June 28, 2009

Steam Properties - I

I have been giving few posts on different properties estimation methods for various uses for process engineers. However, the most useful & most frequently used property is the steam property for which earlier I suggested using an Excel Add-In called water_97.xla.

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But, sometimes if you are using different programs other than Excel, in that case, the add-in will not work & you need to know the correlations for different properties to use in other calculators.

So I thought it would be better to share these correlations for the benefit of all readers. We will cover these properties in few parts of this post. This is the first one for P & T correlation only.

1. Saturation Pressure at Given Temperature

Log(e) (2256500/P) = [7.21379 + (alpha + beta * T + gamma * T^n) (T-483.16)^2] x [647.31 / T - 1]

T = Temperature in Deg K
alpha, beta & gamma are given as below,

alpha = 1.152 x 10^-5
beta = -4.787 x 10^-9
gamma = 0 & n = 0 for t = 0 to 210 °C

alpha = 1.0071 x 10^-6
beta = 1.9312 x 10^-8
gamma = 8.913 x 10^-96 & n = 32 for t = 210 to 374.15 °C

Alternatively more simpler formula is

T = (P ^ 0.25) * 100


P = ( T / 100) ^ 4

Where T is in °C.

One more equation developed by me
This equation is valid from t = 0 to 374 °C

Log10 (Pv) = A t^5 + B t^4 + C t^3 + D t^2 + E t + F

A = 3.482223 x 10^-13
B = - 4.890675 x 10^-10
C = 3.038026 x 10^-7
D = -1.1351158 x 10^-4
E = 0.03090855
F = -2.2016923

List of other property estimation methods on this Blog.

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June 20, 2009

HAZOP - Basic Understanding

Hazard and operability studies are a methodology for identifying and dealing with potential processes, particularly those which would create a hazardous situation or severe impairment of the process. It is commonly known as HAZOP.

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HAZARD any operation that could possibly cause a catastrophic release of toxic, flammable or explosive chemicals or any action that could result in injury to personnel.

OPERABILITY any operation inside the design envelope that would cause a shutdown that could possible lead to a violation of environmental, health or safety regulations or negatively impact profitability.

HAZOP Process
The HAZOP focuses on specific portions of the process called “nodes”. Generally these are identified from the P&ID of the process. A process parameter is identified, say flow, and an intention is created for the node under consideration.

Then a series of guidewords is combined with the parameter “flow” to create deviations. For Example, the guideword “no” is combined with the parameter flow to give the deviation “no flow”. Then focus is on listing all the credible causes of a “no flow” deviation beginning with the cause that can result in the worst possible consequence one can think of.

Once the causes are recorded, a list is made of the consequences, safeguards and any recommendations deemed appropriate. The process is repeated for the next deviation and so on until completion of the node.

Guidewords, Selection of Parameters and Deviations
The HAZOP process creates deviations from the process design intent by combining guide words (no, more, less, etc) with process parameters resulting in a possible deviation from design intent.

These include Flow, Pressure, Temperature, Level, Time, Agitation, Reaction, Start-Up/Shut-Down, Draining/Venting, Utility Failure(instrument air, power), Maintenance, Vibrations etc.

Concept of Point Of Reference
When defining nodes and performing a HAZOP on a particular node it is useful to use the concept of point of reference (POR) when evaluating deviations.

Screening for Causes of Deviations
It is necessary to be thorough in listing causes of deviations. A deviation is considered realistic if there are sufficient causes to believe the deviation can occur. However, only credible causes should be listed.

There are three basic types of causes. They are:

  • Human error which are acts of omission or commission by an operator, designer, constructor or other person creating a hazard that could possibly result in a release of hazardous or flammable material.

  • Equipment failure in which a mechanical, structural or operating failure results in the release of hazardous or flammable material.

  • External Events in which items outside the unit being reviewed affect the operation of the unit to the extent that the release of hazardous or flammable material is possible. External events include upsets on adjacent units affecting the safe operation of the unit (or node) being studied, loss of utilities, and exposure from weather and seismic activity.

By Associate Writer : Ms Nidhi Gupta

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June 13, 2009

How to Read P & ID

Often when you use P&ID you will find that each company or engineering consultant is using his own list of symbols & instrumentation diagrams. This is very confusing for young engineers in the initial age where sometime somebody is using B symbol for tanks, some use D & finally some use T.

Why it is so?

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Equipment & Instrumentation details vary with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provide less detail than fully developed piping and instrumentation diagrams (P&IDs).

Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA’s S5.1-1984 (R 1992) Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, and connecting lines.

Deciphering symbols
ISA S5.1 defines four graphical elements—discrete instruments, shared control/display, computer function, and programmable logic controller and groups them into three location categories (primary location, auxiliary location, and field mounted).

Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded by a square. Computer functions are indicted by a hexagon and programmable logic controller (PLC) functions are shown as a triangle
inside a square.

Adding a single horizontal bar across any of the four graphical elements indicates the function resides in the primary location category. A double line indicates an auxiliary location, and no line places the device or function in the field. Devices
located behind a panel-board in some other inaccessible location are shown with a dashed horizontal line Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard.

Numbers are user assigned and schemes vary with some companies use of sequential numbering, others tie the instrument number to the process line number, and still others adopt unique and sometimes unusual numbering systems.

The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth.

Referring to the Example P&ID diagram, FT 101 represents a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device. A square root extraction of the input signal is applied as part of FIC 101’s functionality.

The output of FIC 101 is an electrical signal to TY 101 located in an inaccessible or
behind-the-panel-board location. The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer). TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature. TIC 101’s output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy.

Often P&ID’s include a cover page where common and typical terms, symbols, numbering systems, etc., are defined. On the example, Typical YIC would likely appear on the cover page and the simplified form of YIC would appear throughout the P&IDs.

Typical YIC indicates an on/off valve is controlled by a solenoid valve and is fitted with limit switches to indicate open (ZSH) and closed (ZSL) positions. All inputs and outputs are wired to a PLC that’s accessible to the operator (diamond in a square with a solid horizontal line). The letter "Y" indicates an event, state, or presence. The letter "I" depicts indication is provided, and the letter "C" means
control takes place in this device.

Adherence to ISA’s S5.1 Instrumentation Symbols and Identification standard ensures a consistent, system independent means of communicating instrumentation, control, and automation intent is developed for everyone to understand.

The article is adopted from Control Engineering magazine with some modifications for the learning of young engineers.

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June 05, 2009

Viscosity of Gaseous Mixtures

In the previous few posts, I discussed about methods for calculating viscosities of pure gases using different methods at low & high pressures. What about mixtures?

Calculating property of any mixture is more tedious than pure components. They vary significantly if components widely vary. Also its very difficult to search for applicable & trusted correlations to calculate the mixture properties.

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So in this part we will discuss the method which are applicable for gaseous mixtures at low & high pressures.

The method used in this case is called Wilke correlation which is very easy for binary mixtures and can be used for multi components mixtures also.

The equation is

vism = Sum (i = 1 to n) [ yi visi / Sum (j = 1 to n) yi phi(i,j) ]

vism = viscosity of mixture
Sum (i = 1 to n) = Sigma sign for i = 1 to n which is represented here in language term due to non availability of symbols in blogger.
yi = mole graction of component i in the mixture
visi = viscosity of pure gas for i compnents
phi(i,j) = double parametric symbol given as below

phi(i,j) = 1 if i = j

phi (1,2) = [ 1 + (vis1/vis2)^0.5 * (M2/M1)^0.25 ]^2 / [8 ( 1 + M1/M2 )]^0.5

Where M1, M2 are molecular weights
Vis1, Vis2 are pure component viscosity


phi(2,1) = vis2/vis1 * M1/M2 * phi (1,2)

So for Binary mixture it reduces to

vism = [ y1 vis1 / (y1 + y2 phi(1,2))] + [y2 vis2 / (y2 + y1 phi(2,1))]

Viscosity of Gas Mixture at High Pressure
The correlation used is of Dean & Stiel for non polar gases mixture.

(vism - vism0) * zetam = 1.08 [ exp(1.439 Drm) - exp(-1.111 Drm ^ 1.858)]

vism = viscosity of mix at high pressure
vis0 = viscosity of mix at atm pressure
zetam = for mixture as defined in the article of Viscosity at High Pressure.
Drm = Reduced density for mixture

To calculate the reduced density of mixture you need to know the critical properties of the mixture for which following general equations are used.

Tcm = Sum (i = 1 to n ) yi x Tci
Zcm = Sum (i = 1 to n ) yi x Zci
Vcm = Sum (i = 1 to n ) yi x Vci
Pcm = Zcm R Tcm / Vcm

So Drm = 1/ Vrm
& Vrm = Vm/Vcm

Yes dear you are right again the unit of viscosity here is micropoise.

List of other property estimation methods on this Blog.

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