or other alloy depends on which ocean and also on regional best practices.

Question: Where have you seen a top inlet and top discharge pump used?

Swetye: In commercial HVAC chiller applications in which the available floor space of the mechanical room is limited.

Question: When do you use suction diffusers, triple duty valves, etc.?

Swetye: Suction diffusers are used to prevent turbulence as water enters the pump suction inlet. This is often encountered when tight mechanical room space places some sort of pipe fitting within too close a proximity to the pump suction flange. The suction diffuser is intended to mitigate this problem. The triple duty valves in a commercial HVAC application perform three roles: 1. balancing, 2. non-return check, and 3. isolation.

Question: What characteristics makes a pump more energy efficient than another?

Swetye: In terms of pure design, it is the combination of the impeller and volute designs, the specific speed of the pump, and its operating speed. Probably most important of all is properly selecting a centrifugal pump to run as near as possible to its best efficiency point—right at the heart of the preferred operating region. Note that larger flow pumps tend to have higher efficiencies than smaller pumps because there is less internal drag. By that I mean that the ratio of flow volume in contact with an internal metallic or other material surface is higher in a small pump.

Question: Are suction diffusers important for pumps?

Swetye: They can help to prevent turbulence as water enters the impeller. Turbulence will result in many different problems, including improper flow, noise, vibration, low head, bearing and seal wear, poor efficiency, etc.

Question: Are all of these pumps eligible for service repair?

Swetye: Most of the pumps we discussed are highly repairable. However, for some of the smaller circulators, the cost of the labor may outweigh the benefit of repair.

Question: The alignment procedures for flex-coupled pumps appear to have become simplified. We have seen recommendations to only use a straight-edge for alignment. Is this correct?

Swetye: The old-timers used nothing but the straight-edge method, and achieved some very long pump lives. But a laser alignment tool can make life simpler. A good tool in the hands of a poor technician will yield poor results.

Question: Are all the pump types covered in this presentation provided with impellers that can be adjusted in place after installation for performance, or are there some more conducive to this than others?

Swetye: As a rule there is virtually no improvement made when an enclosed impeller is adjusted after it has become worn. There are exceptions, but this is the general rule. Talk with your pump supplier to be sure. Many pumps with open, semi-open, or semi-enclosed style impellers can benefit from proper clearance adjustment.

Question: If the lowest bowl on a vertical turbine pump is submerged, but the other bowls are not, is there a potential problem in startup?

Swetye: The turbine pump manufacturer’s literature will state how much submergence is required to prevent vortexing and other issues. The problem with vortex formation is that air can be sucked into the impeller, which will prevent any possibility of pumping at the correct flow rate. If only the bottom bowl is submerged, I would be very concerned about the potential for a problem. Contact the pump manufacturer for advice.

Question: Can VFDs be used with high head applications?

Swetye: Absolutely—yes.

Question: When must we use inertia base?

Swetye: Contact a manufacturer of inertia bases for specific details. Note that inertia bases are more important for pumps that are mounted above people, like on top floors, etc. They are important for roof-mounted equipment like air handlers, etc.

Question: What do we consider when dealing with installing VFD on a pump?

Swetye: The answer to this question is long, complex, and varies by application. It all depends on what you are trying to accomplish. I suggest that you discuss in detail with a representative of a pump manufacturer. There is a lot of training available on this topic.

Question: In the slide with the chilled water system pump comparison, there were two pump types listed: primary and secondary. What pump type do these represent?

Swetye: The answer to your question will vary, depending on how the system is designed and the required flow rates in each area. Frequently the primary piping loop may have constant flow, which will enable the use of a single speed pump. The secondary loops, which are where temperature is controlled in individual spaces, often will use variable speed pumps. In either case, the type of pump used will often depend on the required flow rate. Smaller pumps tend to be inlines, and larger pumps tend to be end suctions and split case. But as we learned during the webinar, there is overlap in the applications of all pump types.

Question: When should an electronically commutated motor (ECM) motor be specified?

Swetye: For hot and chilled water applications, especially when variable conditions of service are specified. The maximum flow is over 500 gpm.

Question: Up to what size are ECM motors available?

