How To Calculate Friction Loss

Prepared for Progressive Dairyman Blog: http://www.progressivedairy.com/index.php?option=com_content&view=article&id=10789%3Ahow-to-calculate-friction-loss&catid=77%3Amanure&Itemid=121 How to calculate friction loss Dairy basics - Manure

Written by Nancy Puck

Tuesday, 18 June 2013 09:59

When planning a hose system or pipeline, we must know what the capabilities of our pump are and what the losses in our system will be. In a previous article "Understanding pump performance curves" we learned how to determine the capabilities of a pump. The pump provides energy as Total Dynamic Head (TDH) that supplies the system. Remember, TDH is measured in feet and can be converted to pressure as PSI by dividing by 2.31. Friction loss is the consumption of energy. By definition, "Friction loss is the loss of energy or 'head' that occurs in pipe flow due to viscous effects generated by the surface of the pipe." We need to know four things to calculate friction loss using the Hazen-Williams Equation (Figure 1): 1. Pipe length (feet) 2. Design coefficient (type of pipe rough/smooth) 3. Flow rate (gallon per minute) 4. Inside diameter (inch) The Hazen-Williams Equation is based on empirical data and gives us reliable outputs: 1. Head loss: Feet 2. Head loss: psi 3. Velocity: Feet per second For an example, let's plan a hose system to service your fields within two miles of the lagoon. You bed with sand, so we will be sure to keep the liquid velocity above 13 feet per second to ensure the sand stays in suspension and doesn't settle out or fill the hose. (Length of pipe: 2 miles x 5,280 feet = 10,560 feet) Lay-flat hose is super smooth, and will swell under pressure, often giving us better results than C-160 pipe. (Design coefficient: 160) We want to operate our hose at 200 psi working pressure. This calculates to 462 feet of head. If we look at our pump performance curve for the Cornell 6NHTB-19 with a 375-horsepower engine, we see 462 feet of TDH will happen at 2,250 gallons per minute. (Figure 2) The energy efficiency of the pump is very good there or at higher flow rates. (Flow Rate: 2,250 gallons per minute) Naturally, a larger diameter pipe will be less restrictive. Choosing from 6-inch, 7-inch or 8-inch mainline hose, we select 8-inch. (Diameter: 8 inches) Hazen-Williams Equation outputs for this scenario: 1. Head Loss: 601 feet 2. Head Loss: 260 psi 3. Velocity: 14 feet per second Then we will add two 660-foot lengths of six-inch drag hose: 1. Head Loss: 304 feet 2. Head Loss: 132 psi 3. Velocity: 26 feet per second Total System Requirements: 601 feet + 304 feet = 905 feet (392 psi) Energy supplied per pump: 465 feet (200 psi) 905 feet ÷ 462 feet = 1.9 Pumps Therefore, two 6NHTB-19 Cornell Pumps are required to fill the requirements of this situation. Elevation gain or loss can be directly added or subtracted as feet of loss (uphill – subtract) or gain (downhill – add). If we compare the same situation with using a 6-inch mainline hose, we see a total of 2,739 feet of head loss at 2,250 gallons per minute, requiring six pumps to overcome the total friction loss. Limiting factors of a system can be the working pressure of the hose or pipe and the velocity of the liquid it can handle. Understanding pumps and friction loss can better inform the investments we make.  PD To learn more about pump placement, contact Nancy Puck using the link below or participate in a PCE Pump School. Nancy Puck  Puck Custom Enterprises, Inc.  (712) 653-3045

Understanding Pump Performance Curves

Prepared For Progressive Dairyman Blog:  http://www.progressivedairy.com/index.php?option=com_content&view=article&id=10280:understanding-pump-performance-curves&catid=77:manure&Itemid=121 Understanding pump performance curves

Dairy basics - Manure

Written by Nancy Puck

As we plan a pumping system to move liquid on a farm, we must know what the capabilities of our pump are, and what the friction losses in our system will be. 

