chemical engineering: August 2007

Wednesday, August 29, 2007

Typical Refinery Flow Sheet

Milt Beychok from www.air-dispersion.com has shared a schematic flow diagram of a typical oil refinery that depicts the various unit processes and the flow of intermediate product streams that occurs between the inlet crude oil feedstock and the final end products. This image has given a brief idea how crude oil is refined. Products included LPG, Butanes, Jet Fuel, Kerosene, Diesel Oil and Fuel Oil and side product Sulfur.

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Tuesday, August 28, 2007

Different Equation for Pitting Resistance Equivalent Number (PREN)

As discussed in earlier post <<Pitting Corrosion - Mechanism & Prevention>>, pitting corrosion is one of the most common localized corrosion attack and most destructive form of corrosion in metal and alloy. Out of so many type of alloy, how to differential the

pitting resistivity of particular metal and alloy compare to the other ? Pitting Resistance Equivalent Number is used.

Pitting Resistance Equivalent Number (PREN) is an index common used to measure and compare resistance level of a particular metal and alloy to pitting corrosion.

PREN can be calculated , using the alloy chemical composition, to estimate relative pitting resistance of metal and alloys.

Common equation for PREN calculation as followed :

PREN = %Cr + m.(%Mo) + n.(%N)

Per experiments, m range from 3.0 to 3.3 whilst n range from 12.8 to 30.

For ferritic grades Stainless Steel, the formula employed is :

PRE = % Cr + 3.3 (% Mo)

For austenitic grades Stainless Steel, the formula employed is :

PREN = %Cr + 3.3(%Mo) + 30(%N)

For duplex (austenitic-ferritic) grade Stainless Steel, the formula employed is :

PREN = %Cr + 3.3(%Mo) + 16(%N)

For high Ni-Cr-Mo alloys e.g. Inconel 625, Hastelloy, etc, the formula employed is :

PREN = %Cr + 1.5(%Mo + %W + %Nb)

where...

Cr - Chromium

Mo - Molybdenum

W - Tungsten

Nb - Niobium

Pitting is one of main problem for material expose to seawater. Minimum PREN required for material expose to seawater is 40. Duplex Stainless steel, Super duplex stainless steel, etc are exhibiting PREN > 40.

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Sunday, August 26, 2007

Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve

Gas expansion through relief valve is ISENTROPIC or ISENTHALPIC process ?

This question has been raised by many engineers and debated in many discussion.

Careful review on the relief valve structure, process fluid passing through a nozzle instead of an orifice. Those many researchers such as Dr. Ron Darby for multiphase flow in Direct Integration method (HDI), Dr Fauske & Dr Henry for two phase relief in Homogeneous Non-equilibrium model (HNE), Dr. Joseph Leung for two phase flow in Omega method, etc have modelled a relief valve as a nozzle.

Classical and conventional studies informed us that vena contracta (VC) will only occur in sharp edge orifice. However, the flow restricting area in a relief valve as decribed by many well knowed specialists is not an orifice, but is actually a nozzle. Thus there is NO vena contracta downstream of the nozzle.

Phenomenon of flow through a nozzle in relief valve is extremely fast. Choked flow is possible occur in some location "A" in the nozzle instead of outside the nozzle (may be closed to the exit end), from the inlet to "A" will be a REVERSIBLE process which generally accepted by most of the specialists. Thus, it is ISENTROPIC process. From location "A" to PSV outlet, the system is expanded and change in state. It will be slow down. There will be transformation energy loss however the enthalpy is maintain constant (ISENTHALPIC). This process is IRREVERSIBLE. "A" is viewed as vena contracta in the relief valve's nozzle. Those in sizing a relief valve, we ignore the frictional loss in the nozzle and consider ISENTROPIC process. However, from expansion process via safety valve (Vena contracta to relief outlet), the process will be ISENTHALPIC.

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Saturday, August 25, 2007

Floating Gas Refinery Unit

You have heard about Floating Production Storage Offloading (FPSO) unit...you have heard about Floating LNG Unit...have you ever heard about Floating Gas Refinery Unit ?

Read HERE...