Swetye: The are available in sizes up to 3 hp.

Question: The slide with the matrix showing pump types for chilled water systems did not show circulators as an option. Why?

Swetye: As a general rule, circulator flow rates are small, whereas chilled water systems require large flow rates because of the small delta T. The classic circulator is a low flow pump. However, if you have a chiller system that can use a circulator, then check to make certain that it can deal with the water temperatures involved.

Question: What are the preferred pumps for firefighting purpose?

Swetye: When an approved fire pump (NFPA, UL/FM) is required, then only specific pump types that are approved can be used. These include split case, end suction, vertical inline, and vertical lineshaft turbines. Note that almost any type can be used for the jockey pumps because these do not have to be approved types. Also, if approved pumps only are not required—as is often the case on a military post or on an off-shore oil rig—then other types have been used.

Question: Is there a significant difference is sound transmission between end suction and vertical in-line?

Swetye: In many cases most of the noise comes from the motor, if the motor size is the same for vertical versus end suction, then it will come down to piping. I think because noise issues stem from vibration and end suctions are probably more likely to encounter things like shaft deflection, maybe there’s a higher likelihood of noise from an end suction arrangement with "poorer" installation practices.

Question: I have a base-mounted horizontal mounted motor. Due to job conditions, is possible to change the motor configuration from one side to other side?

Swetye: If you have a horizontal split case double suction pump, then depending on the manufacturer of the pump, the answer is yes. You can move the motor from one side of the pump to the other. You must also install a new pump shaft assembly changing from, for instance, a right-handed shaft to a left-handed shaft. Be very careful about the direction of rotation of the motor and impeller after the change is made. Also, you may have to pin the shaft sleeves onto the new shaft, to keep them from rotating loose during operation. All of this depends on instructions from the pump manufacturer. And not all manufacturers can accommodate this change.

Question: When are flexible pump connectors required? Are they are source of problems? Sometime the acoustical consultants require these yet we’d prefer to leave out.

Swetye: Some designers feel that flexible connectors should always be used. But note that millions of pumps have been installed without them. However, that if you have only one problem that could have been mitigated by flexible connectors, you will wish you had specified them. I suggest that they be considered. And they should not be used for the purpose of "fixing" misaligned piping.

Question: How close should the first 90 deg bend be to the pump?

Swetye: The seasoned veterans taught me to use a minimum of 8 pipe diameters. So by that reasoning, for a 3-in. pipe, stay 24 in. from the pump flanges. Others will tell you to use 16 diameters. Still others will argue for less than 8 diameters. It is also argued that the use of a suction diffuser helps to reduce the distance. Some pump designs have an integrally cast suction diffuser vane in the suction inlet that also mitigates the problem.

Question: In light of your opening video, what are the ramifications of running two pumps at reduced speed on maintenance costs? We know due to affinity rules, it’s more energy-efficient. What’s your experience? Does running two pumps at 50% speed incur less maintenance costs than 1 pump at full speed in a lead/lag?

Swetye: Some old-timers subscribed to the idea that slower is always better, because the rate of erosive wear is directly proportional to pump life, which seems to be true if you are pumping abrasives. But in a clean water commercial HVAC application, it is much less of an issue. If you were to know that you could get 30 years of pump life at full speed, would you care if you can expect to get 60 years at half-speed? You might—but then again you might not. Also, there will be an efficiency difference between the two and given that electricity could be 70% to 80% the cost of owning that pump, that should be considered as well. Once again—conduct a lIfecycle cost analysis.

Question: Selection for minimizing vibration impacts to the building?

Swetye: If poor piping practices are used, or if there are critical speed conflicts between the machine and the building, the results can be really bad. Those loads can be transferred to the building. If not mitigated, there can be disastrous results. I have been in pump houses where you could not stay because of the loud noises that result from the critical speed issue. You could feel the walls vibrating. So during the selection process, critical speed calculations should be made. Contact the pump manufacturer for critical speed data on the pump, and a structural engineer for the building and its components.

Question: Does Grundfos have a quick selection guide for pump? If yes, how can we get one or is it available online?

Depending on which Grundfos sub-brands you wish to work with, you should visit one of the following websites: