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#1 mohammaddhanse

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Posted 16 June 2009 - 03:38 AM

Currently we have a pump whoes discharge pressure is 31kg/cm2g. the pump discharge piping has design pressure of 27 kg/cm2g & test pressure of 40.5 kg/cm2g. is it possible to use this pipeline for this discharge pressure and also the pump max shut off pressure is 34 kg/cm2g.

#2 riven

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Posted 16 June 2009 - 04:42 AM

QUOTE (mohammaddhanse @ Jun 16 2009, 08:38 AM) <{POST_SNAPBACK}>
Currently we have a pump whoes discharge pressure is 31kg/cm2g. the pump discharge piping has design pressure of 27 kg/cm2g & test pressure of 40.5 kg/cm2g. is it possible to use this pipeline for this discharge pressure and also the pump max shut off pressure is 34 kg/cm2g.


I a word: no. See below

In overly simple terms:
Your design pressure is lower than the maximum outlet pressure of the pump. Design pressure can be regarded as the cut off between safe and unsafe operation whereby safe is below this pressure.

In more detail:
What I assume you means is that
Max discharge pressure of pump is 31kg/cm2g

Maximum allowable design pressure (MADP) of piping = 27 kg/cm2g
Hydrostatic test pressure = 40.5 kg/cm2g

If my definitions are correct (i would suggest you clarify your definitions) then your
Max allowable operation pressure (MAOP) =0.75*MADP = 20.25 kg/cm2g

Note that the MAOP=0.75*MADP is according to the quality standards in force at my company (Europe). Diffeent standards may apply.

#3 djack77494

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Posted 16 June 2009 - 08:20 AM

I largely agree with what riven has said, but depending on your motivation you may have a bit of "wiggle room". First note that pipeline do NOT fall under ASME 31.3. Instead, they are governed by 31.4 and/or 31.8. The rules are somewhat different, so be sure to understand them.

Besides that, you may have other options. Assuming your normal pipeline entry pressure (i.e downstream of the pump discharge) is less than the design pressure (and by a decent margin), then you may want to implement a strategy that would control the discharge piping to below the design pressure. Of course, your methods would have to be very reliable and would quite possibly include a PSV. You don't say anything about line losses between the pump and pipeline, and you imply that the normal pump discharge pressure is greater than the pipeline design pressure. This may very well be totally unacceptable if there are no "mitigating circumstances". Be careful, and if you're not sure, consult with an expert.



#4 Nageswar Kotari

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Posted 19 June 2009 - 03:21 AM

I agree with Jedi,
But from the analyst point let me explain this.

The design pressure is used to know the static load on the piping. For this analysis the Allowable stress factor used will be Max. Allowable Stress from ASME B31.3 or 31.1
But if you are thinking of the test pressure (40 Bar) which has big margin when compared to your pump discharge pressure (31Bar) here is one point I would like to highlight.
The Static load on a piping with test pressure will be checked with Yield stress of the piping (not with Max Allowable stress value). After allowable stress value yield stress will occur. That means test pressure is only the max working pressure. This should be taken into account only once in a piping life time. This should not occur frequently.
After all this I recomend you to back check the design to 31 or frankly speaking 34bar of operating pressure which is giving a design pressure of around 37bar (rough estimation).
The piping may still withstand the pressure while it is checked for as the loading on pump discharge line will have less effect when compared to suction line. That means it would have been checked for a design pressure of 27bar but it may withstand 34bar also. But this should not be estimated although there are PRV's governing the system as this will decrease the life time of piping (yield point and all).

Am I correct Jack?


#5 djack77494

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Posted 19 June 2009 - 08:04 AM

Nageswar,
I'm not sure I fully appreciate your points, and we're on the edge of topics I feel qualified to discuss. In particular, I don't know what you mean when you refer to "static load". Two other matters bother me some. I would normally keep test pressure totally out of the discussion. I know that calculations of the pipe's yield strength and determinations of test pressures should result in numbers much larger than any practical pressures the pipe "will see". That doesn't concern a process engineer such as myself, and I like to think of such things as part of the design margin "I keep in my back pocket". They never enter into the equation in the realm of practical process engineering. So I'm a bit disturbed to see you introducing these terms. To intelligently discuss this topic more fully, I beleive we need more information from the OP such as a schematic of the system with labelled normal and design pressures. I'd like to not see or hear anything regarding test pressures or yield strengths because I don't believe they belong in the conversation.

#6 S.AHMAD

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Posted 22 June 2009 - 03:37 AM

Dear sir
I would like to suggest that first of all identify the flange rating which I believe it is 300#. If this is true then the flange can take up to 35 bar at 350degC. It is even higher at lower temperature. As for the piping, we can determine the maximum allowable working pressure (MAWP) from its actual thickness (sometime the designer put some fat into the design) and back calculate the MAWP. Otherwise you may seriously consider PSV at pump discharge or some sort of pressure control.

