Why and how to maintain trunnion ball valves part 2
In the first article we discussed the spindle seal and whiter or not there spindle was equipped with a lubrication fitting making it possible to lubricate or seal the valve in case of a external leakage out past the stem seal. The next item on the list was lubrication fittings and connecting canals to the seat/ball seal. Looking back 30 years or more when all ball valves was soft seated and easily scratched and worn on the seat seal there was secondary seal injection ports on the valves where sealing component could be injected to seal a valve in case of a internal leak. Now a day it is common to use hard metal or tungsten carbide seal on the seats making the much more resistant towards scratches and operational wear. This in the mind of some makes the lubrication fittings and the connecting canals unnecessary and in fact a danger to the safety of the valves as they are regarded as possible leak-points and therefore taken away.
Lets look into the why and how´s for the use of lubrication fittings in connection with the seats on a trunnion valve. The lubrication fittings have three main functions.
One; to keep the seats loos in the seat pocket enabling them to keep in good contact with the ball to make the valve seal
Two; as earlier mentioned to use sealing component to seal of minor leaks or scratches.
Three. clean the seal surface of the ball an seats.
It´s not that placing a lubrication fitting and drilling some holes solves all the problems, fare from it, it´s not that simple, there is valve manufacturer constructing a good solution that actually function very well but other’s do not work at all. Lets see what works and why.
In figure 7 you can se how the canal from the lubrication fitting connects to the canals in the seat and leads the lubrication to the ball/ seat seal area. As discussed in earlier articles, the seat (blue) needs to be able to float in the seat-pocket to make a seal together with the ball. If the seat is stuck in the seat pocket making it impossible to form a seal with the ball, one can inject in valve cleaner to reduce the friction between the seat and the seat-pocket. When reducing the friction the coil springs in figure 7 will overcome the friction in the seat-pocket and press the seat towards the ball. This has nothing to do with the seat having metal or soft seat seal. Some month ago I was do a valve maintenance job on an installation in the Norwegian sector. On the list were 8 leaky valves ready to be replaced with new valves, but in this time of economical crises and budget cuts they was going to try my method of maintenance and try to save the valves. After injecting valve cleaner and done some hocus pocus seven of the eight valves was 100 % sealed. The eight valve did have a minor leak, due to damages on the seat seal
The last valve was a 6” class 900 valve with only one lubrication fitting to each seat, we tried to seal the valve by injecting sealing component. Managed to reduce the leak-rate by 60% but that was the best we got.
We now have to take a look at the way the canals are drilled in the valve body and the seats. Looking at figure 8 you can see the horizontal canals drilled in the seat, the canal transporting the cleaner or sealing component towards the ball. Imagine there were only 4 of this canals in the seat, how much sealing component did you have to inject to make the sealing component from one hole meet the component coming from the hole next to it to enabling a seal to be formed by the sealing component entering between the ball and seat. Then imagine there are eight or ten horizontal holes in the seat, you would then need a less amount of sealing component to fill the distance between the holes. So the first role is simple, you want as many holes as possible to get a good result when doing maintenance on the valve.
Then we have to think of how many lubrication fittings there are to each seat. In article one in this series I stated that every trunnion valve between 3” and 8“ should be equipped with two lubrication fittings to each seat (on each side), and all 10” and up should have four lubrication fittings to each seat (two on each side).
Valve number eight on my offshore job had only one lubrication fitting. The second rule is that there must sufficient with injection points to get the cleaner and sealant transported around 100% of the seal area. Look at figure 9 and imagine that the sealing component injected into the one canal should be transported into the seat and around the seat to the opposite side of where the injection was taken place. I would think it´s nearly impossible to be able to this without using a huge amount of sealing component. Then imagine there are two lubrication canals on each seat, one on each side, now it is feasible to seal of the valve by using sealing component. I have never been able to seal of a valve with only on fitting to each seat, I have been able to reduce the leak rate but not to seal it complete.
I did a job on 12 valves in a metering-system, they was nearly 30 years old soft seated 24” valves, they all had some medium to small leaks, but as illustrated in figure 10 the valves was equipped with four Injection fittings to each seat, two on each side. After injecting a normal amount of sealant component all 12 valves was sealed. There was no need for valve replacement and the valves were ready for at least 10 more years.
To compare, I had another job on a 24” class 1500 trunnion soft seated ball valve, that valve was equipped with only one lubrication fitting on each seat placed diagonally opposite of each other. The valve had a small internal leak, but it was not possible to get the valve sealed. After injecting 100% more medium sealant component then calculated necessary, the leak rate was reduced by only 40%. It was not possible to seal that valve with only one lubrication fitting there was simply not enough canals and injection points.
If you are going to do maintenance on a ball valve that have been in service for 10 – 15 years you first of all need to clean the sealing surfaces and reduce the friction between the seats and the seat pockets. To be able to do that you need a valve cleaner in gel form. The reason for that is that liquid substances like diesel or kerosene only flows downwards. If looking at figure 9 and imagine you are injecting diesel, the injected liquid will enter the seat pocket and float to the bottom of the valve. Absolutely nothing will go towards the top of the seat. With only one lubrication fitting you can only clean ¼ of the circle. With two fittings you can clean ½ of the circle, you will never be able to clean the whole circle unless the cleaning agent is a semi liquid substance in gel form that actually transport itself upwards and filled the whole seal area and all the canals with cleaning agent. And further more the gel form cleaner will stay on the metal surface for how long as it takes, but the liquid diesel or kerosene will float down and dry out, unless you were constantly injecting lots of it.
To be continued
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