9 replies to this topic

[vinay]

Dear All,

The NORSOK standard P-001 provides following guidelines for estimation of pressure drop in CS piping :

carbon steel (non corroded) : 0.05 mm
carbon steel (corroded) : 0.5 mm

Other reference on web/text books also specify a value in betwen these two extremes.

My question is that what roughness value should be considered for designing a new pumping /piping system in typical refinery environment. There is a vast difference in estimated pressure drop between these two extreme values & it may result in a oversize of pump if we design with 0.5 mm roughness when the piping is new & uncorroded, on the other hand with 0.05 mm roughness used for design it may result in a bottleneck situation as the piping becomes old & start getting corroded.

I have seen many consultants using a value of 0.15 mm & want to know What is the industry practise ?

Regards,

Vinay Mathur

[pleckner]

For clean commercial steel pipe, the CRANE TP 410 roughness value is basically 0.00015 feet (close to 0.05mm). This is probably as close to an industry standard you will find. You are at liberty to increase tihs value if you wish for your particular system. 0.15mm (0.0005 feet) would be more for galvanized iron than clean commercial steel pipe.

[vinay]

Phil,

Thanks for your mail. However my dilema is far from over. It is understood that for new pipes the roughness factor is 0.05 or even less. Real question is what value shall be considered for a new plant piping design ( which is supposed to last for atleast 10-15 years). If I design considering a value of 0.05 mm , with new piping the pressure drop is going to be very low but as piping becomes corroded /getting old , my pipe will become a bottleneck. If I design with 0.5 mm roughness , the size of pipeline/pump will go up & also the increased size will be of no use for the new piping. Hence the dilema. A value of 0.15 mm seems to be suitable considering both sides mentioned above , thats why I put up the question.

Regards,

[pleckner]

I have not nor have I seen other people design for a higher roughness for steel pipe. This is why you don't select a pump with the maximum impeller size but choose one where the design head falls somewhere around the middle sized impeller. If in the future you need to increase the pump head (for whatever reason), you have plenty of room to put in a larger impeller.

In addition, and depending on your system, the frictional loss of piping is small compared to the static head, control valves and equipment losses. So even if the frictional losses in the pipe increase over time, it typicallly does not make that much of an impact on the pump. AND, you don't specify a pump without some capacity margin thrown in. AND, the vendor typically puts their own safety factor on the pump performance.

[jprocess]

Dear Phil,

About your statement,that is putting a larger pump impeller:
What about in gas fluid services?

Can we modify the configuration of compressors? You do not recommend using old roughness values for gas services?

This topic is about piping system. But i want to ask about pipelines. I heard that it is a common pracice for using old roughness values for pipelines.

Thanks in advance.

[vinay]

Dear Jprocess,

Thats a very valid point for gas lines , where frictional pressure drop can be a dominating factor in sizing of compressor.

Does it means we shall size liquid lines (with pumping system) for a roughness value of 0.05 mm & gas lines for higher roughness (0.15 mm -0.5 mm) depending on criticiality of service.

I am wondering why our "guru "Art is silent on this issue ?

Regards,
Vinay

[pleckner]

I haven't worked in the pipeline industry so I don't know what their guides say but at the risk of repeating myself, for process piping, I have not nor have I seen other people design for a higher roughness for steel pipe. My same arguments apply to compressors (just a vapor pump as far as I am concerned).

In addition, if the materials of the pipe are chosen properly, I would expect piping for gas/vapor to show even less corrosion effects over a lperiod of time. And note that I am talking about a long period of time, not just a few years. Your piping roughness should not change significantly until a very long time. Roughness is not an absolute (pardon the pun), the values used are only averages anyways. No one knows what the real roughness are or how quick they may change duirng operation.

I know we all look to Art for his expertiese, as do I, but just to let you know I've been at this game for over 30 years myself (no I'm not insulted that you are asking for Art's input as I too would like to read what he has to say on this). I just say this to put my credentials out on the table.

It boils down to this, if your company has standards, you follow them. If they want their process engineers to design with conservative anything, you do it. If not, then go with the flow.

[vinay]

Phil,

First accept my appologies for making you feel hurt, though in no way I intended to doubt your capabilities /or insult you with my asking for Art's opinion on this issue. I only intended to seek his views as he ,alongwith you is one of the seniormost guys on this forum. I know this subject is very elementry but the doubt existed because of various values used by various people in this world. As you have asked me to check with my organisation, i was not surprised to find that there are many a number floating around within design group. I expected it , & many of them got surprised when I with 12 years of expereience asked this question but none of them could give me an satisfactory answer . Many agreed with use of lower value (0.05 mm) , however other were designing with higher roughness value of 0.15 mm .

As I mentioned in my earlier post I agree with your method of designing piping with 0.05 mm roughness value & keeping provision inpump to raise the impeller dia for any future contingency. As this appears to satisfy both the issues ( lower initial capital cost with future readiness to meet any contingency)

I hope this matter is closed for now. Keep up the good work .

Regards,

Vinay

[pleckner]

@Vinay:

Thank you. As I pointed out in my last post, I was not insulted so no apologies needed.

It comes as no surprise that even within your own organization there appears to be no definitive "standard". Just remember we can do anything we want, technically correct and safe, with the right amount of money.

[JoeWong]

Phil / Vinay,
Please allow me to drop some simple notes...

That's no ONE solution fit for all...that's why engineer like us still in demand to make good engineering judgement.

Type of facilities
Dealing with new facilities, 0.05mm may be used for most services. However, dealing with aged pipe, you may need to obtain more information from operator/maintanence, inspection, etc and justify if a higher roughness to be used. I guest that's why NORSOK has provision on "Corroded pipe"' roughness.

Type of Service
Dealing with critical service such as compressor suction/discharge, tank / venting, flare/vent, pump suction especially positive displacement pump, slurry service, high viscosity fluid, etc, surface roughness of 0.05mm may be used. Additional sensitivity check on high roughness, analyse it's impact, provision of remedy plan, etc should be thought of and documented in philosophy during design phase.

JoeWong