Best regards

#7 riven

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Posted 22 June 2009 - 06:27 AM

Regarding the basics of PED

Hydrostatic test pressure PED 97/23/EC is very clear on this as are the American standards

Hydrostatic test pressure (HTP) = highest of 1.43 MADP or 1.25*MADP*('a temperature ratio').
The HTP is the maximum pressure to which the piping can be subjected to without failure. To this we apply a safety factor (1.43) to get back to MADP.

In either case we can summarise that the regulations ask you first to define your MADP and MAOP. Therefore using pressures beyond this points eats into your safety factor and must not be done.

Regarding yeild stress/strength and other items.
These can be used to determine thickness required for a unit at a given pressure usually the MADP. They are not indicative of allowable working pressures and should never be used as such.


To get back to the original poster it should be checked to see if the MADP defined is actually a calculated value or a requested value. For example for many pilot or experimental systems it is cheaper to use standard sizes of components which actually have a higher pressure rating ability than that defined in the terms of use.

For example I have a unit where I know that the working pressure cannot go beyond 40 bar and this was then taken as the MAOP leaving MADP at ~ 47 bar. According to calculations however the highest MADP is (based on the weakest component of the unit) 75.1 bar.

#8 imtiaz

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Posted 22 June 2009 - 06:47 AM

Mohammaddhanse,
Determine the flange rating as suggested by Ahmad. Is it that the pump's discharge pressure during operation is 31 kg/cm2g and the design pressure is 34 kg/cm2? I assume this is a centrifugal pump and not positive displacement. In this case please check the HAZOP on this aspect of your plant to determine what is the safety design philosphy. Usually, A centrifugal pump will reach the maximum design pressure on a blocked outlet scenario in which case if the lines are rated at a lower pressure these should be protected via PSV located on the pump discharge. Please verify this on your P&ID and the field.
The test pressure quoted(i assume hydrostatic) is approx. 150 % of the design value you quoted. Please also verify this. Plese check your presure code requirements for this piping.
If you need further assistance, you need to give more information for example, a sketch of this section with appropriate safety devices, also the discharge piping class and a short description of this aspect of the process.
Regards,
Imtiaz.

#9 Nageswar Kotari

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Posted 24 June 2009 - 05:26 AM

Good to hear the Clarifications here,
Let me explain the scenario with an example how the loads on pump nozzles will be done.
We check the piping for two cases via...
For example say

1) The operating pressure :- 31Bar
2) Design Pressure:- 34Bar

While checking the loads on piping during this the loads produced by fluid and pipe weight will be counter checked with Allowable Stress on piping
Say allowable stress is around 30000 from pipe material data and loads from piping during design and operating are 25000 and 23000 respectively.
This gives that during design and operating pressures the piping is OK (less than allowable stress value)

But while checking the loads on piping due to Hydro Test pressure which our friend is taking now,say

Hydro Test Pressure :- 46.5Bar

the loads or the stress will be counter checked (designer or analyst practice) with yield stress of that piping material. say yields stress value is around 40000. say the loads generated by fluid and pipe weight is around 31000. Now also the piping is ok.
But my point is here a normal designer or an analyst will check the piping for yield stress as hydro test pressure will be very rare condition.

After all these number of points I want to explain that the piping will not be designed for hydro test pressure case(because 31000 is more than allowable stress of 30000 in example) to occur often. Thats why a back check is needed on the piping inspite of which the piping will not be qualified.

I thought my conversation went wrong ( which I cannot understand where) in my last reply and the statement went wrong. And I say sorry as similar to all of the others I'm not providing any solution but asking my friend to BACK CHECK again

And also all the numbers that are stated above are only for example not the real ones(cannot be compared with the real data too)

Thanks




QUOTE (djack77494 @ Jun 19 2009, 06:34 PM) <{POST_SNAPBACK}>
Nageswar,
I'm not sure I fully appreciate your points, and we're on the edge of topics I feel qualified to discuss. In particular, I don't know what you mean when you refer to "static load". Two other matters bother me some. I would normally keep test pressure totally out of the discussion. I know that calculations of the pipe's yield strength and determinations of test pressures should result in numbers much larger than any practical pressures the pipe "will see". That doesn't concern a process engineer such as myself, and I like to think of such things as part of the design margin "I keep in my back pocket". They never enter into the equation in the realm of practical process engineering. So I'm a bit disturbed to see you introducing these terms. To intelligently discuss this topic more fully, I beleive we need more information from the OP such as a schematic of the system with labelled normal and design pressures. I'd like to not see or hear anything regarding test pressures or yield strengths because I don't believe they belong in the conversation.



#10 riven

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Posted 24 June 2009 - 06:38 AM

QUOTE (imtiaz @ Jun 22 2009, 12:47 PM) <{POST_SNAPBACK}>
Usually, A centrifugal pump will reach the maximum design pressure on a blocked outlet scenario in which case if the lines are rated at a lower pressure these should be protected via PSV located on the pump discharge. Please verify this on your P&ID and the field.


