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Hi Phil,
I have read a lot about balanced safety valves and their function. I dont understand how they contribute by not being affected by the downstream pressure.
In fact I want to see one physically. Even though there are so many in my plant and I asked the mechanical technician about them none is aware about them.
The steam safety valves have their spring exposed that is to prevent the spring from losing its mechnical properties due to the temperature.
Can u suggest me something to look out for to identify if it is a balanced PSV.
Regards
AFD
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Psv
Started by Guest_Guest_afdmello_*, Jul 24 2003 11:55 AM
3 replies to this topic
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#2
pleckner
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Posted 28 July 2003 - 05:47 PM
Many PSVs look the same from the outside. Obviously, those that have the spring exposed are conventional valves so you can ignore them. Look for those with closed bonnets (coverings). Look at the bonnet. The balanced bellows valves will have open vent holes. The conventional valve bonnet may also have a vent hole but it will be plugged.
The BB valve uses a balancing pitston and a bellows to help equalize the pressure on the disk. The disk is basically the opening and closing part of the PSV. If you have a copy of API RP520, there a some good drawings and an explaination of how they work. You can also go to the Curtiss-Wright website at www.cwfc.com and follow the links to FARRIS valves.
One point I want to make. Balanced Bellows are not immune from backpressure problems, they are just less sensitive to them. The capacity of a balanced bellows will begin to fall off if the variable backpressure exceeds 30% of the set pressure (for many manufacturers) or 50% for Farris valves.
The BB valve uses a balancing pitston and a bellows to help equalize the pressure on the disk. The disk is basically the opening and closing part of the PSV. If you have a copy of API RP520, there a some good drawings and an explaination of how they work. You can also go to the Curtiss-Wright website at www.cwfc.com and follow the links to FARRIS valves.
One point I want to make. Balanced Bellows are not immune from backpressure problems, they are just less sensitive to them. The capacity of a balanced bellows will begin to fall off if the variable backpressure exceeds 30% of the set pressure (for many manufacturers) or 50% for Farris valves.
#3
Guest_Ben Thayer_*
Guest_Ben Thayer_*
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Posted 04 August 2003 - 10:57 AM
The response that balanced piston valves will have a vented bonnet can be misleading.
Some valves such as the Consolidated 1543 series will have a hole in the side of the bonnet that is intended as a weep hole and the valve is only intended for steam, air, etc. and NOT for toxics or flammables. This is not a balanced bellows valve.
Also, there is a real tendency amongst some operators to add a plug to the bonnet because "all the other valves have them." This is NOT the right thing to do and can cause the valve to malfunction and damage the bellows.
Model number and serial number and a catalog is the best way to be certain.
Some valves such as the Consolidated 1543 series will have a hole in the side of the bonnet that is intended as a weep hole and the valve is only intended for steam, air, etc. and NOT for toxics or flammables. This is not a balanced bellows valve.
Also, there is a real tendency amongst some operators to add a plug to the bonnet because "all the other valves have them." This is NOT the right thing to do and can cause the valve to malfunction and damage the bellows.
Model number and serial number and a catalog is the best way to be certain.
#4
pleckner
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Posted 05 August 2003 - 05:49 PM
I will agree that to be absolutely sure whether it is a conventional valve or a balanced bellows valve one should get the model number off the tag and look for it in a catalogue. However, the question was how to recognize the valve if one were to be walking through the plant. If it is obvious that the service is air or steam, it will more than likely be a conventional valve. If the tail pipe is to atmosphere, it will probably be a conventional valve. If neither of these are true, then it will probably be a balanced bellows valve. But before anyone wants to write back with specifics, I am saying "probably", not definitely. Yes, get the model number and look it up but what I state above is a quick way of narrowing down the search.
