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Saturday, October 24, 2009

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High frequency acoustic excitation downstream of pressure reducing device potentially results downstream piping failure due to Acoustic Induced Vibration (AIV). Whenever there is pressure drop with mass passing through the valve, internal acoustic energy is generated and transmitted to downstream piping and potentially lead to severe piping excitation, vibration and stresses on downstream piping, in particular at discontinuity section i.e fabricated Tee, small bore connection, welded pipe and pipe support, etc. This acoustic excitation phenomena is generally involve high frequency (more than 1000Hz) acoustic energy. When high frequency acoustic energy is matches with mechanical natural frequency of piping and its component, excitation amplitude is at maximum and lead to increased stress level. AIV phenomenon and assessment methodology have been discussed in previous post. Sound Power Level (PWL) is commonly used in quantifying acoustic energy in AIV. Higher PWL, higher the potential of vibration level and higher the risk of AIV.

Principle in Eliminating & Minimizing AIV Impact
This post will focus in some common measures and techniques to eliminate and/or minimize PWL generation and transmission. The main principles in avoiding and minimizing the impact of AIV are :
  1. Eliminate or reduce vibration level at pressure reduction device
  2. Damper vibration level
  3. Increase resistance to vibration
  4. Minimizing vibration transmission
All shall measures are not exclusive between each and other but all measures shall be taken and apply all (if possible). However, It shall follow the sequence from principle no. 1 to principle no.4.



Application Example
A control valve discharge gas from high pressure vessel to flare header results high vibration level (or sound power level, PWL) and lead to AIV problem. The consideration shall be focused on reducing the PWL by using special trim control valve (principle no. 1). Splitting flow into several control valves in parallel may reduce the PWL (principle no.1). However one shall remember this sometime is not a good idea as common mode failure may worsen the situation. Next may consider a silencer insert downstream of control valve to damper PWL (principle no.2). One may also consider to increase piping resistance to vibration i.e. increase wall thickness (principle no.3). This high resistance piping may be extended to decrease the PWL to certain acceptable limit before it is tie in to downstream piping which is less resistance (principle no. 4).

Concluding Remark
In eliminating and minimizing AIV impact, all principles as mentioned above are not exclusive and shall be applied to maximum (if possible).

In coming post "Measures & Technique In Eliminating / Minimizing PWL", the discussion will focus on
- type of AIV source
- measures in tackling each AIV problem

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Tuesday, October 13, 2009

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FREE Chemical Engineering Digital Issue for Oct 2009 has just been released !

Chemical Engineering Magazine as just released Oct 2009 issue. If you are the subscriber of Chemical Engineering, you should have received similar notification.


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Interesting articles for this month :

Strategies For Water Reuse
Membrane technologies increase the sustainability of industrial processes by enabling large-scale water reuse

Estimating the Total Cost of Cartridge and Bag Filtration
When changeout and disposal costs are added to the purchase cost of filters, the total cost of disposable filters can more than quadruple . A proven method of reducing total lifecycle cost is larger surface area filters

FAYF - Chemical Resistance of Thermoplastics
This one-page guide outlines considerations for choosing thermoplastics in corrosion-resistant applications

Energy Efficiency : Tracking Natural Gas With Flowmeters
Thermal mass flowmeters provide advantages over other options for metering the consumption of natural gas by individual combustion units throughout the facility

Preventing Dust Explosions
Risk management programs are critical for safe handling and processing of combustible dust as well as for OSHA regulatory compliance

Compressed Gases: Managing Cylinders Safely
Follow these recommendations to ensure the safe handling, storage and use of gas cylinders

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If you yet to be subscriber of Chemical Engineering, requested your FREE subscription via this link (click HERE). Prior to fill-up the form, read "Tips on Succession in FREE Subscription".

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Saturday, October 10, 2009

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A mixture is combustible / flammable if and only if the hydrocarbon composition is within mixture LFL/LEL and UFL/UEL as discussed in "Estimate Mixture Flammability & Explosivity At Reference P & T. As the operating pressure (P) and temperature (T) change (from reference P & T, the mixture LFL/LEL and UFL/UEL at P & T will change accordingly. In recent works, found in literature an interesting relationship between number of Carbon (in paraffin hydrocarbon) with LFL/LEL and UFL/UEL. This relationship is pretty useful especially when you have no information on hand.

A paraffin hydrocarbon with NC of Carbon (C), the Upper Explosive Limit (UFL) and Lower Explosive Limit (LFL) can be established with following equations :


Example
A Methane (CH4) contains One (1) Carbon. From literature, the UFL = 15 vol% and LFL = 5%.
From above equations,

UFL = 1 / (0.01337 x 1 + 0.05151)
UFL = 5.6% (compare to 5%)

LFL = 1 / (0.1347 x 1 + 0.04343)
LFL = 15.4% (compare to 15%)

A Propane (C3H8) contains Three (3) Carbon. From literature, the UFL = 10.1 vol% and LFL = 2.1%. From above equations,

UFL = 1 / (0.01337 x 3 + 0.05151)
UFL = 10.9% (compare to 10.1%)

LFL = 1 / (0.1347 x 3 + 0.04343)
LFL = 2.2% (compare to 2.1%)

A Hexane (C6H14) contains Six (6) Carbon. From literature, the UFL = 7.0 vol% and LFL = 1.25%. From above equations,

UFL = 1 / (0.01337 x 6 + 0.05151)
UFL = 7.6% (compare to 7%)

LFL = 1 / (0.1347 x 6 + 0.04343)
LFL = 1.2% (compare to 1.25%)

Above equation is just equations for quick estimation. It may provide some idea of UFL and LFL when no information is available. The error could be large for certain component i.e. Octane. For design and practical use, an in depth method shall be employed.




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