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Picking the Right Pump

11 Questions To Ask Before Choosing A Pump

You need a pump, and you need one quickly.  Now what?  We offer the following tips to make sure you get the most for your investment.

The Starting Point

Determining what materials need to be pumped—e.g., clear water, chemical or dirty water—is the starting point for choosing the right model pump for the application. Most pumps fall into one of these categories:

  • General Purpose/De-watering Pumps

  • Construction/Trash Pumps

  • Multi-Purpose Pumps

  • Submersible Pumps

Most general purpose/de-watering pumps are for moving relatively clear water. Construction—or trash—pumps are used for pumping water contaminated with sticks, leaves, stones and other high solid content. The solids-handling capability of trash pumps allows large-hole-size strainers that are less prone to clogging, to be used.  Multi-purpose pumps move water as well as a variety of approved agricultural and industrial chemicals. Finally, submersible pumps are used for a wide range of residential and commercial sump applications.

Consumers should evaluate the site where the pump will be operated. Factors to consider in the evaluation include: the vertical distance from the surface of the liquid being pumped to the highest point of the discharge hose, the length and material of the hose or pipe, whether a nozzle or sprinklers will be used, and how much discharge volume is needed. Higher elevations also can be a factor in limiting pump performance.

All pumps use basic forces of nature to move a liquid. As the moving pump part (impeller, vane, pistons, diaphragm, etc.) begins to move, air is pushed out of the way. The movement of air creates a partial vacuum (low pressure) which can be filled up by more air or, in the case of water pumps, water. This is similar to sucking on a straw where the mouth creates a partial vacuum.  The liquid is pushed up the straw because of the pressure differences between the inside of the mouth and the atmosphere.

It’s important to keep in mind that engine performance of the operating pump decreases as elevation increases. The higher the elevation, the less air there is available to support combustion. Maximum engine power decreases approximately 3.5 percent per 1,000 feet of elevation gain and, in certain instances, can result in reduced discharge capacity. Additionally, the maximum available suction head will be reduced at higher elevations.

The following questions and answers serve as guidelines for the proper pump selection.

1. What is “Total Head” and how is it determined?

The best predictor of the performance of a centrifugal pump in a specific application is the total dynamic head (or total head), which is the sum of the static suction head, static discharge head, and all additional losses in the system. Losses that should be calculated include, but are not limited to, friction losses due to pipe size, length and material, and losses from sprinklers or a nozzle. The total dynamic head is the actual head on the pump during operation.

Selecting the proper pump can be a challenge. Pump manufacturers typically calculate performance curves using a vacuum gauge on the suction port and a pressure gauge and flow meter connected to the discharge port. For many different total head values, the corresponding discharge capacity is measured. A series of measured data points are then graphed and connected to create the performance curve.

Often times the user will only consider total static head when selecting a pump, but if frictional losses aren’t included in the calculations, it’s possible that pump performance will not meet expectations. The actual discharge performance may be significantly less than predicted by using static head alone due to friction losses in the system.

Performance curves are useful in selecting a particular water pump. When a question regarding the performance of a specific pump must be answered, refer to the pump specifications for the particular model.

Determine how high the pump will sit above the water surface (static suction head). Determine how high the discharge end will be elevated above the pump (static discharge head). Determine what the discharge capacity (gallons per minute) of the pump must be.

Given the total head (suction + discharge), the discharge capacity can be estimated by referring to the performance curve for the specific model of pump.

Pressure can be calculated for total head by multiplying total head by .433. Pressure available at the end of the hose at zero flow for a given total head (less than the maximum total head) can be calculated by multiplying the total head by .433 then subtracting it from the maximum pressure.

Example:
The maximum pressure for a WH20X is 64 psi (.433 x 148 total head in feet). The maximum available pressure at a total head of 100 feet is 64 – 43 = 21 psi at zero flow, where the 43 is determined by multiplying the total head, 100, by .433.

2. How much fluid needs to be pumped and how fast? Where is the discharge going?

Pump performance (capacity or pressure) is highest when the pump is operated close to the water’s surface. Increasing the suction head will decrease the total head.  (If the suction head increases [within the maximum suction head limitation], and the discharge head decreases by the same amount, the total head remains the same—so discharge capacity is not affected). Most important, suction head should be kept to the smallest value possible to reduce the likelihood of cavitation (the sudden formation and collapse of low-pressure vapor [bubbles] across the vanes of the impeller).

