What makes this valve an excellent candidate?
Although lately there have been a lot of articles about this topic there is actually still a lot of confusion about the “real” meaning of DBB and DIB. Many users actually require DIB valves although they specify DBB valves. By Dieter Chlechowitz, Arflu, Spain.
Double Block & Bleed (DBB) and even more Double Isolation & Bleed (DIB) service is a challenge in many applications, as drop-tight sealing on both the upstream and the downstream side is essential in many sectors such as:
• Tank Storage
• Loading/unloading Stations
• Multi-Product Manifolds
• Metering Stations
• Aviation fuel Services
• Custody Transfer Units, and
• Offshore Platforms
The differences are defined in API for everybody to read, but to simplify the matter the essence is that a DBB valve achieves 100% tightness upstream and downstream only when there is line pressure on both sides, a DIB valve does guarantee the tightness regardless of line pressure, so also in case there is pressure only on one side. In the end it comes down to seat tightness and how a particular valve type does achieve it.
In this respect a valve testing standard like API 598 does not give you any guarantee about the valve performance as it only refers to newly manufactured valves, but what happens after the valve is in operation for some time?
Common valve designs like most gate and ball valves depend on line pressure or springs to achieve sealing and cannot guarantee long-term double block & bleed and much less DIB service as their sealing mechanism is based on friction and/or tearing and consequently seal abrasion and scored seating elements may lead to product loss and contamination.
Consider the ball valve: the ball and seats are in permanent contact during each cycle and are in constant contact, rubbing up against each other.
Consequently, after some time of operation, this leads to wear and tear and possible leakage. The same happens in a gate valve, where sealing is achieved by friction between the wedge and the seats. As a result, scratch seat rings and leakage are common consequences.
Even a small leakage may have a huge impact in the performance of an installation. In a multi-product system it may lead to product contamination, in a metering station to a wrong calibration and a deviation in the measuring. So seat tightness is important and apart from API 598 in more and more specifications users refer to the ANSI standard FCI 70-2, which defines leakage classes for different valve types, class 6 being the most stringent, which applies to soft seated valves.
But even leakage class 6 allows a certain (and considerable) leakage, which ultimately means that even a so-called DBB ball valve cannot guarantee complete tightness.
Many users simply ignore these well-known facts and still operate their installations.
DIB VALVES
With standard gate or ball valves, consequently facing important problems like product contamination or inaccurate metering.
In other installations the system to achieve DBB or DIB features is the use of 2 block valves and a spool piece with a bleed valve to verify the seat integrity.
This configuration requires a lot of space and the operation of 2 valves, which especially in the actuated version is a very expensive and maintenance-intensive solution and – although minimizing it – does not really eliminate possible leakage due to wear and tear of the seats.
Advantages of the Dual Expanding
Plug Valve
The principal idea to achieve and guarantee drop-tight shut off and therefore real DBB and DIB service is the elimination of seat wear and tear. The dual expanding plug valve can best be described as a hybrid between a gate and a plug valve, combining a rising stem motion with a 90ºturn.
By means of the plug and attached retractable slips it is constructed in such a way that during the rotation the plug and sealing elements are not in contact with the body or any other part of the valve, thus avoiding any wear and tear of the sealing elements.The rising stem action “wedges” the slips outwards into the seats achieving a purely mechanical sealing on both sides that does not depend on line pressure or springs.
Therefore this valve type with its energized seats does guarantee 100% tightness both upstream and downstream, regardless of any line pressure, and fully complies with DIB requirements.
As a matter of fact, the newest version of API 6D will include a table confirming that the only valve design fulfilling the DIB requirements without restrictions is either the use of 2 block valves or the use of anexpanding plug valve.
The advantage of a single valve solution compared to the use of 2 valves is obvious due to space and weight requirements and easier operation. But there are other benefits as well.
Due to the contactless plug rotation the operating torques are considerably lower than in a comparable ball valve, which result in smaller electric actuators and a consequent cost reduction.
Especially in the field of maintenance, the dual expanding plug valve offers important advantages. Due to its top and bottom
entry design it is easy to access the valve interior without dismantling the valve from the line, which is not possible in a common
ball valve.
In a superior design as show here the stem packing can even be replaced with the line under pressure, which saves a lot of
maintenance time and money.
When it comes to DIB service and/or drop tight sealing with a compact and user-friendly design, there is no real alternative to the dual expanding plug valve. It is already widely used in metering systems, where precision and therefore zero leakage is essential and also many oil terminals and airports rely on the advantages of this valve type.
