chemical engineering: February 2008

Thursday, February 28, 2008

Design Temperature (Td) versus Maximum Allowable Working Temperature (MAWT)

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A question raised related to Design temperature (Td) and Maximum Allowable Working Pressure (MAWT).

"I understood that Maximum Allowable Working Pressure (MAWT)

is the temperature where the vessel material will fail by its internal (design) pressure due to reduced at elevated temperature
is subject to vessel wall thickness. MAWT should be same or higher than design temperature (DT) as fabricated wall thickness would be thicker due to material available in the market or shop or some conservative calculation roundup.
Allowable stresses are same from-20F to 650 F (-29C to 343 C)

Is the vessel is fit from -29 C to 343 C ? Can we declare design temperature of 343 degC ?"

Above analysis is pretty correct to some extent. However, it is not the complete story about the design temperature and MAWT.

MAWT for tank is subject to vessel/tank wall thickness. Above analysis has covered most (if not all) points. However, there are other devices and fitting like flange, instrument, nozzle, etc attached to the vessel and possibly, the MAWT of these devices and fittings are much lower than the MAWT of tank. We shall always refer MAWT for SYSTEM instead of a particular equipment / instrument.

Hence the design temperature of the system is come into the picture ?
As mentioned above, MAWT for equipment, instrument, piping, etc are different. Design temperature for a system is what the process demand, MAWT is what the equipment / device can take. MAWT for ALL equipment, instrument, fittings, etc within a SYSTEM shall equal or more than the design temperature as specified by process engineer.

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What if Engineering Control Not Feasible ?

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Coming Tuesday, CETD, IEM will organised a talk on

"WHAT IF ENGINEERING CONTROL NOT FEASIBLE? “PERSONAL PROTECTIVE EQUIPMENT (PPE)"

Date : 4 March 2008 (Tuesday)
Time : 5.30 pm
Venue : IEM Conference Hall A, Bangunan Ingenieur, Petaling Jaya
Speaker : Mr. Daryl Low

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Wednesday, February 27, 2008

Protective Measures against FIRE other than Pressure Relief Device (PRD)

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In earlier post "Should we consider JET FIRE for Pressure Relief Valve (PRV) load determination ?", there were few main points discussed. Among all, there are

i) Pressure Relief Valve (PRV) may not protecting equipment and/or system from FIRE attack

ii) External FIRE (regardless of POOL or JET fire) for gas filled vessel will probably fail the vessel before the internal pressure reach its PRV set pressure.

Similarly for a high boiling point liquid stored in a vessel or tank exposing to external FIRE, the vessel may fail prior to the internal pressure reach its PRV set pressure. For a gas filled and high boiling point liquid vessel, there shall be other protective measures in place in order to avoid or minimize the consequent of catastrophic failure due to FIRE.

What are other protective measures against FIRE you may consider other than Pressure Relief Valve (PRV) ?

(i) Depressuring (vapor & Liquid)

Depressuring is one of the effective protective measures against FIRE attack. Depressuring (a) relocate hazardous fluid to SAFE area and (b) decrease the internal pressure of vessel or tank under fire attacked. Somehow, please don't misunderstood depressuring.

(ii) Provide Rupture / bursting disc instead of Pressure Relief Valve (for system that will lift PRV)

Rupture disc (RD) will burst in the event of overpressure. With the bursting of RD, it (a) relocate hazardous fluid to SAFE area and (b) decrease the internal pressure of vessel or tank under fire attacked.

(iii) External cooling (deluge & spraying)
In Steam in FIRE, it's been discussed that water is an inert agent it has the potential (vaporization and specific heat) of removing heat released by FIRE. Provide fixed deluge and spraying system around equipment is one of the effective protective measures.

