This question may sound like a provocative statement which it is because some prefer Kp-Lok double block and bleed valve. Recently, several recommendations and articles have been compiled, several tests have been done and there is development of several products. If we take a leap back at the test which was carried out in the 1990’s we would observe the relationship between fugitive emission and valves.
There are two types of seals on a valve and they are dynamic and static seals. The bonnets as well as the flanges are in the same position while the spindle is in motion. The dynamic seal on the spindle is the most likely to be the main leak source of the leaking valves.
It was shown that out of about one thousand valves which are leaking, only 2% were ball valves, 32% gate valves, 24% control valves and 42% globe valves. So to understand this, there is need to understand the way valves are operated. Which type of valve which has the most spindle movement during the operation of the valves? The answer to this is globe valve and this is because it is usually rising and rotating.
Gate valve is the nest in line and in this case, there is a rising spindle movement and does not rotate. The same is applicable to the control valve but quite different from gate valve because it doesn’t move all the way up or down. Also, there is a least spindle movement in all the ball valve and the movement is 90 degrees to and fro. Which of the seal valve is exposed to high wear and tear because of the use? There is no need for you to be knowledgeable in the field before makingthe appropriate conclusion.
There are some people that usually ask whether the situation is the same as it was several years back, the answer to the question is a combination of Yes and No. We can boldly say Yes because the rate of leakage has reduced due to the better quality of packings whenever they are used. The major leak source of the valve is the globe valve spindle area while the gate valves is the second one and the ball valves are in the front. We now have to take a look at the various spindle seals which are functioning. The spindle seals are of two kinds:
- Seals due to the compression of the packing material
- Sealing by means of pressure from the media
The first category is the Lip-seals because the O-rings are not considered in this case. The difference which exist between the Lip-seal and the Chevron rings are mostly a combination of rings which are placed on one another while the Lip-seals is a single ring and in this case, a spindle seal is inside while the bonnet seal is outside. There is need to preload the two systems on the seal so as to function properly. The Chevron ring will get the preload from the compression ring which is above the chevrons after the installation.
The preload of this seal originate from the spring which is found between the lips and the within the ring. There is higher than what is required for the pressure activated seals and there are different kinds of materials which are used for the compressed packaging, but most companies’ deals with graphite because it is the most commonly used materials in the hydrocarbon industry.