This is an overview of pump capabilities as detailed on the pump's performance curve.

On one axis of the pump curve, you will find capacity, and on the other, total dynamic head (TDH). Capacity is measured in U.S. gallons per minute or cubic liters per hour. 

This tells us what the pump is capable of flowing if we fulfill the other inputs: RPM, horsepower and NPSH required.


These inputs are detailed in Figure 1 above, as well as the pump's efficiencies. Click here or on the image at right to view it at full size in a new window.

TDH is measured in feet or meters. By dividing feet of TDH by the 2.31, we can convert feet of head to pressure as psi (100 feet / 2.31 = 43 psi). 

This is useful if you know how much pressure you need, or so you can read the pump's outflow pressure gauge, and in reverse, multiply by 2.31 to estimate the feet of TDH (100 psi x 2.31 = 231 feet). 

You can then better understand how the pump is performing through referencing the pump curve. Higher pressure does not equal higher flow.

Why is the measurement on the pump curve in feet or meters and not pressure? Pressure is related to the type of liquid, its specific gravity. Heaver liquids will create more resistance and therefore more pressure. 

Head is a fluids term that measures the kinetic energy a pump creates. The pump will move any liquid to the same vertical height (feet) if the pump can be spun at the same rpm. 

For liquids with a higher specific gravity than water, more power is required. Pump performance curves are based on clear water at sea-level.

RPM tells us how fast we are rotating the pump. Using the rpm curves, we can find how much TDH the pump will create at a given flow rate at any point on that curve. 

Intersecting the rpm curves are horsepower requirements. Horsepower increases with higher flow rates and higher TDH.

Net Positive Suction Head Required (NPSHr) is the required minimum inflow to keep the pump from cavitation and allow it to work properly. This number helps us calculate how much lift a pump has on the intake side. 

Pumps create a vacuum at the center of the impeller that draws fluid into the pump. After priming the pump, this vacuum continues to work if the pump is within a close enough distance to the liquid allow atmospheric pressure to fulfill NPSHr. 

If not, you may need to provide inflow to ensure this requirement is filled. Cavitation creates massive damage to the pump. We will discuss NPSHr and cavitation in more depth later.  PD


Nancy Puck 
Puck Custom Enterprises, Inc. 
(712) 653-3045

"Twin Line"

Do you have extra hose you leave at home?  Does a significant amount stay on your reel?  Are you wondering how to gain gallons per minute and distance?  Is an underground pipeline limiting your flow rates because of pressure restrictions?

Sometimes, a good solution can be splitting the outflow at your pump into two lines that lay side by side.  Maybe one of those lines jumps into an underground pipeline and it's sister line continues to run above ground.  Then, they meet at the center pivot and join into another pump.  Perhaps that pump is ready to push through the last few pieces of drags to the tractor, the process is repeated with another set of twin line, or they merge through a single hose for a shorter distance.  

The idea is to decrease friction losses by allowing each hose to flow half of our total flow rate.  

Let's say you have a pump unit at the manure storage site that will flow 1750 GPM.  Perhaps it's a PCE PT 2047 with a 4NHTB Cornell Pump and a 275 horsepower JD 6.8 L engine. This works with any pump, just be sure to consult your pump curve.

If we take a look at the pump curve, we can follow the CAPACITY across the bottom to 1750 GPM - that's our goal.  On the left vertical axis, find 2000 RPM at the top. Follow that dark black line over to where it meets 1750 gallons per minute (GPM).  This just over the 250 horsepower requirement, noted on the right side of the chart by the slashed line traveling diagonally up and to the left.  Along the top of the chart 20 FT is noted as NPSH REQUIRED.  The pump will require close to 21 FEET of head coming into the pump to operate correctly at this flow.

The chart shows with all these requirements filled, the pump will produce about 400 FEET of Total Dynamic Head (TDH).  400 divided by 2.31 equals 173 PSI.

