5 replies to this topic

[CS Kang]

I came across this question in my assignment which I think more discussion through this forum can help me understand the piping concepts better.

The question: Assuming the amount of liquid flow through a pipe remains the same, discuss the implications of:
(i) a default pressure drop of 0.5psi/100 feet of pipe used in the pipe sizing. What are the implications if a larger or a smaller guideline has been chosen?

Not to confuse anyone here, the guideline here refers to the pressure drop,∆P. Since I am using a simulation program DESIGNII, I can observe the changes when I increases/decreases the pressure drop. The observation I obtained is if a larger guideline has been chosen, the nominal pipe size (NPS) will be larger and similarly, a smaller guideline will lead to smaller nominal pipe size. For example, for process stream no.1 in my simulation program, the NPS is initially 8in. After I increase ∆P to 1.0 PSI, the simulated results show that the NPS decreases to 6in. On the other hand, as ∆P is decreased to 0.1 PSI, the NPS increases to 10in. My explanation for this question: This is because as pressure drop increases, a larger NPS is required to withstand the increase in pressure drop while also smaller NPS is sufficient to withstand a smaller pressure drop.

Anyone think that there's anything I should have added in my explanation or I have missed out?

[Art Montemayor]

Cher Sung:

I think you've missed out an important point in your observations of what happens when you increase or decrease the allowable pressure drop through a fixed piping installation while maintaining the liquid flow constant. That important point is to always look at and study the effects reflected by your liquid average VELOCITY within the piping installation. In my opinion, you are wise to use the criteria of allowable pressure drop to be the prime factor in selecting an appropriate piping size for an application. However, this should be done with consideration for staying within the recommended velocity range for the specific liquid and the specific application.

You don't identify the specific liquid, but it is wise to note that when liquids contain solids in suspension - especially slurrys - there is a definite tendency for the solids to precipitate out of the liquid at a certain minimum velocity and this can cause havoc in pumping systems since it creates plugging and eventually leads to a shut down due to no flow. Some liquids can be very errosive and cause mechanical damage when forced to undergo velocities beyond a certain maximum. Normally, for the majority of liquids, velocity is preferred to be 5 - 10 ft/sec. Experience is the best criteria to rely on in determining these empirical factors.

Reducing a pipe size beyond a certain diameter can be very impractical and simply won't work in the real world - like for example, when one tries to employ a 1" diameter pipe in a standard plant pipe rack installation that has spacings of 10 - 15 feet. I believe that you will find that you can't support piping that is less than 2" diameter under those conditions and, therefore, that factor must take some priority.

Additionally, capital costs for piping go up as the pipe size is increase.

I hope these comments help out.

[CS Kang]

Thank you Mr Art. Yah, I have realised I have not specified the material used on the piping, the average velocity and the liquid flowing through the piping system. The material used is Carbon Steel. The average velocity is calculated to be approximately 5.13Ft/s, with a range from 4ft/s to 7ft/s. As for the liquid, it is a mixture of hydrocarbons containing PROPANE, I-BUTANE,N-BUTANE,I-PENTANE,N-PENTANE,N-HEXANE. One thing to I want to bring up is fouling factor. Will the fouling causes different degree of erosion on the pipes and results in the need to change the pipe size?

[pleckner]

Fouling factors are typically considered in heat exchange problems, not pipe flow problems. For pipe flow problems we are concerned with a term called the roughness factor that has an impact on the friction factor used in calculating pressure drop. Different pipe materials have different roughness factors. Time will cause an increase in the roughness factor for most metallic pipes because they do start to become corroded and dirty. However in the type of analysis being done, one should assume a constant pipe material and roughness factor, otherwise too many variables are being considered at the same time and besides, they have nothing to do with the question you are being asked.

The main effects are velocity and consequently the pressure drop of the fluid and pipe size as Art pointed out. For a given flow rate and fluid properties, the increase in velocity (smaller pipe size) will result in an increase in pressure drop. Pipe size will affect pipe cost both on purchasing the pipe and installing the pipe.

Note that we do not increase the pipe size to withstand a higer pressure drop. We increase pipe size to reduce the pressure drop.

[CS Kang]

Thank you Mr pleckner for pointing out my misconception on Fouling Factor and the explanation I should use. To add on, I have found out that the use of different pipe thickness will result in change of pressure drop and operating pressure of the pipe. This can be observed by varying the pipe wall code (STD, XS, and XXS). For example, for stream no. 1 in my simulation program, the initial operating pressure is 210.0PSIA and pressure drop ∆P is 0.1618PSI. After varying the pipe wall code from standard (STD) to extra strong (XS), ∆P increases to 0.1804PSI, while the operating pressure remains at 210.0PSIA. Similarly, when pipe wall code is varied to XXS (extra-extra strong), ∆P increases to 0.3474PSI without any change in operating pressure. The above changes are also based on the assumption that the amount of liquid flowing through the pipe remains the same. Can I say this is because the pressure drop is allowed to increase with a stronger pipe wall without changing the operating pressure.

[pleckner]

You can say that the pressure drop will change because any increase/decrease in wall thickness changes the internal diameter of the pipe. It is the same affect as changing the size of the pipe.

The concept you are now describing is what we call the nominal pipe diameter. A 4" pipe is NOT a 4" pipe. We call the pipe nominally 4" but the real flow area is dependent on the wall thickness of this 4" pipe. For example, the outside diameter (OD) of a 4" nominal steel pipe is 4.5" but the inside diameter (ID) of this pipe can be 4.026" (for standard wall thickness) or 3.826" (for extra strong wall thickness).