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How Pilot Operated Safety Valves Work?


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#1 papaya

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Posted 16 February 2009 - 02:49 AM

Been recapping how safety valves work, but realise i haven't fully understood how pilot operated valves works.

Please correct me if i'm wrong:

I assume for conventional safety valves, if the backpressure is high, the pressure tend to "grab" of push the valve downwards resulting tighter valves that require more inlet pressure to lift the valve.

Using balanced bellow, the bellow which is softer than the valve/stem tends to "absord" the high backpressure (up to 30% of inlet set pressure) stem so the backpressure less likely to act on the valve itself thus minimizing the backpressure effects.

About Pilot Operated Safety Valves, I understand it has a tube which brings the inlet pressure towards the upper part of the valve body, and because of the area at the upper part is bigger than the lower inlet part, there is enough force to keep the valve close under normal condition, and when the inlet pressure reach relief pressure the pilot valve will open, thus the top part of the valve looses pressure and the bottom inlet part will have enough pressure to push open the valve. It is stated that pilot operated valves can operated under backpressure up till 80% of inlet set pressure, however I also see in some drawings there are 2 places where backpressure applies: 1.There is a tube from the valve outlet connected to the pilot valve (which i assume is the pilot valve venting to outlet), and 2.the backpressure that plagued conventional valve is still there around the valve. So how does pilot operated valve overcome these backpressure? Won't the backpressure coming back from the outlet into the pilot valve tube affect the valve opening?

Been trying to find explanation but still can't fully understand.

#2 fallah

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Posted 16 February 2009 - 10:23 AM


What is the reference you are talking based on? Please read API 520 (7th Edition) sec 2.2.2 and you will find clear explanation about the operation of all types of pilot operated valves.

#3 papaya

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Posted 16 February 2009 - 10:10 PM

QUOTE (fallah @ Feb 16 2009, 11:23 PM) <{POST_SNAPBACK}>
What is the reference you are talking based on? Please read API 520 (7th Edition) sec 2.2.2 and you will find clear explanation about the operation of all types of pilot operated valves.


I was looking at this article: http://www.spiraxsar...afety-valve.asp

A pity I couldn't find API 520 sec 2.2.2 only manage to find the installation chapter. Will see if my other library have this standard.

#4 JoeWong

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Posted 17 February 2009 - 04:08 AM

Papaya,

If you in the oil and gas industry ( as I presume) and you are serious about it, you should have a copy of API Std 520, part 1 and at least understand the terms, operation, type, & selection of a Pressure relief Device (PRD)...

Spirax sarco is one of the "great" sites for information. There are others: i.e. Crosby handbook. More in the following link.

QUOTE
I assume for conventional safety valves, if the backpressure is high, the pressure tend to "grab" of push the valve downwards resulting tighter valves that require more inlet pressure to lift the valve.


More-or-less "right"... Increase in back pressure to Conv. PSV tends to increase force against the inlet pressure, this result higher inlet force to lift the disc


QUOTE
Using balanced bellow, the bellow which is softer than the valve/stem tends to "absord" the high backpressure (up to 30% of inlet set pressure) stem so the backpressure less likely to act on the valve itself thus minimizing the backpressure effects.


There is area different between the disc facing PRV nozzle and disc facing chamber. The bellow (or piston) is to balance these areas in order remove the impact of backpressure. The Spirax sarco page is pretty clear on this. Also, you can read more here.


QUOTE
About Pilot Operated Safety Valves, I understand it has a tube which brings the inlet pressure towards the upper part of the valve body, and because of the area at the upper part is bigger than the lower inlet part, there is enough force to keep the valve close under normal condition, and when the inlet pressure reach relief pressure the pilot valve will open, thus the top part of the valve looses pressure and the bottom inlet part will have enough pressure to push open the valve. It is stated that pilot operated valves can operated under backpressure up till 80% of inlet set pressure, however I also see in some drawings there are 2 places where backpressure applies: 1.There is a tube from the valve outlet connected to the pilot valve (which i assume is the pilot valve venting to outlet), and 2.the backpressure that plagued conventional valve is still there around the valve. So how does pilot operated valve overcome these backpressure? Won't the backpressure coming back from the outlet into the pilot valve tube affect the valve opening?



The pilot can be "viewed" as another little "balance piston PSV" and it has a much smaller disc area. These make the pilot valve less sensitive to back pressure. Thus, it can take more than 50% back pressure, subject to type of pilot...

The Spirax Sarco image is one type. There are other type of pilot operated PSV. Please read the API Std 520 Part 1.




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