8 replies to this topic

[chemks2012]

Hi All,

Once again, I have a query on relief valve set pressure

We have a reactor whose design pressure is 2.27barg and MAWP is 3.45barg. The reactor has a jacket with the design pressure of 4barg and MAWP of 5.17barg.

The jacket is supplied with steam at 2barg [using pressure reducing valve i.e. steam pressure reduced from 10 to 2barg]

Can I set the set pressure of the jacket relief valve to 5.17barg i.e. MAWP of jacket?

Thanks

KS

[TS1979]

If the MAWP of the jacket is specified as 5.17 barg, sure you can set the jacket pressure relief valve up to 5.17 barg.

[chemks2012]

Hi TS1979,

Thanks but the reason I am asking is that MAWP of reactor is only 3.45barg and I am not sure what is the maximum external the reactor will be able to withstand.

Thanks

[Art Montemayor]

This thread belongs in the Relief Devices Forum, and it is being moved there so that it can receive the specialized attention it deserves.

I am compelled to address this thread and make some comments on the query – as well as on some of the responses so far. I have experienced a lot of applications – in the thousandths – that involved this type of topic and I want to make sure that what I have learned is passed on and fully understood.

The application is a jacketed batch reactor, I believe. My first comment is that, if applicable, the run-away reaction should have been evaluated as a potential overriding case. If that is so, then a very different method of PSV design is called for in deciding the relieving capacity.

The vessel in question has been mechanically designed to withstand both internal AND external pressures. Here, the term “external pressure” does not mean a vacuum – although a vacuum case should certainly be considered for the steam-heated jacket. I am assuming the jacket is a full, cylindrical jacket, and not a dimpled or half-pipe type.
I think what is needed here is to emphasize that the maximum set pressure called for by the ASME code is the MAWP (Maximum Allowable Working Pressure). It is very important to understand what this term means and implies.

TS1979 is correct; the maximum set pressure should be the MAWP. It is not the Design Pressure. I have mentioned and written a lot on this subject in the past, but it seems I have not been understood or I am not communicating correctly. It is very important to fully understand both terms and the fact that the MAWP is the value of record. The MAWP is important because ASME has taken the pain and precaution to make sure that PSVs are set and operate at the point they were intended to activate in order to comply with the scope and intention of their design. What ASME was charged to do when they had to write the Pressure Vessel Code (Section VIII), was to protect human beings from being killed by pressure vessel explosions (primarily steam boilers). To do that, ASME has identified a value of pressure that, in its definition, cannot be exceeded in a given pressure vessel. This is regardless of the time element the vessel finds itself in. In other words, ASME means that by stating “MAWP”, it means the maximum allowable working pressure for that vessel AT THE TIME IT RELIEVES. What that means, then, is that the owner (or operator) of the vessel MUST ENSURE THAT THE MAWP IS KNOWN AT THE TIME OF THE RELIEF AND THE PSV THAT MUST (BY LAW) PROTECT THAT VESSEL RELIEVES AT THAT MAWP. ASME knows that the MAWP will change – in fact, it starts to degrade the moment the vessel leaves the fabrication shop. It is well-known that the Design Pressure (DP) is less than the MAWP of a vessel and that if one sets the PSV at the DP, one is being conservative in protecting the vessel. That’s perfectly OK; however, what that insinuates is that one can forget about the MAWP. That is not so (nor legally acceptable) because the degradation of the MAWP value with time can defeat the DP value – in other words, with erosion and corrosion the MAWP value could drop below the DP value, and the result would be that the vessel would not be protected. That is unacceptable. That is why ASME keeps insisting on and relating to the MAWP. What ASME is indirectly trying to achieve is that the owner or operator must accept the responsibility of knowing what are the maximum stresses that can be imposed on his/her pressure vessels – and subsequently take the steps to see that the mandated pressure relief devices are set according to the known MAWP value. An engineer really doesn’t know what the so-called DP is after years of service on the part of the vessel. The DP is a pseudo-value. It is a value picked by a process engineer prior to the vessel being fabricated. When the vessel is fabricated, the fabricator uses the material and shell thickness that will satisfy the DP value cited on the Data Sheet – always ensuring that the resulting vessel will have the capacity to EXCEED the limits of the DP. Therefore, the DP becomes a fictitious value. It really does not exist – except in the Data Sheet.

