Pipe Design 2

[Lijantropo]

Good Morning,

I have read about the considerations for the design of pipe lines and usually, there are two parameters for the selection of the diameter:

1.velocity
2.Pressure drop

My question is which one must be the main factor to take into account for the final selection?

I mean, the pressure drop has an influence on the discharge pressure of the pumps and a higher velocity might originate vibration and erosion. But, if I have a system in which both parameters -velocity and pressure drop- are in acceptable values, how can I improve my design?
Should I increase the velocity or reduce the pressure drop?

bye

 

[katmar]

Neither of these are the main factor. The aim is to achieve the most economic performance over the life of the pipeline. Velocity and pressure drop both need to be considered in evaluating the economic performance and their relative impact will vary from application to application. Googling "economic piping design" will get you plenty of hits.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lijantropo]

Thank you Katmar for you response

 

[zdas04]

Another way of saying what Katmar said is that "velocity has a physical impact on the system and pressure drop has an economic impact".

I tend to design piplines with velocity between 10-100 ft/sec to control noise, scouring of passivation layers, and pressure drop. Then I look at how much hp is required to restore the pressure lost in friction.

David

 

[dcasto]

since pressure drop is a function of velocity squared, you can't have on without the other and velocity squared drives

 

[CJKruger]

I agree with dcasto, and prefers to design to a pressure drop criteria (psi/100ft).

Only exception is corrosion/erosion considerations, which is usually a velocity criteria.

 

[Lijantropo]

Thanks for your responses.

I have been reading about "economic piping design" and I think -please correct me if I am wrong- this is related mainly with the pumping costs and the installation & maintenance costs. The idea is to find the diameter with the less total costs.

I usually designed with pressure drop (psi/ft), but now I agree with katmar -the "economic" pipe selection- because you could have an acceptable pressure drop but very high costs associated with the I&M.

Obviously, the process may add other factors for the selection (e.g, maximum velocity inside an instrument) and all selected diameters have to take this into account.

 

[katmar]

OK, now that you have seen that economics is the ultimate driver I will admit that in the overwhelming majority of cases the piping designer would not actually consider the economics. In turbulent flow the pressure drop varies with the 5th power of the diameter, and piping only comes in discrete diameter sizes. This means that you are selecting a pipe size and not designing a specific diameter, and there will be a relatively large change in the pressure drop from one size to the next.

Because of the step change in the pipe size there is a step change in the economics and it is not a nice smooth curve that can be optimized using classical calculus. Experience has resulted in various "rules of thumb" that are good enough for you to get close to the optimum economics by sticking to the typical pressure drops. Similarly, experience has shown how far you can go with velocity before things like erosion, noise and water hammer become problematic.

It is really only with very long pipelines that you would put an effort into economic optimization. But how long is long?

Also, most of the literature assumes that you have a constant flow rate for the assumed 20 year lifespan of the pipeline. You will be lucky if things stay constant for 20 months. So we tend to design with a little bit of "safety margin" built in.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lijantropo]

OK. I have a new question. The "appropriate method" is to optimize the curve "pipe size vs economics" using, maybe, numerical methods. Can I simplify this method using the lower value from selected pipe diameters?. I mean for some conditions I evaluate different sizes and then choose the one with the lower cost. (Without do any optimization)

I believe this could be an improvement on the "selection" but it is still a short method to apply (e.g using a spreadsheet), especially when your process has for example 500 lines.

Mr Katmar, would you please tell me something more about the "rules of thumb" used in these cases.

Thank you,
Lijantropo

 

[zdas04]

The rules of thumb tend to be company specific. I have one client that requires that the gas velocity at every point in the line be between 40 ft/s and 120 ft/s. I think their minimum is too high, but not unsafe and I use it.

Another client has a max dP/mile of 5 psi for dehydrated lines and 15 psi/mile for wet-gas lines.

I've never seen any industry-wide rules of thumb.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Remember, being ignorant isn't your fault; staying ignorant is.

 

[katmar]

I mean for some conditions I evaluate different sizes and then choose the one with the lower cost. (Without do any optimization)

Yes - that is exactly what I was trying to convey.

The rules of thumb tend to be company specific.

I agree with David - the more universal the rule of thumb, the less likely it is to be helpful in your specific application. Check with colleagues in your particular industry for what standards they use.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lijantropo]

Thank you all for your comments and your suggestions.
Lij.

 

[CJKruger]

Lijantropo, the following pressure drop criteria are reasonable for the petrochemical industry:

Pump discharge (liq or two phase) = 1 psi/100 ft
Pump suction (liq) = 0.2 psi/100 ft
Gases (P>15psia) = 0.3-1 psi/100 ft depending on pres.
Free draining = 1 ft/s.

Despite their simplicity, these rules cover a multitude of cases.

 

[Lijantropo]

Thank you Mr CJKruger.

 

[HEC]

material handled also has influence on line velocities and pressure drops. Handling conc. sulphuric acid for example, line velocities should not exceed 2m/sec, moving cream around should not exceed 1.5m/sec (you can get butter out if pumped far and fast enough!). So there are economic, energy and physical material conditoins to consider too.

Mark Hutton

 

[rickfer]

I agree with Mark, it is importan to consider what are we moving through the pipe.

e.g. if we are pumping water with sand, it isn't good idea to flow with very high velocity, because it might corrode the pipe (and very fast), but, we don't want it to flow to slow, because sand can settle and clog the pipe.

Also, there is a rule I like to follow. The pressure lost by velocity should be around 15% to 20% of TDH.

So, if pressure lost by velocity is higher than 20%, you must consider to low the velocity. This saves money during operation.
If pressur lost by velocity is less than 15%, you may consider to use a smaller pipe diameter. This will save you money during installation.

 

[Lijantropo]

Thank you Mr. HEC & Mr. Rickfer.

In short, we can said that pipe sizing depends on overall costs. However, it must be taken into account design parameters related with material handled or process requirements.

Regards,

Lij