(iv) Fire Extinguishing agent (with external cooling)

In the fixed deluge and spraying water system, may consider to mix water with fire extinguishing agent e.g. foam. As water mixed with foam is sprayed, foam in water with high surface tension will tends to capture air in the bubble and form barrier between fresh air, fire and material under fire. Other than foam deluge and spraying system which normal directly spray on the equipment, may also consider to provide fixed / portable foam pouring system. It works exactly the same as foam deluge system, however, it target on the material under fire which is normal in stay in bunded area. For fire potential occur in hydrocarbon storage tank, fixed / portable foam pouring system can be used to extinguish fire in the tank.

(v) External Insulation

This is another effective measure. Providing external fire resistance insulation will minimize heat input into vessel or tank and to avoid/ minimize internal overpressure and failure of vessel or tank. It associate with reliability issue of insulation and internal corrosion issue.

(vi) Sand Cover Storage
This method works similar to external insulation where it isolate the vessel and its content from any external fire. However, it associate with corrosion issue.

(vii) Diversion wall for isolation and proper drainage

This is one of the effective measure to minimize / avoid escalation of fire from one area to another area. Basically the concept is to provide bund wall around equipments. In the event overfilling or vessel rupture and caught fire, it content is capture in the bunded area and fire will not escalated to other vessel. Proper drainage system is one is measure should be in place in order to evacuate any hydrocarbon liquid to SAFE area.

(viii) Reliable & Compatible Fire detection system

Conduct detail safety and fire analysis and provide reliable and compatible fire detection system around the high fire potential areas. This includes redundancy of fire detectors (sparing, diferrent detection technology, etc) and compatibility (correct fire detector for type of fire).

So do you have any more to add ? Why not share with us here ?

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Monday, February 25, 2008

Distillation Control & Optimization in Series

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Béla Lipták has presented 7 Part Series on Distillation Control and Optimization in brief. Who is Béla Lipták ? Click to see who is Béla Lipták.

Béla Lipták is...

- editor of the Instrument Engineer’s Handbook
- former chief instrument engineer at C&R (later John Brown)
- recipient of ISA’s Life Achievement Award (2005)
- member of the CONTROL Process Automation Hall of Fame (2001).

His 7 series on Distillation Control and Optimization includes

Distillation Control and Optimization
Pressure Control Optimization
Distillation Control and Optimization
Controlling the Whole Column
Distillation : Multiple Products
Distillation Control and Optimization i
Distillation Control and Optimization ii

If you are operating or designing control system for a distillation, you should take this opportunity for quick view of these series.

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Sunday, February 24, 2008

Guideline on Quick Determination of Flare Stack Support Type

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In recent project, we worked on a project involved in flare system design with reasonable large flaring load (about 1200 mmscfd). During design phase, we have been asked to provide the structure type of the flare stack support and sterile area diameter as project would like to fix the plot plan. Normally, this information is provided by the flare tip vendor. We have received information from several flare tip vendors. The tricky part is some vendor indicated that guyed wire and some indicated derrick type. As the cost of construction and plot area requirement for every type of support are pretty much different, this has given us some level of challenge.

Conventionally three main type of flare stack support structure have been used. There are self support (SS), guyed wire (GW) and derrick support (DS). The following photos shown different types of flare support.

General rule of thumbs for each support type are :

a) Self support (SS)

lease land area
KOD and/or seal drum can be attached at the bottom of flare stack
Wall thickness increases with flare stack height. Maximum recommended height about 76 m (250 ft) [Ref.: JohnZink Combustion handbook, chap 20].
API Std 521 has recommended 60 - 90m (200-300 ft).
Low in construction cost

b) Guyed wire support (GW)

most land area
GW type can goes as height as 180 m (600 ft) [Ref.: JohnZink Combustion handbook, chap 20]. API Std 521 has recommended 180-250m (600-800 ft).
reasonable construction cost

c) Derrick Support (DS)

lease land area
height is more than 200 m [Ref.: JohnZink Combustion handbook, chap 20].
High construction cost

There is a simple guideline provided in the JohnZink Combustion handbook. We have used it as a quick guide and pretty supportive and informative. The following image shows the guideline.

