The Pressure Times - October 2013

Fast Pipeline Losses Calculator

by Mario Chapa

I’ve visited literally thousands of plants in 20 years of being an engineer.  I rarely have seen a well-designed air pipeline!

Usually people will invest a considerable amount of time designing water, vapor or “other than air” lines.  Pressure losses and leaks are evident especially with stinky gases or hazardous liquids.  But rarely anyone designs and maintains the air pipeline up to date.  When a growing company needs more air, typically what happens is that they buy a new compressor and add it to the same pipe that was not necessarily adequately sized in the first place, now with the new compressor, they may very well be losing up to 50% of the energy of that new compressor in friction loses alone.

Why is this such a common mistake?  First, air leaks are invisible.  At the most, you will hear a hiss or a whistle that will camouflage itself in the shop noise.  Air leaks don’t leave a puddle in the shop floor, or at least they shouldn’t! (if they do, you are in desperate need for either a dryer or an automatic drain or both! But that’s another story!)

Air is usually the single biggest energy consumer in most shops, and yet when people want to save money they look into their process and not into their compressed air pipeline.  Adequately sizing and designing your air pipeline can save money in different ways.  Here are a few:

  1. You can consume less energy and get the same amount of pressurized air.
  2. You can increase your production by getting a better air flow.
  3. You can avoid buying a new compressor.
  4. You can avoid leaks.
  5. You can save money by spreading your maintenance costs due to less compressor usage.
  6. You can increase the life of your compressor buy not running it as hot.

We recently finished two projects in the Houston-Galveston area.  In one, we saved the customer from starting his third compressor that is now idling most of the time.  That’s almost 200 HP just by redesigning the header.

In another location, the wrong pipe size caused more than 55% of the energy lost against pressure loses.  Redesigning the pipeline will save this customer almost $9,000.00 a month!  So the morale of the story is that you should take some time to analyze your whole air system that includes:  compressors, dryers, receivers, drains and pipes.  The fastest way to do that is that you give us a call and we will do it for you for free!  Even better, send us an email with the following information and we will do the evaluation within 48 hours:

  1. Compressors size, number and type (i.e. 3 L55 compressors or; we have two 100 HP rotary screw compressors)
  2. Receiver number and size
  3. Dryers number and size
  4. Main pipe diameter, material and approximate length (i.e. 300 ft. of 2” galvanized pipe).
  5. Approximate amount of: elbows, tees and valves in the line.
  6. Is the line a loop or not
  7. Line pressure
  8. Air requirements if possible.

Still, if you want to do the calcs yourself, here are a few pointers:

Once you gather the information required above, you need to calculate an equivalent pipe length.  Each elbow, valve, tee, and reduction causes pressure loses that can be translated into equivalent pipe length.  Usually these are about two to three times the length of the pipe, so if you have a 200’ pipe, chances are the equivalent length of your pipe once you include all the fitting losses, is going to be somewhere between 400 and 600’.  You can compute the pipeline equivalent length using table 1.  Use the equivalent length to calculate the pressure loss using Harris Equation.  Then calculate the cost of pressure drop using equation 2.  This formulas are over simplified to have a quick assessment on how is the overall efficiency of your system.  The further higher your flow is, the more money you are losing in friction loses.

Let’s do an example.  Assume we have a plant with two 100 HP rotary screw compressors .  The plant has a 2” pipe that runs the length of the shop, around 300’.  The plant runs two 8 hour shifts for 5 days a week (4160 hours a year). A rapid walk through gave the following totals: 12 valves, 4 check valves, 18 tees, 26 elbows (90o).  The system pressure is maintained at 110 psi.  A rotary screw will give conservatively 4 scfm per HP.  Beware that you will have to divide scfm by 60 to obtain scfs that Harris requires.

  1. Get equivalent lengths from table 1.
    1. 12 x  2.24 =  26.88
    2. 4   x 23.2 =   92.8
    3. 18 x  3.44 =  61.92
    4. 26 x   5.2 =   135.2
    5. TOTAL = 26.88+92.8+61.92+135.2+300 = 616 feet
  2. Calculate compression ratio
    1. P atm = 14.2 psi (for Houston)
    2. P system = 110 psig = 124.2 psia
    3. R = Ps/Patm = 124.2/14.2 = 8.74
  3. Calculate pressure drop using Harris
    1. P drop = 0.1025 x 616 ft. x (200*4/60)2/8.74/(2.157)5.31 = 21.657 psi
  4. Compute monetary cost using equation
    1. Dollars/ year = 0.1x200x0.108x.7457/0.93*4160 = $7,222 dollars per year!

This means that you are losing 21.657/110 = 19.6% of your compressor’s energy fighting energy losses.

Harris Equation

Pressure Drop       =       0.1025 x L x Q2
                                                r x d5.31

Where,

Pressure drop in psig

L = system equivalent pipe length in feet

Q = cubic feet of free air per second

R = Compression ratio at inlet condition

d = Inner diameter of pipe, inches

Equation 2. Cost of Pressure Drop

Dollars/Year = CE x BHP x PI x 0.7457 /ME x Hours/Year

Where,

CE =  cost of energy,  usually around $0.10 dollars/KW-Hour

BHP = total compressor horsepower

PI = Percentage increase in compressor BHP = Pressure drop (from Harris)/2/100

0.7457 = conversion factor kW/BHP

ME = Motor Efficiency at full load usually 93%


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