Friday, January 23, 2015

Agitator/Mixer Data Sheet

Hi all,

Today I am going to focus on Agitators/Mixers and how to interpret a data sheet for procuring. This article is largely based on my experience.

Note: 

The vendor/manufacturer decides appropriate mixing equipment for your operation to achieve the desired mixing performance. So it is very vital to give the manufacturer the necessary details of the process. This article will help you to understand what goes into an Agitator and understand its basics, so that you can have effective discussion with your vendors and make the right first choice thus avoiding confusions and misconceptions.
I will strictly focus on Interpretation of Agitators data sheet. This being a very vast topic, it is beyond my scope to discuss its various types and specialties in this article. 

Think & ask yourself:

       1.     What is the purpose of an agitator? How many impellers would I need?
       2.      Components of an agitator?
       3.      What basic details would an agitator manufacturer would require?

Purpose:
Agitator or mixers are of various types and the basic purpose of having them in your industry would be one of the following reasons or sometimes may be to serve multiple purposes.
       1.      Uniform mixing of liquids.
       2.      To keep solid in suspension.

Components:

Motor, gearbox, shaft, impeller all of which forms the mechanical parts of an agitator.
Let’s study them in more detail.
First of all what do you need to run an agitator? A Motor right. It’s as easy as ABC.

Motor:

 Now Motors have fixed rpm (900, 1450 and 2900) based on the number of poles it has. For example, 2 pole motor will have rpm of 2900, 4 pole as 1450 rpm and 6 pole offers 900 rpm. This rpm may vary within a small range around the mentioned rpm depending on the losses inside the motor. The rpm required by your agitator may not necessarily be same as motor’s rpm to serve the process purpose. It may be less or may be more compared to motor’s rpm.  What would you do then? You would then definitely look for something that will reduce/increase its rpm in order to meet the agitator’s rpm requirement. This something is – A Gearbox.

Gearbox:

Gearbox is used in Process industries to match the motor speed with that to the process speed requirement. The combination of gears within the gearbox reduces/increases the rpm to match the requirement. It is thus very vital to get the right gearbox for your process and not get an over sized or undersized one, thus details like make, model, reduction ratio, loading rating and capacity of gearbox are included in the data sheet.
Make:
It is the brand type of the gearbox.
Model:
Model number of the gearbox, typically for cross checking the vendor filled data from the brand catalog/website.
Reduction ratio:
  
It is the ratio of motor rpm to the process rpm.
Loading Rating:
The loading rating of the gearbox is the maximum power bearing capacity of the gearbox. It is expressed in the power units - kW or HP.
Service factor: It is the ratio of the Loading rating of the gearbox to the rated power of the motor. It is also called as the safety factor. During the start-up of the system, the torque generated by the gearbox requires a lot of power which is more than the motor power. Thus the gearbox must have this high power bearing capacity in the initial phase and thus service factor is important. It is also known as the safety factor. The minimum service factor usually recommended is 1.5

Coupling:

Coupling ‘couples’ two drive elements to transfer torque/motion between them. The correct gearbox selection ensures that the process runs on required rpm. Thus the output of the gearbox needs to be transferred to the agitator shaft. This transfer is done using Coupling, where the gearbox output is coupled with the agitator shaft.

Shaft:

Gearbox is connected to the shaft. So, the next logical question would be how long my shaft should be? Well, if it is a top entry agitator, then the length of the shaft depends on the vessel height (leaving some clearance at the bottom depending on the type of agitation and the type of fluid handled) including the overhead assembly up to the gearbox.
If it is a side entry agitator, then the minimum length should be half of impeller’s diameter.
Shaft is connected to impeller. So what type of impeller? What would be the diameter of the impeller? It is your call, based on your process application. There are many types of impellers available in the market - pitched blade, hydrofoil type, propeller type, turbine type etc. each having its own characteristics and applicability.
How many impellers would I need? This would depend on the dimension of your vessel and the degree of agitation you aiming for (which would actually depend on the type of system you would be using it for). Discuss it with your vendor before your purchase or place an order, ensuring that the vendor gives you the best fit for your system within your budget and that he guarantees the desired mixing quality as required.
Calculate the pumping flow rate i.e. the rate of mixing (m3/hr) or flow discharged by the rotating impeller.

Critical Speed:

Critical speed is usually considered as 30-35 % over the rpm required in your process. It is at this speed that your agitator would produce a lot of vibrations and noise. Necessary actions should be taken if the process rpm reaches -10% of the critical speed.

For eg:
If the rpm required = 300,
Critical speed should be 1.35*300 = 405 rpm.
Necessary actions to reduce the rpm at -10% i.e. at {405 – (0.1*405)} = 365 rpm. 

Note that, this is just an approximate calculation showing the significance and importance of this parameter. However, in the plant noises and vibration would serve as an indication of increasing speed and immediate action has to be taken at that time to avoid any mishaps.

For top entry agitator:

Pumping rate (m3/hr)



Where,
NQ = Flow number
DI = Impeller Diameter (m)
rpm = Speed of an impeller (rpm)

Bulk fluid velocity can be calculated using the following equation:
 

                     






Where,
D = Diameter of the vessel

Bulk velocity table:

Scale of agitation
Bulk Velocity (m/s)
Application
1
0.03
For uniform mixing of
 miscible fluids
2
0.06
3
0.09

For uniform mixing as well as keeping solids in suspension.
4
0.12
5
0.15
6
0.18
7
0.21

Vigorous agitation
8
0.24
9
0.27





Where,
N
p = Power number
rpm = Speed of an impeller (rpm)
DI = Impeller diameter (m)
S.G. = Specific Gravity of the fluid.

