14 replies to this topic

[Mark-TR]

Dear all,

 

I’m currently studying the design of a system that follows ASME I code.

 

I ‘d like to know if balanced PRVs can be used for the boiler protection.

 

On this code on paragraph PG-67.5  it is stated the following paragraph:

 

“For  high-temperature  water boilers  pressure  relief valves  shall  be used.  Such valves  shall have  a  closed bonnet.  In  addition the pressure relief valves shall be capable

of satisfactory operation when  relieving water at the saturation  temperature  corresponding to  the  pressure at which the valve  is  set to  blow”

 

According to this paragraph it is my understanding that balanced PRVs could be used.

 

Some people state that balance PRV can not be used as per ASME I

 

It is my understanding that in the superheated zone, conventional PRV are mandatory for the requirement stated on PG 68.6:

 

“The pressure relief valve shall have a flanged  inlet connection,  or a  weld-end  inlet connection.  It shall  have  the

seat and disk of suitable heat erosive and corrosive resisting material, and the spring of direct spring-loaded safety

valves shall be fully exposed outside of the valve casing so that it shall  be  protected from  contact with the escaping steam”

 

on this area balanced PRV can not be used. But I do not understand why they could not be used for boiler protection.

 

Your help is appreciated.

 

Kind regards

[fallah]

Mark-TR,

 

Due to having no significant back pressure (open to aimosphere) and possibility of bellows failure, you don't need to use ballanced bellows PSV for boiler protection...

[CMA010]

What exactly from PG-67.5 and PG-68.6 makes you conclude that balanced valves can / can not be used? And why do you even want to use a balanced valve as the discharge is usually to atmosphere?

[Mark-TR]

dear  Cma010,

 

the problem that I have is that I´m reviewing some FEED engineering.

 

If I consider a conventional PRV backpressure is limited to 10%, if I consider a balanced bellow my backpresure con be limited between 30% and 50%.

 

this means that discharge line is increased proportionally with the reduction of bakcpressure.

 

in numbers this means that the discharge lines goes from 14" to 20". it is most costly effective to consider a balanced PRV than a conventional PRV.

 

regarding the codes,

 

PG-68.6 statet that the PRV  shall be fully exposed outside of the valve casing, on this basis a  PRV can not be balanced, as balanced PRV are not open bonnet .

 

PG-67.5 states that a closed bonnet shall be used.

 

nevertheless, some people in my department states that balanced PRV are not used on ASME I services.

 

I have not received an answer based on facts, just opinions.

 

so I'd like to have some feedback for a technical reason.

 

Fallah

 

I understand that balanced bellow should not be use, as water may freeze and block the bonnet vent. this will mean that balanced PRV will not work properly. leading to dangerous situation.

 

Nevertheless, I have seen that in freezing service Balanced piston type are used.

 

I'll appreciate the feedback from the forum on this point.

 

Kind regards 

[CMA010]

The statement of PG-67.5 is for high-temperature water boilers, e.g. water relief, I do not know if this applies to your system. PG-68.6 is for superheaters, or steam relief above 450 °F. The criteria regarding open / closed bonnet do not relate to balanced or conventional in my opinion. I did not find any specific statement about the use of balanced valves in ASME I (note: this does not mean it is not there) nor am I aware of any code cases regarding balanced valves (there is one for pilot operated valves). I think your problem is to find ASME I steam certified valves.

 

Vendors usually specify a temperature range of -450 °F to 1000 or even 1500 °F for their balanced valves, so unless you expect a lot of cycling (temperature) I don't know if bellows failure would be a real concern in your case.

 

In case of sizing a the tail pipe to atmosphere the sound level is usually a concern, so often low mach numbers (or larger diameters) are required. Did you include this in your evaluation (if applicable)?

[Mark-TR]

Dear CMa010

 

Please note that all balanced PRV are closed bonnet by definition, you can not have a balanced bellow PRV with open bonnet in the market.

 

I have also review vendors catalogues, it seems that balanced bellow PRV are not ASME I certified. I do not know the technical reason but it seems it is a fact.

 

The only basis of failure for the bellow is minor leakage. if water is leaking and I'm working under winter conditions, water may freeze at the bonnet vent and then a missoperation may occur. API 520 part I makes a warning regarding this potential problem on balanced PRVs.

 

I have seen that when this may happen a balanced piston type PRV is selected, nevertheless I have not seen any information on vendor catalogues regarding balanced piston type PRV.

 

Furthermore, regarding critical flow my specification allow to go to high mach numbers as this is not a hydrocarbon relief and only steam is relieved. This philosophy is according to ASME b31.1.

 

Therefore, at this point i'd like to confirm if balanced piston could be allowed on ASME I. 

 

I will appreciate if somebody could clarify/comfirm the technical reason why balanced PRVs are not allowed.

 

kind regards

[fallah]

 

you can not have a balanced bellow PRV with open bonnet in the market.

 

 

I will appreciate if somebody could clarify/comfirm the technical reason why balanced PRVs are not allowed.

 

 

Mark-TR,

 

Eventually all balanced bellows PSVs in the market are open bonnet type to keep balancing characteristic in variable back pressure condition. If in any case that open bonnet cannot be directed to the atmosphere (e.g. toxic gas service), the bonnet opening should be piped away to a safe location...

 

When there is small and constant back pressure, no need having a technical reason in hand to noy applying balanced bellows PSV...

Edited by fallah, 11 February 2014 - 03:59 AM.

[Mark-TR]

Dear fallah,

 

Please note that balanced PRV are closed bonnet, but they have a vent routed to atmosphere or a closed system.

