9 replies to this topic

[zoran73]

I have experienced following problems with 4" #1500 ball valves - metal seated, top entry, double-block-and-bleed,
Here is the scenario...

1 Closed ball valve was pressurized from one side with 100 bar of natural gas.
2. As ball valves are double block and bleed, we unscrew plug that is in the middle of the valve connecting to ball cavity to check leakage and
no leakage detected

3. Using 3/4" bypass line we pressurized ball valve from the other side on the same pressure - and still no leakage
on the plug

4. Plug was screwed back and ball valve opened/closed - 3-4 times

5. We tried to open same plug again to release the pressure from valve cavity, but after just small opening it was leaking and not stopping
for 5-6 mins

6. We screw back the plug and tried to release the pressure after the ball valve on 1/2" relief pipe to
atmosphere but the pressure remained 100 bar - like the ball valve is open

This all very strange to me.

I think it might have something with springs and metal sealing when the pressure enters the ball cavity -
maybe that pressure is causing springs and seats not to function well.

Hope for some help on this...

[Art Montemayor]

Zoran:

A “Double-block and bleed” valve arrangement is one where you have two (2) block valves installed in series on the same pipe and a “bleed” valve is installed in the connecting pipe segment between both valves in order to allow venting or draining of that pipe segment between the valves. Please refer to the attached Excel workbook to see a schematic drawing of this valve arrangement.

Note in the schematic sketch that a total of 3 valves are involved in order to have a “Double-block and bleed” configuration. You, however, are referring to one (1) valve only as being of a “Double-block and bleed” configuration. That is difficult to visualize – or to conceive.

I suspect that what you are referring to is a ball valve that has a vent (or drain) valve installed on a vent hole that is drilled into the side of the valve body and that has access to the internal ball cavity when the ball is turned 90 degrees and is in the shut-off position. Am I correct in this assumption?

It is very difficult to follow your written description without having a sound and correct idea of what it is that you physically have. Can you furnish photos or a detailed sketch of the valve configuration that you are referring to? A "cut sheet" of the valve's design would be of help.

When you use the term “plug”, I suspect you are referring to a pipe plug that is screwed into the threaded hole made on the side of the ball valve’s body and gives access to the internal ball cavity when the ball is in the shut off position. If this is correct, then I would not recommend that you resort to checking to see if there is internal pressure inside the ball cavity by unscrewing this plug. That is a very hazardous and dangerous action. If you want to routinely check for pressure inside the ball’s cavity, then you should install a pipe nipple with a small block valve (usually another, smaller ball valve) on this access threaded valve body hole. You can then use the small valve as a test valve for cavity pressure – with or without a pressure gauge.

Await your reply and explanations.

Attached Files

•  Pipe Valves Configurations.xls   43KB   40 downloads

[zoran73]

Dear Art Montemayor

You right about everything.

But, does not API 6D say that double block and bleed can be arranged with single valve but it has to be able to close in both directions?

It is of course better to have one small valve that can be used as bleed and we will install it tomorrow, after pressure is relieved.

Metal seals (left and right) are pushed to the ball with springs which are placed around - not sure about the number of springs.

I will put image later - do not have it at this moment

[zoran73]

Here is the image

Attached Files

•  valve.jpg   136.81KB   49 downloads

[Qalander (Chem)]

Dear Zoran Hello/Good Evening,

• Firstly you have been very correctly guided by our 'Art'
• Secondly in the real sense of application and practical utility the"Double Block& Bleed System" speaks for itself i.e.
• There must be two separate Block or Gate valves(Not Ball or Globe etc.Valves)
• Combined with intermediate space bleed valve of much smaller size

Having said that

• I do recall that we were approached (At my previous employer) by some valve manufacturer also advertised in many big magazines
• with " claims" that their product (one valve) serves as a replacement for 'whole' Double Block^ Bleed system;
• that was never agreed in reality by us and I never agree to the same(probably some 11`12 years back)
• Still one valve was was somehow purchased under pressure of procurement people ;which could never prove it's service Claims/Worth.

Its configuration was similar to double u-turning flow path with throttling at each U point with globe like sealing on seat,with two points identified as plugged bleed(A very similar construction as described by you;if understood correctly!

Now what you have described is reminding me of similar problematic/ unreliable performance which made us reject and scrap the idea for good.

Just one small observation that once you operate such a valve, the plugged areas(points) get exposed-to and experience point pressure of service fluid stream; you should realize and

since no way-out exist the trapped pressure is released as per "quantity trapped" at any plugged spot

Hope this elaborately explains the whole situation with a reference to what we faced 10-11 years back; Feel free to ask if still unclear on anything

Edited by Art Montemayor, 21 October 2010 - 02:40 PM.

[Art Montemayor]

Zoran:

Like Qalander, I have never place my trust in one valve to successfully and reliably maintain a positive seat on two faces of the same ball (when dealing with ball valves). I have always employed 3 independent valves – whether gate, globe, or ball – to form what I call a “true” Double-block and bleed” valve configuration. My experiences with ball valves have led me to trust the fluid pressure to exert sufficient force upon the ball so that it seats on a soft seat and seals because of the fluid’s continued force. This principle is defeated when we apply fluid forces on both sides of the same ball. Which seat will be favored with proper seating? Agreeably, the principle is applied to a trunnion type of ball valve and not to a floating ball type of design. Nevertheless, the possibility arises that one side of the ball will be more favored than the other – leading to possible leakage through the side less favored.

