1 reply to this topic

[Nasiruddin]

Dear all,

 

I have some confusion about pipeline design / operating pressure terminologies. Would any one please clarifies the term Pipeline Maximum Allowable Operating Pressure (MAOP), is this the same as design pressure (DP) or Design pressure will be further higher than MAOP.

 

 

If a pipeline design specification sheet mentioned as:

 

Normal Operating Pressure = 72 bar-g

 

Maximum Allowable Operating Pressure (MAOP) = 83 bar-g

 

 

What will be the design pressure for this pipeline?

 

 

Regards,

 

Nasiruddin

Edited by Nasiruddin, 26 October 2015 - 12:30 PM.

[shantanuk100]

Hello Nasiruddin,
Here is what they actually mean, and how they range.

 

1. The Operating Pressure (OP) - This is the Pressure at which the device /

equipment /pressure vessel is operating at under normal operating conditions.

Simply put, you could say the general Pressure conditions in the equipment on a

regular basis.

 

2. The Maximum Operating Pressure (MOP) - This is the Maximum Operating

Pressure that the engineer considers will encounter in the process operation,

including a margin for any possible surges or fluctuations in the Pressure.

 

3. The Design Pressure (DP) - As per the ASME B31.3 Definition, this is the

most Severe and Coincident Condition, Internal or External due to the service

that the equipment is likely to be subject to. Meaning that it is the worst possible

Temperature and Pressure Case the fluid can exert on the Vessel / Equipment.

This is decided by the Process Design Engineer based on the fluid service

existing, and varies with the fluid too. The engineer decides it based on previous

experience and a sound judgement.

 

4. The Maximum Allowable Operating Pressure (MAOP) - The MAOP is not to

be confused with the MOP. It is a parameter usually applicable to pipelines and

not pressure vessels.

It is used to verify the wall failure conditions for pressurized pipelines.

 

NOTE : This is not applicable to pressure vessels in general since ASME

doesn’t mention it. It is just to understand the term. It’s usage is in general

very limited so this formula is not to be considered for any

design/calculation. It is just to give an understanding. Please don’t use the

formulae below for vessels as they apply to pipelines and not pressure

vessels.

 

It is a parameter that checks for the failure of the pipeline based on the Pressure

experienced. As an example, it is calculated below as shown for Steel Pipes:

P = (2St/D) x F x E x T

Where

S = Grade of Pipe;

T = thickness of Pipe;

D = Diameter of Pipe;

F = Design Factor;

E = Longitudinal Joint Factor;

T = Temperature Derating Factor.

 

NOTE

Pls Refer to the following Links -

http://www.northeast...friend_moap.pdf

http://www.ingaa.org...e.aspx?id=12366

(The information on MAOP is related to Natural Gas Transportation Pipelines, not industrial

process pressure vessels. It doesn’t apply to the ASME VIII pressure vessel

code, which is the almost universal, accepted vessel design code.)

 

5. The Maximum Allowable Working Pressure (MAWP) –

 

This is defined as the Maximum Pressure experienced by the Weakest point

of the Equipment before failure. It is mistakenly referred to as design pressure

but, simply put it tells us the Maximum pressure that the weakest point in the

equipment can withstand as per code (Also referred to as Design Pressure of the

weakest point, but not the general Design pressure as such).

 

(This pressure value is still ALLOWABLE for safe operation. You are simply not

allowed - by code, or law - to exceed it. There is still an operating “cushion” that

is higher than the MAWP before material failure is expected; we simply are not

allowed to go there or tolerate it for safe, common sense reasons.)

 

The temperature stresses have a major role to play in the failure of the

equipment and it is the unpredictability of this factor that makes it difficult to

calculate theoretically. The MAWP is usually determined by the manufacturer of

the equipment based on fabrication procedures, and backward correlations etc.

and is usually stamped on the equipment.

 

The MAWP is subject to change over time due to corrosion wear and thus

reduces over time. So please consult the fabricator of the vessel on changes in

this aspect.

 

The MAWP is a result of the criteria, calculations, welding procedures, and

ultimate plate selection of the vessel fabricator. It is a value that is totally out of

the control or realm of the process design engineer and normally lies in the

hands of the fabrication engineer that ultimately wins the purchase order with a

successful bid.

 

The Range of values and ascension of the above pressures is as mentioned

below:

OP < MOP < DP < MAWP

 

PSV RELIEF SCENARIOS

 

For PSV Relief you have an amount of overpressure allowed above the set

pressure before the valve fully opens. This overpressure is defined and explained

as per API 520, 521, 526.

If the PSV Set Pressure=MAWP, then we call the Overpressure as

Accumulation.

 

1. When we have a single PSV to a pressure vessel (Non Fire Case)

- the Maximum Allowable Set Pressure for the PSV is = 100% X MAWP.

- Allowable Overpressure for this Case is a maximum of 10%

thus making the overall maximum allowable Accumulated Pressure as 110% X

MAWP.

 

2. When we have a single PSV to a pressure vessel (Fire Case)

- the Maximum Allowable Set Pressure for the PSV is still = 100% X MAWP.

- Allowable Overpressure for this Case is a maximum of 21% thus making the

overall maximum allowable Accumulated Pressure as

121% X MAWP.

 

3. When we have multiple PSVs to a pressure vessel (Non Fire Case)

- the Maximum Allowable Set Pressure for the PSV is 100% X MAWP (For the

first PSV) and 105% X MAWP (For any subsequent PSVs)

- Allowable Overpressure for this Case is a maximum of 16% thus making the

overall maximum allowable Accumulated Pressure as 116% X MAWP (For all

Valves).

 

4. When we have multiple PSVs to a pressure vessel (Fire Case), - the

Maximum Allowable Set Pressure for the PSV is 100% X MAWP (For the first

PSV) and 105% X MAWP (For any subsequent additional PSVs), and 110%

X MAWP (For any supplemental PSV's).

- Allowable Overpressure for this Case is a maximum of 21% thus making the

overall maximum allowable Accumulated Pressure as 121% X MAWP (For all

Valves).

 

5. A conservative estimate would be to set the PSV at 90-100 % of the MAWP. An even more conservative estimate would be to set the PSV Set pressure = Design pressure of the equipment since there is no need to go beyond that and use the MAWP.

But take care not to lower the Setpoint too much due to which it will cause the

PSV to open up for every small fluctuation due to being too close to the operating

pressure. Maintain a sufficient Operating Margin between the Operating

Pressure and the Set Point.

 

6. The MAWP is measured and verified and Hydrotests or Pneumatic Tests are

done for the Equipment. The usual Pressure for the Hydrotest of the equipment

is 130% of MAWP (Earlier used to be 150%) and for pneumatic it is around

110% of the MAWP.

 

7. PSV Set Pressure's and overpressures depend on the respective cases so I

would suggest you refer to the Specs I mentioned and understand them

thoroughly. This document isn’t exhaustive since there are unlimited scenarios

where there is a possibility of variation. There are also numerous posts and

spreadsheets on this topic and related, so I would suggest you make good use of

them.

 

Regards,

Shantanu

Edited by shantanuk100, 19 November 2015 - 12:49 AM.