 

Please find Tyco catalogue on this link

 

 http://www.tycovalve...CMC-0334-US.pdf

 

Please look at page 10, you will find open bonnet conventional valves, if you go to page 8 you will see closed bonnet conventional and balanced PRV.

 

The mechanical construction is diffferent as the spring is in contact with atmosphere , where in the closed bonnet it is not.

 

kind regards

[fallah]

Mark-TR,

 

Actually by open bonnet i meant the bonnet is to be vented to the atmosphere by a hole. Then, even with closed bonnet, if it vented to the atmosphere whose inside spring can be in contact to the atmosphere...

[CMA010]

I understand you reasoning regarding balanced and open / closed bonnet, however this is my opinion not the intend of ASME I PG 68.6 (or PG-67.5). In my opinion this statment is related to cooling of the spring, not specifically to disallow (or allow) balanced valves. As there are no open bonnet balanced valves on the market it means you're stuck with conventional valves but I don't know if one could design an open bonnet balanced ASME I certified valve if one wanted.

 

I'm not aware of any limitations or allowances in ASME B31.1 regarding noise levels or exit velocities futhermore do I fail to see any difference between a HC, steam, nitrogen, air or any other relief to atmosphere. Noise levels are usually defined in company standards and / or local regulations or permits. Personally I feel that the noise levels should always be investigated as that in case of discharge to atmosphere with high mach numbers you can end up with noise levels well over 140 dB(A) at 1 m.

[Mark-TR]

Dear CMA010,

 

I agree with you the key statement is the following:

 

In my opinion this statment is related to cooling of the spring, not specifically to disallow (or allow) balanced valves. As there are no open bonnet balanced valves on the market it means you're stuck with conventional valves

 

There are no open bonnet balanced PRV and in consequence I cannot guarantee the cooling effect.

 

In fact API 520 states the following:

 

"Open bonnets are limited to services that are non-hazardous, such as steam, air and water, since the process fluid will escape through the open bonnet upon actuation of the PRV. Open bonnets allow the spring to be cooled by ambient conditions in high temperature applications"

 

Regarding the mach number, This generally limited to HC service. In non hazardous service such nitrogen, steam, air is not limited by some companies  in the industry.

 

This is not an absolute rule but some Companies practices. As far as I know, this is to avoid autoignition because of the high velocity at the end of the pipe.

 

In non hazardous system is not a problem.  Furthermore the level sound limitations in a intermitent service such a PRV is more flexible.

Because the probability to open a PRV is very low, it should nevr open unless there is an emergency.

 

Again the velocity and sound level limitation criteria changes among the Companies and should not be consider as an absolute rule. 

 

Kind regards

Edited by Mark-TR, 14 February 2014 - 07:32 AM.

[jamesgo]

Hi

 

I don't agree with this: For example you can read this on the LESER catalog on the API 526 type :

 

High temperature equipment as J88 option

 

 

For fluid temperatures higher than 400 °C / 752 °F high temperature equipment is necessary to protect the inner parts and

the spring against inadmissible influence of temperature. The maximum inlet temperature is 550 °C / 1022 °F. The equipment

shown is only fitted in Type 5267. For all other Types an open bonnet and a stainless steel bellows is necessary for fluid tem-

peratures exceeding 400 °C / 752 °F up to max. 450 °C / 842 °F

 

and you can see a drawing and materials on the same page

 

Hope it can help you

 

Best regards,

 

Dear CMA010,

 

I agree with you the key statement is the following:

 

In my opinion this statment is related to cooling of the spring, not specifically to disallow (or allow) balanced valves. As there are no open bonnet balanced valves on the market it means you're stuck with conventional valves

 

There are no open bonnet balanced PRV and in consequence I cannot guarantee the cooling effect.

 

In fact API 520 states the following:

 

"Open bonnets are limited to services that are non-hazardous, such as steam, air and water, since the process fluid will escape through the open bonnet upon actuation of the PRV. Open bonnets allow the spring to be cooled by ambient conditions in high temperature applications"

 

Regarding the mach number, This generally limited to HC service. In non hazardous service such nitrogen, steam, air is not limited by some companies  in the industry.

 

This is not an absolute rule but some Companies practices. As far as I know, this is to avoid autoignition because of the high velocity at the end of the pipe.

 

In non hazardous system is not a problem.  Furthermore the level sound limitations in a intermitent service such a PRV is more flexible.

Because the probability to open a PRV is very low, it should nevr open unless there is an emergency.

 

Again the velocity and sound level limitation criteria changes among the Companies and should not be consider as an absolute rule. 

 

Kind regards

[aroon]

Mark,

 

This is just to clarify on your post #4 related to back-pressure. The 10% back pressure limitation for conventional valve is just a preliminary value. There are special type of conventional valves for boiler service, which allows you to have back pressure even more than 20%. I have seen in some of the cases  with allowable back pressure of 22.5% to 27% ..... manufacturers are Crosby, Farris, etc. Please check with manufacturers. They have ASME I certified valve with higher allowable back pressure.

 

I will let you know if I find those particular Catalogs.

[fallah]

aroon,

 

Attched is a fact for allowable back pressure in ASME Sec I valves from Pentair...

Attached Files

•  Pentair.docx   382.69KB   17 downloads

[Mark-TR]

Thanks a lot to all of you for you help it has clarified a lot of things.

 

dear jamesgo i'd like to note that I've visited Leser webpage. API 526 model does resist a high temperatures:

 

http://www.leser.com...s/type-526.html

 

nevertheless, they are not ASME I certified.  thanks any way for your kind help.

 

kind regards