Buying a water pump can be downright confusing. Instead of simply buying by engine size or flow rate, there are several factors to consider, including pressure, head and water type. What do all these terms mean, and what do you need to look for to get the right pump for your needs?
What is Head, and How Does it Affect Performance?
Head is the pump’s maximum pumping height. Static head is the theoretical maximum height a pump can move water. Dynamic head takes friction into account, compensating for losses from hoses, fittings, strainers and other parts connected to the pump. Dynamic head figures that are quoted by manufacturers are based on hoses and other parts included with that model. If you add accessories or use different hoses, it will affect performance.
Head can be split into suction head, the height between the body of water being drained and pump, and discharge head, the height difference between the pump and the end of the discharge hose. If you drop a hose in the water, it fills up to the body of water’s surface level. That means suction head is measured from the water surface to the pump, since that’s where it has to start pumping. The discharge head is the height difference between the pump and the height of the discharge hose.
Pump pressure is measured in pounds per square inch (PSI). You can calculate pressure by multiplying the total head in feet by 0.433. If you have a total head of 10 feet, the pump is generating 4.333 PSI.
You may be surprised to see that most pumps have a maximum total head of 26 feet. That’s because centrifugal pumps depend on atmospheric pressure to help pull water through the pump. Since air is less dense at higher altitudes, there’s less atmospheric pressure. On average, the pump’s maximum head decreases by two feet for every 1,000 foot increase in elevation. Positive displacement pumps use a sealed chamber with a piston to move water. These aren’t affected by atmospheric pressure, but they’re also less efficient.
What’s the Difference Between Standard, Self-Priming and Trash Pumps?
A standard pump is only built to move water. Water must be added to the volute case before starting, or it won’t be able to draw water through the hose.
A self-priming pump isn’t just water tight, it’s also air tight. This lets it pump air through the hoses until water reaches the volute case.
A trash pump has a heavy-duty pump seal and extra space between the impeller and volute case. This lets rocks and other debris pass through without damaging the pump.
Cavitation and Water Hammering
Pressure increases suddenly as water moves between the suction and discharge sides of the pump’s volute case. This is balanced out by using a large intake and matching hose. However, if the water flow is out of balance, it can cause cavitation. This creates vacuum bubbles that exert massive amounts of force on the impeller, splitting it apart. Cavitation is usually caused by using hoses that are too small.
If the discharge hose is blocked, water flow stops, increasing pressure through the hose up to the impeller. The resulting pressure wave is called “water hammer.” This wave can damage the volute case and seals.
Where Can I Get a Quality Water Pump?
Shank’s Lawn Equipment carries the full line of Honda Power Equipment pumps, including everything from 50cc watering pumps to large industrial trash pumps. We service everything we sell, and we carry all the OEM parts and accessories you need for your equipment. Visit us at 4900 Molly Pitcher Highway in Chambersburg, PA. You can also order parts and accessories for your pump by visiting our website, www.shankslawn.com. We ship across the United States and Canada.
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