If we split the 1750 gallon per minute outflow into 2 lines each flowing 875 gpm, lets see how far we can go compared to a single line flowing the full 1750 gpm.

Single 8 inch line @ 1750 = 23.5 feet of head loss per 660 ft. length
400/23.5 = 17 lengths of distance

Single 7 inch line @ 1750 = 45 feet of head loss per 660 ft. length
400/45 = 8.88 lengths of distance

Single 6 inch line @ 1750 = 95 feet of head loss per 660 ft. length
400/95 = 4.21 lengths of distance

Single 5 inch line @ 1750 = 232 feet of head loss per 660 ft. length
400/232 = 1.72 lengths of distance

(8 lengths per mile, or 200 meter lengths)


Twin 8 inch line @ 875 gpm each = 6.5 feet of head loss per length
400/6.5 = 61.5 lengths /2 = 30.7 lengths for each line
divided by 8 = 3.8 miles (compared to 2.125 miles solo with the same pump at the same flow rate)

Twin 7 inch line @ 875 gpm each = 12.5 feet of head loss per length
400/12.5 = 32 lengths /2 = 16 lengths for each line
divided by 8 = 2 miles (compared to 1.1 miles with a solo line at the same flow with the same pump)

Twin 6 inch line @ 875 gpm each = 26.5 feet of head loss per length
400/26.5 = 15 lengths /2 = 7.5 lengths for each line
divided by 8 = 0.94 miles (compared to 0.52 miles with a solo 6 inch hose)

Twin 5 inch line @ 875 gpm each = 64.3 feet of head loss per length
400/64.3 = 6 lengths /2 = 3 lengths for each line
divided by 8 = 0.375 miles

In conclusion, a twin line can help you maximize the ability of a single pump to reach it's best gallons and best distance.

These numbers are based on cool water.  They do not take into account elevation changes, requirements of the next pump in line, variations in temperature or changes in direction.  This is just a guideline to show how a single scenario would work in theory.

Please post questions and comments below or give us a call.  712-653-3045

Thank you!

NPSHr

Net Positive Suction Head Required

All pumps have requirements to their performance, and no pump can perform well if you can't meet it's needs.

Below is part of the Cornell® 6NHTB-19 Pump Curve with a colored over-lay for this discussion.

NPSH REQUIRED is found at the top of the curves in FT.

What does this mean?

For this pump, running at 2100 RPM with its horsepower requirements met, you can pick a flow rate and easily find its NPSH REQUIRED.

1500 GPM = 5.5 FT.
2000 GPM = 8 FT.
2500 GPM = 11 FT.
3000 GPM = 17 FT.

Net Positive Suction Head Required is the required feet of head going into the back of the pump for it to function properly and achieve your desired flow rate.

Easy signs your pump's NPSHR is NOT met: 

• The pump is shaking
• It sounds like you have rocks going through your pump
• Gauges and Valves are falling off
• The unit is vibrating
• The tongue's jack is sunken into the ground from vibration
• Your gauges are broken
• Your suction cover shows physical damage of gouging and wash-out
• Your impeller shows physical damage of gouging and wash-out

CAUTION! Your Pump is Cavitating! 
More about cavitation: http://www.engineersedge.com/pumps/cavitation.htm



MEETING NPSH REQUIRED AT THE SITE:

Atmospheric Pressure – Friction Loss in Intake Hose – NPSHr of Pump = Static Suction Lift

• Static Suction Lift tells us the maximum distance from the top of the water to the center of the pump's impeller we can operate within without causing cavitation.  The pump needs to be primed, but not fed with another pump within this distance.
• Atmospheric Pressure is the force of air pressing down on a body of water - providing us with free FEET OF HEAD: we will use an easy-to-remember and conservative estimation of 30 FT. for atmospheric pressure.
• Friction Loss in Intake Hose is estimated using a C-140 Head Loss Chart.  See Below. We have modified it for 25 FT lengths. example: 10" Hose at 2000 GPM = 0.5 FT
  
  
• NPSHr is found on the pump's curve at a given flow rate. example: 2000 GPM = 8 FT.