Mother Nature also plays an important role in how high water can be pushed. Water is heavy and tends to flow back down to its original source. The mechanical energy of the impeller transmits its force against the water coming in contact with it. This force can be measured in psi at the pump discharge. As the pump discharge head increases in height, the pump capacity (GPM) decreases, and the available pressure at the end of the discharge hose (if the flow is stopped or a sprinkler/nozzle is used) also will decrease. At maximum head, the capacity (GPM) will drop to zero, and there will be no pressure available at the end of the hose to run a sprinkler or nozzle.

3. What piping material is going to be used?

A liquid moving through a hose creates heat due to the friction of the two surfaces (water against hose). Steel pipe will produce more friction than smooth PVC or vinyl pipe. As the length of the discharge hose increases, the water comes into contact with more hose surface and the inner wall of the discharge hose (in contact with the rushing water) will cause friction to build up. The increase in friction will slow the water, decreasing the discharge capacity.

4. How long will the pump be running and will it be monitored constantly?

No matter the job at hand, it is important to purchase a pump that will be reliable day in and day out. Matching your pump spec’s to the job at hand is the best way to ensure longevity. It is also a good idea to monitor any pump you use for best results.

5. Are there any noise constraints on site?

Consumers should check local ordinances to determine if there are any restrictions on where tools or machines, such as pumps, may be used.

6. What are other items to look for when selecting and operating a pump?

  • Make sure you understand all the terminology and the basic pump features (see Pump Terminology list below).
  • Use the proper size hoses and fittings.
  • Use a strainer with the proper hole sizes. The holes must be equal to or smaller than the original strainer included with the pump.
  • Inspect hoses for leaks, weaknesses and kinks. Fittings should be checked to ensure proper suction.
  • Never let a centrifugal pump run dry; this action could damage some types of pumps and/or seals.
  • Make sure the pump has proper mounting or frame protection.
  • Check the engine’s oil or monitor the Oil Alert® indicator featured on some models.
  • Check individual manufacturer recommendations for approved chemical applications.
  • While initial cost is important, also consider operating cost and the lifecycle of the pump. When comparing fuel efficiency, be sure to compare running time and tank size among models.

7. What does self-priming mean?

Self-priming is a term that describes the ability of a pump to create a partial vacuum by purging air from the intake hose and pump casing. Self-priming pumps still require water to be added to the pump casing first to start the priming process.

8. What are good priming tips and practices?

  • Always be sure to use a strainer on the end of the suction hose—not using a strainer can result in catastrophic failure. And, if using a different strainer, make sure the holes in the strainer are the same size or smaller than the holes on the strainer included with the pump.  Place the pump as close to the water surface as possible. The less lift required reduces priming time.
  • Fill the pump case completely with water (never operate a centrifugal pump without water in the pump casing).
  • Start the pump engine.
  • Place the throttle lever in the fastest position to reduce priming time.
  • Make sure there are no air leaks in the suction hose or fittings.
  • Shutting off a pump will allow water to flow out of the suction hose. The pump contains a one-way flapper valve, so water will remain in the pump after shutting off. However, the suction hose will have to re-prime each time the pump is restarted.
  • The use of a foot valve on the end of the suction hose will prevent water from flowing out of the suction hose if the pump is stopped, reducing the time required for the pump to regain its prime. If you do use a foot valve, make sure it includes a strainer with holes equal to or smaller than the original strainer included with the pump.
  • Pump performance and increased time required to prime the pump can occur when the volute and impeller wear out. Regular inspection and maintenance of a pump will maintain peak performance.

9. What operational tips help keep pumps running in top shape?

As a rule of thumb, always follow the maintenance schedule in the owner’s manual.

