7 replies to this topic

[engg]

hi all;

PV=nRT

i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed?

any ideas.

thanks

Edited by engg, 15 September 2011 - 06:43 AM.

[kkala]

Oohps! My understanding on the matter was originally quite different, since "volume" can have the meaning of quantity (at least in my thought), so "volume of piping " was considered by me as piping material take off. Probably I have to erase this post, or leave it to indicate results of misunderstanding! Even though post gives some info on piping design, the matter below is not related to the query, which has been written in a clear form now. Expecting suggestions, the lesson is "ask clarifications before".

hi all; I wanted to know if it is possible to calculate volume of piping from P&ID alone? any ideas. thanks

P&ID is not drawn under scale, neither does it show pipe elevations or downs; it is just a shematic diagram showing all lines. You can do the "line list" based on it (you have to know elevation differences too), but you cannot estimate the length of each pipe. Isometric or piping arrangement drawings are needed for "piping material take off" (revised at least three times during project implementation). Mentioned isometric / piping arrangement drawings are elaborated by piping designers / draftsmen, after having looked into P&ID, layout and existing piping drawings. They usually visit the field for clarifications.
Pipe count (α meters of 3" pipe of class ABCD, β meters of 4" pipe of class CDFD, etc), valve count, count of rest piping elements (e.g. thermal liquid expansion valves, blinds, etc) are concentrated in a list of piping material take off, Revision 0. This may represent only (say) 75% of total piping (of final Revision), since a lot of drains, vents, expansion joints, new utility pipes, are added (by Piping or Other Disciplines). Orders are usually placed after every substantial Revision, since piping is a critical item in the majority of Projects.
Above concerns information from a rather big Petrochemical Project (1981). If it is a simple modification involving only one P&ID, things may be simpler. In a fertilizer factory new piping was approximately estimated from plot plans, when we had to install a gas cleaning system (1980). No isometrics were used (most probably there were not). Maintenance Dept measured, cut, and installed piping on field, connecting equipment as needed. I think this practice would be confusing today for a small refinery Project, due to plurality of pipes and racks. It may be preferrable to elaborate piping drawings first (isometrics & arrangements), then do piping counts.
In brief, you need isometric & arrangement drawings to estimate length of each pipe and count of elbows (at least), even though you could estimate valves and diameter / characteristic of each line from a detailed P&ID (but see above concerning Revisions).

Edited by kkala, 15 September 2011 - 10:14 AM.

[kkala]

hi all; PV=nRT
i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed? any ideas.thanks.

Suppose that the physical length of your line is x. If d=internal diameter, total pipe volume is V=x*π*d^2/4, so ideal gas law can get the form n=(P*π*d^2 / (4*R*T))*x. This means that gas quantity n to fill this pipe line is proportional to line length x. So you cannot specify n without having specified x. Of course you can estimate an approximate length of pipe x and find an approximate quantity of gas n, assuming a pipe routing on a layout plan. Probably this is what you can do now that only PID has been prepared. Or you can measure / estimate pipe length on the field.
If pipe line has several diameters along its length, you have to estimate line length for every diameter and add the corresponding contents n.

Note: Do not consider previous post of mine, it is irrelevant. If I had looked at the label carefully, I might have then suspected the real question.

Edited by kkala, 15 September 2011 - 10:45 AM.

[GS81Process]

hi all;

PV=nRT

i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed?

any ideas.

thanks

kkala's second response is right. The actual volume of gas you need is the actual volume of the pipe to be filled. You need to know the length.

[engg]

hi all;

PV=nRT

i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed?

any ideas.

thanks

kkala's second response is right. The actual volume of gas you need is the actual volume of the pipe to be filled. You need to know the length.

hi all; PV=nRT
i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed? any ideas.thanks.

Suppose that the physical length of your line is x. If d=internal diameter, total pipe volume is V=x*π*d^2/4, so ideal gas law can get the form n=(P*π*d^2 / (4*R*T))*x. This means that gas quantity n to fill this pipe line is proportional to line length x. So you cannot specify n without having specified x. Of course you can estimate an approximate length of pipe x and find an approximate quantity of gas n, assuming a pipe routing on a layout plan. Probably this is what you can do now that only PID has been prepared. Or you can measure / estimate pipe length on the field.
If pipe line has several diameters along its length, you have to estimate line length for every diameter and add the corresponding contents n.

