Metal Seated Ball Valves

Metal-seated Ball valves come in an array of sizes, styles, configurations and quality just as do their soft-seated counterparts. They are not treated quite like a commodity that their soft-seated cousins are because they come in at a significantly higher price point – all the more reason proper valve selection is important. The basic pros & cons are the same as for the soft seated ball valves:

PROS

• it’s a quarter turn device making it much easier to reduce emissions and cheaper & easier to automate.
• it has a relatively large free flow area (Cv) even in the standard/regular port configurations

 CONS

• on every cycle to the closed position the process media is trapped inside the ball and the body cavities which can “freeze” or otherwise have detrimental effects on operation, including increasing valve torques substantially
• high cycle life with tight shutoff is almost impossible with a mediocre quality ball valve, without being maintenance intensive

What makes a quality metal-seated ball valve – it again starts with the sphericity of the ball and a surface finish of as good as 1 RMS. This, again, provides outstanding smoothness and roundness, resulting in a prime contribution to low torque and zero leakage. The seats are the next item to be addressed – the harder the seats, the more demanding the quality of the ball. How the ball is held and how it mates with the seat is the next issue: floating ball and trunnion designs. Almost all of the high-end ball valves are trunnion mounted or a variation thereof, such as a heavy/extended stem guiding system in smaller sizes and lower pressure classes. Since metal-seated ball valves typically see broad service temperature ranges, any thermal distortion in the body translates into a seat leak. Thus, the best of the best metal-seated ball valves have forged body construction.

- the contact pressure
- the accuracy of surface finish on the ball and ball seat
- the contact surface of the seat
- the sealing design and the sealing material.

The contact pressure is built up by the initial stress in the seat (compact or spring supported) and the medium pressure.  The extremely high durability of  sealing systems is achieved on the one hand by a close observance of  manufacturing tolerances, ch are guaranteed by the Quality Assurance System and on the other hand by construction such as e.g. the  Teflon ball seats, backed up with a stainless steel support ring.

The width of the contact surfaces is kept in a balanced proportion between the aspects of safety and economy by an absolute spherical ball, a maximum surface finish of one (metric) micron and a fixing of the ball for 3 inch and larger.
 

       The sealing principle
        of the floating ball
    - Sealing is effected at the
       downstream seat.
    - The ball is pressed against
       the opposite seat by
       the medium pressure.
    - In doing so the seat rings
       have a double function.
       They seal off and at the
       same time serve as a bearing.
    - The seal at the upstream seat
       can be relieved in order
       to avoid a build-up of pressure.

           The sealing principle
           of the fixed ball
      - Sealing is effected at the
         upstream seat.
         The spring supported seat
         is pressed against the fixed
         ball by the medium pressure.
      - Fixing of the ball can be
                                                                                achieved in different ways:
                                                                                by bearing pads in the body
                                                                                by trunnion bearing stems
 
 
 

    A pressure build-up is prevented by the spring supported seats in connection with the fixed ball.  The separation of sealing and bearing functions ensures exemplary operating reliability and durability of  our ball valves.  This design aspect is observed without any compromises.
 

In line serviceable long life double stem system, incorporating primary seal in several material possibilities (e.g. PTFE, Viton)  and the secondary seal ring and bonnet seal in celastic for emergency and fire safety.  Stem supported in bearings to ensure seals are free from valve operating loads.
 

Anti-blow-out

Stems should be designed in a way which prevents them from being blown out when any incompetent operation occurs.

Fire safe

In case of fire, metallic contact surfaces should serve as emergency sealing, supported by heat-resistant sealing materials.  The operating safety can thus be guaranteed in case of emergency up to a temperature of 600 °C and can be verified in with the requirements of API 607 or in accordance with customers' specifications.
 

A stainless steel coil spring between bonnet and stop plate
(< 2") or a stainless steel spring washer between stem and ball (3") ensures earthed continuity between ball, stem and body.

Secondary sealing system

The secondary sealing system comes into action where disconnection from the pipeline can only be achieved with difficulty e.g.: in buried service, or when, during commissioning, the possibility of damage to the ball seats cannot be excluded.  The injection of a lubricant paste into the sealing areas between the ball surface and the seats enables effective sealing of any damaged areas, even against medium/high pressure, for both liquid and gas.

Standards

Depending on type and pressure class,  ball valves are designed in conformity with international standards, e.g.  BS  5351 - API 6 D and ANSI B 16.34. Verifications are continuously carried out by using test programs, which  were developed by BS and API.  Testing organizations such as BS - Lloyd's Register - TOV - DNV - Bureau Veritas,  as well as independent inspection authorities regularly carry out these tests on behalf of  their customers.

Sour service

Material choice and hardness in accordance with NACE MR - 01 - 75.
 

OA/QC

Design and production should be carried out in accordance with the QA manual/QC system which is subject
to continual supervision.