I have to disagree with this.
Firstly you design regarding to pressure requirements on the weakest element. He has clearly stated that the design pressure of the piping is below 'some' pressure of the pump ('some' could be max or even normal we are not sure). It does not matter what the flange is rated for if you components downstream are going to fail.
Furthermore it is not correct to design for 'normal' scenario. A HAZOP is based on worst case scenarios and should require your equipment to be designed for such (within reason). In this case if the max discharge pressure during excursion or unusual event is 34 kg/cm2 (not a particularly high value) I do not see why one would even consider using piping that is below this pressure. Replace the piping to be in a safe situation. Therefore assume that a blocked scenario can and will occur i.e. HAZOP: no flow in piping beyond the pump.

Regarding the use of a PSV: a PSV should only be used for an excursion beyond a safe point. Be very careful on using a PSV until more clarification is offered.
If the pump can be run so that the pressure in the discharge cannot normally exceed the design pressure of the piping then the use of a PSV is ok. But if that is not the case then every tome the pressure approaches MADP the PSV will relieve. The question then becomes is that relief safe (in the case of this possibly happening often, then the answer is no and system needs to be designed).


#11 sunilh

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Posted 25 June 2009 - 03:09 AM

QUOTE (mohammaddhanse @ Jun 16 2009, 02:08 PM) <{POST_SNAPBACK}>
Currently we have a pump whoes discharge pressure is 31kg/cm2g. the pump discharge piping has design pressure of 27 kg/cm2g & test pressure of 40.5 kg/cm2g. is it possible to use this pipeline for this discharge pressure and also the pump max shut off pressure is 34 kg/cm2g.


Hello All,

It was great learning reading all your solutions. but i really dont understand how the design pressure of 27kg/cm2g is fixed. In our regular practice we take discharge piping design pressure same as pump max shut off pressure. In pump shut off condition, piping will severly be loaded and would cause unnecesaary damage and we may need to provide PSV on the discharge piping. In deciding the desgin pressure test pressure should not come into picture, infact test pressure is fixed on design pressure.

It is so simple to straightway consider pump shutoff pressure as discharge piping design pressure.

Please correct me if i have mistaken above.

Regards
Sunil

#12 imtiaz

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Posted 26 June 2009 - 05:22 AM

QUOTE (riven @ Jun 24 2009, 07:38 AM) <{POST_SNAPBACK}>
QUOTE (imtiaz @ Jun 22 2009, 12:47 PM) <{POST_SNAPBACK}>
Usually, A centrifugal pump will reach the maximum design pressure on a blocked outlet scenario in which case if the lines are rated at a lower pressure these should be protected via PSV located on the pump discharge. Please verify this on your P&ID and the field.


I have to disagree with this.
Firstly you design regarding to pressure requirements on the weakest element. He has clearly stated that the design pressure of the piping is below 'some' pressure of the pump ('some' could be max or even normal we are not sure). It does not matter what the flange is rated for if you components downstream are going to fail.
Furthermore it is not correct to design for 'normal' scenario. A HAZOP is based on worst case scenarios and should require your equipment to be designed for such (within reason). In this case if the max discharge pressure during excursion or unusual event is 34 kg/cm2 (not a particularly high value) I do not see why one would even consider using piping that is below this pressure. Replace the piping to be in a safe situation. Therefore assume that a blocked scenario can and will occur i.e. HAZOP: no flow in piping beyond the pump.

Regarding the use of a PSV: a PSV should only be used for an excursion beyond a safe point. Be very careful on using a PSV until more clarification is offered.
If the pump can be run so that the pressure in the discharge cannot normally exceed the design pressure of the piping then the use of a PSV is ok. But if that is not the case then every tome the pressure approaches MADP the PSV will relieve. The question then becomes is that relief safe (in the case of this possibly happening often, then the answer is no and system needs to be designed).


You have some good points, however, the question here is what is the current design? In the absence of a rigorous inherent design, if the downstream piping or section of it is is not rated for the maximum pressure as in a blocked outlet (no flow HAZOP deviation), then you must ensure that adequate engineering control (passive or active) or even procedures can safeguard(or mitigate) against the risk of a rupture. There is insufficient information presented- hence, my questions.
Regards,
imtiaz.


#13 imtiaz

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Posted 26 June 2009 - 06:06 AM


Sunil,
Your plant should have a piping class specification document which lists the different piping classes present together with their rated pressures. You should also have a datasheet for your pump. I am assuming this is a centrifugal pump. You will be able to get the the maximum discharge pressure from this curve. this is the dead head pressure (No flow condition resulting from a blocked dischange). match this pressure against the piping design pressure. If you have a disparity such that the pump will be at a higher pressure then a PSV may be required upstream the block valve. Please note that this should have been captured in the plant HAZOP, so i urge you to look at this section of the document. A P&ID of this section will be useful together with the design information if you require further assistance.
Regards,
Imtiaz.


#14 riven

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Posted 01 July 2009 - 07:19 AM

QUOTE (imtiaz @ Jun 26 2009, 10:22 AM) <{POST_SNAPBACK}>
You have some good points, however, the question here is what is the exact current design?


As have you but the most important question is the one above.






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