Above image has been programed in EXCEL sheet for ease of future used. If you are interested, d ownload from HERE. If you have any comments or improvement, please drop me a note.

Update
Feb 24, 2008 : I have missed the definition of parameter used in the Excel spreadsheet. It has been updated. Thanks to Michael.

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Saturday, February 23, 2008

SteamTab - A simple Executable file for Steam - Water Properties...

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One fine day, you are in the meeting and you have been asked to about the properties for steam at 10 barg & 200 degC. You can either

use the excel add-on Water97_v13 as introduced in "Conduct Steam-Water Balance MANUALLY using Water97_v13"
use the Steam-Condensate calculator as introduced in "Useful Steam - Condensate Calculator"
Steam table (manual calculation)

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Use Excel may required to remember the function and steam-condensate may required you to login to internet. Sometime the server which steam-condensate calculator may down for some reason. Using steam table would definitely time consuming. One of way is to use the SteamTab steam-condensate software which is rather handy for you.

Similar to the excel add-on, you may obtain steam-condensate properties for subcooled, saturated and superheated condition. In additional to common properties, it also calculated other parameter such as surface tension, Gibbs energy, etc.

If you have problem in obtaining this handy tool, please drop me a note.

Friday, February 22, 2008

Nanowire battery... High Potential of Replacing Lithium Battery

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My notebook battery (lithium type) can last for about 2 hours after fully charged. Now, with Nanowire battery, you may expect 10 times longer without stop. This really a great invention and improvement in battery industry.

Stanford researchers led by Yi Cui, assistant professor of materials science and engineering, have found a way to use silicon nanowires to reinvent the rechargeable lithium-ion batteries, known as Li-ion that power laptops, iPods, video cameras, cell phones, and countless other devices.

Read details in Nanowire battery can hold 10 times the charge of existing lithium-ion battery.

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Thursday, February 21, 2008

Don't misunderstood depressuring

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I asked a young engineer. "What is the main purpose of system depressuring ?". The answer given to me was a surprise but seem "logical" to some extent.

Young engineer responded "A conventional depressuring system consist of blowdown valve (BDV) and restriction orifice (RO). The purpose of depressuring system is to limit the flow rate to flare system in order to avoid large capacity flare system. It is similar to the RO provided on the level control valve (LCV), to avoid large gas blowby rate in case LCV failed open."

What do you think about above statement ?

The young engineer made analogy of BDV/RO with LCV/RO. It is true that the RO upstream (or downstream) of LCV is to limit the flow during gas blowby and to minimise relieving load of downstream pressure relief device (PRD). However, this principle may not be the primary purpose for depressuring system.

My opinion...
The main purpose of depressuring is to evacuate the inventory from process system as fast as possible so that the reduced internal pressure stresses is kept below the rupture stress. This has been discussed in "Depressuring within 15 minutes no longer applicable ?". Nevertheless, quick depressuring may lead to other problem such as low temperature embrittlement, excessive noise and vibration, etc. Depressure a high pressure would lead to low temperature of depressured system and failure due to low temperature embrittlement. Higher the depressuring rate, lower the temperature can be experienced by depressured system. Thus, the RO downstream of BDV in depressuring system primarily is to limit flow so that the temperature will not drop below the allowable lowest temperature limit of material.

For some system (generally low pressure), quick depressuring may not be required as the internal pressure deduced stress may still well below the maximum allowable stress of depressured system. Under this scenario, the RO is to limit the flow in order to minimise disposal and flaring capacity (this pretty inline to what the young engineer explained to me).

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Wednesday, February 20, 2008

PSV for Shell-and-Tube HEX Tube Side Overpressure Protection against External Fire Attack ?

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This is a rather doubtful query and have debated "n" times in many forums. Reason being tube is not part of pressure vessel code (i.e. ASME Section VIII) and thus NO Pressure relief device (i,e. PSV) is required. I do agree with the statement that PSV is NOT required IF design and fabrication code for tube side do not demand present of PSV. This solely from CODE application perspective.

