To size a pump, you must define:
- The flow rate of liquid the pump is required to deliver
- The total head of a system that the pump must generate to deliver the required flow rate
- The duty point in order to obtain the impeller size and the HP of the motor.
The total head of a system (Step by Step A-B-C)
- Static Head (HS): Static head represents the net change in height, in feet, that the pump must overcome.
- Friction Head: When fluid flows through any system, friction is caused by resistance in the piping, fittings and valves called friction head. This is also called pressure drop
- Pressure Head: When liquid is pumped from a vessel at one pressure to a vessel at another pressure, pressure head exists.
Use of Hoses in pumps suction and discharge
Most food and beverages industries uses hoses in their transfer process.
Many people don't take into consideration the "friction loss" due surface roughness on the inside of the hose. longer hoses, coils and curves increase friction loss and "reduce the pump performance".
- Hose Friction Loss is considered as a "major loss"
- If a suction hose is used in lieu of hard sanitary tubing, the hose must be non collapsing, largest diameter, short and straight as possible.
- If a discharge hose is used instead sanitary tubing minimize the number curves, coils and length.
One pump is drawing water from an open tank to another open tank 7 feet above the pump at 50 GPM.
Suction and discharge tube diameter is 1 ½”.
Find the total head under these conditions:
A) The static head is 6 ft. – 1 ft.||= 5.00 ft.|
|B) Friction loss from straight tube, valves and fittings:|
|1ft. + 2 ft. + 7 ft. + 2 ft. + 192 ft. = 204 lineal ft. X 0.25 ft./ft.||= 51.00 ft.|
|Friction loss from butterfly valve : 2.32 ft. X 2||= 4.64 ft.|
|Friction loss from elbows :0.23 ft. X 6||= 1.38 ft.|
|= 57.00 ft.|
|C) Pressure head||= 0 ft.|
Total Head ((A)5 +(B) 57 +(C)0)||=62.00 ft.|
Why “head” instead psi?
The main reason for using "head" instead of pressure (psi) to measure a centrifugal pump's energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes.
How can I convert feet to psi?
For example a Sugar syrup @ 60 Brix, have an specific gravity of 1.29
Convert 90 feet of head to psi :
90 feet x 0.433 x 1.29 = 50 psi
The pressure of a column of water is about 0.433 psi per foot of column height.
Why is the discharge pipe diameter of a centrifugal pump smaller than the suction pipe?
A centrifugal pump apply centrifugal force to generate velocity this force converts the energy by accelerating the fluid between suction and discharge, the outlet has a higher velocity due the small area of the tri-clamp compared with the Suction area, but the flow is the same.
Suction lines are generally designed to keep friction losses to a minimum and the discharge size is determined by the head generated by the friction losses and cost considerations.
Is the weight of liquid in comparison to water at approximately 68°F (SG =1).
Is the pressure exerted by a vertical column of water inside of a well casing or a pipe, measured at the bottom of the column.
The pressure of a column of water is about 0.433 psi per foot of column height
Continuing with our example:
We need to know the “duty point”
The point on the curve where the flow and head match the application's requirement is known as the duty point.
A centrifugal pump performance curve is a tool that shows how a pump will perform in terms of head and flow. Pumps can generate high volume flow rates when pumping against low pressure head or low volume flow rates when pumping against high-pressure head. The possible combinations of total pressure and volume flow rate for a specific pump can be plotted to create a pump curve. The curve defines the range of possible operating conditions for the pump. Pump curves plot data on a graph with x and y axes. The x axis (vertical) shows total head while the y axis (horizontal) shows flow capacity, typically GPM.
Flow =50 GPM
Total Head = 62 feet
To determine duty point:
A: Find the intersection point of 50 GPM on the bottom of the graph and 62 feet on the side of head in the vertical scale (Head in Feet)
B: This intersection falls between the 3.75 and 4.0 impeller curve lines. Choose the curve line above the duty point (4.0 inch impeller size).
C. To determine NPSHR (Net positive suction Head required):
Use the NPSHR graph and plot the intersection point of 50 GPM. Follow the horizontally to the left. It reads 6'. (This will be Net positive suction Head required.)
The centrifugal pump that we need for our application is:
Motor HP = 1 1/2 HP Impeller Size = 4”
Note: NPSHA (Net positive suction Available) must be > or = NPSHR
Is the measure of the pressure or force exerted by water expressed in feet. Centrifugal pump curves show pressure as head, which is the equivalent height of water with specific gravity = 1.
Is the vertical height difference from the surface of a water source to the center-line of the impeller.
The vertical height difference from the surface of the water source to the discharge point is known as total static head.
Total Head / Total Dynamic Head:
Is the total height difference (total static head) plus friction losses and demand pressure from nozzles etc. (total discharge head) = total dynamic head.
The amount of fluid the pump will move is determined mainly by the width of the impeller and the shaft speed. Capacity is normally measured in gallons per minute (gpm.)
As the capacity increases, the total head which the pump is capable of developing decreases.
BHP (Brake Horsepower) and Capacity
BHP Is the total power required by a pump to do a specified amount of work.
Generally, the HP increases as we increase the capacity.
BHP = Q x H x Sp.Gr / 3960 x Efficiency
Q = Capacity delivered by the pump
H = Head developed by the pump
Sp. Gr = Specific Gravity of liquid being pumped
BHP = Horsepower required by the pump
3960 is a constant related to the Horse power (33,000 ft lbs / min) with 8.33 Lbs./gallon of water
33000 / 8.333 ( Water density) = 3960
NPSH (Net positive Suction Head)
Net Positive Suction Head is an important element in the proper selection of a centrifugal pump.
The NPSH required is the head of the liquid required at the pump suction nozzle above the vapor pressure of the liquid at that point.
Net Positive Suction Head is directly related problematic phenomenon known as cavitation. Liquid cavitation has a deleterious effect on a pump’s internal parts and as a result reduces the pump’s efficiency, performance, and ultimately, reliability.
Cavities or voids in liquid. Bubbles take up space leading to a drop in pump capacity. Collapsing
bubbles can damage the impeller and volute, making cavitation a problem for both the pump and the mechanical seal.
Is the force exerted by the gas released by a liquid in a closed space. If the vapor pressure of a liquid is greater than the surrounding air pressure, the liquid will boil.
Is the measure of a liquid’s resistance to flow (i.e., how thick it is). The viscosity determines the type of pump used, how fast it can run, and with gear pumps, the internal clearances required.
Capacity, head and efficiency will decrease at higher viscosity.
Required power will increase at higher viscosity.
Is the amount of pressure / head required to force liquid through pipes and fittings
Is the moving element in a pump that drives the liquid.
Is the spiral-shaped casing surrounding a pump impeller that collects the liquid discharged by the impeller.