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Friday, August 24, 2007

Pitting Corrosion - Mechanism & Prevention

Pitting Corrosion on Metal Surface

Pitting is one of the most destructive forms of corrosion as it will potential cause equipment failures due to perforation / penetration. pitting generally occurs on metal surfaces protected by oxide film such as Stainless steel, aluminum, etc. Typically for boiler and feed water system, pitting corrosion rate increase dramatically with the increase of oxygen content in the fluid.

Pitting can occur in any metal surfaces. Following are some pictures of pitting corrosion.

Pitting corrosion on external pipe surface

H2S Pitting corrosion on internal pipe surface

Co2 Pitting corrosion on internal pipe surface

Mechanism

Lets look at figure below, oxygen rich fluid in contact with metal surface (at the top of the pit) will becomes the cathode. At the bottom of the pit, low in oxygen level becomes the anode. this will form a complete circuit where metal at the pit (FE) will be ionized to release electron (e) and form ion Ferum (FE2+), this electron will travel to the top of pit to react with Oxygen (O2) (and water, H2O) to form ion hydroxides (OH-). Ion Ferum (FE2+) will react with ion hydroxides (OH-) to form Ferum Oxide (Fe2O3) which typically a brown rust. Deeper the pit leeser the oxygen content and higher the potential and pitting corrosion rate.

Severity of pitting corrosion
Knowing that pitting can cause failure due to perforation while the total corrosion, as measured by weight lossm might be rather minimal, experience shown that rate of penetration may be 10 to 100 times that by general corrosion, pitting corrosion has been considered to be more dangerous than the uniform corrosion damage because it is very difficult to detect, predict and design against. General metal weight loss method almost impossible to detect the internal pitting corrosion.

Pitting corrosion shape
Pits formed due to pitting corrosion can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal. Following picture demonstrate several types of pitting corrosion shape. This has made it even more difficult to be detected especially undercutting, subsuface and horizontal type.

Preventive measures
There are several preventive approah to avoid pitting. There are :

Proper material selection e.g. SS316 with molydenum having higher pitting resistance compare to SS304
Use higher alloys (ASTM G48) for increased resistance to pitting corrosion
Control oxygen level by injecting oxygen scavenger in boiler water system
Control pH, chloride concentration and temperature
Cathodic protection and/or Anodic Protection
Proper monitoring of oxygen & chloride contents by routine sampling
Agitation of stagnant fluid

Thursday, August 23, 2007

How a Chloride Stress Corrosion Cracking Lookslike ?

The following are some images of metal experienced Chloride Stress Corrosion Cracking.

Inter granular SCC of an Inconel heat exchanger tube

Trans granular SCC of 316 stainless steel chemical processing piping system

CSCC occured on insulated vessel

CSCC occured on insulated vessel

CSCC occured on Condenser tube

CSCC on pipe

Inter granular SCC of a pipe

Wednesday, August 22, 2007

Chloride Stress Corrosion Cracking & Use correct MOC for seawater service

Chloride stress - corrosion cracking (CSCC) is initiation and propagation of cracks in a metal or alloy under tensile stresses and a corrosive environment contains Chloride compounds. Once the crack is initiated, it will propagate rapidly and potentially lead to catastrophic failure.

Factors that influence the rate and severity of cracking include

chloride content
oxygen content
temperature
stress level
pH value of an aqueous solution

Higher chloride content in process fluid will increase potential of CSCC.

It has been established that oxygen is required for CSCC to occur. Detail may refer to HERE.

The severity of cracking increases with temperature. Figure below shows several Stainless Steel materials increases it susceptibility to CSCC as temperature is increased.

Source : Sandvik Material Technology

SAF 2205 (UNS 31803) = Duplex Stainless Steel
SAF 2507 (UNS 32750) = Super Duplex Stainless Steel

Material under pressure without Post weld heat treatment will experience high stress level. Higher the stress level, higher the potential of CSCC.

Acidic process(low pH) with chloride content in it tends to increase the CSCC potential.

