Pressure Drop per Unit Length 1

[Jestoni]

Hi guys. I got one small question. What does the quantity pressure drop per unit length mean (bar/100m, for example) and what is it for? Thanks a lot!

 

[Artisi]

I assume you are refering to pressure drop for fluids thru a pipelne - if so - it only means that for flow "x quantity" thru "y diameter" pipe you need a pressure of "z bar per 100 lineal metre of pipelin.

 

[Latexman]

If you multiply pressure drop per unit length times the length of your pipe run, you get the total pressure drop.

Good luck,
Latexman

 

[Jestoni]

Thanks for your very prompt replies.

@Artisi: Yes, I am referring to pressure drop for fluid thru a pipeline.

@Latexman: I'm sorry but from what I see in the hydraulic calculation results, this value is a lot higher than the "plain" pressure drop. If I do multiply this pressure drop per unit length to the pipe length, I am not arriving to the total pressure drop of the pipe.

What's its use anyway? I'm just overly curious about the term because we continue to include it in the calculation results. Sure I do understand pressure drop but about the term I'm asking, I do not get its sense at all.

Again, thanks a lot. I've been reading a lot in this site so your names are familiar to me. I'm so glad to get an answer from you guys.

 

[zdas04]

People use drop/unit for two things (both are gas specific, I've never seen the term used in liquids lines): (1) it is a surrogate for velocity that is more scaleable; and (2) to plan for compressor stations on cross country pipelines.

The first use is one that I personally use quite often. The number that works for me is on pipes smaller than 16 inch I use 15 psi/mile. On larger pipes I use 5 psi/mile. Both numbers result in acceptable velocities at a wide range of operating pressures. The reason for the lower acceptable velocity in big pipes is the (occasionally invalid) assumption that big lines tend to be dehydrated and the risk of liquids accumulations is lower so slower velocities are effective.

If I have a 500 mile pipeline that starts at 1,000 psig and the minimum acceptable pressure is 600 psig and I'm willing to accept 5 psi/mile then I need a compressor booster station about every 80 miles. If the acceptable pressure drop is 2 psi/mile then I can go 200 miles between stations but the same size pipe can move less gas.

David

 

[Jestoni]

Thanks David. I have a clearer understanding now. You kinda nailed it because we use this criteria side by side with the allowable velocity for certain services. Btw, in our company, we also use this criteria for liquids.

 

[Latexman]

Since I can't see the values you are talking about, I don't understand where you are coming from with "If I do multiply this pressure drop per unit length to the pipe length, I am not arriving to the total pressure drop of the pipe". If there are fittings and elevation changes and diameter changes, of course not. You have to correct for those things. If I run a pressure drop calc for water in 100 ft. of straight, horizontal, sch. 40 pipe I get identical results to the table in Crane's TP 410. If your hydraulic calculator cannot duplicate the values in your pressure drop/length table, when using the same basis, there is a problem somewhere.

David - it's common practice in the chemical industry to use pressure drop/length for liquids.

Good luck,
Latexman

 

[25362]

Pipe fittings' resistance can be converted into piping equivalent lengths. See, for example,

http://www.engineeringtoolbox.com/resistance-equivalent-length-d_192.html

 

[zdas04]

I don't do much work with commercial liquids. The only liquid I deal with regularly is water from gas wells and the typical level of Engineering in that field is about the same as you'd find on a family farm in the 1850s.

Pressure drop/length is a scoping tool. It is far from precise. Adding fittings and equivalent pipe lengths is much like adding micro meter marks on a meter stick. I use it for statements like "going into a compressor station with 100 psig suction I need to be able to live with 250 psig at a well about 10 miles away, what size pipe can do that for the projected volumes (and what compression hp will I need at the well)". It gets me into the design universe and then I do Engineering to decide on pipe sizes, wall thickness, required water removal equipment, etc.

David

 

[Latexman]

Yep, that's how we use it with liquids. Then I add 30% if there's a control valve and add/subtract for significant elevation rise/fall. I usually can ignore velocity changes. It quickly gets us in the ballpark.

Good luck,
Latexman

 

[SNORGY]

zdas04:

"The only liquid I deal with regularly is water from gas wells and the typical level of Engineering in that field is about the same as you'd find on a family farm in the 1850s."

That's still ahead of the technology in our civilization north of the border. We have only recently discovered fire. Accordingly, we often welcome finding water in our gas wells.

Regards,

SNORGY.

 

[zdas04]

I spend a lot of time on that side of the border (in Calgary twice last month). The only difference I see is winter down here is a time of scaling back and dealing with mud. Winter up there is a time of ramping up to avoid mud.

On the other hand, water systems up there are just as primitive as everywhere else in the world. You can't sell produced water so management doesn't want to spend any money on it. It amazes me how much gas gets shut in because there is no place to take water.

David

 

[SNORGY]

There is a good sub-industry in making produced water boil-off tanks.

That said, in a lot of fields and wells, the economics and margins are such that some gas just isn't worth going after.

Regards,

SNORGY.

 

[rpedde01]

Hi

Pressure drop/unit length is the pressure that you loose when any fluid ( gas, fuel oils, water) flow trough a pipe.

This information will be mainly used to acureately size the equipment upstream and down stream of pipes.

upstream equipments could be compressors, pumps.

Down stream equipment will be the burners etc.

so pressure drop is not equal to the line pressure.

but if you add the pressure drop and outlet pressure you should total pressure or the inlet pressure.

hope this explains your question.

 

[zdas04]

rpedde,
Have you read the posts in this thread? It is really bad Internet etiquette to read the original question and skip to the end where you post zero information that hasn't been provided in the posts you skipped.

David