30 FT. – 0.5 FT – 8 FT. = 21.5 FT

21.5 FT = the maximum distance from the top of the body of water to the center of the pump impeller that this pump can operate at without being force-fed and without cavitation at 2,000 GPM.  If you drop the flow rate, these numbers change and it has less NPSHr, and more Static Suction Lift

Call or write with Questions!

712-653-3045 - PCE Office

nancypuck@gmail.com - Nancy
bpuck@puckenterprises.com - Ben
jpuck@puckenterprises.com - Jeremy

A Comparison of 3 Pumps

Let's do a comparison of 3 Cornell Pump Curves:

4414 - 4 inch outflow, 4 inch intake, 14 inch impeller

4514 - 4 inch outflow, 5 inch intake, 14 inch impeller

4NHTB - 4 inch outflow, 5 inch intake, 17 inch impeller

I know it's bad to start with a disclaimer, but please realize these numbers are found using WATER, AT SEA LEVEL.  ESTIMATIONS HAVE BEEN MADE. This is simply an exercise in reading pump curves.

 ***NOT GUIDELINES OF HOW TO PLACE PUMPS.***

Below is the curve for the 4414 Pump. Notice the US. GALLONS PER MINUTE across the bottom. The last number listed is 1600 GPM.  Follow the 1600 line straight up, and it says if you run this pump at 2300 RPM, you could achieve 320 FT of TOTAL DYNAMIC HEAD. The pump is running at about 77.5% efficiency way out there in the top right corner (almost the best efficiency this pump can run). But it has to be fed (at an almost impossible amount to be a lead pump at this GPM). The NPSH (net positive suction head) is greater than 24 ft. -- I'm estimating closer to 30 ft. or more to keep it from cavitation.  

On flat terrain, wishing to travel 1 mile, and wishing to use 4414 pumps to get there, we will use 6" mainline hose, with a Head loss at 1600 GPM to be about 90 FT per 660 FT. length of hose.

320ft of TDH leaving pump 1. Pump 2 will need 30 ft NPSH to keep from cavitation. 

320' ÷ 90' (Head loss) = 3.55 lengths of hose

3 lengths x 90' = 270' of TDH used

320' - 270' = 50' of TDH remaining

Pump 1 (with it's NPSH fulfilled), 3 lengths of 6" hose, Pump 2, 3 lengths of 6" hose, Pump 3, you get the idea.

Lets say you drop into 5 inch drags after the last 4414 pump. Head loss jumps to 200' per 660ft length. No NPSH required to the applicator so:

320' ÷ 200' = 1.6 lengths of drags

BELOW -- 4514 Cornell Pump Curve

The top right corner of this curve ends right around 1950 GPM, at 2300 RPM, with about 30' NPSH 

required, providing 290ft. of TDH.

Again - 4514 Pumps in line with 6" mainline hose on flat terrain.

At 1950 GPM - Head loss in 6" hose is about 130' ft.

290' TDH ÷ 130' = 2.23 lengths of hose

2 x 130' = 260' TDH used

290' - 260' = 30 ft. TDH remaining to fulfill NPSH Requirements for the next pump.

SO: Pump 1, 2 lengths of hose, Pump 2, 2 lengths of hose, Pump 3, 2 lengths of hose, Pump 4, 2 lengths of hose -- this is silly...

Lets try this 4514 at 1500 GPM, still 2300 RPM, still in the 72% efficiency with less NPSH Required (19') and more TDH (330 ft). Head loss is less, at 1500 GPM is 80ft.


330' ÷ 80' = 4.125 lengths of hose
4 x 80' = 320' of TDH used
330' - 320' = 10 ft remaining -- NOT ENOUGH - 19 ft is needed for the next unit.


