Lesson 5:
Suction Lift (Fire Pumps)

 

 

Objective

In this lesson we will learn about the concept of suction lift. This concept is most easily understood, by the example of a fire pump placed into a stream in rural areas to pump water to fight fires. This is a submersible type of pump. We also will discuss the use of a “suction lift” set-up for pumping chemicals from a barrel into the system.

 

Reading Assignment

Read the online lesson as well as the supplemental material found at here.

 

Lecture

Introduction

Suction lift deals with the maximum distance to the intake of a pump. Fire pumps and others may lift about 5' to 10' of suction. You must lower the pump continually towards the water to keep them pumping. This creates a water risk, and when they put it back in, it pumps for a while, and if it quits again, then the same process must be repeated until it is pumping properly. Pumps operating at a negative minimum inlet pressure are capable of creating a suction lift (non-self-priming). The suction capacity is approximately equal to the level of the negative minimum inlet pressure minus a 1 m safety factor.

 

 

Suction Limitations

Regardless of the extent of the vacuum, water can only be “lifted” a set distance or height due to it’s' vaporization pressure. As the pressure above the water is reduced, the water will tend to rise as a result of the atmospheric pressure, which is tending to push the water into the pump suction piping. The theoretical maximum suction lift for water is 33.9 feet. From a practical standpoint, in consideration of the friction loss of the piping, the altitude of the station, etc., the normal maximum lift for any pump is approximately 25 ft. However, it must be remembered that cavitation of the impeller increases as the suction lift increases , and therefore, the pump, where possible, should be located so that the suction line is submerged at all times.

Pumps lift water with the help of atmospheric pressure, then pressurize and discharge the water from the casing. The practical suction lift, at sea level is 25 feet. Most pump manufacturers will list this as the maximum suction lift. Static suction lift is the maximum distance from the water level, to the centerline of the impeller. The main type of pump used for suction lift is a vertical shaft turbine pump.

Suction lift exists when a liquid is taken from an open tank to an atmospheric tank where the liquid level is below the centerline of the pump suction.

The following relationships may help to better understand Suction Lift:

Total Dynamic Head = Total discharge head + Total Suction Lift

Total suction lift = static + friction

Study the following diagram of Suction Lift Installation , because it is a very common “set-up” used in water or wastewater plants to feed a liquid chemical from a barrel into the treatment process.

 

 

 

Study this suction lift calculation method to help assist with installation:

Pump Performance

The performance of a pump is measured in volume as gallons per minute (GPM) and in pressure as head. A trade off generally occurs between head and flow. The head will increase causing a decrease in flow and vice versa.

To define head, it refers to gains or losses in pressure caused by gravity and friction as water moves through the system. It is most commonly expressed as feet of water, but can be measured in lb/in² (PSI).

To illustrate this, consider a pump is rated with a maximum head of 90 feet. This pump must produce 1 PSI to push the water vertically 2.31 feet. To provide the maximum pressure capability of the pump, divide the maximum head rating by 2.31.

90 (ft/head) ÷ 2.31 (ft/head) = 38.96 PSI

You can also multiply 2.31 by the maximum pressure capability of the pump to provide the maximum head rating.

2.31 ft/head ÷ 38.96 PSI = 90 ft/head

Suction lift and head may be referred to as static or dynamic, depending on how the measurement was taken. When the measurement does not account for the friction caused by water moving through the pipe this is considered static. When the friction losses are factored in, this is known as dynamic.

The following terms are used when referring to lift or head:

Static Suction Lift: The vertical distance from the water line to the centerline of the impeller.

Static Discharge Head: The vertical distance from the discharge outlet to the point of discharge or liquid level when discharging into the bottom of a water tank.

Dynamic Suction Head: The Static Suction Lift plus the friction in the suction line. Also referred to as a Total Suction Head.

Dynamic Discharge Head: The Static Discharge Head plus the friction in the discharge line. Also referred to as Total Discharge Head.

Total Dynamic Head: The Dynamic Suction Head plus the Dynamic Discharge Head. Also referred to as Total Head.

 

Suction Lift Chart

Suction lift is the vertical distance from the pump to the surface of the water.

 

Review

In this lesson we learned about the concept of “suction lift.” The most basic example of this would be in rural areas where there are no fire hydrants, and submersible pumps are placed into a creek or stream to pump water out into a fire pumper truck to help fight a fire. Also, “Suction Lift” is a common set-up used when small amounts of chemicals are needed and can easily be fed into the treatment process, by placing a pump on top of the barrel with a tube placed into the liquid in the barrel to pump out the chemical and add it into the treatment process.

The maximum suction lift at sea level is approximately 25 feet. The static suction lift is the vertical distance from the water level to the centerline of the pump impeller. The vertical shaft turbine pump is most often used to accomplish “suction lift.” We showed a diagram to help with the set-up for suction lift, a very common process.

We discussed the relationships of Total Dynamic Head = Total discharge head + total suction lift, and that total suction lift = static + friction.

 

 

Sources

Pump Terminology For Beginners - Contractors Depot, Fla.

Water Distribution Operator Training Handbook - AWWA

LMI - Milton Roy Pump Company - Customer Support Drawings

Pump World - Suction Lift

Pumps-in-Stock - Pump Terminology

 

 

Assignment

Answer the following questions and either mail or fax to the instructor.

Label the Diagram below

 

Choices:

Solution Drum
Flow
Injection Check Valve
Tubing Straightener Assembly
Flow
Anti-Syphon Pressure Relief Valve
2 inch Space for Insolubles to Settle
LMI Pump

1. Cavitation of the impeller increases as the suction lift __________________.
2. Why is the location of the pump so important?
3. Should the suction line be positioned all the way to the bottom of the barrel or drum?  Why?
4. What is the maximum lift for most pumps (at sea level)?
5. What type of pump is used for "suction lift"?

 

 

Quiz

Answer the questions in Quiz 5 . When you have completed the quiz, print it out and either mail or fax to the instructor, Harriett Long. You may also take the quiz online and directly submit it into the database for a grade.