Building Principles: A “Primer” on Pumps

Richard "Rick" Stryker, PE

This month, we’re going to address where pumps work for you at camp, matching the equipment to the application, and some simple troubleshooting steps to get the most out of your pumps.

Pumps move all sorts of fluids around camp: chlorine to the pool, wastewater from here to there, and water out of the well. Despite their similarities, each of the different fluids requires very different considerations. For starters, the physical characteristics of the fluid make a huge difference in what pump arrangement is “right” for any particular job. Water (and as we’ll see shortly, wastewater, too) moves through pipes differently than the chlorine injected into the pool water. Muddy water contains sandy grit, which is abrasive to pump components. Wastewater solids must pass through pumps and pipes without clogging. Those are just a few of the physical attributes of the fluids. Chemically, chlorine, groundwater, and wastewater are all different from one another — requiring that those pumps be manufactured from materials that aren’t subject to corrosion and deterioration.

Even though wastewater contains solids, it is mostly (about 94 percent) water by volume. When ground into slurry, wastewater moves through pipes just like water does. You can imagine though, that wastewater pumps may be expected to perform two functions at the same time: grinding AND pumping. As wastewater is reduced by bacteria, it releases corrosive and caustic compounds and gasses. On top of that, gravity sewer collection systems are seldom truly water tight, meaning that groundwater entering the system picks up bits of gravel, sand, and soil as it enters (or “infiltrates”) the underground pipes. So though they perform hydraulically just like any other water-moving pump, wastewater pumps must be made of materials that are very resistant to abrasion (for the grit), corrosion (for the hydrogen sulfide), and clogging (for incompletely ground stringy material). The physical arrangement of the components is arranged specifically to account for these and many other criteria.

Contrast this with a well pump. Well pumps aren’t designed to deal with any of the troubles of a sewage pump, even though they also move water. Sandy groundwater will literally shred a well pump’s guts. At the same time, well pumps lift water from deep, deep underground, far deeper than any sewage pump station. They are closely machined to very tight tolerances, allowing them to move more grit-free water to greater heights (pressures), at the same horsepower as their sewage counterparts.

Next, we’ll talk about the workhorses around the construction site. They go by a bunch of different names, but common ones are “mud hog” and “trash pump.” These are used to extract muddy water from excavations in order to construct foundations or other below-grade structures. To work reliably, these combine several of the different features of both sewage and well pumps, including resistance to abrasion and moving large volumes of water for a long period of time.

And finally, we shouldn’t miss talking about one more type of pump that keeps camp running — the chemical feed pump. Usually used to inject a chlorine solution into the pool, drinking water, or wastewater, these fall broadly into two different categories: the positive displacement and the peristaltic. Instead of having internal parts that spin like a fan blade, the positive displacement pump has a motor attached to a syringe. On the back stroke, it sucks the chlorine liquid from a reservoir. On the forward stroke, it squirts the fluid into a pipe containing the water to be disinfected. A peristaltic pump has a set of rollers that squeeze a tube that contains the pumped chlorine. Much like massaging toothpaste from the bottom of the tube, these rollers push the chlorine downstream from outside the flexible tube. The rollers never come into contact with the chlorine, and so aren’t subjected to the harsh chemical. Lots of operators prefer peristaltic over positive displacement pumps because of that.

Since most people’s experience with pumps involves only water, they take certain things for granted. Like, what does it mean to “prime the pump,” and “why don’t all pumps need to be primed”? To explain the answers, let’s look just at the mud hog. Because they can move all kinds of gritty mud, their internal parts don’t operate as close to each other as, say, a well pump. The gap between the fan part (called the “impeller”) and the body of the pump (called the “volute”) is wide enough that air can slip through but water can’t. In priming, water is dumped into the pump intake to fill that space. This gives the pump something to move and creates suction at the intake, drawing the muddy water up like a drinking straw. Once there’s a continuous, air-free stream of water from the source through the intake, the pump can push a lot of mud along the discharge pipe. If you allow the intake to break the surface of the puddle being pumped out or if there’s a crack in an intake (either of which will allow air in), the pump will lose suction and stop working. Some pumps are self-priming.

Well pumps and submersible sewage pumps are installed completely beneath the water surface, and are primed all the time by default. Other types of sewage pumps (called “dry well mounted”) have their intake far below the pump itself. This allows for maintenance without entering the wet-well, but it also requires that the pump be able to draw a vacuum sufficient to draw up the liquid until it gets to the pump — hence the term “self-priming.” An interesting engineering fun-fact about self-priming pumps is under the most perfect conditions, no pump can draw water higher than 22.7 feet (at sea level). That’s because the vacuum (very low pressure) necessary to get the water any higher causes the water to turn into water vapor, like steam.

