1 reply to this topic

[sahas]

In context of the last discussion about backpressure on RVs it became evident that increasing variable supeimposed backpressure or built up pressure during discharge a balanced bellows valve is preferable ( or must).

a) Is their any industry wise guidelines for this selection between conventional and balance bellow sealed valves which could be used instead of undergoing detailed backpressure calculations.

In case of balanced belllow valves the bonnet needs to be vented safely. Is their any piping /sizing guidelines for this vent line to for monitoring for bellow leaks .

c) for corrosive application unbalanced bellow sealed valves are used. Does this mean the bonnet vent is plugged for a bellow sealed valve ?

Your inputs is appreciated.

Samit

[pleckner]

(A) Let's look at this backpressure thing in a little more detail.

When sizing a relief valve, we do our sizing calculations not on the set pressure of the valve but on the relieving pressure. The relieving pressure is the set pressure plus allowable accumulated (over)pressure. For a single valve installation, this over pressure is 10% for all "what can go wrong" scenarios except for fire. For fire, it is 21%. If more than one valve is installed and in operation, i.e. the second valve is not just an installed spare, then the 10% is increased to 16% but the maximum is still 21% for fire.

Now, a conventional valve works completely on the dynamics of the spring. There is a force pushing on it to keep the valve's seat opened and then there is the backpressure which is pushing against it the other way to try to force it closed. As long as the two forces are balanced, the valve stays opened. But again, the forces need to be balanced at a minimum and the inlet forces must be greater than the outlet and opposite forces at a maximum. Also, these valves will not relieve at their capacity unless the overpressure exists. But notice I said "capacity". This is not the same as the required relieving rate, which must be less than or equal to capacity. So what happens if during relief the backpressure exceeds 10% of set pressure? Well, the force will start to close the valve and the capacity will decrease. But will the capacity decrease be great enough such that its value falls below that of the required relieving rate? This is the $64,000 question. As long as the capacity is equal to or greater than the required relieving rate, the valve will still protect the system even if it has been de-rated.

Also, what happens during a fire? The overpressure will be 21% of set pressure. So if the backpressure goes up to, say 20% of set pressure, what will happen? Nothing!!! The force keeping the valve opened is still greater than the force trying to close it. So with all due respects to the future posts telling me how wrong I am, I'm not. In older editions of API RP520, there used to be a de-rating chart for conventional valves. It has since been removed but API has never removed the paragraph that describes the situation; see 7th Edition, January 2000 of API RP 520, Paragraph 3.3.3.1.3.

So, to answer your question, yes you can just make a call on guidelines without doing the calcualtions (I'll give it to you in a second) but you shouldn't. The system should be analyzed so that you have the most efficient system you can design.

Now for the guideline. If you have valve that relieves into a header where there will be other relieving devices also relieving into the same header, go with the balanced bellows valve.

( In my years of experience, I've never seen a pipe (or tube) connected to the bonnet. Perhaps I've just been deprived but I've never seen it. Perhaps someone else will be able to enlighten us with their experience.

© I'm not quite sure what you mean. In a corrosive environment, I would make sure the materials of constructiion are compatible. For a balanced bellows valve to work properly, you must never seal off the bonnet. Again, I await enlghtenment from others but I will be surprised if someone says different.