8 replies to this topic

[imtinan mohsin]

I just want an opinion regarding the hydraulics of pipelines.
If pressure rating is already fixed i.e. ANSI 300# then calculation of MAOP is required or not?
Please advice.

Edited by imtinan mohsin, 25 October 2009 - 09:26 AM.

[fatimah]

Hi Imtinan Mohsin

i think the better way is to check the MAOP. do not expect the pressure rating is correct all the time. mistakes can happen unexpected, anywhere. so be responsible for each thing that you may control.

[VikingUK]

I just want an opinion regarding the hydraulics of pipelines.
If pressure rating is already fixed i.e. ANSI 300# then calculation of MAOP is required or not?
Please advice.

The pressure rating of pipelines is not the only consideration when fixing a MAOP.
You need to consider what else in the process and what would happen if pressure escaped into adjoining plant.
example if your pipework downstream of your PSV is the same rating as your hydraulics

[kkala]

If pressure rating is already fixed i.e. ANSI 300# then calculation of MAOP is required or not?
On this topic I would like to say an opinion, even late:

The pressure rating is determined by Max allowable working pressure (MAWP), which has to cover max pressure potentially formed (as the name denotes). Although pressure rating may give some idea of potential MAWP (also called design pressure), MAWP (design pressure) has to be specified in the line lists. This is also supported by the other contributors to the topic.
Let us assume an ANSI 300-lb class pipeline, flanged, of normal carbon steel, handling fluid of 750 F design temperature. MAWP=505 psig, according to Perry (7th ed, Table 10-45). For same class of 300-lb, MAWP would differ to 505 psig in case of difference in material or design temperature (as the table indicates).
Besides the pipeline may have a fitting or instrument of lower MAWP (even though today all piping components are tried to have same MAWP). Then the whole pipeline gets this lower MAWP.
At any case, above MAWP is theoretical and rather a maximum value. True MAWP of the pipeline is the one concluded from the certified hydrostatic test, realized after pipeline construction. The pressure of this test can be set to a corresponding MAWP lower than the above, especially in the case when there are pieces of equipment connected to the pipeline. Then this is the certified MAWP.
Line list specifies (among others) nominal pressure rating, hydrostatic test pressure, MAWP (design pressure). I think the latter can be significantly lower to MAWP calculated as above (from Perry).

Note: Required hydrostatic test pressure for piping is set at MAWP x f (f=1.5 for ambient temperature, f>1.5 for higher temperatures, depending on piping material. Connected equipment may have lower f, which is confusing to me).
Note 1: Exceeding MAWP (design pressure) by a rather small percentage is permissible per some piping codes for a short time.

Edited by kkala, 06 December 2009 - 12:51 PM.

[fallah]

At any case, above MAWP is theoretical and rather a maximum value. True MAWP of the pipeline is the one concluded from the certified hydrostatic test, realized after pipeline construction.

I think there is no theoretical MAWP and true MAWP is one calculated and realized after pipeline construction.

[narendrasony]

Kkala,
I disagree with you that MAWP (referred as true MAWP) is less than design pressure.
Design pressure and MAWP are two separate things. Design pressure is fixed by the process design engineer based on certain margin (typically 10%) over normal operating pressure. MAWP (Initial MAWP) is fixed by the vessel fabricator based on various factors like vessel material, actual thickness, welding techniques etc. and is always more than design pressure.
Periodic vessel inspections are done to check actual MAWP which may reduce from initial MAWP due to corrosion. BUT at no point of time it can be less than the Design pressure.

Regards
Narendra

[riven]

The answer to the first question is simply yes; MAOP can depend on other conditions depending on the typie of regulations you are based upon. For example how will corrosion or temperature affect your system. In my case based on European pressure equipment directive (PED) 97/23/EH my normal reexamination for a particular device is 4 years for non-corrosive applications and 1 year for mildly corrosive applications (less than 0.5mm/year).

The design process is that you fix your MAOP (correct term for Europe) or MAWP which then lets you set your MADP (design pressure).

MAOP =0.85*MADP at the application of PSV or
MAOP = 0.75*MADP at the application of break devices

From here the necessary hydrostatic test pressure can be calculated with the other information of stresses at operational temperature, volumes thicknesses etc etc.
Our engineering department uses COADE engineering software in this process.

