8 replies to this topic

[pleckner]

In my opinion, yes, you need to use a balanced bellows PSV for steam almost all the time. You will have choked flow at the exit nozzle of the PSV on steam relief just about every time and that will throw you over the 10% builtup backpressure allowed for a conventional valve.

However, most plants put in conventional valves for this service?!? And they all seem to work! My best guess is that the controlling scenario is by far very conservative and you never really get the relieving rate you calculate. Nevertheless, the calculations say you should use a balanced bellows valve and that is what I specify unless over ruled by someone willing to put his/her/its own name on the specification.

Stand your ground!!!

[pleckner]

Talk to the vendor about the PSV they sell for steam service. Most of these were designed for boilers which falls under ASME Section 1, not Section VIII. I believe (I won't bet my pay check though) that many of them are built differently from standard conventional valves (I never could get a good explaination from the vendors I've dealt with on this).

Also the requirements for Section 1 are not the same as Section VIII. As a matter of fact, most people size the discharge line for steam service per ASME B31.3, not by using the standard adiabatic or isothemal flow equations for gas/vapor service. This way, they assume the conventional valve is OK and use B31.3 to size the discharge line without even thinking about the built up backpressure.

Again, these conventional valves do work so there must be something to their construction although I haven't hit upon it yet. And again, I would use the gas flow calculations and put in balanced bellows valves.

One point I would like to make is that the cost difference between most standard size balanced bellows valves and conventional valves, of equal size, is not significant. I would try to standardize on the balanced bellows valves anyways and avoid all sorts of potential problems.

[sahas]

I have been enjoying this article but I am missing something here. The original question

"If I have a relief valve set at 300 psig relieving steam (saturated) to atmosphere, do I need to specify a balanced bellows because I can never meet the 10% backpressure on the discharge side?"

What is the limitation that prevents u sizing your SRV vent stack to meet the 10 % back pressure rule for Conventional valve ? What do you mean by never ?

Balanced Bellows SRV are required to be vented for proper operation and the bonnet also is required to be vented. This is a hazard !!! since most of the bellows are prone to failure and u can take chances with stream.

Theoritically u are correct but I guess practically size adeqaute Vent line for a conventional RV

Thanks

[pleckner]

To Guest:

No, I never said that the PSVs for steam service don't require the same analysis as other PSVs. On the contrary, my original response was:

"In my opinion, yes, you need to use a balanced bellows PSV for steam almost all the time. You will have choked flow at the exit nozzle of the PSV on steam relief just about every time and that will throw you over the 10% builtup backpressure allowed for a conventional valve."

I am always willing to admit my mistakes but please don't try to mis-quote me or put words where they aren't. I will admit that my strong knowledge base is more in tune with chemicals rather than utilities (but that will change shortly). And that is why I was not exactly definitive in my response as to why it seems conventional PSVs in steam service appear to work fine even though the calculations say otherwise.

For cost comparison, my apologies becuase I should have clarified with "of similar design" as well as equal size. Yes, my statement was more in line with Farris 2600 series and not when comparing 2600 to 1800 series valves.

Concerning standardization? We will just have to agree to disagree. There is always exceptions and I did use the word TRY in my response. (The operative word here is "TRY"). And, using your example, I am comparing "6Q8 PSVs with balanced bellows and in Hastelloy C or some such exotic material, with 12" inlet and 16" discharge piping" against the same valve but in a conventional design. Oh, a company I used to work for was doing exactly what I said, standardizing on balanced bellows PSVs where it made sense to do so. It's not such a far fetched idea!

Finally, I do agree 100% with your last statement.
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To Sahas:

Let me repeat one thing I said in a previous response. You will have choked flow at the EXIT NOZZLE of the PSV on steam relief just about every time and that will throw you over the 10% builtup backpressure allowed for a conventional valve. Note what I am saying. The EXIT NOZZLE. You haven't even gotten into the piping yet and you already blew it! There is nothing you can do with the piping to make this work.

Is it a given that the choked pressure will always be such that the built up backpressure will exceed 10% of the set pressure? Only death and taxes are "always". However, do some checking and in most cases (especially with a 300 psig relieving pressure) you will not be able to use a conventional valve PER THE CALCULATIONS.

I'm not too sure why you are so afraid of balanced bellows valves. They are used all the time! Also, I don't know where you get your statistics from that shows "...most of the bellows are prone to failure..." If you have a real hazardous system, this is the time to put in a rupture disk upstream of the PSV.

Whew! Long enough response for all of our loyal (and new) readers/contributors???

[pleckner]

Just wanted to add my two cents to the last response from "Guest". I've met Paul Papa (had lunch with him a couple of years back) and had a lot of dealings with Josh Kolenc in the past as well. They are two very good people and I concur with the recommendation.

I do want to add that there is nothing wrong with getting advice from these message boards. It makes for a great learning experience for all of us in the field. Saying this, there is nothing like getting the manufacturer's recommendation for the type of valve to use in specific environments.