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D. Pressure


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#1 process101

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Posted 12 August 2008 - 09:11 AM

Hi every one:
what are the effect if the design pressure of vessel or any equipment is higher then it supposed to be.
for example if the design pressure of cooler or condenser is 75psig but it is increased to 100psig.
Thank you very much for help.

#2 djack77494

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Posted 12 August 2008 - 02:23 PM

process,
I find your question to be confusing. The process engineer should specify the design pressure of a vessel based on the various pressures that would be expected in that vessel. I word my response like this to indicate that there could be a variety of pressure/temperature combinations that the vessel may experience during various operations - normal and otherwise. The process engineer should ascertain these conditions. Alone or with the help of equipment specialists, the process engineer then determines the most severe of these conditions and specifies these as the design conditions. If he/she cannot determine the most severe, then they might specify several conditions. This essentially notifies the equipment designer that they must ensure that their design is suitable for ALL of the conditions. (Some may disagree that this is an option since the process engineer is essentially failing to fulfill his/her responsibilities in this scenario. I'm OK with this if the engineer is sure that a competent and qualified designer will complete the work.)

Now, when you say, "what are the effect if the design pressure of vessel or any equipment is higher then it supposed to be", then I'm confused. THe design pressure is "supposed to be" adequate for the task. THere is no required upper limit - only one established by economic considerations. Higher than necessary design pressure is fine/no problem.

Now, I get further confused when you say, "but it is increased to 100psig". If you're talking about an existing peice of equipment, it's a pretty big deal to rerate the equipment from a DP of 75psig to 100psig. It's a big deal even if nothing needs to be physically done to the equipment because a rather lot of specialized calculations will be needed, followed by testing and then restamping (for most equipment). This is not a trivial task. If you have a "paper design", then it may be quite simple. Of course you can simply change a "75" to a "100", but an engineer worth anything would consider all the ramifications of any changes he/she is proposing. Here a consultation with the equipment engineer or designer would be needed as the first step in determining the feasibility of the proposed change.

#3 katmar

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Posted 13 August 2008 - 12:45 AM

The main effect of over-specification of the design pressure (at the design stage) is to potentially make the exchanger more expensive than necessary. I say potentially because the higher pressure will theoretically require thicker material, but for practical reasons the plate actually used may not change. For example, if the design pressure change increased the required plate thickness from 8,1mm to 9,7mm you would probably use 10,0mm plate in both cases, making no change to the final implementation of the design. There was a thread recently that went into this in more detail.

However, as a general rule over specification is wasteful. I cannot think of an instance where over specification of the design pressure would lead to a badly performing piece of equipment, but certainly over specification of flow rates and volumes can lead to equipment that does not work as well as a properly specified item.




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