CASE STUDIES

Hot gas (Shell) is cooled by seawater (Tube) from 220 degC to 180 degC in a Shell & Tube heat exchanger. Seawater is being heated from 30 degC to 35 degC and return to sea. The Shell and Tube material of construction are Carbon steel (CS) and Duplex Stainless Steel (DSS) respectively. After 2 months in operation, cracks occurred at the tube (DSS) and leads to major platform shutdown. Investigation found crack was caused by CSCC at tube.

Why a CSCC occurred at DSS tube although the seawater temperature only 35 degC maximum ?

Eventhough the inlet and outlet temperature are below 150 degC, thermal designer may design the heat exchanger with high heat flux in order to reduce the heat exchanger area and this result tube skin temperature exceeded 150 degC. Condition with Seawater which contains ~20,000 mg/l Chloride, high in dissolved oxygen, slightly acidic and skin temperature exceeded 150 degC is perfect combination conditions for CSCC to occur for DSS. Those heat exchanger designer shall always check skin temperature profile especially for low flow condition or specify better material i.e. Super DSS for above service.

Further Reading

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Tuesday, August 21, 2007

Unified Numbering System for Metals and Alloys

What are the differences between Duplex Stainless Steel, Medium Alloy Duplex, 22% Cr, SAF 2205 and UNS 31803 ?

They are refer to same metal. Duplex Stainless Steel and Medium Alloy Duplex is general (layman) term and commonly used across discipline. Material specialist like to call it 22% Cr. SAF 2205 is the trade name where procurement people like put it in purchase order. Different terms used sometime may results confusion and miscommunication. Thus, Unified Numbering System (UNS) has been created for standardization and easy administration. This system is widely use in North American included Canada.

“The Unified Numbering System for Metals and Alloys (UNS) provides a means of correlating many internationally used metal and alloy numbering systems administered by societies, trade associations, and those individual users and producers of metals and alloys. It provides the uniformity necessary for efficient indexing, record keeping, data storage and retrieval, and cross-referencing.”

Above was extracted from book <<Metals & Alloys in the Unified Numbering Systems >>. This book (in CD) provides information on :

UNS number
Description
Common trade names and alloy designations
Cross-reference organization
Cross-reference specifications
Chemical composition

The UNS is managed jointly by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE).

The UNS number (for "Unified Numbering System for Metals and Alloys") is a systematic approach where each metal is designated by a LETTER followed by five NUMBERS. The number is unique and composition-based of commercial materials. It is used for material reference but it does not guarantee any performance specifications and/or exact composition.
Following are overview of common commercial metals / alloys using UNS system :

Axxxxx - Aluminium Alloys
Cxxxxx - Copper Alloys, including Brass and Bronze
Fxxxxx - Iron, including Ductile Irons and Cast Irons
Gxxxxx - Carbon and Alloy Steels
Hxxxxx - Steels - AISI H Steels
Jxxxxx - Steels - Cast
Kxxxxx - Steels, including Maraging, Stainless, HSLA, Iron-Base Superalloys
L5xxxx - Lead Alloys, including Babbit Alloys and Solders
M1xxxx - Magnesium Alloys
Nxxxxx - Nickel Alloys
Rxxxxx - Refractory Alloys
R03xxx- Molybdenum Alloys
R04xxx- Niobium (Columbium) Alloys
R05xxx- Tantalum Alloys
R3xxxx- Cobalt Alloys
R5xxxx- Titanium Alloys
R6xxxx- Zirconium Alloys
Sxxxxx - Stainless Steels, including Precipitation Hardening and Iron-Based Superalloys
Txxxxx - Tool Steels
Zxxxxx - Zinc Alloys

Typical example :

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Monday, August 20, 2007

Heat Transfer Coefficient For Air-Cooled Heat Exchangers

Basic heat transfer relationships is apply to Air-Cooled Heat Exchanger (ACHE) . The fundamental heat transfer equation :

Q = U. A. LMTD.F

where
U = overall heat transfer coefficient
A = Heat transfer area
LMTD = Log mean temperature difference
F = Correction factor

Typical heat transfer coefficient for Air-Cooled Heat Exchangers

Source : Hudson Product Corporation (HPC)

Source : DELTA T

Further Reading

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Sunday, August 19, 2007

Air Cooled Heat Exchanger Control using Variable Pitch Fans

There are several ways of controling process fluid temperature of an Air Cooled Heat Exchanger. They are tabulated as follow :

i) Fluid bypass (manual)
ii) On-off fan operation (manual)
iii) Two-speed fans (manual or automatic)
iv) Louvers or shutters (automatic)
v) Variable pitch fans (automatic)
vi) Variable speed fans (automatic)

Out of all methods, variable speed fans control is considered the most reliable as compare to others. Nevertheless, R.C. Monroe from Hudson is proposing variable pitch fans...Read more in the following article.