And as we’ve seen, pumps intended to move water have to move liquid water — they cannot prime with water vapor and so will not work. The same principle is true for mud hogs mounted above the water level and not submerged in the puddle they’re draining. So here’s the first troubleshooting hint: If your pump won’t hold prime, try lowering it closer to the water level from which you’re pumping. Demanding less intake vacuum pressure makes the water less apt to vaporize and helps keep the pump primed.

Priming and vapor pressures bring us to another troubleshooting idea. Chlorine solution has different physical properties than water, so certain aspects of pumping that are often ignored when pumping water become critical in this application. Because concentrated chlorine solutions are heavier and thicker than water, positive displacement pumps moving chlorine need all the help they can get to stay primed. The tubing supplied includes a funny-looking basket arrangement at one end. More than just a strainer, that basket contains a small ball bearing, which seals the intake line when the pump is on the forward stroke. This is a simple stopping arrangement called a “check valve.” In addition to using the pressure of the pushing piston, it relies on gravity to seat itself against the lower end of the intake tube. So unless it’s hanging STRAIGHT DOWN in the feed tank, it won’t seal properly and the pump cannot prime itself.

All of this is no big deal until you learn that new pumps are usually supplied with five or six feet of intake tubing, but the solution tanks are seldom more than four feet deep. Not knowing any better, many operators simply connect the furnished length of tube and allow it to curl around the bottom of the tank. Not only does the system attempt to suck in all of the precipitates in the bottom of the tank (almost guaranteeing a clog shortly), but it won’t draw properly because the check valve won’t seat!

What are the lessons to be learned here? First, READ THE INSTRUCTIONS. Without exception, the pump manufacturer’s instructions will tell the installer that the tubing MUST be cut to fit the geometry in place and that the tubing MUST hang straight down. No kinks. No curls. No bends. One trick I’ve seen is to let the tube hang in the sun to take the curl out of it, which remains after shipping.

Another bit of trouble that operators get into is not maintaining chemical feed pumps. Whether you have a peristaltic or a positive displacement arrangement, they need care and replacement parts regularly. Think “annually.” Yes. I know that your water system or pool only runs a couple of months each year. Again, sodium hypochlorite is highly caustic, and that deteriorates the parts inside the pump whether you’re pumping or not. At the very least, the pump should be thoroughly flushed with clear water at the end of each season, but even that’s no guarantee that chemical attack has stopped. This is one of the big benefits of using a peristaltic pump, since replacing a short piece of tubing is much simpler than tearing a diaphragm, piston, and intake apart to rebuild it.

And while we’re at it, let’s talk about repair kits. The pump in your well house feeds chlorine solution, but that very same pump can be configured to pump hundreds of different chemicals. The only difference is the materials that will contact the chemical being pumped. It’s absolutely critical to install a repair kit that is intended to withstand the chemical to be pumped. It will be supplied by the pump manufacturer and will be confirmed to withstand the attack of what you’re planning to pump. This means that despite being identical (appearing) pumps, you cannot simply use a rebuild kit intended for the unit at the pool (feeding chlorine) on the one at the wastewater plant to feed magnesium hydroxide.

All of this may sound overwhelming and confusing. But it doesn’t have to be. Each and every pump you buy comes with detailed instructions that explain all of these considerations and more. You paid for everything that goes with the pump, including the instructions. Don’t treat them like more packing material! Use them, keep them handy, and refer to them when the pump is installed, operated, and maintained. For example, the enclosed pump literature always has a place to write important information about the pump itself, including the serial number, electrical information, the date, price, and supply source. Once the pump’s been installed, getting information from the data plate will be darn near impossible, and warranty claims aren’t going to happen without it. But I seldom, if ever, see the blanks filled out on the manual. Many times, they’re still sealed in the plastic envelope in which they were shipped, which tells me that the pump was installed without reading the instructions.

Pumps are neither as simple as they appear nor hopelessly complex. Rely on experts in the field for help choosing equipment that matches the job, maintain it regularly, and follow the instructions. Now you’re on the right track toward long, efficient lives from your pumps at camp!

Rick Stryker is a professional engineer with a particular passion for helping camps with infrastructure, planning, and regulatory issues. He can always be reached at campfc@ptd.net or 570.828.4004.

Originally published in the 2012 July/August Camping Magazine. 
 

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