The how (and why) of maintenance part 6
The case with the six valves in the previous article is only on of many examples where the lack of proper maintenance / preservation of the valves was the reason behind the problems. There are many different agendas when it comes to building a plant and one is to get the investment cost as low as possible, but as we all know we will get what we pay for. I don´t say you should go bananas, but you should carefully look at the reasons for your valve problems and find out what the root causes are. It´s something true in the old saying: Nothing is to expensive if you get what you need, but it´s a total loss if you cant use what you bought cheap. And if you add a shut down and the time loss of production you end up using fare more than the early “too expensive” solution would have cost you.
As we all know designing and producing a high-pressure valve is difficult. The dimensions, material quality, weight, flexibility of the material, pip stress, pressure and temperature are all making their contribution the challenges. And on top of it all there are the challenges of the soft seals. The valve needs soft seal to allow for the operation, movement of the stem witch are a static seal that suddenly goes dynamic, and then we have the radial seals on the seats.
As stem seal there are several solutions like O-rings (fig. 26), Graphite rings (fig. 27), Lip seals fig. 28) or Chevron rings (fig. 29) or it could be a combination of two or more like illustrated in fig. 30 witch combine lip seal as the primary seal and O-ring as the secondary seal. In fig. 31 there is a combination of graphite rings and Chevron rings of different qualities. Some of the seal types are compressible like the graphite and some are not compressible like the Lip seal or Chevron rings. The non-compressible seals, witch seal by the mean of pressure must have a pre-tension towards the stem and bonnet for the seal to function, as it should.
In many cases it´s stated from the valve manufacturer that the stem seal are lubricated for life and for that reason need no lubrication fitting. The first part is true and the only objection to that is that the life can be short. The second part is totally wrong; most valves should be equipped with a lubrication fitting to the stem. In all cases lubrication will reduce operational weir and prolong the lifetime of the seals. But in addition to the lubricating possibilities of the stem seal you will also have the possibility of using sealing component in case of a stem leak.
If you are planning for maintenance in case of a stem leak in the future, you also must select the proper type and arrangement of the stem seal. The seal arrangement and seal type depends on the medium and type of stem on the valve.
The stem can be a stationary one on a parallel gate valve or on a ball valve. In the case of a stationary stem on a parallel gate valve there can be from a few up to hundred rotations on the stem, compared to a ball valve witch only will have ¼ turn from open to closed. The stem can also be a rising stem witch moves up and down. The worst stem movement are rising and rotating as on most of the globe valve; witch probably is the cause to the fact that the globe valves are the valve type with most reported stem leaks.
Looking at the ¼ turn of the trunnion mounted ball valve there are many seal solutions that could be chosen, like the one in fig. 32 witch is a combination of O-rings and Lip seal, or the one in fig. 33 witch is double set of O-rings. But it is not the seal type that is important in this case, what is important is the fact that both solutions do have a lubrication fitting installed in the bonnet. This enables you to inject a non-hardening sealing component in between the stem seals. In fig. 32 the seals are installed in a lantern witch allows the sealing component to be spread all around the stem and create a good seal.
In both fig. 32 and 33 the injection canal enters in between the two seals. I would prefer the canal to enter below the lower seal allowing the sealing component to go down to the thrust bearing of the stem. When the sealing component enters in between two seal there can sometime be difficult to fill the whole volume between the two seals with sealing component. It´s very important that the sealing component do not dry out, it must be soft and flexible otherwise it will shortly start to leak.
There is a difference between rising and rotating stems when it comes to parallel gate valves. The stationary stem in fig. 34 only moves by rotating but the rising stem in fig. 35 are moved up an down when opening and closing the valve. Do to the movement of the stem the seals must be slightly different. The upper Chevron stack are installed with the lips pointing down on both valves, but the lover stack in fig. 34 points down and the lower stack in fig 35 points up.
The reasons for this is easy to se if one imaging injecting sealing component to seal of a leaking stem. If the seal instalment in fig. 34 were on a rising stem the injected sealing component would be dragged down when the stem was operated down-word and the leak would re-occur. But if the lower stack were installed like in fig 35 the sealing component would be locked in between both stacks when the stem was moving up and down. As long as the stem only rotates like the one in fig. 34 the cavity pressure will hold the sealing component in place and seal of any stem-leak. If the seal stack contained a lantern-ring like in fig. 31, it would be easier to inject the proper amount of lubricant/ sealing component all around the stem. But there are drawback one must be aware of: When injecting sealing component into the construction in fig. 34 excessive sealing component will pas the lower stack and just go into the cavity, but if injecting sealing component too fast or too much there can be a pressure build up that prevents the valve from operating properly.
If injecting anything into the stem or the seat lubrication fittings the personnel must be trained for the job and the pump should always be equipped with a gauge so the person doing the job are in control over the situation.
To be continued
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