Tuesday, August 25, 2015

Dynamic Simulation Controller Tuning parameters

While putting controllers in the dynamic mode (other things to put are valves and size of the equipment's), you need to put basic controllers in your simulation while switching from steady state to Dynamic modal.

Here is the suggested initial vales to be given for controllers : But before doing that don't forget to save your steady state modal in a separate file .( An experienced person only knows that Simulation files usually closed with out saving ( when something go wrong , you want to start again from the last good configuration 

Unfortunately Simulators still do Not have Ctrl +Z option  


One useful Tip along with These Controller tuning parameters : While giving Specification for dynamic mode, do not give both Pressure and flow specification to a stream.


Friday, August 21, 2015

Typical Pressure Drop values across process equipment and piping

Typical Pressure drop values across process equipment and piping is can be taken as:

Process Equipment
  1. COOLING TOWERS: Tower fill is of a highly open structure so as to initialize pressure drop, which is in standard practice a maximum of 2 in of water. (0.5 kPa.)
  2. DISTILLATION AND GAS ABSORPTION: Pressure drop per tray is of the order of 3 in. of water or 0.1 psi. . (0.75 kPa.)

Service                                   
Pressure Drop                        
(in. H20/ ft packed depth)
Absorbers and Regenerators 
(Non-foaming System           
0.25 to 0.40 (.06 to .1 kPa)
Absorbers and Regenerators  (Moderate Foaming Systems)
0.15 to 0.25 (.04 to 0.6 kPa)
Fume Scrubbers
(Water Absorbent)
0.40 to 0.60 (.06 to .8 kPa)
Fume Scrubbers (Chemical Absorbent)                 
0.25 to 0.40 (.06 to .1 kPa)
Atmospheric or Pressure
Fractionators
0.40 to 0.80 (0.1 to 0.2 kPa)
Vacuum Fractionators
0.15 to 0.40 (.04 to .1 kPa)

  • HEAT EXCHANGERS: Pressure drops are 10 kPa  for boiling and 20 to 60 kPa9 psi for other services 
  • Wire mesh type mist eliminator:     < 0.25 kPa of water column
 PIPING
  • Line velocities and pressure drops, with line diameter D in inches: liquid pump discharge, (5 + D /3) ft/sec, 2.0 psi/100 ft; liquid pump suction, (1.3 + D /6) ft/sec, 0.4 psi/100 ft; steam or gas, 20D ft/sec, 0.5 psi/100 ft. 
  • Steam Allowable pressure drop vary from 1 psi to .3 psi per 100 ft. it is more in laterals than main line and more with Steam pressure.


Laterals
Mains

Pressure,  psig
600
175
30
600
175
30
dP,  psi (kPa)/100 ft
1  (7)
0.7 (5)
0.5 (3.5)
0.7 (5)
0.5 (3.5)
0.3 (2)


Determining Vessel Design Pressure and Design Temperature

Vessel Design Pressure can be considered as Follow: 

 What should be my Vessel Design Pressure This Question is very basic and usually answered by thumb rule, based on experience.

The design pressure of a vessel shall be taken as of the following;

Operating Pressure (barg)                     Design Pressure (barg)       
                                              
1. P=0- 10                                                 MOP + 1 bar MOP z Maximum Process Operating

2. 10 - 50                                                   MOP t 10 % Pressure

3. 50 - I00                                                  MOP + 5 barg

4. > 100                                                      MOP+5%
  • · Vessels subject to vacuum during operation shall be designed for the maximum external Operating pressure plus a margin of 0.15 bar.
  • If the internal pressure Is 0.35 bara or less the vessel will be designed for full vacuum.
  • Design pressure for pump discharges shall be calculated by taking 120 % of the normal Pump (DP) when operating at design conditions.
Vessel Design Temperature can be considered as Follow:
  • Maximum design temperature = max. Operating Temp + 15 °C
  • Minimum design temperature = min. operating Temp - 5 °C or minimum ambient temperature

Common Mistakes in Steady State Process Simulation

Common Mistakes in Steady State Process Simulation

(Some points are ASPEN HYSYS Specific but can be generalized for other process Simulators)
Better Check before starting rather than trapped in the simulator’s un-convergence frustration

1. Not use correct property package

Now days ASPEN HYSYS 7.3 new version itself can tell you about the best suitable property package for the component of the system but still one should have done literature survey for the correct package for the process.

2. Not have done homework on paper like mass balance & azeotrope formation.

When you need to specify the variables it should be a smart guess rather than any arbitrary value exception that simulator will calculate the correct value. Simulation Engine converges within the vicinity of the guess given and if there is no minima than it will not converge (read numerical methods for how simulator works).

