The most common pump design available, centrifugal pump, uses centrifugal force to discharge fluid into a pipe. Coupled to the engine's crankshaft, a rotating disk with vanes slings liquid outward, producing a pressure change that causes liquid to flow through the pump. This mechanism, called an impeller, is housed within a volute. The volute collects and directs the flow of liquid from the impeller and increases the pressure of the high-velocity water flowing from the vanes of the impeller.
Engineers can and have tweaked this basic design several ways. But ultimately, how much and how fast a pump moves liquid — referred to as “head” (see glossary) — also depends on how well you understand a few basic scientific principles.
Atmospheric pressure is what enables a pump to lift water. Mother Nature plays an important role by exerting 14.7 psi on any body of water at sea level. This limits the suction head of any pump to 33.9 feet, but only if perfect vacuum occurs in the pump. Thus, suction head is limited to about 26 feet.
In addition, engine performance decreases by approximately 3.5% per 1,000 feet of elevation. The engine may be rejetted so that it can operate optimally at high altitudes. This process involves altering the air/fuel mixture and usually overcomes any altitude-related losses in discharge capacity or suction head.
A liquid moving through a discharge hose or pipe creates heat due to the friction of the two surfaces: water against the inner wall of the hose. As length increases and/or diameter decreases, the increase in friction slows the flow of liquid and decreases discharge capacity.
Minimize “friction loss” by using the largest and shortest hoses possible for your application. Steel pipe produces more friction than smooth PVC or vinyl, so if steel is better suited for the liquid you're pumping, you'll want large-diameter hoses that are shorter in length.
Friction losses are included in dynamic discharge head and dynamic suction (see the glossary). Combining the two determines a pump's total head.
DEAD-HEAD Stopping the flow out of the pump without shutting off the engine. Centrifugal pumps should be dead-headed for no more than about five minutes to increase the pump's maximum rated pressure. Dead-heading for an extended period of time will cause the water or liquid in the pump to eventually heat up and cause damage to the mechanical seal. Never dead-head a positive displacement pump.
DYNAMIC DISCHARGE HEAD The static discharge head (see static discharge head entry) plus the additional discharge head created by friction or resistance — usually referred to as losses — from the liquid flowing through the hoses, fittings, sprinklers, nozzle, etc.
DYNAMIC SUCTION HEAD The static suction head (see static suction head entry) plus the additional suction head created by friction from the liquid flowing through the hoses, fittings, etc.
HEAD Height of a column of water that can be supported by the pressure or vacuum exerted at the pump.
MECHANICAL SEAL A spring-loaded seal, consisting of several parts, that seals the rotating impeller in the pump case and prevents water from leaking into and damaging the engine. Mechanical seals are subject to wear when pumping water containing abrasives, and will quickly overheat if the pump is run without filling the pump chamber with water before starting the engine.
PRESSURE Force per unit area, usually listed in psi. Pressure often is included in pump performance curves. Pressure and head are directly related when referring to pump performance. The pressure exerted (in psi) at the base of a column of water is 0.433 x head (in feet). If you attach a pressure gauge at the base of a pipe measuring 100 feet tall filled with clear water, you would measure 43.3 psi. The maximum pressure (at zero discharge) of any pump can be determined by multiplying the maximum head by 0.433.
SELF-PRIMING Most centrifugal pumps require the pump casing to be filled with water before starting. Self-priming pumps can purge air from the case and create a partial vacuum, allowing water to begin flowing through the suction hose.
STATIC DISCHARGE HEAD The vertical distance between the pump's discharge port and the point of discharge, which is the liquid surface if the hose is submerged or pumping into the bottom of a tank.
STATIC SUCTION HEAD The vertical distance between the pump impeller and the surface of the liquid on the suction side of the pump.
WATER HAMMER Energy transmitted back to the pump due to the sudden stoppage of water flowing from the pump. More likely to occur when using a long discharge hose. If the water flow at the end of the discharge hose is shut off in less than the critical time, energy is transmitted back to the pump causing a large pressure spike in the pump housing. Often results in damage to the pump casing.
Mother Nature also plays a role in how high water can be pushed.