[kkala]

Initial question:If pressure rating is already fixed i.e. ANSI 300# then calculation of MAOP is required or not?
Please advice.
....
kkala:"Let us assume an ANSI 300-lb class pipeline, flanged, of normal carbon steel, handling fluid of 750 F design temperature. MAWP=505 psig, according to Perry (7th ed, Table 10-45)...At any case, above MAWP is theoretical and rather a maximum value. True MAWP of the pipeline is the one concluded from the certified hydrostatic test, realized after pipeline construction. The pressure of this test can be set to a corresponding MAWP lower than the above, especially in the case when there are pieces of equipment connected to the pipeline. Then this is the certified MAWP".
---I think there is no theoretical MAWP and true MAWP is one calculated and realized after pipeline construction.
---I disagree that MAWP (referred as true MAWP) is less than design pressure.
Design pressure and MAWP are two separate things. Design pressure is fixed by the process design engineer based on certain margin (typically 10%) over normal operating pressure. MAWP (Initial MAWP) is fixed by the vessel fabricator is always more than design pressure. Periodic vessel inspections are done to check actual MAWP which may reduce from initial MAWP due to corrosion. BUT at no point of time it can be less than the Design pressure.
---The design process is that you fix your MAOP (correct term for Europe) or MAWP which then lets you set your MADP (design pressure).
MAOP =0.85*MADP at the application of PSV or
MAOP = 0.75*MADP at the application of break devices
From here the necessary hydrostatic test pressure can be calculated.

I (kkala) will try to clarify some aspects, subject to others' criticism and contribution for full clarification:
1. In the above example by kkala: let us assume that the mentioned ANSI 300-lb pipeline is the discharge of a pump (transferring liquid of 750 F design pressure and) having 250 psig max shutoff pressure, plus surge of 25 psig. And that pipeline hydrostatic test is carried out at such a pressure (e.g. 605 psig) to ensure everything OK for liquid of 275 psig at 750 F. Then pipeline MAWP is 275 psig, not 505 psig (theoretical) indicated from its class. MAWP of the pipeline could increase up to 505 psig, only in case it is sustained by a new hydrostatic test at the proper pressure.
2. There is no official term for "theoretical" or "actual" MAWP, used here to clarify the initial question through the example.
3. I think that design pressure =MAWP dictated by hydrostatic test. A past article by Cheresources explains that a vessel MAWP can be higher than its design pressure, but (to my interpretation) this concerns the design phase before hydrostatic test. Chemical Engineers seem to have no practical difference between MAWP and design pressure; at least here (Greece and around) I have not seen the term "MAWP" in process manuals or line lists, except in API.
4. Probably mentioned MAOP (lower than design pressure, as formula indicates) is not same as MAWP. Does MAOP show max acceptable value of operating pressure? I am not familiar with EN.

Additional note: 29 Apr 2012: Look into http://www.eng-tips...cfm?qid=165400 .
Also into http://cr4.globalspe...ressure-as-MAWP '> http://cr4.globalspe...ressure-as-MAWP .

Edited by kkala, 29 April 2012 - 07:37 AM.

[ankur2061]

I just want an opinion regarding the hydraulics of pipelines.
If pressure rating is already fixed i.e. ANSI 300# then calculation of MAOP is required or not?
Please advice.

This is rather a`late response to a`query raised in 2009 but then today itself I have responded to another post related to MAOP and I feel that clarity is required on the OP's post.

The MAOP should be defined as the maximum limit of the pressure in the system which will allow control of the system, the system could be a vessel, a pipe or a pipeline. Beyond the MAOP, there is no control over the system and there can be unforeseen consequences with respect to the operation.

In context to a pipline MAOP can be useful by providing a overpressure protection device to prevent the pressure exceeding the MAOP if there are unforeseen consequences of the pressure exceeding MAOP.

Providing an overpressure protection device with a set pressure corresponding to the MAOP is not obligatory. The overpressure protection could be provided to activate at the MAWP or DP of the pipeline if it is ascertained that pressure exceeding the MAOP can be tolerated from the operations viewpoint and the main concern of the overpressure protection is system integrity.

Thus the engineer has the flexibility of considering MAOP for any system protection or ignore it in favor of the MAWP or DP. This answer's the OP's query by providing him a choice to consider MAOP or overlook it.

Regards,
Ankur.