How about 3 lengths?
3 x 80' = 240'
330' - 240' = 90 ft remaining


So: Pump 1, 3 lengths of hose, Pump 2, 3 lengths of hose, Pump 3, 3 lengths of hose....




BELOW: 4NHTB Cornell Pump Curve


With this pump we're going to follow the 1875 GPM line up and meet 2000 RPM and receive 390 ft of TDH. NPSH Required is estimated at about 25' - 27'.

Head loss at this flow through 6" mainline is about 120 ft. So:
390' ÷ 120' = 3.25 lengths
3 x 120 = 360' used
390' - 360' = 30' remaining  (25' to  27' needed)


Head loss through 8" mainline at this rate is about 51 ft.
390' ÷ 51' = 7.647 lengths
7 x 51' = 357' used
390' - 357' = 33' remaining (25' to  27' needed)


6" hose:
Pump 1, 3 lengths of hose, Pump 2, 3 lengths of hose, Pump 3,
**Head loss through 5" drags at 1875 GPM is about 285 ft.  - giving you 1.368 lengths to the tractor


8" hose:
Pump 1, 7 lengths of hose, Pump 2, 3 lengths of 6" - mainline/drags


Theses are very basic and incomplete examples. Estimates were made on Head loss through the hose. Estimates were made on NPSH Required.  Terrain, elevation, and temperature were not considered. MANURE WAS NOT CONSIDERED.

Efficiencies of the Engine matched to run the pump was not considered.  These examples do not show how PCE puts pump/engine units together nor how PCE would utilize these pumps.

If you'd like to go back through and see all these pumps at 1000 GPM, just use 38 ft. of Head Loss per 660 ft. of 6" hose.  The distances improve. 

Thanks,

Nancy

6NHTB-19 PUMP -- 8 INCH HOSE -- 2,500 GPM Flow

We just finished up our second session of Pump School and we just can't say thanks enough to everyone who attended. Hopefully it was worth the drive for those of you traveling from out of state.


A quick re-cap of some key points about the Pump Curves:

Example: 6NHTB-19 PUMP and 8 INCH HOSE -- 2,500 GPM Flow

NPSH - Net Positive Suction Head

With this pump, we are required to provide 11 feet of NPSH to the inlet side of the pump (see in red below) .  We have 34 feet of head available at sea level that is provided to us (free of charge) by atmospheric pressure.  This is based upon an open body of water.  Because we are not at sea level, and because of friction loss in the suction hose, we like to estimate 30 feet in Iowa.  This means that if we are required to have 11 feet, we can reach from the center of the impeller to the top of the body of water 19 feet or less. (30' available - 11' required = 19')  

If we are pumping 22 feet deep, we only have 8 feet of NPSH available.  At this depth the pump will only flow 2000 GPM - as indicated on the pump curve below.

TDH - Total Dynamic Head

     -- at sea level, the distance a pump can push water vertical, no flow

Estimating TDH at 460 ft. (follow the RED line above)


460 x 0.42 = 193.2 PSI -- a check that this number is less than the 200 PSI of operating pressure on the 8 inch hose


At flows of 2500 GPM we estimate friction loss in 8 inch hose to be 60 ft of TDH per 660 ft length of hose - with no elevation change.


The pump is outflowing 460' TDH at 2500 GPM.
460 ÷ 60 = 7.75 lengths of hose


We can lay out 7 lengths of 8" hose and use 420ft of our 460ft available.
(7 x 60 = 420)


460-420 = 40ft of TDH remaining


Now return to the pump curve, and notice NPSH REQUIRED.


This number must be fulfilled to keep your pump from cavitation. Cavitation is the process of liquid turning to vapor due to a lack of pressure. This causes damage inside the pump, false pressure readings and high inefficiencies.
see: http://en.wikipedia.org/wiki/Cavitation


NPSH REQUIRED is 11ft if we have another 6NHTB-19 pumping 2500 GPM inline. 40ft fills this requirement.


Just a partial overview, but hopefully a helpful example. Thanks for attending!


Nancy