Nevertheless, we shall also review from other relevant codes compliance and non-code perspectives.

i) API STD 521 compliance - Nowadays in general most plant design to API STD 521 compliance. API STD 521 2007, section 4.3.1 stated "The process-systems designer shall define the minimum pressure-relief capacity required to prevent the pressure in any piece of equipment from exceeding the maximum allowable accumulated pressure. The principle causes of overpressure of overpressure listed in 4.3.2 through 4.3.15 are guides to generally accepted practices." and 4.3.14 stated "plant fire" is one the potential cause of overpressure. As S&T HX tube side front end and rear end is potential expose to direct external fire heat input and there is potential of heat input from external fire to shell side fluid which further transfer to tube side, thus there is potential of overpressure cause by external fire and it demand pressure relieving.

ii) API RP 14C, 2001 compliance - Oil & facilities are commonly designed to API RP 14C, 2001 compliance. Following appendix A.10. Heat Exchanger (Shell-Tube), A.10.C.2 "Each input source is protected by a PSV set no higher than the maximum allowable working plessure of the heat exchanger section and a PSV is installed on the heat exchanger section for fire exposure and thermal relief.", a PSV may be required on tube side.

iii) Pinhole leak - There are many heat exchanger has pinhole leak due to thermal shock, thermal stress, flow induced vibration, erosion, corrosion, etc. These kind of pinhole leak may not be detected during normal operation as the leakage is rather small as compare to process fluid flow. In the event of fire, entire S&T HEX may be totally isolated. External heat input to Shell side and result pressure increase. Pinhole leak would lead to continuous flow from Shell side to tube and subsequent overpressure the tube side.

iv) Inventory associated Risk - Requirement of PSV shall NOT be 100% judged from the design code itself. The RISK and CONSEQUENCE e.g. INVENTORY associates risk & consequences shall come into consideration to define if a overpressure protection i.e PSV is required. Although the code does not call for it but from risk point of view and insurance premium perspective, provision of PSV may be a more practical way of handling it. Read more in "Requirement of overpressure protection devices on system design to PIPING code".

Thus, it is advisable to consider providing a PSV protecting tube side overpressure cause by external fire.

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Tuesday, February 19, 2008

Steam in FIRE...

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Why inject steam into fire and flare tip ?

I guess many of you as Chemical or Safety engineer should be aware of the reason. This post is reiterate the response to query raised by a young engineer.

Steam in Fire
Initiate and maintain continuous fire required 3 major elements. There are fuel, oxygen and heat (see famous 3 ring-symbol below).

From chemistry, without any one of the element, a fire will NOT form. Those the first principle in fire fighting is separate or create barrier between them.

As Steam (H2O) is non-combustible When you inject steam into the a fire, it does at least two things

i) Steam expands as it absorbed heat, displace air (oxygen rich) and form a layer of barrier to avoid/minimise the contact between Fuel/Heat and oxygen. This strategy is removal of OXYGEN.

ii) Steam at 100-120 degC @ ATM injected into Fire (>1000 degC ) will have the potential of absorbing heat from fire. Once it gain the heat, it rise and disperse to non-fire region. This is removal of HEAT. Similar on this principle, inject water @ (<>

Steam in Flare

Steam is used in FLARE TIP as smoke suppression. It promote extraction of air from ambient into the flame and increases the combustion efficiency and effectiveness. Nevertheless, steam injected at a proper location (flame zone) and correction direction with correct quantity and proper device will create turbulence and entrain air into the flame zone via steam jets. This improve Oxygen concentration in fire and promote combustion.

In many cases steam will properly injected into the flare in 3 location.

Upper - to minimize wind caused irregular flame which potential lead to flame-out
Centre - to prevent internal combustion
Internal - to entrain air (using external eductor) into flare gas and promote combustion

STEAM helps in put out fire as well as promoting combustion. It is very much subject to how a steam is introduce into fire a well as the quantity.

