Submersible electric motors can be a vital component in many pump systems; however, many users have common questions that prevent or limit them from maximizing the equipment’s contributions to their applications. In this month’s SWPA Insight, current member—and past SWPA President—Barry Jongsma of Pentair answers the skeptics on submersible electric motors and addresses their benefits.
Even though this technology has been in use for over five decades, do you still encounter skeptics who doubt an electric motor-powered pump can perform underwater? How do you re-educate them?
Of course, whenever you mix electrical equipment with water there are skeptics and concern. Over time, however, the advantages of the submersible motor driven pump over the traditional dry pit application have changed most skeptics in to converts. For new construction, submersible pump wet-pit applications are now the norm for pumping stations up to 15MGD. Stations in the 30MGD and higher range are less frequent; however, the manufacture’s product range has expanded to meet even this demand.
SWPA has been the on the forefront of education for decades—we take this seriously! These are some of our highlights on the education front:
The Submersible Sewage Pumping Systems Handbook, first released in 1986, is now on its fourth edition. More than 40,000 copies of this publication are in circulation. Some of our readers refer to this as ‘The Bible’ and we get annual orders from college institutions because they want to teach these important topics in the classroom.
In 2001 SWPA pioneered the Submersible Pump Test Standard that is now adopted and distributed worldwide through the Hydraulic Institute.
Since 2001, we have offered extensive two-day regional training to more than 1,000 people with our traveling Pump Systems Training Seminar. Nobody offers this type of extensive training for submersible wastewater pump systems. We get shinning reviews from attendees and they are enthusiastically requesting more advanced material.
Recently, SWPA has sponsored free one-hour webinars over the internet, training more than 3,000 people on important SWPA related topics with this format.
This year SWPA is launching a web-based technical resource center with on demand education programing that includes: live training seminars, pre-preprogramed webinars, white papers, technical resources, and CEU’s.
In many applications, an exposed motor can be the cause of noise or on-site accidents. What are some of the advantages that submersible electric motors provide for noise reduction and safety?
In a pumping system, the motor, pump, and piping all generate sound. Acoustics is a highly technical field, but let’s start with some basic science. The pumping system sound is transmitted through a medium by acoustic pressure pulsations. The method that these sound pressure pulsations travel through air and water medium are very similar; however, the intensity of the sound for an equivalent unit of pressure is highly dependent on the medium density and the speed of sound through that medium. The product of the speed and density is known as acoustic impedance. The speed of sound through water is 4.4 times faster than air and the density of water is about 820 times greater than air; therefore, the impedance contrast ratio between water and air is more than 3600. The second piece of science deals with the water to air boundary layer. The acoustic impedance between the water and air is so high that almost no energy will cross the boundary surface. In fact, the water—air boundary surface is often referred to as a perfect reflector. As kids, we all ducked our head under water to avoid hearing Mom tell us it’s time to get out, right? If only I had the foresight to explain that the problem wasn’t me, but the acoustic impedance contrast!
The submersible pump motor does not have a noise producing cooling fan, a significant noise contributor in the dry-pit station. More importantly, the majority of the submersible pumping system is under water where the acoustic impedance and reflective boundary surface work to an extreme advantage and very little noise will pass to the human ear. This results in peace and harmony for the neighbor nearby.
One important safety contrast for workers between the submersible pump station and dry-pit station is confined space. The dry pit station often requires a worker to enter a confined space to dismantle piping joints before it can be removed from service. Most municipalities have a confined entry protocol where the air quality is measured, permits are filled out, ventilation applied, harness equipment is used and a spotter is required. The submersible pump does require confined space protocol because it is easily hoisted out of the station without entry.
In most submersible applications, the motor benefits from being cooled naturally while performing its job surrounded by water. How does this extend motor life and improve reliably over the motor’s use?
Compared to air, water does an excellent job of removing heat from the motor. A classic example is to heat two balloons with a lighter; one filled with air, the other water. The air filled version immediately pops because the air inside does a poor job of dissipating the heat at the surface of the balloon. The water filled version does such a good job of removing heat from the balloon surface it stays intact. In a similar matter for motor applications, the ability to remove heat directly impacts the rated power. For instance, consider a 60 horsepower motor rated for continuous duty submerged in water with a 104 degree Fahrenheit (40 degree Celsius) ambient temperature. If you place that motor in 104 degree Fahrenheit (40 degree Celsius) air under a 60 horsepower load, it would soon over heat and trip the thermostats because air cannot remove a sufficient amount of heat.
For any type motor, submerged or not, life and reliability are dependent on factors such as the quality of manufacture, the type of load that is applied, the operating temperature, starting methods, starts per hour, environmental conditions and routine maintenance. The submersible pump motor is designed and tested for a specific purpose and environmental conditions. The best way to improve reliably of the system is source from a quality manufacture, understand and operate within the confines of the intended purpose, and follow good installation and maintenance protocol.
What are some of the control options available for submersible electric motors, and how should users’ assess their applications to choose the right one for their needs?
Controls for the basic submersible pump involve a starter, level control, thermal protection, and moisture detection. Decisions for each of these subsets are highly varied. For instance, should I use a common FVNR starter (full voltage non-reversing magnetic starter), auto-transformer, SSRV starter (solid state reduced voltage) wye-delta starter or VFD? In addition to the basic controls, one may want additional sensors on the equipment, like bearing temperature and vibrations sensors. Maybe complex PLC topography to monitor and control multiple stations remotely is required. Since the variations are endless, the best approach to assess and choose is to get educated. The first resource I use is the Submersible Sewage Pumping Systems Handbook and SWPA training ( SWPA has a wealth of material and guidance available that has been developed from the foremost experts in the industry for all aspects of the submersible motor pumping system.
How important is motor sizing for submersible applications? What considerations should users turn to as a general guideline for selecting the right motor?
As a duty specific product, the good news is that the submersible motor and pump are designed, tested and validated as a combined unit. As such, most of the sizing decisions are made by the pump manufacturer. These units are available worldwide at all voltage and frequency power supply configurations. From an application standpoint, the main focus should be understanding the pumping system requirements and characteristic and then do a thorough job of communicating these needs to the manufacturer’s sales representative. ◆
For More Information:
The Submersible Wastewater Pump Association (SWPA) is a national trade association representing and serving the manufacturers of submersible pumps for municipal and industrial wastewater applications. Founded in 1976, the association’s primary focus is on industry guidelines, education and promotion. For more information, contact SWPA headquarters at 847.681.1868 or visit
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