At approximately 8.3 ppg, water is heavy. It wants to flow back down to its original source. Pump pressure is thus greatest when the unit is operated close to the water's surface.
As liquid level falls, the pump has to shoot farther and pressure drops. To maintain the speed of the liquid flow, you'll be tempted to decrease suction head. Don't. Keep it to the smallest value possible to reduce the likelihood of damaging the pump via cavitation.
|KEEP YOUR PUMP IN TOP SHAPE
When the surface pressure on a liquid becomes low enough, the liquid will begin to boil (even at room temperature). With centrifugal pumps, this can occur when the suction vacuum becomes great enough to allow water vapor or bubbles to begin forming at the impeller. Cavitation is the sudden formation and collapse of low-pressure vapor bubbles across the vanes of the impeller.
When this water vapor travels through the rapid-pressure increase across the impeller, a large amount of energy is released that can cause impeller damage. Minimizing suction head and using the largest practical suction hose diameter reduces the likelihood of cavitation. Also, never use a suction hose with a diameter smaller than the pump's suction port.
— Eric Queen is the product planner for Honda pumps within American Honda's Power Equipment division (www.pumps.honda.com), the U.S.-based sales and marketing arm for Honda power products, engines, and outboard marine engines.
|PRIMING BEST PRACTICES
A few pump selection tips
Determining what material will be pumped - i.e., chemicals, clean water, trash water, sewage - will lead you to the type of pump you need:
- Most general-purpose dewatering pumps are for moving relatively clean water.
- Construction, or trash, pumps are ideal for pumping water contaminated with sticks, leaves, stones, and other waste materials.
- Multipurpose pumps move water as well a variety of agricultural and industrial chemicals.
- Finally, submersible pumps are used for a wide range of residential and commercial sump applications.
When specifying a pump, you'll need to evaluate your jobsite to determine:
The vertical distance from the top of the source of the liquid being pumped to the highest point of the discharge hose to determine the total head lift. The length of hoses or pipes will vary by application.
- Whether or not a nozzle or sprinklers will be used . Nozzles and sprinklers are traditionally only used with a high-pressure pump, for irrigation or fire suppression.
- How much discharge volume is needed . The speed of seepage and volume of water or other material being pumped is the determining factor in choosing between a lightweight general-purpose, high-pressure multipurpose, trash, or diaphragm pump. Also important to know during pump selection is the inverse relation between discharge volume, or gallons per minute (gpm), and pound per square inch (psi).
Finally, operating and fuel costsWhile initial cost is important when choosing between pumps, you should also consider operating costs. Selecting the most fuel-efficient and reliable pump for your application will reduce overall operating costs by keeping fuel costs low and keeping the tool dependable and in operation. Time is money, and machine downtime due to excessive repair equates directly to a loss in productivity.
Concerning fuel efficiency, be sure to compare running time and tank size among models. Buyers should look for a durable, quality pump that has an excellent track record for reliability. Also, selecting a pump that is backed by a good warranty and dealer network support is also vital for the user's long-term product satisfaction.
What total head meansHead pressure describes how powerful a pump is. Although many manufacturers provide a head-capacity performance curve for their pumps, it can be difficult to predict the exact point on the curve where the pump will perform. Typically, “total static head” is considered when selecting a pump. However, this method can be unreliable due to frictional losses. In many cases the pump performance will not meet expectations.
Static suction head is the vertical distance between the pump impeller to the surface of the liquid on the suction side of the pump. Static discharge head is the vertical distance between the pump's discharge port and the highest point of the discharge hose, which is the liquid surface if the hose is submerged or pumping into the bottom of a tank.
The selection process becomes even more complicated when a nozzle or sprinklers are used. To accurately predict the performance of a centrifugal pump in a specific application, the total dynamic head - the sum of the static suction head, static discharge head, and all additional losses in the system - must be calculated. These losses include, but are not limited to, friction losses due to pipe size, length, and material, and losses from sprinklers or a nozzle.
Simply put, the total dynamic head is the actual head on the pump during operation. Your dealer can help you calculate the total head you need. For instance, Honda Power Equipment provides customers with software that calculates the pump performance. To use this software, you'll need to provide the info you've gathered from your site evaluation. (See “A few pump selection tips”) PW