These points are:

• 1. Any damage even in very small dimension to the profiled sealing elastomer element which is located on the rim of the disk will cause leakage
• 2. Any repair job needed requires removing butterfly valve from the pipeline and to do this you need to drain the pipeline upstream and downstream of the valve, in order to take the valve out.
• 3. Butterfly valves have preferred sealing direction which means they seal on one direction better than the other direction, and this can cause the valve to leak in the non-preferred directions. Especially when the pressure is high.
• 4. Because of double eccentric design of butterfly valves and interference seating mechanism these two causes very large operating torques to overcome dynamic and seating-unseating torque.

To overcome all these problems MECHANIC AB Company, Tabriz/Iran, has designed, fabricated, and tested a special kind of butterfly valve with energizable and contractible sealing element located in the body of valve instead of disk rim: the T-Butterfly valve.
In T-Butterfly valves the sealing element has no contact with the valve disk in its whole course of rotation and keeps a 2-3 mm clearance with the disk and only when the disk moves into its full close position the seal is energized by a small hand or electrical pump which uses water to press the sealing element to contract and seal the valve. Upon unseating the valve, the sealing element is depressurized letting the seal to expand and move away from disk rim leaving the 2-3 mm clearance and then the valve start to open. This causes long life of the sealing element and lower required torque. This characteristic also eliminates the need for by-pass valve in order to balance the pressure upstream and downstream of the valve and so it saves money by making valve room smaller and unnecessary valves and fitting for by-pass line.
On the other hand, because the sealing element is energized from outside almost any kind of damage to the sealing element has no effect on the valve complete sealing in both directions. In other words the T-Butterfly valve has no preferred sealing direction. Also a single damage with the size in Figure 2a to the sealing ring of a normal butterfly valve causes severe leakage. Three damages with the sizes in Figure 2b to the sealing ring of a single T-Butterfly valve however does not effect complete sealing of the valve.
T-Butterfly valve is a centric valve and this causes much lower dynamic torque. These valves need 30-40% less torque than normal butterfly valves. T-Butterfly valve is produced in two versions: single sealing and double sealing type. In double sealing version the disk is by plane type caring two sealing trims each, and two sealing elements one each side of the valve, the sealing mechanism in the valve inlet is used as stand by sealing element and the sealing element in the valve outlet side is normally operated and if for any reason the sealing mechanism requires replacement the sealing mechanism in the inlet side comes to operation so without any need to drain or depressurize the pipeline at the upstream side of the valve you can conduct sealing replacement on the downstream side of the valve. T-Butterfly valves are produced in sizes of 300-3000 mm and pressures in 10,16,25,40 bar they can be either hand operated or electrically operated. T-Butterfly valves are produced in single seal and double seal versions.
Summarizing the above mentioned the T-Butterfly valve offers:

• 1. Complete sealing in both directions even with damaged sealing element
• 2. 10-15% lower weight because of shorter face to face dimension
• 3. 30-40% lower operating torque
• 4. No need to by-pass line to balance the pressure before opening the valve.
• 5. No need to remove valve from pipeline to replace sealing element
• 6. Much longer service life of sealing element because of no rubbing action on it.
• 7. Much more dependability than normal butterfly valve.