Miscellaneous features:

Packing, Material of construction, tank coupling, motor weight, total weight, torque, bending moment all are provided by the vendor.

Packing:
Mechanical seal or gland packing.

Material of construction:
 
MOC is to be defined for the wetted parts (parts in contact with the liquid) which include the shaft and the impeller. MOC is selected by studying the nature and the characteristics of the fluid to be agitated.


Weights:
Motor weight and the total weight of the agitator is required not from process but from structural point of view. The structures/columns/beams are designed to bear the full load of the equipment and its accessories and thus the weight of the equipment is necessary to be included in the specification sheet.  




Saturday, August 10, 2013

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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Friday, April 26, 2013

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.

Monday, March 18, 2013

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.

READ MORE ...

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

READ MORE ... 


Wednesday, February 20, 2013

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.

READ MORE ...

Sunday, November 04, 2012

NPSH



NPSH:



Net positive suction head measures the difference in head (differential head) & not the difference in pressure.


NPSHA
NPSHR
Absolute Pressure required at the pump suction above the vapor pressure of the liquid at that temperature.
(Psuction > Pvap)
It is the minimum absolute pressure required at the pump suction to avoid vaporization.
(Psuction = Pvap)

It is the function / requirement of your actual process / system
It is pump specific.
Hence it is calculated with the help of process parameters & conditions.
It is provided by pump manufacturer. It is calculated using water at room temperature by the manufacturer.


It is worth noting that while NPSHa must be greater than NPSHr, NPSHr test values stem from a procedure defined by Hydraulic Institute.
When the NPSHr value is determined by the pump manufacturer using water, the pump performance has already decreased by 3% in order to measure the change & so, NPSHa must actually be atleast a few feet/meter greater than NPSHr, not just equal.

READ MORE...

Friday, August 31, 2012

Pumps - Affinity Laws


Affinity laws allows engineers to estimate changes in critical performance parameters like

  • Flow rate (Q)
  • Head (H)** / Pressure (P)
  • Brake horsepower (BHP)
due to variation in
  1. Shaft / motor speed (N)
  2. Impeller diameter (D)
** Head refers to Total dynamic head (static head + losses OR differential head), since static head (process requirement) does not change with either shaft speed or impeller diameter or with flow rate.

Affinity laws are applicable on centrifugal pumps, fans or turbines (these are applicable only for centrifugal pumps, laws for fans are different) assuming the points on the system curve have approximately the same efficiency. The system curve changes with change in either shaft/motor speed or impeller diameter.

The following affinity laws are for a specific centrifugal pump.

Affinity law set 1
Affinity law set 2
Constant impeller diameter (d)
Constant motor speed (N)
Q α N
Q α D
H α N2
H α D2
BHP α N3
BHP α D3

Usually, there is no appreciable change in efficiency with range of normal operating speeds. Hence Affinity law set 1 can be considered accurate & reliable.

Whereas, on the other hand set 2 laws (same casing size, but different impeller diameter) are not as accurate as set 1 because large diameter reductions involve changes in the geometry of the blades (outlet width, blade angle, blade length) thus increasing the mismatch with the casing volute, which in turn causes change in the efficiency.

Note: The new impeller diameter should not be more than 10-20% of the original diameter.

The following affinity laws are for a geometrically similar pumps (meaning the pumps will run with same specific speed but with different impeller size.)

For set of geometrically similar pumps
Q α ND3
H α N2D2
BHP α N3D5

Tuesday, May 08, 2012

Absorption

ABSORPTION

Falling Liquid solvent absorbs the gas in the absorption column & is then sent into any one of he following unit:- Distillation, stripping section, removal through precipitation & settling, neutralization, oxidation, reduction & hydrolysis.


Purpose: 
  • Gas purification
  • Gas separation
  • Product recovery
  • Solvent recovery
Solvent Properties:
  • Solubility of gas should be high in selected solvent. ( If not, then absorption is a waste!! )
  • Low volatility organic liquid ( but water is preferred in many cases, due to easy availability & removal of water soluble gases like, HF, HCl, SiF4 ).
  • Low vapor pressure ( to reduce evaporative loss of solvent )
  • non-toxic, non-flammable, non- corrosive 
  • Low viscosity
Types of columns:





    Packed Column

  • Smaller column diameter application.
  • Large inter facial area for mass transfer
  • Simple and cheap in construction
  • Preferred for corrosive gases because of availability of ceramic / plastic MOC packing
Material & types of packing



                  
Internals of Packed tower
  1. Packing support plate:                                                                                                                   It must bear the weight of the packings & allow unrestricted flow of down coming liquid. Drawback is that, that the packing blocks some holes, thus reducing the tower capacity. 
  2. Liquid distributor:                                                                                                                      Placed 6-12 inch above packing for allowing gas disengagement from the bed. Absorption & stripper columns require only one distributor, whereas, Distillation column requires 2 (feed & reflux)
  3. Liquid re-distributor:                                                                                                                        Some part of entering liquid flows through the wall without coming in contact with the gas flowing counter currently, thus we need a liquid re-distributor to collect the down coming liquid and distribute it uniformly throughout the bed and thus increase the efficiency of the tower.
  4. Demisters / Entrainment separators: Generally installed in exit gas streams for arresting the liquid droplets entrained in the gases. If demisters are not installed they can corrode / choke downstream equipments like heat exchangers tubes, damage tube sheets, contaminate products etc.
Types of packing :


Material of construction:
  1. Against all acid gases, metal tower, metal plates, metal packing or any other metal internals can have deleterious effect.
  2. Same is the case with organic liquid and plastic packing.
  3. Ensure internal cooling facility is available when materials and gases releases high heat of absorption.
Dry & wet random packing:


Packed & plate column:






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