When an owner or operator are forced to inspect and re-rate their pressure vessels to ensure that the stress values imposed on the vessel are indeed protected by a PSV, they either confirm the value of the original MAWP, or they are forced to revise it (almost always to a lesser value) and note it on a revised ASME stamped plate. Upon doing that, nothing is done to the DP value – because at this point, it is a moot point; it doesn’t enter into any calculations. All that matters is that if the MAWP has to be revised downward, then the related PSV also have to likewise be revised. This is called legally protecting the personnel and the vessel. Note that the sole and main purpose of the PSV is to avoid that the internal pressure does not exceed the MAWP – although it might exceed the design pressure - of a vessel.
You are always free to set your PSVs below the MAWP value; usually this is done by using the DP value as the set point (at the initial time of operation). However, after the vessel operates for some years, it is always safer and more prudent to set the PSVs to the MAWP (as identified at that time).

I apologize for the lengthiness of this post, but I believe that it is important for our members (who are professional engineers) to be perfectly aware and in tune with the intentions, spirit, and mandates that ASME and other similar codes try to enforce. It is the design engineer – as well as the operations engineer – who are charged with implementing safe and legally correct actions that will secure the safety of their personnel.

[chemks2012]

Hello Art,

Thanks very much for your detailed explanation. Much appreciated.

Yes, the jacket of the vessel is the full jacket and it is used only for heating using low pressure steam at 2 barg.

After following your comments in last few threads, now I really understand that why we need to select the MAWP as set pressure for PSV. Thanks again.

However in our case, there are two PSVs, one PSV1 is protecting the reactor which will be set at reactor MAWP of 3.45 barg. And another PSV2 is protecting the jacket and as you suggested we can set its value to the MAWP of jacket which 5.17 barg.

Now if I set value of PSV2 to 5.17 barg, will it not exert pressure on reactor cell which has MAWP of only 3.45 barg? And therefore we need to set PSV2 to 3.45 barg? Or I am cometely missing the plot here?

[fallah]

chemks2012,

Generally, in a jacketed reactor DP/MAWP of each side has to be set such that the vessel can withstand the worst case in the pressure point of view, i.e. : simultaneous maximum internal pressure and minimum external pressure

Considering that the MAWP of the jacket is higher than that of the reactor itself, it seems that the jacket has been desined for FV due to the possibility of sudden steam condensation there. Then, setting the PSV2 to 5.17 barg will not exert abnormal pressure on reactor cell, probably due to this matter that the reactor would never be subject to partial/full vacuum and will provide higher external pressure for jacket than the jacket for the reactor.
Am i right?

Fallah

[Art Montemayor]

This seems to be a unique case where the steam jacket on the reactor has a higher MAWP rating than that on the reactor itself. That’s OK. I don’t know the reason for that paradox, but I’m sure there is a perfectly good engineering justification for that type of construction in your application. Or perhaps the reactor was originally fabricated for another specific application that necessitated the difference in MAWP’s.

Regardless of the reason, if the existing physical conditions of the reactor and the jacket are still in such a state so as to justify the stated MAWPs, it is my opinion that the PSV on each section should be set in accordance to their respective MAWP. Allow me to cite API 520, Part 1:

From:
Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries
Part I—Sizing and Selection
API RECOMMENDED PRACTICE 520
SEVENTH EDITION, JANUARY 2000
“The maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The maximum allowable working pressure is the basis for the pressure setting of the pressure relief devices that protect the vessel. The MAWP is normally greater than the design pressure but must be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.”

