Pressure controlling or Pressure Containing? 2

[bmoorthy]

Hello All

In a gate valve, is the gate Pressure controlling or Pressure containing.

In a Ball valve is the ball pressure controlling or Pressure containing.

The above question is asked in the context of applying the requirements (For material, NDE etc). Whether the treatment of the balls and gates are to be different from that of Valve Body.

By the way how are Gates and Balls designed. Gate thickness is determined by what process.

Instead of Solid ball, if some body decides to use a fabricated ball using plates, how to determine the thickness. (Is it permitted to use Hollow ball (Fabricated from Plate)

Standards such as B 16.34, API 6A does not seem to address the above issue.

 

[jsummerfield]

The valve body and bonnet are pressure containing components. I do not regard valve trim as pressure containing.

John

 

[Sircrashalot]

This is a debate I have often with clients and internally as certification requirements always revolve around the pressure containing components.

In many cases, they have included the balls/gates, in other cases not. In almost all cases, the STEM is included as a pressure containing part from the point of view of certification. In general, as the ball or gate is a key component of the valve design and corrosion or other failure of this part will lead to operational problems, the client wants the QA back-up for these parts. I believe API6D counts the ball and stem as part of the pressure containing parts.

I have even had clients consider the bonnet and body bolting and the gland packing retainer as 'pressure containing', so it really depends on your client's definition.

I have only seen reference to hollow balls in a client specification where they specified that they were not allowed. All of the balls I've seen (from 1/2" to 30"+) have been solid forgings.

Hope this helps,

Andy

 

[bmoorthy]

Thanks a lot for your view point.

This debate started because the client spec calls for NDE (Ultrasonic testing) of the pressure containing components.

On analysing this clause one of the gentle man opined that the Gate or the ball contains the pressure when they are in closed position.

Now traditionally NDE for the Balls are not performed by most manufacturers and there is no cross reference for NDE for the ball in any of the standard.

The pressures are very high say in the range of 250 to 300 Bar and the sizes are big in the range of 24 to 40 inches. The balls will be like solid huge rock (Forged) but no NDE as per any reference standard? but we have this statement in the client specification hence the doubt.

 

[Sircrashalot]

We have a saying in my company "The client may not always be right, but the client is ALWAYS the client".

This is a case where it can be technically argued either way, but the client being the client, they will usually win the argument.

Sounds like you are dealing with some pretty serious valves, which also also lends itself to being more cautious and doing extra NDE, regardless of what the spec usually calls for.

I have not had any of the balls UT'd on any of the valves I have supplied - just Dye Pen to check for cracks in any overlay or hard facing. I'm sure you and your client can come to an agreement on what standard to apply for UT of the balls, or see if there is another method of NDE that he would like to employ.

Real problem is that they can usually only do the UT after the ball has been forged and machined - if a problem is found you are looking at significant schedule delays to get a new ball (not to mention the cost).

Really it boils down to Quality ASSURANCE - what can be done to show that the ball has been manufacturered correctly and contains no hidden defects that could be detrimental to performance. How can you give your client the 'warm and fuzzy feeling' that the valve is going to work, and giving him the paper to back this up? That is your real task.

Andy

 

[cityone1]

According to ASME Sec. III Class 1, valve disc is pressure containing part. see para. NB3546.2