October 21, 2010
Pumping Systems for Landscaping Pumps, Controls and Accessories Mark Snyder, PE
Pump Station Design Purpose of Pump Stations Pump stations are designed to boost water pressure from a lower pressure to a higher pressure The discharge pipe is commonly called a Force Main or Pressure Pipe Pump stations are used when City Pressure is not sufficient
Pump Station Design Ask or measure existing pressure Calculate head loss through pipes Determine head pressure at last sprinkler heads Determine elevation difference Determine boost needed Pump stations should have at least 2 x pumps each designed to handle to the full design flow. Designing each pump for full flow with alternating spare will buy operations time in the event of pump failure
Types of Pump Stations Contractor Built Package Built
TYPES OF PUMPS Vertical Multistage End Suction Horizontal Split-Case Vertical Turbine Submersible
Force Main Design Force main (discharge line) remains under pressure after pumps shutoff For irrigation pumps, typically 2 diameter or larger Typical materials of construction include PVC, HDPE, or ductile iron (check pressure requirements) Force main should be designed with minimum liquid velocity exceeding 8-10 ft per second
Centrifugal Pumps - Basic Hydraulics
This section will show the basic design criteria used to make pump selections, review basic pump terms and formulas, and give examples on proper pump selection. We will discuss: Head Static, Dynamic, and Total Dynamic Head how to convert PSI (pounds per square inch) to feet of head How to read pump curves and properly make pump selections.
Head Head, or feet of head is the most common way to express pressure generated by a pump. Imagine a pipe shooting a jet of water straight up into the air, the height the water goes up would be the head Most pump curves use Feet of Head to express pressure. Converting PSI to Feet of Head. Feet of Head = PSI X 2.31 Specific Gravity (S.G.) Converting Feet of Head to PSI. PSI = Feet of Head x Specific Gravity (S.G.) 2.31 We are almost always discussing water at ambient temperature. With 68 degree F water, SG = 1.0
Head Once we know the flow rate we need from the pump, we must then calculate the Head required i.e. the required discharge pressure. There are a number of different terms describing Head Components: Static components Suction Head Discharge head (Elevation overcome on the discharge side) Pressure head (Hp) Total Head Dynamic head Friction head (Friction losses) Total Dynamic Head the pressure we must calculate to select correct pump which will pump desired amount of water through our system.
Flooded Suction Positive Suction Head The preferred configuration!!
Discharge Head (Static) The vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank. Static loss is the difference in elevation from point A to point B in a piping layout.
Discharge Head lifting up in elevation
Discharge Head (aka Static)
Total Head Total Head = Discharge Head - Positive Suction Head Total Head = Discharge Head + Negative Suction Head
Friction Loss (H f ) = the Dynamic Losses in the piping system Friction Head (Hf) aka Friction Loss. It is the energy in feet necessary to overcome the friction loss caused by flow of a liquid through piping and fittings. Friction head loss in a pumping system is a function of pipe size, length, number and type of fittings, liquid flow rate and nature of the liquid. Simply stated, this is the drag created when flowing water comes in contact with the inside wall of the piping, valves and fittings of a system. With the friction loss chart on the next slide, increased flow in the same size pipe will increase friction loss These same types of charts are available for valves and fittings.
Friction Losses We must consider four factors when determining friction losses in the system: 1. Size of the pipe 2. Amount of flow through the pipe 3. The length of the pipe 4. The C factor of the pipe. The C factor is the coefficient of friction, and represents the roughness of the inside of the pipe Smooth clean pipe has a high C factor, is easier to pump thorugh Dirty rough pipe has a low C factor, and requires more energy to pump through
Friction Losses
At 100 GPM, the friction loss is only.094 feet per 100 feet of 6 pipe. At 650 GPM, the friction loss is now 3.38 feet per 100 feet of 6 pipe.
Finally, Calculating Total Dynamic Head As easy as 1+2+3! Calculate your Total Dynamic Head (TDH) in 3 easy steps. To choose the right pumping system you MUST calculate the "Total Dynamic Head or T.D.H. Total Dynamic Head (T.D.H) takes into account the suction side water conditions. Total Dynamic Head (T.D.H) takes into account the discharge elevation we must overcome. Total Dynamic Head (T.D.H) takes into account the Friction Loss created by the movement of water through the delivery pipeline.
Total Dynamic Head Positive Suction Head: Total Dynamic Head = Discharge Head Positive Suction Head + Friction Loss + System Pressure Demands. Negative Suction Head (Suction Lift): Total Dynamic Head = Discharge Head + Negative Suction Head + Friction Loss + System Pressure Demands. Total Dynamic Head the pressure we must calculate to select correct pump.
Basic Centrifugal Pump Terms Understanding and Reading Pump Curves The following slides will list common pump terms and formulas used in the pumping industry. Each will be explained and examples will be given. It is important that you take the time to understand these terms and formulas, as you will run across many of these on a daily basis.
Design Conditions or Conditions of Service When you hear the term design conditions, conditions of service, or design point being referred to, reference is being made to the GPM and TDH required on any given pumping application. In the previous example, the conditions of service were 750 GPM @ 298 of head. Can you read the pump curve on the following slide and determine what the conditions of service are for the point selected? The correct answer can be found on the slide after the pump curve.
Centrifugal Pump - Curve Shape You will notice that as the pump curve moves to the right - Flowing MORE water The curve is also slowly dropping (LESS pressure). The reason for this is that as the flow increases, the same water remains inside the impeller for a shorter period of time, and the centrifugal force has less time to impact the passing water. You will notice in the future that some curves are flatter than others, and some curves drop off quicker as you flow more water. The reason for this difference is in the internal impeller design. The following slide points out the natural curve shape of a centrifugal pump from beginning to end for a 13 inch impeller.
Pump curve shape.
Example 4 zones max zone flow 200 gpm City pressure is 30 psi Sprinkler Pressure required 40 psi Elevation change is 50 Force main is 4 and 500 pvc schedule 40
Example Need 2 pumps at 200 gpm Zone Pressure minus City Pressure is 10 psi Elevation Change is 50/2.31=21 psi Losses thru 100 of 4 pipe is 7.6 (5X7.6) So total loss is 38 (16 psi) Pump should be 200 gpm at 47 psi (109 )
WHAT TO REALLY DO Call a reliable pump company Give them the flow and number of pumps Give them City Pressure and Discharge Pressure (determine Boost) Give Voltage Requirements Special Requirements, etc VFD, Valves Interface with Sprinkler Control