Consider Variable Pitch Fans

R.C. Monroe

Hudson Products Corp. Houston, Texas

Saturday, August 18, 2007

Design of Quiet Air-Cooled Heat Exchangers

Many industrial facilities are required to meet stringent noise requirements. These requirements are imposed to protect workers’ hearing and/or to meet community ordinances. The facility designer must pay careful attention to the noise level of all industrial equipment, including air-cooled heat exchangers.

Air-cooled heat exchangers are a source of plant noise. Therefore, it is important to design each unit to produce the minimum amount of noise while still meeting the thermal requirements at a reasonable cost.

Design of Quiet Air-Cooled Heat Exchangers

S. Chapple & A. Pinkerton, Hudson Products Corp. Houston, Texas

This paper discusses the major noise sources of an air-cooled heat exchanger, the factors affecting the noise from each source, and how the source affects the overall noise level of the air-cooled heat exchanger.

Further reading

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Friday, August 17, 2007

HPC - Air Cooled Heat Exchanger & Axial Fans - P3

Tuf-Lite
Axial Flow Fans for Air Cooled Heat Exchanger & Cooling tower from HUDSON Product Incorporation (HPC)

Read The Basics of Axial Flow Fans for basic understanding of Axial Fan especially Tuf-Lite series. You may check-out the differences between these 3 generation of Tuf-Lite axial fans with following links :

If you are plant / maintenance or isntallation engineer of axial fans, your may be interested to read Axial Fans Manual, Fan assembly & installation instruction, Field inspection Guidelines and Proper Belt Tension.

Further reading

Thursday, August 16, 2007

HPC - Air Cooled Heat Exchanger & Axial Fans - P2

Apart from FREE software for download, HUDSON also share other very useful articles in it website...

Basic of Air-Cooled Heat Exchanger (ACHE)

This is very good article for a beginer involve in design and operation of an air-cooled heat exchanger. In this article, many aspects of air-cooled heat exchanger has been discussed. Those aspects include Components in Air-cooled heat exchanger, construction of forced draft fan and induced draft fan air-cooled heat exchnager, tube bundle construction, type of fins, comparison between forced draft fan and induced draft fan, thermal design of air cooled heat exchanger, Typical Heat Transfer Coefficients for Air-Cooled Heat Exchangers, fan selection, controls, etc.

Other very useful articles available for FREE download in HUDSON website are :

Further Reading

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Tuesday, August 14, 2007

HPC - Air Cooled Heat Exchanger & Axial Fans - P1

HUDSON Product Corp. (HPC) a well known Air cooled Heat Exchanger. Establish since 1939, has designed and manufactured air-cooled heat exchanger equipment to serve the oil, gas and petrochemical processing industries. HPC is pioneer in this field, has developed several international recognized air cooled Heat exchanger product :

Fin-Fan ® Air-Cooled Heat Exchangers
Hy-Fin ® Extruded Finned Tubing
Tuf-Lite ® FRP Axial Flow Fans for air coolers and cooling towers.
Tuf-Lite I I® FRP Axial Flow Fans for air coolers and cooling towers.

FREE Air-cooled Heat Exchanger and Fan rating Softwares available for download :

Basics of Air-Cooled Heat Exchangers - This software is designed to familiarize users with the types, components, and features of air-cooled heat exchangers.
Fan-Rating Program - The Tuf-lite Fan Selection Program is designed to assist customers in selecting the appropriate axial flow fan for their application.

Further reading

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Monday, August 13, 2007

Is pressure drop increase with pipe schedule ?

Identical flowrate passing 2" pipes with different pipe schedule (40 & 160), will pressure drop increase on schedule 160 ?