Remember simulator does not solve the process itself. It’s a gun to shoot the problem but you are the one have to aim the target.

3. Not create Case specific Unit set by customizing standard unit set

While putting valves in different unit set than the values of your initial guess ( homework) there would certainly one- or two times you would landed up by putting the value mistakenly in wrong unit and even you don’t know where you have made that mistake.

4. Over specifying the simulation. 
 
This is the most common mistacke in simulation and hardest one to identifiy.
Do not make the full flow sheet at once and then start converging unit operation one by one until you are quite familiar with the whole process. Instead of that make flow sheet bit by bit and make it converge step by step and proceed. After giving all input to the unit operation. Check with the degree of freedom of the unit operation and then carefully give the specification which you desire out of that unit operation (Equipment duty). This way you will give minimum specification and never conflict the variable specified and variable calculated.

Page 2

Common Mistakes in Steady State Process Simulation page 2

5. Using Recycle without recycling block.

It won’t be possible for simulation to converge the whole system or run the simulation without using it recycle block. Use lots of recycle block but initial value should be again a smart guess again (approximate to the value you expect as a final value after conversion.

6. Not Checking Final result with expected or estimated value.

Engineering judgment is always require to check weather Simulation results are expected.

7. Not knowing their simulator accuracy and capability for a particular unit operation. e. g.

· HYSYS value for compressor power is still not reliable (thumb rule calculation are still use to report compressor power consumption.

· Horizontal Separator don’t have hemispherical head type vessel

· For scrubbers or separators, Standard Flow for Actual
rule of thumb : Actual volume =~ Standard volume / pressure (barg)

· When dealing with gas compressors (especially reciprocating), people have the tendency to believe that compressor discharge pressure is set by the compressor.

· Absorber and reactors modeling is still very property so we cant realy on HYSYS results except initial estimation

Your comments are welcome and I will discuss more on Dynamic Simulation on my next blog

8. Trying to make robust simulation at once.

Start with simple over all model of process, converge it properly. Check with the system inlet and outlet streams and check their value. Than one should start complexion the model by replacing the cooler & heater with exchangers, Shot cut column to proper distillation column etc.

Take outside all around view , be confident than go inside

Guidelines for Changing from Steady State to Dynamic Simulation



Guidelines for Changing from Steady State to Dynamic Simulation

1.       Add a resistance unit Operation (e.g. valve, Pump, Compressor) between all pressure nodes in the flow sheet.
a.       Internal Flow rates will be calculated by the pressure gradients throughout the flow sheet
2.       One P/F specification should  be made on each boundary streams ( Feeds and products)
a.       Make Pressure Specification n boundary streams attached to pressure equipment that use a resistance equation to calculate flow rates (e.g. valve, Pump, Compressor, heat exchangers)
b.      Recommendation :-Add Valve to all boundary streams
·   (You will use them for flow controllers anyway once you add the control Strategy to the Dynamic Model.)

Saturday, August 8, 2015

WATER BATH HEATER DESIGN


Today I would like to tell you about water bath heater calculation. i.e. How to design a Water bath heater.

I assume you have enough Idea about water bath heaters BEFORE deciding to design a one, if not that may be just read on Wikipedia.

Read API 12K for design, Its short ( Total 40 pages) and easy but you won’t be able to figure out from where to start for design.

Below is the Figure showing various parts of a Water bath heater:





Here is the steps :

1. What is the duty required i.e. how much process fluid is to be heated, flow, pressure & composition, and check phase
change is occurred or not during temperature rice.

2. Calculate the no of process coils , OD, Length :

a. Assume heat transfer Coefficient h = 100

b. Water bath temperature 90°C

c. We already know process fluid inlet and required out let temperature.


i. Calculate LMTD


d. You will get area ,


e. Now assume diameter of tube and no of tubes


i. check velocity in it ( should be in the range of 10 to 15 m/sec, if fluid is clean , check for erosion


velocity if this contain sand particles.)


f. assume standard length of 6 , 8 of 10 ft and check the total area is more than the area required by heat


transfer in step d.

3. fuel consumption : what is the fuel calorific vale? divide heat duty by this and get mass flow rate of fuel, fuel system


is designed based on this , line size, valve sizes etc.

4. Fire tube design :


a. The heat given to process fluid is divide by the heater efficiency ( typically 80-90%). this will give you the heat


given by fire tube.


b. Assume Diameter and length of fire tube ( i.e. 20” and 10 Ft) , calculate this area


i. Divide The Heat calculated for fire tube from this area and get the heat flux,


ii. This should be in the range of 10000 to 12000 btu/hr/ft2 as per API 12 K , so adjust dia and length.