  • Never operate a centrifugal pump dry, since water is needed to prevent the mechanical seal from overheating.
  • If possible, avoid pumping water containing abrasives.
  • Always drain the pump housing after use—allowing water to freeze in the pump will cause pump case failure.
  • When pumping salt water or water containing silt and mud, flush the pump housing with clean water after each use.
    • If you plan to pump salt water, be sure to choose a multipurpose or stainless submersible pump and not a pump with an aluminum housing.
  • Also, avoid driving over and collapsing the discharge hose when the pump is operating and/or the hose is full of water (damage can even occur with the pump off, if a nozzle is shut off and the discharge hose is full of water).
    • Doing so can force water and/or a shock wave (water hammer) back to the pump, causing damage to the pump.
    • If you must place the hose across a roadway, position boards on each side of the hose to prevent it from collapsing when a vehicle drives over it.

10. Are there routine steps an owner should take after operating a pump or when preparing a pump for storage?

After use, the operator should turn off the fuel valve and drain the pump case (flush if pumping salt water or water containing mud or silt). If the pump is going to be stored, refer to the storage procedure in the owner’s manual. It is especially important to add gasoline stabilizer and/or drain the carburetor to prevent fuel system damage due to deteriorated gasoline.

11. Can the flow out of the pump be stopped without shutting off the engine?

All centrifugal pumps can be deadheaded for a brief period (as a general rule, no more than about five minutes). During this period, the pump pressure will increase to the pump’s maximum rated pressure.  However, deadheading the pump 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 deadhead a positive displacement pump. This practice can cause severe damage to the pump.

Quick-Reference Chart on Choosing a Pump

Pump Terminology

Cavitation

The sudden formation and collapse of low-pressure vapor (bubbles) across the vanes of the impeller. When the surface pressure on a liquid becomes low enough, the liquid will begin to boil (even at room temperature). With centrifugal pumps, cavitation can occur when the suction vacuum becomes great enough to allow water vapor or bubbles to begin forming at the impeller. When this water vapor travels through the rapid pressure increase across the impeller, a large amount of energy is released which can cause impeller damage. Minimizing suction head and using the largest practical suction hose diameter will reduce the likelihood of cavitation. Pump operators should never use a suction hose with a diameter smaller than the pump’s suction port.

Centrifugal Pump

A pump that uses centrifugal force to discharge fluid into a pipe, typically by mechanical means such as a rotating impeller held within a volute and pump housing.

Diaphragm Pump

A pump that uses positive displacement to discharge a fluid into a pipe by means of a combination of a reciprocating diaphragm and check valve system.

Dynamic Discharge Head

The static discharge head 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 plus the additional suction head created by friction from the liquid flowng through the hoses, fittings, etc. Atmospheric pressure enables pumps to lift water. As a result, an atmospheric pressure of 14.7 psi at sea level limits practical dynamic suction head lift to less than approximately 26 feet for any pump (with the amount of head lift decreasing as altitude increases).

Friction Losses

The additional pressure or head created at the pump due to the friction of the liquid flowing through the hoses, pipes, fittings, etc. Friction losses always occur when a liquid is flowing through pipes and becomes greater as the length of pipe increases and/or the diameter decreases. Friction losses result in reduced pump output and can be minimized by using the largest and shortest hoses possible. Friction losses are included in dynamic suction and dynamic discharge head.

Head

Refers to the height of a column of water that can be supported by the pressure or vacuum exerted at the pump.

Impeller

An impeller is a rotating disk containing vanes coupled to the engine’s crankshaft. All centrifugal pumps contain an impeller. The impeller vanes sling liquid outward through centrifugal force, causing a pressure change. This pressure change results in liquid flowing through the pump.

Mechanical seal

This is 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 that contains abrasives. They will quickly overheat if the pump is run without filling the pump chamber with water before starting the engine. Also, deadheading the pump 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. 

Pressure

Pressure is force per unit area and is usually listed in psi (pounds per square inch). 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 is a term often used to describe pumps that have the ability to 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.

Total Head

The dynamic suction head plus the dynamic discharge head.

Volute

The volute is the stationary housing enclosing the impeller. 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.

Water Hammer

Water hammer is energy transmitted back to the pump due to the sudden stoppage of water flowing from the pump. Water hammer is more likely to occur when using a very long discharge hose. The most common cause of water hammer discharge damage is driving over the discharge hose when the pump is running. If the flow of water 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. Water hammer often results in damage to the pump casing. Water hammering can be avoided by (slowly) closing a valve located at the end of the discharge hose.

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