Friction Factor - Confusion

This topic has often caused a lot of confusion in many people while calculating pressure drop of fluids in pipe. Many people are even unaware of the presence of 2 types of friction factor, wherein one is 4 times the other.

Moody's friction factor (also known as the Darcy friction factor) and Fanning friction factor are the two types of friction factor used in this world for pressure drop calculations.

Relationship:

f = Friction factor.
fd = Darcy friction factor
ff  = Fanning friction factor.

NOTE: The third friction factor (which is mentioned above in the correlation) is used in Coulson & Richardson Vol. 6 for pressure drop calculation.

In text books you'll come across various pressure drop equations, the only difference lies in the type of friction factor used. Darcy / Moody friction factor is most widely used for pressure drop calculations, as they can be easily interpreted from moody's chart.

To keep it simple and straight forward, note the following points and equations very carefully to avoid any confusion on this topic henceforth.

Equations:


   1.  Darcy Friction Factor --- Laminar condition

 


Pressure drop equation:

Moody's Chart representing Darcy Friction factor:



The graph above is a Moody's chart for calculating friction factor using Reynolds Number (Re) and relative roughness. The " f '" on Y-axis is Darcy friction factor and not Fanning friction factor.


TRICK:
Follow the red line marked on the graph, it indicates that for Re = 1000, f = 0.064 which satisfies the Darcy's equation for laminar case. Thus this moody's chart gives us the Darcy friction factor and now I can, without any hesitation, use this " f D " value to calculate the pressure drop by using the corresponding pressure drop equation.


2.  Fanning Friction Factor --- Laminar condition

 

   



    Pressure drop equation:

BEWARE !! Many a times the graph that you might be referring to will give a value of f = 0.016 for Re = 1000 i.e. using f = 16 / Re, so this is fanning friction factor. The following Moody's chart shows the aforementioned case.




3) Friction Factor: 

According to Coulson and Richardson vol. 6, we have the following equation for pressure drop calculation and for that particular equation if you are using, make sure you use the correct graph for that equation which is shown below.

NOTE: Its not necessary to have Laminar condition to use this equations / graphs as I have highlighted and focused only on the laminar region. Its just to make my point clear of explaining you the trick to help you to check whether the curve that you are referring to gives you Darcy or Fanning friction value and which graph and equation you need to use for calculation of pressure drop.

 I have here made it easy by putting the pressure drop equation inline with their respective graphs.

I Hope, the point of this article is clear and friction factors won't be any confusion in any minds from now on. I follow the same practice while determining the value of friction factor and this check has never failed me.

Fix your steps of calculating pressure drop, but I insist on using Darcy equation i.e. Darcy friction factor. This should not lead to a conclusion that fanning friction factor should not be used. Remember either of it can be used, and both will lead to correct pressure drop, provided you use the right set of equations and keep the relationship in mind

Just in case you get confused again, you can re-visit this article and if you have any doubts in mind you can leave a comment here.


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Ball Valve

                      

Purpose: ON-OFF / Isolation valve

P&ID symbol:

General points:

• Ball valves can be manufactured from minimum of 1/4" to maximum of 10" (some manufacturers also give upto 16")
• Due to cost and bulkiness of the valve,Butterfly valves are always preferred over globe and ball for larger sizes i.e. more than 6" or 8".
• For smaller size Ball valves are favorite of all, from cost, handling and installation and service point of view. 
• In most household applications, ball valves are used. 

Types of Ball valve:

1. Full bore: Valve size = line size 
   
   
   
2. Reduced port: Valve size < Line size (by 1 inch)

Advantages: 

1. Requires only 1/4th turn to operate.
2. Excellent for shut-off application & preferred over gate & globe valve (for sizes < 60 mm)
3. Ease of operation
4. Supports & sustain high P (<10,000 psi), T (<200 C) & Q (flow).
5. Long service life.
6. Better sealing.
7. Sturdy device.
8. Relatively low cost.
9. Inspection & repair of seats & seals can be done without removing of valve body from the pipeline. 

Disadvantages: 

1. Flow control is not possible. 
2. Tightens with age, implies require more maintenance.
3. Regular replacement of seal is required. 
4. Abrasive solids will damage seal & ball surface.

Pump Encounter - Visitors contribution # 3

Pump Encounter:

Mark Brein

General Manager 

Penguin Pumps

In the following article Mr. Mark Brein, General Manager, Penguin Pumps shares with us one of his personal experience and also gives us wise advice's. Its a great way to learn form the mistakes of other's and I thus, sincerely request you to go through this article and appreciate Mr. Mark's efforts of writing all through this and sharing with us his personal experience to help us learn and grow. 

       The following brief pump encounter took place in the       wet fume scrubber Industry dating back 35 years.  For many years this industry has used Cantilevered vertical pumps to pump liquid from an open reservoir, through spray nozzles,  back to the reservoir. The tanks holding the liquid were taller than they were wide by a 2 to 1 margin or greater. The liquid level height inside these tanks was always 3 ft minimum.  The volume of liquid inside these tanks was always 3-4 times the pumping flow rate. Pump problems encountered over the years were practically zero. 

READ MORE ... 

Most pump troubles should never happen - Visitors Contribution # 2

MOST PUMP TROUBLES SHOULD NEVER HAPPEN:

“GENERAL RULES OF THUMB”

Mark Brein

General Manager 

Penguin Pumps

• Size a pump to operate on the midpoint, plus or minus ¼, on its performance curve.  This simple rule has been around for many years, and it still holds true.

• Pump suction conditions:  

The piping on the suction side of the pump is much more important than the piping on the pump discharge. If any mistakes are made on the discharge side, they  can usually be compensated for by increasing the performance capability from the pump.  Problems on the suction side, however, can be the source of ongoing and expensive difficulties, which may never be traced back to that area.

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NPSH - Visitor Contribution # 1

NET POSITIVE SUCTION HEAD—NPSH

Mark Brien,

General Manager,

Penguin Pumps

This phenomenon can get complicated if allowed to do so and is a subject about which complete books have been written.  So let’s just accept the premise that every impeller requires a minimum amount of pressure in the liquid being supplied in order to perform without the liquid being pumped vaporizing inside the pump, which we may simply define as cavitation. 

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Gate Valve

                                    GATE VALVE:

Introduction

Gate valves are used in most of chemical process industries, predominantly in Petroleum industries, because of the attention that it has gained as a good flow isolation valve with very little or negligible leakage. Nearly 70% of the valves in petroleum industries are gate valves. They are suitable for services which require ON/OFF application. Throttling is not preferred with this valve type.

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