5. Check fire box rating and as per API 12k

6. Determine Tube thickness as per API 12k


a. Which material to be used and what would be the hoop stress of that (‘S’)


That’s All for beginner, but you really need to do a real life situation design to fully understand the pain of designing this. the above

article is meant to use as a guide path for the same.







Please comment to enrich the same.

Saturday, August 1, 2015

Tray Tower-Distillation Column Design rule of thumbs

Tray Towers   
   
A.  For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressures.
  
B.  Tower operating pressure is most often determined by the cooling medium in condenser or the    
     maximum allowable reboiler temperature to avoid degradation of the process fluid.
   
C.  For sequencing columns:   
    1.  Perform the easiest separation first (least trays and lowest reflux)
    2.  If relative volatility nor feed composition vary widely, take products off one at time
         as the overhead
    3.  If the relative volatility of components do vary significantly, remove products in order
         of decreasing volatility
    4.  If the concentrations of the feed vary significantly but the relative volatility do not,
         remove products in order of decreasing concentration.

D.  The most economic reflux ratio usually is between 1.2Rmin and 1.5Rmin
   
E.  The most economic number of trays is usually about twice the  minimum number of trays.   
    The minimum number of trays is determined with the Fenske-Underwood Equation.

F.  Typically, 10% more trays than are calculated are specified for a tower. 
 
G.  Tray spacings should be from 18 to 24 inches, with accessibility in mind
  
H.  Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures,    
     or 6 ft/s (1.8 m/s) under vacuum conditions. 
 
I.  A typical pressure drop per tray is 0.1 psi (0.007 bar)   
J.  Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and   
     stripping typically have efficiencies closer to 10-20%   
K.  The three most common types of trays are valve, sieve, and bubble cap.  Bubble cap trays are    
      typically used when low-turn down is expected or a lower pressure drop than the valve or sieve   
      trays can provide is necessary.   
L.  Seive tray holes are 0.25 to 0.50 in. diameter with the total hole area being about 10% of the total active tray area.   
M.  Valve trays typically have 1.5 in. diameter holes each with a lifting cap.  12-14 caps/square foot of tray is a good benchmark.     
    Valve trays usually cost less than seive trays.
N.  The most common weir heights are 2 and 3 in and the weir length is typically 75% of the tray diameter
   
O.  Reflux pumps should be at least 25% over-designed
   
P.  The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00
  
Q.  Reflux drums are almost always horizontally mounted and designed for a 5 min holdup at half of the drum's capacity.   

R.  For towers that are at least 3 ft (0.9 m) is diameter, 4 ft (1.2 m) should be added to the top for vapor release and 6 ft (1.8 m) should be added to the bottom to account for the liquid level and reboiler return   

S.  Limit tower heights to 175 ft (53 m) due to wind load and foundation considerations.   

T.  The Length/Diameter ratio of a tower should be no more than 30 and preferrably below 20 
 
U.  A rough estimate of reboiler duty as a function of tower diameter is given by:   
    Q = 0.5 D^2   for pressure distillation
    Q = 0.3 D^2 for atmospheric distillation
    Q = 0.15 D^2 for vacuum distillation
    where Q is in Million Btu/hr and D is tower diameter in feet
Refrigeration and Utilities

A. A ton of refrigeration equals the removal of 12,000 Btu/h (12,700 kJ/h) of heat

B. For various refrigeration temperatures, the following are common refrigerants:

Temp (0F)        Temp (0C)               Refrigerant
0 to 50                -18 to -10               Chilled brine or glycol
-50 to -40        -45 to -10                  Ammonia, Freon, butane
-150 to -50        -100 to -45              Ethane, propane
          
    
C. Cooling tower water is received from the tower between 80-90 0F (27-32 0C) and should be returned between 115-125 0F (45-52 0C) depending on the size of the tower. Seawater should be return no higher than 110 0F (43 0C)

D. Heat transfer fluids used: petroleum oils below 600 0F (315 0C), Dowtherms or other synthetics below 750 degree F (400 degreeC), molten salts below 1100 degreeF (600 degreeC)

E. Common compressed air pressures are: 45, 150, 300, and 450 psig

F. Instrument air is generally delivered around 45 psig with a dewpoint that is 30 °F below the coldest expected ambient temperature

Pump Funda's

Pumps

A. Power estimates for pumping liquids:
  • kW=(1.67)*[Flow (m3/min)]*[Pressure drop (bar)]  / Efficiency

  • hp=[Flow (gpm)]*[Pressure drop (psi)]  / 1714*(Efficiency) 

