how to calculate the maximum flow velocity of flammable liquid? 1

[navycaptain]

Dear all:
I'd like to ask you a question: how to calculation the flammable liquid DMC(Dimethyl carbonate CAS No. 616-38-6) which MSDS is attached, it's designed flow rate is 12.5m3/h, DN40 304 stainless steel pipeline which velocity is 2.7m/s, but there is an opinion which is velocity should not be high than 1m/s for flammable to aviod static electricity, i looked up API2003 and NFPA 77, it mentioned that VD<0.38 (V is velocity, D is diameter) is OK while under on circumstance is higher than 7m/s
My question is how to calculate the maximum velocity for flammable liqiud? for example, how to determine the maximum velocity of DMC Dimethy carbonate? thank you a lot!

 

[itsmoked]

I doubt that is a calculatable value. They probably find the numbers empirically. Or more likely, they find the value empirically for that chemical or a close relative and then infer what the similar chemical should be. In all cases they include a safety factor to cover any discrepancies in the assumptions.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

You normally need to be up to about 7 m sec to start getting serious static issues.

A lot depends on the ability of the static charge to dissipate from the liquid and avoiding any means of either creating a spark or getting an explosive atmosphere.

I'd never seem that equation before but the number sounds right to me.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[navycaptain]

Hi itsmoked! thank you for your reply, in actual design scenario, normally, process engnieer will calculate a economic velocity, but if this velocity is too high for safety engineer, then there should be a way to resolve.

 

[navycaptain]

Hi LittleInch, thank you for your reply, maybe it's hard to find a solution to accurately check if the velocity is fine for the static electricity risk, and it's not worth it to do so, but there is an argument that: if we choose high velocity, then we can select smaller diameter pipe, and it's cost effective, but maybe increase the safety concern, so if there is a convincing way to calcuate the static electricity velocity, then there will be no concern.

 

[Compositepro]

Static in a closed pipe is rarely an issue. Problems occur when the high velocity flow enters a tank and falls through air. The air gap breaks the conductive path back to the grounded pipe wall, so charge can accumulate where oxygen is present. Slowing the flow before entering the tank and not having an air gap should take care of any static. Proper grounding of everything is also important. Non-conductive coatings inside pipe and tanks cause static problems.

 

[navycaptain]

Hi compositepro, thank you for your reply, it's true that there is raraly a static electricity issue for a clsoed pipe system like you mentioned, and both API2003 and NFPA77 put emphisis on tank loading velocity, not higher than 7m/s. but there is normally a requirement for pipe bonding and grouding if the liquid is flammable to eliminate the static electricity.
thank you again for your professional reply.

Youth is not a time of life, it's a state of mind.

 

[navycaptain]

Hi Compositepro, I found one description about velocity for some chemical:" At normal pumping rates of about
3 m/s the generation of static electricity by acrylonitrile should be minimal, but plant handling this chemical should be earthed."

is this means, for acrylonitrile this chemical, the velocity should not be higher than 3m/s or at least 3m/s? thank you a lot!

Youth is not a time of life, it's a state of mind.

 

[moltenmetal]

No, it means that typically people target an "economic velocity" for ordinary liquids of about 10 ft/s or 3 m/s. Static discharge isn't a problem in closed metallic piping as you've already been correctly told. It isn't something that you need to concern yourself with. It can be an issue in tanks as mentioned, and in nonmetallic materials such as lined pipe and nonmetallic hoses that are not sufficiently conductive.

 

[navycaptain]

moltenmateal, thank you for your response, i see there is a "economic velocity" for ordinary liquid. Can i ask you an additional question: for lined, nonmmetallic pipe. are there any specific velocity requirement? for example, if DMC Dimethyl carbonate CAS No. 616-38-6 is used in lined pipe? what velocity is accpetable?

Youth is not a time of life, it's a state of mind.