Note: Do not consider previous post of mine, it is irrelevant. If I had looked at the label carefully, I might have then suspected the real question.

hi all;

PV=nRT

i want to fill up a line (of unknown length, as i have only P&ID) with N2 at know pressure and constant temperature. How can i calculate the volume of N2 needed?

any ideas.

thanks

kkala's second response is right. The actual volume of gas you need is the actual volume of the pipe to be filled. You need to know the length.

thanks guys. your answer is very clear. KKala no matter
, i appreciate your response and the time you spent in writing your first response, i read it very carefully, probably i should i have been clear in my question but its fine.

i have another stupid question. I need to do a pressure test. For some pipes at high N2 pressure and for some at low pressure. My question is, assuming the same legth of pipe (and temperature), such that Pipe A is done at high pressure and Pipe B is Done at low pressure. than which pipe will need more nitrogen A or B? i.e. does increase in pressure reduces volume of nitrogen needed or increses? as per boyles law as pressure increases volume reduces but i feel its not applicable in this case is it?

Edited by engg, 18 September 2011 - 12:15 AM.

[GS81Process]

Pipe A will need more nitrogen than pipe B on a mass basis but if the volume of pipe to be tested is the same then the actual volume of nitrogen required will be the same. The density of gas in pipe A will be greater.

Say for example that the absolute pressure required for pressure testing pipe A is two times than what is required for pipe B and that pipe A and B are the same volume. Assuming that the pressure testing is under conditions of pressure and temperature where the ideal gas law is applicable, then the density of nitrogen in pipe A will be twice that in pipe B. PV=nRT or density= MP/RT

Here's a better question: why are you pressure testing with nitrogen (a compressible fluid) instead of water (an incompressible fluid). It is safer to pressure test with water because if the pipe fails then there is no (or little) energy of decompression released. There are many historical references on this subject.

Is this for a pipeline?

[engg]

Pipe A will need more nitrogen than pipe B on a mass basis but if the volume of pipe to be tested is the same then the actual volume of nitrogen required will be the same. The density of gas in pipe A will be greater.

Say for example that the absolute pressure required for pressure testing pipe A is two times than what is required for pipe B and that pipe A and B are the same volume. Assuming that the pressure testing is under conditions of pressure and temperature where the ideal gas law is applicable, then the density of nitrogen in pipe A will be twice that in pipe B. PV=nRT or density= MP/RT

Here's a better question: why are you pressure testing with nitrogen (a compressible fluid) instead of water (an incompressible fluid). It is safer to pressure test with water because if the pipe fails then there is no (or little) energy of decompression released. There are many historical references on this subject.

Is this for a pipeline?

thanks G8,

its a pipeline and production platform. We need to helium test to quantify the leak

[kkala]

Matter has been clarified by GS81Process; moreover let us have an arithmetic example, with arbitrary data.
Suppose 500 m of 6'' (Sch 80) pipe A has to be tested to 30 Barg (31.013 Bara), while another 500 m of 6" (Sch 40) pipe B has to be tested to 15 Barg (15.013 Bara). Ambient air / N2 temperature is 20 oC.
1. Volume of pipe A: 500*74.190E-4=3.71 m3.
N2 to fill the pipe :3.71 m3 (30 Barg, 20 oC), or 3.71*(31.013/1.013)*(273/293)=105.8 Nm3 (0 Barg, 0 oC), or 105.8/22.4=4.72 kgmol N2 = 132 kg N2.
2. Volume of pipe B: 500*82.124E-4=4.11 m3
N2 to fill the pipe: 4.11 m3 (15 Barg, 20 oC), or 4.11*(15.013/1.013)*(273/293)=56.8 Nm3 (0 Barg, 0 oC), or 56.8/22.4=2.53 kgmol N2 = 71 kg N2.