The science of pumping

by Jenny Lescohier
Created: September 26, 2007

Understanding what it takes to move water can help you to better help your customers.

To start with, atmospheric pressure on Earth at sea level is 14.7 psi (one bar, or one barometric unit). The pressure becomes lower as altitude increases and higher in areas lower than sea level. With (distilled) water weighing 8.33 pounds per gallon, atmospheric pressure will force that water to rise 33.9 feet in a cylinder at sea level if all the air in the cylinder is evacuated. Likewise, mercury (Hg) weighs 112 pounds per gallon and atmospheric pressure will force it to rise to 29.9 inches in a cylinder at sea level if all the air is removed. So, what does all this mean? Pumps often have an Hg rating and what this means, for example, is 1" Hg equals the ability to move 1.13 feet of water (basic rule of thumb is 1" = 1'). Therefore, a 20-inch Hg rating means the pump will lift approximately 20 feet of water.

Thompson says pumps are designed to do two things: move the liquid in and move it out. This is achieved through creating a difference in pressure between the suction and discharge sides of the pump. The pump must create low pressure on the suction side of the system. The combination of low pressure in the pump and atmospheric pressure results in positive suction head.

Likewise, the pump must create high pressure on the discharge side of the system. This results in water being thrown out of the pump in the case of variable displacement models, or being pushed out, as in the case of positive displacement pumps. Variable displacement means that at a given speed, the volume displaced varies with pressure. Positive displacement means that at a given speed, an equal volume is displaced per cycle, regardless of pressure.

Bottom line
According to John Michael Paz, president of Godwin Pumps of America, which operates 24 company-owned stores around the country and supplies pumps to rental businesses throughout North America, renting pumps is more than handing over a piece of machinery when the customer needs it. In a lot of cases, the customer's job is complicated and involves people's lives or livelihoods. In cases like this, your business is supplying a solution to a problem as well as the machine to move water. Having a knowledge and understanding of basic hydraulics and pump technology is helpful in making that happen.

Pump terminology
Head: Centrifugal pump curves show 'pressure' as head, which is the equivalent height of water with S.G. = 1. This makes allowance for specific gravity variations in the pressure to head conversion to cater for higher power requirements. Positive Displacement pumps use pressure (i.e., psi or kPa) and then multiply power requirements by the S.G.

Static Head: The vertical height difference from surface of water source to centerline of impeller is termed as static suction head or suction lift. ("Suction lift" can also mean total suction head.) The vertical height difference from centerline of impeller to discharge point is termed as discharge static head. The vertical height difference from surface of water source to discharge point is termed as total static head.

Total Head/Total Dynamic Head: Total height difference (total static head) plus friction losses & "demand" pressure from nozzles, etc. (i.e., Total Suction Head plus Total Discharge Head = Total Dynamic Head).

NPSH: Net Positive Suction Head is related to how much suction lift a pump can achieve by creating a partial vacuum. Atmospheric pressure then pushes liquid into the pump. This is a method of calculating if the pump will work or not.

Friction Loss: The amount of pressure/head required to "force" liquid through pipe and fittings.

Flow: A measure of the liquid volume capacity of a pump. Given in gallons per hour (GPH), gallons per minute (GPM), liters per minute (L/min), or milliliters per minute (mL/min).

Pressure: The force exerted on the walls of a tank, pipe, etc., by a liquid. Normally measured in pounds per square inch (psi).

Prime: Charge of liquid required to begin pumping action when liquid source is lower than pump. Held in pump by a foot valve on the intake line or by a valve or chamber within the pump.

Self-Priming: A type of pump that requires the casing to be filled with water before starting.

Dry-Priming: A type of pump that does not need water to start. This pump can operate "dry."

Information provided by Thompson Pump & Manufacturing.