API Specification 6D, Twenty-second Edition, January 2002, mentions this in:

“3.7
double-block-and-bleed (DBB) valvevalve with two seating surfaces which, in the closed position, blocks flow from both valve ends when the cavity between the seating surfaces is vented through a bleed connection provided on the body cavity.”

I have never had enough confidence in this type of design to trust it to work properly as designed. My purpose in using a DB&B arrangement has always been to ensure that there is positively NO COMMUNICATION of fluids between both sides of the DB&B. Consequently, I have never had the justification to try it by using only one valve, such as your type of ball valve. I am not surprised that you have discovered that there is leakage between one sealing face – or maybe both.

[zoran73]

I agree that three valves in DB&B configuration will be better, but this is not a option at this moment.
I understand that gas trapped in valve cavity remains under pressure until you relief it - that is why we will use another ball valve connetcted to plug that is drilled down to valve cavity.

But is seams that once pressure is in cavity - seats are not working correctly.

Now, as you can notice seats are pushed with springs that are placed around them. Those springs are making the ball valve seal, not just pressure that you apply from one side to another. It is in my opinion much different situation than you have with soft seated valves with seats fixed in valve body when pressure from one side or another is making ball move from seats and valve might leak.

I will try today to pressurized valve cavity with nitrogen in our lab to see if the pressure in cavity is causing springs to move and "open" valve... It is my assumption for now.

[Qalander (Chem)]

Dear,

Sorry.

Actually, the correct reply is the "Bitter/Harsh way indeed which we had to finally resort to" i.e., the one valve trial. pipeline area flanges/ fittings were again cut, removed, and normal system restored.

thereby very high costs and downtimes incurred;idea practically scrapped.

[Art Montemayor]

Zoran:

Your .jpg picture is difficult to interpret because even when I blow it up, the details on the seals are not very sharp and in detail. It is difficult to determine just how many springs there are and what the F. S. Ring does. However, I am very familiar with the basic design, so I can assume the basic scope of design and tell what is subject to going wrong or not functioning as planned.

As I explained in my prior post, this is a trunnion type of ball valve design and, as such, depends on the seats doing the “moving” and positioning to effect the expected seal on the ball. The ball cannot move because of the trunnion and the seats are “hard” seats. This is the part that I don’t like about the design. I am not criticizing it, I just want to point out the weak points:

Refer to my attached short table of the basic, sealing components used in your valve. Note that since the ball is fixed with respect to lateral movement and the seats are hard, the seat is expected to move and adjust to the ball in order to form a seal with its surface. To do this, the seat is positioned by springs that seem to push the “F.S. Ring” and the seat into position. But this alone is not sufficient to form a positive seal against fluid pressure. Because the seat is now moving, the seat mechanism must also form a seal against the valve as it slides forward. O-Rings are used to form this secondary seal, and this is what I consider the main culprit in this application. Depending on the fluid’s cleanliness and the conditions of the O-Ring, it may or may not form a seal as it moves. I just do not trust O-Rings to form positive, reliable seals and I strongly suspect that this is the primary cause of your problem. There are two O-Ring seals involved here – one on each side of the valve. Therefore, the probability of failure is doubled.

I suspect your valve leakage is not due to the ball not forming a seal with the seat; it may be due to the seats’ O-rings not being effective in sealing against fluid pressure due to damage, design, deformation, fluid compatibility, etc., etc.

Good Luck!

Attached Files

•  Trunnion type Ball Valve Components.doc   31.5KB   29 downloads

[zoran73]

Art, here is some conclusion that we made after yesterdays testing at our lab.

Ball valve has two holes that are drilled to the valve cavity. On one hole we placed gauge meter. On the other we connected nitrogen from 250 bar bottle.
With ball in closed position we pressurized ball cavity and at about 10 bar leakage left and right from the ball detected.

Now the question is:
1. Is the leakage caused by pressure acting on the seat and moving it away from ball, overcoming force in spring?
2. Is the leakage caused due to malfunction of O ring (pos. 4) in my drawing?

I think it is the first one!
It is because the diameter where seat is touching the ball is smaller than diameter where the O ring (pos. 4) is placed. This induces the force acting counter the springs and seat opens.

Now, to answer your questions.

There are 12 springs. FS Ring is just lubrication that can be done from outside of the valve. This lubrication can act as seal but actually O-ring pos. 4 is holding pressure. That O-ring is placed closest to the ball.

The ball valves are intended to be used on gas storage facility where dirt is very common - I saw what is going out from these pipes and it is very very dirty water... with some gas in it I hope ... Also we witnessed water coming out of these drilled holes to ball cavity, one plug was moved out. So metal seats were I think required solution.
Also with 250 bar working pressure I am not sure that floating ball type valve would be possible - as once you have such big pressure pushing ball against seat, it is very difficult to open/close valve.

We will today test to find if the problem is with o-ring or it is with pressure acting against sprigs.
Also we try to add more springs - double (24 pcs), but I feel it can not be sufficient to withstand 250 bar once we had problem on 10 bar.

Also we will try to reduce the diameter of seat where o-ring is making that diameter equal to one that seat is making contact with the ball equalizing the forces on seat.

Edited by zoran73, 23 October 2010 - 01:39 AM.