Now allow me to also cite another source that has been battered about as a credible reference when employing engineering decisions regarding PSVs:

From:
Coulson & Richardson’s CHEMICAL ENGINEERING; VOLUME 6
R. K. Sinnott
“13.4.1. Design pressure
A vessel must be designed to withstand the maximum pressure to which it is likely to be subjected in operation. For vessels under internal pressure, the design pressure is normally taken as the pressure at which the relief device is set. This will normally be 5 to 10 per cent above the normal working pressure, to avoid spurious operation during minor process upsets. When deciding the design pressure, the hydrostatic pressure in the base of the column should be added to the operating pressure, if significant. Vessels subject to external pressure should be designed to resist the maximum differential pressure that is likely to occur in service. Vessels likely to be subjected to vacuum should be designed for a full negative pressure of 1 bar, unless fitted with an effective, and reliable, vacuum breaker.”

Obviously these two sources are at odds regarding how PSV are to be set. I bring this forth because the fact is that no serious engineering design house or organization is going to design according to the recommended practices of Mr. R.K. Sinnott. The norm and standard in the industry is, however, using API 520 (both parts) and API 521 as the recommended practice on PSV applications.

I use this to emphasize - once more – the importance of MAWP over design pressure and the fact that the vessel in question must be documented accurately with the specific, applicable MAWP. By this, I mean that you should make sure that the MAWPs you cite are veritably the ones that apply to the sections of the reactor you mention. I am assuming that you have the reactor's mechanical calculations, Data Sheet, and fabrication drawings - all of which clearly support the fact that the MAWPs you cite are the ones for the jacket and the reactor.

[Robert Montoya]

Dear Art. this is very important discussion and I think that depend of each company.
According with ASME sec. VIII there is not any difference between the design pressure and the MWAP in fact provide that MWAP is at least the design pressure ie will never the design pressure will be greater than MWAP
For another way ASME also establishes "All pressure vessels other than unfired steam boilers shall be protected by a pressure relief device that shall prevent the pressure from rising more than 10% or 3 psi, whichever is greater, above the maximum allowable working pressure"
Is very clear that the set point of the PSV never can be higher that the MWAP, in some companies set criteria more conservative based on the safety of theirs plants between the design pressure and MWAP like the MWAP is 105% de design pressure and even establish that the MWAP is only to construction purpose and is used by ASME while the design pressure is only used to calculate the minimum thickness and = set point of the PSV in order to avoid that the pressure increases above of the design pressure inside the vessel. This is my point of view regarding this point.

Now regarding the doubt of CHEMKS2012; the problem is that when there is a problem in a jacket and the pressure increases above 3.45 barg the reactor perhaps result with some damage. I really think that the design case of the jacket is failure in the FV. However I think that a relationship exists between the pressure increase in the coil and the increased pressure in the reactor so that when there is high pressure in the coil so there will also in the reactor (due to increase in the amount of heat) anyway it would be desirable to know more details, such as occurs in the reactor when there is an increase in temperature?

Otherwise you should reconsider the set point of the jacket since for low pressure side of the jacket that can happen is to be triggered under certain situations but more important is the protection provided the side the reactor.

[skearse]

Hey all-been a while since I've posted here but this caught my eye and I thought I'd add my two cents...take them for what they're worth. Based on my experience, here they are...

In order to rate the jacket for the MAWP, all components that comprise the jacket must be designed for that MAWP. In this case, that would include the 'shared' component of the reactor wall. I've seen some situations similar to this in the past, where the reactor and jacket have different MAWPs. Normally what I've seen is that the limiting component for the rating of the reactor is an unjacketed portion of the vessel, [again] usually the top head. By reviewing the design of the entire vessel (or having it reviewed by a PV deisgner/engineer), one should be able to confirm whether or not this is true in this case. If that is indeed the case here, I would have no problem setting the jacket at the higher MAWP.