Most of us may already aware of the answer. Pressure drop will increase.

Reason being,

"The OD of the pipe is basically fixed (+ or -) but as you have already determined, the schedule number is related to the thickness, with the higher numbers indicating thicker pipe, smaller pipe ID and therefore a smaller cross sectional area of the flow path. A 2" pipe has a fairly constant OD of 2.375". The 2" schedule 40 pipe has an ID of 2.067" but a 2" schedule 160 pipe has an ID of 1.687". Obviously the schedule 160 pipe has a smaller flow path than does the schedule 40 pipe and thus for your example the velocity (and pressure drop) will be greater in the schedule 160 pipe.", by Phil LECKNER.

The post has triggered me to create some handy links...

What's PIPE Schedule ?

For all pipe sizes the outside diameter (O.D.) remains relatively constant. The variations in wall thickness affects only the inside diameter (I.D.). Details...

NPS - "Nominal Pipe Size" and DN - "Diametre Nominel"
The size of pipes, fittings, flanges and valves are often given in inches as NPS - Nominal Pipe Size, or in metric units as DN - "Diametre Nominel"

Pipe Data CHART

Carbon, Alloy and Stainless Steel Pipes - ASME/ANSI B36.10/19

Pipe sizes, inside and outside diameters, wall thickness, schedules, moment of inertia, transverse area, weight of pipe filled with water - U.S. Customary Units

Pipes Fractional Equivalents
Comparing fractions and inches for pipes

Pipe Equations
Calculate cross-sectional area, weight of empty pipes, weight of pipes filled with water, inside and outside surface area

Calculating Pipes Weight

If the outside diameter and the wall thickness of a steel pipe is known, the weight per foot can be expressed as:

m = 10.68 (do - tw) tw

where
m = weight per foot (lbs/ft)
do = outside diameter (inches)
tw = wall thickness (inches)

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Friday, August 10, 2007

Use of conventional type PSV with back pressure exceeded 10% set pressure

Back pressure is seriously affects the operational and capacity of Pressure Relief valves (PRV/PSV). As the backpressure to spring loaded direct acting PSV (conventional type) is increased, the driven force which is a function of differential pressure across a PSV will drop and reduce the ability to flow across the PSV. impact on flow capacity is minimal as long as the back pressure is below it critical pressure.

Another phenomenon which common understood by everyone is that as the backpressure is increased, additional force apply on the PSV disc and reduce disc lift, it reduce discharge capability. This clearly explained in figure 22 in API RP 521 Part-I.

However, the impact is insignificant at low backpressure. API 521 Part-I has recommended that in a conventional PSV application, built-up backpressure SHOULD NOT exceed 10% of the set pressure at 10% allowable overpressure (refer 3.3.3.1.3.). This statement is recommendation instead of mandatory requirement per API. Those conventional type PSV still can be used if the built backpressure > 10% of set pressure for specific events and selected PSVs.

A studies <<Back pressure effects on safety valves operating with compressible flow>>, conducted by Vincenzo Dossena has clearly proved above statement.

This report has showed that
i) ratio of measured discharge coefficient (Kd,m) and theoretical discharge coefficient (Kd,t) (at 10% overpressure) is in the range of 0.95 to 1.02 at 10% built-up backpressure

ii) ratio of measured discharge coefficient (Kd,m) and theoretical discharge coefficient (Kd,t) (at 10% overpressure) maintain at / above 1 for some PSVs at >10% built-up backpressure

Point (i), with the minimum ratio of 0.95, normal Kd factor for PSV is 1, the actual Kd may be conservatively 0.95.
API 520 Part-I has recommended Kd factor of 0.975. SHELL DEP has taken conservative approach where Kd factor of 0.9 to be used.

Point (ii), this implies that conventional PCV may be used even the back pressure is exceeded 10% built-up back pressure. Designer/engineer shall take EXTRA attention if conventional PSV is used when the back pressure > 10% set pressure and shall always seek advice & confirm with PSV vendor.

Reference :
i) API RP 520 Part-I, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Seven Edition, an 2000

ii) Vincenzo Dossena, “Back pressure effects on safety valves operating with compressible flow”, published in VALVE WORLD.