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Monday, February 18, 2008

Blast rocks Texas oil refinery

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An explosion rocked a Texas oil refinery located a Big Spring Texas, Monday morning (about 8.20am local time), shaking homes three miles away. Massive cloud of smoke forming over the refinery. One worker was injured in the blast, and all workers had been accounted for. Read more news and video clip... (Click HERE).

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Sunday, February 17, 2008

Why Centrifugal pump NPSH required increases with flow ?

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Why centrifugal pump Net Positive Suction Head required (NPSHr) increases with flow ?

There is a simple answer. NPRHr is a parameter fixed during design and fabrication of a centrifugal pump. The NPSHr is increased with flow. Main reason to this increase is increase in frictional lose at entrances nozzle and turbulence friction lose at vane tip.

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Let's look at the following image.

The pressure at different point location from entrance pipe to impeller is indicated as red line in the curve for normal flow. From point A-C, the friction lose cause by nozzle whilst turbulence friction lose from point C-D at impeller center. The pressure increases from D-E as impeller is adding head into the liquid. As the flow increases from normal flow and the physical configuration is fixed, the frictional lose and turbulence friction lose will increase with flow. The pressure profile from entrance nozzle to pump impeller center and outlet will be shifted downward as indicated in blue line. The lowest pressure at the impeller center is getting closer or even cross the vapor pressure of the fluid. This potentially results cavitation.

Pump manufacturer would needs to design a pump with the Suction specific speed (Nss) below 10000 (normally) to **ensure a healthy life span as high Nss would leads to rough operation and shorten life span. Some manufacturer may even design for 8000-8500 .

Suction specific speed ( N_ss) of a pump is a dimensionless number expressed as

N_ss=( N* Q ^0.5 ) / (NPSHr)^0.75

Where

N_ss : Suction Specific speed
Q : Flow rate (gpm) at the Best Efficiency Point
N : Pump rotational speed (rpm)
NPSHr : Net Positive Suction Head required (ft)

Since the normal pump speed is 1450 rpm for low speed pump and 2950 rpm for high speed pump, it is obvious that the NPSHr is increased as flow rate increased.

Updated on Feb 21, 2008 : ***

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Must Read Flare Handbook

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Flare system is a common safe and environment friendly gas disposing facilities in Oil & Gas development, Refinery and Petrochemical plants.

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If you are looking for a Flare Handbook, i would recommend you The John Zink Combustion Handbook, a publication of John Zink, "grand daddy" in Flare. Chapter 20 "FLARES" describes in details of

Flare Systems - description of flare application, types & components
Factors Influencing Flare Design - How flow rate, composition, temperature, etc affecting flare design
Flare Design Consideration - Aspect to be considered in flare design
Flare Equipments - Details description of flare burner, pilots system, KO drum, seal drum, etc
Flare Combustion Products - Overview of combustion efficiency, emission and dispersion

This is a great handbook for a young engineer to get the basic understanding and fundamentals of flare system. You may preview (Click HERE) or get copy for yourself (Click HERE).

Friday, February 15, 2008

Nanowerk - NanoTech News & Database Center

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Nanowerk, one of the leading nanotechnology and nanosciences portal. Its Editor content is one the feature area to inform readers on what’s hot and new from around the globe. The content extended from introduction to nanoscience, understanding current developments, and advanced reviews of leading-edge nanotechnology research. On average 70-100 news articles published every week.

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Nanomaterial Database™ available in Nanowerk is a powerful, free tool for the nanotechnology community to research nanomaterials such as carbon nanotubes, nanoparticles or quantum dots from over 120 suppliers worldwide. If you are nanotechnology researcher or engineers, this is a must visit portal.

If you are new comer in nanotechnology and would like to have quick understanding of nanotechnology, there is a 27-page introduction section, complete with a Carbon Nanotube 101 available HERE...

