Bonnet Bleed Valve

[SVpower]

We have multiple valves thoughout the plant that has a smaller bonnet bleed valve. The question came up as to what postion these valves should be in. Does one leave the valve open at all times; or do you open the valve prior to operation and then shut it?

For example we have a 8" Newco gate valve (feedwater control valve outlet) and there is a smaller 1" globe that taps into the bonnet and goes to the downstream side of the larger valve.

 

[Zapster]

I am not an expert in this area; however, small changes in temp can create huge pressures for fluids in confined spaces. If bonnet is isolated and liquid packed when valve is closed, and then there is an increase in the trapped fluid temp, excessive pressure in the bonnet will lead to seal or bonnet failure.

The small globe valve should be open when the gate valve is closed if fluid can be isolated in the bonnet when the valve is closed.

I believe that swagelok makes some of the small PSVs that I have seen on some bubble tight valve that have isolated pockets when the valve is closed. I don’t recall if there was a common piping arrangement either upstream or downstream for the connection after the PSV. If I remember the valve number, I will post later.

 

[zdas04]

The bonnet bleed is a response to the fact that for an isolated water-full system, pressure will increase about 100 psi for every 1F temperature increase--you can get very high pressures within the bonnet on start-up that can jam the valve or even break the valve casting. Many start-up procedures require draining the bonnets of valves that remain shut for part of a cold-plant start-up. Draining them at shutdown is generally not effective because they can tend to fill back up over time due to minor seepage.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Zapster]

Again, I am no expert here so look at my comments with caution. I thought that for a wedge gate valve, its bonnet only had positive isolation from the main stream fluid when the valve was closed. Fluid would be trapped between the wedge and the bonnet stem seal. I do not understand how fluid is sealed in the bonnet when the valve is open.

Anyways, if my concerns are correct, the pressure rise concerns should be similar to the General Twin Seal Valve’s bonnet. This is a valve that I am very familiar with. Go to

http://www.coopercameron.com/cgi-bin/ccc/common/showdocw.cfm?OID=40000379

and read page 18. There is a lot of misunderstanding about just what the bleed port is for in these valves.

 

[zdas04]

Zapster,
Bonnet bleed valve are there for protection of shut valves. Open gate valves don't trap fluid (but open ball valves can).

David

 

[Zapster]

My understanding is that trunnion ball valves can trop fluids both open and closed; however, the ball seals are spring-loaded and self-energized. So any trapped fluid in the isolated cavity will be relived past the spring-loaded/self-energized ball seals. Basically one seal on a trunnion ball valve only has an effective seal in one direction. No problem because there is a seal on both the upstream and downstream end of the ball. For floating ball valves, no fluid can be trapped.

Zdas04, reading your first post, you said, “Many start-up procedures require draining the bonnets of valves that remain shut for part of a cold-plant start-up. Draining them at shutdown is generally not effective because they can tend to fill back up over time due to minor seepage.” I do not understand what you are recommending here.

 

[SVpower]

So if the valve is to be normally left open during operations then there should be no worries of leaving the bleed/drain valve closed. But at the same time it could be said that it doesnt really hurt to leave the valve open either. Only during cold starts on valve that is shut should I be concerned about the postioning of the bonnet bleed/drain valve.

BTW Thanks for the inputs!!!

 

[Zapster]

SVpower, no, the statement, “Only during cold starts on valve that is shut should I be concerned about the positioning of the bonnet bleed/drain valve,” is incorrect. The position of the bonnet bleed should be of concern any time the gate is closed and the bonnet is liquid packed. Because this is liquid service that means any time the gate is closed. I am not sure what zdas04 was saying in his post regarding the startup.

I just tried the address for the reference I gave above and found that it did not work. So I assume that you could not read the document. Go to

http://www.coopercameron.com

and locate the document, “General Valve Twin Seal Operation and Maintenance Manual (GV) “ using their search engine. Page 18 of the English version of this document clearly gives the scope, background, and how the cavity bleed system should works.

 

[zdas04]

Zapster,
Gate valves are not a bubble-tight seal. Consequently if you shut a gate valve and drain the body cavity and shut the bonnet bleed a few hours later there will be fluid in the cavity again due to seepage past the seats. A few days later the body cavity will usually be full. With the body cavity full of liquid, if you add heat to one side of the valve during startup you'll risk overpressurizing the bonnet.

If the body cavity is full and at the same temperature as the surrounding fluid then you don't have much risk of overpressurizing the bonnet, but the risk is not zero.

You're statement abouve about trunnion ball valves only sealing in one direction doesn't really jibe with my experience. There are a lot of very stiff springs behind the seat. The seat fits very tightly in its channel and it has considerable stiffness. I certainly wouldn't bet my life that pressure between the sides of an open ball valve will equalize across the seat. I also don't know where you are coming from with your statement that the seats on a trunnion ball valve only seat in one direction. I've taken valves from several manufacturer's apart and I sure don't see a "check valve operation" on the seats.

David

 

[Zapster]

zdas04, In contrast to gate valves which I know a little about, I do know something about plug and ball valves. Typically the differential body cavity pressure needed to lift the seal off the ball is around 200 psi. Hence, it is considered that most trunnion ball valves body cavities are considered to have automatic internal relief of body pressure. In contrast, differential pressure in the other direction only pushes the seal more tightly against the ball. That is why I made reference to only an effective seal in one direction. For any valve installation, follow the manufactures recommendations.

Now for a Newco 8” valve that is the main topic of this thread, I made an assumption that this was a wedge gate valve. I do not believe there is anything to lift when the wedge is pushed tightly against the seat ring unlike most trunnion ball valves (keep in mind that this in something that I am not positive about). This would make the cavity sealed like a General Twin Seal plug valve. Hence, the recommended explanation from Cooper Cameron on body cavity overpressure protection. I would not count on this valve to have a leaking seat that is not bubble tight to ensure that the bonnet is not overpressure in liquid service when ambient temperatures change. Machining tolerances are getting better all the time and fluid does not flow past an imperfection in a seat like most gases. What zdas04 is saying that there will be some guaranteed leakage between the wedge and seat ring that protects the bonnet from all ambient temperature changes; however, it will not protect the bonnet when heat flows from the conduction of stagnant fluid down the line during a startup. I would consult with the valve manufacturer to confirm that this is acceptable and meets their installation requirements so that warrantee claims will not be compromised. This is not to imply that there would be a warrantee claim. I am not saying that zdas04 is wrong, only that I would verify his recommendations with the valve manufacturer.

 

[Hipressure]

Is there any possibility the valve is just a "start-up" by pass to equalize pressure around the gate, and make opening the valve easier? I have had little exposure with bonnet drains in the power industry. But your description sounds like a bypass with the exception of bonnet (could be body) tie in location?