Note that Efficiency expressed as a fraction in these relations


B. NPSH
              = (pressure at impeller eye-vapor pressure)/(density*gravitational constant)

Common range is 1.2 to 6.1 m (4-20 ft) of liquid


C. An equation developed for efficiency based on the GPSA Engineering Data Book is:

Efficiency = 80-0.2855F+.000378FG-.000000238FG^2+.000539F^2-.000000639(F^2)G+
.0000000004(F^2)(G^2) where

Efficiency is in fraction form,
F is developed head in feet, and
G is flow in GPM

Ranges of applicability are F=50-300 ft and G=100-1000 GPM Error documented at 3.5%


D. Centrifugal pumps: 

  • Single stage for 0.057-18.9 m3/min (15-5000 GPM), 152 m (500 ft) maximum head;
  • For flow of 0.076-41.6 m3/min (20-11,000 GPM) use multistage, 1675 m (5500 ft) maximum head; 
Take Efficiencies of 45% at 0.378 m3/min (100 GPM), 70% at 1.89 m3/min (500 GPM),

80% at 37.8 m3/min (10,000 GPM).


E. Axial pumps can be used for flows of 0.076-378 m3/min (20-100,000 GPM)

Expect heads up to 12 m (40 ft) and efficiencies of about 65-85%


F. Rotary pumps can be used for flows of 0.00378-18.9 m3/min (1-5000 GPM)

Expect heads up to 15,200 m (50,000 ft) and efficiencies of about 50-80%


G. Reciprocating pumps can be used for 0.0378-37.8 m3/min (10-100,000 GPM)

Expect heads up to 300,000 m (1,000,000 ft).

Efficiencies: 70% at 7.46 kW (10 hp), 85% at 37.3 kW (50 hp), and 90% at 373 kW (500 hp)

Evaporation Funda's


Evaporation















A.  Most popular types are long tube vertical with natural or forced circulation.  Tubes range from 3/4" to 2.5"  (19-63 mm) in diameter and 12-30 ft (3.6-9.1 m) in length.









B.  Forced circulation tube velocities are generally in the 15-20 ft/s (4.5-6 m/s) range.









C.  Boiling Point Elevation (BPE) as a result of having dissolved solids must be  accounted for in the differences between the solution temperature and the temperature of the saturated vapor.









D.  BPE's greater than 7 °F (3.9 °C) usually result in 4-6 effects in series (feed-forward) as an economical solution.  With smaller BPE's, more effects in series are typically more economical, depending on the cost of steam.









E.  Reverse feed results in the more concentrated solution being heated with the hottest steam to minimize surface area.  However, the solution must be pumped from one stage to the next.









F.  Inter stage steam pressures can be increased with ejectors (20-30% efficient) or mechanical compressors (70-75% efficient).

Calculating Vertical tank with dished (or flat) ends

Dear Reader

I just customize an standard Vertical tank with dished (or flat) ends volume calculation. You just need to put values in Blue font cell. The tank volume is required for various purposes like Calculating relief load .

Tank Claffification as per pressure rating:

  • Above Ground

Atmospheric — Atmospheric pressure tanks are designed and equipped for storage of contents at atmospheric pressure. This category usually employs tanks of vertical cylindrical configuration that range in size from small shop welded to large field erected tanks. Bolted tanks, and occasionally rectangular welded tanks, are also used for atmospheric storage service.

Low Pressure (0 to 2.5 psig) — Low pressure tanks are normally used in applications for storage of intermediates and products that require an internal gas pressure from close to atmospheric up to a gas pressure of 2.5 psig. The shape is generally cylindrical with flat or dished bottoms and sloped or domed roofs. Low pressure storage tanks are usually of welded design. However, bolted tanks are often used for operating pressures near atmospheric. Many refrigerated storage tanks operate at approximately 0.5 psig.

Medium Pressure (2.5 to 15 psig) — Medium pressure tanks are normally used for the storage of  higher volatility intermediates and products that cannot be stored in low pressure tanks. The shape may be cylindrical with flat or dished bottoms and sloped or domed roofs. Medium pressure tanks are usually of welded design. Welded spheres may also be used, particularly for pressures at or near 15 psig.

High Pressure (Above 15 psig) — High pressure tanks are generally used for storage of refined  products or fractionated components at pressure above 15 psig. Tanks are of welded design and may be of cylindrical or spherical configuration.

  • Underground
Gas processing industry liquids may be stored in underground, conventionally mined or solution  mined caverns. No known standard procedures are available for this type storage; however, there are many publications and books covering the subject in detail.

Vertical tank volume Calculation