Watch an introduction to nanotechnology, starting with Richrd Feynman's classic talk in December 1959 "There's Plenty of Room at the Bottom - An Invitation to Enter a New Field of Physics."

Watch an animation of various nanotubes and a fullerene (buckyball):

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Thursday, February 14, 2008

Conduct Steam-Water Balance MANUALLY using Water97_v13

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Recently i was supervising a young engineer in the design of Steam - Water system for an LNG plant. This kind of system can be easily simulated in any Process Simulator such as HYSYS, PRO-II, etc and generate a Heat & Material Balance (HMB). If you have several years experience, you may notice that nowadays young engineer tends to use Process simulator and ignoring the basic behind. Due to this reason, i insisted the young engineer to conduct Heat & Material Balance for Steam -water system MANUALLY using Excel spreadsheet. Without any surprise the young engineer having difficulties...

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In "Useful Steam - Condensate Calculator", there is an Excel Add-in (Water97_v13.xla or Alternative download) available FREE for download, is very useful for calculating thermodynamic and transport properties of water and steam using the industrial standard IAPWS-IF97. In this post, i will elaborate a little bit on the method to conduct steam-water balance manually using above add-in.

Problem statement :
High pressure steam (HS) at 20 barg @ saturated condition mix with Boiler Feed Water (BFW) at 15 barg @ 60 degC to 1000 kg/h Low Pressure Steam (LS) at 3.5 barg @ Saturated condition. Find quantity of HS & BFW.

Material Balance :

M1 + M2 = M3
M2 = M3 - M1

Heat Balance :

M1 x h1,vs + M2 x h2,T = M3 x h3,vs
M1 x h1,vs + (M3-M1) x h2,T = M3 x h3,vs
M1 x h1,vs + M3 x h2,T - M1 x h2,T = M3 x h3,vs
M1 x h1,vs - M1 x h2,T = M3 x h3,vs - M3 x h2,T
M1 x (h1,vs - h2,T) = M3 x (h3,vs - h2,T)
M1 = M3 x (h3,vs - h2,T) / (h1,vs - h2,T)

a) For HS Steam : Get Specific enthalpy (h1,vs) for Saturated steam at pressure 20 using enthalpySatVapPW (P)

[e.g. enthalpySatVapPW(20+1.01325)]

b) For BFW : Get Specific enthalpy (h2,T) for Subcooled liquid at 15 barg @ 60 degC using enthalpyW(T,P)

[e.g. enthalpyW(60+273.15, 15+1.01325)]

c) For LS Steam : Get Specific enthalpy (h3,vs) for Saturated steam at pressure 3.5 using enthalpySatVapPW (P)

[e.g. enthalpySatVapPW(3.5+1.01325)]

d) As M3 is 1000 kg/h, M1 & M2 can be obtained by modeling it in the EXCEL sheet.
For M1 :

=1000 * (enthalpySatVapPW(3.5+1.01325) - enthalpyW(60+273.15,15+1.01325))/ (enthalpySatVapPW(20+1.01325) - enthalpyW(60+273.15,15+1.01325))

For M2 :

= 1000 - M1

Another method is using GOAL SEEK in EXCEL sheet...
M1 & M2 can be adjusted until the enthalpy difference between INLET section (e.g. M1 x h1,vs + M2 x h2,T ) and OUTLET section (e.g. M3 x h3,vs) equal to zero AND M1+M2 = M3. The adjust function can be easily setup using GOAL SEEK feature in EXCEL.

Details may refer to following image.

It is rather simple programming as seen from above and no simulator is required.

Young engineer is always encourage to conduct above calculation as least 2-3 times so that you are sure yourself understand heat & material balance in depth. By doing this kind of calculation, you will surprise your depth understanding. Later stage, you can always simulate balance with process simulator to save time.

For those who are interested in the EXCEL spreadsheet, you may drop a note to me.

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