The Art of Line Sizing

As process engineer, we have responsibility in determining the pipe size. Both at proposal and project stage, the adequacy of line size shall be reviewed by process team. I am really sure all of us already familiar with line sizing. We know well both the methods and criteria. Here, we will learn together that actually, the understanding of line sizing philosophy is very important. Line sizing is not just about GOOD and NOT GOOD as we can show in the spreadsheet.

Acceptable Size

The acceptable line size is purely depends on the line sizing philosophy. Here, I’ll give some example. The velocity of suction pump shall be keep low enough to prevent vortex at outlet of vessel or tank, and to prevent excessive pressure drop along the suction line. The velocity of reciprocating compressor suction shall be not too high so that leading to the surge. The velocity of drain line shall be maintained high enough to prevent deposit forming. The pressure drop of discharge pump and compressor shall be low enough to prevent high operating cost. The velocity of two phase flow shall not be leading to the pipe erosion. Etc.

Fortunately, many philosophies are already summarized in the parameter that clearly defined. For instance, to avoid deposit at drain line we need velocity more than 1.1 fps, to avoid surge on the reciprocating compressor we need suction velocity less than 40 fps etc. To achieve optimal operating cost of cooling water pump, the water header pressure drops shall be less than about 0.3 bar/100 m. Etc. Those data parameter are very useful. It’s helpful for us to imagine our pipe condition.

It is not surprise for me that those parameters are not the exact value. I mean, the value is not always 1.1 fps, 40 fps. Etc. Here simple example, for liquid sub cooled, suction pump has parameter of maximum velocity 8 fps, we may choose to use 10 pipe size with the velocity 8.1 fps rather than 12 inch with the velocity 7.0 fps. I just say ’may’, not ‘better’ because it is depends on some consideration. Please continue reading. I hope you got it after read this fully article

Many companies make difference in determining the parameter value. for instance, based on my experience, there are various velocity criteria for pump discharge velocity used. Each Client have a specific design criteria. Client A state that velocity shall be less than 29.5 fps, B use 19.6 fps, C say 12 fps, D use 16 fps. Sometimes, the client has his own standard procedure to evaluate economic velocity of the discharge pump. If we refer to API 14E, it stated that economical velocity pump system reached when the discharge pumps velocity is not exceeding three times of the suction velocity. Based on those reality, I think, it’s not forbidden for us to use parameter around those values as long as a good engineering judgment based on experience is used.

Many companies agreed only for two phases flow parameter. They agreed to avoid slug flow and erosion with same parameters called flow induce vibration and erosion. We can refer to the formula stated in API 14E for calculate erosional velocity-[erosional velocity = Constant/mix density^0.5]-. In some cases we cannot avoid slug flow due to erosional velocity limitation. In this case, good understanding of line sizing philosophy is helpful. Actually, slug flow can be acceptable if we can make the pipe stand using more strength support. But, erosion is something we cannot avoid without change the pipe size. Once erosion continued happen, your pipe is under dangerous condition. So, we shall use flow regime and an erosional velocity for limitation of acceptability pipe size for two phase line.

Optimal Size and Get Competitive Cost

At proposal stage, we need to optimize the pipe size to achieve competitive cost.

Regarding of reviewing line sizing result, I suggest for job leader to give more attention for the following line :
1. Large diameter pipe size.
2. Long pipe.
3. Stainless steel or higher material line
4. Line having a large number of valves or and instrumentation.

For example, for large size duplex gas line, carefully reviewing of the line sizing result is very important. Optimize if it is possible. No doubt, the cost will be significantly reduced if we can optimize duplex or stainless steel pipe with many valve installed along that pipe. We can choose the lower pipe size as long as it is reasonable and the philosophy is achieved.

Other example, for air system, there are small pipe size in the system, and the material usually CS or CS galvanized is used. If you have short time of proposal duration, I suggest you to check the adequacy only. Don’t waste your time to optimize this small line.

Sometimes, we are also able to optimize the pipe size for some cases depend on the service condition. Here I give an example.
Look at this picture.

The service line is fuel gas to burner, the line in high pressure, with 8 inch pipe size. If you conduct line sizing calculation, and the result show that NOT GOOD due to pressure drop. We can ignore it. Actually we don’t need the requirement pressure drop limitation. The downstream pressure will be reduced by PCV even more than pressure drop in the pipe. Your spreadsheet always says “I don’t care; my result formula is NOT GOOD”. You must know this. Don’t forget to check velocity or Mach number if you want to reduce this line size.

That’s all I can share to you, let’s keep in our mind, line sizing is not just inputting the data, and then looking at column GOOD and NOT GOOD. Even API 14E says “Calculated line sizes may need to be adjusted in accordance with good engineering judgment”

Hopefully, this article is useful for us. Please correct me if I am wrong. Thank you.